wc/94/065r supergene geochemistry of arsenic, antimony and … · 2004-03-26 · supergene...
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BRITISH GEOLOGICAL SURVEY
TECHNICAL REPORT WC/94/65/R Overseas Geology Series
Supergene geochemistry of arsenic, antimony and associated elements at Globe and Phoenix Mine, Kwekwe, Zimbabwe.
Martin Williams and Barry Smith Minerals and Geochemical Surveys Division
A report prepared for the Overseas Development Administration under the ODA/BGS Technology Development and Research Programme, Project 9216.
0 D A classification Subsector: Others Subject: Geoscience Therm: Mineral Resources Project title: Environmental Impacts of Gold and Complex Sulphide Mining
Bibliographic reference: Williams T M & Smith B: 1994: Supergene geochemistry of arsenic, antimony and associated elements at Globe and Phoenix Mine, Kwekwe, Zimbabwe.
Cover illustration: Tabex Mining Operation, Globe and Phoenix, Kwekwe.
Keyworth, Nottingham, British Geological Survey, 1994
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SUMMARY
The Globe and Phoenix mine, Kwekwe is historically the second largest producer of gold in Zimbabwe. Mineralisation is carried by quartz veins in a "W-trending shear system close to the contact of the Kwekwe Ultramafic Complex and the Rhodesdale Batholith, within an antimony-rich province of the Midlands Greenstone Belt. Arsenic, lead and zinc also occur in the zoned ore assemblage. In contrast to many gold mines in the Midlands region, Globe and Phoenix is located in an area of high population density. The potential human and environmental impact of the mobilisation of toxic trace elements such as As and Sb from the site is, therefore, considerable.
Microprobe and XRD data for pristine ore, gossan and tailings samples from Globe and Phoenix indicate that the primary sulphides degrade to a haematitic-limonitic assemblage in which As and Sb are held as oxides (e.g. tripuhyite), oxyhydroxides (e.g. stibiconite) and arsenates (e.g. scorodite). A complex partitioning of Pb is indicated by the simultaneous occurrence of Pb phosphates, sulphates and carbonates. In excess of 100 mg/l As and Sb was detected in hydrocyclone water used for laboratory size/density fractionation of tailings samples, suggesting the presence of readily soluble secondary phases. These may be amenable to dissolution by infiltrating water in the Globe and Phoenix waste pile.
The hydrochemistry of the north-flowing drainage emanating from Globe and Phoenix is temporally variable and reflects the relative contributions derived from spoil runoff (acidic) and tailings re-processing eftluent (alkaline). In March 1993, acidic discharge (min. pH 3.7) was recorded under moderate flow conditions (c. 2 m3/s) downstre'm of the mine. In February 1994, a very low downstream discharge (<1 m3/s) was found to be near-neutral or alkaline (max. pIi 9.5) in character. High concentrations of As and Sb (max. 7.4 mg/l and 15.5 mg/l respectively) characterised the upper reaches of the drainage under both alkaline and acid conditions, with values generally declining by an order of magnitude within 1 km of the source. The high solubility of As and Sb across a wide pH range is, in part, attributable to the low ambient concentrations of dissolved Fe (c1 mg/l). The formation of soluble organo-Sb complexes in the waste pile may also be signilicant.
High concentrations of hcavy metals were rccordcd in the uppermost sector of the drainage system during both sampling campaigns. Under acid discharge conditions, the mobility of Cu and Zn (m'w. 2085 pgA and 236 pg/1 respectively) appeared to be directly controlled by pH. Under alkaline conditions, high dissolved concentrations of several metals (16 mg/l Ni, 0.64 mg/l CO, 34 mg/l Cu, 29 mg/l Zn) were detected at the head of the system, with solubility controlled by the presence of cyanic complexes (derived from re- processing eflluent). Such complexes are inherently unstable and degrade, with resultant precipitation of metals, within a few hundred metres of the source.
Equilibrium-speciation and SI calculations for As and heavy metals (based on the code WATEQ4F) indicate a dominance of As as HA SO^^- in alkaline discharges, with major elements present as sulphates and uncornplexed ions. Despite the high SO4 loading, most heavy metals are likely to be stablized as hydroxyl or carbonate complexes at high pH. The strongly alkaline (pH 9.5) headwaters are theoretically undersaturated with respect to As oxides such as arsenolite and the higher-solubility arsenates (notably Ca(As04)~). Saturation with respect to scorodite is unlikely due to the low abundance of dissolved Fe. Simulative scoping of As and heavy metal speciation under acid conditions (pH c4) has shown that As could be stablized as H2AsO4, with heavy metals forming sulphates or uncomplexed ions.
X-ray fluorescence data for drainage sediments and flcxxlplain soils have confirmed the presence of >loo0 pg/g As, >5000 pg/g Sb and anomalous heavy metal coilcentrations in these media to the west and north of the Globe and Phoenix mine. Sequential extraction data for sediments indicate that up to 78% of the total As burden is immobilized through binding within crystalline Fe-oxides, although a significant fulvic organic component is also evident at some highly contaminated sites. Over much of the floodplain to the north of the mine, soils appear to be enriched with As and Sb as a consequence of the physical outwash of tailings, and show a multi-element signature broadly cornparable to samples from the waste pile. A weakly sorbed, water-soluble soil As-Sb component was isolated during size fraction analysis.
Despite the observed enrichment and mobilisation of As and Sb, the toxicological impact of the Globe and Phoenix operation may be relatively minor. Dissolved As predominantly occurs in the least toxic of the common inorganic forms (arsenate). A proportion of the total As and Sb loadings reported may also be organic, with very low toxicity. The impact of particulate As and Sb contamination may, however, warrant attention. Preliminary studies of bioassimilation by invertibrates and vegetation have shown that
translocation of As and Sb through local food chains can occur. A thorough medical or epidemiological assessment of human exposure could thus be justified.
Any future strategy for improved pollution control at Globe and Phoenix should account for the following unusual characteristics of the site:- (i) Intense AMD is not evident. (ii) Dissolved Fe concentrations are lower than those normally associated with mine drainage (during both acid and alkaline drainage episodes) and ochre precipitation is not an important regulatory process. (iii) As and Sb are the principal aqueous contaminants. Both occur primarily as oxyanionic species and are mobile across a wide pH range. (iv) Heavy metal contamination is localised, often associated with cyanic alkaline waters, and of limited environmental significance. (v) Particulate As and Sb dispersal exceeds aqueous transport as an environmental hazard.
While many conventional methods of mine water (AMD) decontamination are not appropriate for the site, bioremediation systems and induced precipitatiodsorption methods (using positively charged resins or other surfactants) may prove applicable.
CONTENTS
1: BACKGROUND: 1.1: INTRODUCTION 1.2: CHEMISTRY AND TOXICOLOGY OF ARSENIC AND ANTIMONY
1.2.1 Physical chemistry 1.2.2 Hydrochemistry 1.2.3 Soil chemistry 1.2.4 Toxicology
1.3: PHYSIOGRAPHY 1.4: GEOLOGICAL SETTING 1.5: MINING HISTORY
2: PRESENT STUDY: 2.1: ORE AND WASTE MATERIALS 2.1.1 Primary assemblages 2.1.2 Gossan mineralogy 2.1.3 Waste slimes
2.2.1 : Methodology 2.2.2: Temporal flow variability 2.2.3: pWEh regime 2.2.4: Arsenic and antimony 2.2.5: Major cations 2.2.6: Anions 2.2.7: Minor and trace elements 2.2.8: Sebakwe river chemistry 2.2.9: Geochemical modelling
2.3.1: Bulk geochemistry 2.3.2: Partitioning
2.2: HYDROGEOCHEMISTRY
2.3: SEDIMENT CHEMISTRY
2.4: SOIL GEOCHEMISTRY
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3: DISCUSSION 3.1: PRINCIPAL CONTROLS ON 39 GLOBE AND PHOENIX DRAINAGE QUALITY.
3.1.1: Chemistry of cyanic waters 3.1.2: Chemistry of sulphidic
waste drainage 3.1.3: Mixing products of
alkaline and acid waters. 3.2: TOXICOLOGICAL RISK 4 4 3.3: OPTIONS FOR IMPROVED 4 5 ENVIRONMENTAL QUALITY
3.3.1: Bio-remediation 3.3.2: Chemical removal
3.4.: FUTURE PRIORITIES 47
ACKNOWLEDGEMENTS REFERENCES APPENDIX
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LIST OF FIGURES
Figure 1: Location and physiography of the Globe and Phoenix mine, Kwekwe.
Figure 2: Simplified geology of the Kwekwe area
Figure 3: X-ray microprobe maps of pyrite grains from Globe and Phoenix ore.
Figure 4: Tabex open-pit gossan mining operation (viewed from the west).
Figure 5: Schematic illustration of Globe and Phoenix mine and associated drainage showing the position of water, sediment and soil sampling stations.
Figure 6: Present day tailings impoundment and overflow drainage (viewed from the west)
Figure 7: Deeply incised gulley carrying intermittent drainage from tailings sands into the principal NNW-flowing river emanating from Globe and Phoenix mine.
Figure 8: pH and Eh profile extending NNW from Globe and Phoenix mine.
Figure 9: Figure 10: Figure 11:
As & Sb profile extending NNW from Globe and Phoenix mine. Major cation profile extending NNW from Globe and Phoenix mine. Major anion profile extending NNW from Globe and Phoenix mine.
Figure 12:
Figure 13: Figure 14:
Piper diagram showing major cation-anion characteristics and total dissolved solid content of water samples GPW 101.102,104 and 110. Fe, Mn, Ni & CO profile extending NNW from Globe and Phoenix mine. Cu & MO profile extending NNW from Globe and Phoenix mine.
Figure 15: pH-Eh diagram for arsenic (the broken line shows the stability field of barium arsenate).
Figure 16: Modelled aqueous speciation of Cu in samples GPW 101,102,104 and 110, showing the characteristic dominance of metal hydroxides over sulphates.
Figure 17: Sequential extraction scheme for the determination of particulate trace element speciation (after Breward and Peachey, 1983).
Figure 18: Variation of As, Sb, W and Pb (ug/g) in sub-surface soil along E-W trending transect (1): Globe and Phoenix mine.
Figure 19: Variation of Al2O3, LOI and MnO (%) in sub-surface soil along E-W trending transect (1): Globe and Phoenix mine.
Figure 20: Comparative major element geochemical signatures of oxidized waste sands (Globe and Phoenix site GPX 150) and alluvial soil from site GPS 30, transect (2).
Figure 21: Comparative trace element geochemical signatures of oxidized waste sands (Globe and Phoenix site GPX 150) and alluvial soil from site GPS EO, transect (2).
Figure 22: Aqueous partitioning of HCN and CN- - defined by pH.
Figure 23: pH dependency of abiotic F$+ oxidation (from Singer and Stumm, 1970).
Figure 24: pH-controlled variation of HFO sorption efficiency in mine drainage from Reynolds Tunnel, Colorado, predicted using a combined equilibrium- speciation and GLTM modelling approach (Smith et al., 1993).
LIST OF TABLES
Table 1: Ore mineralogy and thin section description of selected mineralised samples from Level Six, Globe and Phoenix Mine.
Table 2: Major and trace element composition of auriferous gossan: Phoenix open pit working.
Table 3: Major and trace element composition of processed tailings: Globe & Phoenix dumps
Table 4: Practical ICP-AES detection limits for elements determined in Globe and Phoenix waters (mg) .
Table 5: Geochemistry of surface waters collected in the vicinity of Globe and Phoenix mine in March 1993 and February 1994.
Table 6: Hydrogeochemistry of Sebakwe River (mg/l)
Table 7: XRF data for stream sediments in the vicinity of Globe and Phoenix mine (major element values quoted in %; trace elements in pg/g)
Table 8: Partitioning of As and selected major and trace elements between sedimentary phases (all data are given in pg/g)
Table 9: Major element geochemistry (%) of soils in the vicinity of Globe and Phoenix Mine
Table 10: Minor and trace element geochemistry (pg/g) of soils in the vicinity of Globe and Phoenix mine.
Table 11: Partitioning of As, Sb and Pb between different soil size fractions: site GPS 15.
Table 12: Selected water quality standards, stipulated by WHO (1959) and US-EPA. (1989). All element values are give in mg/l.
1.1: INTRODUCTION
Metalliferous mining accounts for over 10% of Zimbabwe's GDP (1991 Mining Annual Review statistics). While continued investment in the minerals sector is officially encouraged, the requirement for environmental monitoring and post-extractive remediation to minimise the adverse impacts of mining is increasingly being recognised. Potential contaminants can be resolved into two distinct categories:- (i) hazardous mineral processing agents (e.g. Hg, CN compounds) and (ii) toxic trace elements occurring naturally within ore assemblages (e.g. As, Sb, Pb, Zn, Cu, Cd). In Zimbabwe, legislative controls cover the use and disposal of cyanide, and Hg availability is restricted by licencing. However, limited data exist to indicate the magnitude, spatial extent and impact of natural contaminants mobilised through the oxidation of sulphide waste. The formulation of advisory or legislative guidelines covering this important area is, therefore, currently difficult.
Studies of the supergene dispersal of potentially toxic elements around gold and complex sulphide mining operations in tropical regions were initiated by BGS in 1992, as part of a Technology Development and Research (TDR) programme funded by the British Government Overseas Development Administration (ODA). The principal aims of the programme are:- (i) to examine the dispersal mechanics and fate of selected trace elements associated with gold and complex sulphide mineralisation, (ii) to establish the role of geological setting, ore mineralogy and mining/processing technology as determinants of drainage geochemistry, and (iii) to formulate practical, cost-effective remediation strategies for contaminated sites. To date, over 50 mining areas in south-east Asia and southern Africa have been investigated. In Zimbabwe, study sites have included a diverse range of vein system, shear-zone-hosted and stratabound Au deposits in the Midlands, Shamva and Harare greenstone belts.
As Zimbabwe's second largest gold producer (120 t) and a major supplier of Sb (1830 t), the Globe and Phoenix mine, Kwekwe, provides an ideal setting in which to appraise the supergene geochemistry and toxicology of As, Sb and related trace elements. In contrast to many mining operations in the Midlands region, Globe and Phoenix is located in a high population density area. The Fate of contaminants mobilised during mining is, therefore, particularly important. Field investigations in the Globe and Phoenix catchment area were undertaken in March 1993 and February 1994. In this report, mineralogical, geochemical and ecotoxicological data are presented, with some preliminary assessment of their environmental implications.
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1.2: CHEMISTRY AND TOXICOLOGY OF ARSENIC AND ANTIMONY
1.2.1: Phvsical chemistry and geochemistrv Arsenic and antimony are metalloid (semi-metal) elements in Group V of the periodic table. Both occur at relatively low average crustal abundances of 1.8 pg/g and 0.2 pg/g respectively, ranking 51st and 62nd of all naturally occurring elements (Greenwood and Earnshaw, 1984). The physical chemistries of As and Sb are closely analogous. Both possess four oxidation states (ranging from M-3 to M+5), of which the trivalent and pentavalent states dominate under most surface environment conditions. The M-3 gas phases, AsH3 and SbH3, are stable only at extremely low redox potentials (around -1 V). Native As and Sb occasionally precipitate in hydrothermal assemblages under acutely alkaline-reducing conditions.
Arsenic and antimony are strongly chalcophile, and sulphides such as realgar (As&), orpiment (As2S3) and stibnite (Sb2S3) are the favoured 'mono-metallic' primary mineral phases under most pH-Eh conditions. In most ore assemblages, mixed sulphides such as arsenopyrite (FeAsS), enargite ( C U ~ A S S ~ ) , gersdorffite (NiAsS), tetrahedrite (CuSbS3) and jamesonite (FePbqSb6S 14) are, however, more common. Arsenides and antimonides of Ni, CO and Fe also occur widely. The principal carrier of As in non-mineralised lithologies (and in many sulphide ores) is pyrite (FeS2), in which up to 0.5% As may be present through lattice substitution for sulphur.
A diversity of secondary As and Sb minerals is derived from low temperature alteration of detrital sulphides such as arsenopyrite, realgar and stibnite. Hydrous arsenates and sulpharsenates, notably scorodite (FeAs04.2H20) and beudantite (PbFe(AsO4)(SO4)(OH)6), are the most common secondary As minerals. Oxides such as arsenolite (As2O3) have also been noted as coatings on FeAsS surfaces in disseminated sulphide deposits, and on native arsenic (e.g. Breward and Williams, 1994). The weathering of primary Sb sulphides in the surface environment typically yields oxides or hydrous oxides such as stibiconite (Sb204.HzO) and servantite (Sb2O4).
1.2.2: Hydrochemistry The natural concentration ranges of As and Sb in unpolluted fresh waters are 1-10 pg/l and 0.5-5 pg/l respectively (Fergusson, 1990). In sulphide mining provinces, the typical As range is 100-5000 p g l . The aqueous chemistry of As and Sb differs significantly from most true metals. Binary compounds either react with water, or are insoluble. Both elements predominantly form oxyanions in solution. At moderate or high redox potentials, As can be stablized as a series of pentavalent (arsenate) oxyanions: H3As04 (pH<3 Eh >500 mV), H2As04- (pH 3-7 Eh >250 mV), HASO$- (pH 7-12, Eh >-300 mV) and As043- (pH >12 Eh >-600 mV). However, under most reducing (acid and mildly alkaline) conditions, the trivalent
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arsenite species (H3As03) predominates. At extremely low Eh values and under alkaline conditions, As02- forms a stable aqueous phase. Antimony forms a hydroxyanion, Sb(OH)6-, under mildly acidic or alkaline pH conditions (>5) at most naturally occurring redox potentials. Antimonite (H3S b03) and polyantimonite salts (e.g. NaSb02) are stable in solution within an Eh field similar to that of arsenite. Disequilibrium between inorganic As (and Sb) species in natural waters is common due to kinetic constrainsts on M(In) - M(v) transformations. Fergusson (1990) has reported the persistence of arsenite for several weeks in oxic marine and (near-neutral) lake waters, following transport from deeper anoxic waters by seasonal currents or thermal overturn.
Arsenic and antimony form stable covalent bonds with carbon to yield a range of alkyVmethy1 species and associated salts. Their contribution to the total As and Sb budget of natural waters is however, usually small ( d o % , Andreae 1986). Methyl As and Sb species are mainly produced by microbial metabolism of inorganic species (S-adenosylmethionine constituting the most common As methylation agent), although abiotic methylation reactions are also known (Abernathy, 1993). In fresh water, the dominant phases are monomethyl and dimethyl compounds; CH3H2As03 (monomethylarsonic acid), CH3H2S b03, (CH3)2HAs02 (dimethylarsinic acid) and (CH3)2 HSb02.
The retention of As and Sb in solution is constrained by co-precipitation with elements such as Fe, Ba, CO, Ni, Pb and Zn, of which Fe and Ba are particularly important due to their low arsenate solubility products. Colloidal reactions may be involved in the precipitation of scorodite (signified by the common occurrence of green X-ray amorphous gel coatings on FeAsS-rich detritus). The sorption of As and Sb by hydrous oxides (or related gel complexes), clays and humic organics has been widely documented (e.g. Pierce and Moore, 1982). The relative efficiency with which individual As species are sorbed to amorphous oxides is pH dependent, but generally decreases from arsenate through organoarsenic species to arsenite (Bowell, 1994). Hydrous Fe oxide gels and colloids are particularly instrumental in scavenging As and Sb oxyanions, due to their strong positive charge characteristics across a wide pH range (Boyle and Jonasson, 1973).
1.2.3: Soil chemistry In soils, the normal concentration ranges for As and Sb are 0.1-40 (mean 6 ) and 0.02-10 (mean 1) pg/g respectively (Ure and Berrow, 1982; Adriano, 1986), rising to 2500 pg/g As and >50 pg/g Sb over many sulphide ore deposits (e.g. Adriano, 1986, Williams et al, 1994). The enhancement of soil As by phospate fertilizers and sewage sludges has also been widely reported (e.g. Fergusson, 1990). In free draining profiles, As resides predominantly as arsenate (AsO4) and is extensively sorbed to clays and ferric oxides. Arsenates of Fe and AI, the dominant phases in acid soils, exhibit low solubility relative to Ca3( As04)2, which
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dominates in many calcareous soils. The mobility of As in oxic soils is thus strongly influenced by Fe content and pH. In anaerobic soils, As may be mobilised and leached through bacterial or abiotic reduction of As04 compounds to soluble arsenite salts. Few investigations have been made of the geochemistry of Sb in soils, but trivalent phases are thermodynamically favoured under most conditions.
Down-profile As and Sb distributions vary markedly in accordance with the local soil regime. Adriano (1986) suggested that both elements are characteristically enriched in the surfkial levels (upper 20 cm) of unpolluted temperate soils. Similar profiles in freshwater sediments have been ascribed to sub-surface anoxia and the resultant leaching of reduced species (e.g. Farmer and Lovell, 1986). However, Bowell (1993) has shown that enrichment of As in the O/A horizon of rainforest soils in Ghana can be related to complexation with organic matter. Soil profiles with maximum As concentrations in the B horizon, or in the interfacial saprolite, have been reported from gold mining localities in Malaysia (Williams et al., 1994). In these instances, the As distribution correlates closely with the concentration of fine-grained ferric oxide.
1.2.4: Toxicolow and human exposure The toxicities of As and Sb are highly dependent on chemical form. Both exhibit greatest toxicity in the M-3 (gas) state. With regard to the more commonly occurring forms, a decrease of toxicity by an order of magnitude through each stage of the sequence M3+ > Ms+ > methyl As/Sb is evident (Abernathy, 1993; Chen et al, 1994). In most fauna, As3+ and Sb3+ toxicity results from reactions with thiol (HS-) groups to form stable thio-AdSb derivatives which inhibit enzyme functions. Arsenate toxicity is manifested through substitution for phosphate. Mammals possess variable abilities to detoxify inorganic As through a sequence of reduction (of Ass+ to As3+), methylation and excretion via the kidneys. In humans, both MMA and DMA are produced, while in most other animals, DMA is the main metabolite (Buchet and Lauwerys, 1993; Vahter, 1993).
Arsenic and antimony are normal components of the human diet. Concentrations of As and Sb in grain, meat and vegetables lie in the ranges 10-200 ng/g and 0.02-40 ng/g respectively, with substantially higher As values (c. lpg/g) present in most seafood (Fergusson, 1990). The US-EPA has estimated an average As intake of 53 pg/d for US citizens, of which up to 80% may be organoarsenic (derived from fish, shellfish and vegetables) and 20% is in inorganic form (derived from meat and grain; Abernathy, 1993). In uncontaminated areas, the average ingestion of As via drinking water is around 5 pg/d (Fergusson, 1990).
Soluble inorganic As and Sb species are readily absorbed in the gastrointestinal tract and are generally considered 100% bioavailable for purposes of risk assessment calculation (Chaney
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et al., 1993). In contrast, in vivo, in vitro and mineralogical assessments of particulate As in soil and mine waste (e.g. Chaney et al., 1993; Tsuji, 1993; Davis et al., 1993) indicate relatively low bioavailabilities ( d o % ) . For populations not subject to occupational exposure, dietary ingestion (including drinking water) is thus more significant than inhalation as a mechanism of As and Sb intake.
A detailed review of the human effects of As ingestion has been provided by Abernathy (1993). Acute oral exposure typically induces gastro-intestinal irriation, loss of peripheral nerve response and, ultimately, cardiovascular failure. The estimated lethal dose in adults is 70-180 mg. Acute Sb toxicity induces liver, kidney and heart malfunctions. Data regarding precise dose- response thresholds are not available.
Examples of chronic As poisoning associated with regionally contaminated water have been documented from numerous countries including Taiwan (Tseng et al., 1968; Chen et al., 1988), India (Chakraborty and Saha, 1987), Mexico (Espinoza, 1963; Cebrian et al., 1983), Chile (Borgono et al., 1980) and Thailand (Fordyce and Williams, 1994). The characteristic effects of long-term consumption of water containing >50- 1000 pg/l are hyperkeratosis, hyperpigmentation, malignant melanoma and peripheral arterioschlerosis (Black foot disease). The role of As exposure in the development of bladder, liver and kidney cancers (notably, unique to humans) has recently been highlighted through clinical (e.g. Sasieni and Cuzick, 1993) and epidemiological (e.g. Chen et al., 1992) studies.
Current UK, US-EPA and EC standards for As and Sb in potable waters stand at 50 pgA and 10 ug/l respectively, although a future reduction of the US-EPA threshold is anticipated in response to evidence of chronic impacts at lower exposures (e.g. Chen et al, 1992; van Leeuwen, 1993). An international interim guideline for As of 10 pg/l has recently been proposed by the WHO (1993).
1.3: SITE PHYSIOGRAPHY
The Globe and Phoenix mine, Kwekwe, lies approximately 170 km south-west of Harare in the Midlands region of Zimbabwe (Fig. 1). The area is characterised by flat or gently undulating topography, with an average altitude of around 1200 m. The regional drainage is dominated by two north flowing rivers, the Kwekwe and Mbembeswana, both of which join the west-northwest flowing Sebakwe-Munyati system around 8 km north of Kwekwe town. An additional network of minor (ephemeral) streams drains north-westward from the Globe and Phoenix mine to the Sebakwe.
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Figure 1: Location and physiography of the Globe and Phoenix mine, Kwekwe.
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The population of Kwekwe is approximately 100, OOO. High density townships lie to the east (along the Mvuma road) and on the western edge of the town, adjacent to the Globe and Phoenix dumps. Much of the land around Kwekwe is utilised by industry, including a large chrome smelter, refractory ore roaster and brewery. Maize and other subsistence crops are grown on most undeveloped land, including a large area between the Western Townlands extension and the Globe and Phoenix mine.
1.4: GEOLOGICAL SETTING
The geology of the Kwekwe region (Fig. 2) comprises a diverse range of basement granitoids, volcanic and metasedimentary rocks, intruded by igneous lithologies of varying age and composition (see Harrison, 1970, for details). The area to the east of Kwekwe is dominated by the Rhodesdale Batholith; a composite ellipsoid mass of basement gneisses and younger 're- generated' granites. The western margin of the batholith is in contact with a narrow outcrop of serpentinites and talc-carbonates, forming the Kwekwe Ultramafic Complex. Serpentinised dunites, often heavily silicified and carbonated, occur along north-trending fractures parallel to the regional strike and form the principal host for the auriferous reefs at the Globe and Phoenix and Gaika mines. Surface silicification has further altered the carbonate zones, and outcrops typically consist of brown chert, or spongy limonitic rocks.
Figure 2: Simplified geology of the Kwekwe area (from Porter, 1991).
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The Kwekwe Ultramafic Complex is overlain by Upper Bulawayan metavolcanic and metasedimentary rocks, trending N-S for a distance of approximately 150 km between Kadoma to Shurugwi. Immediately west of Kwekwe, a basal zone of altered basaltic pillow lavas (the Mafic Formation) forms the contact with the Kwekwe Ultramafic Complex. Interbedded jaspilites give rise to a series of N-S trending topographic highs (the most conspicuous geographic features of the Midlands greenstone belt). Further west (and south- west), the pillow lava and jaspilite outcrops are enclosed within a broad wedge of andesites, dacites and felsites. This heterogeneic sequence probably reflects cyclic volcanicity ; basaltic members at the base grading progressively to more felsic segregations.
The Upper Bulawayan volcanic sequence was folded along a NNW-trending axis and eroded substantially prior to the onset of Shamvian sedimentation. Metagreywackes, phyllites and grits of the Shamvian Group outcrop extensively to the west of the Upper Bulawayan greenstone belt.
Gold mineralisation in the Midlands of Zimbabwe occurs in both stratabound (iron- formation) and non-stratabound (vein and shear-zone) settings. Regionally, vein deposits at the margin of the Rhodesdale Batholith are primarily confined to a major "W-trending shear-zone, characterised by quartz-sericite (Foster et al., 1986), and hosted exclusively within the marginal facies of the Greenstone Belt. This trend is conspicuously contradicted at Kwekwe, where mineralisation in the NNW-trending shear system cuts obliquely across a wide range of lithologies (Porter, 1991).
The Globe and Phoenix deposit comprises a series of veins in a ductile shear system focussed at, or close to, the contact of the Kwekwe gneisses (marginal Rhodesdale Batholith facies) and the Kwekwe Ultramafic Complex. Shear directions are diverse, but are predominantly NW- and NE-trending. The shearing was primarily confined to ultramafic facies, reflecting the contrasting ductility of the serpentinites and adjacent gneisses. Fissure development is evident for a distance of up to 25 m from the contact. Shears are typicaIly complex, showing narrow areas of intense deformation, enclosed within broader, less comprehensively fractured zones. Detailed information regarding the deformation characteristics and vein development at Globe and Phoenix has been provided by Porter (199 1). The mineralisation shows strong zonation with depth. Pyrite and jamesonite dominate near the surface, with stibnite and tetrahedrite common to 1000 m, and a galena-sphalerite-bearing assemblage prevalent at greater depths.
8
1.5: MINING HISTORY
The Globe and Phoenix area shows extensive surface workings attributed to activities of the 'Ancients' (600-1700 AD), and was subsequently pegged by Europeans in 1894. The Globe and Phoenix reefs were initially worked independently, but milling was undertaken on a combined basis with no record retained of the relative tonnages taken. Production from the Globe system ceased in 1907, with effective exhaustion of the Phoenix sector some 70 years later. The Phoenix reefs were worked to a depth of over 1400 m (43 levels), producing a total of around 120 t Au at an average grade of 27.6 g/t.
The main Phoenix shaft remains operational to the present day. Access is possible to the uppermost 12 levels, below which the mine is flooded. Current gold production is largely based on heap-leach cyanidation of carbonated and silicified limonitic gossan, extracted from an open pit (effectively a surface expression of part of the Phoenix reef system). Reprocessing of tailings (at a rate of around 30000 t per month) is also being undertaken by pressure hose extraction and tank cyanidation, yielding an average grade of 0.5- 1 g/t.
2.1: ORE AND WASTE CHARACTERISTICS
2.1.1 : Primary assemblages A total of 21 pristine mineralised rock specimens was recovered from two Em- and NNW- trending vein systems exposed at level six of the Globe and Phoenix mine. Optical and microprobe investigations of polished thin sections of four 'representative' samples (Table 1) were undertaken by staff of the BGS Mineralogy and Petrology Group, and are reported in detail elsewhere (Naden and Bland, 1994). Results confirm that As and Sb are principally hosted in gersdorffite (NiAsS) and tetrahedrite-tennantite ((CuFe)l2(SbAs)4S13). Boulangerite (PbsSb4S1 I), jamesonite (Pb4FeSbgS14) and arsenopyrite (FeAsS) are all present as minor components of the mineralised assemblage. Other ore minerals recorded include pyrite, pyrrhotite, mackinavite, sphalerite, chromite and galena.
Microgeochemical maps of samples GPR 13 and 14 indicate a persistent zonation of pyrite with respect to Ni and CO (Fig. 3). Fracturing and veining of pyrite with Sb minerals (notably tetrahedrite) is also common. A paragenetic sequence, comprising early pyrite-arsenopyrite- pyrrhotite, followed by Sb-phases (tetrahedrite, boulangerite and jamesonite) and base metals (galena) is widely apparent.
9
I
I ' ..
L.,
4.- , .
a.;'
;:- A
.
L > ..- .J
'. .
0 0
0 0 a
0 a a e 0 0 a a
a a
e
a 0 0 0 a a a 0 0 a a 0 0 0 a 0 0
Table 1:
Ore mineralogy and thin section description of selected mineralised samples from Level Six, Globe and Phoenix Mine.
SAMPLE ORE MINERALS SECTION DESCRIPTION
GPR 11 PYRITE Quartzairhate vein with minor sulphides (<1%). Approximately equal proportions of quartz and carbonate. Principal sulphide is pyrite, typically subhedral with a grain size of c. 1 mm. Pyrite is not veined and replaced by base-metal sulphides. No secondary alteration phases evident. Rare occurrence of chromite.
.n.~......~.....~o...........~.n.~....~......n...~~~n..~...n..~..~n~.o..o~~.oo~on......n..oo.~~......~~..........~~~~~~.~
GERSDORFFITE CHROMITE
GPR 12
GPR 13
GALENA TETRAHEDRITE PYRITE
Vein quartz with minor (c. 5%) sulphides occurring along micro- fractures. Dominant sulphides are galena and tetrahedrite (95%) with minor pyrite. Pyrite is locally fractured and veined by galena and tetrahedrite. Sulphides are fresh, with no signs of low-temperature alteration. A minor green mica (fuchsite) and carbonate occur in close association with sulphides.
TETRAHEDRITE SPHALERITE Ni-PY RITE GERSDORFFITE CHROMITE
Tetrabedrite is the dominant sulphide with intergrowths of sphalerite and galena, and commonly associated with carbonate (magnesite). Nickeliferous pyrite occurs rarely as euhedral grains and is commonly veined and replaced by other sulphides.
GPR 14 CHROMITE Quartz-carbonate vein with thin folded band (1 mm) of sulphides and minor broken chromite. Carbonate is the main gangue phase. Pyrite is the dominant sulphide, locally cemented by gersdorffite with small inclusions of nickel-sulphides.
NI-PY RITE GERSDORFFITE
2.1.2: Gossan m ineralogv a nd geoc hemistry The auriferous gossan currently being worked by Tabex Mining yields c. 0.5-1.0 g/t Au. The open pit (Fig. 4) has a maximum diameter of >500 m and extends to a depth of c. 20 metres. The gossan shows typical haematitic and limonitic coloration, and is variably carbonated and silicified. Massive quartz-carbonate veins and minor veinlets remain clearly identifiable. The unweathered facies exposed at the base of the pit dominantly comprise talc-schists and serpentini tes.
Samples of oxidized detritus were collected at varying depths from wall sections of the Phoenix pit during visits in 1993 and 1994. Qualitative mineralogical investigations (microprobe and X-ray diffraction) confirmed the presence of haematite, goethite, limonite and manganite as the dominant secondary constituents. Pyromorphite (Pb4(PbCl)(P04)3), anglesite (PbS04), cerussite (PbC03) , tripuhyite (Fe2S b2O7). stibiconite ((Sb3+Sb4+2)06(OH)), scorodite (FeAs042H20) and a range of other hydrated Sb and As minerals occur widely as minor accessories.
1 1
0
0 0
0 0
0 0 0
0
0 0
0 0 0 0
0 0
0
0
0 a 0 0 0 0
0 0
0 0
0 0
Multi-element data (Table 2) for freshly exposed gossan, recovered from 2 m and 5 m depths at the north-west margin of the Phoenix pit, were obtained by X-ray fluorescence.
Table 2: Major and trace element composition of auriferous gossan: Phoenix open pit working.
GPX 151 Ni Cu Zn As MO Pb Bi V Cr 5682 561 317 1901 18 298 nd 193 2238
GPX 152 3311 26 83 240 11 26 4 155 3255
GPX 151 GPX 152
A P Cd Sb Ba Majors = % 6 nd 302 456 7 Traces =ppm 6 nd 59 69 6
The vertical geochemical zonation within the gossan is consistent with intense tropical leaching. De-silicification of superficial material (such as occurs during laterite pedogenesis) is accompanied by relative enrichment of transition metals (Mn, Fe, Ni, Cu, Zn), As and Sb, bound primarily in hydrous oxides. Chromium is enriched in the deeper levels of the profile, in association with spinels and primary resistate minerals. It is notable that the total abundances of the metalloids As and Sb, plus certain heavy metals (notably Pb) are low in the gossan profile relative to the average abundances recorded in mine tailings and overburden (see below). Dissolution and mobilisation of these elements during deep weathering is therefore inferred.
2.1.3: Tailings composition
Tailings material from an actively used sector of the Globe and Phoenix dumps (adjacent to the township access road to the west of the mine) was collected during the 1993 field visit for use in granulometric and mineralogical investigations (see Naden and Bland, 1994 for details). Total abundances of As (2100 ppm), Sb (3997 ppm) and Pb (4022 ppm) were determined by XRF analysis, after which the material was size-fractioned into -60 and -120 BSI mesh fractions and re-analysed. Results indicate that approximately 70% of the total As is contained in the coarser fraction (probably as primary arsenopyrite), while Sb is preferentially associated with finer phases. Analysis by XRF of evaporated residues from the wash-water used during the sieving process confirmed the presence of significant water- soluble As and Sb components (probably secondary oxides).
a 0
12
0
0
0
a 0
0
0
0
a 0
a 0 a e 0 a a a a 0
0
0 a 0 0 0 0
0
0
0
0 0
0
0
150, coupled with the observed liberation of substantial ( A 0 0 mg/l) As and Sb concentrations in wash-water during sample sieving, suggest that some, or all, of the preferred secondary phases are amenable to dissolution by permeating water.
Table 3: Major and trace element composition of processed tailings: Globe & Phoenix dumps
Ni Cu Zn As MO Pb Bi V Cr GPX149 5808 >1% >1% 19027 35 21108 nd 18 922 GPX 150 411 87 159 456 12 173 nd 43 493
& Cd Sb Ba U Majors = % GPX 149 166 108 >I% 21 nd Traces =ppm GPX 150 1 nd 1305 114 4
2.2: STREAM HYDROCHEMISTRY
2.2.1 : Methodology Surface water samples were collected from 7 stations along a NW-flowing drainage network emanating from the Globe and Phoenix site, and from an additional site on the Sebakwe River (15 km away) downstream of inputs from the Globe and Phoenix catchment area (Fig. 5). Surface water pH and Eh was determined in the field (using a temperature-compensated pH electrode, Pt Eh electrode and a Radiometer Instruments meter). Calibration of electrodes was undertaken prior to each site measurement using a series of commercial buffer solutions (pH 4, 7 and 9) and Zobell Eh standard. The suite of samples collected at each site for geochemical analysis included:- (i) 30 ml unfiltered HN03-acidified water, (ii) 30 ml filtered (0.45 pm Millipore) unacidified water, (iii) 30 ml filtered (0.45 pm) HN03-acidified water and (iv) 30 ml filtered (0.45 pm Millipore) HCI-acidified water. Additional samples, collected only at selected localities, included (v) 30 ml filtered (0.20 pm Millipore) HNO3- acidified water and (vi) 30 ml (0.45 pm) filtered water with 5 ml dipiridyl reagent. Filtered waters were obtained by drawing 30 ml of water into a sterile syringe and passing it through a sealed filtration cartridge (loaded with a filter of appropriate pore-size) into a Sterilin storage tube. Acidification of samples was carried out within 4 hr of collection through the addition of 0.3 ml of concentrated ARISTAR grade acid (HNO3 and HC1).
14
0
0 0
0 0 0
0 e
a m
e a 0
a
a 0 a
0 0
0 0
0 0
a 0
e 0 0
0
0
Figure 5: Schematic illustration of Globe and Phoenix mine and associated drainage, showing the position of water, sediment and soil sampling stations.
0
(B) drainage from former spoil (A) drainage from tailings impoundment and process tanks
Analyses of As and Sb were carried out by hydride-generation (ICP-AES). Total abundances were determined using HNOyacidified samples, with HCl-acidified waters used for the analysis of As3+ and Ass+. Arsenite was determined following the suppression of As5+
species (including organoarsenicals) by saturation with Zr. Arsenate (plus methyl As) was then calculated by subtracting the As3+ value from the total As concentration. A solvent extraction technique for As3+ removal (using ammonium pyrolidinedithiocarbamate as a complexing ligand; Chen et al., 1994) was used to cross-check the Ass+ data for three samples (GPW 101, GPW 104, GPW 1 lO), and showed good agreement (~10% variation) in each case. Analyses of additional major and trace elements (including K, Ca, Na, Mg, Fe, Mn, P, Al, Cu, Zn, Pb, CO, Ni, Ba, Si, MO, B and Cr) were made by ICP-AES. Practical detection limits for all ICP-determined elements are given in Table 4. Data for NO3, SO4 and C1 were obtained by ion chromatography. Analyses of HCO3 were carried out by HC1-titration. Unless otherwise indicated, all laboratory analytical data presented in this study relate to 0.45 pm filtered waters.
15
8 8 8 8
8 8 8 8 8 8 0
0
0
I)
0 0
0 0
0 0 0
0
0 0 0
0
0 0 0
0
0
0
0
Table 4:
waters (mg/l). Practical ICP-AES detection limits for elements determined in Globe and Phoenix
99D-DDD~9ooo9---DDoo9o9-9~~ooo99o9Dooo99Dooo9o99Doo999oooooooooooo99oooo9o9o99DoDoooo~oo~~~o9oo9DDooo9999D~w
Na 0.020 Mg 0.030 K 0.100 Al 0.030 P 0.030 so4 0.100 Ca 0.020 Cr 0.010 Mn 0.005 Fe 0.010 C O 0.010 Ni 0.010 Cu 0.007 Zn 0.005 Pb 0.030 As 0.0002 B 0.013 MO 0.006 Si 0.100 Sb 0.001 Ba 0.002
Temporal variations of water chemistry were investigated through the repeated collection and analysis of samples from selected sites (GPW 101, 102, 103) during the 1993 and 1994 field campaigns. Data for both temporal suites are given in Table 5.
2: Tempo ral flow va riabili ty The natural "W-f lowing watercourse which drains the Globe and Phoenix site along its western margin is fed by numerous seepages, gulleys and pipelines, draining dominantly westward from the present-day tailings impoundment and former waste sandshailings dumps. The most significant tributaries (in terms of discharge) are indicated on Figure 5 as:-
(A) a NNW trending system carrying water from the present-day tailings impoundment (Fig.
6).
(B) a deeply incised WNW-trending gulley draining an extensive area of older waste sands lying to the north of the Tabex open pit operation (Fig. 7). These tributaries converge approximately 250 m upstream of sampling site GPW 101, which was therefore considered an appropriate station for monitoring temporal discharge variability.
In March 1993, discharge at site GPW 101 was estimated as 2 m3/sec. Contributions were derived from both of the above tributary sources (A and B). Regular rainfall and surface- runoff (or shallow interflow) during the 'wet season' (November-March) is likely to result a dominance of inputs from the larger source area (B) throughout this period. Contasting flow conditions (discharge at site GPW 101 <lm%ec) in February 1994 reflected a (seasonally uncharacteristic) period of low rainfall for 2-3 weeks prior to sampling. No contribution was evident was from source area (B) at this time. It is likely that similar conditions persist throughout much of the 'dry season' (typically extending from April-October), with discharge through tributary (A) maintained only by pumped outflow from the tailings dam and adjacent process water tanks (see Fig. 5).
16
9
C
@ r
C s T C T
<
C 5
0 7
0
PI
0 9
d
4 Ei 3 a c -
0 0 0
0
0
0 0 0 0 0 0 0
0 0 0
0 0 0 0 0 0 0
0
@ 0 0
0 0 0 0
0 8 0 I)
2.2.3: pH and Eh regime 1993 Data: The pH data collected at sites GPW 101-103 during March 1993 (Table 5 ) show the prevalence of strongly acidic conditions in close proximity to the Globe and Phoenix dumps (pH 3.7 at GPW 101). Such values are entirely attributable to AMD generation through sulphide oxidation within the tailings. Buffering of this drainage is evident approximately 400m downstream at site GPW 103 (pH 6.6), in response to mixing with a NE-trending outflow from the Kwekwe Townlands extension (and associated sewage treatment plant).
I994 Dntn: Contrasting pH conditions were recorded in outflow drainage from the Globe and Phoenix site in February 1994 (Table 5 and Fig. 8). Strongly alkaline conditions (pH 9.5 at GPW 110) were noted in drainage waters immediately below the active tailings impoundment (tributary-source A; see Fig. 5) , falling to pH 8.0 at site GPW 101. Conditions remain alkaline at sites GPW 103 and 104, following the mixing of minewaters with Kwekwe Townlands drainage. Such values indicate that AMD ceases to influence overall water quality during low flow conditions. The very high pH level recorded at site GPW 110 is attributable to seepage of limed process-waters, derived from spoil cyanidation activities. An overflow canal draining the south-west sector of the Globe and Phoenix site (GPW 106) yielded a pH value of 7.77.
Figure 8: pH and Eh profile extending NNW from Globe and Phoenix mine (distances between sites are not to scale: GPW 110 - 104 = 2 km).
Eh (mV) PH 10 I
‘ 1 U 6 -
5 - - U w
4 - - 8 7
3 . I 1 I I I
GPW 110 GPW 108 GPW 101 GPW 102 GPW 103 GPW 104
400
300
200
1 00
Redox data collected during February 1994 (Table 5 and Fig. 8) depict a weakly-oxic regime. Values remain in a relatively narrow range (240-352 vV) between the present-day tailings impoundment (GPW 1 10) and the coni-hence with drainage from the Kwekwe Townlands (GPW 103). A depression of Eh downstream of the Townlands sewage inflow (site GPW 104, 105 mV) reilects increased 0 2 consumption by organic matter. At this Eh, redox- mediated dissolution of of Mn and As phases could be thermodynamically favoured.
19
0 0 0 0 0 0 0
0 0 0 0
0 0 0
0 0 0 0 0 0 0 0 I)
D 0
8
I)
8 8 8 8 8
Reduction and mobilisation of Fe may also occur in response to a depression of pH to ~ 6 . 0 (ie. the levels recorded in 1993), given no adjustment to Eh.
2.2.4: Arsenic and antimonv I993 Data: Data obtained for total As and Sb at three water sampling stations (GPC 101, 102 and 103) during an 'acid drainage' episode in 1993 (Table 5) highlight significant enrichment of both elements close to the perimeter of the mine operation (248 pg/l As and 2833 pg/l Sb at GPW 101), declining progressively with increasing distance from the site (169 pg/l As, 942 pg/l Sb at GPW 103). Inverse relationships between As &Sb concentrations and pH, and a positive correlation with dissolved Mn suggest pH-induced precipitation andor scavenging by hydrous oxides during dispersal.
1994 Data: Substantial enrichment of As (to 7.4 mdl) and Sb (to 15.5 mgh) was recorded at 5 minewater sites upstream of the Kwekwe Townlands inflow in February 1994 (Table 5; Fig. 9). Such values confirm that mobilisation of metalloids from the Globe and Phoenix waste piles and/or in process-waters occurs across a wide pH range (ie. mobilisation is not exclusively associated with AMD generation). There is no clear relationship between downstream As or Sb variation and pH. A reduction of dissolved loadings to 230 pg/l As and 21 10 pg/l Sb occurs following the mixing of mine-drainage with outflow from the Kwekwe Townlands. This reduction (in line with that shown by most other analytes) corresponds to dilution rather than precipitation. Relatively low ( d o 0 pg/l) concentrations of As and Sb were recorded in the canal draining the south-west sector of the Globe and Phoenix site.
Arsenic speciation data for the 1994 water suite indicate that As introduced to the Globe and Phoenix drainage via process water is predominantly in the form of arsenate (andor organo- arsenic compounds). Ratios of As3+/As5+ increase progressively downstream, from CO. 1 at site GPW 108 to 0.42 at site GPW 104. The overall dominance of arsenate is consistent with thermodynamic calculations for equilibrium As speciation in alkaline waters at positive redox potentials (Brookins, 1988). At site GPW 104, the substantial arsenite component probably reflects the depression of Eh (105 mV) following the inflow of (organic-rich) effluent from the Kwekwe Townlands extension. The possibility of selective sorption of Ass+ species by particulate ferric oxides cannot, however, be discounted.
Low As/Sb ratios are a notable feature of minewaters collected during both field campaigns (1993 range: 0.08-0.17; 1994 range 0.03-0.57). The persistence of such values in surface drainage is unusual, given the low solubility of primary Sb minerals such as stibnite (Sb2S3), and secondary phases such as Sb(OH)3 and cervantite (Sb2O4). Interactions with soluble organic phases may be particularly critical in enhancing solubility. The long established use of soil layers to stablize the Globe and Phoenix tailings piles provides a conducive
20
0 0 0 0 0 0 0 0 0 0 8 0 0 0 0 0 8 0 I)
0 8 8 0 8 8 8 8
8 8 B
8 B
environment for organo-Sb complexation. Weak cynanic discharges from the tailings re- processing tank adjacent to GPW 110 may also be particularly Sb-rich. Once in solution, the high pH-moderate Eh conditions prevailing during the 1994 sampling campaign could favour the persistence of Sb(OH)6, or possibly polyantimonite salts. .
It is notable that ratios of As/Sb are very low (c. 0.3) in mineralised bedrock and tailings (but not gossan).
Figure 9: As & Sb profile extending NNW from Globe and Phoenix mine. (distances between sites are not to scale: GPW 110 - 104 = 2 km).
As (ug/l) Sb (ug/l) 8000
6000
4000
2000
n
20000
10OOO
n I I I I
GPW 108 GPW 101 GPW 102 GPW 103 GPW 104
2.2.5: Maim cations: Major cation data for waters collected during the 1994 sampling campaign are given in Table 5 and Figure 10. The waters are characteristically Mg-dominated, reflecting the abundance of degraded ferromagnesian phases (derived from ultramafic lithologies) in the mine waste. Values of 467-2045 mg/l prevail at all sites upstream of the confluence with the Kwekwe Townlands outflow (near GPW 103). Low CdMg ratios (0.2 1-0.28) characterise all samples in this sequence except site GPW 110, in which a relatively low Mg concentration (467 mg/l) increases the value to >1. The dissolved Ca concentration between sites GPW 110 and GPW 103 is derived, in part, from the addition of CaCO3 (and/or CaOH) to process-water as a buffering agent. High Na concentrations (493-632 mdl ) at sites upstream of the confluence with the Kwekwe Townlands outflow are attributable to process-water contamination, compounded by high evaporation rates and visible evaporite-crust formation across much of
21
0
0
0
0 0 0
0
0 0
0
0
0 0 0
0 0 0
0 0
0
0 0 0 0 0 e 0 0 0
0
0 0 0
0
the tailings pile. Marked dilution of Mg-Ca-Na-rich minewater by outflow drainage from the Twekwe Townlands is evident at site GPW 104, in which the aggregate loading of these elements is c. 170 mdl . Potassium is present as a minor component of the major cation balance in all analysed waters, concentrations ranging from 4.89 mg/l (GPW 106) to 24.1 mg/l (GPW108).
Variations of total major cation concentration are accurately depicted by electrical conductivity values (Table 3, which range between 11000 pS at site GPW 108 and 900 pS at GPW 104.
Fig. 10: Major cation profile extending NNW from Globe and Phoenix mine. (distances between sites are not to scale: GPW 110 - 104 = 2 km).
4 Mnn
GPW 110 GPW 108 GPW 101 GPW 102 GPW 103 GPW 104 GPW 106
Site
Fig. 11: Major anion profile extending NNW from Globe and Phoenix mine. (distances between sites are not to scale: GPW 110 - 104 = 2 km).
100000 so,
10000
1000
100
10
1
-1 GPW 110 GPW 108 GPW 101 GPW 102 GPW 103 GPW 104 GPW 106
Site
22
0
0 0
0 0
0 0 0 a 0 0
0 0 0
0 e 0 0 0
0 0 0 0
0
0 a 0 a 0 0
0 e 0
0
2.2.6: Maior anions; Data showing the major anion composition of waters collected in February 1994 are
presented in Table 5 and Figure 11. Sites located upstream of the confluence with the Kwekwe Townlands outflow are highly S04-rich (3988-9194 mg/l; common in AMD but
uncharacteristic of alkaline waters). A close spatial correlation between SO4 and Mg indicates that MgS04 is the principal dissolved species. The solubility of this phase is high relative to Cas04 in most natural waters (saturation: MgS04 = 260 gh; Cas04 = 2.09 g/l at pH 7), and is likely to be maintained across the seasonal pH range observed at Globe and Phoenix. Chloride is strongly enriched (through evaporative concentration) in all samples draining directly from the Globe and Phoenix site. Equilibrium calculations indicate that NaCl could account for all dissolved Na, with the exception of site GPW 110 in which an excess of Na is probably present as sulphate. Values of 222-748 m d l HCO3 in waters upstream of site GPW 103 are ascribed to (CaC03) liming of spoil cyanidation process-waters near site GPW 110. The marked increase of HCO3 immediately downstream of GPW 110 (e.g. GPW 108) may signify CO3 liberation associated with the breakdown of CN- complexes. Nitrate abundances are generally low or moderate (0.1-33 mg/l), but show a significant increase (from 1 to 12 mg/l) in response to the influx of the Kwekwe Township outflow between sites GPW 103 and GPW 104 (also accompanied by a minor increase in P).
A summary of the major cation and anion characteristics of samples GPW 101, 102, 104 and 110 is provided Fig. 12, in which proportionate concentrations and TDS data are plotted on a conventional Piper diagram.
Figure 12: Piper diagram showing major cation-anion characteristics and total dissolved solid content of water samples GPW 101. 102,104 and 110.
100 v 100
Circle Diameter is log TDS ( P P ~ )
Scales are percent of t o milliequivalents per litre
G P W l 10
100 Ca 0 0 CI 100 CATIONS ANIONS I
23
0
0 0 a 0 e 0
0
0 0
0 0
0 0 0 0 0
0 0
0 0
a 0 0
0 e 0 0 e 0
a 0 0
0
2.2.7: Minor and trace elements: Elements forming the principal aqueous redox-couples, Fe and Mn, are present in Globe and Phoenix drainage waters at low concentrations (c 1 m d l and 5 mg/l respectively; Table 5; Fig. 13). In the drainage sequence extending between site GPW 110 to GPW 104, highest Fe concentrations occur at the uppermost location (0.43 mdl), fed directly by waters emanating from the tailings impoundment and process water tank. Lowest values occur downstream of the confluence with the Kwekwe Townlands outflow. In contrast to most mine waters, dissolved Fe loadings appear to remain low during both acid (1993) and alkaline (1994) drainage episodes. Data indicating the relative abundances Fe(I1) and Fe (111), based on colorimetric determination of dipiridyl pigmentation (Table 5) , indicate the prevalence of between 25% - 60% of the total Fe in the lower valency state. It should be noted, however, that the precision of Fe(I1) determinations in waters with low total Fe content is poor. On the basis of established thermodynamic calculations (e.g. Brookins, 1988), Fe(II1) species could be dominant under the range of pH-Eh conditions recorded upstream of site GPW 104 (pH >7 Eh; >200 mV) in February 1994.
Several heavy metals, including Ni, CO, Cu and Zn, display covariant behaviour in solution. All attain extremely high concentrations (16.1 mg/l Ni, 0.64 m d l CO, 34 mg/l Cu and 29 mg/l Zn) at site GPW 110, confirming a strong contaminant influence from the tailings impoundment and/or tailings reprocessing efkluent. At the high ambient pH ( 9 3 , metal solubility is almost certainly controlled by complexing agents such as cyanides or amines (N. Breward, pers. comm), the degradation of which induces abrupt precipitation and reduction of dissolved Ni, CO, Cu and Zn by 2-4 orders of magnitude within 200 m of the source (site GPW 108). Under alkaline conditions, the precipitation of metal hydroxides and carbonates is favoured, although the effect of this on the dissolved HCO3 budget is masked by the large flux of CO3 derived from the breakdown of CN- compounds (see sections 2.2.6 and 3.1.1).
Data obtained for Cu and Zn at sites GPW 101-103 in March 1993 (Table 5 ) indicate substantially enhanced dissolved loadings relative to the con-esponding 1994 values. At other mines in Zimbabwe (and elsewhere) the mobilisation and solubility of these metals has been shown to be strongly pH-dependent. Concentrations are thus likely to increase routinely during acid discharge episodes. This trend contrasts with that shown by the metalloids, As and Sb, which maintain solubility over a wider pH range.
Despite the high abundances of Pb and Cr in ore and waste materials at Globe and Phoenix, both elements occur at sub-detection limit concentrations in minewaters collected during 1993 and 1994.
24
0
e 0
a 0
0
0
0
a a 0
a 0 a 0 0 a a 0 0
0 0
0 0
0 e 0 e 0
0
0 0 0
0
Fig 13: Fe, Mn, Ni & CO profile extending NNW from Globe and Phoenix mine.
100000 1
10000
1000
100
10
1
.1
.01
.001 GPW 110 GPW 108 GPW 101 GPW 102 GPW 103 GPW 104
Fig. 14: Cu & MO profile extending NNW from Globe and Phoenix mine.
100000 3
10000
1000
100
10
1
.1
.01
.oo 1 I GPW 110 GPW 108 GPW 101 GPW 102 GPW 103 GPW 104
2.2.8: Sebakwe river hydrochemistrv Water samples from the Sebakwe river were collected from a location approximately 5 km downstream of the confluence with the tributary draining the Globe and Phoenix catchment and around 15 km (total channel distance) from the mine. Hydrogeochemical data for 1994 (Table 6) show the concentrations of As and Sb (16 and 2 pg/l respectively) and heavy metals to fall within WHO limits for potable waters. The influence of the Globe and Phoenix operation (and numerous other gold mines in the area) on the inorganic hydrochemistry of this major river is thus apparently limited.
25
a 0
0
e a 0
0
0
0
0
0
e 0 0 0
0 0
0
0
0 a 0
0
0
0 0
0
0
0
0
0
0 0
0
Table 6: Hydrogeochemistry of Sebakwe River (mg/l)
2.2.9: Geochemical modelling The range of aqueous metal species, their stability and degree of saturation in four Globe and Phoenix drainage waters (GPW 101, 102, 104, 110) was assessed using the equilibrium code WATEQ4F (Ball and Nordstrom, 1987). From a theoretical viewpoint, the model requires that (i) the aqueous system is in full chemical equilibrium (between solid, aqueous and gas phases) and (ii) lunetic constraints on speciation adjustments do not operate. In the temporally fluctuating pWEh regime of the Globe and Phoenix waters, these requirements will not be met. With respect to elements such as As, disequilibrium between trivalent and tetravalent species can be particularly pronounced, due to the slow kinetics of arsenite oxidation.
The thermodynamic constants used for calculating chemical equilibria were unchanged from those in the original WATEQ4F dataset. Correct operation of the Fortran code was ensured by running and cross-checking results against output for standard test cases for seawater, surface water and an acidic volcanic condensate (Ball et al., 1980). Empirical input data included pH, Eh (temp. corrected), Ca, Mg, Na, K, C1, HCO3, SO4, Fe, Mn, Cu, Zn, Ni and As. Modelling of Sb speciation and mineral saturation indices was precluded by the lack of thermodynamic data for Sb phases in the WATEQ database. Sample density was assumed to be 1.00 g/cm3 in all cases. Charge-balance calculations for all samples were undertaken prior to modelling. Errors in the range -13.01 meq/l (GPW 101) and +3.18 meqn (GPW 102) are all below 10% of the total balance, and convergence to 0.1% was obtained during computation.
Chemical speciation: WATEQ4F speciation output for samples GPW 101, 102, 104 and 110 is provided in Appendix 1. The most notable observations can be summarised as follows:-
(i): The predicted speciation of As at all sites (within thepH range 7.5-10) is dominated by arsenate as HAs0d2- (>90% total As). Under alkaline conditions, the theoretical stability
26
field for this phase extends from highly oxidizing to moderately reducing (Eh c. -250 mV, Fig 15).
Figure 15: pH-Eh diagram for arsenic (the broken line shows the stability field of barium arsenate).
-075-
2 4 6 8 1 0 1 2 PH
(ii) Anionic complexes dominate in the Globe and Phoenix drainage waters at pH values of S.0. Under these circumstances metal sorption via cation exchange is impeded, and the CEC of particulate surfaces will exert only a minor influence on metal mobility. However, the importance of anionic complexes falls markedly with decreasing pH (ie from site 110 to 101), with neutral species becoming dominant at pH 8.0.
(iii) The major element chemistry of the drainage waters is dominated by sulphate complexes (CaSO4, MgSO4, FeSO4) and uncomplexed Ca2+, Mg2+ and Na+ ions (together accounting for up to 80% of the budget for these elements). High ratios of NdC1 (particularly in GPW 110) indicate that the Na input is not from salt. Uncomplexed Fe2+ is significant (>30% total) at <pH 9.0, but is absent in the highly alkaline regime of site GPW 110.
(iv) Despite high SO4 loadings, the aqueous chemistry of numerous trace elements is dominated by hydroxyl and carbonate complexes, The former appear particularly dominant in the highly alkaline water GPW 110. Modelled speciation data for Cu show 33% as CuCO3 and 59% Cu(OH)2 at site GPW 102, compared to 3% CuCO3 and 95% Cu(OH)2 at site GPW 110 (Fig. 16).
(v) Re-calculated Cu speciation data for site GPW 101, following the incorporation of TOC data (38 mdl) into the input dataset, suggest that organic carbon exerts a minimal influence at pH values of >8.0.
27
Figure 16: Modelled aqueous speciation of Cu in samples GPW 101,102, 104 and 110, showing the characteristic dominance of metal hydroxides over sulphates.
IB- C u ( ~ P
m m 0 C u ( W
GPW 101 - total C u = 21 ugA: GPW 102 - total C u = 0.5 ugn: pH = 8.1 pH = 8.0
GPW 104 - total c u = 0.09 ugn: DH = 8.3
GPW 110 - total C u = 34467 ugll: pH = 9.5
Saturation indices: The equilibrium model WATEQ4F facilitates the calculation of saturation indices (SI) for some 200 minerals for which data are held within the database. It should be stressed that SI values signify the extent of theoretical dis-equilibrium (in terms of under- or over-saturation). In the absence of kinetic data, SI values indicating over-saturation with respect to a given mineral phase need not imply that precipitation will actually take place. By convention, SI = log Q - log K, where log Q is the activity quotient which is equal to log K,
28
e 0 0
a 0
0 0 0
0
0 0 e 0 0 0
0 0 0
0 0 0 0
0
0 0 0
0 0 0 0
0 0 8 I)
the equilibrium constant at equilibrium. Accordingly, SI <o = under-saturation; SI = 0 at equilibrium; SI >O = over-saturation.
Comprehensive SI data (Log IAPIKT) are provided in Appendix 1. All waters are under- saturated with respect to As oxides (e.g. arsenolite, SI -92) and high solubility arsenates such as Ca3(As04)2. Data for the low solubility arsenate Ba3(AsO4)2 (solubility product 7.7 x
10-51) suggest persistent over-saturation (SI = 11.61 at GPW 101, falling to 9.87 at GPW 104). Although this phase exerts a major constraint on As mobility in many natural and contaminated systems (Fergusson, 1990), it's role in As removal from the Globe and Phoenix drainage waters will be negligible on account ot the low (<0.2 mgA) total Ba concentration. Saturation indices for other low solubility arsenates such as scorodite (Fe3(AsO4).2H20) and Cr3(AsO4)2 were not calculated, but their influence on As solubility is, again, likely to be restricted by the low dissolved concentrations of Fe and Cr.
The aqueous geochemistry of Fe is dominated by the extremely low solubilities c?f ferrihydrite (SI = 2.05 at GPW 110, falling to 1.1 at GPW 104), goethite (SI >7 at GPW 110 and GPW 104) and related hydroxides across the pH range recorded during the 1994 field campaign. Precipitation of Cu(OH)2 may occur at site GPW 110 (SI = 1.95), but under-saturation is evident under the less oxidizing Eh conditions and lower total Cu content at site GPW 104.
Despite the high ambient pH and SO4 loadings, all stations are under-saturated with respect to soluble sulphates such as MgSO4 and CuSO4. However, hydroxy-sulphates such as brochantite ( C U ~ ( O H ) ~ S O ~ ) may precipitate under high pH conditions in the upper sectors of the drainage system. Waters GPW 110 and 101 are close to equilibrium with respect to Cas04 (GPW 110 SI = -0.012). Precipitation of gypsum could thus be anticipated in response to any further addition of Ca. Under-saturation is pronounced at site GPW 104 (SI = -1.21). Precipitation of ZnSOq is indicated at station GPW 110, but reduced Zn concentrations yield negative indices at all sites downstream. Barium solubility is strictly controlled by saturation with respect to barite at all sites.
2.3: SEDIMENT GEOCHEMISTRY
2.3.1 : Bulk geochemistry Samples of -100 BSI ( 4 5 0 pm) sediment were collected from sites 101, 102, 103, 104 and 109 (coded GPC) during an 'acid discharge' episode in March 1993. Analyses for selected major and trace elements were carried out by XRF, following the ignition of organic matter and preparation of 12 g pressed pellets (Table 7). At all sites, the sediments were dominantly composed of clean quartzose sand, silt and alluvium, much of which may have been derived
29
from the outwash of fine tailings and waste sands. Evidence of Fe and Mn staining was limited in all cases.
Major element and LOI data for site GPW 109 indicate a significant combustable fraction (1 1.9%), comprising inter-lattice water and organic matter. The inorganic matrix is confirmed as predominantly siliceous (64.4% Si@) with a relatively low Fe and Mn content. The ratio of MgFe is unusually high.
The metalloids As and Sb display strongly anomalous concentrations at sites GPC 101-103, but fall by almost an order of magnitude downstream of the confluence with the Kwekwe Townlands outflow (site GPC 104). Pronounced sedimentary As and Sb enrichment (865 and 5934 pg/g respectively) at site GPC 103 coincides with a marked surface water pH increase (from 4.1 at GPC 102 to 6.6 at GPW 103) and loss of both elements (notably Sb) from solution (see 1993 data; Table 5). Enrichment through (sorptive) scavenging by organics andor hydrous oxides is thus likely.
Table 7: XRF data for stream sediments in the vicinity of Globe and Phoenix mine (major element values quoted in %; trace elements in pg/g)
GPC 101 GPC 102 GPC 103 GPC 104 GPC 109 1-1---1.--9)1-1--1-1------.--. .--,- ,--~~~~,~~,~~,~,~~--~~.~~.~.~~.~~~~-,------,- ,---,~-9-9---~----999--9
LOI 11.9 Si02 64.4 Ti02 0.32 A1203 4.59 Fe203 6.34 7.80 6.26 7.30 5.34 MnO 0.1 1 0.13 0.11 0.10 0.07
9.15 1.64
MgO CaO Na20 0.36 K20 0.81 P205 0.03 As 696 552 865 98 Sb 3486 1262 5934 542 Ni 438 404 637 278 c u 108 237 167 59 Zn 1090 758 1681 82 Pb 656 459 1099 35
The sedimentary trends (and probable solid-solute interactions) displayed by Pb and Zn are closely analogous to those described for As and Sb.
30
0
'Shake for 1 hr wth 0 1 M NHZW HU in 0.001M HN03
2.3.2: Solid-Phase partitioning Sequential extractive procedures formulated by Breward and Peachey (1983) were applied to sediments from sites GPC 101-104 to determine the partitioning of As and selected major and trace elements between geochemical phases (Fig. 17). Oxidation of sample material was avoided prior to analysis to preclude adjustments of speciation during storage. For As, the initial stage of the leaching sequence entailed shaking 1 g of sediment (dry wt.) in 1M NaOAc (pH 7) to remove adsorbed/exchangeable phases. Organic components of the residue were then removed with 1M NH4OH and subsequently separated into fulvic and humic fractions by HCI precipitation. The inorganic residuum remaining after the N H 4 0 H application was leached for 1 hour with 0.1M NH2.0H.HCl to remove Mn-oxide bound As.
A Tamm's Reagent (oxalic acid and ammonium oxalate mixture) application was then used for the dissolution of Fe-oxide-bound phases. Washing of the residuum was undertaken between each stage to avoid cross-contamination andor reduced efficiency of extractants. The abundance of As held in resistate (silicate and non-volatile sulphide) minerals was estimated by subtracting the sum liberated during the five leaches from the 'total' concentration value derived from XRF analysis. For all other analysed elements, a modified procedure incorporating only the fulvic, humic and Fe-oxide extraction stages was applied.
0 a
e
a
a 0 e
a a 0 a
7
0 0
e 0
0 0 0 0
0
Figure 17: Sequential extraction scheme for the determination of particulate trace element speciation (after Breward and Peachey, 1983).
Solid
solution hydrochlonc acid
I 1 Solid
1
Secondary
31
0 0
0 0 0
0
0 0 0 0
0 0 0 0
0
0 0 0
0 0
0 0
0 0
0
0 0 0
0
0
0 0
0
0
Table 8: Partitioning of As and selected major and trace elements between sedimentary phases (all data are given in pg/g)
As Adsorbed Fulvic Humic Mn-oxide Fe-oxide Residual
Mn
Fe
Zn
c u
Fulvic Humic Fe -oxi de
Fulvic Humic Fe-oxide
Fulvic Humic Fe-oxide
Fulvic Humic Fe-oxide
Pb Fulvic Humic Fe-oxide
6.82 59.65 0.30 1.72 267.50 362
0.95 0.40 45.00
129.15 63.57 485 1
30.20 4.87 222.4
nd 2.77 43.0
2 1.62 nd 265.6
13.42 9 1.22 5.45 1.45 300.50 140
4.60 0.62 62.40
253.25 114.02 6366
5 1.90 3.35 307.5
13.45 14.15 243.4
28.6 nd 241.1
7.80 125.70 6.72 4.00 623.02 99
4.40 1.10 109.5
355.20 168.70 13457
98.50 18.72 635.6
nd 10.27 69.6
56.87 nd 446.0
4.15 3.57 2.35 0.15 10.70 78
0.70 1.52 97.6
50.70 36 1.75 1705
2.50 9.85 16.3
nd 2.72 31.5
nd 2.45 91.9
All leachates were analysed for As by hydride generation AAS, and for other elements by ICP-AES. The results (Table 8) can be summarised as follows:
Leach I : NaOAc extraction: All analysed samples yielded low concentrations of As (4-13 mg/l) in the NaOAc extractant, indicating that exchangeable or adsorbed phases are of limited significance as metalloid hosts (4% total sedimentary loading).
Leaches 2-3: Ammonium extract: Organo-metallic complexation and/or surface sorption to organic matter is an important control on the non-detrital partitioning of As. In the highly enriched samples GPC 101-103, fulvic organics dominate over humics as As complexing agents or sorption sites by at least a factor of 15, accounting for c. 16% of total As in sample GPC 102 and 14% in sample GPC 103. However, the leaching by ammonia of secondary As sulphates and thiosulphates from AMD-affected sediments has been noted previously (e.g. Williams et al. 1994). Such influences may compound the high fulvidhumic ratios reported here. Fulvic and humic organics are relatively unimportant as camers of Mn, Fe, Zn, Cu or Pb
32
0
0 0
0
0 0 0 0
0 0
0 0
0 0
0
0 0
0
0
0 0 0
0
0 0
0
0 0
0 0
0
0
0
0
(do% total). With the exception of Cu, for which humic fractions dominate, these metals show persistent frac tionation into fulvic organic complexes.
Leach 4: NH;!.OH.HCZ extract. The very low abundances of As extracted by NH2.0H.HCl (4 pg/g) indicate that manganese oxides are of negligible significance as As carriers.
Leach 5.- Tamms reagent extraction : Crystalline Fe-oxides constitute the dominant non- detrital carrier of As and all other analysed elements in Globe and Phoenix drainage sediments. With respect to As, the relative contribution of these phases increases with the total sedimentary As loading, accounting for 38% of total As as site GPC 101, rising to a maximum of 78% total As at the most enriched site, GPC 103, and declining to around 10% of total As at site GPC 104. This trend supports the formerly postulated removal of As from solution around site GPC 103 by (pH-induced) Fe-oxide scavenging (section 2.3.1).
2.4: SOIL GEOCHEMISTRY
Thirty five soil samples were hand-augered at 50 m intervals along two independent transects located to the west of the Globe and Phoenix compound. Samples GPS 1-12 were extracted from approximately 30 cm depth along an E-W trending line between the western perimeter fence and the Kwekwe Townlands extension (soil line 1, Fig. 5) . The characteristic down- profile zonation along this transect comprised a thin (c. 1 cm) limonitic A horizon, underlain by poorly zoned brown silty clay, extending to a depth of 9 . 5 m. Samples GPW 14-40 were collected from a depth of c. 10 cm along a N-trending transect following the outwasldfloodplain of the river flowing NW from the Globe and Phoenix mine (soil line 2, Fig. 5). The floodplain is extensively masked by silty yellow alluvium (up to 15 cm thick), underlain by an inpenetrable hard-pan. The surface vegetation along both transects included maize and leguminous crops. All soils were sieved to remove coarse (>2 mm) fractions, and were analysed by for As, Sb, LOI, major oxides and 12 additional trace elements by XFW following the preparation of 12 g pellets.
The results of XRF analyses of soil samples GPS 1-40 are given in Tables 9-10. With the exceptions of A1203 & MgO, the major oxide composition of the entire sample suite shows limited variability. All soils are characteristically siliceous (Si02 >60%) and poor in labile oxides (Fe203 and MnO <7% and 1% respectively).
The E-W trending transect ( 1 ) shows considerable enhancement of As (>500 pg/g), Sb (>4800 pg/g) and a range of heavy metals at it's western extremity (sites GPS 1-2; adjacent to the Globe and Phoenix perimeter fence and only c. 100 m from the waste dump
33
a rn 0
0
0
0
0 0
0
0 0
0
0
0 0 0
0
0 0
0
0 0
0
0
0 0
0
0
0 0
0
0 0
0
embankment). To the east of this anomalous zone, concentrations fall by 1-2 orders of magnitude (sites GPS 3-1 1: As and Sb e70 pg/g). While this abrupt pedogeochemical transition could define the extent of westward As, Sb and heavy metal dispersal from the Globe and Phoenix compound, the coincidence of reduced soil concentrations with the onset of deep furrowing and maize production (west of site GPS 2) requires emphasis. Depressed soil As concentrations have been noted in sub-surface soils from planted sectors of transects over mineralised terrain elsewhere in the Zimbabwe Midlands (notably the RTZ Battlefields EPO area). The phenomenon can be ascribed to physical processes of downwash of primary sulphides or surface-enriched clays, instigated by increased infiltration. This influence is inferred by the closely covariant patterns displayed by metalloid elements (As and Sb) and W (which is primarily hosted in the immobile detrital phase scheelite) along transect (1) (Fig. 18). Mobilisation of As, Sb and transition elements by chemical dissolution of labile oxides is unlikely, as soil disturbance would characteristically increase oxidation. From the data presented in Table 9 and Fig 19, it is evident that the effect of soil disturbance on the total MnO content is negligible.
Figure 18: Variation of As, Sb, W and Pb (U&) in sub-surface soil along E-W trending transect (1): Globe and Phoenix mine.
10000
1000
100
10
1
- s b - w - P b
GPSl GPS2 GPSG GPS7 GPS8 GPSS GPS10 GPS11 GPS12
Figure 19: Variation of Al2O3, LOI and MnO (%) in sub-surface soil along E-W trending transect (1): Globe and Phoenix mine.
100
10
A1203 1 - LOI - MnO
-'I - .01 ! I I I 1 I I I
I 1
GPSl GPS2 GPSG GPS7 GPSB GPSS GPSlO GPSl l GPSl2
34
~ B m m ~ ~ ~ ~ ~ ~ o m ~ ~ N ~ m 0 ~ ~ ~ ~ ~ ~ ~ ~ m 4 m ~ ~ ~ m m m ~ ~ 0 o o o o o o o o P - l o o o N o o o o o o o o o o o o ~ ~ o o o o o o o o a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 CJ . . . . . . . . . . . . . . . . . . . . . . . . I . . . . . . . . . .
O ~ ~ P - l m ~ N m P ~ 4 ~ N O ~ d N w ~ O w P m N w w w ~ d m w P ~ P m N o N N N 4 w ~ ~ O P ~ O ~ ~ N 0 N N N m N N N N N N N N 0 N N N m N N m m (Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
z 00000~~00d0000000000000000000000000
0 8 P P ~ ~ ~ O P m ~ m ~ ~ P d ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ P m P P m m ~ c 00000d000d000r4000000000000000000000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZZ 00000000000000000000000000000000000
~ B ~ 0 w d 0 ~ ~ ~ ~ ~ 0 ~ ~ m ~ 0 N ~ o ~ r - i ~ N N N o ~ o N ~ ~ ~ d 0 N 0 ~ m ~ ~ ~ m ~ m ~ 0 m N N N N " N N d ~ N N N N ~ ~ N N N N N N N N
E 00000000000000000000000000000000000 -4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-34
Q4 0 h E c, e n
Trace element data for transect (2) show no clear lateral trends across the c. 2.5 km floodplain area to the north of the Globe and Phoenix compound (Table 10). All samples are highly enriched in As and Sb, with values of over 1000 pg/g and 5000 pg/g respectively recorded close to the northern end of the transect (site GPS 39; some 3 km from the Globe and Phoenix perimeter). High soil concentrations of Cr (max. 943 pg/g), Ni (max. 830 pg/g), Zn ( m u . 1586 pg/g) and Pb (max. 906 pg/g) are also widely recorded.
The high concentrations of As, Sb and heavy metals in transect (2) samples relative to most of transect ( l) , although partially attributable to agricultural disturbances over the latter (see above), also reflect differences of regolith composition, structure and sampling depth. The high proportion of alluvial outwash over the transect (2) area is particularly notable. This material has the appearance of mine waste sands, probably transported from the Globe and Phoenix mining area over a period of c. 100 years by fluvial and aeolian processes. A comparison of the major and trace element signatures of transect (2) soils collected up to 2 km from the Globe and Phoenix site (e.g. GPS 30) and oxidized tailings (GPX 150, described in section 2.1.3) illuminates a marked geochemical similarity (Figs 20-2 1). Due to hard-pan development beneath the alluvium, sampling and analysis of deeper horizons was not undertaken. It is likely that such an inpenetrable layer will also constrain the rooting depth of natural vegetation (mainly arrow grass) and subsistence crops (principally maize) in this area.
Preliminary XRF data showing the distribution of As, Sb and Pb between different grain size fractions of soil sample GPS 15 (transect 2) have been reported by Naden and Bland (1994) and are summarised in Table 11.
Table 11 Partitioning of As, Sb and Pb between different soil size fractions: site GPS 15.
(All trace element data are given in pug)
37
0
0 0
0 0
0
0
0
0
0 0
0
0 0
0 0
0 0
I)
8 8 8 8 8 8 D
I)
B B D
Figure 20: Comparative major element geochemical signatures of oxidized waste sands (Globe and Phoenix site GPX 150) and alluvial soi1 from site GPS 30, transect (2).
Conc. ug/g
GPX 150
Figure 21: Comparative trace element geochemical signatures of oxidized waste sands (Globe and Phoenix site GPX 150) and alluvial soil from site GPS 30, transect (2).
Concentration YO 100
GPS30 10
1
I 1 1 1 1 1 1 1 1
Si02 T i02 Fe203 MnO M@ Cao NaX) WO p205
Sub-2 mm sample material was initially wet sieved to separate A 2 0 pm, 63-120 pm and e63 pm fractions. Superpanned (heavy mineral) splits of each fraction were then obtained and subjected to XRF analysis. The light (tail) component of the c63 pm fraction was subsequently hydrocycloned to isolate a e10 pm fraction. Washwater used during the hydrocyclone process was evaporated to dryness, and the residue analysed by XRF to provide an indication of water-soluble As. Sb and Pb.
Probe analyses of (superpanned) heavy mineral fractions in the 60-120 pm class have confirmed that As and Sb are present as discrete detrital sulphides, oxides (Sb only), arsenates (As only) and amorphous adsorbed surface coatings. Within the 63 pm fraction (56% of the total analysed mass), a volumetrically small heavy mineral component is strongly As- enriched (8400 p s / ~ ) . High concentrations of As (2234 pg/g) and Sb (868 pg/g) are also
38
evident in the c10 um fraction which, at 41% of the sample mass, is the principal contributor to the overall soil As-Sb burden. The pronounced fractionation of Sb into c10 pm material is interpreted as reflecting a dominance of secondary adsorbed Sb-oxides and over detrital sulphides. A significant water-soluble As-Sb component in the matrix is indicated by concentrations of 193 pg/g As and 143 pg/g Sb in evaporated residues from the supernatant.
3.1: PRINCIPAL CONTROLS ON GLOBE AND PHOENIX DRAINAGE QUALITY
Studies of gold and base-metal mining operations in North and Latin America (e.g. Plumlee et al., 1993) and Asia (e.g. Williams et al., 1994) have shown that individual deposit types produce hydrochemical signatures which are usually a predictable function of the first-order controls:- (i) pyrite abundance, (ii) sulphide weathering rate, (iii) ganguehost rock buffering capacity and (iv) mine waste permeability. Marked temporal fluctuations between acid and alkaline drainage such as occur at Globe and Phoenix have not formerly been documented. In this unusual situation, physico-chemical processes continue to exert an impact, but the water quality cycle is ultimately controlled by climatic and technological factors. These govern the proportion of total discharge derived from (i) cyanic process water and (ii) drainage from sulphidic waste.
3.1.1 : Chemistry of cyanic process waters: In the absence of a natural baseflow component, dry season drainage from Globe and Phoenix is maintained by discharge from mineral processing activities. Sources include underflow seepage from the tailings impoundment, process-water from tailings cyanidation tanks (see Fig. 3, waste-water from pressure-hose (hillwash) activities on older waste-sands and waste effluent from heap-leach pads. The last noted source is located to the north of the site, and is thus potentially influential only downstream of site GPW 101.
The presence of CN- compounds in the upper reaches of the Globe and Phoenix drainage system has been inferred from the hydrogeochemical data and is a predictable consequence of the use of NaCN for Au recovery from gossanous ore (by heap leach) and old tailings (by agitated suspension) at the site. While of no direct environmental concern, the aqueous stability and degradation of CN complexes exerts a first-order control on trace element mobility under low-flow drainage conditions.
Cyanidation-based methods of Au recovery have been used worldwide for almost a century. The basic chemistry is described by Elsner's equation:-
4Au + 8NaCN + 0 2 + H2O = 4NaAu(CN)2 + 2NaOH
39
In the absence of other inorganic complexing agents, effective Au recovery can be achieved using low CN- concentrations, but in the presence of other cyanide-complexing metals (notably first-row transition metals) cyanide consumption is increased. Thiocyanate (SCN) complexes form concurrently with metallo-c yanides during the processing of ore containing reactive sulphides (including FeAsS). In discharge waters such as emanate from Globe and Phoenix, cyanide will occur initially as free CN- and as metal complexes incorporating Na, K, Hg, Zn, Cu, Fe, CO, Ni etc. (of which Cu(CN)2 and Ni(CN)4 are moderately strong, and Co(CN)6 very strong). With the rapid reduction of pH from c. 9.5 - 8.0 downstream of site GPW 110, considerable losses of cyanide will occur through CN- - HCN transformations and resultant volatisation (Fig. 22). Oxidation of CN- (and consequent generation of nitrogen gas) will also occur regardless of pH. The depression factors of Cu, Ni (>3000) and CO (<lOOO) at site GPW 108 (relative to site GPW 110) suggest that metals precipitate in order of increasing aqueous complex stability.
The role of cyanide as a determinant of As and Sb mobility is less pronouced. Cyanidation, while effective for metal recovery from minerals such as enargite, arsenopyrite (65% Cu/Fe extraction) and to a lesser extent tetrahedrite (21% Cu extraction, Scott and Ingles, 1987), does not complex metalloids in the same way. Free As and Sb ions (or oxyanions) are generated during SCN formation, but their subsequent mobility is independent of cyanide behaviour. Consequently, volatisation of HCN (and oxidation of CN-) at site GPW 108 has no impact on total dissolved As and Sb loadings.
Figure 22: Aqueous partitioning of HCN and CN- - defined by pH.
40
3.1.2: Chemistry of sulphidic waste drainage: At Globe and Phoenix, sulphide oxidation and the attendant liberation of As, Sb and heavy metals from primary minerals will be continuous (although seasonal adjustments of oxidation rate in the tailings pile will occur in accordance with fluctuations of ground- or pore-water levels). The hydrochemical effects of the process are, however, confined to periods of rainfall (principally November-April). Particularly acidic discharges may characterise the early stages of the wet season, due to the dissolution and flushing of slcllphate crusts (the product of dry season oxidation and evaporation) from the tailings pile surface.
The oxidation of sulphides in mine waste is initiated by the interaction of aqueous solutions at the mineral-fluid interface, facilitated by cracks, fissures, cleavage and intergranular boundaries (e.g. Thornber and Taylor, 1992; Bowell, 1994). The process typically involves minerals which possess semi-conductor properties (Thornber and Wildman, 1994), with simultaneous cathodic reduction and anodic oxidation. Thus the cathodic reduction of dissolved oxygen:-
may occur concurrently with anodic sulphide oxidation (in this case pyrite):-
At higher electrode potentials (and pH >2) the reaction may be:-
FeS2 + l l H 2 0 -> Fe(OH)3 + 2SO4 + 1YH+ + 15e' (3)
or: -
In AMD systems, the products of reaction (2) are particularly critical. In addition to net acidity, ferrous iron is liberated which, on oxidation to Fe3+, acts as a further oxidant of mineral sulphides in accordance with the reaction:-
Singer and Stumm (1970) proposed that the oxidation of Fez+ constitutes the primary rate- controlling step in the formation of AMD. In abiotic systems, this process is highly pH dependent and is markedly suppressed at low pH (Fig. 23). Observations of Fe2+ oxidation at rates in excess of theoretical abiotic maxima in AMD (e.g. Silverman and Erlrich, 1964) have confirmed the influence of several catalysts, of which microbial-mediators such as Thiobacillus Ferrooxidms (Suzuki et al., 1993) are particularly important.
41
0 e e 0 0 e e 0 0 e 0
e 0 0 0 0 0 0 0 0 0 8 I)
D 8
I)
I)
B B
D B
In addition to it's role as an oxidant, Fe3+ also generates acidity through hydrolysis reactions:-
Fe3+ + H20 > Fe(OH)2+ + H+ or (6)
Fez+ + 0.2502 + 2.5H20 > Fe(OH)3 + 2H+
A similar reaction involving ferrous iron can be expressed as:-
(7)
Figure 23: pH dependency of abiotic Fek oxidation (from Singer and Stumm, 1970).
0
- 1 .o
-2 -3
-3 .O n
t 1
.- c 5 -4.3
0 -5.0 1 M
2
-6.0
-7 c
-a.o
-3.c
-1
T .erno = 2 5 J C
The dissolution of As-bearing phases such as arsenopyrite in the waste pile, or within overburden, should yield soluble usenate in accordance with the reaction:-
In alkaline conditions and/or in the presence of ferric iron, free arsenate is typically immobilised through precipitation with Fe to form scorodite and related phases:-
42
While precipitatiodsorption by ferric oxides is clearly implicated in the removal of As and Sb from the Globe and Phoenix drainage waters (notably by the dominance of ferric-oxides as sedimentary carriers of both elements), such processes are inherently restricted by the low ambient concentrations of dissolved Fe (in many instances, dissolved As concentrations actually exceed Fe by up to a factor of 35). This unusual mine water situation may contribute to the high presence of As in both surface drainage and soil pore-waters.
The behaviour of tennantite-tetrahedrite series minerals ((CuFe)l2( S bAs)4S 13)) during oxidation is of particular relevance to conditions at Globe and Phoenix, as these phases are major carriers of As and Sb (see sections 2.1.1 - 2.1.3). A summary of observations of alteration products has been provided by Bowel1 (1994). These phases show high reactivity (particularly tennantite) relative to most detrital sulphides. Trellis-textured boxworks are commonly formed during weathering, with infilling by tetrahedrite, chalcopyrite and scorodite in tennantite matrices. In tetrahedrite, phases such as bindhemite (Pb2Sb2(0,0H)) and stibiconite (Sb3+Sbb206(OH)) may be preserved in the boxwork.
The lowest pH values noted in drainage at Globe and Phoenix (3.7) are considerably higher than reported for many other Zimbabwean gold and base-metal mining operations investigated under BGS-ODA TDR project 92/6 (for example, massive pyrite & Au workings at Iron Duke generate drainage with pH ~ 0 . 6 ) . This trend is consistent with the high buffering capacity of the deposit, derived from (easily weathered) serpentinised ultramafics and talc- schists. With the exception of localised Cu and Zn enhancement at site GPW 101 (1993 data), heavy metal mobility is restricted and the AMD constitutes no significant environmental hazard. Dispersal of As and Sb, although of greater concern, occurs independently of AMD and should thus be addressed separately.
3.1.3: Mixing products of alkaline and acid waters: During wet season flow conditions, the chemical characteristics of sites downstream of site GPW 108 are the product of mixing of alkaline process water and acidic drainage from sulphidic spoil. Data collected during 1993 show the resultant pH to be low (GPW 101 = 3.7), suggesting either a hydrochemical or a volumetric dominance of the spoil-derived input. However, details regarding the equilibrium speciatiordsaturation characteristics of the mixed water are not known.
Simulative 'scoping' using WATEQ4F provides a valuable mechanism for assessing the relative influence of the two discrete water inputs on downstream drainage chemistry. In the absence of data for undulterated spoil seepage, data for an underground acid seepage (Globe and Phoenix level 6) have been used to represent the acidic end member in this study (pH 3.5, Eh 600 mV, SO4 5000 mg/1, Ca 490 mg/l, As 3 mdl , Sb 5 mg/1, Cd 0.06 mdl , Fe 1 0 0
43
0
0 0
0 0
0
0 0
0
0 0 0
0 0 0
0
0
0
e 0
0 0
0 0
e 0 0
0
0 a 0
0 e 0
mg4, Zn 10 mg/l, Cu 5 mg/l, Ni 5 mg/l). Modelled output for a 5050 mix of this water with alkaline water GPW 1 10 yielded a pH of 3.8 (close to the field determination obtained at site GPW 101 in 1993). A dominance of As is predicted as H2As04, and most heavy metals persist as uncomplexed (M)2+ or sulphate species (>75% Cu, >95% Ni).
3.2: TOXICOLOGICAL RISK
The toxicological impact of the mobilisation of As, Sb and heavy metals from Globe and Phoenix can be evaluated in relation to (i) the spatial extent of dispersal, (ii) speciation and (iii) biological uptake. With regard to dispersal, As and Sb maintain dissolved concentrations which exceed international water quality standards (Table 12) for at least a kilometre downstream of the mine (GPW 104, 230 pg/l As, 2.1 mg/l Sb; 1994). At the time of sampling, surface drainage was ill-defined downstream of site GPW 104, but contaminated interflow through the floodplain alluvium could be anticipated. Analytical and modelled data for speciation indicate that the least toxic of the common inorganic species, arsenate, is routinely dominant in the mine water. It is also possible that a substantial component of the As and Sb loading of the alkaline discharge is, in fact, organically complexed, and hence of very low toxicity.
The impact of particulate As and Sb contamination, (associated with the northward outwash of enriched alluvium) warrants greater attention. The recipient floodplain to the north of Globe and Phoenix is extensively cultivated, producing maize and vegetables for consumption in the Kwekwe Townlands area. While at least 50% of the As loading is bound to poorly reactive ferric oxides, the presence of a substantial water-soluble soil component and fulvic sediment component indicates a marked potential for bioassimilation. Analyses of As uptake by flora and fauna at Globe and Pheonix (Weeks et al., in prep) have shown that mass specific concentrations in several earthworm samples exceed the median for soils, and that As (and Sb) concentrations reach 100 x average background abundances (Fergusson, 1990) in maize.
The toxicological hazard associated with heavy metals in Globe and Phoenix drainage waters is strictly limited. Under alkaline drainage conditions, attenuation of metals such as Ni, Cu as Zn occurs through the breakdown of CN-compounds (to non-toxic end-products) within a few hundred metres of the source, with the resultant reduction of concentrations to below international standards for potable waters (Table 12) at the outflow from the Tabex site perimeter (GPW 101). While values for SO4 and TDS exceed established drinking water standards as far downstream as site GPW 104, no significant environmental risk can be inferred.
44
0
0 0
0
0
0 0 0
0
0 0
0
0 a 0
0 0 0
0 0
0
0 0 0 0 0
a 0 0
0
0
0 a 0
Table 12: Water quality standards for selected parameters ,stipulated by WHO (1959) and US-EPA. (1989). All element values are give in mgA
3.3: OPTIONS FOR IMPROVED ENVIRONMENTAL QUALITY
In contrast to many gold and base metal mining operations in tropical developing countries, the requirement for environmental remediation technology at Globe and Phoenix is not critical. Any future strategy for improved pollution control must, however, account for the following site charactc=ristics:-
(i) Intense AMD is not evident, due to buffering by ultramafic host lithologies (or waste derivatives) and alkaline processing effluent. (ii) Dissolved Fe concentrations are low (during both acid and alkaline drainage episodes). (iii) Heavy metal contamination is localised (often associated with cyanic alkaline waters) and environmentally unimportant. (iv) Dispersal of As and Sb is the principal environmental hazard. Mobility of these elements is retained across a wide pH range and is independent of cyanide geochemistry. (v) Particulate As and Sb dispersal exceeds aqueous transport as an environmental hazard.
Under these unusual circumstances, conventional methods for decontamination of mine waters through regulation of the pH regime (e.g. limestone drains) are inappropriate. Biotic systems and induced sorption techniques offer greater potential.
3.3.1 : Bio-remediation: Wetlands provide effective matrices for simultaneously removing particulate and dissolved metals from contaminated mine drainage (e.g. Alpers and Blowes, 1993). The systems utilise the high cation exchange capacity of species such as Sphagnum, coupled with the tendency for metal sulphides to precipitate in anoxic substrates. Sulphate reduction reactions consume protons, thus serving to raise the drainage pH. Sedimentation of particulate metals and flocculates is promoted by reducing flow velocities. Adsorption, ion exchange, oxidation of sulphides, organo-metallic complexation and bio-assimilation processes are also operative. The accumulation of base- and precious metals to economic concentrations has been reported
45
in several instances (Smith et al., 1993). Low flow rates are a fundamental requirement for effective metal recovery in wetlands, and this condition is adequately met at Globe and Phoenix. Bio-reactors, such as the Pittsburgh or Palmerton systems (Dvorak et al., 1992), operated at low Eh, may have considerable applicability, given the low discharge volumes monitored during the 1993 and 1994 field seasons.
3.3.2: Chemical removal: The induced precipitation of metal hydroxides, which subsequently act as heavy metallmetalloid sorption sites, offers considerable potential for the decontamination of As
(and Sb)-polluted water at low discharges (c2m3/s). In acidic, Fe-rich mine waters, Fe is precipitated through the addition of an oxidant (Cl) or by mechanical aeration, with addition of a buffer to counter Fe hydrolysis. In alkaline or Fe-poor waters (such as prevail in the upper reaches of the Globe and Phoenix drainage system), synthetic oxides or resins may be utilised for sorption. In either case, the efficiency of the process is determined by the charge characteristics of the oxidehesin surface. For all phases, electro-negativity increases with pH above the isoelectric point (pH 3.2 for natural FeOOH, pH 5.4-6.9 for natural Fe2O3; Parks, 1965). Hydrous Mn oxides may serve an analogous scavenging function, but require higher pH levels (>8.0) for complete precipitation. For the removal of oxyanions of As and Sb, positively charged surfactants are required.
A predictive model based on the equilibrium-speciation model MINTEQ and a GTLM sorption code (Smith et al., 1993) facilitates prediction of gross metal/metalloid retrieval by the above processes. Ferric oxide precipitation and trace metal adsorption are simultaneously modelled across a variety of pH levels. The GTLM code assumes that sorption reactions are specific to coordination sites on the oxidehesin surface, and that pH is the first-order control on binding of metals at these sites.
Figure 24: pH-controlled variation of HFO sorption efficiency in mine drainage from Reynolds Tunnel, Colorado, predicted using a combined equilibrium-
speciation and GLTM modelling approach (Smith et al., 193).
Yo sorption
80 60 40 20 0
2 3 4 5 6 7 8 PH
Model assumes 100% precipitation of dissolved Fe loading (310 ppm) as HFO. Trace concentrations = 3 ppm As, 10 ppm Pb, 20 ppm Cu, 20 ppm Zn.
46
e e e a 0
e 0
0
0
a 0
0
0 a
a a
0
0
0 0 0
0
a 0 0 0
e e 0
e 0
0 0 0
The concentration of coordination sites is estimated for a given volume of hydrous ferric oxide (HFO) precipitates, from which a sorption constant can be calculated. The application of the model to mine drainage from a site in Summitville, Colorado is outlined in Fig. 24. The precipitation of 100% of Fe (total 310 mg/l) as HFO is shown to induce significant recovery of As at pH 2.0 and total recovery at >pH 6 (it is notable that recovery of heavy metals is less efficient).
3.4: FUTURE PRIORITIES
This study was founded on only two sampling campaigns, and is thus inherently limited by its probable failure to account for the full range of short-. and long-term fluctuations of hydrology and hydrochemistry. While some information regarding surface water quality has been provided, an evaluation of the pH, Eh and dissolved metal characteristics of tailings pore- waters, and the possible formation of contaminant plumes within the groundwater system remains to be undertaken. Potential remediation methods, including biological and chemical decontamination procedures, could ideally be pilot-tested rapidly, with a view to identifying optimum procedures for long-term drainage control after mine closure. Although the data presented in this study give little cause for concern, a thorough evaluation of local human exposure to contaminants such as As and Sb (via dust, water and the food chain) may be w man te d.
47
ACKNOWLEDGEMENTS
Sincere thanks are extended to the Zimbabwe Geological Survey Department for the provision of local expertise, field staff and related logistic support for the duration of fieldwork at Globe and Phoenix. Valuable support and guidance was also provided by resident BGS TC officers Dr P Pitfield and Dr P Lowenstein. The British High Commission generously provided a vehicle for use throughout the 1993 and 1994 field campaigns. Permission to sample within the area of the current mining operation was kindly granted by Tabex Mining. Discussions with Jean Francois Milian (Chield Geologist, Tabex Mining) were particularly critical to the collection of representative samples (underground and at surface).
48
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51
0
e e 0 a 0 a 0 0
0
0 0 0 0
0
0
0
0
e 0
a 0 0 0
0
0
0 0
0
0
0 0
0
0
APPENDIX 1
Modelled speciation and saturation indices for samples GPW 101,102, 104 and 110, calculated using WATEQ4F. Note - TOC data
were not incorporated in speciation calcuIations.
52
a, U U w
1 0 0
1 0
10
I 0
1 0 0
I . 0
1 0 d
I
1 0 0
1 0
1 0
I 0
1 0 0
I . 0
1 0 d
1
1 0 0
1 0
1 0
I 0
1 0 0
I ' 0
1 0 r(
I
1 0 0
1 0
1 0
I 0
1 0 0
I ' 0
1 0 rf
0 aO 1 0
0 I .
0 I
1 0 0
1 0
1 0 0
I d
I 0
1 0 0
I - 0
I
1 0 0
1 0
1 0 0
I d
I 0
1 0 0
I . 0
I
1 0 0
I 0
1 0 0
I d
I 0
1 0 0
I ' 0
I
I d CO
I d
I w
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I 0
1 0 0
I * 0
1 0 rt
I
1 0 0
1 0
1 0
I 0
I 0 0
I ' 0
1 0 rl
0
1 0 0
I . 0
I
1 0 0
1 0
10 0
I d
I 0
1 0 0
I . 0
I
1 0 0
1 0
1 0 0
I d
I 0
1 0 0
I ' 0
I
1 - 3 m
1 . 3
I N rt
I 1
I 0
1 0 0
I . 0
I
I d CO
I d
I W
0 - P a t - X o \ acn o u
x m 2 . .
k m I U ' ZTY
W W 0 0 U
U
2
w o c n c n c n w o c n c n C O m m o m C O m m o P P a m d c m P w m e O m P o I ' O m I ' o
0 0 0 d 0 0 0 0 r f . . . . . . . . .
dccnmdcmcnwCOdc m <U cu nl m 0 0 0 4 I I I I I I I I I w w w w w w w w w
d I - l m m 4 m L n I ' C o P I ' m d d c m o c n q dc m cu Pa P '3 rf d Ln
W I n P a r t d P a m m P a . . . . . . . . .
o o o o o m w d o o o o o o m c n P o
o o o o o o c o o o . . . . . . . . .
cn
c n c n o c n c n o m m o m TYLnPadCm 0 I ' m o p
d O O d 0 . . . . .
d c m c o m c n 0 0 0 0 0
I I I I I w w w w w d c o d c T Y P a m d c o I ' c n m m L n c n *
d P a I ' c v r n . . . . .
d c w I ' c n m m w m m w c n O d 0 0 P P a r n
. . . . .
dcLnI'mcn 0 0 0 0 0
I I I I I I I I I I I I I I w w w w w w w w w w w w w w m w W P d P a r I c n d c P a w m d m w u 3 C O e C O d P a d P a m m w c v . . . . . . . . .
d( I' 0 0 I I w
w m W m
d I'
0 0 0 0 0 P N O C O c n
m c n ooooodc W O
o o o o o o m o 3 d c m m o o U o o o o o o m o o cnoooo d . . . . . . . . . . . . . . cd o o o o o o r f o o p m o o o U
E 0 0 0 0 0 P a d o In0
8 ooooodcrfcno P a T Y P a o o
0 0 m 0 P 4
d 0
o m P a d o d m P a d o o o d P a o d I l l I 1 1 I
c n m r l c n m c n c n c n a 3 m a 3 w m m o o m o w o m m m o m w m m m m m m m d m d m m m d c c u c u m d c m m c u c u P o m o P P P o P m P o P P m m
o r l o t - 4 o o o r l o o o r l o o o o . . . . . . . . . . . . . . . .
m P m d d r l a , m m m m m t - 4 O c u P ~ 0 0 0 0 d 0 0 0 0 0 0 d t - 4 r l 0
I I I I l l l l l l l l l l l l w w w w w w w w w w w w w w w w m d a 3 c u P m d m m w d a 3 m m w O m w 0 0 0 r l 0 d m d m m m w 0 P P t - i P c u d d c o m m t - 4 m m c n O w r l a 3 m r l c u t - 4 p c n d w a 3 m m m w w d . . . . . . . . . . . . . . . .
o d m w o o o m o r l a 3 w o o o o O d p r l I n O O m O w P m 0 0 0 r l
o p t - 4 h ) r l o o * o c u m o o o o o . . . . . . . . . . . . . . . .
w m d o P rl
I l l l l l l l l l l l l l i l
w r l r l w a 3 d o o a 3 t - t - c o O m m d w w w w w w w w w w w w w w w w w w a 3 r i u 3 m o P o i n i n * c y d * c y d m O r l d a 3 m m w d O d c u O o w d d m c u r l m r l d m c u P m m a , c y m . . . . . . . . . . . . . . . .
m 0
I w t-i rl a,
m
0 0 0 o m m ot-ia, O t - i d
0 0 4 0 nl
. . .
0 0 0
m a, m
m m r l d c o m
O L n O P m c o o o m w o o 0 - 0 0 c u d
. . . .
m 0 I
m 0 w cn
w cu 0 I w
P 0 rl
rl
m P m 0 0 0 o r l o
o a , h ) P w o o o m w r l c u n l r 1 o o o h ) w o a 3 o r l o o o o ~ P d c w c u o o o o a m r l h ) m m d
. . . . . . . . .
JJ
2
r-l cd U ~ 0 0 0 0 c u ~ ~ m m 0 0 m m w w a , r l 0 d o o d o c u ~ 0 0 0 0 r l w r l r l w 0 0 0 0 I n t - i m a 3 0 * o o w o m W 0 0 0 0 d d 0 0 0 0 0 0 0 a 3 0 m a 3 0 r l o o P o r l 0 t-4 a3 m h ) m m
de
. . . . . . . . . . . . . . . . . . . . . . . .
r-i cd
r-i
2 r-l cd c 4
m r l o o E d c o o o acuo 0 0 a 0 0 0 0
0 0 0 0 u o o o o d . . . . cdoooo U
0 0 0 0 0 0
F a , m w m w
E 0 0 m CL t-i r(
hl a 2
w o 4 0 m o
m n l r l m m o o r l m m d t - l o o o m o o 4 o o w h ) o o P o o . . . . . . .
cu
m a, m
w m a 3 o m c O h ) d ( L n O P c u 0 0 0 0 0 0
0 0 0 0 0 0 . . . . . .
0 LO r-i
0
o m w o O d O O ocooo o c u o o m o o o o r l o o o o t - i
0 0 0 0 0 . . . . .
dc 0 I w a3 P rl
w
dc 0 0 0 0 O P o m O d
cy
. .
0 0 m m c\l
0
8 m -
cncno o m m - m m o r - m 4 0 0 . . .
o e w
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . m c n P L n ~ c v c v ~ 0 0 h l d d d m 0 0 m r l r f w d a d c d l - i c v P ~ d d w d w d
Fi cd L, 0 0 0 0 ~ 0 0 0
0 0
#
. . . w o o 0
4
0 0 ~ m d h l L n P O ~ o O d ~ 0 0 0 0 0 0 d 0 0 ~ o c n ~ o o ~ c n r 1 ~ o o c n o d c v d c o d m o ~ o o ~ d o o o o o d ~ l - i ~ h l m o o d o r l o o o c v
a 3 0 0 d d o c n m o o o o o m o o o o o 0 0 m o o w o o o 0 0 r l o o o o o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . dc e a Ln d c n a m
(v 0
I W rn 0 w
Ln
Ln 0
I w w cn dc
m
hl 0
I W hl dc hl
hl
0 0 0 c n O ohlclo o w o o ocnoo 0 0 0 0 0 0 0 0
0 0 0 0 . . . .
0 0 0 0 0 0 o o o p o ooocoo
m P o o o d o ~ o d c h l o o o o o o d c c n m o c v o o o o o c n w h l h l o o o o o o o w h l . . . . . . . . . .
P d
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