a comparison of selective extraction soil geochemistry and biogeochemistry in the cobar area

.Journal of Geochemical Exploration 61 1998 173189

A comparison of selective extraction soil geochemistry andbiogeochemistry in the Cobar area, New South Wales

D.R. Cohen a,), X.C. Shen a, A.C. Dunlop a, N.F. Rutherford ba Department of Applied Geology, Uniersity of New South Wales, Sydney, NSW 2052, Australia

b Rutherford Mineral Resource Consultants, 87 Brook St, Coogee, NSW 2034, AustraliaReceived 30 March 1997; accepted 18 November 1997

Abstract

.In parts of the deeply weathered and semi-arid environments of the Cobar area NSW, Australia , detection ofmineralisation using conventional soil sampling and total metal analysis is impeded. This is due to the intense leaching oftrace elements within the weathered profile, discontinuous coverage of transported materials and the existence of diffuseregional geochemical anomalies of ill-defined source. Selective chemical extractions, applied to various regolith compo-nents, and biogeochemistry offer a means of isolating localised geochemical patterns related to recent dispersion of trace

.elements through the overburden. Lag geochemical patterns across the McKinnons deposit Au and Mrangelli prospect .PbZnAs reflect mechanical dispersion processes and minor hydromorphic effects. Concentrations of more mobileelements tend to be higher in the non-magnetic fraction, due to higher proportions of goethite and poorly crystalline hematitethan in the magnetic fraction. The subdued soil geochemical responses for metals extractable by cold 40% hydrochloric acid .CHX and for total element concentration reflect the leached nature of the residual profile, low grade of mineralisation,dilution by aeolian components and disequilibrium of fine fractions with coarser, relict Fe-oxides. The stronger contrast forCHX for most metals, compared with total extraction, indicates surface accumulation of trace elements derived fromunderlying mineralisation. Enzyme leach element anomalies are intense but generally located directly over bedrock sourcesor major structural breaks, irrespective of the nature of the overburden. Though mechanisms for the dispersion of traceelements extracted by enzyme leaching are not well established, the lack of lateral transport suggests vertical migration of

.volatile metal species atmimorphic dispersion . The strong, multi-element response to mineralisation in cypress pine needlesindicates significant metal recycling during the present erosional cycle. However, a comparison of the trace element

.concentrations in vegetation cypress pine needles and selective extractions of soils indicates that recycling by the plants isnot the dominant mechanism for transportation of metals through the overburden. The vegetation may be responding tohydromorphic dispersion patterns at depth. The use of selective extractions may be useful in detecting mineralisation throughdeeply leached profiles, but offers even greater potential when integrated with biogeochemistry to detect targets buried bysignificant thickness of transported cover. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: selective extraction; biogeochemistry; lag; enzyme leach; McKinnons

) Corresponding author. E-mail: [email protected]

0375-6742r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. .PII S0375-6742 97 00052-6

( )D.R. Cohen et al.rJournal of Geochemical Exploration 61 1998 173189174

1. Introduction

Single step and sequential, selective chemical ex-tractions are used in geochemical exploration tocharacterise element dispersion mechanisms, to im-prove geochemical contrast by screening out traceelement components not derived from local miner-alised sources, and to detect subtle leakage haloesemanating from deeply buried mineralisation. Inmany geochemical terrains, weak geochemical dis-persion haloes are actively forming in the top fewmetres of overburden due to groundwater movement,capillary action, diffusion of volatile compounds 1 .atmimorphic dispersion , natural electrochemicalgradients, vegetation recycling and the action oforganisms. Metals transported by these mechanismswould typically be associated with recently devel-oped, transient and metastable secondary minerals,such as amorphous Fe and Mn oxyhydroxides, espe-cially metals that are not readily incorporated intothe crystalline mineral phases. Such element accumu-lations may represent only a small fraction of thetotal trace element content of the overburden; hence,geochemical contrast would be suppressed if totalmetal extractions were used.

In the absence of detailed mineralogical studies,most extractions are, in effect, only operationally

.defined Hall et al., 1997 , the amount of traceelements released being dependent on the degree ofcrystallinity and purity of the mineral phases present,dissolution conditions and digestion time. The newlydeveloped enzyme leach appears to selectively dis-solve amorphous manganese oxides and other solu-ble phases, and yet is slow to attack crystallinemanganese oxides, amorphous or crystalline iron

oxides or carbonates Clark, 1993; Cohen et al., in.prep . In the early stages of reaction, strong solutions

.of cold hydrochloric acid CHX primarily attackmanganese oxides and poorly crystalline Fe oxides,rather than more crystalline Fe oxides. The latter,including hematite, are more gradually attacked by

.CHX Xie and Dunlop, 1998 .Geochemical anomaly detection in the Cobar area

is impeded by the presence of transported overbur-

1 .From the Greek atmi n,f s vapour.

den, truncation of weathered profiles, removal ofzones of trace element palaeo-accumulations anddevelopment of diffuse regional geochemical haloes.Although stream sediments and lag have been used

in regional reconnaissance surveys Dunlop et al.,.1983 , conventional sampling procedures have had to

be adapted to accommodate the problem of Quater-nary and recent siltation of drainage channels in-

cluding augering to zones containing buried lag;.Schmidt, 1990 . At the prospect scale, various soil

fractions and regolith components have proven effec-tive sampling media in erosional landforms domi-

nated by residual overburden Scott et al., 1991;Chaffee and Scott, 1995; Cairns et al., 1995; Cohen

.et al., 1996 . In many cases, sampling has focussedon an ill-defined B-horizon, developed in the mas-sive red soils of the region, with the objective ofdetecting metals suspected to be accumulating underthe present cycle of weathering. Selective extraction

on soils by weak hydroxylamine hydrochloride Chao.and Zhou, 1983 has provided interpretable patterns,

particularly for Au, near mineralisation at McKin- .nons Rugless and Elliot, 1995 . Although such ap-

proaches have proven effective over some truncatedprofiles and in areas with thin transported cover, it isless certain that such an approach will delineatemetal sources under thick transported cover.

An alternative to regolith sampling is the analysis .of groundwater Giblin, 1995 . This, however, is

dependent on the availability of water bores, whichare sparse in the Cobar area. Another alternative toregolith sampling is biogeochemistry. Phreatophytesassimilate trace metals from large volumes of soiland weathered bedrock, via interaction between theirextensive root systems and groundwater or contactwith soil particles. Biogeochemistry has been able todelineate mineralisation in various Australian ter-

rains Baker, 1986; Cohen et al., 1995; Lintern et al.,.1995; Marshall and Lintern, 1995 , in some cases in

the absence of strong anomalies in adjacent soils .Smith and Keele, 1984 .

This study compares the response of a relict .medium lags with that of vegetation and the total,

CHX and enzyme leach soil geochemistry over the .McKinnons deposit Au and the Mrangelli prospect

.AsPbZn . This provides a basis for discussion ofthe nature and extent of dispersion processes, andfuture planning and interpreting geochemical surveys

( )D.R. Cohen et al.rJournal of Geochemical Exploration 61 1998 173189 175

in areas of transported cover in regions with charac-teristics similar to those of the Cobar area.

2. Site descriptions

The Cobar area is part of an extensive palaeo-plainin southeastern Australia, which evolved while Aus-

tralia was part of the Gondwana supercontinent Ol-.lier and Pain, 1994 . This feature is known locally as

the Cobar Pediplain and its antiquity is indicated bythe presence of remnant olivineleucitite flows, 1416 Ma in age, which extruded subsequent to deep

.weathering Byrnes, 1993 . Humid conditions ex- .tended until the Mid Miocene Martin, 1991 , fol-

lowed by more arid conditions during the LateMiocene and MidLate Pliocene. Dissection of theCobar Pediplain weathering surface has produced agradation from relict, erosional domains to deposi-

.tional domains Senior, 1992 . Although the timingof the onset of this dissection is poorly constrained,widespread climatic change elsewhere to a more aridregime and mild tectonism suggests a Mid-Mioceneage, with further adjustments during the Pliocene andPleistocene.

The area has low topographic relief. Local zonesof silicification, associated with mineralised struc-

tures or more resistant lithologies sandstones and.conglomerates form topographic highs. Deep weath-

ering may extend below 100 m, and has generallyproduced a quartzkaolinitemuscovitesaprolite as-semblage. Ferruginous, residual cappings are notwidely developed, although heterogeneous ferricretesof detrital character are common on or near thesurface.

A veneer of lag, of diverse origin and morphol-ogy, is widespread in the erosional domains Alipour

.et al., 1996 . The transformation of surface Fe-oxidesto maghemite, attributed to bushfire activity by Fitz-

.patrick 1988 , probably dates from the LateMiocene. In localised, depositional domains alongmore densely vegetated drainage lines and adjacentplains, colluvial and alluvial mantles rest on trun-cated weathering profiles. The alluvium may be morethan 50 m thick and contain interfingering silts andgravels. The character of the gravels is comparablewith the adjacent lag, and typically includes a signif-icant proportion of maghemite-rich clasts.

Soils are typically massive red earths Walker,.1978 which show little profile differentiation and

are interpreted to have a significant aeolian compo-nent. Recent deflation of interfluve areas has con-tributed large quantities of silty colluvium along thepresent drainage lines. Thin, discontinuous, surface

.calcrete zones nodules and cements occur at thebase of slope and along drainage lines, with somedeeper lenses in transported cover observed in over-burden drill samples. The calcrete is probably devel-oped as a result of evapotranspiration.

The southern Cobar area is dominated by the .association of bimble box Eucalyptus populnea

. and white cypress pine Callitris columellaris Cun-.ningham et al., 1981 . Cypress pine is the most

uniformly distributed species, forming dense standsalong drainages, and is the first tree to recolonisecleared land.

2.1. McKinnons

The McKinnons gold deposit is located 37 kmsouth west of Cobar and was the first deposit discov-ered on the western edge of the Cobar Basin. In thisstudy, soil, lag and vegetation samples were col-lected, at 100 to 200 m spacing, from two 8-km

.traverses: line A 79600N which passes 400 m to .the north of the mine and line B 76000N which

.passes 3 km to the south Fig. 1 .The general area was targeted for follow-up ex-

ploration on the basis of anomalous Au, Ag, As, Sband Pb contents in -180 mm stream sediments,although streams adjacent to the deposit displaybackground metal values. The deposit was discov-ered in 1988 by Norgold Ltd., through rock chipsampling of a quartz-veined, silicified outcrop. Min-eralisation is hosted by weakly metamorphosed, sili-cified and brecciated, laminated to massive siltstones

and fine sandstones of the Amphitheatre Group part.of the early Devonian Cobar Supergroup . The min-

eralisation appears to follow the axial trace of thenorthwesterly trending Nullawarra Anticline Rug-

.less and Elliot, 1995 which is intersected around20000E on line A. The mineralisation assemblagecommences with Au near-surface and progresses toan AuAg assemblage at depth; PbZnAg mineral-isation is common at depths greater than 80 m .Bywater et al., 1996 . The deposit had an original


Fig.

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ore reserve of 2.2 mt at 1.91 grt Au, which is nowmined out. An intersection of 20 m at 1 grt alongline A indicates an extension of the mineralisedstructure some hundreds of metres to the north of the

.pit Maxey, 1995 . This structure appears to extendto the south of the pit.

The pre-existing profile has been strongly trun-cated, with only the lower saprolite zone preserved.Contact between the saprolite and fresh bedrock is ata depth of about 60 m. The area is gently sloping toundulating, with a thin cover of residual and collu-vial soil. Interspersed drainages contain a basal layerof mixed residual soil, colluvium and alluvium, over-

lain by up to 15 m of transported material mainly.silts . Outcrop in the area is rare. Lithic and pisoid

lags are common in the erosional landforms. To-wards drainage channels, the abundance and size oflag decrease, whereas the quartz content and propor-tion of magnetic to non-magnetic lag increase.

2.2. Mrangelli

The Mrangelli prospect is located 25 km west ofCobar at the western end of the eastwest trendingAmphitheatre Dome. Soil, lag and vegetation sam-

.ples were collected along line C 9400N , at 100 m .sample spacing Fig. 2 .

The area is dominated by silicified, resistant ridgesof Biddibirra Formation sandstone with local eleva-

.tion up to 50 m . Mineralisation is composed of clotsof disseminated pyrite, galena and sphalerite, hostedby strongly silicified sandstone and a quartz-veinedfault zone. Coincident PbAs lag and rotary air blast

.drill RAB geochemical anomalies were delineatedby Dominion Mining above weathered CSA Silt-stone. Peak lag values of 2000 ppm As and 500 ppm

.Pb above respective backgrounds of 50 ppm occurnear outcropping mineralisation.

The lower slopes are dominated by variablyweathered siltstone. Intense weathering of the profileup to 10 m depth is observed in RAB holes, but welldeveloped mottling and local ferruginisation mayextend to 20 m below surface. Pisoid lags are abun-dant, whereas lithic, silicified and subangular peb-bles of sandstone increase in abundance towards theelevated outcrops. Pedogenic calcrete is common atthe edge of the erosional landforms and in streambeds.

3. Sampling and analysis

Lag samples were collected at intervals of 100 to200 m and obtained by sweeping the ground surfaceat a number of locations within 10 m of the nominalsite. The samples were sieved to the 211 mm sizerange. The magnetic and non-magnetic componentswere separated and a large proportion of the quartzcontent removed from the non-magnetic fraction by

.hand. Representative portions 50 to 100 g of eachfraction were milled to -75 mm.

Soils were collected from 1020 cm depth toavoid the more recent transported material. Samplesfor the CHX digestion and total extraction werecollected at 100-m intervals and composited to pro-vide 200-m spacings. Soil samples used for theenzyme leach work were collected from the samesites as those for lag and vegetation. Soils weresieved to -180 mm for total extraction and enzymeleach, and to -75 mm for CHX extraction.

Cypress pine needles were selected as the princi-pal biogeochemical sampling medium due to theirstrong response to mineralisation compared with

.other species Pahlow, 1995 , limited seasonal vari-ability and resistance to dust contamination Cohen

.et al., 1996 . In July 1995, samples of needles werecollected from the lower branches of three to fiveindividual trees at each site. Samples were dried andmacerated, but left unashed. Seasonal variation testsites have since been established above mineralisedand background sites at McKinnons.

Total metal contents of the lag and soil weredetermined by a combination of INAA andHFrHClO rHNO digestion followed by AAS anal-4 3ysis. The CHX extractions for soils were performedby a commercial laboratory. Samples of 4 g weredigested at room temperature for 4 h in 10 ml of a40% HCl solution, with occasional mixing. Analysiswas performed using ICPMS. A 1-h enzyme leachwas also performed by a commercial laboratory andthe solutions centrifuged. For the selective extrac-tions, solutions were analysed by ICPMS, cali-brated against in-house reference materials. For thevegetation, 10-g samples were encapsulated in poly-thene vials and analysed by INAA. Vegetation sub-samples from Mrangelli were also digested in aqua

.regia 10 g in 100 ml and analysed by AAS for Pband Zn.


Fig.

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Table 1 .Analytical detection limits ppm

Element Soils and lag Vegetation

INAA AAS ICPMS Enzyme leachrICPMS INAA AASAs 1 0.2 0.005 0.05Au 0.005 0.0001 0.0001 0.0003Cu 1 1 0.005 1Fe 500 500 200 200 200La 0.5 0.001 0.01Pb 1 0.2 0.001 1Sb 0.2 0.1 0.001 0.01Zn 2 0.3 0.01 1

Analytical quality control was monitored usingprimary and secondary reference materials and sub-sample duplicates. Combined sub-sample and analyt-ical variance was less than 10% at concentrations

above three times the analytical detection limits Ta-.ble 1 .

4. Results

4.1. McKinnons

On lines A and B, both magnetic and non-mag-netic lag display a series of spot Au anomalies 710

.ppb , extending east from the main northeast-trend- .ing mineralised zone Fig. 3 . A similar pattern is

observed in the total Au contents of the adjacentsoils, though anomalies on line A are restricted to thezone of mineralisation and adjacent drainage. On lineA there is a contiguous set of CHX-Au anomalies .0.30.7 ppb , extending 250 m either side of theprojection of the structure hosting the mineralisation.CHX-Au values are slightly elevated in the drainageon the eastern end of the line, and in general, appearto be slightly elevated across the alluvium filledchannels. A spot enzyme leach anomaly of 1.2 ppboccurs above the trend of mineralisation, with asubsidiary peak in the adjacent drainage. A consis-tent, 2 km wide Au anomaly is observed in thecypress pine needles. Concentration peaks at 4.2 ppb .above a background of 0.8 ppb and the anomaly iscentred on the projection of the McKinnons mineral-isation. The Au spike in the needles at 23400E,

located at the top of a low ridge, cannot be related toknown mineralisation.

Well developed and consistent Au anomalies arenot observed in the soils on line B for either thepartial or the total extractions. The exception is the12 ppb anomaly above a background of -2 ppb forthe total extraction and 0.25 ppb above a background

of 0.15 ppb for CHX at 22200E with a weak.anomaly in the needles . In the cypress pine needles,

the most prominent feature is a group of five low-contrast anomalous sites spanning an alluvium-filledchannel between 19400E and 20000E, reaching 1.03.0 ppb. More subdued features reaching 1.2 ppb,situated near 18000E and 22000E, are associatedwith marginally elevated Au contents in CHX andtotal soil extractions. Several of these anomalies arerelated to fold hinge zones and quartz veining near18000E. Between 20400E and 20800E, thin, residualsoils partially cover an antiform hinge zone associ-ated with a persistent zone of en-echelon, narrow

shear zones possibly an extension of the Fence.Shear . These are also located adjacent to prominent,

coarse-grained, quartzose sandstone-grit units, withinwhich intermittent, narrow quartz veins, increasedcleavage development and adjacent silicification arecommon. Previous work by Burdekin Resources in-dicates broad, regional, low-level As and Sb anoma-lies to be derived from this structure.

Arsenic, Sb and La values in soils and lags on .line A Figs. 3 and 4 are much more subdued than

Au, with no obvious reflection of the McKinnonsmineralisation. At the edge of the eastern drainageline, near 23500E, lags contain 80240 ppm As and2040 ppm Sb in the non-magnetic fraction and soils


.Fig. 3. Comparison between Au and As contents of lags, soils total, CHX- and enzyme-extractable components and cypress pine needlesalong lines A and B, McKinnons. The profile is based on surface mapping, RAB drill holes and the projected location of mineralizationfrom the pit.

display a low-amplitude feature of 810 ppm totalAs and 11.6 ppm Sb. Elevated CHX-Sb concentra-

.tions 0.350.50 ppm overlap the total extractionfeature, and correspond with a peak in the otherwiseragged pattern of enzyme leach-As values. Althoughthe area of elevated As and Sb at the eastern end ofthe line is covered by soil and alluvium, it is situatedalong the northward projection of the OwlOsterly

Fault and regional anticlinal axis, along which othersoil and drainage sediment geochemical anomaliesoccur. The prominent enzyme leach-La anomaliestend to occur at the edge of drainages either side ofthe ridge containing the projected zone of mineralisa-tion, though there is some suggestion of the rabbitears style of anomaly on both lines for La, Cu andZn. Arsenic and Sb patterns in cypress pine needles


closely follow the trend for Au in the needles and theprojection of the main structure hosting Au minerali-sation at McKinnons mine 400 m to the south, withstrongly elevated values between 20000E and

21500E. The zone of anomalous La values and.other REEs in the needles is displaced from the Au

anomalies, commencing above mineralisation at20800E and extending across the adjacent drainageto 19500E.

On line B a prominent AsSb anomaly in nearlyall media is situated at 20400E20800E Figs. 3 and

.4 , with a subsidiary peak at 21400E, associated withthe Fence Shear. Arsenic and Sb patterns are almostidentical for the various media; however, the anoma-lous zone tends to be broader for the CHX extractionthan the total values. In lags, the main anomalyreaches 130250 ppm As and 2565 ppm Sb in the

magnetic fraction compared with 75 ppm As and 20.ppm Sb background , whereas the non-magnetic

fraction reaches 100500 ppm As and 2045 ppmSb. In the soils, a distinct, relatively high-contrastfeature for the total metal contents of 2045 ppm As

.Fig. 4. Comparison between Sb and La contents of lags, soils total, CHX- and enzyme-extractable components and cypress pine needlesalong lines A and B, McKinnons.


and 24 ppm Sb is located above the projectedmineralisation. This feature is repeated as a 0.40.9ppm As-CHX anomaly, but is not repeated for Sb.Beyond the strong enzyme leach anomaly of 1540ppb As, a number of single point anomalies above abackground of 1020 ppb extend east as far as24000E. The pattern for As in cypress pine needlesis subdued, but there are a number of peaks up to 0.1

ppm though contrast is restricted by the 0.05 ppm.detection limit .

Despite the presence of CuPbZn-bearing sul-phide zones in the base of the mine pit, there is noobservable Cu response in any media and the only

.Zn response is in the lag on line A Fig. 5 . On lineB, there is a series of Cu and Zn anomalies in thenon-magnetic lag, east of the projected zone ofmineralisation. Narrow zones of anomalous concen-trations of CHX-Zn and total-Cu also occur in thesoil zones near 20800E, and broadly correspond withthe lag anomalies. Despite high local variability,low-contrast Zn anomalies for total and CHX extrac-tions are observed between 18000E and 19000E.These may relate to the Nullawarra Anticline, westof McKinnons. Rabbit-ear enzyme leach anomaliesappear to have developed for Cu, Zn and Pb aboutthe projected zone of mineralisation on both lines,

.Fig. 5. Comparison between Cu and Zn contents of lags and soils total and enzyme-extractable components along lines A and B,McKinnons.


but the response is somewhat erratic. Most enzymeleach and CHX peaks correspond with the edges ofthe erosional landforms at which groundwater seep-age and surface accumulation of carbonate is com-mon.

With the exception of the halogens and somemetals which are mobile as oxyanions, the enzymeleach patterns are nearly identical for the remaining42 elements. For the majority of these elements,most analyses were below detection limits. However,intense, multi-element enzyme leach spot anomaliesoccur directly above mineralised zones, prominentstructural discontinuities and the edges of the largerdrainages. Other features not indicated in the figuresare the large series of anomalous trace element con-tents, including the REEs, in the cypress pine nee-dles corresponding to the main section of Au anoma-lies on line A.

4.2. Mrangelli

A series of 50200 m wide zones between 9400Eand 10350E, displaying elevated As levels 50270

.ppm at ;6 m depth in RAB holes, were defined byDominion Mining. Although RAB data indicate nosignificant Au component to the mineralisation atMrangelli, 3 ppb Au and 3 ppm Sb soil anomaliesare coincident with weak cypress pine needle Au

anomalies of 0.30.5 ppb above -0.1 ppb back-.ground to the east of the Biddibirra Fault at 8500E

.Figs. 2 and 6 . A series of irregular, 150300 ppmAs peaks extend from 9200E to 11000E in themagnetic fraction and 120660 ppm As from 9400E

to 10200E in the non-magnetic fraction compared.with ;100 ppm background in both lag fractions .

These broadly coincide with the highest RAB values.The sub-crop positions and the relative strength of

.Fig. 6. Comparison between Au, As, Sb, Pb, Zn and La contents of lags, soils total and enzyme-extractable components and cypress pineneedles along line C, Mrangelli. The profile is based on surface mapping and RAB drill hole data.


RAB geochemical features cannot readily be deter-mined. The adjacent lag values of about 100 ppmshould be considered anomalous in a regional con-text, suggesting a broad, mechanical dispersion haloin both lag fractions. Total As levels in soils define asubtle, but distinct anomaly of 1040 ppm between9500E and 10200E. Enzyme leach patterns are simi-lar to the soils near the strongest RAB As featurenear 10000E, with values peaking at 6090 ppb,compared with 2030 ppb background. Marginally

enhanced As values extend west to the vicinity of theBiddibirra Fault. In cypress pine needles, a promi-

nent peak of 0.100.15 ppm above a background of.- 0.05 ppm is situated between 10100E and

10700E, and is offset 250 m to the west of thesub-outcrop of the strongest zone as defined by theAs RAB geochemistry.

The pattern for Pb in lags shows prominent peaksof 300500 ppm at 9700E and 10100E in the non-magnetic fraction. In the magnetic fraction more

. .Fig. 7. Plots of Au, As, Pb and Zn versus Fe in lags total analysis and soils total, CHX- and enzyme leach-extractable components for thecombined McKinnons and Mrangelli data sets.


subdued peaks of 180200 ppm occur at the same .locations. Shoulders to these peaks 100120 ppm ,

over the interval 8800E11600E, form a broad haloabout the mineralised zone. These main peaks coin-cide with the strongest base-of-hole RAB Pb fea-tures. In both enzyme leach- and total-Pb soil con-tents, a single point feature of 110 ppb and 90 ppm,respectively, is situated at 10100E. This reflects thebroadest zone of RAB Pb features of 150200 m at230 to 500 ppm Pb, as compared with a 50 m zonewith 1701100 ppm at 9700E. The pattern of Pb incypress pines is somewhat irregular, with local peaksof 22.6 ppm as compared to a background of0.82.0 ppm. A prominent, low-contrast featurereaching 2.6 ppm at 10300E10500E is offset 250 mto the west of the most extensive RAB Pb geochemi-cal feature.

There is an irregular distribution of Zn peaksaround the mineralised zone. In the magnetic lagfraction, values reach 270430 ppm and 570620ppm in the non-magnetic fraction compared with4080 ppm background. The main peak in both themagnetic and non-magnetic fractions at 9700E coin-cides with the highest RAB Pb values, and with Znlevels above 500 ppm in the upper part of theunderlying RAB holes. The broadest zone of highRAB Pb values is one of marked Zn depletion inboth magnetic and non-magnetic lag values. Soilpatterns are somewhat similar, with an enzyme leachsingle point feature of 150 ppb at 10100E coincidentwith the previously mentioned RAB Pb peak. TotalZn in soil shows a low-amplitude feature of 8085

.ppm compared with -50 ppm situated over theBiddibirra Fault. The Zn content of cypress pine

needles ranges from 8 to 15 ppm without discernibleanomalies. As with McKinnons, the CHX soilanomalies tend to be slightly broader than the totalsoil anomalies, or have elevated shoulder valuesadjacent to the peak anomalous values.

4.3. Iron controls

The relationship between Fe content and the Au,As, Pb and Zn concentration of the magnetic andnon-magnetic lags, total and CHX-extractable soils

.vary among the elements examined Fig. 7 . There isno correlation between Au and Fe in lags or totalsoils, and a very weak correlation between CHX-ex-tractable Au and Fe. Apart from a few anomaloussamples, the As contents are correlated with Fe inthe lags and total soils but not correlated strongly forCHX. Lead is also strongly correlated with Fe in thelags and follows the trends observed at Mrangelli

.and other sites in the region by Alipour et al. 1997 .Zinc is weakly correlated with Fe in lags, morestrongly correlated in the total soils, but uncorrelatedfor the CHX extraction.

Median element concentrations for samples fromMcKinnons and the CHXrtotal metal ratios are pre-sented in Table 2. The proportion of total traceelements released by CHX extraction of the amor-phous Fe-oxides varied markedly between elements.Though a median of 6.1% of the total Fe content was

extracted by CHX indicating a low proportion of.poorly crystalline Fe-oxides in the soils only 4.1%

of the As appears to be bound up in the amorphousFe-oxides. Conversely, over 20% of Cu, Pb and Sbappear bound up in this component.

Table 2Median concentrations of Au, As, Sb, Ba, Cu, Pb, Zn and Fe in lags and soils from McKinnons

. . . . .Element Magnetic lag total ppm Non-magnetic lag total ppm Soil CHX ppm Soil total ppm Soil CHXrtotal %As 60 58 0.25 6.05 4.1

.Fe % 39 22 0.21 3.45 6.1Zn 45 92 1.70 25.00 6.8

.Au ppb 1 1 0.12 1.4 8.6Ba 430 480 29 300 9.7Cu 22 22 4 20 20.0Pb 76 51 4.0 18.0 21.2Sb 17 13 0.28 1.13 24.8

ns110.


5. Discussion

Geochemical features in the Cobar area are devel-oped over a thick, relict weathering profile, datingfrom the early Tertiary. Most deposits discovered byexploration geochemistry have been located in ero-sional regimes where anomalies can be detected in awide range of geochemical sampling media. Geo-chemical anomalies related to mineralised structureswithin the three antiforms in the vicinity of the

.McKinnons deposit AuAs"Sb"Zn"Pb , and aweakly mineralised zone of silicification at Mrangelli .PbZnAs , have been detected by a range ofmedia and extraction techniques.

The patterns for Au in the lags at McKinnons areerratic and reflect relict dispersion patterns. Theexistence of a series of spot Au anomalies in lag to

the east of mineralisation at McKinnons 21000E to.25000E , in the absence of soil or vegetation anoma-

lies, and scattered patches of gravels, suggests topo-graphic inversion has altered the drainage trend fromeastwards to northwards. The response in both lagfractions is consistent with the Au being particulateand most likely derived from local quartz vein-hostedmineralisation and silicified wall rocks.

Except for sites directly over projected miner-alised zones in erosional regimes, geochemical con-trast in total and CHX-extractable soil components is

.subdued in the order of 2 to 20 . Potential causes forthis include the leached nature of the relict profile,the low grades of mineralisation, dilution by aeoliancomponents and disequilibrium of fine fractions withcoarser relict Fe-oxide-rich fragments. The generallypoor correlation between trace element patterns inthe non-magnetic lag and total extraction for soils at

Mrangelli given the magnitude of the lag trace.element values may be attributed to the highly

siliceous nature of the non-magnetic lag that wouldlimit the loss of trace elements to soil. The weak-to-moderate spatial correlation between the trace ele-ment contents of the non-magnetic lags and theCHX-extractable soils at McKinnons and the slightbroadening of the anomalous zones for CHX com-pared with the totals for soils around the main

.anomalies also observed in the Fe-contents , sug-gests that a portion of the amorphous Fe may bederived from the degradation of adjacent lags andsubsequent dispersion in surface soils. In most in-

stants, CHX geochemical anomalies developed abovemineralised zones are more extensive than thosedisplayed by the total metal contents of the soils.

The comparatively high CHXrtotal metal ratiofor Cu and Pb in the soils, compared with As and Sb,may be attributed to stronger retention of Cu and Pbin the poorly crystalline Fe-oxides. These patternsare consistent with the dissolution patterns for vari-ous elements in lags from Mrangelli and nearby

.Yarrawonga, reported by Alipour et al. 1997 . Thepoor-to-insignificant correlation between trace ele-ments and Fe in both total and CHX extracts pre-cludes direct application of regression against Fe toenhance anomaly patterns. Extensive hydromorphicdispersion and multi-stage lag development are thelikely causes of the low-level As, Bi, Sb and Znanomalies which are present in the soils and lags ofdrainage systems surrounding discrete sources in theCobar area, including drainage lines peripheral to theweakly mineralised antiforms in the McKinnons area,

. Elura Schmidt, 1990 and Wagga Tank Scott et al.,.1991 .

The distinct multi-element response to mineralisa-tion in the cypress pine needles downslope frommineralised sites indicates significant metal recyclingduring the present erosion cycle. However, the rela-tive positions of the vegetation, enzyme leach andtotal element concentration peaks indicate that recy-cling by the plants is not the dominant mechanismfor introduction of metals at surface. In many in-stances, trace element anomalies in the vegetation orthe limit of vegetation anomalies is displaced 200 to600 m downslope from equivalent features in thesoils or lag. The vegetation may be responding to thepresent or palaeo-hydromorphic dispersion patternsthat have been defined at depth by RAB and augersampling. Elevated trace element contents in refrac-tory Fe-oxide-rich lag are not generally reflected byeither soils or vegetation, though this may relate tothe small proportion of the total profile volumecontributed by lag. The lack of correlation betweenthe vegetation and CHX soil patterns indicates thatthe metals released into the skeletal A-horizonthrough the shedding of needles, twigs and bark ordirect plant exudations are being rapidly flushed intodrainages and are not adsorbing to the Fe-oxides inthe upper B-horizon soils.

Enzyme leach anomalies tend to be of the apical


or rabbit-ear types, with the most intense anomaliesoccurring directly above potential sources, such asmineralisation or major structural discontinuities.There is no indication of the lateral or down-slopedisplacement of anomalies evident in the adjacentvegetation, soils or lag geochemistry. This featurehas also been observed in transported overburdennear the CSA mine and in a variety of other terrains .Clark and Cohen, 1995 .

Although no sites were available with definedmineralised sources covered by thick, transportedoverburden without lateral contributions from ex-posed mineralisation, the geochemical patterns ob-served may have some implication for exploration inareas of thick cover in the Cobar and similar dis-tricts. The relatively narrow and discrete vegetationAs peaks associated with drainage lines on line B atMcKinnons suggest that dispersion is predominantlydown-drainage, whereas on line A, the 1.42.2 kmwide AuAsSbBaLa feature in the needles indi-cates that a broad hydromorphic fan at depth is beingtapped. Some geochemical dispersion may be de-rived from palaeo-clastic dispersion of lags and soilsalong broad drainage lines, but the lack of correla-tion between vegetation and lag patterns, togetherwith the results over PbZnSb features in palaeo-drainage lines suggests the metals may be in arelatively refractory form not released to vegetation.

6. Conclusions

The strong, multi-element biogeochemical re-sponse to mineralisation, with a consistent zone ofanomalies extending up to 1 km either side of theprojected zone of mineralisation at McKinnons, isindicative of substantial sub-surface hydromorphicdispersion and biological transport of metals to sur-face. The lack of correlation between trace elementsand Fe in both the total or CHX soil extracts and theweaker response to mineralisation in soils than in thevegetation indicate either lateral hydromorphic dis-persion does not extend into the upper soil horizons,or dilution by transported materials.

The dissection and modification of the weatheringprofile has generated lag in which geochemical pat-terns are consistent with predominantly mechanicaldispersion and minor hydromorphic effects. The dif-

ferences between the trace element contents of themagnetic and non-magnetic lag fractions may beaccounted for by the retention characteristics of themineral phases present and the provenance of the lag .Alipour et al., 1997 . Patterns relating to poorly

crystalline Fe-oxides operationally defined by the.CHX leach , compared with total element contents of

near surface soils, suggest some metal mobility inthe present cycle of weathering, perhaps related tothe same evapo-transpiration processes which areproducing calcrete.

Although some apical enzyme leach metal anoma-lies have developed directly over major structures ormineralised zones, some patterns appear to be of therabbit-ears type. This pattern is ascribed by Clark .1993 to the effect of electrochemical cells relatedto oxidizing mineralisation. Although mechanismsfor the dispersion of trace elements extracted byenzyme leaching are not well established, the verticalalignment of multi-element enzyme leach anomalieswith major structures or mineralised zones, the lackof detectable lateral dispersion for Au, and the lackof correlation with the patterns for the biogeochemi-cal anomalies suggest the possibility that vertical

migration of trace elements that are subsequently.trapped by transient, amorphous Mn-oxides could

include a component of vapour transport atmimor-.phic dispersion .

In the erosional regimes, even with thin veneersof transported material, total analysis of various re-golith components appears to provide as strong aresponse to mineralisation as the vegetation. In areaswith deeper cover of transported material, it is con-cluded that, although the use of partial digestionsmay be an effective means of detecting mineralisa-tion, they may offer even greater potential whenintegrated with biogeochemistry to detect targetsburied by significant thickness of transported cover.The added analytical costs for exploration programsare likely to be outweighed by the increased proba-bility of detecting weak dispersion haloes in regionalsurveys and improvements to the interpretation ofgeochemical anomaly patterns. One possible sam-pling strategy in areas of deep, transported cover isalternate lines of cypress pine needles and eitherCHX or enzyme leach extractions from shallow soils,collected as composites over widely spaced intervals .200400 m . The strong geochemical contrast of


enzyme leach anomalies would allow compositingover a number of sites.

Acknowledgements

The authors thank Burdekin Resources, CobarMines, Dominion Mining and CRA Exploration fortheir support and permission to publish the data.Analyses were provided by Activation Laboratories . .Denver and Becquerel Laboratories Sydney . Partof this project was funded by a grant to DRC fromthe Australian Research Council.

ReferencesAlipour, S., Dunlop, A.C., Cohen, D.R., 1996. Morphology of

lags in the Cobar area, N.S.W. AGSO J. Aust. Geol. Geophys.16, 253262.

Alipour, S., Cohen, D.R., Dunlop, A.C., 1997. Geochemicalcharacteristics of lag in the Cobar area. N.S.W. J. Geochem.Explor. 58, 1528.

Baker, W.E., 1986. Gold in vegetation as a prospecting method in .Tasmania. In: Carlisle, D. et al. Eds. , Mineral Exploration:

.Biological Systems and Organic Matter Rubey v. 5 .Prentice-Hall, Englewood Cliffs, N.J., pp. 150158.

Byrnes, J.G., 1993. Metallogenic study and mineral deposit sheets.Bourke 1:250,000 Metallogenic Map. Geological Survey ofNew South Wales.

Bywater, A., Johnston, C., Hall, C.R., Bell, P.W., Elliott, S.M.,1996. Geology of McKinnons Gold Mine, Cobar, New South

.Wales. In: Cook, W.G. et al. Eds. , The Cobar Mineral FieldA 1996 Perspective. Aust. Inst. Min. Metall., Melbourne,pp. 279291.

Cairns, C., McQueen, K.G., Taylor, G., 1995. Regolith geochem-istry and mineralogy over two mineralised shear zones nearCobar, NSW. Abstr. Regolith 94, Broken Hill, AGSO Record1994r56, p. 13.

Chaffee, M.A., Scott, K.M., 1995. Primary and secondary elementand mineral dispersion in the Wagga polymetallic deposit,New South Wales, Australia. Abstr. 17th IGES, Townsville,May 1995, pp. 230231.

Chao, T.T., Zhou, L., 1983. Extraction techniques for selectivedissolution of amorphous iron oxides from soils and sedi-ments. Soil Sci. Soc. Am. J. 47, 225232.

Clark, J.R., 1993. Enzyme-induced leaching of B-horizon soils formineral exploration in areas of glacial overburden. Trans. Inst.Min. Metall. 102, B19B29.

Clark, J.R., Cohen, D.R., 1995. Innovative enzyme leach providesoverburden penetration. Abstr. 17th IGES, Townsville, May1995, pp. 311314.

Cohen, D.R., Garnett, D., Rutherford, N.F., 1995. A comparisonof stream sediment and vegetation geochemistry in the DrakeArea, New South Wales. In: Dunn, C.E., Hall, G.E.M., Scagel,

.R.K., Cohen, D.R. Eds. , Biogeochemical Methods for Explo-

ration and Environmental Geochemistry. Association of Ex-ploration Geochemists.

Cohen, D.R., Rutherford, N.F., Dunlop, A.C., Alipour, S., 1996.Geochemical Exploration in the Cobar Region. In: Cook,

.W.G. et al. Eds. , The Cobar Mineral FieldA 1996 Per-spective. Aust. Inst. Min. Metall., Melbourne, pp. 125155.

Cohen, D.R., Kelley, D.L., Clark, J.R., in prep. Selectivity andkinetics of some selective geochemical extractions.

Cunningham, G.M., Mulham, W.E., Millthorpe, P.L., Leigh, J.H.,1981. Plants of Western N.S.W. NSW Soil ConservationService, 766 pp.

Dunlop, A.C., Atherden, P.R., Govett, G.J.S., 1983. Lead distribu-tion in drainage channels about the Elura zincleadsilverdeposit, Cobar, NSW, Australia. J. Geochem. Explor. 18,195204.

Fitzpatrick, R.W., 1988. Iron compounds as indicators of pedo-genic processes: examples from the southern hemisphere. In:

.Stucker, J.W. et al. Eds. , Iron in Soils and Clay Minerals. D.Reidel, Dordrecht, pp. 351396.

Giblin, A.M., 1995. Groundwater geochemistry in Australiaanexploration window to concealed ore deposits. Abstr. 17thIGES, Townsville, May 1995, pp. 319322.

Hall, G.E.M., Vaive, J.E., Beer, R., Hoashi, M., 1997. Selectiveleaches revisited, with emphasis on the amorphous Fe-oxyhydroxide phase extraction. J. Geochem. Explor. 56, 5978.

Lintern, M.J., Butt, C.R.M., Scott, K.M., 1995. Gold in vegetationand soilthree case studies from the goldfields of south-west-ern Australia. In: Dunn, C.E., Hall, G.E.M., Scagel, R.K.,

.Cohen, D.R. Eds. , Biogeochemical Methods for Explorationand Environmental Geochemistry. Short Course Manual, As-sociation of Exploration Geochemists.

Marshall, A.E., Lintern, M.J., 1995. Biogeochemical investiga-tions in the Murchison and Telfer regions of arid WesternAustralia. In: Dunn, C.E., Hall, G.E.M., Scagel, R.K., Cohen,

.D.R. Eds. . Biogeochemical Methods for Exploration andEnvironmental Geochemistry. Short Course Manual, Associa-tion of Exploration Geochemists.

Martin, H.A., 1991. Tertiary stratigraphic palynology and palaeo-climate of the inland river systems in New South Wales. In:

.Williams, M.A.J., DeDekker, P., Kershak, A.P. Eds. , TheCainozoic in Australia: a Reappraisal of the Evidence. Geol.Soc. Aust. Spec. Publ. 18, 181194.

Maxey, A., 1995. Burdekin goes elephant hunting. Gold Gazette3, 2526.

Ollier, C.D., Pain, C.F., 1994. Landscape evolution and tectonicsin southeastern Australia. AGSO J. Aust. Geol. Geophys. 15,335345.

Pahlow, K.M., 1995. Geology and Mineralization of the Larsens, .Hartmans and NE Prospects, Girilambone, NSW. BSc Hons

Thesis, UNSW.Rugless, C.S., Elliot, S.M., 1995. Multielement exploration in

deeply weathered terrain: the McKinnons gold deposit nearCobar, NSW, Australiaa case study. Abstr. 17th IGES,Townsville, May 1995, pp. 100102.

Schmidt, B.L. 1990. Elura zincleadsilver mine, Cobar. In: .Glasson, K.R., Rattigan, J.H. Eds. , Geological Aspects of the


Discovery of some Important Mineral Deposits in Australia.Monogr. 17, Aust. Inst. Min. Metall., pp. 161170.

Scott, F.M., Rabonne, G., Chaffee, M.A., 1991. Weathering andits effects upon geochemical dispersion at the polymetallicWagga deposit, N.S.W., Australia. J. Geochem. Explor. 40,413426.

Senior, B., 1992. Regolith photogeology of the CobarrWrightvillearea, NSW. Report to Dominion Mining Ltd., 23 pp.

Smith, B.H., Keele, R.A., 1984. Some observations on the geo-chemistry of gold mineralization in the weathered zone atNorseman, W.A. J. Geochem. Explor. 22, 120.

Walker, P.J., 1978. SoilsCobar District. In: Technical Manual,Soil Conservation Service of New South Wales, pp. 5.15.13.

Xie, J., Dunlop, A.C., 1998. Dissolution rates of iron oxides:Implications for sampling ferruginous materials with signifi-

.cant relict iron oxides. J. Geochem. Explor. this issue .

a comparison of selective extraction soil geochemistry and biogeochemistry in the cobar area

Documents

total extraction

recent dispersion of

selective extractions

selective chemical extractions

dispersion of traceelements

use of selective extractions

enzyme leach element

total element concentration