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GEOCHEMICAL AND PARAMAGNET EXPLORATION

AN INTEGRATED GEOCHEMICAL AND PARAMAGNETIC EXPLORATION AT THE MCKINNONS GOLD DEPOSIT,COBAR, NSWAUNG PWA 1 G M BAILEy2, J C VAN MOORT 1 AND K G MCQUEEN 3

1 School of Earth Sciences; University of Tasmania, Hobart, TA52 Physics Division, ANSTO, Lucas Heights; NS W

3 CRC LEME, Faculty ojApplied Science, University oj Canberm, Canberm, ACT

ABSTRACTA geochemical and paramagnetic study has been carried out at the McKinnons gold mine to investigate thepotential of acid insoluble residues from regolith samples as a sample medium in the search for buried golddeposits Residues derived by sequential treatment of regolith samples with hot aqua regia and sulphuric acidare composed of quartz with a minor amount of muscovite

Zones of enrichment in Cu, Zn, Pb, Ag, As, and Sb (as indicators of mineralisation) and depletion in K, AI, Fe,Mg, Ca, Na, Ti and Sr (as indicators of wall-rock alteration) in acid insoluble residues from regolith samples outlinethe gold deposi t and also extend to the surface Regolith samples also preserve a geochemical zonationcharacterised by enrichment in Cu, Pb, Ag, As, Sb, Ba, Bi and W, and depletion in Zn and Ni at the upper levelof the deposit The geochemical indices of Zn x Pb x As, Cu x Pb x As, Zn x Pb, Ca x Na x Sr, (Zn x Pb x As)/(Cax Na x Sr), (Zn x Pb)/(Na x Sr), (Cu x Pb x As)/(Zn x Ni) and (Pb x Ag x Sb)/(Zn x Ni) are useful indicators ofmineralisation and associated hydrothermal alteration in the area studied These indices define the goldmineralisation, and clearly distinguish between gold mineralised and barren areas Electron paramagneticresonance (EPR) signatures of the acid insoluble reSidues also show significant differences between mineralisedand barren areas A strong paramagnetic halo outlines the gold mineralisation, while barren areas have weakparamagnetism The EPR spectra are considered to reflect lattice imperfections in quartz related to substitutions,with mineralised quartz containing more imperfections and substitutions

Significant geochemical and paramagnetic expressions in acid insoluble residues from regolith samples at thesurface above and adjacent to the McKinnons gold deposit demonstrate that this integrated exploration methodhas potential for detecting buried gold deposits in highly weathered terrainsKey words: regolith, gold exploration, acid insoluble residues, geochemistry, EPR, McKinnons gold deposit

INTRODUCTIONThe McKinnons gold deposit, located 37 km southwestof Cobar, NSW (Figure 1) occurs in a highly prospectiveregion for gold and base metal mineralisa tion This studyused acid insoluble residues of regolith samples for anintegrated geochemical and paramagnetic explorationprogram The objectives were: to determine the spatialdistribution patterns of major, minor and trace elements,and EPR signal intensity, in the acid insoluble reSidues ofregolith samples around the McKinnons gold mineralisationand its associated wall rock alteration; to examinegeochemical zonality of the mineralisation system; and todefine exploration vectors for buried ore depositsThe method was developed from a series of studies by

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To Broken Hill

o

XElura

CSA

* cKinnonsGold Mine20km

~~ S Y D N E

Figure 1: location oj McKinnons gold depso!t

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Aung Pwa et al (1992), Aung Pwa (1995), Russell andvan Moort (1998), van Moort et al (1995) and Aung Pwaand van Moort (1998) on VHMS and gold mineralisationThe study at McKinnons follows initial regolith studies ongold deposits in the Tanami Desert, NT, Mt Percy,Kalgoorlie and the Rand Pit, Meekatharra, WA (Xu Li andvan Moort, 1998; Stott et ai , 1998)Detailed geochemical and mineralogical studies of theMcKinnons deposit have been undertaken by Rugless andElliott (1995), Tan (1996) and Marshall et al (1996) Theyall described geochemical and mineralogical Signatures inrelation to the McKinnons gold mineralisation andassociated hydrothermal alteration A largeepithermal/mesothermal hydrothermal alteration systemassociated with the McKinnons gold mineralisationcoincides with multielement geochemical halos in theprimary zone These are retained within the regolith Thesaprolitic profile has preserved primary lithic fabricsRugless and Elliot (1995) found that both soil and laggeochemical techniques were useful in the initialexploration phase in the McKinnons area The goldmineralisation is defined by multielement anomaliescharacterised by positive Sb, Mo, Ba, Ag and As, andnegative Mn, Cu, Zn, Sn, P, V and Fe values Marshall et al(1996) indicate that the regolith profile over mineralisationcan be distinguished from a barren one by the presence ofillite The barren profile contains mUSCOVite and kaolinite

GEOLOGY AND MINERALISATIONThe McKinnons depOsit is located near the westernmargin of the Cobar Basin, and occurs in sedimentaryrocks of the Amphitheatre Group of the Devonian CobarSuper group These and similar rocks also host basemetal and gold-copper depOsits (e g Elura Pb-Zn, CSACu-Pb-Zn and Peak Au-Cu depOSits) on the easternmargin of the basin (Glen, 1987) The host rocks mostlyconsist of weakly metamorphosed Siltstones andsandstones depOsited as distal turbidites in a shallowmarine environment The McKinnons depOSit contained22 Mt at 19 gl t Au (135, 000 ozs) in the weatheredzone (Bywater et ai, 1996)The deposit is structurally controlled, and mineralisationappears to follow the NW trending Nullawarra Anticlinewhich has been effected by NE-SW and NS structures(Rugless and ElliOt, 1995) Detailed geological mapping(Figure 2) has revealed four major structures associatedwith mineralisation (Bywater et ai, 1996): a subvertical toeasterly dipping 330 20 trend; a subvertical 010 10trend; a subvertical to east dipping trend at 03SO 10;

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/ Fault, , / Bedding & beddingparallel fault/Breccia veining

~ ; ~ ; ~ : e ~ ~ ~ ~ ~ S ~ o r ; z o n[ j Siltstone and fineFigure 2: Interpretative geology map o/McKinnon mine pit at180Rl (20 metres below the surface) Modified after Bywater etal (1996)

and a subvertical 090 20 trend The intersection ofthese trends produced zones of intense fracturing andjointing and provided favourable conditions for ore fluidflow and consequently major mineralisation andassociated alteration (silicification) Gold mineralisationoccurs in the breCCiated and silicified sedimentsassociated with pyrite mineralisation Numerous quartzveins are recognised near and and around themineral ised zone Silicification, sericitisation and clayalteration are the dominant alteration featuresassociated with the McKinnons depOsit (Rugless andElliott, 1995; Bywater et ai , 1996) Figure 3 shows goldmineralisation (more than 0 3 ppm; Marshall et ai ,1996) along the mine section 15150 N Themineralisation system at the McKinnons depOSit is zonedwith gold mineralisation near surface changing to goldpoor Zn-Pb-Ag sulphide mineralisation in unweatheredrock at depths below 100 m (Bywater et ai , 1996)

WEATHERING PROFILEWeathering profiles of the McKinnons area have beenstudied by Rugless and Elliott (1995), Tan (1996) andMarshall et al (1996) Detail mineralogical andgeochemical variation in the regolith horizons were citedin the last two papersOver the mineralised area the profile consists of brownpisoliths at the topl; overlying an upper layer composed

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15150 N

Au> 0 3 ppm \ Drill hole and sample point10m

Figure 3: Locations oj drill holes an d samples selected for thisstudy (from Marshall et ai , 1996)

of quartz, illite with a minor amount of kaolinite,muscovite and hematite/goethite; overlying whitesaprolith, consisting of quartz, clay minerals (mainly illite)and muscovite, and a lower zone of grey brown saprolitecomprising quartz, clay mineral (illite), muscovite; pyrite,goethite and hematite There is a significant difference inmineralogical composition between barren andmineralised profiles The mineralised profile is mainlycomposed of quartz and illite in the upper level While thebarren profile contains quartz, kaolinite and muscovite(Marshall et al , 1996) Mineralogical compositions at thelower levels of both profiles do not show significantdifference between each other

SAMPLE SELECTION AND COLLECTIONThis study involved a total of one hundred and oneregolith samples from fourteen reverse circulationpercussion drill holes Eighty-eight powdered regolithsamples from thirteen drill holes, of which twelve holesare located along the mine section 15150 N (Figure 3),were obtained from Marshall et al (1996) The other drillhole is MB4 which is known to be a barren hole Anadditional thirteen regolith samples were collected bythe authors from drill hole MKD 64 and crushed in a jawcrusher and ground in a swing mill to 75 jJm

SAMPLE PREPARATION AND ANALYSIS

SAMPLE PREPARATIONTwo grams of each powdered sample were digested in20ml of aqua regia (3 HCI: 1 HN03) at 60C for 12 hoursin test tubes in order to remove iron and manganese

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oxides and hydroxides, sulphides, carbonates and mostsulphates The test tubes were intermittently stirred witha glass rod The acid was discarded, and the residues inthe test tubes were washed once with distilled water Thenthe residues in the test tubes were digested withconcentrated H2S04 (15 ml) at 60C for 12 hours todissolve clay minerals The acid was discarded, theresidues in the test tubes were washed with distilled waterfour times, and dried in an oven at 90C for 12 hours

ANALYSISThe study involved mineralogical determination by semiquantitative X-ray diffraction (XRD), EPR measurement byelectron spin resonance spectroscopy and chemicalanalyses by proton induced X-ray emmision (PIXE) andproton induced gamma ray emission (PIGME) and ICP-MS

X-ray diffraction (XRD) revealed that the residues aremainly composed of quartz (probably more than 90%),and a minor amount of mica The surface soil samplesconsist partly of microcrystalline chalcedony aggregateswith inclusions of iron oXide, and the deeper samplesconsist of quartz, with only a few inclusions Quartz andmica are considered to be the sources of the variouselements in the residues of the regolith samples Thechemical composition of the samples reflect thecomposition of these minerals, their lattice substitutionsand also their inclusions

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EPR measurement was made on 100 mg of the residualpowders from the acid leach These were placed in aWilmad annealed silica EPR tube inserted in amultipurpose cavity of a JEOL FE3X ESR spectrometer ofthe Central Science Laboratory at the University ofTasmania Samples were subjected at room temperatureto a microwave frequency of 9 15 GHz, 10 mWmicrowave power, modulation frequency at 100 KHzmodulation width at 0 2 mT, amplitude at 5 x 10 and amagnetic flux density with a median value of 32 6 5 mTSpectra of the first derivative of the absorption of themicrowave power were run over sweeps of 326 5 5 mTand 326 5 100 mT The first spectrum (326 5 5 mT)provides details of the shape of the central section of thespectrum The large sweeps are all very similar and willnot be further considered in this report Instrumentsettings for EPR measurements were, except forinstrument gain, the same as the ones used by vanMoort and Russell (1987) for their study of theBeaconsfield gold deposi t The sweeps over 32 6 5 5mT all show a rather strong signal at g - 2 0025Because of the strength of the signal, the gain(amplitude) of the instrument was set at 5 x 10, fivetimes less than in the standard condition routinelyapplied in our other EPR studiesFor chemical analysis (proton induced X-ray emission(PIXE) and proton induced gamma ray emission(PIGME)), reSidual powder samples were weighed andmechanically mixed with spectrographically puregraphite powder in a ratio of 4: 1 (eg 40 0 mg sample:100 mg graphite powder) in an acrylic vial (containing anacrylic ball) for 10 minutes in a SPEX mixer The mixedpowder was pressed into a small aluminium cap (12 mmin diameter) Each sample was analysed for 22elements, K, Fe, Ca, Ti, Mn, CI, Ga, Rb, Sr, Zr, Y,Cu, Zn,Pb, Ni, As, V and Mo by PIXE, and AI, Na, Mg and F byPIGME on the 3MV van de Graaff accelerator at ANSTO,Lucas Heights, Sydney A detailed discussion of thisanalytical method was recently presented by van Moortet al (1995) The PIXE method is not suitable for someelements contained in the regolith residues from theMcKinnons area (eg Ag, Sb, Ba and W), particularly forlow level concentrationsDetermination of Ag, Ba, Sb, Bi, W, Pb and TI by ICP-MS for66 acid insoluble residue samples from 12 drill holes along15150 mN section was arranged by Becquerel Laboratories,Lucas Heights, Sydney These analyses were deisgned toexamine possible geochemical zonality It should be notedthat Pb was determined by both PIXE and ICP-MS analysesand was used to compare the distribution pattems

184

ELEMENT DISTRIBUTION IN ACID INSOLUBLERESIDUES FROM MINE THE SECTION15150 NMine section 15150 N was selected to illustrate thespatial distribution of K, AI, Fe, Mg, Ca, Na, Ti, CI, F, Ga,Rb, Sr, Zr, Y, Cu, Zn, Pb, Ni, As, V and Mo (by PIXE/PIGME)and TI (by ICP-MS) in acid insoluble residues from theregolith across the McKinnons gold depOsit

DISTRIBUTION OF K, A L, FB, CA, NA, M G AND TIThe major and minor elements are dealt with as a specialgroup as they represent essentially the silicate mineralsin the sediments and their alteration products Spatialdistributions of K, AI, Fe, Mg, Ca, Na and Ti in the acidinsoluble residues of regolith samples along the 15150 Nsection reveal depletion of all these elements at thecentre of the gold mineralisation which occurs along theintense fracture zones (between 4900 mE and 5000 mE)trending NS (Bywater et al , 1996) The concentrations ofthese elements tend to increase toward the eastem andwestern peripheries of the mineralised zone Figure 4ashows a depletion zone of K 1 5 %) within the goldmineralised zone, and an increased concentration towardthe eastern and westem peripheries

DISTRIBUTION OF Cu, ZN, PB, NI, As AND MoThe spatial distributions of Cu, Zn, Pb, Ni, As and Mo inthe acid insoluble residues from regolith along the15150 N section demonstrate tha t these elements showhigh values within the mineralised zone, although theirdistribution patterns are different to each other Zinc andNi are generally concentrated at the lower level of themineralisation, and Cu, Pb, As and Mo are relativelyenriched at the upper level A zone of high Cu (> 4 ppm)occurs at the upper level and extends to the surface(Figure 4b) A zone of high Zn (> 5 ppm) occurs at thelower level of mineralised zone (Figure 4c) Lead alsoshows a significant zone of more than 50 ppm at theupper level (Figure Sa) These elements demonstrate ageochemical zonation aSSOCiated with the goldmineralisation

OTHER TRACE ELEMENTSChlorine, F, Ga, Rb, Zr, Y, V and TI are relatively depletedwithin the gold mineralised zone, and increase towardthe periphery of the mineralised zone Stront ium isenriched in concentration in the upper regolith anddepleted at the lower levels of the mineralised zone

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Mineralised zone (Au> a3 ppm)10m

Figure 4(a): Distribution ofK

(e)

Zn in ppm (by PIXE).(::> Mineralised zone (Au> 0 3 ppm)

10m

Figure 4(c): Dzstribution oj Zm

i 10MULTIPLICATIVE INDICES AND RATIOS OF THEELEMENTS DETERMrnNEDA multiplicative index is defined as a multiplication ofindicator elements (to detect mineralisation or wall rockalteration) and a multiplicative ratio is defined as a ratioof multiplicative indices Halos, as defined bymultiplicative indices are, in some cases, larger andmore distinct than the geochemical halos defined bysingle elements Multiplicative indices of multi-elementdata have been successfully used to delineategeochemical halos related to mineralisation (Grigorian,1974; Beus and Grigorian, 1977; Rugless, 1983;Govett, 1983; Gundobin, 1984; Aung Pwa, 1995; AungPwa and van Moort, 1995) They are often used as analternative to computer-based statistical methods suchas discriminant analysis or factor analysisMulti-element indices used in this study includemultiplicative indices and multiplicative ratios of bothelements. The indices are presented as expressions ofconcentrations in ppm Copper, Zn, Pb, As and Ni, which

185

o-"/ Mineralised zon e (Au> 0 3 ppm)10mFigure 4(b): Distribution oj eu

"'ov

03 ppm)10m

Figure Sea): Distribution oj pb

(b)

eu in ppm (by PIXE)

4

(a)

Pb in ppm (by PlXE)100

are commonly enriched in the McKinnons mineralised zoneare used as the indicators of the mineralisation, while Ca,Na and Sr are generally depleted in the alterationassociated with mineralisation The McKinnons gold depos itis well defined by multiplicative indices of Zn x Pb, Zn x Pbx As, Cu x Pb x As and Zn x Ni as indicators for mineralisation,and multiplicative ratios of (Zn x Pb x As)/(Ca x Na x Sr), and(Zn x Pb)/(Na x Sr) possibly indicating the combined effect ofmineralisation and alteration associated With themineralisation The distributions of Zn x Pb and (Cu x Pb xAs)/(Ca x Na x Sr) are shown in Figures 5b and 5c, and theMcKinnons mineralised zone is associated with high values ofZn x Pb (> 50) and (Cu x Pb x Asy(Ca x Na x Sr) x 10-2 (> 1)The above multiplicative indices and ratios (except for Znx Ni) reveal significant and extensive geochemical haloson the surface of the mineralised zone at McKinnons Asignificant zone of high Zn x Ni occurs at the lower levelof the mineralisat ion These indices and ratios implyenrichment in Cu, Zn, Pb, As and Ni, and depletion in Ca,Na and Sr within the mineralised zone

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c.":;> Mineralised zone (Au> 0 3 ppm)- 10m

Figure 5(b): Distribution o/Zm xpbi 100

COMPARISON OF GEOCHEMICAL SIGNATURES BETWEENMINERALISED AND BARREN REGOLITH SAMPLESGeometric means of element concentrations in acidinsoluble residues determined by PIXE/PIGME were usedfor a comparison of elemental abundance andgeochemical indices between mineralised and barrenareas Most of the elements, except those associatedwith gold and base metal mineralisation, are depleted inthe mineralised regolith The depleted elements includeK, AI, Fe, Mg, Ca, Na, Ti, CI, F, Ga, Rb, Sr, Zr, Y and V (Table1) Copper, Zn, Pb, As, (Zn x Pb), (Zn x Pb x As), (Cu xPb x As), and (Cu x Pb x As)/(Zn x Ni) as indicators for thegold and base metal mineralisation, exhibit higher valuesin the mineralised zone compared to the barren areaFigure 6 shows the binary relation of (Zn x Pb) and (Cu xPb x As) between mineralised and barren zones Morethan 80% of the mineralised and barren samples arecorrectly classified as their respective group

0 3 ppm)10m

(c)

(Zn xP b x As)/(Ca x Nax Sr)x 10-2i 3Figure 5(c): Disribution o/(Zm x ob x As)I(Ca x Na x sr )100 : Mineralised regolith samlj /e . . i i10 = 0 Barren regolith sample III.

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GEOCHEMICAL ZONALITYGeochemical zoning within the McKinnons deposit wasinvestigated by examining the distributions of Pb, Ag, Sb,Ba, Bi and W (by ICP-MS), and Zn, Ni, As and Mo (by PIXE)in the acid insoluble residues of the regolith samples SiXty-six regolith samples from 12 drill holes along the 15150 Nsection were involved in this studyThe distributions of Pb, Ag, Sb, Ba, Bi and W in the acidinsoluble residues of regolith samples (by ICP-MS)demonstrate that these elements are enriched at the upperlevel of the McKinnons mineralised zone and at the surfaceabove the gold deposit compared to the lower level Zonesof high Ag (> 1 ppm) and Sb (> 20 ppm) occur at the upperlevel of the McKinnons mineralised zone (Figures 7a and7b) The distribution ofPb by ICP-MS (not shown) is identicalto that by PIXE As discussed before, As and Mo (by PIXE)are also concentrated at the upper level, while Zn and Ni (byPIXE) are enriched at the lower level of mineralisation

Mineralised zone (Au> 0 3 ppm)- 10m

Figure 7(a): Distribution oj Ag

o Mineralised zone (Au> 0 3 ppm)10m

(a)

Ag in ppm (by ICP-MS)

3

Sb in ppm (ICP"MS)i 3 0(b)

Figure 7(b): Distribution of Sb (by ICP-MS) in the acid insolubleresidues oj regolith samples from 15150 N section, McKinnonsgold depositThe multiplicative index (Pb x Ag x Sb) and ratios (Pb x Agx Sb)/(Zn x Ni) and (Cu x Pb x As)/(Zn x Ni) are high at theupper level of the mineralised zone, and extensive on thesurface, and decrease downward, indicating enrichment inPb, Ag, As and Sb and depletion in Zn and Ni at the upperlevel of mineralisation The distribution of the multiplicative

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ratiO values (Cu x Pb x As)/(Zn x Ni) x 10- 1 shown in Figure8 reveals a significant halo greater than 1 associated withthe McKinnons mineralised zone The halo is extensive atthe surface and narrows at depth

C.J Mineralised zone (Au> 0 3 ppm)10m

(Cu x Pb x As)/(Zn x Ni) x 10"1

K. Al. Fe, Mg, Ca,>Na, Ti,CI,F,Ga,> Rb, Zr'. Y, V, II< Cu, Zn, Ph, Ag, Ni< As, Sb, Mil, Bi

Fracture zone

tPossible flowof ore solution

FRESHROCK1Figure 9: Geochemical zonality model associated with theMcKinnons gold deposit

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ELECTRON PARAMAGNET IC RESONANCEThe EPR study is based on the major signal in quartzpowder EPR spectra, observed between an effectivespectroscoping splitting factor g = 2.0127 and 19969in X-band spectroscopy (Russell and van Moort, 1998)The intensities of these signals were measured onanalogue charts The signal is generally absent in quartzno t related to mineralisation and represents latticeimperfections in general, and the presence ofsubstitutional AI in the quartz lattice in particularComparison of the spectra of barren and mineralisedsamples shows an obviously stronger signal in themineralised samples compared to the barren hole (MB4) sample (Figure 10) It is considered that the EPRspectra reflect the lattice imperfection in the quartz dueto impurity substitutions which occur duringmineralisation and alteration processes (van Moort andBarth, 1992)

Mineralised samples

Barren sample s134487 (SR 357

1134502 (SR 364

Figure 10: EPR spectra (observed in 3265+5mT sweeps) ojmineralisedan d barren regolith samples

DISTRIBUTION OF EPR SIGNAL INTENSITIES IN THERESIDUES OF REGOLITH SAMPLES FROM MINE SECTION15150 NAs with the geochemical study, samples were separatedinto two groups representing mineralised and barrenareas respectively The EPR signal intensities (measuredat magnetiC flux density sweeps 326 5 5 mT) forregolith samples from the barren group vary from 0 to5 5 cm and have a geometric mean of 1 2 cm Thosefrom the mineralised group range from 0 1 to 12 5 cmand have 5 3 cm geometric mean

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The distribution map of EPR intensities (measured atmagnetic flux density sweeps of 326 5 5 mT) in thedrill core regolith samples from the 15150 N section(Figure 11 ) reveals a significant EPR halo or zone definedby values greater than 5 cm which outlines theMcKinnons gold mineralised zone The halo opens to thewest suggesting mineralisation extends downward to thewest There is no significant extension of the EPR halo tothe east beyond the gold mineralisation

o Minerallsed zone (Au> 0 3 ppm)- 10m

EPR intensity (em)ii 55-6>6Figure 11: Distribution oj EPR signal inensities measured atmagnetiC flux density sweeps 32655mT in the acid insolubleresidues of reg ith samples from 15150N section, McKinnons golddeposit

DISCUSSION AND CONCLUSIONS

SAMPLE MEDIUM AND GEOCHEMICAL ANALYSESAcid insoluble residues of regolith and soil samples (aftersequential treatment with hot aqua regia and H2S04 ) areevaluated as an exploration medium by geochemical andEPR analyses Semi-quantitative XRD analysis of theseresidues indicates that these are mainly quartz (> 90%)with minor mica The chemical compositions of thesamples thus reflect the composition of these minerals,their lattice substitutions and their inclusions In previousstUdies of the Rosebery volcanic-hosted Zn-Pb-Cu-Ag-Aumassive sulphide deposit, the residues of quartz powdersamples (after hot HN03 treatment) showed enrichmentin K, Rb, Zn, Pb, Mn and Ba in the mineralised areacompared to the barren area (Aung Pwa, 1995)Muscovites have principal isomorphous replacements ofK by Na, Rb, and Ca; AI by Mg, Fe+2, Fe+3, Mn, Li, Ti andV; (OH) by F (Deer et ai, 1965) Variations of F, Na, AI,K, Rb and possibly Ti and V in acid leached silicateUasperoid) powder at Carlin, Nevada, relate to thepresence of sericite (van Moort et al , 1995) and this isalso likely to be the case at the McKinnons gold deposit

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GEOCHEMICAL SIGNATURESThe elements in the acid insoluble residues of regolithfrom the McKinnons gold deposit display bothenrichment and depletion patterns within themineralised zone The enriched elements include Cu ,Zn, Pb, Ag, As, Sb, Bi and Mo which are associated withgold and base metal mineralisation They form a specificvertical geochemical zonation characterised by enrichmentin Cu, Pb, Ag, As, Sb, Ba, Bi, Mo and W, and depletion in Znand Ni at the upper level of the mineralisationHydrothermal gold deposits are commonly showgeochemical zoning, and vertical zonation features indicatethe direction of the ore solution from depth (e g Beus andGrigorian, 1977; Boyle, 1979) through fracture zonesDepletion in K, AI, Fe, Mg, Ca, Na, Ti, CI, F, Ga, Rb, Zr, Y, Vand TI are considered to be related to the hydrothermalalteration involving destruction of primary muscovite duringthe mineralisation processes which occurred along theintense fracture zones (between 4900 mE and 5000 mE)trending NS (Bywater et ai, 1996) Strontium is enriched inthe upper regolith over the mineralised zone and depletedat the lower level This may be related to the presence ofbarite Gundobin (1984) also found that Sr concentrates inthe peripheral zones and upper level of Au-Ag depositsThe geochemical Signatures at the McKinnons gold depos itare conSidered to be related to primary geochemicaldispersions which were preserved in quartz and mica in theregolith This supports the cotention of Rugless and Elliott(1995), that a large hydrothermal alteration system whichcoincides with multielement geochemical halos in theprimary zone of the gold deposit is retained within theregolith Robertson and Taylor (1987) described extensivedepletions in alkalis (Li, Na, K, Rb) and alkaline earths (Sr,Ba), Ti and C related to destruction of feldspar and whitemica in fresh rocks around Cobar-style mineralisation Thisstudy extends these depletions into the weathered zone

EPK SIGNATURESThe EPR spectra reflect the lattice imperfections in thequartz due to substitution of impurities duringmineralisation and alteration The mineralised samplesshow stronger signals than the barren samples inaccordance with other published EPR data (Russell andvan Moort, 1998; van Moort et ai , 1995)The distribution map of EPR intensities in the acidinsoluble residues of the regolith samples from the15150 N section reveals a significant EPR halo or zoneoutlining the McKinnons gold mineralised zone

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IMPLICATIONS FOR EXPLORATION

(1) The acid insoluble residue of regolith is a usefulsample medium for chemical analysis, as itpreserves primary geochemical signatures from thefresh rock into the regolith

(2) Acid insoluble K, AI, Fe, Mg, Ca, Na, Ti, CI, F, Ga, Rb,Zr, Y, V and TI are depleted in the regolith in andaround mineralisation indicating associated wall rockalteration

(3) Copper, Zn, Pb, Ag, As, Sb, Bi and Mo are enrichedin the mineralised zone, and are useful indicatorelements together with the multiplicative indices of(Zn x Pb), (Zn x Pb x As), (Cu x Pb x As), (Pb x Ag xSb) and (Zn x Ni), and the ratiOS of (Cu x Pb xAs)/(Zn x Ni) and (Pb x Ag x Sb)/(Zn x Ni) Themultiplicative ratios of (Zn x Pb x As)/(Ca x Na x Sr),and (Zn x Pb )/(Na x Sr) possibly indicate thecombined effect of mineralisation and alteration andhence also define the McKinnons gold deposit

(4) High EPR intensities measured on the acid insolubleresidues of the regolith are indicative of Significantmineralisation

ACKNOWLEDGEMENTSC.R M Butt (CRC LEME/CSIRO Exploration and Mining,Wembley W A) initially encouraged us to investigategeochemical and EPR respone in the regolith samplesfrom the McKinnons gold deposit The assistance of S MMarshall and K M Scot t, CRC LEME/CSIRO Explorationand Mining, North Ryde, NSW, who provided most ofsamples for this study, is acknowledged A Bywater(Burdekin Resources) assisted in the collection of somedrill core samples They all provided information on thegeology of McKinnons deposit, and on regolithgeochemistry and exploration in general The AustralianInstitute of Nuclear Science and Enginneering (AINSE)grant 97/079 provided the PIXE/PIGME analyses Xu Liassisted with sample collection of McKinnons and atCSIRO, North Ryde Xiachu Shen (Department of AppliedGeology, University of New South Wales) is thanked forproviding some samples from the McKinnons area

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REFERENCESAUNG PWA 1995 Application of rock geochemistry and

electron paramagnetic resonance (EPR) of rock inmineral exploration, Rosebery mine area, WestemTasmania Ph.D thesis, Geology Department,University of Tasmania, 303pp (Unpublished)

AUNG PWA and VAN MOORT J C 1995Lithogeochemistry and electron paramagneticresonance (EPR) of rocks in the Hercules mine area- exploration and alteration studies GeologyDepartment, University of Tasmania, unpublishedreport to Pasminco Exploration, Bumie, December,1995, 50 pp

AUNG PWA and VAN MOORT J C 1999 Electronparamagnetic resonance (EPR) spectroscopy of rockpowder in massive sulphide exploration, Roseberymine area, Westem Tasmania, Australia J GeochemExplor (in press)

AUNG PWA, NASCHWITZ W, HOTCHKIS M and VANMOORT J C 1992 Exploration rock geochemistry inthe Rosebery mine area, Western Tasmania BullGeol Surv Tasm 70: 7 -16

BEUS A.A and GRIGORIAN S V 1977 GeochemicalExploration Methods for Mineral Deposits AppliedPublishing Company, Wilmette, III , 187 pp

BOYLE R W 1979 The geochemistry of gold and Itsdeposits Geol Surv Can, Bull 280, 584 pp

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