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Mineral hosts for gold and trace elements in the regolith, Boddington and Mt Percy gold deposits, Western Australia

AuScope
CRC LEME (Point of Contact) Terry Rankine (Point of Contact) le Gleuher, M. (Author)
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ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Adc&rfr_id=info%3Asid%2FANDS&rft_id=http://portal.auscope.org/gmap.html&rft.title=Mineral hosts for gold and trace elements in the regolith, Boddington and Mt Percy gold deposits, Western Australia&rft.identifier=0c569efb-6c1e-4a93-890e-85b2d516deab&rft.publisher=AuScope&rft.description=Mineral-trace element associations have been investigated in the regolith developed onmineralised rocks, at the Boddington Cu-Au deposit and Mt Percy gold deposit, WesternAustralia. A detailed characterisation of the mineralogy and geochemistry has been achievedusing in situ microanalyses on regolith materials. The use of Laser Ablation-InductivelyCoupled Plasma-Mass Spectrometry (LA-ICP-MS) for the in situ quantification of traceelements in clays and other regolith minerals has been assessed, and a method to process thedata was developed.BoddingtonMineralogyThe weathering profiles consist of a saprock, saprolite, clay zone, bauxitic horizon, duricrustand a layer of loose pisoliths. The saprock is composed of abundant muscovite and quartz,epidote, chlorite, titanite and ilmenite. Chlorite-vermiculite and a biotite-chlorite interstratifiedmineral have been identified in the clay fraction. The Cu and Fe-sulphides are often partiallydissolved or oxidised. The saprolite consists largely of microcrystalline kaolinite and quartz,with minor goethite, high charge corrensite, biotite-vermiculite interstatified mineral andvermiculite. Iron sulphides are replaced by goethite and hematite. In the overlying clay zone,kaolinite is replaced by microcrystalline gibbsitic and goethitic plasmas. The duricrust typicallycomprises a fragmental duricrust overlain by a pisolitic duricrust, characterised by the in situinduration of plasmic domains. Angular concretions, nodules and pisoliths contain abundanthematite, goethite and gibbsite, and are set in a soft gibbsitic matrix. Anatase-rich domainsresulting from the weathering of ilmenite and titanite are scattered in the duricrust and the loosepisolitic material. Maghemite is occasionally present in the fragmental duricrust, and becomesmore abundant in the loose pisolith along with boehmite and corundum.Trace element-mineral associationsNew data about natural concentrations of trace elements in regolith minerals are reported. In thesaprolite, trace elements are concentrated in minerals of marginal abundance such as corrensite,vermiculite and Fe oxides. The trace elements directly related to the Cu-Au mineralisation (Cu,Au, As, Pb, Bi, Mo and W) released by the dissolution of the sulphides are incorporated ingoethite and hematite, with a preference for the former. Corrensite and vermiculite containlarge amounts of Cu. Tungsten, Bi and Mo are also hosted by anatase. Trace elements related tothe nature of the host rock are either residual in primary minerals or in their alteration products.Manganese, originally contained in chlorite, is retained in vermiculite.In the duricrust, Co, Zn, As and Mo are concentrated in goethite in the cutans and matrix of thenodules. Tungsten occurs as residual scheelite, in the matrix goethite and in anatase. Gold ispresent as microparticles scattered in the Al-Fe microplasmas. Copper, Zn, Ni, Co and Mn arealso concentrated in goethite in the loose pisolith cortex.Kaolinite and gibbsite do not scavenge any significant amounts of trace elements.Mt PercyMineralogyThe regolith is over 60 m thick, and comprises a saprolite, a clay upper saprolite, a mottled andplasmic clay zone, a lateritic duricrust and gravel layer, and a top loamy soil containingpedogenic carbonates. The saprolite is composed of fuchsite, quartz, sulphide relicts replacedby Fe oxides, minor rutile and Cr-spinels. The clay saprolite is characterised by the replacementof fuchsite by kaolinite and the development of kaolinitic and goethitic rich microcrystallineplasmas. In the vicinity of the sulphide relicts, the plasmas and quartz grains are dissolved andreplaced by alunite. The overlying mottled and plasmic clay zone is strongly ferruginised inplaces, and cutans consisting of kaolinite, gibbsite and goethite fill cracks and voids.Trace element-mineral associationsThe trace elements used as pathfinders have multiple hosts. Antimony and W are hosted inrutile and in Fe oxides replacing the sulphides. Arsenic is strongly concentrated in goethite.Copper and Zn are contained in goethite in the saprolite and mottled and plasmic clay zone.Alunite hosts copper in the clay saprolite, and Zn is present in Cr-Zn spinels. The traceelements related to the parent-rock such as Ni, Co and Mn are concentrated in goethitereplacing the sulphides, in vermiculite and fuchsite in the lower saprolite. Higher up in theprofile, Ni, Co and Mn are held by goethite in the ferruginised rock and cutans.The elements associated with the fuchsitic-carbonate alteration such as Ba, K and Rb arelocated in fuchsite in the saprolite. In the clay saprolite, alunite hosts Ba and kaolinite containsRb.Implications for explorationThe in situ mineralogical and chemical studies of the Boddington and Mt Percy regolithmaterial have provided data of relevance for Au and mineral exploration. It has shown thattargeting individual minerals and mineralogical phases provide a potential tool to enhancegeochemical anomaly detection. Kaolinite and gibbsite do not scavenge any significantamounts of trace elements and should therefore be disregarded as a sampling medium. Theimportance of the clay fraction (&rft.creator=le Gleuher, M.&rft.date=1970&rft.coverage=northlimit=-32.75222231; southlimit=-32.75222231; westlimit=116.3594441; eastLimit=116.3594441; projection=WGS84&rft_rights=There are no access constraints associated with this collection.&rft_subject=Regolith&rft_subject=Trace Elements&rft_subject=Mineral Hosts&rft_subject=Weathering&rft_subject=Geochemistry&rft_subject=Mineralogy&rft_subject=Gold&rft_subject=Boddington&rft_subject=Western Australia&rft_subject=Mt Percy&rft_subject=Report&rft_subject=Dwellingup&rft_subject=Kalgoorlie&rft.type=dataset&rft.language=English Go to Data Provider

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Mineral-trace element associations have been investigated in the regolith developed onmineralised rocks, at the Boddington Cu-Au deposit and Mt Percy gold deposit, WesternAustralia. A detailed characterisation of the mineralogy and geochemistry has been achievedusing in situ microanalyses on regolith materials. The use of Laser Ablation-InductivelyCoupled Plasma-Mass Spectrometry (LA-ICP-MS) for the in situ quantification of traceelements in clays and other regolith minerals has been assessed, and a method to process thedata was developed.BoddingtonMineralogyThe weathering profiles consist of a saprock, saprolite, clay zone, bauxitic horizon, duricrustand a layer of loose pisoliths. The saprock is composed of abundant muscovite and quartz,epidote, chlorite, titanite and ilmenite. Chlorite-vermiculite and a biotite-chlorite interstratifiedmineral have been identified in the clay fraction. The Cu and Fe-sulphides are often partiallydissolved or oxidised. The saprolite consists largely of microcrystalline kaolinite and quartz,with minor goethite, high charge corrensite, biotite-vermiculite interstatified mineral andvermiculite. Iron sulphides are replaced by goethite and hematite. In the overlying clay zone,kaolinite is replaced by microcrystalline gibbsitic and goethitic plasmas. The duricrust typicallycomprises a fragmental duricrust overlain by a pisolitic duricrust, characterised by the in situinduration of plasmic domains. Angular concretions, nodules and pisoliths contain abundanthematite, goethite and gibbsite, and are set in a soft gibbsitic matrix. Anatase-rich domainsresulting from the weathering of ilmenite and titanite are scattered in the duricrust and the loosepisolitic material. Maghemite is occasionally present in the fragmental duricrust, and becomesmore abundant in the loose pisolith along with boehmite and corundum.Trace element-mineral associationsNew data about natural concentrations of trace elements in regolith minerals are reported. In thesaprolite, trace elements are concentrated in minerals of marginal abundance such as corrensite,vermiculite and Fe oxides. The trace elements directly related to the Cu-Au mineralisation (Cu,Au, As, Pb, Bi, Mo and W) released by the dissolution of the sulphides are incorporated ingoethite and hematite, with a preference for the former. Corrensite and vermiculite containlarge amounts of Cu. Tungsten, Bi and Mo are also hosted by anatase. Trace elements related tothe nature of the host rock are either residual in primary minerals or in their alteration products.Manganese, originally contained in chlorite, is retained in vermiculite.In the duricrust, Co, Zn, As and Mo are concentrated in goethite in the cutans and matrix of thenodules. Tungsten occurs as residual scheelite, in the matrix goethite and in anatase. Gold ispresent as microparticles scattered in the Al-Fe microplasmas. Copper, Zn, Ni, Co and Mn arealso concentrated in goethite in the loose pisolith cortex.Kaolinite and gibbsite do not scavenge any significant amounts of trace elements.Mt PercyMineralogyThe regolith is over 60 m thick, and comprises a saprolite, a clay upper saprolite, a mottled andplasmic clay zone, a lateritic duricrust and gravel layer, and a top loamy soil containingpedogenic carbonates. The saprolite is composed of fuchsite, quartz, sulphide relicts replacedby Fe oxides, minor rutile and Cr-spinels. The clay saprolite is characterised by the replacementof fuchsite by kaolinite and the development of kaolinitic and goethitic rich microcrystallineplasmas. In the vicinity of the sulphide relicts, the plasmas and quartz grains are dissolved andreplaced by alunite. The overlying mottled and plasmic clay zone is strongly ferruginised inplaces, and cutans consisting of kaolinite, gibbsite and goethite fill cracks and voids.Trace element-mineral associationsThe trace elements used as pathfinders have multiple hosts. Antimony and W are hosted inrutile and in Fe oxides replacing the sulphides. Arsenic is strongly concentrated in goethite.Copper and Zn are contained in goethite in the saprolite and mottled and plasmic clay zone.Alunite hosts copper in the clay saprolite, and Zn is present in Cr-Zn spinels. The traceelements related to the parent-rock such as Ni, Co and Mn are concentrated in goethitereplacing the sulphides, in vermiculite and fuchsite in the lower saprolite. Higher up in theprofile, Ni, Co and Mn are held by goethite in the ferruginised rock and cutans.The elements associated with the fuchsitic-carbonate alteration such as Ba, K and Rb arelocated in fuchsite in the saprolite. In the clay saprolite, alunite hosts Ba and kaolinite containsRb.Implications for explorationThe in situ mineralogical and chemical studies of the Boddington and Mt Percy regolithmaterial have provided data of relevance for Au and mineral exploration. It has shown thattargeting individual minerals and mineralogical phases provide a potential tool to enhancegeochemical anomaly detection. Kaolinite and gibbsite do not scavenge any significantamounts of trace elements and should therefore be disregarded as a sampling medium. Theimportance of the clay fraction (<2 ?m) in kaolinite-dominated samples has been demonstrated.Phyllosilicates such as interstratified minerals and vermiculite provide the potential to trap largequantities of trace elements. Goethite is the ideal target for analysis as it has a high capacity toscavenge pathfinder elements. Tungsten and Sb are mainly hosted by resistant minerals and arenot reliable indicators of mineralisation.

116.3594441,-32.75222231

116.3594441,-32.75222231

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