Regolith geochemical exploration using acid insoluble residues as a samplemedium for gold and base metal deposits in the Cobar region, NSW, Australia

Acid insoluble residues of regolith samples after treatment with aqua regia (3HCl:1HNO(3)) and subsequently with H2SO4 have been tested as a sampling medium for determining geochemical signatures around gold and base metal mi neralisation at the McKinnons and Wagga Tank deposits and at the Lower Tank prospect, in the Cobar region, N.S.W., Australia. The residues, composed o f quartz, chalcedony and minor mica, retain primary geochemical signatures related to mineralisation/alteration events. Gold and base metal deposits i n the Cobar region occur within clastic sediments and volcaniclastics of th e Early Devonian Cobar Basin and its equivalents (e.g. Mt Hope Trough). Com mon alteration styles are silicification and chloritisation with some carbo nitisation and sericitisation. Within the regolith, clay mineral alteration is significant. At both the McKinnons and Wagga Tank deposits the weatheri ng profile is about 80-100 m thick. The regolith at McKinnons consists of r ed-brown soil containing ferruginous pisoliths underlain by saprolite consi sting of quartz, clay minerals (mainly illite) and muscovite, pyrite, goeth ite and haematite. in the Wagga Tank area, the weathering profile consists of soil overlying a residual ferruginous layer underlain by a kaolinite-ric h zone which passes into goethitic saprolite containing alunite-jarosite fa mily minerals. Distributions of elements in the acid insoluble residues of rock and regolith (including surface soil) exhibit similar geochemical sign atures at the McKinnons and Wagga Tank deposits. There an two main groups o f elements of interest. The first group (Cu, Zn, Pb, Ni, As, Mo, Ag, Sb, Ba , Bi and W) are ore-related and are enriched in and around the mineralisati on. The second group (K, Pe, Ca, Ti, Mn, Cl, Ga, Rb, Sr, Zr, Y, V,Al, Na, M g, Tl and F) are generally depleted in acid insoluble residues of the rock and regolith near and around the mineralisation. The depletion of some elem ents such as K, Al, Ca, Na, Rb and Sr is considered to reflect element loss from the hydrothermal system as a result of feldspar and mica destruction followed by silicification. These signatures are considered to be mainly th e result of primary geochemical dispersion and despite the weathering effec t are retained within the regolith. The previously defined regolith-hosted base metal anomalies at the Lower Tank prospect are also reflected by anoma lous Cu, Zn, Pb, Ni, Mo and As in the acid insoluble residues of both sapro lite and drainage sediments. However, K, Fe, Ca, Na, Rb, Sr and Ga are not depleted, which does not support the possibility of wall rock alteration an d associated buried mineralisation. (C) 1999 Elsevier Science B.V. All righ ts reserved.