Regolith geochemical exploration using acid insoluble residues as a sample medium for gold and base metal deposits in the Cobar region, N.S.W., Australia

Abstract

Acid insoluble residues of regolith samples after treatment with aqua regia (3HCl : 1HNO 3) and subsequently with H 2SO 4 have been tested as a sampling medium for determining geochemical signatures around gold and base metal mineralisation at the McKinnons and Wagga Tank deposits and at the Lower Tank prospect, in the Cobar region, N.S.W., Australia. The residues, composed of quartz, chalcedony and minor mica, retain primary geochemical signatures related to mineralisation/alteration events. Gold and base metal deposits in the Cobar region occur within clastic sediments and volcaniclastics of the Early Devonian Cobar Basin and its equivalents (e.g. Mt Hope Trough). Common alteration styles are silicification and chloritisation with some carbonitisation and sericitisation. Within the regolith, clay mineral alteration is significant. At both the McKinnons and Wagga Tank deposits the weathering profile is about 80–100 m thick. The regolith at McKinnons consists of red-brown soil containing ferruginous pisoliths underlain by saprolite consisting of quartz, clay minerals (mainly illite) and muscovite, pyrite, goethite and haematite. In the Wagga Tank area, the weathering profile consists of soil overlying a residual ferruginous layer underlain by a kaolinite-rich zone which passes into goethitic saprolite containing alunite–jarosite family minerals. Distributions of elements in the acid insoluble residues of rock and regolith (including surface soil) exhibit similar geochemical signatures at the McKinnons and Wagga Tank deposits. There are two main groups of 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 mineralisation. The second group (K, Fe, Ca, Ti, Mn, Cl, Ga, Rb, Sr, Zr, Y, V, Al, Na, Mg, Tl and F) are generally depleted in acid insoluble residues of the rock and regolith near and around the mineralisation. The depletion of some elements 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 the result of primary geochemical dispersion and despite the weathering effect are retained within the regolith. The previously defined regolith-hosted base metal anomalies at the Lower Tank prospect are also reflected by anomalous Cu, Zn, Pb, Ni, Mo and As in the acid insoluble residues of both saprolite 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 and associated buried mineralisation.