A COMPARISON OF SELECTIVE EXTRACTION SOIL GEOCHEMISTRY AND BIOGEOCHEMISTRY IN THE COBAR AREA, NEW-SOUTH-WALES

In parts of the deeply weathered and semi-arid environments of the Cob ar area (NSW, Australia), detection of mineralisation using convention al soil sampling and total metal analysis is impeded. This is due to t he intense leaching of trace elements within the weathered profile, di scontinuous coverage of transported materials and the existence of dif fuse regional geochemical anomalies of ill-defined source. Selective c hemical extractions, applied to various regolith components, and bioge ochemistry offer a means of isolating localised geochemical patterns r elated to recent dispersion of trace elements through the overburden. Lag geochemical patterns across the McKinnons deposit (Au) and Mrangel li prospect (Pb-Zn-As) reflect mechanical dispersion processes and min or hydromorphic effects. Concentrations of more mobile elements tend t o be higher in the non-magnetic fraction, due to higher proportions of goethite and poorly crystalline hematite than in the magnetic fractio n. The subdued soil geochemical responses for metals extractable by co ld 40% hydrochloric acid (CHX) and for total element concentration ref lect the leached nature of the residual profile, low grade of minerali sation, dilution by aeolian components and disequilibrium of fine frac tions with coarser, relict Fe-oxides. The stronger contrast for CHX fo r most metals, compared with total extraction, indicates surface accum ulation of trace elements derived from underlying mineralisation. Enzy me leach element anomalies are intense but generally located directly over bedrock sources or major structural breaks, irrespective of the n ature of the overburden, Though mechanisms for the dispersion of trace elements extracted by enzyme leaching are not well established, the l ack of lateral transport suggests vertical migration of volatile metal species (atmimorphic dispersion). The strong, multi-element response to mineralisation in cypress pine needles indicates 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 pl ants is not the dominant mechanism for transportation of metals throug h the overburden. The vegetation may be responding to hydromorphic dis persion patterns at depth. The use of selective extractions may be use ful in detecting mineralisation through deeply leached profiles, but o ffers even greater potential when integrated with biogeochemistry to d etect targets buried by significant thickness of transported cover. (C ) 1998 Elsevier Science B.V. All rights reserved.