Geochemical effects of deeply buried Cu—Au mineralization ontransported regolith in an arid terrain

摘要

The presence of transported regolith commonly restricts the disper-sion of trace elements from mineralized bedrock or in situ regolith to surface,especially in arid terrains. Where dispersion through transported regolith does occur,low-concentration haloes derived from mineralization may be masked by othersurficial geochemical processes. Substantial changes to the mineralogy and mineraldistribution within transported regolith, as a result of 1-1+ released from oxidizingsulphide mineralization and other reactions, has been recognized in a number ofregolith settings. Various approaches to detection of such features have beendeveloped, but their application has met with limited success in arid environmentstypical of Australia. The Mandamah Cu—Au deposit, in central—western New SouthWales, is covered by c. 50 m of transported regolith and c. 30 m of saprolite. The top2 m of regolith in the region is characterized by an organic-rich surface layer withneutral pH, followed by a high-pH zone with pedogenic carbonate development anda lower selvage of gypsum, and an underlying low-pH zone displaying Fe mottling.The principal effects of mineralization in the upper transported regolith aredepletion of ammonium acetate-extractable Ca, S and Na, a reduction in the cationexchange capacity, the presence of non-carbonate alkalinity and a low soil electricalconductivity. There is no indication of transport to surface and accumulation ofelements related to the mineralization, such as Cu, Mo, Au or S. A model to accountfor these patterns, based partly on existing dispersion models, involves a progradestage of alteration of clay mineralogy with redistribution of carbonates and someelements as a result of the development of an acid chimney above the oxidizingmineralization during periods of elevated water tables and a subsequent retrogradestage involving a redistribution of mobile elements into the area of the former acidchimney following the onset of more arid conditions.