Geochemical effects of deeply buried Cu–Au mineralization on transported regolith in an arid terrain

Abstract

ABSTRACT The presence of transported regolith commonly restricts the dispersion 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 other surficial geochemical processes. Substantial changes to the mineralogy and mineral distribution within transported regolith, as a result of H + released from oxidizing sulphide mineralization and other reactions, has been recognized in a number of regolith settings. Various approaches to detection of such features have been developed, but their application has met with limited success in arid environments typical of Australia. The Mandamah Cu–Au deposit, in central–western New South Wales, is covered by c . 50 m of transported regolith and c . 30 m of saprolite. The top 2 m of regolith in the region is characterized by an organic-rich surface layer with neutral pH, followed by a high-pH zone with pedogenic carbonate development and a lower selvage of gypsum, and an underlying low-pH zone displaying Fe mottling. The principal effects of mineralization in the upper transported regolith are depletion of ammonium acetate-extractable Ca, S and Na, a reduction in the cation exchange capacity, the presence of non-carbonate alkalinity and a low soil electrical conductivity. There is no indication of transport to surface and accumulation of elements related to the mineralization, such as Cu, Mo, Au or S. A model to account for these patterns, based partly on existing dispersion models, involves a prograde stage of alteration of clay mineralogy with redistribution of carbonates and some elements as a result of the development of an acid chimney above the oxidizing mineralization during periods of elevated water tables and a subsequent retrograde stage involving a redistribution of mobile elements into the area of the former acid chimney following the onset of more arid conditions.