Can geophysics and geochemistry combine to detect mineralisation under transported cover?

Abstract

The oxidation of Fe-containing sulfide mineralization can lead to the development of natural galvanic cells that are characterized by a reduced acid chimney above the mineralization itself and pH-redox controlled chemical haloes surrounding the chimney. Such chimneys may extend through overlying transported regolith cover of varying thicknesses and composition, altering regolith mineralogy (changes to clays, loss of carbonates), element distribution patterns and geophysical characteristics such as electrical conductivity (EC). This paper compares EM and geochemical profiles in transported regolith over porphyry style Au-Mo-Cu mineralization at Mandamah (NSW) and MVT style Pb-Zn mineralization at Pitchi (Iran). At Mandamah, mineralisation is buried under 30 m of residual regolith and a further 50 m of re-weathered alluvium. Mineralised zones that intersect the base of weathering are characterized by low EM and EC in the upper 6 m of the transported regolith profile. This can be related to changes in clay mineralogy and the destruction of carbonates caused by an acid chimney that formed during a "prograde" phase of regolith development. These patterns are also variably reflected in soil pH and the distribution of selectively-extracted Ca, Mg, S, Ba and REE in the upper part of the transported regolith profile, but generally not in the elements of economic interest (Au, Cu or Mo). At Pitch carbonate-hosted Pb-Zn mineralisation is covered by 80 m of barren dolomitic sedimentary rocks and a thin layer of alluvium and colluvium. There is an increase in the EM and EC as well as Na and S in the transported regolith above mineralisation and adjacent low EC values, but again no detectable patterns in the main ore-related metals. The combined geochemical and (EM) geophysical approach provides a new approach to exploration in areas with economically prospective geology but where mineralization is buried by various forms of cover including transported regolith.