Geochemistry and depositional environment of the gold-mineralized Proterozoic Koolpin Formation, Pine Creek Inlier, Northern Australia: a comparison with modern shale sequences

Abstract

The Paleo- to Mesoproterozoic Koolpin Formation is a lithologically distinct sequence in the sedimentary pile of the Pine Creek Inlier in Northern Australia. This succession hosts several gold deposits such as the Cosmo Howley Gold Mine (Matthäi et al., 1995a). Part of the Koolpin Formation has been referred to as ‘iron formation’ and interpreted as an ‘exhalative’ deep water sediment. By analogy with the Homestake gold deposit (South Dakota), a syn-sedimentary origin of the gold-enrichment has been proposed for Cosmo Howley. This paper presents a detailed investigation of the sedimentology, petrography, and geochemistry of the Koolpin Formation at and in the vicinity of Cosmo Howley. It is shown that the sequence consists mainly of metamorphosed shales with narrow intercalations of formerly carbonate-bearing mudstones in which chert-concretions formed during periods of subaerial weathering. In the hanging wall, the sequence grades into carbonaceous slates which formerly contained diagenetic dolomite. Depositional structures indicate that the older part of the Koolpin Formation at Cosmo Howley formed in a low-energy, inter- to supra-tidal environment dominated by detrital input of terrestrial provenance. The younger carbonaceous slates seem to reflect a transition to shallow-marine conditions. The chemical character of the metasediments and their REE-signatures are comparable to modern-day analogs, despite potassium enrichment and Ca-, Na-depletion which is inferred to be a consequence of fluid flow during mineralization and contact metamorphism. Iron formations as defined by James (1954) are not present at Cosmo Howley. Iron enrichment to present values (⩽ 27 wt.% total Fe) is shown to have occurred from a weakly diagenetically enriched protolith due to silica loss during nodule formation, volatile loss during metamorphism, and epigenetic sulfide precipitation during retrograde alteration. No gold enrichment can be demonstrated for the protolith. Similar conclusions may be reached for the iron-rich metasediments which host gold mineralisation at Homestake, South Dakota.