GEOCHEMISTRY AND DEPOSITIONAL ENVIRONMENT OF THE GOLD-MINERALIZED PROTEROZOIC KOOLPIN FORMATION, PINE CREEK INLIER, NORTHERN AUSTRALIA - A COMPARISON WITH MODERN SHALE SEQUENCES

The Paleo- to Mesoproterozoic Koolpin Formation is a lithologically di stinct sequence in the sedimentary pile of the Pine Creek Inlier in No rthern Australia. This succession hosts several gold deposits such as the Cosmo Howley Gold Mine (Matthai et al., 1995a). Part of the Koolpi n Formation has been referred to as 'iron formation' and interpreted a s an 'exhalative' deep water sediment, By analogy with the Homestake g old deposit (South Dakota), a syn-sedimentary origin of the gold-enric hment has been proposed for Cosmo Howley. This paper presents a detail ed investigation of the sedimentology, petrography, and geochemistry o f the Koolpin Formation at and in the vicinity of Cosmo Howley. It is shown that the sequence consists mainly of metamorphosed shales with n arrow intercalations of formerly carbonate-bearing mudstones in which chert-concretions formed during periods of subaerial weathering, In th e hanging wall, the sequence grades into carbonaceous slates which for merly 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 ch aracter of the metasediments and their REE-signatures are comparable t o modem-day analogs, despite potassium enrichment and Ca-, Na-depletio n which is inferred to be a consequence of fluid flow during mineraliz ation and contact metamorphism. Iron formations as defined by James (1 954) are not present at Cosmo Howley. Iron enrichment to present value s (less than or equal to 27 wt.% total Fe) is shown to have occurred f rom a weakly diagenetically enriched protolith due to silica loss duri ng nodule formation, volatile loss during metamorphism, and epigenetic sulfide precipitation during retrograde alteration. No gold enrichmen t can be demonstrated for the protolith. Similar conclusions may be re ached for the iron-rich metasediments which host gold mineralisation a t Homestake, South Dakota.