SEDEX ZINC DEPOSITS, SIDERITE AND SULFUR-POOR SEAS

Late Palaeoproterozoic to Early Mesoproterozoic sedimentary rocks in northern Australia host several supergiant and giant zinc deposits (Large et al., 2005 Economic Geology 100th Anniversary Volume). The prevailing consensus is that these represent a spectrum of deposits types that form either during active extension and sedimentation, or at the earliest stages of basin inversion. The relative importance of syngenetic-exhalative (SEDEX) versus diagenetic (early through late) processes may vary between deposits (e.g., dominantly SEDEX for HYC cf deep diagenesis for Century). All the deposits have reduced (pyritic and/or carbonaceous) fine grained hosts, and all have Mn and Fe-rich carbonate (siderite or ferroan dolomite) as the dominant carbonate in siltstones within and around the sulfide lenses (Large & McGoldrick, 1988, Journal of Geochemical Exploration; Large et al., 2000, Journal of Geochemical Exploration). The spatial coincidence of Fe-carbonate with other primary chemical and isotopic dispersions around the deposits implies link between Fe-carbonate formation and ore formation.

Previous chemical modelling (Cooke et al., 2000, Economic Geology) has shown, for the types of fluids implicated in metal transport in these deposits, that high partial pressures of CO₂ are needed to stabilize siderite. However, this also requires substantial pressure to prevent the loss of CO₂ by de-gassing. This is not an issue for deep diagenetic or very deep water SEDEX settings, but may be difficult to achieve for deposits formed in shallower settings.

We will present an alternative explanation for the origin of siderite in the SEDEX deposits. It has recently been recognized that the Late Paleoproterozoic world ocean had an order of magnitude less sulfur than the modern ocean (Shen et al., 2003, Nature; Kah et al., 2004, Nature) which allowed anoxia and euxinia to be much more common. Precipitation of large amounts Fe- and base metal sulfides when ore fluids exhaled into these euxenic marine environments may have effectively removed all locally available S and stabilized siderite at quite low partial pressures of CO₂. Sulfur isotope patterns from the Lady Loretta deposit will be used to support such an explanation.