Geobiology of northern Australian mid-Proterozoic sedex Zn-Pb-Ag deposits

The northern Australian mid-Proterozoic McArthur and Mount Isa Basins host four supergiant sediment-hosted Zn±Pb±Ag deposits, several smaller deposits and a number of sub-economic prospects. These formed at, or very close to, the sea floor when coolwann, saline, oxidized brines 'exhaled' into anoxic (ferruginous) bottom water.

Host sediments are variably dolomitic, sideritic, pyritic and carbonaceous shales and siltstones, mostly deposited sub-stonn wave base. Ores are present at several stratigraphic levels and ages range from ∼1 .65 Ga (Mount Isa) to ∼1 .59 Ga (Century).

The hosts and ores contain a variety of textures interpreted to have fonned directly, or indirectly, by microbial activity. Chemical, isotopic and mass balance arguments suggest the (fonner) presence of vast quantities of microbial biomass. This prokaryotedominated vent biota likely played a key role in the formation of the deposits by facilitating the production of reduced sulfur needed to fix base metals as metal sulfides.

Early eukaryotes living in these microbial communities may have experienced unique selection pressures in response to the deep, dark anoxic conditions in which they lived.