PRECAMBRIAN BASIN-RELATED MVT DEPOSITS: POSSIBLE RELATION TO EARLY HYDROSPHERE AND ATMOSPHERE COMPOSITIONS

Mississippi Valley (MVT) Pb-Zn-(F) mineralization of Precambrian age is found in Neoarchean rocks in the Brushy Park, Pering, Genandenal and Zeerust districts in platform carbonates of the Cambellrand and Malmani Subgroups (2.6-2.5 Ga) in South Africa, and in Proterozoic rocks in the Little Ramah Bay South prospects in the ~2 Ga Reddick Bight Dolomite in Labrador, Canada, the Esker district in the 1.89 Ga Rocknest Formation Nunavut, Canada, the Coxco deposit in the 1.6 Ga McArthur Group, Northern Territory, Australia, the Gayna River district in the 1.08 to 0.78 Ga Little Dal Group, NWT, Canada and the Nanisivik deposit in the 1.2 to 0.72 Ga Uluksan Group, Baffin Island, Canada. Possibly related mineralization is found in Neoproterozoic rocks of the Bambui Group (Morro Agudo) and Irece Basin in Brazil and possibly the Paterson orogen in Australia (Warrabarty). Microbolite reefs hosting these deposits range from typical Paleoarchean-Proterozoic layered build-ups to more nearly, Phanerozoic-type zoned frameworks with growth cavities and internal sediment. Preliminary geochemical data on the older deposits do not differ significantly from their younger counterparts, and some old deposits contain fine-grained, colloform sulfides.

Although the age of most of these deposits is poorly constrained, the chemistry of ore-forming basinal fluids that formed them was probably controlled in part by their sedimentary host rocks, which reflect the composition of the atmosphere and hydrosphere in which they were deposited. Models for formation of most Phanerozoic colloform MVT deposits involve oxidized metal-bearing brines that deposit ore by mixing with sulfide of internal (sulfate-reduction) or external (fluid mixing) origin. This model is less likely for deposits in Neoarchean to Paleoproterozoic rocks if they formed when a reducing atmosphere limited the abundance of marine sulfate. Thus, either early deposits of this type had a different hydrothermal origin or sulfate-rich brines were available in Neoarchean-Paleoproterozoic time.