The Rosie pyrite hydrothermal system in the McArthur Basin: tectono-sedimentary constraints on mineralization and alkali metasomatism in stratiform sediment-hosted sulfide deposits of northern Australia

The world-class McArthur River Zn-Pb-Ag deposit is the type example of stratiform sediment-hosted deposits found in the Proterozoic basins of northern Australia. However, the McArthur River deposit is enigmatic in that, aside from a few sub-economic deposits within the same camp, other deposits of this type have not been discovered regionally. Understanding the key components of McArthur River-type mineral systems and the regional distribution of these components is crucial to further discovery. The recent discovery of the similar but pyrite-dominated system at Rosie ~ 90 km north of the McArthur River Mine sheds new light on the formation of this deposit class and provides a counterpoint to compare and contrast with the high base metal endowment at McArthur River. Stratiform pyrite mineralization at Rosie is finely laminated and is affected by erosional sedimentary processes superficially indicating it formed during sedimentation to early diagenesis and at or near the sediment–water interface. The conventional ore horizon in the McArthur River camp is the HYC Pyritic Shale Member in the lower Barney Creek Formation (~ 1640 Ma) within sub-basin architecture. Rosie is contained within a similar sub-basin architecture, but it occurs in the upper Barney Creek Formation. The absence of mineralization in the lower Barney Creek Formation at Rosie is attributed in part to prohibitive tectono-sedimentary conditions during early sub-basin development. The HYC Pyritic Shale Member equivalent is relatively thin beneath the Rosie mineralization and is only weakly enriched in hydrothermal elements. Its deposition was abruptly superseded by infill of the sub-basin by a thick barren sequence of fine turbidites. The fine turbidites were a rapid phase of accommodation space fill that diluted brines entering the sediment pile and effectively precluded hydrothermal sulfide mineralization as well as low temperature alkali metasomatism. Permissible tectono-sedimentary conditions did not return until late in sub-basin development, and by this time, sub-basin geometry had broadened and shallowed, and the hydrothermal fluid had evolved. Rosie mineralization is subsequently poorly focused and base metal deficient. The observation that sedimentation rate may play a role in mineralization is a significant addition to the field of basin-hosted mineral systems.