Synsedimentary mineralization in the Paleoproterozoic McArthur River (HYC) Zn-Pb-Ag deposit, McArthur Basin, Australia

The McArthur River (HYC) Zn-Pb-Ag deposit of the McArthur Basin, Northern Australia is a superbly preserved example of stratiform shale-hosted mineralization. Despite extensive research on the deposit, the question of whether the sulphide minerals are synsedimentary or epigenetic remains unresolved. Here, we examine sedimentological, textural and geochemical features of the deposit to constrain the timing and environment of sulphide mineral precipitation. Results indicate that most of the base metal sulphide minerals at McArthur River are of synsedimentary origin and were precipitated at or just below (within millimetres of) the sediment–water interface. Common sedimentological features of the ore lenses include small-scale erosion surfaces truncating sphalerite micronodules, sphalerite hardgrounds and crusts, and reworked clasts of sphalerite. All of these features indicate synsedimentary precipitation of base metal sulphide minerals. Several paleoredox proxies (including Mo, U, Cu, V, pyrite framboid size) for the host HYC Pyritic Shale Member suggest deposition occurred under a dominantly oxic-dysoxic water column, rather than in a euxinic/anoxic setting as has previously been assumed. A sulphide mineral precipitation mechanism reminiscent of some modern marine settings is suggested for HYC metal sulphide precipitation. Sulphate reduction below the sediment–water interface, driven by a supply of organic matter, led to a buildup of H 2 S in the sediment pore waters. This H 2 S acted as a trap for chalcophile metals (Zn, Pb, Cu) diffusing downwards from the water column, leading to metal sulphide mineral precipitation in the near-surface sediments. This precipitation mechanism may be globally important in formation of stratiform ore deposits of this type.