STRATIFORM AND STRATABOUND ZN-PB-AG DEPOSITS IN PROTEROZOIC SEDIMENTARY BASINS, NORTHERN AUSTRALIA

In terms of zinc, lead, and silver metal endowment, the Proterozoic sedimentary basins of northern Australia rank number one in the world. The Mt Isa-McArthur basin system hosts five supergiant, stratiform, sedimentary rock-hosted Zn-Pb-Ag deposits (McArthur River, Century, Mt Isa, Hilton, and George Fisher), and one supergiant stratabound Ag-Pb-Zn deposit (Cannington). The major stratiform Zn-Pb-Ag deposits exhibit many similar geological and geochemical features that include: 1) location close to regionally extensive normal and strike-slip synsedimentary faults, 2) organic-rich black shale and siltstone host rocks, 3) laminated, bedding-parallel synsedimentary sulfide minerals, 4) stacked ore lenses separated by pyritic and Fe-Mn carbonate-bearing siltstones, 5) lateral zonation exhibiting an increasing Zn/Pb ratio away from the feeder fault, 6) vertical zonation exhibiting decreasing Zn/Pb ratio up-stratigraphy, 7) an extensive stratabound halo of iron- and manganese-rich alteration in the sedimentary rocks surrounding and along strike from ore, 8) a broad range of δ³⁴S values for sulfide minerals, from about 0 to +20‰, with pyrite exhibiting a greater spread than base metal sulfides, and 9) lead isotope ratios that indicate derivation of lead from intrabasinal sources with interpreted lead model ages being similar to the measured zircon U-Pb ages of the host rocks. These common features demonstrate that the stratiform Zn-Pb-Ag ores formed approximately contemporaneously with sedimentation and (or) diagenesis.

Numerical modelling of fluid flow and heat transport in sedimentary basins suggests that free convection, driven by density changes in sedimentary brines, may be the most important process in forming giant SEDEX deposits. In response to periods of active tectonism during the sag phase of sedimentation, extensional faults are reactivated such that they penetrate the shale and/ or carbonate cap rocks to depths of several kilometres and access fluids contained in the rift phase clastic reservoir. The deep metalliferous brines ascend the faults and discharge on, or close-to, the basin floor. Metal precipitation occurs in the anoxic, organic-rich sub-basins developing adjacent to the syn-sedimentary feeder faults.