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 strata-bound Ag-Pb-Zn deposit (Cannington). These superbasins consist of units deposited during three nested cycles of deposition and exhumation that occurred in the period from 1800 to 1580 Ma. The cycles took place in response to far -field extension and subsidence associated with a major northward-dipping subduction zone in central Australia. All major stratiform zinc-dominant deposits occur within rocks of the sag phase of the youngest Isa superbasin, which was deposited between 1670 and 1580 Ma. The strata-bound silver- and lead-rich Cannington deposit is hosted by highgrade metamorphosed clastic sedimentary rocks that are temporal correlatives of the basal extensional phase of the Isa superbasin. It exhibits distinct differences from the stratiform zinc-dominant deposits but shows similarities with Broken Hill-type deposits.
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 upstratigraphy, (7) an extensive strata-bound 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 per mil, 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. The exact timing of mineralization relative to these processes varies from deposit to deposit. However metamorphic overprints in some deposits (e.g., Mt. Isa, Hilton, Dugald River, Lady Loretta) have lead to recrystallization of sulfide minerals, making it difficult to interpret primary paragenetic relationships and absolute timing of mineralization. Mount Isa is the only northern Australian stratiform Zn-Pb-Ag deposit that has spatially associated highgrade copper mineralization.
Textural and isotopic data for the stratiform Zn-Pb-Ag deposits suggest there is a spread of ore depositional processes from synsedimentary exhalative to syndiagenetic replacement. At McArthur River, for example, the highgrade laminated ores principally formed by synsedimentary exhalative processes. However, there is good evidence that the lower grade ores at the margins of the deposit formed at shallow depth in the organic-rich muds by syndiagenetic replacement and open-space fill. At Century, on the other hand, the textual and lead isotope evidence indicate the major mineralization probably formed by syndiagenetic replacement about 20 m.y. after sedimentation. At Mt. Isa, Hilton, and George Fisher, overprinting metamorphism precludes determination of the precise timing of ore deposition relative to sedimentation and diagenesis, but recent studies at the least metamorphosed George Fisher deposit suggest that syndiagenetic replacement was likely dominant.
The lack of footwall stringer zones or hydrothermal vent complexes in the Zn-Pb-Ag deposits, coupled with the lateral and vertical Pb-Zn metal zonation, suggest the ores are of the vent-distal type, forming at some lateral distance from the hydrothermal vent or feeder fault. The laterally extensive strata-bound Fe-Mn car bonate halos indicate significant hydrothermal fluid volumes that have interacted with the sea-floor and sub-sea-floor sediments. These halos provide an important vector for exploration. Basin-scale, fluid-flow modeling has emphasized the importance of (1) early rift phase volcanic and volcaniclastic rocks as potential deep sources for metals, (2) clastic units at the top of the rift package that act as aquifers for basin-wide hy drothermal fluid flow, (3) evaporitic units that lead to high fluid salinity, which enhances metal transport, (4) thick packages of fine-grained dolomites and siltstones in the overlying sag phase sequence, which act as a seal over the fluid-rich reservoir rocks (rift clastics), and (5) deeply penetrating faults that provide the fluid conduit from the fluid reservoir and metal source area, located deep in the sedimentary basin, to the organic-rich trap rocks at the top of the section. Fluids were oxidized, low- to moderate-temperature (100°–250°C), near-neutral pH brines, with sulfate reduction in organic-bearing trap sites being the principal cause of zinc- and lead-bearing sulfide deposition.
