Abstract
AbstractThe McArthur River (HYC) Zn-Pb-Ag deposit in the Carpentaria Zn belt, northern Australia, is one of the world’s largest and most studied sediment-hosted base metal deposits, owing to its lack of deformation and preservation of sedimentary and ore textures. However, the ore formation process (syngenetic vs. epigenetic) is still a subject of controversy. In this paper we focus on key characteristics of the HYC deposit that remain unexplained: preservation of sedimentary carbonate (dolomite) and its association with Zn, and the role of thallium (Tl) and manganese (Mn) distribution in the orebody.Our findings demonstrate a sequence of events during ore formation: Tl is hosted almost exclusively within euhedral pyritic overgrowths around early diagenetic pyrite; sphalerite mineralization occurred after Tl-bearing pyrite overgrowths, in association with acid dissolution (replacement) of laminated and nodular dolomite across the subbasin; and outer rims are enriched in Mn on preserved dolomite at the dissolution reaction front in contact with sphalerite. New thermodynamic fluid chemistry modeling demonstrates the metal distribution and paragenesis can be explained by acidic, oxidized ore fluids entering the pyrite-dolomite host lithology, allowing reduction and pH buffering by acid carbonate dissolution, resulting in stepwise metal deposition in an evolving fluid.We argue this represents strong evidence for epigenetic ore formation at HYC. Furthermore, the primary control on ore deposition is not synsedimentary faulting in the subbasin; rather, the chemical potential of sedimentary carbonate within reduced, sulfidic lithologies appears to be of critical importance to precipitation of sphalerite.
Highlights
The super-giant McArthur River Zn-Pb-Ag deposit is one of the world’s largest known sediment-hosted base metal deposits
The first scenario, which is central to the synsedimentary exhalative interpretation of ore formation, we argue is unlikely for the following reasons: 1. Sphalerite only rarely occurs in the carbonaceous-pyrite beds in laminated facies, and where it does it only occurs in thin sphalerite interlaminae between pyrite laminae
It might be argued that carbonaceous material is absent from the sphaleritic laminae because of a potential role of bacterial sulfate reduction (BSR) that oxidized organic matter, but this would not explain the absence of pyrite in these areas, which would almost certainly be present in such a case
Summary
The super-giant McArthur River Zn-Pb-Ag deposit (commonly known as “Here’s Your Chance,” or “HYC,” as it is referred) is one of the world’s largest known sediment-hosted base metal deposits It is hosted in the Proterozoic McArthur-Isa superbasin (Fig. 1), which forms the Carpentaria Zn belt in northern Australia, the world’s largest Zn-Pb province. A sedex formation process assumes that mineralization occurred following exhalation of a metalliferous brine into the water column during sedimentation and synsedimentary faulting, with resultant deposition of laminated base metal sulfide ore as a chemical sediment. This interpretation of ore formation influences exploration approaches, as it implies a direct control of the sedimentary environment and synsedimentary tectonic
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
