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Broad geochemical and mineralogical haloes surround base metal mineralisation at Mount Isa in northwestern Queensland. With the haloes being cryptic in nature, all aspects of the host rock geology were taken into account to distinguish regional and background effects from those related to mineralisation processes. To this end, variation was accounted for in the host rock through detailed drill core logging, sedimentological analysis, correlation between drill cores, construction of an internal stratigraphy, and geochemical comparison along correlated horizons.
The Urquhart Shale is host to Zn-Pb mineralisation at Mount Isa. Detailed analysis shows that six discrete, laterally continuous stratigraphic members record variations in depositional environment. Most of the Urquhart Shale consists of dolomitic, carbonaceous and pyritic laminated siltstone and mudstone that was deposited on a carbonate slope, offshore from a sabkha or saline mudflat. Units deposited on a carbonate slope are host to all of the high-grade orebodies in the Mount Isa mine. Facies analysis shows that only one particular sedimentary facies, the Rhythmite facies, in particular the Muddy Shale and Silty Shale subfacies, hosts Zn-Pb mineralisation.
Both mineralogical and geochemical haloes are defined for Zn-Pb mineralisation at Mount Isa. Mineralogical haloes are based on variations in mineral abundance, variations in mineral composition, and/or changes in mineral textures. Other mineralogical changes are the result of late stage hydrothermal alteration, with characteristic mineral assemblages that overprint Zn-Pb mineralisation and are related to late-stage Cu mineralisation.
There are several distinct types of geochemical haloes that can be related to base metal sulfide mineralisation. Sulfide-related elements show strong positive correlation with mine proximity, whereas silicate-related elements show variable negative correlation with mine proximity due to dilution by sulfides. Carbonate-related elements show variable trends with mine proximity, largely due to changes in carbonate chemistry as a result of interaction with the mineralising fluids. Carbon and oxygen isotope depletion with mine proximity shows minor differences between sedimentary facies, but is largely related to later hydrothermal Cu mineralisation (as indicated in previous studies). Sulfur isotope compositions for fine-grained pyrite vary laterally, but consideration of textural constraints and sulfur isotope systematics shows that this trend is unrelated to the Zn-Pb-mineralising event(s) and is a result of finite reservoir effects during fine-grained pyrite genesis.
Detailed consideration of aspects of the sedimentology, mineralogy, petrology, stratigrahy and economic geology define a specific model for mineralisation at Mount Isa involving discrete Fe, Zn-Pb and Cu mineralisation events. Fine-grained pyrite is the earliest generation of pyrite and the most abundant sulfide within the Urquhart Shale. The pyrite is intimately interbanded with ore grade Zn-Pb mineralisation at Mount Isa mine but is also abundant north and south of the mine. Comparison of the sulfur isotope systematics of fine-grained pyrite (δ34S = -3.3 to 26.3 per mil) and spatially associated base-metal sulfides from the Mount Isa Zn-Pb and Cu orebodies indicates contrasting formation mechanisms, with a common sulfur source of ultimately marine origin for all sulfide types. Late diagenetic, possibly abiogenic fine-grained pyrite mineralisation was consumed and enhanced by subsequent hydrothermal Zn-Pb mineralisation. Finally, syn-tectonic hydrothermal Cu mineralisation both overprinted and remobilised pre-existing Zn-Pb mineralisation.
Features important for the identification of Mount Isa-style deposits are proximity to a regional fault system, the presence of a fine-grained sedimentary package that is carbonaceous, dolomitic to Fe-dolomitic, predominantly laminated siltstone to mudstone, and contains an intrasedimentary sulfur source. Alternating aquifer and aquitard facies are important for focussing of the mineralising fluid, and metamorphism, deformation and associated hydrothennal alteration may result in enhanced Zn-Pb grades; however, the latter are not essential for ore grade mineralisation. Each of the geochemical and mineralogical haloes listed in this manuscript can be used as vectors to mineralisation. With these methods, indicators of Zn-Pb mineralisation can be detected over 13 km along sfrike from a Mount Isa-sized orebody. Amongst the most effective vectors is a geochemical ratio defined here as the Isa Vector (T1/Ge.(FeOdo1 + 10MnOd), where T1 and Ge are whole rock concentrations, and FeOdo1 and MnOd represent Fe and Mn content of dolomite respectively). This vector increases systematically with proximity to the centre of the mineralising system, and can distinguish Zn-Pb anomalies on the margin of a larger Zn-Pb mineralising system.
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