The Mount Isa deposit is the largest known accumulation of Cu in the Mount Isa region. Adjacent to high grade Cu mineralisation sit multiple high-grade stratabound Zn-Pb-Ag orebodies. The relationship between the Cu and Zn-Pb-Ag mineralisation has been the subject of intense debate. Over the years three separate metallogenic models have been proposed to explain the relationship between the Cu and Zn-Pb-Ag mineralisation. These models are: (1) syn-sedimentary Pb-Zn mineralisation overprinted by epigenetic syn-deformational Cu mineralisation (Gulson et al., 1983; Smith et al., 1978); (2) syn-sedimentary Cu and Pb-Zn mineralisation (Finlow-Bates & Stumpfl, 1980; McGoldrick & Keays, 1990); and (3) Epigenetic syn-D4 Cu-Pb-Zn (Davis, 2004; Grondijs & Schouten, 1937; W. G. Perkins, 1997).
Syn-sedimentary Cu & Zn-Pb-Ag Mineralisation
The second model suggests that both Cu and Pb-Zn mineralisation formed in a SEDEX-like environment at ca. 1650 Ma. A geometric model produced by Finlow-Bates and Stumpfl (1980) argued that the ore-bearing solution boiled its way up through the stratigraphy, resulting in the brecciation of the pre-mineralisation silica-dolomite lithology. Cooling of the ore solution resulted in the precipitation of chalcopyrite within the stockwork, and fluids containing Pb and Zn were expelled across the ocean floor, precipitating galena, then sphalerite. The temperature drop associated with the seawater mixing was used to explain the deposit scale-zonation of Cu → Pb → Zn, consistent with the temperature-dependent solubility of the metals (Finlow-Bates & Stumpfl, 1980). Based on various volatile and whole rock trace-element data, McGoldrick and Keays (1990) proposed a direct genetic link between Cu and Pb-Zn mineralisation. The source of S was proposed as sedimentary with a relatively cool (<200 °C) oxidized brine responsible for Cu and Pb-Zn mineralisation. The Cu → Pb → Zn zonation was attributed to the relative solubility of these metals (McGoldrick and Keays, 1990). It was argued that the stratabound nature of the Cu ore was subsequently destroyed by post-ore fluids that channelled up through the Paroo fault and re-distributed the Cu to higher levels to form the smaller 500 and 650 Cu-orebodies (McGoldrick and Keays, 1990).
Syn-sedimentary Zn-Pb-Ag with Overprinting Cu Mineralisation
The most widely accepted model suggests that Pb-Zn-Ag mineralisation formed in a syn-sedimentary SEDEX-like environment at ca. 1650 Ma. The Pb-Zn-Ag orebodies were then significantly remobilised during lower greenschist facies metamorphism and D2 deformation of the Isan Orogeny before being overprinted by subsequent Cu mineralisation at ca. 1520 Ma (C. Perkins et al., 1999). This model has been favoured by numerous studies due to the largely stratiform nature of the Pb-Zn ore relative to the discordant Cu ore, which is a key criterion in the identification of exhalative ore types (Leach et al., 2005). Smith et al. (1978) showed that a large range in 34S/32S isotope values were given by samples containing Pb-Zn mineralisation, contrary to a relatively small range in values from Cu-bearing samples. This study suggested that samples containing sulphides from Pb-Zn mineralisation were in isotopic equilibrium, while sulphide-bearing samples from Cu mineralisation were in disequilibrium (Smith et al., 1978). This evidence was used to suggest that Pb-Zn was deposited before regional metamorphism and epigenetic Cu mineralisation formed after regional metamorphism (Smith et al., 1978). Carr et al. (2004) used an Pb isotope growth curve controlled by the Pb-isotope composition of feldspars from independently dated granites to differentiate the sources of Cu and Pb-Zn mineralisation. As Pb isotopes from Pb-Zn mineralisation plotted close to ca. 1650 Ma reference line, syn-sedimentary Pb-Zn mineralisation was favoured. Differences in the Pb isotope composition from Cu-bearing samples was used to imply a second, separate Cu mineralising event.
Epigenetic Zn-Pb-Ag & Cu Mineralisation
The third model suggests that Cu and Pb-Zn mineralisation is both epigenetic and coeval. Based on petrology completed on 250 polished blocks, Grondijs and Schouten (1937) suggested that Cu and Pb-Zn-Ag ores replace carbonate, while stratiform Pb-Zn-Ag mineralisation replaced bedding-parallel carbonate, deformed shale, and fine-grained pyrite-rich shale. Analysis of the microstructural and textural evidence of Pb-Zn-Ag ore by Perkins (1997) favoured a coeval and epigenetic Cu-Zn-Pb-Ag model. It was suggested that an evolving alteration system occurred alongside the formation of layer-parallel and cross-cutting steep cleavages on the carbonaceous Urquhart Shale during D4 deformation of the Isan Orogeny (Perkins, 1997). This was subsequently overprinted by a short-lived Cu-Zn-Pb-Ag mineralisation event (Perkins, 1997). Davis (2004) focused on the structural controls on Zn-Pb-Ag mineralisation, favouring a syn-D4 Zn-Pb-Ag mineralisation event. This study showed that the Zn-Pb-Ag orebodies correlate with the presence of D4 folds, with the highest Pb-Zn grades centred on the short limbs and hinges of D4 folds. By reconstructing the pre-deformation architecture of the deposit, it was shown that a syn-sedimentary model could not account for the geometry of the Pb-Zn orebodies (Davis, 2004). Recently, Cave et al (2020) assessed the textural relationship of Cu-Zn-Pb-Ag sulphides across the transition from the Cu to Zn-Pb-Ag deposits. This study found that sphalerite, galena and chalcopyrite co-crystallised, consistently enveloping and/or replacing pre-mineralisation silica-dolomite, siderite, calcite, arsenopyrite, fine- and coarse-grained pyrite (Cave et al., 2020). This study also favoured an epigenetic model where the zonation from Cu and Zn-Pb-Ag mineralisation is explained by the presence of a temperature gradient out from the deposit-scale Paroo Fault. It must be noted that the timing of Cu mineralisation is also debated. While previous studies have dated Cu-mineralisation at ca. 1520 Ma, Re-Os geochronology performed on multiple Cu-bearing samples produced an age of ca. 1380 Ma (Gregory et al., 2008), consistent with a ca. 1380 Ma age recently published monazite U-Pb geochronology performed on a sample from the Cu orebody (Cave et al., 2023).
Conclusion
Decades of research on the Mount Isa deposit has resulted in three genetic models in order to explain the relationship between the significant Cu ore deposit and the adjacent stratabound Zn-Pb-Ag orebodies. The models are: (1) syn-sedimentary Pb-Zn mineralisation overprinted by epigenetic syn-deformational Cu mineralisation (Gulson et al., 1983, Smith et al., 1978); (2) syn-sedimentary Cu and Pb-Zn mineralisation (Finlow-Bates and Stumpfl, 1980, McGoldrick and Keays, 1990); and (3) Epigenetic syn-D4 Cu-Pb-Zn mineralisation (Davis, 2004, Grondijs and Schouten, 1937, Perkins, 1997, Cave et al 2020). There is still no clear consensus on the origin and age of Zn-Pb-Ag mineralisation, with a syn-sedimentary and epigenetic models still being debated in the literature. Overall, there is a populist consensus that Cu mineralisation was deposited during late-stage deformation of the Isan Orogeny, however the overall timing of the Cu event is still debated.
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