Geodynamic evolution and genesis of the Cannington Broken Hill-type Ag-Pb-Zn deposit, Mount Isa Inlier, Queensland

The Cannington Ag-Pb-Zn deposit is located in the southeastern area of the Palaeo- to Mesoproterozoic Mount Ise lnlier, northwest Queensland. The orebody is totally concealed beneath 10- 60m of flat-lying, semi-lithified Cretaceous to Recent sediments of the Eromanga Basin. The deposit constitutes an economic resource of 43.8 million tonnes (Mt) at 11.6% Pb, 4.4% Zn and 538g/t Ag, and currently the mine is the world's largest Pb and Ag producer. The deposit has been classified as a classic Broken Hill-type (BHT) Pb-Zn deposit based on remarkably similar geochemical, mineralogical and host rock affinities to the giant -280Mt Broken Hill deposit in New South Wales, Australia; the holotype of the BHT classification. Conjecture over the genesis of the Cannington deposit has arisen because of two disparate lines of thought. Metamorphogenic models assume that ore textures, mineralogies and chemistries are a record of the primary ore formation process, whereas syngenetic models consider that the same characteristics have been extensively modified by post-depositional deformation and metamorphism. As a result, premetamorphic and metamorphic-metasomatic models have been proposed. This thesis contributes to this debate by integrating geological, paragenetic, geochemical and isotopic constraints to identify the most plausible genetic model. Cannington is hosted by migmatitic gneiss with intercalated pegmatites, amphibolites, minor quartzites and rare Fe-Mn silicate (± sulphide) units. Various lines of evidence indicate that the gneiss represents metamorphosed, immature, siliciclastic sediments that were deposited during the early stages of intracontinental rifting ca. 1675 Ma. The migmatitic gneiss grades into a 250m-wide sillimanite-garnet schist and garnetiferous quartzite ore envelope that is interpreted to be a metamorphosed hydrothermal alteration halo genetically related to ore formation. Gahnite-bearing schists are a less common, but important component of the envelope. The host succession was subjected to polydeformation and high-P/low-T metamorphism related to dominantly west-vergent folding and thrusting during the lsan Orogeny between ca. 1600-1480 Ma. Using a combination of mineral equilibria and cation exchange thermobarometers, M1-M2 metamorphism is characterised by an anti-clockwise P-T-t path, with peak metamorphic conditions (Mi) of -730-750°C and 5-6kbar; consistent with upper amphibolite to transitional granulite facies conditions. It was during MI that substantial anatectic partial melting of the host metasedimentary succession occurred ca. 1580 Ma. This was followed by a retrograde M2 event that involved the ingress of metamorphically-derived, acidic fluids and the development of characteristic secondary sillimanite shear fabrics (S2). Third generation recumbent folding and thrusting resulted in the development of the Cannington Synform and Footwall Shear; an interpreted fold-thrust pair. The Cannington Synform controls the gross geometry of the deposit, resulting in repetition of ore lenses in the structural hanging wall and footwall. The Footwall Shear truncated ore lenses in the lower, attenuated limb of the Cannington Synform. A fourth generation of deformation (D4) appears to be related to a transient thermal perturbation (M3) initiated by the intrusion of voluminous granitic melts of the Williams-Naraku Batholith ca. 1520 Ma. D4 has a moderate effect on the geometry of the deposit, and is manifested by an open, upright synform. D5 represents a change in deformation style to a brittle regime. Two generations of subvertical faults overprint the orebody and locally offset ore lenses by up to 50m. The youngest of these generations comprises a conjugate fault set. Mineralised rocks can be divided into graphitic, Fe-Mn silicate and siliceous types, based on differing mineralogies, metal zonation and Fe, Mn, Pb and Zn concentrations. Textures are extremely variable depending on the ore type, and include: i) laterally continuous, millimetre to centimetre-scale compositional banding; ii) granoblastic sulphide-silicate textures with spheroidal sulphide inclusions in silicate minerals; iii) durchbewegt textures; iv) anastonnosing mylonitic zones in fluorite-magnetite-rich ores, and; v) quartz-sulphide breccias. The paragenesis of the deposit has been divided into three principal stages based on mineralogical assemblages and temporal relationships to deformation and metamorphic events recorded in the host metasediments and proximal garnetiferous alteration halo. Stage /: peak metamorphic assemblages comprise anhydrous minerals in sulphide-bearing graphitic, olivine and hedenbergite ores; Stage Ha: retrograde syn- to post-D2 metasomatism associated with anhydrous alteration assemblages (hedenbergite-garnet-quartz-pyroxferroite) and silicification; Stage Ilb: amphibole veining and alteration, and minor sulphide deposition; and Stage Ilc: syn- to post-M3 pyrosmalite-magnetite alteration associated with abundant secondary sulphides, sulphosalts and antimonides; Stage ///: syn to post-D5 quartz-carbonate-chlorite alteration, temporally and spatially associated with two generations of subvertical faulting. The Stage Ila hedenbergite-garnet-quartz metasomatism and silicification produced a range of protolith-dependent metasomatic ore types such as siliceous Pb-Zn-Ag and siliceous Zn ore breccias. Overall, syn- to post-D2 metasomatism resulted in reworking and zone-refining of the deposit via localised solution-associated remobilisation of sulphides. Granoblastic sulphide-silicate textures and spheroidal sulphide inclusions in Stage I olivines and magnetite are consistent with the presence of sulphides prior to peak metamorphism. The presence of pseudomorphic bow-tie textures and relic inclusion trials (Si) in garnet porphyroblasts in graphitic ores, indicate that the timing for ore formation can be traced back through prograde metamorphism to pre- Ism Orogeny times. This is supported by cation exchange thermometric estimations that record the prograde evolution of the ores. Compared to migmatitic gneiss, sillimanite-garnet and gahnite-bearing schists have higher K/Na ratios and anomalous Mn, Pb and Zn concentrations. The most significant mass changes associated with alteration of gneissic protolith to sillimanite-garnet and gahnite-bearing schist involved mass gains in K, Mn, Pb, Zn and Rb, and losses in Si, Fe, Mg, Na, Sr and Ba, with average, absolute net mass gains of 0.8g/100g and 5.9g/100g respectively. On the basis of immobile elements (Ti, Zr, Al) graphitic ores contain a pelitic detrital component. Mass balance calculations between an average pelitic gneiss precursor composition and graphitic ores indicate absolute net mass gains between 25.0g/100g for samples that contain the highest proportion of pelitic detritus, to >1000g/100g that contain the least. Such large net mass changes can be explained by the presence of hydrothermal components in the ores, in addition to pelitic detritus. Chondrite-normalised REE signatures are characterised by LREE-enrichment and strong positive Eu anomalies, similar to hydrothermal fluids and chemical sediments proximal to mid-ocean ridge hot springs. Calculated hydrothermal REE signatures are characterised by strong positive Eu anomalies when normalised to the local host gneiss. The data are consistent with the interpretation that graphitic ores are metamorphosed chemogenic sediments. 613C signatures of graphite in graphitic ores range between -27%. to -25%., consistent with a biogenic origin. The preservation of carbon is best accounted for by an anoxic environment if a synsedimentary origin is assumed. The data do not support a retrograde metasomatic model for ore formation. Paragenetically constrained Pb isotope signatures were determined on various ore types, and host rocks. Stage I and II galenas plot as a distinct linear cluster oriented along the machine fractionation trend, but within the limits of the precision ellipse. Most of the data plot just below the average crustal growth curve, and compared to other Proterozoic sediment-hosted Pb-Zn deposits in northern Australia (e.g. McArthur River and Mount lsa), have more primitive p values similar to other BHT deposits, such as Pegmont (30km west of Cannington) and Broken Hill in New South Wales. A strong correlation between Pb isotope signatures from Stage I and II galenas indicates that syn- to post-D3 metasomatic fluids sourced Pb from pre-existing Stage I ores and that significant exotic Pb was not introduced. Stage III galena and amazonite in lode pegmatites inherited their Pb isotopic signatures from pre-existing galena, as well as from radiogenic Pb in gneissic and garnetiferous rocks. The Pb isotope data also reveals that Stage I Pb was not sourced from the nearby granites, or the host metasediments. The available evidence suggests a more primitive, exotic source that is not exposed in the region. Development of a terrain-specific Pb isotope model for the Mount Isa region suggests that mineralisation was deposited at -1675 Ma. This age coincides with maximum depositional ages of ca. 1680 Ma (zircon U-Pb SHRIMP date). It provides further evidence that the orebody formed in a synsedimentary environment. Ore textures, thermobarometric estimates, 813C, Pb isotope systematics and geochemical affiliations, combined with U-Pb zircon ages, provide firm evidence for a synsedimentary timing for ore formation. This is further supported by the fine-scale laterally continuous and compositionally diverse banding in graphitic ores that are interpreted to represent chemogenic sediments with a variable detrital pelitic component. Precursor authigenic hydrothermal assemblages are likely to have been dominated by variable proportions of Fe-Mn carbonates (manganoan siderite and ankerite), fluorapatite, fluorite, magnetite, Fe-Mn chlorites (chamosite, greenalite) and clays, quartz, biogenic matter (kerogens) galena, s...