Geology and mineralization of the Cannington Ag-Pb-Zn deposit; an example of Broken Hill-type mineralization in the eastern succession, Mount Isa Inlier, Australia

Abstract

The Cannington Ag-Pb-Zn deposit represents an important new discovery of Ag-rich Broken Hill-type mineralization in the Eastern succession of the Mount Isa inlier. The deposit is concealed beneath 10 to 60 m of Recent and Cretaceous cover, and there is no oxidation profile preserved at the basement subcrop. Mineralization is hosted by amphibolite facies migmatitic quartzofeldspathic gneisses, and is characterized by intense deformation and metamorphism, with complex metasomatic and retrograde overprints. Lithostratigraphic correlations of the host lithologies with other units in the Eastern succession are unclear. Limited dating of probable stratigraphic equivalents has given an age of 1677 + or - 9 Ma, which is broadly coeval with host depositional ages for Pb-Zn-Ag mineralization at Mount Isa, HYC and Broken Hill.The orebody is divided on the basis of late structural displacement into Northern and Southern zones. The Southern zone is the focus of current development, and mineralization occurs as crudely strata-bound massive sulfide lenses that display complex brittle and ductile disruption. A large-scale isoclinal D 2 synform within the Southern zone appears to control broad repetition patterns between ore lenses. Grade control within individual ore zones can also be related to zones of ductile strain and metasomatism influenced by strain partitioning around the termination of the Core Amphibolite.Mineralization within the Cannington Southern zone is divided into five main economic lode horizons that incorporate 10 mineralization types. These types are defined on the basis of distinctive zonations in Pb/Zn ratios, and Fe-rich versus siliceous gangue lithologies. Fe-rich mineralization types are characterized by coarse-grained, equigranular hedenbergite, Mn-Fe pyroxenoid, magnetite, olivine, and fluorite mineralogies, zones of amphibole, almandine, ilvaite, pyrosmalite-dominant mineralogies with sulfide- and fluorite-rich ductile breccias are associated with extensive postpeak metamorphic metasomatism and retrogression. Siliceous mineralization types represent late-stage metasomatism, and are associated with further modification of mineralization and retrogression of Fe silicates. Siliceous mineralization types exhibit a distinctive low abundance of magnetite and fluorite.Dominant sulfides are galena and sphalerite, which show multiple generations and variable intergrowths. Subordinate magnetite-pyrrhotite with minor arsenopyrite-lollingite-chalcopyrite are characteristic of Fe-rich mineralization types. Pyrite is generally absent and is only locally associated with late structural and low-temperature metasomatic overprints. Extreme Ag enrichment is a consistent association of all mineralization types in the Cannington deposit, and is related to argentiferous galena with freibergite inclusions. High levels of Sb, Cd, As, Cu, and F are also a feature of specific mineralization types. When in full production, Cannington will be one of the world9s largest Ag producers.Cannington shows many similarities with the Broken Hill Main lode (New South Wales), and represents an important new example of a Broken Hill-type classification. However, the Ag enrichment that characterizes Cannington is unusual even for previously considered Ag-rich members of the classification. A genetic model is proposed that involves high-temperature metasomatic zone refining of a preexisting Fe-Ca-Mn-Pb-Zn-Ag-rich mineralized system.