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

The Cannington Ag-Pb-Zn deposit represents an important new discovery of Ag -rich Broken Hill-type mineralization in the Eastern succession of the Moun t Isa inlier. The deposit is concealed beneath 10 to 60 m of Recent and Cre taceous cover, and there is no oxidation profile preserved at the basement subcrop. Mineralization is hosted by amphibolite facies migmatitic quartzof eldspathic gneisses, and is characterized by intense deformation and metamo rphism, with complex metasomatic and retrograde overprints. Lithostratigrap hic correlations of the host lithologies with other units in the Eastern su ccession are unclear Limited dating of probable stratigraphic equivalents h as given an age of 1677 +/- 9 Ma, which is broadly coeval with host deposit ional 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 No rthern and Southern zones. The Southern zone is the focus of current develo pment, and mineralization occurs as crudely strata-bound massive sulfide le nses that display complex brittle and ductile disruption. A large-scale iso clinal D-2 synform within the Southern zone appears to control broad repeti tion 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 mai n economic lode horizons that incorporate 10 mineralization types. These ty pes are defined on the basis of distinctive zonations in Pb/Zn ratios, and Fe-rich versus siliceous gangue lithologies. Fe-rich mineralization types a re characterized by coarse-grained, equigranular hedenbergite, Mn-Fe pyroxe noid, magnetite, olivine, and fluorite mineralogies. zones of amphibole, al mandine, ilvaite, pyrosmalite-dominant mineralogies with sulfide- and fluor ite-rich ductile breccias are associated with extensive postpeak metamorphi c metasomatism and retrogression. Siliceous mineralization types represent late-stage metasomatism, and are associated with further modification of mi neralization and retrogression of Fe silicates. Siliceous mineralization ty pes exhibit a distinctive low abundance of magnetite and fluorite. Dominant sulfides are galena and sphalerite, which show multiple generation s and variable intergrowths. Subordinate magnetite-pyrrhotite with minor ar senopyrite-lollingite-chalcopyrite are characteristic of Fe-rich mineraliza tion types. Pyrite is generally absent and is only locally associated with late structural and low-temperature metasomatic overprints. Extreme Ag enri chment is a consistent association of all mineralization types in the Canni ngton deposit, and is related to argentiferous galena with freibergite incl usions. 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 t he world's largest Ag producers. Cannington shows many similarities with the Broken Hill Main lode (New Sout h Wales), and represents an important new example of a Broken Hill-type cla ssification. However, the Ag enrichment that characterizes Cannington is un usual 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.