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
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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.Keywords:
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In the Southern Alps, the PTB(Permian-Triassic Boundary) is settled within the basal part of the Werfen Formation(Changhsingian-Induan Tesero Member) conformably overlying the Bellerophon Formation, although temporal gap between the formations is a subject of dispute.The succession in the Tesero and Bulla sections of the western Dolomites is designated as the hypostratotype and parastratotype of the PTB, respectively and most intensively studied stratigraphically, paleontologically, geochemically and magnetostratigraphically(e.g., Magaritz et al., 1988;Scholger et al., 2000;Farabegoli et al., 2007) .Both sections are well exposed along the road surrounded by a magnificent panorama of the Southern Tyrol.Repeated detailed field works by many earth scientists are recorded directly on a rock face as markings of a sample level with various manners, round holes, cylindrical tracks and others.
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During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ 18 O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ 18 O magmatic fluids caused δ 18 O depletions in the gneisses within the migmatite zone. These low δ 18 O fluids were derived from the young (<4 Ma), relatively low δ 18 O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40 Ar/ 39 Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.
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