Abstract

The Proterozoic Cannington Ag-Pb-Zn deposit in the Mount Isa inlier is hosted by a polydeformed sequence of quartzofeldspathic gneiss, pegmatites, amphibolites, quartzites, garnetiferous quartzites, sillimanite-garnet schists, and minor Fe-Mn silicate units that display close mineralogical and geochemical similarities to rocks hosting the Broken Hill deposit in New South Wales. The timing of mineralization at Cannington is controversial; premetamorphic and metamorphic-metasomatic models have been proposed. This paper aims to contribute to this debate from the point of view of paragenetic relationships on a deposit scale.Sulfide-bearing rocks include graphitic, Fe-Mn silicate, and siliceous types, based on differing mineral contents, metal zonation, and Fe, Mn, Pb, and Zn concentrations. Fe-Mn silicate and siliceous types are subdivided according to the relative proportions of Pb and Zn. Graphitic and quartz-rich, banded olivine-pyroxenoid types display laterally continuous, fine compositional banding over meter scales, and granoblastic sulfide-silicate textures with spheroidal sulfide inclusions in all silicates. Deformation is evidenced by D 2 structures such as folds, boudinage, sulfide-filled extensional veinlets and piercement veins, and the preferred alignment of graphite, biotite, ovoid garnet poikiloblasts, fayalite and pyroxmangite aggregates, and in some instances, sulfide grains. Quartz-poor olivine-pyroxenoid and hedenbergite types comprise contrasting brittle-ductile breccias with anastomosing D 2 mylonites in fluorite-rich zones. In fluorite-poor zones, granoblastic sulfide-silicate-magnetite textures predominate, with spheroidal sulfide inclusions in Fe-Mn silicates and magnetite. Quartz-rich, banded hedenbergite types are characterized by syn- to post-D 2 hedenbergite veins and associated hedenbergite-quartz-garnet alteration that overprints compositional banding in banded olivine-pyroxenoid types. The alteration is commonly localized around D 2 structures such as quartz-hedenbergite-garnet-carbonate-filled tension gashes. Siliceous Pb-Zn-Ag type is characterized by quartz veins and sulfidic breccias comprising nebulous, ragged clasts of silicified and weakly mineralized graphitic and, to a lesser extent, banded olivine-pyroxenoid types in a matrix of recrystallized sulfide. The siliceous Zn type consists of sulfidic breccias and quartz veins in strongly silicified metasediments. Quartz veins in siliceous Pb-Zn-Ag and Zn types display a syn-to post-D 2 timing.The paragenesis has been divided into pre- to synpeak metamorphic (stage I), syn- to postpeak metamorphic retrograde alteration (stage II), and near-surface hydrothermal alteration (stage III) assemblages. Stage II has been subdivided into high-grade anhydrous (stage IIa), high-grade hydrous (stage IIb) and low-grade hydrous (stage IIc) assemblages. Stage I peak metamorphic assemblages comprise anhydrous minerals in sulfide-bearing graphitic and olivine-pyroxenoid types. Granoblastic sulfide-silicate textures and spheroidal sulfide inclusions in stage I Fe-Mn silicates indicate that sulfides were present prior to metamorphism. Stage IIa retrograde syn- to post-D 2 hedenbergite veins and hedenbergite-quartz-calcic almandine alteration overprint stage I assemblages. Stage IIb retrogression involved post-D 2 amphibole alteration and minor sulfide deposition, and was followed by stage IIc pyrosmalite and pyrosmalite-magnetite alteration associated with abundant secondary sulfides, sulfosalts, and antimonides. Stage III involved quartz-carbonate-chlorite alteration temporally and spatially associated with D 4 subvertical faulting.The presence of premetamorphic sulfides indicates that the deposit has in part a premetamorphic origin. Subsequent modification during syn- to post-D 2 retrogression resulted in reworking and possible zone refining of the deposit via localized remobilization of sulfides. It is likely that exotic components were introduced during the retrograde stage by regional metasomatic fluids.

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