Recognizing and quantifying metamorphosed alteration zones through amphibolite facies metamorphic overprint at the Key Anacon Zn-Pb-Cu-Ag deposits, Bathurst Mining Camp, New Brunswick, Canada

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June 2016
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The Key Anacon deposits, Bathurst Mining Camp, New Brunswick, are hosted in upper greenschist- to amphibolite-facies felsic volcanic rocks. The occurrence of cordierite-biotite and garnet-biotite-muscovite assemblages parallel to the regional tectonic fabric in the metamorphosed hydrothermal alteration zones point to a pre-metamorphic mineralization event that was synchronous with sub-aqueous volcanism. A combination of textural, mineralogical, lithogeochemical alteration indices and vectors, molar element ratios, and P-T diagrams have been used to recognize the effects of medium-grade metamorphism and establish the mass compositional changes associated with pre-metamorphic hydrothermal alteration. Modelling the altered felsic volcanic rocks in a K2O-Fe2O3-MgO-Al2O3-SiO2-H2O-TiO2 (KFMASHT) system and comparing the observed peak metamorphic assemblages with those produced in a petrogenetic grid allows us to interpret the style of pre-metamorphic hydrothermal alteration related to deposit formation. The compositional change in the stratigraphic footwall (structural hanging wall) is characterized by mass gains of 0.1 to 4.0 wt. % Fe2O3 (Total), 0.7 to 22.2 wt. % MgO, and 0.5 to 55.2 wt. % CaO, and mass losses of 25.1 to 56.7 wt. % SiO2, 0.2 to 2.0 wt. % Na2O, and 0.3 to 3.8 wt. % K2O. Variable gains and losses of Zn, Pb, and Cu are characteristic of the footwall alteration zones with Zn displaying gains proximal to the sulfide lens, and losses distal to the massive sulfide lens. The alteration indices (AI) values increase as the massive sulfide lens is approached from either the footwall or hanging wall, whereas the Ghandi index (GI) discriminates the intensely chlorite-altered rocks proximal to mineralization from the sericitic altered rock in more distal areas. Overall, there is an increase of the GI from the weakly to moderately altered zone (GI =1.3 to 6.0) to the more intensely altered zone (GI= 6.1 and 60). Although the Chlorite-Carbonate-Pyrite index (CCPI) is similar to the GI, it is preferable to adopt the more robust GI, because it is a ratio of the added chemical components (Fe2O3 (Total) +MgO) to those lost from the system (Na2O+K2O) during the most intense hydrothermal alteration. These physical and geochemical observations are consistent with early feldspar-destructive alteration followed by chloritization proximal to the sulfide lens and accompanied by sericitization alteration distal prior to sulfidation and oxidation during prograde metamorphism