Abstract

The Amulet Upper A orebody is a stratiform pyritiferous copper–zinc deposit situated within a sequence of andesitic pillow lavas. Underlying the massive sulphides is a cylindrical alteration pipe containing sulphides plus cordierite–anthophyllite-bearing rock. Hydrothermal alteration considered coincident with the formation of the overlying massive sulphides resulted in the development of a chlorite–quartz assemblage occupying the center of the alteration pipe, with a sericite–epidote–quartz assemblage at the fringes. Contact metamorphism resulting from the intrusion of the Lac Dufault granodiorite isochemically transformed the chlorite–quartz assemblage to cordierite–anthophyllite, and the sericite–epidote–quartz assemblage to biotite–epidote–quartz. This caused the development of four mineralogical zones within the alteration pipe: (1) anthophyllite zone, (2) cordierite zone, (3) biotite zone, and (4) stockwork zone.Pillows within the alteration pipe and exterior to it were studied using whole-rock analyses. A statistically balanced sampling procedure was adopted such that chemical and mineralogical variations within a single pillow could be compared with variations between adjacent pillows and finally with pillows situated in different locations within the deposit area. The largest source of variation was found to be a function of sample position within the alteration pipe. Ancillary variations include: (1) variations on the scale of an individual pillow resulting from the inward penetration of hydrothermal solutions and (2) variations between adjacent pillows due to differing degrees of alteration and veining.Proceeding into the alteration pipe, CaO, Na2O, Ba, Cr, Ni, and Sr were removed, Fe2O3T, MgO, K2O, H2O+, S2−, Cd, Cu, Li, and Zn were added, and Al2O3, TiO2, and V remained relatively constant. These changes were largely a chemical manifestation of the introduction of sulphides, plus the breakdown of plagioclase and pyroxene to form chlorite and quartz in the center of the alteration pipe, and sericite, epidote, and quartz at the fringes. Temperature appears to have been the dominant factor in controlling the distribution of the alteration assemblages, although sulphur fugacity did have an effect on the sulphide assemblages.

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