Abstract

The Allard stock, which hosts a porphyry Cu–Ag–Au–Pt metals deposit, is an epizonal Laramide pluton at the southwest end of the Colorado mineral belt. The stock and associated mineralization evolved by a complex interaction of igneous and metasomatic processes as follows. Stage 1: forceful emplacement of syenite plutons. Stage 2: intense hydrothermal argillization of stage 1 syenites. Stage 3: intrusion of mafic syenite plutons, some of which are layered, along shear zones. Stage 4: violent escape of volatiles, producing intrusive breccia pipes. Stage 5: intense K-metasomatic alteration of the older stock units with production of a fenitelike alteration halo extending 1000 m beyond the stock and simultaneous intrusion of carbonate-bearing alkali feldspar dikes. Stage 6: emplacement of trachyte, syenite pegmatite, and mafic pegmatite dikes. Stage 7: deposition of metallic minerals, sanidine, quartz, calcite, and fluorite as veinlets and disseminated replacement masses from hydrothermal solutions.The stock shows enrichment of volatiles (CO2, F, and S), high concentrations of Ba and Sr, low concentrations of Rb, high concentrations of Cu, Ag, Au, Bi, Te, Pt, and Pd, with little or no enrichment in Nb, U, Y, and Zr.The Allard stock and its associated mineralization are unlike the calc-alkaline porphyry copper deposits of the western United States and Canada. Although the Allard syenites are silica saturated (quartz-normative), some aspects of Allard alteration and mineralization show affinities with copper and (or) precious metal deposits hosted by alkaline silica-undersaturated (nepheline- or leucite-normative) plutons and carbonatite complexes including the "alkaline suite" porphyry copper deposits of British Columbia, the Palabora, South Africa, copper deposit, and the Cripple Creek, Colorado, gold telluride deposit. Computer-assisted evaluation of major-element analyses of igneous rocks suggests that Allard-type Cu – precious metal deposits may occur in other North American alkaline rock provinces. Epithermal gold telluride deposits in subvolcanic alkaline rocks may grade downward into alkaline porphyry Cu – precious metal deposits.

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