Petrogenesis of the Volcanic and Intrusive Rocks Associated with the Bingham Canyon Porphyry Cu-Au-Mo Deposit, Utah

Abstract

Abstract Recent examination of volcanic rocks near the Bingham Canyon Cu-Au-Mo deposit, Utah, suggests that primitive alkaline magmas are an important factor in the formation of this world-class porphyry copper deposit. The Bingham deposit is spatially associated with a monzonitic intrusive complex emplaced at 39 to 37 Ma into Paleozoic sedimentary rocks. Coeval igneous rocks vented to the surface and formed a volcanic pile, part of which is preserved on the eastern flank of the Oquirrh Mountains. Bingham volcanic rocks are divided into three chrono-lithologic suites: an older volcanic suite, a nepheline minette-shoshonite suite, and a chemically distinct younger volcanic suite. Petrographic and geochemical data indicate that the intrusive complex and older volcanic suite are largely comagmatic. This relationship is substantiated by similarities in ages and proximity of the older volcanic rocks to the intrusive suite. No significant chemical differences occur between these two suites, except where hydrothermal alteration has increased the concentration of Cu and K in the intrusions at the expense of Na. The nepheline minette lava flows extruded at ~38 Ma are primitive. They are characterized by Mg # >65, high concentrations of volatiles, LILE, and LREE, and strongly compatible elements. They contain relatively low concentrations of Ti, Nb, and Zr on a mantle-normalized basis, 15 to 18 percent normative nepheline, and 1 to 15 percent normative leucite. The nepheline minette magmas may be primary melts that are products of small degrees of partial melting of metasomatized, lithospheric mantle. Associated shoshonites lack normative nepheline and normative leucite. At comparable SiO2 contents, the younger volcanic rocks emplaced at ~32 Ma are generally more aluminous and sodic, and less magnesian and potassic than the older volcanic and intrusive suites. All magmas except the nepheline minette were sulfide-saturated when erupted and contain magmatic sulfides as 1 to 100 micron-diameter elliptical inclusions in mafic mineral phenocrysts. Analytical data suggest that sulfides accommodate most of the Cu and Ag that are present in the magmas. Resorption and oxidation of the sulfides may have made these metals available to the hydrothermal system. The sulfide-undersaturated character of the primitive, alkaline magmas may have allowed them to rise through the crust with almost no loss of S, Cu, and other chalcophile metals. Fractional crystallization, magma mixing, and assimilation all played roles in determining the composition of the magmas in the Bingham system. Trace-element modeling shows that the minette flows are not related to the other volcanic suites or to the intrusive suite by fractional crystallization. However, mixing of the minette magma with latitic magma could have created the shoshonites and the magma of the older suite. Trace-element modeling also indicates that late mineralized dikes may be formed by mixing of about 10 percent minette magma and 90 percent calc-alkaline magma. Degassing of mixed minette magma underlying the calc-alkaline magma may have contributed even larger proportions of sulfur, volatiles, and metals to the ore-forming system.