Geology of the Alta stock, Utah

Abstract

The Alta stock of granodiorite is closely associated in space and time with many of the Pb, Zn and Ag ore deposits of the Park City-Cottonwood mining area in the central Wasatch Mountains, Utah. The purpose of this study was to determine if fractional crystallization of the Alta magma ultimately resulted in a metal-rich ore-forming fluid. The Alta magma was intruded in two pulses into pre-Triassic sedimentary rocks at a depth of approximately 21,000 feet and at a temperature in excess of 720°C. Detailed study of texture indicates that the first phase of magma solidified with a nonporphyritic texture, crystallizing from the wall progressively toward the center of the stock. The rock at the wall is rich in mafic minerals but grades inward within a few hundred feet to light-gray medium-grained biotite-hornblende granodiorite. Early in the history of the first pulse of magma, convection currents carrying growing crystals of early minerals formed stratified gravity accumulations rich in hornblende, biotite, sphene, magnetite and apatite. These layered masses were subsequently broken and displaced. They now form tabular inclusions of mafic layered granodiorite in the light-gray nonporphyritic granodiorite. At a late stage in the crystallization history, the Alta stock had an essentially solid shell of nonporphyritic granodiorite at least 1200 feet thick that surrounded a partially liquid but semirigid crystal mush at the center of the stock. Structural adjustments at this late stage re sulted in close-space jointing and emplacement of aplite-pegmatite dikes in the nonporphyritic granodiorite shell. After emplacement of the dikes but before the interstitial liquid of the magma at the center of the stock had completely crystallized, the semi-rigid crystal mush and solid shell were intruded by a second pulse of the magma. The distribution of strongly porphyritic texture indicates that loss of volatiles through fractures in the overlying material caused this later phase of the magma to finish crystallization with a fine-grained groundmass. Modal analyses supported by gravimetric chemical analyses show that the porphyritic granodiorite has a slightly more silicic composition than the nonporphyritic granodiorite. The Flagstaff-Emma ore zone in the Little Cottonwood district is on the trend of a westward dike like projection of the porphyritic granodiorite. The fluids that were released during the emplacement of this late phase of the magma conceivably may have been the same fluids that deposited the ore minerals in the Flagstaff-Emma ore zone. In order to determine the trend of the ore-metals during crystallization of the Alta magma, 76 mineral separates of orthoclase, plagioclase, biotite, hornblende, magnetite, sphene, quartz and chlorite were analyzed by the emission spectrographic method for Cu, Pb, Zn, Ag, Co, Ni and Sn. Comparison between samples of the same mineral show that Co and Ni are slightly less abundant and Cu, Pb and Sn are more abundant in the later phases of the Alta igneous suite. No trend is apparent for Ag and Zn. Cu, Co, Ni, Ag and Zn were added to the solid phases of the rock during chloritic alteration. The release of volatiles associated with the late-stage intrusion of porphyritic granodiorite, the structural relation of the adjacent ore deposits to this porphyritic granodiorite and the trend in trace-element abundances suggest that an ore-forming fluid may have been produced at a late stage in the crystallization of the Alta stock.