Sulfide minerals in intrusive and volcanic rocks of the Bingham-Park City belt, Utah

We report here the results of a reconnaissance study of sulfide inclusions in some intrusive and volcanic rocks from the Bingham-Park City belt in cen tral Utah. The study was undertaken to determine whether primary sulfides w ere preserved in these rocks and, if so, to assess their role in the geoche mistry of copper in the magmas. Numerous samples were studied from the Alta , Clayton Peak, and Flagstaff stocks and a few samples were also studied fr om the Little Cottonwood, Last Chance, and Soldier Canyon stocks, as well a s from the Phoenix dike and the Keetley Volcanics. Our results show that su lfides are present in unaltered intrusive and volcanic rocks. Although most of these inclusions lack hematite and pyrite that are typical products of degassing, at least some inclusions appear to have been modified, possibly by reaction with enclosing iron-bearing host minerals. Measurements of the sizes of inclusions suggest that sulfides contain only 20 to 50 percent of the copper in the rock, whereas sulfide selective analyses suggest that all of the copper in the rock is hosted by sulfides. Despite this uncertainty, compositions of sulfides vary systematically from unit to unit. The distri bution of sulfide inclusions in different minerals suggests that mafic intr usions were saturated with sulfur over a longer period of time during their crystallization history than were felsic intrusions. Although there are ex ceptions, chalcopyrite is most abundant in intrusions emplaced at deeper le vels, bornite is found in intrusions emplaced at intermediate levels, and p yrrhotite is found in the shallowest intrusions and volcanic rocks. Finally , apparently unaltered samples of the Last Chance, Soldiers Canyon, and Pho enix bodies contain talnakhite, a copper-bearing sulfide mineral that was n ot observed in the other intrusive rocks. In view of the close association of the Last Chance and Phoenix bodies with the Bingham porphyry copper depo sit, this relationship could be of use in evaluation of the mineral potenti al of felsic intrusive rocks.