MINERALOGY AND SULFUR ISOTOPE GEOCHEMISTRY OF THE APEX AND BONANZA PROSPECTS AT THE GOLDEN SUNLIGHT MINE, MONTANA

The Golden Sunlight mine, located 50 km to the east of the famous Butte porphyry/lode deposits, is the largest gold mine in Montana, and has produced over 3 million ounces of gold in its 35+ year history. Most of this gold has come from the Mineral Hill breccia pipe (MHBP), a west dipping, cylindrical body of brecciated latite and country rock fragments of the Precambrian LaHood and Greyson Formations. The breccia pipe is late Cretaceous in age (84±18 Ma, DeWitt et al., 1986), is silicified, pyrite-rich, and is mineralized with gold, silver, and minor base metals. Because the MHBP has largely been mined out, the present owners of the mine, Barrick Gold, have been exploring new prospects called Apex and Bonanza to the north of the main MHBP ore body. The Apex deposit exists near the unconformable contact between the Proterozoic Greyson Fm. and the overlying Cambrian Flathead Fm. Both sedimentary units are mineralized, although exploration has focused more on structures in the Greyson formation. The Bonanza prospect is exploring the contact between latite porphyry and the Greyson Formation. The purpose of the present study is to examine the mineralogy and stable isotope geochemistry of Apex and Bonanza to see how it relates to the MHBP. Reflected and transmitted light microscopy and SEM-EDS analyses of polished core samples from Apex and Bonanza reveal a complex paragenesis of ore and gangue minerals that is similar to that described by previous workers for MHBP. Pyrite is the dominant sulfide, accompanied by common sulfides such as galena, sphalerite, chalcopyrite, bornite, and tetrahedrite-tennantite, as well as less common phases such as pearceite (Cu(Ag,Cu)6Ag9As2S11), tetradymite (Bi2Te2S), aikinite (CuPbBiS3), goldfieldite (Cu10Te4S13), and electrum (Au-Ag alloy). Gangue minerals include quartz, barite, anhydrite, adularia, dolomite, siderite, sericite, rutile, althausite and magnetite. Analyses of drill core with a Terraspec Halo SWIR device reveal that illite, muscovite and kaolinite are the dominant alteration minerals, with several detections of the NH4-rich K-feldspar, buddingtonite. The S-isotope composition of pyrite and molybdenite in veins from the North Pit, Bonanza and Apex deposits overlap with pyrite from the MHBP (as determined by previous workers). In the deposits, there is a cluster of δ34S values in the range of -10 to -5 ‰, with a tail to more positive values \u3e 0 ‰. The heavier S appears to be associated with sedimentary pyrite in the Precambrian Belt sediments whereas the lighter S is of hydrothermal origin. The fact that the mineralogy and S-isotopic compositions at Apex, Bonanza, and MHBP are similar implies that the deposits formed in the same ore-forming event. The light S-isotope composition of sulfides at Apex and MHBP may have been caused by separate processes: 1) assimilation of isotopically light sulfide at a deeper crustal level by ascending magmas and/or 2) isotopic fractionation between sulfide and sulfate in an oxidized (SO2-stable) environment