Mineralogy and sulfur isotope geochemistry of polymetallic, porphyry-epithermal mineralization peripheral to the Golden Sunlight gold mine, Montana

Abstract

The Golden Sunlight mine, located 50 km east of the famous Butte porphyry/lode deposit, is the largest gold mine in Montana, and has produced over 3.6 M oz of gold in its 36-year history. Most of this gold has come from the Mineral Hill breccia pipe (MHBP), a west plunging, cylindrical body of brecciated latite porphyry and siliciclastic sedimentary rocks of the Mesoproterozoic Belt Supergroup. This paper focuses on four areas of mineralization outside the MHBP termed the Apex, Bonanza, North Pit, and 102/Sunlight zones. The Apex and 102/Sunlight areas consist of auriferous, pyrite-rich veins cutting Precambrian sediments and the Cambrian Flathead Quartzite, whereas the Bonanza and North Pit areas are polymetallic Mo-Cu-Ag-Au prospects hosted in latite porphyry and Belt sediments. Recent dating (40Ar/39Ar of igneous biotite) shows the porphyry at Bonanza was emplaced at 78 ± 1 Ma, which is several m.y. younger than the porphyry associated with the MHBP (81.9 ± 1.9 Ma, zircon U/Pb), and several m.y. older than the main phase of the Boulder Batholith that hosts the Butte deposit.Overall, the ore and gangue mineral assemblages in the peripheral deposits are similar to those described previously for the MHBP deposit. Pyrite, locally As-rich, is abundant in all settings, with varying amounts of galena, sphalerite, chalcopyrite, bornite, molybdenite (at Bonanza), tetrahedrite-tennantite, and traces of pearceite, tetradymite, aikinite, calaverite, petzite, goldfieldite, buckhornite, gold and electrum. Gangue minerals include quartz, carbonates (Fe-dolomite, siderite, magnesite), barite, anhydrite, adularia, and alumino-phosphate sulfate (APS) minerals. Spectral analysis indicates that some of the adularia is ammonium rich, i.e., buddingtonite.Values of δ34S for sulfide minerals from the peripheral deposits overlap with values obtained by previous workers from the MHBP. The bulk of the analyses (including all samples from the porphyry-hosted Bonanza deposit) have δ34S values between −12 and −4‰, with a tail to heavier values >+5‰. It is postulated that most of the lighter S was introduced into the district by late Cretaceous magmatic/hydrothermal fluids, whereas the heavier S is attributed to sedimentary pyrite in the Precambrian Greyson and LaHood formations. A weak negative correlation exists between gold grade and the δ34S of associated pyrite: all samples in this study that contain >0.1 oz/ton Au have δ34S < −5.9‰. Two possible explanations for the isotopically light S at Golden Sunlight are explored: 1) assimilation of biogenic pyrite from underlying sedimentary units; and 2) fractionation of S isotopes between SO4 and H2S in a system where oxidized S > reduced S.