Petrologic and stable isotope study of the gold-bearing breccia pipe at the Golden Sunlight Deposit, Montana

Abstract

The Golden Sunlight, a gold-silver deposit in the Whitehall mining district, Jefferson County, Montana, is centered on a silica-cemented breccia pipe with fragments of both Belt Supergroup sedimentary rocks and a latite porphyry. Native gold and gold tellurides occur finely disseminated within gangue minerals that have filled open spaces in the breccia pipe.Alteration in and around ( approximately 0.8 km radius) the pipe is principally quartz-sericite. Within the pipe, breccia fragments are also characterized by a highly silicified outer rind. Openspace filling is divided into four periods as follows according to the presence of the following minerals: (I) quartz and pyrite, with minor hematite, chalcopyrite, bornitc, and gold minerals; (II) chalcopyrite, with lesser amounts of galena, sphalerite, and martasite; (III) additional gold-silver tellurides and tennantite; and (IV) barite, quartz, and sericite with minor carbonates and pyrite. Isotopic study shows delta 34 S values of sulfides between -12.2 and +3.1 per mil, whereas barite values range from 1.2 to 5.9 per mil. Oxygen isotope values of whole-rock breccia samples range from 6.5 to 13.8 per mil, quartz from 10.5 to 14.3 per mil, sericite from 9.7 to 15.5 per mil, and unaltered country rock from 12.3 to 16.6 per mil. Fluid inclusion homogenization studies indicate a temperature of deposition for period I mineralization of approximately 200 degrees C. The coexistence of sylvanitc and petzite in period III suggests a temperature less than 170 degrees C. Values of delta 18 O (sub H 2 O) computed from quartz and temperature data and sericite delta 18 O values range from -2.4 to +4.0 per mil. Hydrogen isotope values of fluid inclusions in period I quartz and water in equilibrium with late sericite fall betweem -56 and -33 per mil. Fluid inclusion freezing study of one sample of period I quartz suggests a fluid salinity of <1 equiv. wt percent NaCl.Isotopic and geologic evidence is consistent with tbrmation of a breccia pipe caused by exsolution of a vapor from a crystallizing magma. Oxygen and hydrogen isotope data suggest that extensive isotopic exchange between a magmatic fluid and igneous rocks occurred prior to open-space filling. Initial brecciation was accompanied by an isoenthalpic temperature decrease, which coupled with conductive cooling led to the precipitation of large amounts of silica. Textural features indicate that brecciation and fracturing, followed by fluid infiltration, occurred episodically. Sulfur isotope data is also consistent with derivation of sulfur from a magmatic source characterized by a delta 34 S (sub Sigma s) value between 0 and 5 per mil. Evidence from mineral assemblages and sulfur isotopes suggests that gold was most likely transported as a chloride complex and was deposited due to a pH increase accompanied by decreases in f (sub O 2 ) and possibly temperature.