Abstract

The Golden Sunlight gold-silver telluride deposit, hosted primarily within the Mineral Hill breccia pipe, is spatially related to a high-level, Late Cretaceous multiphase, alkaline to subalkaline porphyry system (latite porphyries, quartz monzodiorite, and lamprophyres). Base metal veins and manganese (rhodochrosite) mineralization occur up to 2 km from the Mineral Hill breccia pipe and form part of a regional mineral zonation pattern genetically related to a low-grade porphyry molybdenum system. Alteration associated with this system consists of a proximal zone, pervasive in style (disseminations and veinlets), that is core to the alkaline igneous rocks and a distal zone that is vein controlled. Proximal alteration is composed of an early barren zone of quartz, hematite, pyrite, and barite as well as a pervasive zone composed of quartz, sericite, K feldspar, base metal sulfides, tellurides, molybdenite, and native gold. Distal alteration is characterized by the same mineralogy as the precious and base metal zone of the proximal zone except that it is vein controlled and does not contain molybdenite; a base metal and rhodochrosite zone occurs farthest from the center of the porphyry system. K feldspar alteration occurs as selvages to quartz-molybdenite mineralization and as a pervasive replacement of clasts in the Mineral Hill breccia pipe (early), whereas quartz-sericite alteration is dominantly associated with auriferous veins and the breccia pipe (late).Molybdenite occurs in high veinlet-density quartz veins at the margins and deeper portions of the breccia pipe, in clasts in prebreccia pipe quartz-pyrite-K feldspar veinlets, as fine-grained disseminations in breccia fragments, and as rims to fragments of xenolithic latite. Previously four periods of hypogene mineralization in the breccia pipe were identified. Stages I and IV constitute [asymp] 99 percent of the mineralization; native gold, calavertite, and Bi tellurides-sulfosalts occur in stage I whereas Au-Ag tellurides developed in stage III. Minor amounts of base metals are present in stage II whereas barite, dolomite, magnesite, trace kaolinite, and sericite develop in stage IV. Proterozoic rocks of the LaHood Formation and the informally named Bull Mountain group host the breccia pipe and contain strata-bound sulfides-sulfosalts (up to 50% pyrite with minor to trace amounts of chalcopyrite, tennantite, pyrrhotite, sphalerite, galena, and molybdenite).Fluid inclusion homogenization temperatures of primary and pseudosecondary aqueous liquid-vapor inclusions (constant liquid to vapor ratios) in quartz-pyrite-molybdenite veins range from 131.8 degrees to 398.2 degrees C. Primary and pseudosecondary inclusions with highly variable liquid-vapor ratios that coexist with rare three-phase CO 2 -H 2 O and multiphase inclusions (one or more solid phases in addition to liquid and vapor) also occur in quartz-pyrite-K feldspar-molybdenite veins and indicate periods of intermittent boiling. Homogenization temperatures for fluid inclusions in stage I auriferous pyrite veins range from 145 degrees to 345 degrees C with salinities of [asymp] 1 to 10 wt percent NaCl equiv. Values of delta 34 S for sulfides in the Mineral Hill breccia pipe, quartz-pyrite-K feldspar-molybdenite veins, Proterozoic sedimentary rocks, and distal base metal veins range from -12.2 to 3.1 per mil, -8.1 to 0.8 per mil, 0.7 to 6.1 per mil, and -8.9 to 9.5 per mil, respectively, and suggest a mixed magmatic-sedimentary sulfur source. Calculated delta 18 O values for water in equilibrium with late-stage sericite at 170 degrees C vary from 0.7 to 2.8 per mil, whereas delta D water values range from -67 to -8 per mil.Geologic, paragenetic, fluid inclusion, and stable isotope studies are consistent with an early magmatic fluid, associated with quartz-pyrite-K feldspar-molybdenite veins, that subsequently mixed with meteoric water during stages I to IV breccia pipe-hosted and auriferous pyrite vein formation. Ore-forming components (e.g., Au, Ag, Te, Cu, Bi, Mo, and much of the S) were most likely derived from the Late Cretaceous intrusive system with possible contributions from the Proterozoic host rocks.

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