Geologic and isotopic investigation of the South Willow creek gold prospect, Madison County, Montana

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6326836

Saunders, M M; Ripley, E M

The South Willow Creek gold prospect is located in the southwestern part of the Potosi tungsten district, southwest Montana. Gold-quartz veins occur in the Bismark shear zone within granodiorite host rocks of the late Cretaceous-early Tertiary Tobacco Root batholith. Four well developed zones of progressive alteration are found adjacent to faults and veins in the area. Gold occurs in solid solution with silver as inclusion in pyrite and alone with quartz. The highest gold assay values occur closest to the Bismark shear zone. Sulfur isotope values for pyrite, galena, and molybdenum range from -6.5 to +1.7 per thousands. Pyrite-galena and pyrite-molybdenum ..delta.. values indicate sulfide deposition temperatures ranging from 220 to 265/sup 0/C. Isotopic values suggest that deposition occurred from a fluid with a delta/sup 34/S value of near 0 per thousand, at redox conditions near those of the ..sigma..sulfate/..sigma..sulfide boundary. Mineral assemblages and delta/sup 34/S values are consistent with gold transport as a chloride complex under acid oxidizing conditions. Increases in pH and decreases in temperature accompanying wallrock alteration are the likely causes of gold deposition. Sericite deltaD values are variable, ranging from -69 to -136 per thousand. Computed delta/sup 18/O/sub H/sub 2/O/ and deltaD/sub H/sub 2/O/ values are most easily interpreted as indicating a predominantly magmatic fluid source, with locally variable contributions of meteoric water. However, non-equilibrium delta/sup 18/O values of coexisting quartz and sericite, and the deltaD values of sericite, both suggest that isotopic exchange rates were variable, and limit the unambiguous determination of fluid source.

• Research Organization: Indiana Univ., Bloomington (USA)
• OSTI ID: 6326836
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

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CENOZOIC ERA
CHALCOGENIDES
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
CHEMICAL REACTIONS
CHEMISTRY
CRETACEOUS PERIOD
FEDERAL REGION VIII
FLUIDS
GALENA
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC FAULTS
GEOLOGIC FRACTURES
GEOLOGIC STRUCTURES
GEOTHERMAL FLUIDS
GEOTHERMAL SYSTEMS
GEOTHERMOMETRY
GOLD ORES
GROUND WATER
HYDROGEN COMPOUNDS
HYDROTHERMAL ALTERATION
IRON COMPOUNDS
IRON SULFIDES
ISOTOPE RATIO
ISOTOPES
LEAD COMPOUNDS
LEAD SULFIDES
MAGMA SYSTEMS
MAGMATIC WATER
MESOZOIC ERA
MINERALIZATION
MINERALOGY
MINERALS
MOLYBDENUM ISOTOPES
MONTANA
NORTH AMERICA
ORES
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
OXYGEN ISOTOPES
PH VALUE
PYRITE
QUANTITATIVE CHEMICAL ANALYSIS
QUARTZ
REDOX REACTIONS
ROCK-FLUID INTERACTIONS
SILICON COMPOUNDS
SILICON OXIDES
SULFIDE MINERALS
SULFIDES
SULFUR COMPOUNDS
TERTIARY PERIOD
TRANSITION ELEMENT COMPOUNDS
USA
WATER