2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 15
Presentation Time: 11:55 AM


NUTT, C.J., U.S. Geol Survey, P.O. Box 25046, M.S. 905, Denver, CO 80225 and HOFSTRA, A.H., U.S. Geological Survey, P.O. Box 25046, M.S. 905, Denver, CO 80225, cnutt@usgs.gov

The intrusion-related Au deposits at Bald Mountain are in Cambrian-Ordovician and Mississippian sedimentary rocks and Jurassic intrusive rock. Reconstruction and fluid inclusions indicate they formed at depths of 6–3 km. The 159–Ma Bald Mountain stock and associated ore deposits are localized by a NW–fault zone. The isotopic composition and mineralogy of the I–type quartz monzonite porphyry stock indicate that magma evolved from an oxidized to reduced state by assimilation of country rocks. The deposits occur in a concentric mineral and geochemical zonation pattern around the stock. Oxide and sulfate minerals are absent. Proximal deposits at 6 and 3 km depth are enriched in Cu, Au, Bi and Te and contain chalcopyrite and native gold. Distal deposits at 6 km depth are low in base metals and contain disseminated auriferous-arsenopyrite and –pyrite with stibnite. At ~6 km, fluid inclusions are carbonic or aqueous, but at 3 km, they are hypersaline or vapor-rich. d18 O and d34 S data suggest the deposits formed from magmatic fluid that obtained S from country rocks. The larger proportion of Au and smaller amounts of Cu and base metals in the ores indicate they formed from H2 S-rich fluids. They are similar to reduced intrusion-related Au deposits.

The Carlin-type Au deposits at Alligator Ridge are distributed for 40 km along the N-striking Mooney Basin fault zone, but are restricted to a 250 m interval of Devonian to Mississippian (D-M) strata. Their age is bracketed between silicified ~45 Ma sedimentary rocks and unaltered 36 to 34 Ma volcanics; the silicification is linked to ore by its similar d18 O values and trace element signature. Reconstruction indicates the deposits formed at £ 300 to 800 m. Mineralogy, silica textures, fluid inclusions, and stable isotopic data indicate they formed from meteoric water (MW) that became an H2 S-bearing ore fluid via reactions with sedimentary rocks at low W/R ratios and elevated T with ore deposition at T=230º to 100ºC. Deep circulation of MW along the Mooney Basin fault zone may have been due to initial uplift of the Ruby Mountains metamorphic core complex. In comparison to other Carlin-type Au deposits, they have lower Au/Ag, Au grades, and Au contents, more abundant jasperoid, and evidence for deposition of amorphous silica. These differences likely result from their shallow depth of formation.