METALLIC MINERAL DEPOSITS AT BULL MOUNTAIN: A RELICT OF MESOZOIC ARC MAGMATISM IN SOUTHWESTERN MONTANA

A remnant of the Late Mesozoic Cordilleran arc is preserved at Bull Mountain (BM), a Miocene Basin and Range horst located north of Whitehall, MT ~40 km east of Butte. We report results from studies of metallic minerals in dikes, breccia, and veins in the BM fault block. At the southern end, near Whitehall, gold ore formed in Elkhorn Mountains Volcanics rhyolite breccia between ~80 and 77 Ma at the Golden Sunlight Mine. 25 km to the north, metallic minerals occur in high-K mafic dikes and sills that follow NE- and NS-striking fissures, and a 20-30° east-dipping volcanic foliation in the Elkhorn Mountains Volcanics. Formation of the mafic dikes and sills at BM is constrained by an age of 78.57 +/- 0.17 Ma (⁴⁰Ar-³⁹Ar on hornblende) from a diorite sill. Chemical analyses of mafic dikes and sills show Sb and Te enrichment of 50-250 times that of average basalt. Sb and Te enrichments suggest that the system may have contributed to development of metallic mineral veins. Metallic mineral veins also occur in marble at the northeast corner of BM, where they follow the east margin of the Boulder Batholith. The veins contain galena, sphalerite, and pyrite. Spot analyses (n=5) of metallic vein material using pXRF yielded average concentrations of 2.0 +/- 0.04 wt. % Pb, 4.9 +/- 0.10 wt. % Zn, and 55 +/- 14 ppm Ag. One sample contained 110 +/- 28 ppm Au. Metallic mineral veins likely formed between 78.6 and 76.5 Ma in the northern part of BM., an interval of arc evolution marked by mafic magmatism prior to emplacement of the main pluton (Butte Granite) of the Boulder Batholith.