Bull Mountain-Elkhorn

Region West, Northwest
Mineral systems
Deposit types
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Magmatic hydrothermal, terrestrial hydrothermal (Boulder Batholith and Elkhorn Mountains Volcanics) including precious-base metal veins and skarns. The focus area is of conspicuous economic importance. Bull Mountain has historical precious and base metal producers located at both its southern and northern ends, at the Golden Sunlight mine and Elkhorn district, respectively. Specifically, the Golden Sunlight mine is a Late Cretaceous multiple intrusive, alkaline to subalkaline porphyry system (Spry and others, 1997).
Identified resources Identified resources of copper and gold. Historical production of gold.
Production Elkhorn mines (1881-1899, intermittently in the 1930s): produced over $15 million in ore from deposit types that include: Ag, Pb, Zn (Au) carbonate replacement deposits, Fe-oxide skarns, sulfide-rich and sulfide poor Au-Bi skarns, tourmaline-bearing breccia pipes and an unmined porphyry Mo (Cu) deposit (Brown, Gammons and Poulson, 2019); Golden Sunlight (1983-2019): the largest Au producer in Montana, has poured over 3 million ounces of gold since 1983 (Oyer and others, 2014).
Status Past mining; no active mining at historical producers; recent exploration.
Estimated resources Exploration drilling between 1984 and 1993 delineated an open pit geologic resource of 1,935,750 oz at a grade of 0.073 opt Au and 20,568,000 lbs Cu (Everson and others, 2005). In 2014, the Golden Sunlight mine had Proven and Probable reserves of 318,000 oz Au (Oyer and others, 2014).
Geologic maps Lewis (1998a), scale 1:250,000; Weeks (1974), scale 1:48,000; Scarberry (2016), scale 1:24,000; Scarberry and others (2017), scale 1:24,000; Olson and others (2016), scale 1:24,000; Klepper and others (1957), scale 132,250; Prostka (1966), scale 1:24,000; Mahoney and others (2008), scale 1:24,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Silver Nail pluton (Kelsey porphyry and skarn).
Deposits Golden Sunlight mine (MRDS dep_id: 10148740), Elkhorn (MRDS dep_id: 10069281).
Evidence from mineral occurrences MRDS.
Geochemical evidence The focus area formed in a Continental Arc geologic environment. Critical metals are associated with the primary metals (Au, Ag, Cu) that form in these settings. Mineralogy at the Golden Sunlight mine discussed in Spry and others (1997).
Geophysical evidence Unknown.
Evidence from other sources None.
Comments Existing studies suggest that it’s conceivable that “blind” magmatic hydrothermal ore deposits, specifically ~80 Ma quartz porphyry and breccia pipe ore bodies that contain concentrated As, Sb, Te, and Bi, and are buried beneath the Elkhorn Mountains Volcanics in the Bull Mountain region between the Elkhorn and Golden Sunlight mines. A uniform igneous stratigraphy in the region will help target an area for a modern aeromagnetic study.
Cover thickness and description Exposed at the surface and extends to unknown depth.
Authors Kaleb C. Scarberry, Stanley L. Korzeb, Jay A. Gunderson.
New data needs Geophysics, lidar, geochemistry, geologic mapping, trace element analysis on the mafic intrusions, skarns, and veins for critical elements.
Geologic mapping and modeling needs Updated geologic mapping.
Geophysical survey and modeling needs High-resolution, Rank 1 aeromagnetic and radiometric surveys.
Digital elevation data needs Lidar incomplete.