LATE CRETACEOUS MAGMATISM AND MINERAL DEPOSITS OF THE ELKHORN 7.5-MINUTE QUADRANGLE, JEFFERSON COUNTY, MONTANA

1:24,000-scale geologic mapping in the Elkhorn 7.5′ quadrangle provides context for the timing of magmatism and mineralization in the Late Cretaceous Boulder Batholith of southwestern Montana. Mineral deposits occur in a 2 km-thick section of Mesoproterozoic to Mesozoic sedimentary and igneous rocks, including 750- m of Late Cretaceous Elkhorn Mountains Volcanics (EMV). Mined ore deposits are found in low-grade Mo-Cu porphyry type, gold and iron skarns, breccia pipes, and carbonate replacement deposits. Deposit types include: (1) Porphyry type – The Turnley Ridge stock in the heart of the Elkhorn district is the only porphyry system exposed in the quadrangle. The stock is a quartz monzonite system that crystallized around 85.5 Ma and is associated with basaltic to andesitic volcanism persisting until about 83.6 Ma. The stock is a weak single-phase porphyry system based on the orthoclase and quartz-rich central portion cropping out on the surface and the weak grades. (2) Breccia pipe deposits – The Skyline mine is located at the northern edge of the Elkhorn mining district at the head of Queen Gulch, southeast of Crow Peak. The breccia pipe cuts and is hosted in andesitic to dacitic welded tuff and breccia deposits. The andesite – dacite rocks are up to 450 m thick and represent the Middle Member EMV, a regionally extensive package of caldera-forming ignimbrites; (3) Skarn deposits – There are two types of skarn deposits within the Elkhorn quadrangle: reduced gold skarns and reduced iron skarns. Skarn deposits are hosted by carbonate rocks near the margins of diorite and granodiorite intrusions. Pyroxene- and hornblende-bearing mafic stocks at Black Butte and Cemetery Ridge formed at about 80 Ma and align on a northwest-striking fissure north of Elkhorn; and (4) Carbonate replacement deposits – mineral deposits that formed proximal and outward of skarn deposits near the margin of the 76.1 Ma Butte Pluton, the main mass of the Late Cretaceous Boulder Batholith. Most historic silver and base-metal production came from ore in carbonate replacement deposits at the Elkhorn, C&D, and Sophia mines. All country rocks are brecciated, assimilated, and contact metamorphosed along the length of their contact with the massive 76.1 Ma Butte Pluton. This final orogenic event may have upgraded any pre-existing mineral deposits.