STRUCTURAL AND MAGMATIC DEVELOPMENT OF LATE CRETACEOUS MAGMAS AND MINERAL DEPOSITS IN THE ELKHORN 7.5’ QUADRANGLE: RESULTS FROM EARTH-MRI (MAPPING RESOURCES INITIATIVE) GEOLOGIC MAPPING IN SOUTHWESTERN MONTANA.

2 km of Mesoproterozoic to Mesozoic sedimentary and igneous country rocks, including about 750 m of the Late Cretaceous Elkhorn Mountains Volcanics (EMV), are deformed and mineralized in the Elkhorn 7.5′ quadrangle in southwestern Montana. Field studies provide insight into deformation and magmatism between 85.5 - 76.1 Ma. New mapping, bulk geochemical data (50), and U-Pb zircon ages (7) define four stages in the geologic evolution of a Cretaceous magma system that hosts historic production from porphyry type, breccia pipe deposits, skarns, and carbonate replacement deposits. The four stages are: (1) 85.5 – 83.6 Ma – Quartz porphyry intrusion (Kqp) and mafic volcanism (Keml) – Emplacement of quartz monzonite porphyry and eruption of basaltic to andesitic lavas, tuffs and breccia of the Lower Member EMV. Angular unconformity between the EMV and older country rocks indicates that shortening was underway before magmatism started at about 85 Ma; (2) 83.6 – 80.3 Ma – Intermediate and silicic volcanism (Kemm) – Andesitic to dacitic tuff of the Middle Member EMV, a regional package of caldera-forming ignimbrites that are up to 450 m thick in the quadrangle; (3) 80.2 – 79.5 Ma – Diorite and granodiorite intrusion (Kdi) – Pyroxene- and hornblende-bearing mafic intrusions, including stocks, dikes, sills, and breccia at Black Butte and Cemetery Ridge; (4) 76.1 Ma Granite batholith (Butte Pluton) emplacement (Kbg) – Country rocks older than the 76.1 Ma Butte Pluton are brecciated, assimilated, and contact metamorphosed along the length of the massive intrusion. The Butte Pluton intrudes Mesoproterozoic Belt metasedimentary rocks (Ybem) through Pennsylvanian - Permian sandstone to quartzite (PPq) over a distance of about 8 km south of Elkhorn Peak. Emplacement of the Butte Pluton into folded country rocks at 76.1 Ma drove contact metamorphism, faulting, and mineralization related to reaction of ore fluids with carbonate country rocks in high-permeability NE-SW-trending fold hinges.