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Timing and characteristics of skarn-hosted copper-gold mineralization at the Blue Copper project, Ophir district, Montana, USA
Shervey, Erin M.
Shervey, Erin M.
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2025
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The Blue Copper Cu-Au prospect, located in the Ophir District on the northern border of the Boulder batholith in Montana, USA, hosts several mineralized skarn zones. These zones are developed along the contact between the Blackfoot City granodiorite intrusion and carbonate rocks of the Mississippian Madison Group, Devonian Jefferson Formation, and the Cambrian Meagher and Pilgrim formations. Characteristics of skarn alteration and copper mineralization are poorly constrained in this district. Geologic mapping, petrographic analyses, fluid inclusion analysis, and radiometric age dating were carried out to constrain formation conditions and to characterize mineralogical and geochemical of alteration and mineralization at Blue Copper. Outcropping skarn zones vary from 3 to 15 m thick and can be traced along several hundreds of meters of strike length. Skarn alteration is often a proximal prograde skarn comprised of red to dark red garnet and minor copper sulfides. Other skarn types observed include pyroxene dominated skarn, retrograde skarn, magnesian skarn, and endoskarn Garnet samples and zircons from intrusive units were dated by LA-ICP-MS U-Pb geochronology. Garnets yielded ages ranging from 82 ± 3 Ma to 85 ± 2 Ma, indicating older skarn alteration than previously reported within the Boulder batholith. Zircon samples yielded ages from 77.4 ± 0.9 Ma to 81.4 ± 0.8 Ma, indicating multiple phases of intrusive emplacement
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