Geology of the Mercur Gold Mine, Oquirrh Mountains, Utah

Abstract

Abstract Production from the Mercur mine was completed in March of 1997. From September 1982 through March 1997, over 2 million ounces of gold were produced from 22 million tons of ore. Gold production prior to 1982 is estimated to be approximately 1.5 million ounces. The gold deposits of the Mercur mine are situated in the southwest Oquirrh Mountains, approximately 30 miles south of Tooele, Utah. The gold deposits of Mercur lie along the eastern limb of the NNW-trending Ophir anticline. The anticline is one of a series of NNW-trending folds that have an asymmetrical shape due to folding of upper plate rocks during Sevier-age thrusting. The Ophir anticline plunges both to the north and south, forming a dome. The Mercur, Sacramento, Marion, and Golden Gate pits were the main deposits developed at Mercur. Mineralization developed along east-to northeast-striking faults and the north- to northwest-striking Mercur fault. Dilation and extension are seen along the east- to northeast-striking faults. It is postulated that the east-to northeast-striking faults represent extensional features that began with the doming of the Ophir anticline. The north- to northeast-striking Mercur fault is inferred to be related to the regional folding. Gold mineralization at Mercur occurs as sediment-hosted, disseminated (Carlin-type) deposits. Mineralization is hosted in a 200- to 240-ft sequence of thin-bedded, fossiliferous limestones and siltstones that form the top of the otherwise micritic Topliff Member of the Mississippian Great Blue Formation. This sequence of rocks, known locally as the Mercur Mine Series, is conformably overlain by the Long Trail Shale Member, which acted as an impermeable cap to the deposits. Intense oxidation, decalcification, argillization, and silicification are evident. Many parts of the deposits remain unoxidized. Carbon flooding is also present. Fracturing and decalcification were critical in preparation of the host rocks, increasing permeability and allowing access for mineralizing fluids. Calcite and quartz veinlets are common in the deposits, along with sulfides which include pyrite, realgar, and orpiment. Jasperoid is present in many areas as a conformable mass along the base of, and extending up into, the lower half of the Mercur Mine Series. The Porphyry Peak granodiorite (36.7 Ma) crops out north of Mercur as a series of NNW-striking dikes. The Eagle Hill rhyolite (aplite; 31.6 Ma) forms a combination sill and dike complex that bounds the Sacramento deposit on the south and west sides. Several hydrothermal breccias have been encountered at Mercur, the largest of which, the Thickener Breccia, accounted for over 90,000 ounces of gold production. The breccia is a heterolithic assemblage of rocks belonging to the Mercur Mine Series as well as to units above and below the Mercur Mine Series. Also present in the breccia are clasts of the two igneous rock types present in the district. Gold mineralization at Mercur is interpreted to have taken place after the emplacement of the Eagle Hill rhyolite. The presence of igneous clasts in the mineralized Thickener Breccia and the presence of rhyolite in a mineralized collapsed flower structure in the Sacramento pit suggest possible post-intrusion age mineralization.