Mount Baldy-Ohio Mining District, Piute County, Utah, USAi

Regional Level Types
Mount Baldy-Ohio Mining District	Mining District
Piute County	County
Utah	State
USA	Country

The Mount Baldy–Ohio mining district is located on the east flank of the Tushar Mountains of Piute County about 5 mi southwest of Marysvale. The district is a large Au-Ag producer having significant Zn-Pb. The district ranks as roughly the sixth largest Au producer in Utah. Total district metal production at modern metal prices is estimated at $333 million. The Deer Trail underground mine on the eastern base of Deer Trail Mountain is by far the most important producer.
The Mount Baldy–Ohio district covers part of the Marysvale volcanic field in the transition zone from the Basin and Range Province to the west and the Colorado Plateau to the east. Upper Paleozoic and Mesozoic sedimentary strata occur along the eastern base of the range and are unconformably overlain by rocks of the Marysvale volcanic field. The volcanic rocks are apparently domed by an unexposed Miocene Mount Belknap alkali rhyolite plug (14 Ma) under Deer Trail Mountain (Beaty and others, 1986). The mineral deposits of the district show a crude, overlapping, but consistent vertical zonation from alunite deposits at the top of the mountain above 10,800 ft elevation, to small, epithermal Au-Ag veins containing minor base metals above 9200 ft, to polymetallic veins and replacement deposits below 9200 ft. Most of the production in the district (about 80%) comes from the precious-metal-rich, polymetallic replacement ores in the Permian Toroweap Formation at the Deer Trail mine on the east face of Deer Trail Mountain (USGS Model 22c). The Deer Trail ore averaged approximately 12% Zn, 5% Pb, 514 ppm Ag, and 3.43 ppm Au. The common ore minerals are sphalerite, galena, tennantite, tetrahedrite, chalcopyrite, and rare native gold. Trace elements in Deer Trail massive sulfide ores average roughly 1.5% F, 1000 ppm As, 1000 ppm Sb, 100 ppm Te, 100 ppm Se, 50 ppm Mo, 10 ppm Bi, and 3 ppm Hg. Four U.S. Geological Survey National Geochemical Database rock samples in Bullion Canyon report over 100 ppm Mo (150, 300, 300, and 500 ppm) with one sample also having strongly anomalous Sn, 150 ppm. Wulfenite is reported from several of the mines in the district (Bullock, 1981). Beaty and others (1986) envision a pair of porphyry cupolas west of the Deer Trail manto and under the alunite veins and structural dome near the top of Deer Trail Mountain.
The Mo and fluorite associated with the Deer Trail manto, the high Mo and Sn in Bullion Canyon, and the rhyolite dikes associated with the alunite mines all indicate a possible porphyry Mo signature. The porphyry system is likely under the alunitic alteration zones at Deer Trail Mountain and/or Mt. Brigham (Beaty and others, 1986). Because of the large amount of relief in the district, a deposit 3000 ft deep would still be accessible via a horizontal tunnel from near the Deer Trail mine.