Sierra Madre Mining District, Box Elder County, Utah, USAi

Regional Level Types
Sierra Madre Mining District	Mining District
Box Elder County	County
Utah	State
USA	Country

The Sierra Madre mining district is a minor district in the Wasatch Range about 3 mi southeast of Willard and lying mostly in Box Elder County. The district stretches along the Wasatch Front for over 5 mi, including a few prospects in adjoining Weber County on the southwest face of Ben Lomond Peak. The district was developed in very steep terrain and some of the workings were connected by two long, aerial tramways in the early 1900s. However, despite splashy reports in the Salt Lake Mining Review, the district had very limited production of Cu-Au-Ag (205 tons of ore) and only a couple of car loads (less than 100 tons of ore) of Pb-Zn-Ag (Butler and others, 1920; Doelling, 1980). Total district metal production at modern metal prices is estimated at $16,000. None of the mines had any appreciable production.
The Wasatch Range is a part of the Middle Rocky Mountain province. The lower section of the Sierra Madre district is Paleoproterozoic (~1.8 Ga) Farmington Canyon Complex orthogneiss. The gneiss is a light-gray, medium-grained, massive to weakly foliated, biotite-hornblende quartz monzonite cut by narrow, ptygmatic pegmatite dikes and quartz veins. The gneiss is unconformably overlain by Lower and Middle Cambrian Tintic Quartzite, Ophir Formation shales and silty limestone, and Maxfield Limestone.
Mineralization occurs mostly as veins in Paleoproterozoic rocks and to a lesser extent as replacement deposits in the Middle Cambrian Ophir Formation and Maxfield Limestone. Narrow Cu-Ag fissures in the gneiss contain minor pyrite, chalcopyrite, and bornite with minor sericite replacing the original hornblende and biotite. Replacement mineralization is reported in a lower, white, coarse-grained dolomite in the Ophir Formation or basal Maxfield Limestone. Fine-grained galena, sphalerite, and pyrite occur with a gangue of quartz and sericite in small, irregular lenses (Butler and others, 1920; Doelling, 1980). Assays of ore from the upper El Dorado mine average about 5% Pb, 6% Zn, and 10 ppm Ag.