Tecoma Mining District, Box Elder County, Utah, USAi

Regional Level Types
Tecoma Mining District	Valley
Box Elder County	County
Utah	State
USA	Country

The Tecoma (Jackson) mining district straddles the Utah-Nevada state line in extreme west-central Box Elder County 100 mi due west of Brigham City. The district has a modest history as a Pb-Ag producer, but all production has come from the Nevada portion of the district (Hill, 1916). Sediment-hosted Au exploration in the late 1970s and early 1980s resulted in the discovery of jasperoid hosting the TUG (Tecoma Utah Gold) Au-Ag deposit in 1980. Delineation drilling of TUG has continued intermittently to the present day.
The Tecoma district lies on the southern termination of the Grouse Creek Mountains of the Basin and Range Province. The unmined TUG distal disseminated Au-Ag deposit (USGS model 19c) is the most important mineralization in the Tecoma district. The TUG mineralization is localized along a detachment fault separating the underlying Devonian Guilmette Formation from the overlying Mississippian Chainman Shale and Diamond Peak Formation. The fault/ore dips about 25° to the northeast. The overlying Mississippian Chainman Shale and Diamond Peak Formation are considerably attenuated by Miocene-Pliocene-age low-angle faulting which cuts out several hundred feet of section including the Devonian-Mississippian Tripon Pass/Pilot Shale (Selway and others, 2012). The ore-bearing fluids at TUG rose along northwest-trending, steeply northeast-dipping normal faults and flooded out at the intersection with the low-angle fault and more permeable section. Quartz monzonite porphyry dikes commonly occupy these northwest-trending faults. Alteration consists of dolomitization of the Guilmette Formation carbonate rocks, silicification ±barite of the Guilmette Formation and Chainman Shale, destruction of carbonaceous material in the Chainman Shale, and argillization of the Miocene (?) quartz monzonite porphyry dikes (Douglas and Oriel, 1984). Mineralization in the jasperoid is associated with barite, pyrite, arsenic-oxides, dussertite, aguilarite, and acanthite.
About 70% of the Au-Ag ore in the TUG deposit is in silicified rock (up to 70 ft thick), which typically shows evidence of multiple episodes of brecciation/silicification. Unsilicified shale ore has thin quartz veinlets, weak silicic zones, and disseminated sulfides. This ore is slightly lower in average Au grade and much lower in Ag. Dolomitic ore is more baritic and gossanous having thin silicified zones and quartz-barite veinlets. Dolomitic ore is about average grade in Ag, but below average grade in Au (Douglas and Oriel, 1984). Drilling programs over the past few decades have delineated a stratiform, subeconomic inferred resource of 8.1 million tons at 1 ppm Au and 31 ppm Ag (Selway and others, 2012).