Henry Mountains Mining District, Garfield County, Utah, USAi

Regional Level Types
Henry Mountains Mining District	Mining District
Garfield County	County
Utah	State
USA	Country

The Henry Mountains mining district is in eastern Garfield County about 25 mi south of Hanksville. The Henry Mountains is a small district that has had minor, sporadic production from the initial discoveries made in the 1880s. Total district metal production at modern metal prices is estimated at $25 million. The largest producer was the Bromide Au-Cu-Ag mine (about 500 oz Au) on Mount Ellen; however, the Crescent Creek Au placers downstream to the east were also nominal producers (Hunt and others, 1953).
The Henry Mountains are a series of north-northwest-trending, Oligocene (about 28 Ma), calc-alkaline stocks and laccoliths intruded into the Henry Basin on the Colorado Plateau. Compositionally the stocks are diorite to quartz monzonite porphyry and the adjoining laccoliths are mostly diorite porphyry. The stocks are often surrounded by shatter zones of brecciated igneous and sedimentary rocks (Hunt and others, 1953). Mineralization in the district is largely associated with the three central intrusive stocks near Mount Ellen, Mount Pennell, and Mount Hillers, from north to south, and is typically not associated with the surrounding laccoliths. The best mineralization occurs in the Bromide Basin cirque on the southeast side of Mount Ellen (Hunt and others, 1953). The dominant igneous rocks in Bromide Basin are equigranular diorite and feldspar-hornblende porphyry with small dikes and plugs of andesite porphyry, crowded feldspar porphyry, and quartz porphyry. These intrusives are largely subvertical and have been subjected to weak propylitic alteration with the development of epidote-chlorite-calcite.
At Bromide Basin, a series of scattered, small, north-northeasterly (N. 5°–25° E.) trending, very steeply dipping, native gold-bearing breccia veins cut the Mount Ellen stock. The veins range from a few inches to about 10 ft in width and average about 2 ft. These breccia veins have narrow zones of weak propylitic and argillic alteration. In addition to visible native gold, the primary metallic minerals are pyrite, chalcopyrite, bornite, pyrrhotite, specularite, magnetite, and rare molybdenite. The small, high-grade ore shoots in the Bromide vein can reportedly average about 68 ppm Au, 100 ppm Ag, and 8.5% Cu (Hunt and others, 1953). Other veins are primarily magnetite, chalcopyrite, and Au. The results of about 70 rock samples from Bromide Basin show no other element consistently correlates with Au, the best correlation for Au is with Cu-Fe-La, and there is no significant correlation to As, Ba, Cd, Hg, Mo, Pb, Sb, or W. Similar, albeit even smaller veins, also occur in the Pennell monzonite porphyry stock and the Mount Hillers diorite porphyry stock. Most of the more recent work has focused on extracting very small tonnages of high-grade Au ore from these old mines (Doelling, 1975). About 300 to 350 ounces of placer gold have been produced from the Crescent Creek placers downstream from Bromide Basin (USGS Model 39a). The gold occurs as fine flakes in black, sandy streaks at the base of surface gravels.