Abstract

The Bingham mining district played a crucial role in the history of Utah, mining in the American West, and the development of large porphyry copper deposits. Following an initial discovery of mineralization about 1850, the discovery of galena, which ultimately lead to the exploitation of the Canyon’s ore deposits, was made in 1863. Many of the early engineering developments of large scale, open-pit copper mining were initiated in the early 1900s by the adjoining Utah Copper and Boston Consolidated Companies at Bingham. Bingham also played an important role in the Allies efforts during World Wars I and II. The Bingham mining district is Utah’s largest producer of copper, gold, molybdenum, silver, lead, and zinc and one of the most productive districts in the world.

The Bingham mining district is centered on a giant, high-K calc-alkaline porphyry copper-gold-molybdenum-silver deposit. This deposit is associated with a small, mafic-rich, composite monzonite stock dated at about 38 Ma. This stock intrudes a thick, intercalated sequence of Pennsylvanian-Permian marine quartz sandstones and limestones. A 400-to 1,000-foot wide quartz monzonite porphyry dike and a swarm of narrower, east-northeast-trending dikes cut the early monzonite stock. The hypogene porphyry mineralization is concentrically zoned around the quartz monzonite dike from a deep, inner, low-sulfide core through progressively overlapping hypogene molybdenite, bornite-chalcocite, chalcopyrite, and pyrite zones. These mineral assemblages range from a deep, low-sulfidation bornite-chalcocite-covellite zone through the intermediate-sulfidation disseminated chalcopyrite-pyrite copper shell to local zones of high-sulfidation pyrite-nukundamite in quartzite. The inverted cup-shaped copper shell is largely coincident with potassic alteration and garnet skarns. The pyrite halo is spatially associated with propylitically-altered rocks. Each of three major phases of porphyry intrusions is followed by cycles of fracturing, veining, alteration, and metal introduction. The cyclic or episodic vein formation resulted in complex vein relationships partly constrained by age dates and crosscutting relationships. The quartz monzonite porphyry stock and younger dikes are spatially coincident with the highest grades of copper and gold in the ore body. The exposed quartz monzonite porphyry stock is too small to have provided the fluids and metals for the district, suggesting a large magma chamber at depth, confirmed by aeromagnetic data. The controls on gold and copper grades appear to be (1) decreasing temperature away from the center of the deposit, (2) host rock conditions favoring the formation of low-sulfidation bornite-digenite, (3) the degree of fluid access afforded by fracturing, and (4) reactive lithologies.

The outermost fringe of the pyrite halo is overprinted by the inner margin of a 3,000-foot wide, intermediate-sulfidation, sphalerite-galena±tetrahedrite manto-vein zone where alteration is largely confined to the immediate vein walls. The outer perimeter of the lead-zinc veins locally contains rhodochrosite and/or barite. The Barneys Canyon and Melco distal disseminated gold deposits lie over four miles north-northeast of the center of the district, completely outside Bingham’s megascopically recognizable sulfide and alteration system, but on the outer fringe of a weak arsenic-gold geochemical halo.