Geology of the Bingham Mining District, Salt Lake County, Utah

The Bingham deposit is centred on a small 40 Ma stock of older, generally equigranular, monzonite that is cut by quartz monzonite porphyry, latite, and quartz latite porphyry dykes. The surrounding country rocks are quartzite and minor but important limestone. A body of fractured rock formed over the top of the monzonite as it cooled leaving a weakly fractured core below a dome of strong fracturing. Early fluids entered this fractured mass at about the time of the intrusion of the first porphyry resulting in an undetermined amount of alteration and mineralisation. This fracturing is a major control on the location of the ore shell and the concentric zoning pattern of alteration and mineralisation. Five porphyry intrusives have been described and each is followed by a cycle of veining, alteration and mineralisation. The porphyries all trend north-easterly across the northern half of the deposit, forming the porphyry trend. At least three overlapping centres of fracturing, alteration and mineralisation seem to be present within the stock, one centred in the fracture dome and two or more in the porphyry trend.

Alteration consists of Bingham Main stage biotite and K feldspar and Late stage sericite/clay. The Bingham Main stage mineralisation includes boraite, chalcocite, chalcopyrite and pyrite. Pyrite is dominantly in an outer halo with both potassic and phyllic alteration. Chalcopyrite lies inside the pyritic zone with potassic alteration. Bornite/chalcocite forms an annulus within the chalcopyrite zone with extreme concentrations in two centres within the porphyry trend. Gold is strongly correlated with bornite in the porphyry trend. There is a core of weakly fractured, weakly veined and mineralised rock in the centre of the deposit at the present level of exposure. Much of the molybdenite formed in quartz veins without alteration selvages at the end of the Bingham Main stage. A Late stage of quartz-sulphide-sericite is the last episode of mineralisation and includes lead-zinc-silver veins and replacements in and beyond the pyritic zone and copper sulphides in the outer ore shell. The Late stage is characterised by chalcopyrite and pyrite plus or minus bornite and chalcocite/ covellite with sericitic alteration of the adjacent wallrock. Mineralisation accompanying the Late stage is economically significant. The Bingham Main stage formed during a period of about a million years.

Ore controls are the hot mass of the stock, temperature variations outward, host rock chemistry or reactivity and fracture controlled fluid access. The outstanding example of host rock effects is the limestone beds, which altered to garnet skarns and captured higher-grade copper mineralisation.