The Pine Creek Tungsten mine offices, mill and main adit to the mine are located at the confluence of Pine and Morgan Creeks at an elevation of 8,100 feet in Pine Creek Canyon. The mine workings are underground, about two miles farther up Morgan Creek, extending from an elevation of 8,100 feet to the surface at 12,000 feet. The ore contains the tungsten-bearing mineral scheelite (CaWO4). Operated by Union Carbide Corporation, it is the largest tungsten mine in the United States. The Pine Creek mine and the Climax mine near Leadville, Colorado accounted for about 93 percent of all domestic tungsten production in 1977 (Stafford, 1980).
The Pine Creek ore body occurs at the northern end of the Pine Creek pendant. The pendant is a raft of metasedimentary and metavolcanic rock intruded by Sierran granitoids (Fig. 12). It is almost 11 kilometers long and one-and-a-half kilometers wide, extending from Mt. Tom to Wheeler Crest. The southern one-third is mostly unmineralized metavolcanic rock. The northern two-thirds is metasedimentary rock divisible into three distinct units. The oldest unit is composed of pelitic horn-fels, micaceous quartzite, and vitreous quartzite. The next youngest is light gray marble, which in turn is overlain by a unit of micaceous quartzite. They are folded into a tight syncline, whose limbs are nearly vertical at the north end and shallow to the south. The rocks have been correlated with those in the Mt. Morrison pendant to the north, which have been dated by fossils as Pennsylvanian and Permian (?) (Bateman, 1965).
The pendant is in contact with three granitic intrusives. The two most important are the Tungsten Hills quartz monzonite, dated as Triassic (Bateman, 1978) and the Wheeler Crest quartz monzonite, 96 m.y. (Kistler and others, 1965). Most of the tungsten mineralization in the Bishop District is thought to be related to the Tungsten Hills quartz monzonite because of its close association to the Pine Creek ore body and numerous other tungsten deposits (Bateman, 1965). An older body of quartz diorite has little or no associated tungsten mineralization.
The Pine Creek ore deposit occurs along the western margin of the pendant, at the northernmost contact between the marble unit and the Tungsten Hills quartz monzonite. It is a contact metasomatic deposit of a scheelite-bearing garnet-pyroxene rock called tactite. The scheelite usually occurs in the tactite as disseminated crystals up to one cm in diameter. When illuminated with a black light, the tactite often shows bands and swirls of these crystals. In a few places, the scheelite occurs in seams tens of centimeters wide, almost 90 percent pure.
Tactite occurs only along the northernmost 1,000 meters of the contact between the quartz monzonite and the marble. Bateman (1965) believed the formation of tactite was controlled by three factors. First, it usually formed from a rock rich in calcium, preferably clean marble. Second, it also commonly formed in embayments into the intrusive contact. Third, and probably the most important factor allowing the formation of tactite is an abundance of fractures in the marble, enough to pass the mineralizing fluids.
The skarn commonly has quartz monzonite in contact with a narrow band of quartz. The quartz is adjacent to the tungsten-bearing tactite. The tactite grades into white, bleached marble, which is in contact with the unmineralized gray marble. Relative thicknesses of each of these units may vary considerably. The quartz and tactite may also contain significant amounts of molybdenite and copper sulfides which, when abundant, are mined for their own sake.
Tungsten mining first began at the Pine Creek mine in 1918. Because of fluctuations in the tungsten market, operations only lasted a few years. The demand for tungsten increased in the mid-1930's and at the same time ultraviolet light was introduced as a prospecting tool for tungsten minerals. Subsequently, numerous tungsten deposits were discovered in the Bishop area. The Pine Creek mine was acquired and reopened by the U.S. Vanadium Company in 1936. This company later changed its name to Union Carbide Corporation. In 1948, an access tunnel was completed 1,500 feet below the previous main level. The ore was shipped by aerial tramway to the present mill. In 1967, a two-mile tunnel was completed from the present mill site to the main workings. Known as the "Easy Going Drift," it provided easy access to the active parts of the mine. A small ore body was accidentally discovered only a short distance in from the portal, while drilling this tunnel. It is called the "Bend" ore body.
Production figures of the mine and mill are now confidential, but up to 1953 it had produced 10,575 short tons of WO3, 1,968 tons of copper, and 3,075 tons of molybdenum (Bateman, 1965). Tungsten is produced at the mill as ammonium paratungstate (APT), and copper and molybdenum as ore concentrates. Most of the APT produced at the Pine Creek mill is used for the production of tungsten metal for light bulb filaments.