The Bingham District, Utah: The Bingham district, situated in the north central part of the state of Utah, is in the Oquirrh Range about 20 miles southwest of Salt Lake City. Its measured reserves of copper ore are probably the largest in the United States. It has produced also large amounts of silver, lead, and gold ore. This copper field includes an oblong area of about 24 square miles. It lies between the Jordon Valley on the east and the Oquirrh range of mountains on the west. The terranes are Carboniferous quartzites and limestones that have suffered extensive intrusion, intense fissuring, and partial burial beneath an andesite flow. The sediments, though showing in general a northerly dip, and northeast-southwest strike throughout the region, vary in their strike from east-west on the western slope to north-south on the eastern, so that they form a synclinal basin, with northward pitch. The limestones of the lower member, averaging 200 feet in thickness, have been highly marbleized, and carry large bodies of copper ore, and the calcareous carbonaceous shales of the upper member sometimes carry it as well.
The Bingham district, near Salt Lake, Utah, is now most widely known by the extensive mining operations of the Utah Copper Co. Two laccoliths and stocks of monzonite, as well as sills and dikes of diorite porphyry, invade the Carboniferous quartzite and limestone. The Carboniferous quartzites and limestones are intruded by monzonite and monzonitic porphyry and covered in part by andesites, andesitic porphyries, and breccias. The quartzite series ("Bingham quartzite") is several thousand feet thick. It contains at least seven limestone lenses, some of them 300 feet thick.
The region is crossed by many faults and fissures which trend in all directions. The faults are both normal and reverse, and some carry ore. Extensive fissuring and some faulting has taken place also after the deposition of the ores. The copper ore bodies are centered in the localities which have undergone the most extensive intrusion and fissuring in or near the intrusive monzonite or monzonitic porphyry. In many cases the copper ore is closely associated with the intrusives. Two types of copper deposits are recognized, viz. : (1) great tabular replacement masses in limestone, lying roughly parallel with the bedding, and showing sometimes an extent of several hundred feet along the strike, as well as a thickness of even 200 feet; (2) disseminations in a large monzonite laccolith, especially in the fractured, fissured, and altered portions of the same.
The contact replacement deposits have been important ones in the past, but the enormous bodies of low-grade disseminated ore in the monzonite are now the most important . The limestone ores consist of primary pyrite and chalcopyrite, enriched in some cases by chalcocite and tetrahedrite. Quartz is the chief gangue mineral, but as might be expected in a contact deposit, garnet, epidote, tremolite, specular hematite, pyrrhotite, sphalerite, galena, etc., are also present. The primary ore of the disseminated type consists of grains and veinlets of pyrite and chalcopyrite, distributed through both shattered and altered monzonite porphyry and quartzite. The commercial ore is due to secondary enrichment, and the zone containing it underlies the leached or partly leached capping, and overlies the primary ore. In this ore zone, whose average thickness is about 165 feet, the secondary sulfides are covellite and chalcocite. The average thickness of the capping was 115 feet.
The ore deposits include fissure veins in several formations, bedding-plane replacement deposits in limestone, and disseminated deposits in shattered porphyry. The ore bodies that were most productive in the earlier history of the district are large replacement deposits of sulfide ore in limestone. These ores consist chiefly of pyrite, chalcopyrite, sphalerite, and chalcocite and their oxidation products. Pyrrhotite and a little garnet occur in some of the ores. The replacement type copper ore occurs in flat lenses in the metamorphosed limestones.
The copper ores of the Highland Boy mine near the surface were extensively oxidized and carried concentrated gold. The mine was first exploited for gold, but deeper developments disclosed great bodies of copper ore containing gold and silver. The copper ores there are large lenticular bodies of chalcopyrite in the fissured marble adjacent to the intrusives and carry little chalcocite and are in the main primary. The Highland Boy ore occurs disseminated through the igneous rocks, and limestone. Other large deposits in this region carry lead and silver.
The theory of origin advanced for the formation of the ore is that the quartzites and limestones were intruded by the monzonite in Mesozoic or early Tertiary times, producing contact metamorphism of the limestone and replacing it with sulfides. After the upper portion of the monzonite intrusion was partly cooled, the inclosing rocks were fractured by northwest-southeast fissures, along which there ascended heated aqueous solutions from the deeper, uncooled portions of the magma. It is thought that heated, aqueous, mineral bearing solutions, rich in carbon dioxide and potassium oxide, rose along strong northeast and southeast fracture zones, altered their walls by adding quartz to quartzite, impregnating marble with metallic sulfides and specular hematite iron ore, and silicifying, seriticizing, and impregnating monzonite with metallic sulfides and depositing lode copper ores in largest volume between calcareous and carbonaceous walls, mainly by filling, and partially by replacement. These solutions not only altered the fissure walls, but deposited additional metallic sulfides, thus enriching the limestones as well as altering the monzonite by the addition of copper, gold, silver, pyrite, and molybdenite. The same author states that "it is probable that the principal source of the copper ore in the limestone was the magma of the intrusive, that the mineral elements were transported by the intrusives and by the thermal solutions and vapors emitted from both their superficial and deeper portions and that the ore was deposited by molecular replacement of a metamorphosed, at least partially marmorized and silicified, country rock".
The lead silver ores in the area are deposits of galena that generally carry a high content of silver ore. They occupy veins in igneous and sedimentary rocks and replace limestone. The largest deposit in the Bingham district is that of the Utah Copper Co. It covers an area of 211 acres, has an average thickness of 414 feet, and contains 338,000,000 tons of ore which carries 1.4 per cent, of copper. It lies like a thick blanket, covered by a mantle of rock leached of copper which averages 110 feet in thickness. This mantle rock is removed, and the ore is then mined with steam shovels. The ore is disseminated in shattered sericitized porphyry. The principal minerals are chalcopyrite, chalcocite, and covellite which appear as shots and stringers. The ore has been enriched by descending waters, which dissolved the copper from the mantle and from the overlying porphyry that has since been eroded, and deposited it below replacing the chalcopyrite and pyrite of the low-grade porphyry. In 1914 the ore treated at the mills of the Utah Copper Company had an average copper content of 1.425 per cent, with an average recovery of 66.04 per cent.
Return To: Copper Ore Deposits of the USA
