Geology of the Bingham Canyon Porphyry Cu-Mo-Au Deposit, Utah
John P. Porter, Kim Schroeder, Gerry Austin, 2012. "Geology of the Bingham Canyon Porphyry Cu-Mo-Au Deposit, Utah", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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The Bingham Canyon porphyry Cu-Mo-Au deposit is located in the central part of the Oquirrh Mountains, 30 km southwest of Salt Lake City, Utah. The Cu-Au-Mo deposit lies at the center of a large polymetallic district, which includes proximal Pb-Zn-Ag replacement and distal sediment-hosted gold mineralization. Open-pit copper mining commenced in 1906 and through 2011 has produced 2.666 billion tonnes of ore averaging 0.74% Cu, 0.035% Mo, 0.448 g/t Au, and 3.29 g/t Ag.
Bingham lies near the west end of an 80-km-long belt of petrochemically similar Eocene to Oligocene intrusions and coeval volcanic rocks, which host base and precious mining districts. The Bingham district is centered on the Bingham stock, a multiphase mid-Eocene intrusion emplaced into a thick succession of folded mid- to upper Paleozoic siliciclastic and carbonate rocks. The 2 × 2 km Bingham stock consists mainly of premineral equigranular monzonite that was intruded successively by a 2-km-long dike-like body of quartz monzonite porphyry (QMP), long narrow dikes and sills of intramineral latite porphyry (LP), and narrower dikes of late-mineral quartz latite porphyry (QLP). The intrusions and associated porphyry-style mineralization dip steeply to the northwest and persist from the pre-mine surface at 2,390 m to below sea level. U-Pb zircon dating indicates a 38.6 Ma age for the equigranular monzonite and suggests that the subsequent porphyry intrusions were emplaced at ∼ 38 Ma. Porphyry intrusion and mineralization were approximately synchronous.
Early actinolite-stable alteration in igneous and sedimentary rocks is flanked by distal chlorite-epidote alteration. Magnetite-destructive secondary biotite alteration is pervasive in intrusive rocks that contain > 0.7% copper but typically is fracture-controlled and overprints earlier actinolite alteration in lower grade intrusive rocks and quartzites. K-feldspar alteration accompanies the highest grade copper and gold mineralization. Sericitic alteration is locally pervasive but, more typically, it is restricted to the margins of late quartz-pyrite veins. Late, low-temperature montmorillonite ± kaolinite alteration is ubiquitous in the intramineral porphyries and adjacent equigranular monzonite.
An outer zone of 0.35 to > 0.70% Cu mineralization surrounds and overlaps an inner zone of 0.05 to > 0.15% Mo mineralization, which in turn forms a cupola around a barren core. Intense bornite-chalcopyrite-chalcocite-gold mineralization (> 0.7% Cu and > 1 g/t Au) occurred directly above the barren core but has largely been removed by mining. The copper mineralization in the flanks of the deposit is dominated by chalcopyrite. Copper and gold grades decrease downward but quartz-molybdenite stockwork mineralization grading > 0.05% Mo extends to the lower limit of drilling. Equigranular monzonite is the predominant ore host, containing 53% of the ore mined to date.
Vein types proceed in a sequence from early biotite through quartz-sulfide to late anhydrite and zeolite. Early copper-bearing quartz veins are truncated by the LP and QLP dikes, but later copper-bearing veins cut the dikes. Cathode luminescence studies indicate that copper-bearing sulfides were deposited within micro-fractures and vugs created by dissolution of earlier quartz. Quartz-molybdenite veins postdate the copper-bearing veins and cut all of the porphyries. Vein densities reach > 10 vol % in the barren core and decrease to ∼ 1 vol % at the outer boundary of the > 0.35% Cu zone.
Published fluid inclusion and vein density studies, together with modeled grade distributions, show that mineralization was precipitated by a plume of metal-bearing fluid approximately 2 km in diameter, centered near the southeastern edge of the QMP. The barren core apparently represents an upflow zone near the top of which supercritical fluids separated into vapor and brine. Copper-bearing sulfides and gold precipitated as vapor-dominant fluids cooled from 430° to 350°C. Inclusions in quartz from the deep flanks of the copper deposit indicate similar fluid compositions, but less separation into vapor and brine.
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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.