Abstract Recent examination of volcanic rocks near the Bingham Canyon Cu-Au-Mo deposit, Utah, suggests that primitive alkaline magmas are an important factor in the formation of this world-class porphyry copper deposit. The Bingham deposit is spatially associated with a monzonitic intrusive complex emplaced at 39 to 37 Ma into Paleozoic sedimentary rocks. Coeval igneous rocks vented to the surface and formed a volcanic pile, part of which is preserved on the eastern flank of the Oquirrh Mountains. Bingham volcanic rocks are divided into three chrono-lithologic suites: an older volcanic suite, a nepheline minette-shoshonite suite, and a chemically distinct younger volcanic suite. Petrographic and geochemical data indicate that the intrusive complex and older volcanic suite are largely comagmatic. This relationship is substantiated by similarities in ages and proximity of the older volcanic rocks to the intrusive suite. No significant chemical differences occur between these two suites, except where hydrothermal alteration has increased the concentration of Cu and K in the intrusions at the expense of Na. The nepheline minette lava flows extruded at ~38 Ma are primitive. They are characterized by Mg # >65, high concentrations of volatiles, LILE, and LREE, and strongly compatible elements. They contain relatively low concentrations of Ti, Nb, and Zr on a mantle-normalized basis, 15 to 18 percent normative nepheline, and 1 to 15 percent normative leucite. The nepheline minette magmas may be primary melts that are products of small degrees of partial melting of metasomatized, lithospheric mantle. Associated shoshonites lack normative nepheline and normative leucite. At comparable SiO 2 contents, the younger volcanic rocks emplaced at ~32 Ma are generally more aluminous and sodic, and less magnesian and potassic than the older volcanic and intrusive suites. All magmas except the nepheline minette were sulfide-saturated when erupted and contain magmatic sulfides as 1 to 100 micron-diameter elliptical inclusions in mafic mineral phenocrysts. Analytical data suggest that sulfides accommodate most of the Cu and Ag that are present in the magmas. Resorption and oxidation of the sulfides may have made these metals available to the hydrothermal system. The sulfide-undersaturated character of the primitive, alkaline magmas may have allowed them to rise through the crust with almost no loss of S, Cu, and other chalcophile metals. Fractional crystallization, magma mixing, and assimilation all played roles in determining the composition of the magmas in the Bingham system. Trace-element modeling shows that the minette flows are not related to the other volcanic suites or to the intrusive suite by fractional crystallization. However, mixing of the minette magma with latitic magma could have created the shoshonites and the magma of the older suite. Trace-element modeling also indicates that late mineralized dikes may be formed by mixing of about 10 percent minette magma and 90 percent calc-alkaline magma. Degassing of mixed minette magma underlying the calc-alkaline magma may have contributed even larger proportions of sulfur, volatiles, and metals to the ore-forming system.

Abstract 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.

Abstract A comprehensive look at the closely spaced blast hole samples, exploration drill data, and a resource model interpolated from diamond exploration drilling data show that the distinct molar tooth shape of the Bingham Canyon orebody has strong lithologic controls at depth. At and below the level of current mining Cu occurs in the Bingham stock as a ring approximately 4000 feet in diameter, centered on a barren core located at the eastern end of the quartz monzonite porphyry (QMP). A smaller ring of Cu (about 2500 feet in diameter) is centered on the southwest end of the QMP. The mineralized cylinders are broken where chemical and physical differences between the mineralized intrusive rocks and the barren quartzites on the south and west sides of the Bingham stock cause gaps in the regular Cu distribution, leading to the distinctive root zones of the Bingham porphyry system. These root zones are artifacts of favorable host rock and not products of multiple mineralization centers or structural control. Blasthole and diamond drill hole assay data show broadly continuous grade through monzonite (MZ), QMP and latite porphyry (LP) dikes, though locally grade changes occur across LP contacts. A drop in grade within quartz latite porphyry (QLP) dikes is common. The overall consistency of grade across intrusive contacts including the LP suggests that the main pulse of mineralization, associated with the QMP, spanned the period of subsequent LP intrusion. Mineralization had notably waned by the time of QLP emplacement, leading to lower Cu grades in the QLP throughout the system.

Abstract The Bingham Canyon porphyry copper-gold-molybdenum deposit is one of the largest and highest-grade porphyry orebodies in the world. This study focused on the northwest side of the deposit where quartz monzonite porphyry (QMP), the first and largest porphyry intrusion, hosts the bulk of the high-grade copper-gold ore (>1.0% Cu, >1.0 ppm Au). The north-northeast–trending, high-grade zone had pre-mining dimensions of 1,500 m strike, >300 m vertical, and 500 m width and contained more than 500 million tonnes (Mt) of ore associated with potassic alteration and abundant quartz veins. The lack of superimposed sericitic alteration yielded ideal exposures in which to study the early, high-temperature stages of ore formation, a style of mineralization that in many porphyry deposits represents the major period of copper introduction. We mapped multiple porphyry dikes in the sequence: (1) QMP, (2) latite porphyry (LP), (3) biotite porphyry (BP), (4) quartz latite porphyry breccia (QLPbx), and (5) quartz latite porphyry (QLP). Porphyry dikes, faults, and quartz veins are steeply dipping and have two dominant orientations; north-northeast– and northwest-striking. Dikes have a north-northeast strike but they thicken and develop northwest-trending apophyses and host high-grade copper-gold zones at intersections with northwest-faults, indicating that magmatic-hydrothermal fluids were focused by these structural intersections. Each porphyry intrusion was accompanied by a similar sequence of veins, potassic alteration, and sulfides. Biotite veinlets were followed by fractures with early dark micaceous (EDM) halos of sericite, K-feldspar, biotite, andalusite, and local corundum containing disseminated bornite-chalcopyrite-gold. EDM halos are cut by multiple generations of A-quartz veins representing the main Cu-Au ore-forming event. Postdating all intrusions are quartz-molybdenite veins followed by quartz-sericite-pyrite veins. Cathodoluminescence (CL) petrography identified distinct A-quartz veinlets consisting of dark-luminescing quartz filling fractures and dissolution vugs in earlier A-quartz veins and adjacent porphyry wall rock. These veinlets contain abundant bornite and chalcopyrite and minor K-feldspar and are closely linked in time to the introduction of the bulk of the copper and gold. Although a similar sequence of veins was repeated on emplacement of all porphyry intrusions, the vein density and intensity of potassic alteration declined with time. The youngest porphyry, QLP, is mostly weakly mineralized and locally unaltered. These observations indicate that magmatic-hydrothermal fluids underwent a similar physiochemical evolution during and immediately following emplacement of each of several porphyry dikes. The relationship between EDM veins and A-quartz veins requires that the flux of magmatic fluid from the magma chamber occurred in an episodic manner as opposed to a continuous discharge. Vein truncation relationships coupled with abrupt changes in copper-gold grades, sulfide ratios, and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive porphyry intrusive-hydrothermal cycles, presumably due to depletion of metals and volatiles in the underlying magma chamber.

Abstract Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast–elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same lithologic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes and a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry dikes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system.

Abstract Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ∼1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (∼0.6 g cm −3 ) and a salinity of ∼7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CO 2 . The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu–rich center near the top of the orebody contains low-density vapor inclusions (∼0.2 g cm −3 ) coexisting with brine inclusions containing ∼45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ∼9/1. The vapor contained Cu as its dominant cation (∼1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (∼36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. This interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow subvolcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor.

Abstract The Bingham Canyon porphyry copper deposit, prior to mining, is estimated to have contained 40.4 million ounces of gold and 289 million ounces of silver within 3,105 million tons of ore. The gold-mineralized zone mimics the shape of, and is coaxial with, the copper ore shell, but the zone of highest-grade gold mineralization lies at a slightly lower elevation than the zone of highest-grade copper mineralization. Geological observations suggest that high gold grades are associated with abundant bornite. The distribution of silver corresponds closely with that of copper. Gold and silver recoveries for Bingham porphyry ore have averaged 68 and 81 percent, respectively, since the introduction of froth flotation in 1920. Electron microprobe examinations of head and concentrate samples suggest that most gold occurs as particles less than 25 microns across and is alloyed with less than 15 wt percent silver. Most gold particles occur on the boundaries of chalcopyrite and bornite grains and as inclusions within those minerals. Gold, silver, and bismuth tellurides are present but rare. The mode of occurrence of silver is not well established, but based on the high silver recovery and the response of concentrator products to leach tests, most of the silver is inferred to be present in solid solution in copper sulfide minerals. Sequential leach tests suggest that approximately 10 percent of the gold in the ore is encapsulated in pyrite and approximately 5 percent is encapsulated in silicate minerals.

Abstract Four molybdenite samples from veins in the Bingham Canyon porphyry copper deposit yield a weighted average Re-Os age of 37.0 ± 0.27 Ma. The molybdenite ages all overlap at the 2σ level. Previous investigations have suggested that mineralization at Bingham Canyon is coincident with and genetically related to the emplacement of the Bingham stock within the Bingham intrusive complex. However, the molybdenite ages are younger than all reported ages for magmatism in the Bingham porphyry deposit (~39.8 to 37.6 Ma). This suggests either that all the reported ages of the Bingham intrusions are disturbed and the actual ages of the intrusive rocks are younger or that molybdenite mineralization is not directly related to emplacement and cooling of any dated intrusion exposed at Bingham. The latter scenario would suggest the Bingham stock is just the focal point for the superposition of exsolved fluids from deeper in the system.

Abstract The molybdenum grade of the ore mined from the Bingham Canyon porphyry Cu-Mo-Au deposit has historically been about 0.05% Mo. Molybdenum has been an important byproduct since 1936 when the first molybdenite recovery plants were constructed. Recent drilling has provided a clearer picture of the distribution of molybdenum at depth. An inverted-cup shaped zone, or “shell”, of >0.09% Mo mineralization is present. The molybdenum shell overlapped part of the >0.35% Cu shell within the portion of the deposit that has already been mined. The molybdenum shell has a lower center of mass and a smaller diameter than the copper shell, and below the bottom of the current pit, the Mo and Cu shells show little spatial overlap. Most of the deep molybdenum mineralization occurs in quartz-molybdenite veins, with a lesser amount occurring as molybdenite-only veins. The quartz-molybdenite veins typically contain rare pyrite and/or chalcopyrite grains, and below detection levels of gold and silver. The veins lack obvious alteration halos. Whereas quartz veins containing significant chalcopyrite and bornite are more abundant in earlier intrusions than in later intrusive phases, the molybdenite-rich veins cut all intrusive phases; they therefore appear to be younger than most or all of the copper mineralization, a finding consistent with previously published Re-Os ages. Narrow, transparent, quartz-molybdenite veins, 1 to 4 mm in width, with high concentrations of molybdenite, cut more common, wider, milky-white veins. Because the pit has mined the top of the inverted-cup shaped shell, the remaining molybdenum-rich zone takes the form of a thin walled annular cylinder. Only part of that cylinder has been drilled out to date. Based on blast hole sampling, the >0.09% Mo zone is a maximum of 200 meters in width. At its outer edge molybdenum grade decreases abruptly from 0.2% to less than 0.05% Mo over a distance of about 35 meters. Molybdenite within the Bingham deposit contains elevated levels of rhenium. If the rhenium is assumed to occur entirely within molybdenite, rhenium contents for molybdenite, based on drill core geochemistry, are 308 ppm in an upper Cu-Mo-Au domain (Mo >0.05%, Cu >0.35%), 117 ppm in a deep, molybdenum-rich and copper-poor domain (Mo >0.05%, Cu <0.35%), and 135 ppm in the barren core (Mo <0.05%, Cu <0.35%). Although several lines of evidence indicate that most of the molybdenum mineralization is younger than most or all of the copper mineralization, the copper and molybdenum ore fluids appear to have ascended the same zone of permeability. The conditions under which molybdenite precipitated are not yet well understood but the remarkably sharp outer boundary of the higher-grade molybdenum mineralization suggests that the molybdenum may have been transported by a plume of buoyant fluid that was ascending rapidly through significantly cooler fluids. That plume appears to have had a diameter of approximately 1.8 km, and been generally coincident with the boundaries of the composite Bingham Stock.