Mineralisation was first discovered in the Bingham district in 1863. Since then it has been, at various times, a gold, silver, lead-zinc and copper producer. The early development of mines was stimulated by the completion of rail access in 1873. The original discovery was of oxidised lead and zinc, in lodes and carbonate replacement skarns peripheral to the yet to be recognised porphyry Cu orebody. Silver, lead and both placer and lode gold production continued through the late 19th century. At the turn of the twentieth century, copper was being produced from high grade skarn and fissure deposits, in and around the porphyry deposit. Shoots carrying up to 15% Cu were exploited, with around 40 companies operating in the canyon in 1906. These mainly underground operations continued until the mines were absorbed by Kennecott (Babcock, et al., 1994).
The Boston Consolidate Mining Company opened a copper smelter in the district in 1899. They first recognised the porphyry deposit at Bingham Canyon, and estimated that their property had a reserve of 300 Mt of ore at a grade of around 2% Cu. This large, low grade resource attracted the attention of Daniel Jackling, who in 1903 incorporated the Utah Copper Company to exploit reserves on the neighbouring property. In 1906-07 the Utah Copper Company and Boston Consolidated both commenced open pit mining and operations at their Magna and Arthur Mills respectively. Both were based on the relatively high grade supergene blanket that originally capped the orebody. Early production had an average grade of around 1.5% Cu, 0.5 g/t Au and 5 g/t Ag. In 1906 ASARCO built a smelter nearby, on the shores of the Great Salt Lake, to take advantage of these operations. In 1910, the Boston Consolidated and the Utah Copper Company were merged to form Utah Consolidated. Between 1904 and 1915, 34 Mt of ore @ 1.46% Cu (mostly supergene ore) had been mined from the "porphyry" operations. In 1915, the Kennecott Copper Corporation, which had been incorporated in the same year to mine copper in Alaska, acquired 25% of Utah Consolidated. In 1923 this interest was increased to 77%, with complete ownership being achieved by 1936 (Babcock, 1991).
The originally discovered oxide Ag-Pb mineralisation on the peripheries of the porphyry copper orebody changed to sulphide as the mines went deeper into the fissures and skarns. Other companies continued these until as late as 1971 when the last mine, the Lark, closed. The Anaconda Company had purchased the Carr Fork mines in 1949 and commenced exploration which continued through the 1950s and 1960s. They subsequently mined high grade Cu skarn ore at Carr Fork from 1979 until 1981. During the 1950s Kennecott discovered several deep porphyry shoots and skarn ore in the neighbouring North Ore Shoot. By the mid 1980s Kennecott had acquired the mineral rights to the entire district. In 1981 Kennecott was purchased by Standard Oil which became part of the British Petroleum Group (BP) in 1987. The mine was closed for the first time in its history in March 1985, restarting in September 1986. The reopening followed a decision in late 1985 to modernise. This USD 400 million upgrade was completed in 1988. BP subsequently sold most of their mineral interests, including Kennecott, to RTZ in 1989 (Babcock, et al., 1994) and was subsequently operated by the Kennecott Copper Corporation as a subsidiary Rio Tinto.
By 1991 the open pit covered an area of 7.7 km2, was approximately 4 km across and 900 m deep. To that date approximately 5 Gt of material, ore and waste, had been removed since its inception in 1906. Some 12 Mt of Cu metal had been produced from that ore (Kennecott information brochure, 1991).
The Bingham district is near the western end of the Uinta Axis, a generally ENE-WSW to east-west oriented structural trend that appeared in the Proterozoic and remained an active element until the Tertiary. The Uinta Axis marks the boundary between the Archaean Wyoming Province to the north and Proterozoic basement rocks to the south (Babcock et al., 1997). The north-south Wasatch fault to the east of Bingham marks the eastern limit of the Basin and Range fault block mountain province.
The Bingham Canyon porphyry copper deposit is located in the Oquirrh Mountains of north-eastern Utah. It lies near the crest and towards the western end of the generally ENE-WSW to east-west oriented, 25 km wide Uinta Axis, which appeared in the Proterozoic and remained an active element until the Tertiary. The Uinta Axis marks the boundary between the Archaean Wyoming Province to the north and a post 1400 Ma Mesoproterozoic rift basin filled with 8000 m of supra-crustal clastic rocks to the south (Babcock et al., 1997). It also lies ~30 km west of the Wasatch Fault Zone, which forms the eastern margin of both the basin and range province of the Great Basin, and of the Sevier-Cordilleran Foreland Thrust Belt (Presnell, 1991; Hoffman 1989). The Uinta Axis was reactivated as several epeirogenic uplifts during the Palaeozoic. The host Bingham Stock also lies on the trace of the NW-SE trending Uncompahgre Lineament, part of the continental scale Olympic-Wichita Lineament.
The igneous history of north-central Utah is dominated by the 'Uinta Trend', a belt of middle late Eocene to late Oligocene plutons and coeval volcanics developed along the Uinta Axis (Presnell, 1991).
The more than 6400 m of Cambrian to late Carboniferous (late Pennsylvanian) sedimentary rocks in the Bingham district are allochthonous, apparently thrust eastward during the Jurassic 'Elko Orogeny' phase of the broader Jurassic to Cretaceous Sevier Orogeny. These sediments were deposited in the Cordilleran Trough, just west of the western margin of the Interior Platform. They include around 4600 m of Permo-Carboniferous rocks, overlying lower to middle Palaeozoic platformal sediments, with some Tertiary and Quaternary cover (Lanier 1978; Presnell, 1991; Gunter, 1991).
While Proterozoic rocks are found to the east, and lower to middle Palaeozoic sequences outcrop in the Southern Oquirrh Mountains, the sequence in the Bingham district of the Central Oquirrh Mountains commences with upper Carboniferous (Pennsylvanian) rocks, and is as follows, from the base (Lanier 1978; Presnell, 1991; Gunter, 1991):
Upper Carboniferous (Pennsylvanian)
Oquirrh Group, which has been sub-divided into:
West Canyon Limestone - of lower Pennsylvanian age, the oldest unit exposed in the immediate district.
Butterfield Peaks Formation - which is of lower to middle Pennsylvanian age, and is composed of tan to light grey calcareous quartzite and platy calcareous sandstone, cyclically interbedded with medium to dark grey sandy bioclastic limestone containing thin bedded lenses of black chert.
Bingham Mine Formation Lower Member, 900 m thick - of upper Pennsylvanian age, which is tan to light grey, and comprises orthoquartzite, calcareous quartzite and calcareous sandstone. These lithologies are locally finely banded and cross-bedded, and are inter-bedded with fine grained sandstone and sandy limestone, with two main thick cherty limestone units near the base. These are the Jordan and Commercial Limestones, the hosts to ore at the Carr Fork Skarn described below. The Jordan Limestone marks the base of the unit.
Bingham Mine Formation Upper Member, 700 m thick - which is also tan to light grey, and is lithologically similar to the upper part of the lower member, except that it contains more orthoquartzite and calcareous quartzite.
Permian - Lower Permian sediments cap the sequence, comprising the:
Curry Peak Formation, light grey to tan quartzites and orthoquartzites, interbedded with calcareous sandstones and siltstones containing fossilised worm burrows.
Freeman Formation, grey to tan, well bedded orthoquartzite with interbedded grey to tan sandstone, carbonaceous siltstone and mudstone.
Kirkman-Diamond Creek Formation, composed mainly of sandstone, comprising buff to tan, massive to bedded, calcareous sandstone, interbedded with coarse, tan, sedimentary breccia and grey carbonate.
Park City Formation, 150 m thick - local remnants of this unit are found at the top of the Kirkman-Diamond Creek Formation. It is made up of cherty and fossiliferous dolomite and dolomitic siltstone.
Locally thick quartzite and thinner limestones of Permian and Upper Carboniferous (Pennsylvanian) age constitute the country rocks in the immediate mine area, underlain by most of the remainder of the Palaeozoic succession in the surrounding region. Mesozoic rocks are absent, except for the peripheries of the district.
The large thrust faults and extensive folding of the Cordilleran Orogeny that deformed all of these rocks are important ore controlling features within the mine area, particularly the Jurassic Elko and the Cretaceous Sevier Orogenies produced a complex series of northerly trending folds and associated thrust faults in the Bingham district (Presnell, 1992), which folded and thrust faulted the Carboniferous and Permian quartzites and limestones that are the country rocks to the Tertiary intrusions associated with the deposit. During the late Eocene and early Oligocene, the region underwent NE-SW compression as the Farallon plate was subducted along the western edge of the continental mass (Best et al., 1989).
Faulting in the district is dominated by the SW-dipping, NE vergent Midas thrust mapped at the surface to the NE of the deposit, displacing all of the pre-Tertiary rocks. Numerous younger, high angle structures with a general north-easterly trend cut the intrusive as well as the sedimentary rocks and are often referred to as fissures. The fissures usually have very little displacement but may have widths of a metre or more and are important localisers of veins and replacements where they cut the limestone beds.
At ~40 Ma the 'Uinta trend' igneous intrusions responsible for the mineralisation in the Park City, Cottonwood, Bingham and Stockton mining districts were emplaced along the western extension of the Uinta Axis. A thick section of comagmatic volcanic rocks occurs east and SE of the mine along the eastern flank of the Oquirrh Mountains as latite flows, breccias and related volcaniclastics. These were the outflow of an eroded volcanic cone well over 1.5 km in height above the present surface (Moore, 1973). At about the time of these intrusions, Tertiary extension and normal faulting began, culminating in the Basin and Range topography that dominates much of the western United States. This extensional stress is reflected in the trend of many mineralised structures in the Bingham district (Presnell, 1997). Phillips and Kruhulek (2003) recognise an orthogonal, ENE and NW structural grain around Bingham. This pattern is most evident in the NW trending Ohio Copper porphyritic quartz monzonite and ENE trending quartz monzonite porphyry and later dykes as well the general boundaries of the Bingham stock.
Mineralisation at Bingham Canyon is associated with the irregularly shaped Eocene Bingham Stock, dated at between 39.8 and 38.8±0.4 Ma. The Bingham stock outcrops over a triangular area of ~3000 m along the base, by ~2000 m to the apex. The composite stock of older, generally equigranular, monzonite is cut by quartz monzonite porphyry, latite and quartz latite porphyry dykes, and a number of minor, igneous phases. The adjacent Last Chance stock, which is composed of similar lithologies of the same age, and is of a similar size, is found over 1 km to the south-west of, and is connected to the Bingham Stock by a broad dyke. A third, slightly smaller intrusive outcrops to the south of the Bingham Stock. A series of other Eocene stocks of varying size are also found within the Oquirrh Mountains. The Bingham Stock is localised within the core of the north-south Copperton Anticline, below the flat Midas Thrust whose surface trace is to the north-east, within a zone of north-east trending faulting. The Jordan and Commercial Limestones beds in the basal sections of the Bingham Mine Formation Lower Member occur on the limbs of the anticline, which appear to control the limits of the Bingham stock to the north-east and north-west (Lanier, et al., 1978).
The Bingham porphyry copper system was intruded into the Butterfield and Bingham Mine Formations, below a cover of 2300±300 m, although some phases apparently vented, forming a volcanic pile of similar age and composition (mainly biotite-hornblende latite, with andesites and rhyolites) that is found to the east of the range (Lanier, et al., 1978). Phillips (1991) divides the intrusive into four main units, from oldest to youngest of monzonite, quartz monzonite porphyry, latite porphyry and quartz latite porphyry, all of which have been altered and variably mineralised, and all of which may contain ore grade copper.
The composite monzonite intrusion of the main Bingham stock is crudely circular, with extensions or protrusions along the northwestern boundary to both the NE and SW. The strong north-easterly trend of this side of the stock is reflected in all subsequent intrusions and is referred to as the 'porphyry trend'. The first intrusion was of 'monzonite', an equigranular to porphyritic monzonite or quartz monzonite and may have been in part a diorite. It is generally a medium to dark grey augite-actinolite-biotite monzonite with 30% K feldspar, 33% plagioclase and 7% quartz. This rock forms many dykes and sills, the contacts are irregular with extensive assimilation of the wall rocks.
A body of fractured rock was 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.
The four main porphyry intrusives, listed above, were progressively intruded, each followed by a cycle of veining, alteration and mineralisation. The quartz monzonite porphyry intrudes the initial monzonite along the northwestern margin of the stock having a thick (~1500 x 350 m) dyke like shape with a north-easterly trend. It is a light grey, amphibole-biotite-quartz monzonite porphyry, originally composed of 32% plagioclase, 32% orthoclase, 23% quartz, and 14% mafic and accessory minerals. Latite porphyry cuts both of the older intrusions, and comprises a light to medium grey hornblende-augite-biotite quartz latite porphyry. It is in turn cut by a series of dykes that are mapped as quartz latite porphyry, a medium grey and light green in colour, and comprises a hornblende-biotite quartz latite porphyry. The latite and quartz latite porphyry dykes are also limited to the northern half of the deposit. The porphyry trend apparently was the result of the regional Tertiary extensional stress associated with regional block faulting (Presnell, 1997). At the eastern end of the quartz monzonite porphyry, irregular masses of the latite porphyry may have been emplaced in rocks that were still hot enough and deep enough to yield plastically. These 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.
Mineralisation and Alteration
The ore is present as both disseminated and vein/veinlet style porphyry mineralisation, and as two major skarn orebodies within Palaeozoic carbonates. The character of alteration at Bingham is dependent on the amount of fracturing and veining, the host lithology, and location within the zoning pattern. Ground preparation prior to mineralisation was extensive, with the sedimentary sequence having been strongly folded and fractured prior to intrusion, while intense fracturing accompanied emplacement of the intrusive and hydrothermal system (Phillips, 1991). The spacing between fractures and veins influences the pervasiveness of rock alteration as well as the quantity of sulphide present. In the porphyries, north-easterly trending sheeted veins may locally constitute 50% or more of the rock over widths of a metre or more and are closely related to high-grade copper/gold mineralisation.
Prior to mining only limited exposures of the quartz-monzonite porphyry outcropped, being largely concealed by the Palaeozoic sediments, with the bulk of the mineralisation being concentrated around the apex of the Bingham Stock. A leached cap was present in the upper outcrop, although it appears that the surface was close to the top of the mineralisation, and above the higher grade primary Au. In general there is a close correspondence between the distribution of Au and Cu mineralisation (Phillips, 1991).
Alteration has been subdivided into the Bingham Main stage potassic biotite and K feldspar and Late stage phyllic sericite/clay. The Bingham Main stage mineralisation includes bornite, chalcocite, chalcopyrite and pyrite. Pyrite is dominantly found in an outer halo with both potassic and phyllic alteration. Chalcopyrite lies inside the pyritic zone and is accompanied by 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 low grade core to the porphyry deposit of weakly fractured, poorly veined and mineralised rock in the centre of the deposit at the current level of exposure. This core is potassically altered and typically contains less than 0.5% total sulphides as chalcopyrite, molybdenite and subordinate bornite, accompanied by primary chalcocite and minor covellite. The core is centred at the monzonite/quartz monzonite porphyry contact and with depth, expands with both the sulphide and metal contents decreasing dramatically.
Molybdenite and copper sulphides are essentially coextensive, although the peak molybdenite content lies just inside the core, adjacent to the copper shell. Molybdenite grades diminishes both into the core and outward to the pyrite halo. Much of the molybdenite formed in quartz veins without alteration selvages at the end of the Bingham Main stage.
The copper grade shell surrounds the low-grade core, defined by the 0.35% Cu contour which roughly bounds the upper and outer margin of the shell, crudely parallel to the stock contact, overstepping the contact into the quartzite for a hundred metres or more on the east and north. Fractures and/or veins are more closely spaced than in the low-grade core causing the biotite selvages of veins to overlap in the monzonite with the result that biotite alteration becomes pervasive. Much of the sulphide is disseminated, and is neither in a vein or fracture nor can it be related the envelope of a specific vein. This disseminated character holds true in thin section as well as in hand specimen. Disseminated sulphides commonly account for more of the mineralisation than do vein sulphides, especially in the high-grade areas. Potassic alteration is typical throughout the copper shell. Bornite extends from the barren core to the core and extends into the medial copper shell, from which chalcopyrite dominates until pyrite increases to the outer margin of the shell into the pyrite halo.
The pyrite halo (>1% pyrite, py>cpy) overlaps the outermost copper shell and extends outward for a kilometre or more. The chalcopyrite content of the copper shell decreases as pyrite increases to about 1 vol.% near the copper shell limit. Pyrite increases rapidly from 1 to >2 vol.% over a distance of a hundred metres or more. The pyrite occurs as veinlets or fine-grained disseminations and may be accompanied by minor chalcopyrite.
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 ± 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.
The Upper Palaeozoic limestones, although only a minor part of the sequence, are important loci of skarn alteration and sulphide mineralisation. Two main limestone bed are mapped, the upper Commercial Limestone and the lower Jordan, each ~30 m thick, separated by ~100 to 150 m of quartzite. On the northern side of the monzonite stock, the limestones are only present in the lower plate of the Midas thrust, although at other locations in the deposit, the limestone beds are in both upper and lower plates. The upper bed is the Commercial Limestone and the lower bed is the Jordan. Ore in the limestone beds will be discussed later. Other thin limestone members, the Alphabet series, are present south of the open pit and are the hosts to lead/zinc replacements.
Early stage contact metasomatism affected the wall rocks of the Bingham stock and resulted in the development of wollastonite, with variable amounts of idocrase and garnet in the major limestone beds, and diopside in quartzite and calcareous quartz sandstone. This mineralogy in quartzite/sandstone changes outward, with diopside being replaced by tremolite and talc, and the addition of calcite. In quartzite, Early stage alteration minerals are overprinted by Bingham Main stage quartz and sulphide veinlets with biotite selvages near the stock and actinolite at greater distance. In the limestones, the Bingham Main stage replaces the early wollastonite alteration with andradite garnet, diopside, quartz, magnetite, hematite and copper sulphides. In a late hydrous or retrograde phase of the Bingham Main Stage, garnet is altered to actinolite with major sulphide mineralisation. The Late stage alteration of the skarn (later than all biotite-K feldspar alteration) converted previously formed calcsilicates to chlorite, clays, sericite and talc. Within the east side quartzite, Inan and Einaudi (2002) described the Bingham Main Stage as K feldspar plus biotite while Late stage alteration is an assemblage of sericite-kaolinite-quartz overprinting and perhaps extending outward from the potassic alteration. The main skarn orebodies North Ore Shoot/Fortuna and Carr Fork are not separate ore bodies, but segments of one extensive mineralised skarn developed in the Commercial and Jordan Limestones (Harrison and Reid, 1997) along the northern and eastern stock contact, and as such are part of the zined Bingham Canyon porphyry mineralised system. Carr Fork is predominantly in steeply to moderately dipping upper plate beds. The larger and higher grade North Ore Shoot/Fortuna mineralisation is in the tightly folded anticlines of Jordan and Commercial Limestones in the lower plate.
Production, Reserves and Resources
The Bingham Canyon mine first operated in 1904 with the supergene cap being exploited with grades of around 1.5% Cu and 0.5 g/t Au. Since the very early years all production has been from hypogene ore.
Historic production of the porphyry mineralisation to 1972 was 1.240 Gt @ 0.91% Cu,
In 1995 reserves were - 1.020 Gt @ 0.59% Cu, 0.38 g/t Au.
In 2005 total reserves were - 0.665 Gt @ 0.54% Cu, 0.32 g/t Au (Rio Tinto Annual Report, 2006).
plus total resources (in addition to reserves) of - 0.96 Gt @ 0.7% Cu, 0.3 g/t Au (Rio Tinto Annual Report, 2006).
The two skarn orebodies contained (1995) 61 Mt @ 1.9% Cu, 0.38 g/t Au and 81 Mt @ 2.8% Cu, 1.6 g/t Au.
JORC compliant ore reserves and mineral resources at December 31, 2011 (Rio Tinto, 2012) for the North Rim skarn deposits were:
Measured + indicated + inferred mineral resources - 20 Mt @ 3.65% Cu, 1.62 g/t Au, 20.95 g/t Ag.
Ore production is from a large open pit, from which some 52.4 Mt of ore were extracted in 1994, with a head grade of 0.63% Cu,
to yield 310 100 t Cu, 15.86 t Au, 8700 t Mo and 135 t Ag.
In 2005 production amounted to 220 600 t of Cu, 11.48 t Au, 15 600 t Mo and 123 t Ag (Rio Tinto Annual Report, 2006).
JORC compliant open-pit ore reserves and mineral resources at December 31, 2011 (Rio Tinto, 2012) were:
Proven + probable ore reserves - 0.835 Gt @ 0.48% Cu, 0.041% Mo, 0.20 g/t Au, 2.1 g/t Ag, plus,
Measured + indicated + inferred mineral resources - 24 Mt @ 0.22% Cu, 0.009% Mo, 0.12 g/t Au, 1.57 g/t Ag.
(Source: Porter GeoConsultancy, http://www.portergeo.com.au/, 1998)