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Carr Fork, North Ore Shoot (Bingham Canyon)
Utah, USA
Main commodities: Cu Mo Ag

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The Carr Fork mine was developed on section of the Carr Fork/North Ore Shoot skarn deposits which are part of the Bingham Canyon porphyry system in the Central Oquirrh Mountains, near Sat Lake City, northern Utah, USA. They occur in the immediate contact zone of the Bingham Stock, adjacent to the Bingham Canyon open pit.

Published reserve and production statistics are as follows:

• 61 Mt @ 1.89% Cu, 0.38 g/t Au, 10 g/t Ag, 0.03% Mo (Anaconda Carr Fork - Pre-mining),
• 81 Mt @ 2.81% Cu, 1.57 g/t Au, 21 g/t Ag, 0.03% Mo (Kennecott North Ore Shoot),
4 Mt @ 2.5% Cu, 76 g/t Ag, 1.7 g/t Au, 5.7% Pb, 2.9% Zn (Highland Boy-Apex - Prod. to 1947).


The Carr Fork Skarn and related North Ore Shoot are developed on the north-western margin of the Eocene Bingham Stock within both the Commercial and Jordan Limestones. These two units are found at the base of the Upper Carboniferous (Pennsylvanian) Bingham Mine Formation Lower Member, immediately above the Butterfield Peaks Formation. The two mineralised bodies (Carr Fork and North Ore Shoot) are separated by the shallowly south-west dipping Midas Thrust which juxtaposes the carbonate beds of the upper plate directly against the same units in the lower plate. The North Ore Shoot is exclusively within the lower plate, while the Carr Fork Skarn and the up-plunge older Highland Boy-Apex Mines are within the upper plate (Reid, 1991).

The un-altered Commercial and Jordan Limestones are grey to black, fine to medium bedded, carbonaceous limestones containing bedded to nodular chert and fossil debris. Clastic quartz is present as sand or silt sized grains in varying amounts. Both limestone beds have abrupt footwall and gradational hangingwall contacts. In the North Ore Shoot area, the Jordan and Commercial Limestone Beds are 43 m and 33 m thick respectively, separated by 75 m of quartzite. Regionally however, they may respectively be up to 70 m and 60 m in thickness, although the intervening thickness of quartzite remains the same. The Carboniferous above the Commercial thins significantly in the lower plate. These units are folded, commonly with overturned axes, in both plates, with associated bedding plane slippage and fracturing. The fold noses and overturned limbs are fractured and broken while in the gently dipping limbs they are more competent. Pre-intrusion faults, such as the 15 m thick Smelter Fault, are present as gauge and breccia zones which localise latite dykes (Reid, 1991).

Skarn Alteration & Mineralisation

Skarn mineralisation occurs along the northern contact of the Bingham Stock, particularly the quartz-monzonite porphyry phase, with alteration zones being broadly draped over the northern lobe of the stock. The distribution of alteration was influenced by folds, bedding plane faults and original lithology, and consequently generally follows the bedding. Skarn tends to be concentrated along the footwall contact of the Jordan and hangingwall of the Commercial, being strongest in the former which is closer to the intrusion (Reid, 1991).

The skarn apparently formed in two main stages; 1). an early dolomite, talc, tremolite, diopside and wollastonite phase, listed in order of occurrence from the outside of the aureole inwards, and 2). the main stage, which began with the formation of andradite garnet, followed by sulphide mineralisation (Reid, 1991).

Previous authors quoted by Reid (1991) recognise three skarn zones as the porphyry is approached, namely: 1). a tremolite-carbonate zone, 2). a garnet zone with increasing amounts of amphibole and pyroxene towards the porphyry, and 3). an amphibole, albite, pyroxene, garnet zone close to the contact. The alteration zoning may also be traced in the quartzites and carbonate interbeds as well as in the main carbonate units (Reid, 1991). This alteration is also described above in the Bingham Canyon section.

Four styles of skarn alteration are utilised by the Kennecott geologists, in the following order, as the intrusive contact is approached, namely,

i). Marble, with associated tremolite.
ii). Leached Limestone, a distinctive black, soft, porous rock composed of quartz sand and sooty black carbonaceous material, from which most of the original calcite has been leached. It is a zone of 'decalcification'. This zone may contain massive pyrite mineralisation. One such interval of 20 m of massive pyrite in the Commercial Limestone contains 10 g/t Au.
iii). Garnet Zone, containing both garnet-skarn and garnet-clay-skarn. The Garnet-skarn is composed of a massive, hard, brown, red or green rock dominantly composed of andradite garnet with ubiquitous pyroxene and subordinate quartz, calcite and magnetite. In the North Ore Shoot, pyroxene is present as early cores to garnet grains and late alternating garnet and pyroxene veining. Early pyroxene formation preceded main stage garnetisation in both deposits, with pyroxene being largely replaced in the North Ore Shoot, while in Carr Fork un-replaced pyroxene is present at the outer edge of the garnet zone. The later pyroxene seen in the North Ore Shoot is absent from Carr Fork. The garnet-clay-skarn is a soft, friable, brown to orange rock with textures varying from massive garnetite with clay and chlorite, to predominantly clay (commonly montmorillonite) with pockets of garnetite. Pyrite and chalcopyrite occur with magnetite in veins within this phase, with associated quartz, calcite, actinolite, epidote, biotite and orthoclase. The chalcopyrite is variable ranging from <1 mm to >1 cm crystals and patches.
iv). Retrograde Skarn ore types, representing hydrous alteration, sulphidation, oxidation or reduction reactions, are present as the three main ore types described below, namely 'iron oxide' 'actinolite' and 'massive sulphide'. The retrograde skarn forms a convex zone between the garnet and marble zones and seems to be strongly controlled by the anticlinal structure. The ore types occur as follows,

a). Iron Oxide Skarn, which varies from dark grey with blocky fracture to a brown to red to orange friable material, typically exhibiting rough banding with alternating layers of hematite, magnetite, pyrite and siderite in a quartz-siderite gangue. Pyrite is the most abundant opaque, while hematite and magnetite are relatively minor. The clay variant of this skarn, which contains variable amounts of clay and limonite, is soft, friable and earthy brown. Sulphides occur both in layers and in quartz-calcite veins, while chalcopyrite is present as disseminations which are abundant in some bands and sparse in others. Some hypogene replacement of chalcopyrite by chalcocite and bornite is observed. The distribution of the iron oxide skarn suggests it formed as a massive replacement along the garnet-marble contact, with the earliest phase being magnetite replacement of garnetite. Sulphide minerals are commonly found in the centres of magnetite veins, while many sulphide veins cross-cut magnetite veins.
b). Actinolite Skarn, is a dark green, massive, moderately hard rock, composed of amphibole with subordinate magnetite, calcite, quartz and sulphides. The clay variant of the actinolite skarn is lighter green to tan in colour and more friable due to higher chlorite and clay. The amphibole is generally 0.8 tremolite and 0.2 ferro-actinolite, with the Fe content increasing from grain centres to the rim. Sulphide minerals, mainly pyrite and chalcopyrite, tend to be disseminated rather than vein controlled. Massive actinolite skarn may represent un-garnetised remnants of pyroxene skarn.
c). Massive Sulphide, which generally comprise 60 to 90% pyrite. Pyrite and chalcopyrite are disseminated in an interlocking quartz matrix with minor calcite (and other carbonate minerals), magnetite, hematite and pyrrhotite. Consolidated massive sulphide is grey to brassy, hard and massive. The sulphides display an irregularly banded texture, while un-consolidated massive sulphide is similar in colour and composition, but has a friable, porous, vuggy texture and contains minor calcite, siderite, hematite and limonite. The latter also includes a dark green to brassy, soft, friable material composed of pyrite and chalcopyrite in a dense matrix of actinolite and chlorite with minor quartz and calcite. In the Jordan Limestone the massive sulphide is concentrated in the centre of the skarn, and expands to virtually the entire width of the unit at the marble front. The Commercial Limestone contains lenses of massive sulphide parallel to contacts. Mapping shows a transition from unconsolidated to consolidated massive sulphide towards the stock.

Copper mineralisation is present largely as chalcopyrite, accompanied by minor bornite and chalcocite, with associated pyrite, molybdenite, pyrrhotite and marcasite.

Nearly 90% of the Cu-Au orebody occurs in massive sulphides or garnet skarn in the North Ore Shoot, as follows: Unconsolidated Massive Sulphide - 49%; Consolidated Massive Sulphide - 7%; Garnet-clay Skarn - 13%; Garnet Skarn - 19%; Actinolite-clay Skarn - zero; Iron-oxide-clay Skarn - 11%; Quartzite - 1%. The main control of Cu-Au mineralisation is apparently the distance from the contact with the quartz-monzonite porphyry. Grade predictions were accurately estimated on the basis of the distance from the contact, while the presence of actinolite alteration is a guide to locating high sulphide zones in the carbonates. Cu-Au grades for the whole limestone beds increase to a maximum at approximately 250 m from the porphyry, then decrease again away from the contact such that the last >1% Cu lies approximately 400 m from the porphyry contact. Massive sulphides and iron oxides ore are less significant in the Carr Fork deposit.

A strong correlation exists between Cu and Au, with Au occurring as fine inclusions in chalcopyrite. Cu and Ag show a poorer correlation. Mo is found in fractures within the skarn, varying from 0.01 to 0.1% MoS2.

An extensive wollastonite zone found in the upper sections of the Carr Fork deposit of the upper plate is not seen in the lower plate. The wollastonite separates the limestone/marble from the garnet zone. Massive sulphides are absent where wollastonite is developed.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this decription was dated: 1995.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Cameron D E, Garmoe W J  1987 - Geology of skarn and high-grade Gold in the Carr Fork mine, Utah: in    Econ. Geol.   v82 pp 1319-1333

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