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Pine Creek, Bishop |
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California, USA |
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W
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Super Porphyry Cu and Au
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The Pine Creek mine at Bishop, which is located in the Sierra Nevada province of north-eastern California, USA, has historically been one of the worlds larger skarn scheelite deposits, and a major source of North American tungsten supply. Production ceased in 2000 due to low prices.
It is a scheelite bearing skarn developed within altered limestones (mainly in a Permo-Carboniferous marble) in a sequence of Cambrian to Late Triassic age, adjacent to calc-alkaline intrusives of the Late Triassic to Mid Cretaceous Sierra Nevada Batholith.
The total deposit probably contained around 16 Mt @ 0.56% WO3, comprising,
12 Mt @ 0.6% WO3 (Production to 1978, Mine visit, 1978);
4.125 Mt @ 0.5% WO3 (Reserves, 1978, Mine visit, 1978)
The Pine Creek orebody is hosted by the 11 x 1.5 km Pine Creek pendant which is found in the roof of the Cretaceous Sierra Nevada Batholith. The Sierra Nevada Batholith is a major composite batholith, some 500 km long and up to 100 km wide. It is made up of lesser late Jurassic (from 154 Ma) mafic granitoids, including gabbro, quartz-diorite and tonalite; and more voluminous upper Cretaceous (to 80 Ma) granodiorite and quartz-monzonite. It is an amalgam of individual plutons, comagmatic plutonic and lesser volcanic suites and dykes which range widely in size and shape (Cowan and Bruhn, 1992).
The batholith intrudes Proterozoic and Cambrian sandstones, shales and carbonates on its flanks. The lithologies of the roof pendant however are predominantly upper Palaeozoic and Mesozoic in age, preserved as contact metamorphics. These include Carboniferous pelitic hornfels, micaceous and vitreous quartzite, and marble, with lesser chert, gneiss, schist and conglomerate. These are unconformably overlain by Triassic and Jurassic felsic and mafic meta-volcanics with lesser marble, calc-silicates, quartzite and conglomerate.
The Pine Creek orebody is located on the northernmost section of the western limb of the Pine Creek Pendant, and in the axial zone of the fold nose of the syncline making up the pendant. The mineralised horizon is within the marble unit, immediately adjacent to the granite contact, being less well developed where it is adjacent to the quartz diorite. This unit dips eastwards at 80 to 85°. The axial zone of the syncline is variable, but overall plunges at 45° to the south. The rock sequence from the quartz-monzonite eastwards, is as follows:
In the mine area the quartz monzonite is 30% quartz, 30% plagioclase 30% orthoclase and 10% biotite, with a grain size of 2 to 3 mm.
On the margin of the quartz-monzonite, a band a few cms to 2 m thick of highly siliceous "granite" is developed. This has a reasonably sharp contact with the quartz monzonite, with the change being affected over a 1 to 2 cm interval. The siliceous granite is pale grey in colour, being almost entirely fine quartz with traces of epidote. In places it is a fine quartzite while in others it exhibits a granitic texture.
Adjacent to the siliceous granite there is a quartz-epidote rock from 10 cm to 2 m thick. This has sharp contacts with the enclosing bands, and carries up to 40% epidote as aggregates of fine crystals less than 1 mm in diameter.
The quartz-epidote rock is rimmed along a knife sharp contact by a high garnet zone 2 to 10 cm thick. This comprises 90% garnet as fine grained aggregates. The garnet is 60% grossularite with the remainder being in the range grossularite to spessartine.
This is followed by the "normal" tactite, which comprises 60 to 70% garnet (composition as for the high garnet-zone) with the remainder largely being pyroxene (predominantly hedenbergite). It is usually well banded with 1 to 2 cm thick bands of fine garnet alternating with thinner green pyroxene bands (the latter making up 30 to 40% of the rock). Pods of calcareous sandstone up to 0.5 m thick and 20 m long are occasionally encountered within the tactite. These usually have a surrounding 2 to 3 cm rim of garnet.
The normal tactite has a sharp contact with the unaltered marble. This contact is concordant overall, but in detail cuts across bedding as well as inter-digitating. The marble is well bedded with 2 to 3 mm up to 1 cm thick laminae. In places pods of completely unaltered marble, are found within the normal tactite. These pods are often up to 1 m thick and 20 to 30 m long, once again with a knife sharp contact.
Within the marble, usually close to the tactite contact, bands of wollastonite calc-silicates are found. These range in width from 10 to 20 cm, up to 20 m.
On the northern end of the orebody there is a narrow tabular breccia zone striking north-south and plunging to the south at 80˚. This zone is very irregular in outline and cuts both the intrusive and metasediments. It contains fragments of the immediately adjacent lithologies and has a groundmass of quartz and calcite. In the vicinity of the breccia, scheelite levels have apparently been depleted.
In the nose zone of the pendant syncline, dykes of quartz-monzonite cut the marble. Cross cutting zones of mineralised tactite (ie. quartz epidote, garnet and normal tactite) cut the bedding parallel to the intrusive contacts.
In the vicinity of the orebody the zoning outwards from the quartz-monzonite contact comprises:
i). a 2 m thick band of siliceous 'granite';
ii). 0.1 to 2 m of quartz-epidote rock;
iii). a 2 to 10 cm band of 'high garnet' composed of 60% grossularite with associated spessartine;
iv). well banded 'normal' tactite composed of 60 to 70% garnet with the same composition as the "high garnet" zone, accompanied by pyroxene, mainly hedenbergite;
v). un-altered marble;
vi). lenses of wollastonite rich calc-silicates from 10 cm to 20 m thick, usually close to the "normal" tactite.
Mineralisation occurs as scheelite within the "normal" tactite, with none being found within the adjacent quartz-monzonite. Scheelite is generally present as bands parallel to the banding of the tactite, and average 1 mm in grain size, with some flakes up to 3 to 4 mm across, while elsewhere significant areas carry finer 0.5 mm crystals. The orebody ranges from zero to 50 m in thickness, averaging 2 to 3 m. The ore zone originally outcropped over a 1.5 km strike length, and persisted over a 1.1 km vertical interval, plunging to the south at around 20°.
The most recent source geological information used to prepare this decription was dated: 1982.
Record last updated: 4/12/2012
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.
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Selected References:
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Brown P E, Bowman J R, Kelly W C 1985 - Petrologic and stable isotope constraints on the source and evolution of skarn-forming fluids at Pine Creek, California: in Econ. Geol. v80 pp 72-95
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Newberry R J 1982 - Tungsten-bearing skarns of the Sierra Nevada. I. The Pine Creek Mine, California: in Econ. Geol. v77 pp 823-844
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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