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Thompson Creek
Idaho, USA
Main commodities: Mo

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The Thompson Creek porphyry molybdenum deposit is located ~30 km SW of Challis, in the Bayhorse Mining District, Custer County in east-central Idaho, USA (#Location: 44° 18' 59"N, 114° 33' 12"W).

The deposit lies near the eastern border of the ~100 to 75 Ma late Cretaceous ldaho Batholith, within a complexly deformed sequence of Palaeozoic sedimentary rocks. Molybdenum mineralisation occurs within the Thompson Creek Intrusive Complex, a composite biotite granodiorite-quartz monzonite stock considered to be genetically related to the Idaho Batholith intrusive event. Section of the host stock and intruded Carboniferous country rocks are exposed in an erosional window within the unconformably overlying Challis Volcanic Sequence in Pat Hughes Creek, ~2.5 km upstream from its junction with Thompson Creek. The stock intrudes thick bedded, dark grey to black, carbonaceous, and locally lime-bearing argillites of the Lower Carboniferous (Mississippian) Copper Basin Formation. Argillites near the intrusive contact have undergone some silicification and hornfelsing, whilst isolated outcrops of light-grey to yellowish-grey quartzite that are locally completely shattered and brecciated, are interpreted to represent either the Lower Carboniferous Wood River Formation, or another undifferentiated member of the Copper Basin Formation. The intrusive and sedimentary rocks are unconformably overlain by the extensive, post-mineral, extrusive, Eocene Challis Volcanic Sequence. This volcanic cover sequence, which is locally up to 300 m thick, is a heterogeneous mixture of lithic tuffs, andesite flows and agglomerates covering a major portion of the orebody (Schmidt et al., 1983).

Two main phases have been recognised in the Thompson Creek Intrusive Complex. These phases are chemically similar, but have some textural variations. They range from biotite granodiorite to biotite quartz monzonite, with the former generally forming a border zone surrounding a core of quartz monzonite. The granodioritic rocks form a complex series of tabular arcuate 'rinds' lacking systematic textural and compositional zoning, but generally becoming younger and more felsic inward. These igneous phases are elongated at ~315° and plunge ~23°NW. Individual phases have contacts that are either sharp or gradational over distances of tens of metres. Chill and breccia textures are rare, and breccia pipes are absent. These granodioritic rocks of the outer zone are petrographically similar to one another, and are phaneritic and medium-grained, with plagioclase varying from oligoclase to andesine. Anorthite and quartz vary within a narrow range from 25 to 35%. The rocks differ in relative grain size, in their plagioclase to K feldspar ratio, and in biotite content (8 to 12%). Accessory minerals are apatite, sphene, zircon and monazite. The central core of the complex is a leucocratic quartz monzonite, the Thompson Creek Quartz Monzonite, which is the youngest igneous phase of volumetric significance, and it is interpreted to be cogenetic with the granodioritic rocks of the border zone. The Thompson Creek Quartz Monzonite is interpreted to have developed from the original stock by late magmatic growth of K feldspar, quartz and biotite. It occurs as an elongate, NW trending intrusive with arcuate upper contacts. It essentially comprises 25 to 35% quartz, plagioclase (An28-32), 57 to 67% K feldspar and <8% biotite. The quartz monzonite texturally differs slightly from the granodioritic phases in that it is medium to coarse grained with a weakly porphyritic texture. In addition, unidirectional crystallisation textures of coarse biotite and oriented K feldspar laths at phase contacts indicate the magma from which the quartz monzonite crystallised was volatile-rich, a texture absent from the granodioritic rocks. Two separate bodies of apparently identical quartz monzonite are mapped within the central and northwestern portions of the intrusive complex, apparently coalescing to the SE where the quartz monzonite is in steep contact with similar leucocratic granodioritic rocks. These may represent unrelated igneous events, or alternatively suggest the lower Thompson Creek Quartz Monzonite body may be underlain by a deep granite porphyry mass (Schmidt et al., 1983).

Surface mineralisation and related rock alteration has been outlined over a NW elongated 750 x 2500 m area. Silicification and pyrite mineralisation is present in the argillite of the Copper Basin Formation throughout this zone. Original carbonate-rich units are now converted to massive skarn and/or calc-silicate rocks. Molybdenite occurs primarily in coarse quartz-biotite-feldspar-muscovite-pyrite veins and veinlets trending at 300 to 315° and dipping at 60 to 80°NE. Flat-lying quartz-molybdenite veinlets are also well exposed underground. Several major post-mineral fault sets have affected the geometry of the mineral deposit. NW-trending normal faults dropped a block of weakly mineralised material 30 to 60 m downward on the NE side, whilst a flat NE-trending fault set with possible reverse movement has been recognised in the NW. Dip-slip displacements of 30 to 60 m are suggested by these structures. Prominent NE-trending normal faulting partly controls basaltic andesite dyke intrusion to the NW of the underground workings. These andesite dykes cut the Challis Volcanic Sequence and may be local feeders for these extrusive rocks (Schmidt et al., 1983).

Apart from weak but pervasive argillic alteration, silicate alteration minerals are restricted to narrow zoned vein selvages that rarely persist for more than a few centimetres into the host rock from the vein margin. To the untrained eye, the host rock in the ore zone appears to be unaltered. Three main hydrothermal alteration zones have been identified. However, three complex and locally overlapping alteration zones have been recognised in these vein selvages, from the central core to the upper portions of the igneous and hydrothermal system:
i). a lower potassic zone characterised by veins of salmon-coloured orthoclase/microcline and quartz with minor biotite;
ii). a main potassic zone comprising quartz, coarse secondary biotite and white- to cream-coloured orthoclase/microcline;
iii). a upper phyllic zone containing coarse sericite/muscovite and pyrite in quartz veins, and also includes partial replacement of the early potassic minerals by the phyllic assemblage.
Weak, pervasive argillic (smectite) alteration of plagioclase and clay, chlorite and calcite concentrations occur on late barren fractures and is of lesser significance, whilst a peripheral propylitic halo, areas of silicification, and quartz-magnetite veining appear to be absent (Schmidt et al., 1983).

The three major alteration zones/assemblages are complex and locally overlapping, and rather than reflecting separate events, probably represent a fundamentally continuous evolution of the hydrothermal fluids with time and distance from source. The orebody is essentially coincident with the main potassic alteration zone. Whilst numerous vein events are represented, the veining in the barren lower potassic zone apparently grade upward into the molybdenite-rich veins of the orebody in the main potassic zone. These molybdenite-rich veins, in turn, then grade mineralogically upward with a diminution of molybdenite and biotite. Retrograde phyllic alteration is found locally throughout the system, but is more intense in the uppermost parts of the system (Schmidt et al., 1983).

The molybdenum mineralisation is associated with coarse biotite, K feldspar and minor pyrite of the main potassic alteration zone, occurring as a network of quartz-biotite-pyrite veins and veinlets with a potassic selvage that display strongly preferred orientations rather than crackle breccia-like randomness. The dominant vein set strikes at 300 to 320° and dips at 30 to 85°NE, parallel to the long dimension of both the intrusive and the orebody, implying that the same or a similar stress field played a role in controlling both the intrusion of the igneous rocks and opening the space occupied by the veining (Schmidt et al., 1983).

Molybdenum is the sole product of the Thompson Creek mine, with the ore generally accompanied by <100 ppm Cu. Molybdenite occurs as either i). coarse 2 to 4 mm rosettes within vein selvages abutting the potassic minerals or ii). in the centre of quartz veins. Vein thicknesses average between 1 and 3 cm with a vein density that is highly variable, depending upon the location within the deposit (Schmidt et al., 1983).

Dating of hydrothermal biotite from quartz-molybdenite vein alteration assemblage of the main potassic alteration zone yielded a K-Ar age of 86.9±3 Ma, and muscovite from the upper phyllic alteration zone gave a K-Ar age of 85.9±3 Ma (Marvin et al., 1973). The K-Ar age determinations of various granodioritic and quartz monzonitic phases of the Thompson Creek Intrusive Complex cannot be distinguish differences in their ages. A granodioritic sample yielded a primary biotite age of 88.4 ±3 Ma, whilst primary biotite from leucocratic quartz monzonite yielded an age of 86.5 ±3 Ma. These ages fall within the ~100 to 75 Ma range of the nearby Idaho batholith (Armstrong et al., 1977). The rock dates appear to overlap the alteration ages, suggesting that molybdenum mineralisation was more or less coeval with crystallisation of the last igneous phases. Except for some intermineral granite porphyry dykes, all other granitoid phases are pre-mineral, and deposition of the hydrothermal minerals is interpreted to have begun shortly after crystallization of the Thompson Creek quartz monzonite and the granite porphyry dykes (Schmidt et al., 1983).

The orebody has plan dimensions of around 1300 x 300 m and extends over a vertical interval of 550 m, with associated intense potassic alteration (USBM, from unpublished Cyprus reports). The ore appears to straddle the transition zone between the quartz-monzonite core and the granodiorite in its upper sections, being present in both lithologies, near the contact with the intruded meta-sediments (Tooker, 1991).

The Thompson Creek mine commenced operations in 1983. It was initially operated by the Cyprus Minerals Company. In 1992 production amounted to 6750 t of Mo (Am. Mines H'book, 1994, p 66). In 2007 the mine was owned by Blue Pearl Mining through its holdings in the Thompson Creek Mining Company acquired in late 2006. The operation has passed via other owners to now (2021) being held by Centerra Gold Inc. The mine and concentrator were placed on care and maintenance in December 2014 and remained a such in 2021.

Schmidt et al., 1983 quote a non-NI 43-101 compliant reserve estimate, based on ~95 500 m of drilling and 2620 m of underground development work, at a 0.05% Mo cut-off, of:
  ~200 Mt @ 0.11% Mo, with a waste to ore stripping ratio of ~3:1.

Published NI 43-101 compliant resources and reserve estimated in 2006 were (Blue Pearl Mining website 2006):
  Measured + Indicated Mineral Resources - 178.6 Mt @ 0.094% Mo
  Inferred Mineral Resources - 34.5 @ 0.06% Mo
  Proved + Probable Ore Reserves - 64.5 Mt @ 0.119% Mo.

Remaining NI 43-101 reserves and resources as at 31 December, 2020 (Centerra Gold Reserves and Resources Report, 2020) were:
  Total Measured + Indicated Mineral Resources - 117.143 Mt @ 0.04% Mo;
  Inferred Mineral Resources - 0.806 Mt @ 0.04% Mo;
  As the Ore Reserves calculated were no longer regarded viable, they were re-classified as mineral resources and no Ore Reserves are quoted.

The most recent source geological information used to prepare this decription was dated: 2006.     Record last updated: 10/3/2021
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.

Thompson Creek

  References & Additional Information
   Selected References:
Schmidt, E.A., Broch, M. and White, R.,  1983 - Geology of the Thompson Creek molybdenum deposit, Custer County, Idaho: in   The genesis of Rocky Mountain ore deposits; changes with time and tectonics, The Denver Region Exploration Geologists Society symposium, 1983,   Proceedings, pp. 79-84.

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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