W SKARNS / K05

Cortesia del Gobierno de BRITISH COLUMBIA. Ministerio de Enegia y Minas

Metallics And Coal

GEOLOGICAL CHARACTERISTICS	ECONOMIC FACTORS
EXPLORATION GUIDES	REFERENCES

IDENTIFICATION

Pyrometasomatic or contact metasomatic tungsten deposits.

W (Mo, Cu, Sn, Zn).

(British Columbia - Canada/International): Emerald Tungsten (082FSW010), Dodger (082FSW011), Feeney (082FSW247), Invincible (082FSW218), Dimac (082M123); Fostung (Ontario, Canada), MacTung (Yukon, Canada), Cantung (Northwest Territories, Canada), Pine Creek and Strawberry(California, USA), Osgood Range (Nevada, USA), King Island (Tasmania, Australia), Sang Dong (South Korea).

GEOLOGICAL CHARACTERISTICS

Scheelite-dominant mineralization genetically associated with a skarn gangue.

Continental margin, synorogenic plutonism intruding deeply buried sequences of eugeoclinal carbonate-shale sedimentary rocks. Can develop in tectonically thickened packages in back-arc thrust settings.

Mainly Mesozoic, but may be any age. Over 70% of the W skarns in British Columbia are related to Cretaceous intrusions.

Pure and impure limestones, calcareous to carbonaceous pelites. Associated with tonalite, granodiorite, quartz monzonite and granite of both I and S-types. W skarn-related granitoids, compared to Cu skarn- related plutonic rocks, tend to be more differentiated, more contaminated with sedimentary material, and have crystallized at a deeper structural level.

Stratiform, tabular and lens-like orebodies. Deposits can be continous for hundreds of metres and follow intrusive contacts.

Igneous textures in endoskarn. Coarse to fine-grained, massive granoblastic to mineralogically layered textures in exoskarn. Biotite hornfelsic textures common.

(Scheelite ± molybdenite ± chalcopyrite ± pyrrhotite ± sphalerite ± arsenopyrite ± pyrite ± powellite. May contain trace wolframite, fluorite, cassiterite, galena, marcasite and bornite. Reduced types are characterized by pyrrhotite, magnetite, bismuthinite, native bismuth and high pyrrhotite:pyrite ratios. Variable amounts of quartz-vein stockwork (with local molybdenite) can cut both the exo and endoskarn. The Emerald Tungsten skarns in British Columbia include pyrrhotite-arsenopyrite veins and pods that carry up to 4 g/t Au.

Exoskarn alteration: Inner zone of diopside-hedenbergite (Hd60- 90, Jo5-20) ± grossular-andradite (Ad 10-50, Spess5-50) ± biotite ± vesuvianite, with outer barren wollastonite-bearing zone. An innermost zone of massive quartz may be present. Late-stage spessartine ± almandine ± biotite ± amphibole ± plagioclase ± phlogopite ± epidote ± fluorite ± sphene. Reduced types are characterized by hedenbergitic pyroxene, Fe-rich biotite, fluorite, vesuvianite, scapolite and low garnet:pyroxene ratios, whereas oxidized types are characterized by salitic pyroxene, epidote and andraditic garnet and high garnet:pyroxene ratios. Exoskarn envelope can be associated with extensive areas of biotite hornfels. Endoskarn alteration: Pyroxene ± garnet ± biotite ± epidote ± amphibole ± muscovite ± plagioclase ± pyrite ± pyrrhotite ± trace tourmaline and scapolite; local greisen developed.

Carbonate rocks in extensive thermal aureoles of intrusions; gently inclined bedding and intrusive contacts; structural and/or stratigraphic traps in sedimentary rocks, and irregular parts of the pluton/country rock contacts.

Sn (K06), Mo (K07) and Pb-Zn (K02) skarns. Wollastonite- rich industrial mineral skarns (K09).

W skarns are separable into two types (Newberry, 1982): reduced skarns (e.g. Cantung, Mactung), formed in carbonaceous rocks and/or at greater depths, and oxidized skarns (e.g. King Island ), formed in hematitic or non-carbonaceous rocks, and/or at shallower depths. Late retrograde alteration is an important factor in many W skarns because, during retrogression, the early low-grade mineralization is often scavenged and redeposited into economic high-grade ore zones (e.g. Bateman, 1945; Dick, 1976, 1980). Dolomitic rocks tend to inhibit the development of W skarns; consequently magnesian W skarns are uncommon. In British Columbia they are preferentially associated with Cretaceous intrusions and hosted by calcareous, Cambrian age cratonic, pericratonic and displaced continental margin rocks in the Cassiar, Kootenay-Barkerville, Dorsay and Ancestral North American terranes.

EXPLORATION GUIDES

W, Cu, Mo, As, Bi and B. Less commonly Zn, Pb, Sn, Be and F geochemical anomalies

ECONOMIC FACTORS

Grades range between 0.4 and 2 % WO3 (typically 0.7 %). Deposits vary from 0.1 to >30 Mt.

Skarn deposits have accounted for nearly 60 % of the western world's production, and over 80 % of British Columbia's production.

REFERENCES

• Bateman, P.C. (1945): Pine Creek and Adamson Tungsten Mines, Inyo County, California; California Journal Mines Geology, Volume 41, pages 231-249.

• Dawson, K.M., Panteleyev, A. and Sutherland Brown, A. (1991): Regional Metallogeny, Chapter 19, in Geology of the Cordilleran Orogen in Canada, Gabrielse, H. and Yorath, C.J., Editors, Geological Survey of Canada, Geology of Canada, Number 4, pages 707-768 (also, Geological Society of America, The Geology of North America, volume G-2).

• Dick, L.A. (1976): Metamorphism and Metasomatism at the MacMillan Pass Tungsten Deposit, Yukon and District of MacKenzie, Canada; unpublished M.Sc. thesis, Queens University, 226 pages.

• Dick, L.A. (1980): A Comparative Study of the Geology, Mineralogy and Conditions of Formation of Contact Metasomatic Mineral Deposits in the Northeastern Canadian Cordillera; Unpublished Ph.D. Thesis, Queen's University, 471 pages.

• Eckstrand, O.R. (1984): Canadian Mineral Deposit Types: A Geological Synopsis; Geological Survey of Canada, Economic Geology Report 36, 86 pages.

• Einaudi, M.T. and Burt, D.M. (1982): Introduction - Terminology, Classification and Composition of Skarn Deposits; Economic Geology; Volume 77, pages 745-754.

• Einaudi, M.T., Meinert, L.D. and Newberry, R.J. (1981): Skarn Deposits; in Seventy-fifth Anniversary Volume, 1906-1980, Economic Geology, Skinner, B.J., Editor, Economic Geology Publishing Co., pages 317-391

• Kwak, T.A.P. (1987): W-Sn Skarn Deposits and Related Metamorphic Skarns and Granitoids; in Developments in Economic Geology, Volume 24, Elsevier Publishing Co., 445 pages.

• Kwak, T.A.P. and White, A.J.R. (1982): Contrasting W-Mo-Cu and W-Sn-F Skarn Types and Related Granitoids. Mining Geology. Volume 32(4), pages 339-351.

• Lowell, G.R. (1991): Tungsten-bearing Scapolite-Vesuvianite Skarns from the Upper Salcha River Area, East-central Alaska; in Skarns - Their Genesis and Metallogeny, Theophrastus Publications, Athens, Greece, pages 385-418.

• Newberry, R.J. (1979): Systematics in the W-Mo-Cu Skarn Formation in the Sierra Nevada: An Overview; Geological Society of America, Abstracts with Programs; Volume 11, page 486.

• Newberry, R.J. (1982): Tungsten-bearing Skarns of the Sierra Nevada. I. The Pine Creek Mine, California; Economic Geology, Volume 77, pages 823-844.

• Newberry, R.J., and Swanson, S.E. (1986): Scheelite Skarn Granitoids: An Evaluation of the Roles of Magmatic Source and Process; Ore Geology Review, Number 1, pages 57- 81.

Mineral Deposit

Cortesia del Gobierno de BRITISH COLUMBIA. Ministerio de Enegia y Minas