Fe SKARNS / K03

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 iron deposits.

Magnetite (Cu, Ag, Au, Co, phlogopite, borate minerals).

(British Columbia - Canada/International): Tasu (103C003), Jessie (103B026), Merry Widow (092L044), Iron Crown (092L034), Iron Hill (092F075), Yellow Kid (092F258), Prescott (092F106), Paxton (092F107), Lake (092F259); Shinyama (Japan), Cornwall Iron Springs (Utah, USA) Eagle Mountain (California, USA), Perschansk, Dashkesan, Sheregesh and Teya (Russia), Daiquiri (Cuba), San Leone (Italy).

GEOLOGICAL CHARACTERISTICS

Magnetite-dominant mineralization genetically associated with a skarn gangue (includes calcic and magnesian Fe skarns).

Calcic Fe skarns: Intra and non-intraoceanic island arcs; rifted continental margins. Magnesian Fe skarns: Cordilleran-type, synorogenic continental margins.

Can be of any age, mainly Mesozoic to Cenozoic. Typically Early to mid-Jurassic in British Columbia.

Calcic Fe skarns: Fe-rich, Si-poor intrusions derived from primitive oceanic crust. Large to small stocks and dikes of gabbro to syenite (mostly gabbro-diorite) intruding limestone, calcareous clastic sedimentary rocks, tuffs or mafic volcanics at a high to intermediate structural level. Magnesian Fe skarns: Small stocks, dikes and sills of granodiorite to granite intruding dolomite and dolomitic sedimentary rocks.

Variable and includes stratiform orebodies, vertical pipes, fault- controlled sheets, massive lenses or veins, and irregular ore zones along intrusive margins.

Igneous textures in endoskarn. Coarse to fine-grained, massive granoblastic to mineralogically layered textures in exoskarn. Some hornfelsic textures. Magnetite varies from massive to disseminated to veins.

Calcic Fe skarns: Magnetite ± chalcopyrite ± pyrite ± cobaltite ± pyrrhotite ± arsenopyrite ± sphalerite ± galena ± molybdenite ± bornite ± hematite ± martite ± gold. Rarely, can contain tellurobismuthite ± fluorite ± scheelite. Magnesian Fe skarns: Magnetite ± chalcopyrite ± bornite ± pyrite ± pyrhhotite ± sphalerite ± molybdenite.

(both calcic and magnesian): High Fe, low Mn, diopside- hedenbergite clinopyroxene (Hd20-80) and grossular-andradite garnet (Ad20-95), ± epidote ± apatite. Late stage amphibole ± chlorite ± ilvaite ± epidote ± scapolite ± albite ± K-feldspar. Magnesian Fe skarns can contain olivine, spinel, phlogopite, xanthophyllite, brucite, serpentine, and rare borate minerals such as ludwigite, szaibelyite, fluorborite and kotoite.

Calcic Fe skarns: Extensive endoskarn with Na-silicates ± garnet ± pyroxene ± epidote ± scapolite. Magnesian skarns: Minor pyroxene ± garnet endoskarn, and propyllitic alteration.

Stratigraphic and structural controls. Close proximity to contacts between intrusions and carbonate sequences, volcanics or calcareous tuffs and sediments. Fracture zones near igneous contacts can also be important

Cu porphyries (L03, L04); Cu (K01) and Pb-Zn (K02) skarns; small Pb-Zn veins (I05).

In both calcic and magnesian Fe skarns, early magnetite is locally intergrown with, or cut by, garnet and magnesian silicates (Korzhinski, 1964, 1965;. Sangster, 1969; Burt, 1977). Some calcic Fe skarns contain relatively small pockets of pyrrhotite-pyrite mineralization that postdate the magnetite; this mineralization can be Au-rich. Byproduct magnetite is also derived from some Sn, Cu and calcic Pb-Zn skarns. Over 90% of the 146 Fe skarn occurrences in British Columbia lie within the Wrangellia Terrane of the Insular Belt. The majority of these form where Early to mid-Jurassic dioritic plutons intrude Late Triassic limestones.

EXPLORATION GUIDES

Calcic Fe skarn: enriched in Fe, Cu, Co, Au, Ni, As, Cr. Overall Cu and Au grades are low (<0.2% Cu and 0.5 g/t Au). Magnesian Fe skarn: enriched in Fe, Cu, Zn, Bo.

Strong positive magnetic, electromagnetic and induced polarization anomalies. Possible gravity anomalies.

Magnetite-rich float. In the Wrangellia Terrane of British Columbia, the upper and lower contacts of the Late Triassic Quatsino limestone (or equivalent units) are favorable horizons for Fe skarn development.

ECONOMIC FACTORS

Grades are typically 40 to 50 % Fe. Worldwide, calcic Fe skarns range from 3 to 150 Mt whereas magnesian Fe skarns can be larger (exceeding 250 Mt). In British Columbia, they reach 20 Mt and average approximately 4 Mt mined ore.

Worldwide, these deposits were once an important source of iron, but in the last 40 years the market has been increasingly dominated by iron formation deposits. Nearly 90 % of British Columbia's historic iron production was from skarns.

REFERENCES

• Burt, D.M. (1977): Mineralogy and Petrology of Skarn Deposits; Rendiconti, Societa Italiana di Mineralogia e Petrologia, Volume 33 (2), pages 859-873.

• Cox, D.P. and Singer, D.A. (1986): Mineral Deposit Models; U.S. Geological Survey, Bulletin 1693, 379 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. (1982): General Features and Origin of Skarns Associated with Porphyry Copper Plutons, Southwestern North America; in Advances in Geology of the Porphyry Copper Deposits, Southwestern U.S., Titley, S.R., Editor, Univ. Arizona Press, pages 185-209.

• 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

• Korzhinski, D.S. (1964): An Outline of Metasomatic Processes (trans. M.E. Bergunker); International Geol. Review, Volume 6, pages 1713-1734.

• Korzhinski, D.S. (1965): The Theory of Systems with Perfectly Mobile Components and Processes of Mineral Formation; American Journal of Science, Volume 263, pages 193-205.

• Meinert, L.D. (1984): Mineralogy and Petrology of Iron Skarns in Western British Columbia, Canada; Economic Geology, Volume 79, Number 5, pages 869-882.

• Meinert, L.D. (1992): Skarns and Skarn Deposits; Geoscience Canada, Volume 19, No. 4, pages 145-162.

• Podlessky, K.V., Vlasova, D.K. and Kudrja, P.F. (1991): Magnetite-bearing Skarns of Mongolia; in Skarns - Their Genesis and Metallogeny, Theophrastus Publications, Athens, Greece, pages 265-298

• Ray, G.E. and Webster, I.C.L. (1991a): Geology and Mineral Occurrences of the Merry Widow Skarn Camp, Northern Vancouver Island, 92L/6; B. C. Ministry of Energy, Mines and Petroleum Resources, Open File Map 1991-8.

• Ray, G.E., and Webster, I.C.L. (1991b): An Overview of Skarn Deposits; in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera; McMillan, W.J., compiler, B. C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 213-252

• Sangster, D.F. (1969): The Contact Metasomatic Magnetite Deposits of British Columbia; Geological Survey of Canada, Bulletin 172, 85 pages.

Mineral Deposit

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