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Ash, Chris (1996): Podiform Chromite, in
Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic
Deposits, Lefebure, D.V. and Hõy, T, Editors, British Columbia Ministry of
Employment and Investment, Open File 1996-13, pages 109-112.
IDENTIFICATION
SYNONYMS: Alpine type; ophiolite
hosted chromite.
COMMODITIES (BYPRODUCTS):
Chromite (may contain platinum group elements Os, Ir and Ru).
EXAMPLES (British Columbia (MINFILE #) -
Canada/International): Castle Mountain Nickel (082ESE091) and
Scottie Creek (092INW001); Guleman ore field (Turkey); Kalimash -
Kukes-Tropoje district, Bulquize and Todo Manco - Bater-Martanesh district
(Mirdita ophiolite, Albania); Tiébaghi ophiolite and Massif du Sud (New
Caledonia), Acoje and Masinloc-Coto (Zambales range/ophiolite, Luzon,
Phillipines); Batamshinsk, Stepninsk, Tagashaisai and Main SE ore fields (Kempirsai
massif, Southern Urals, Russia); Xeraivado and Skoumtsa mines (Vourinos
ophiolite, Greece); Semail ophiolite (Oman); Luobusa, Donqiao, Sartohay,
Yushi, Solun, Wudu and Hegenshan deposits (China) all > 1.5 Mt.
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Deposits of
massive chromitite occur as pods, lenses or layers within ophiolitic
ultramafic rocks.
TECTONIC SETTING: Obducted fragments
of oceanic, lower crustal and upper mantle ultramafic rocks within
accreted oceanic terranes.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: Formed as a primary magmatic differentiate during early
olivine and chrome-spinel crystal fractionation of basaltic liquid at an
oceanic spreading centre; (1) as massive to disseminated pods and lenses
of chrome-spinel surrounded by a dunite envelope within depleted mantle
harzburgite; or (2) as massive to disseminated cumulate layers in dunite
at the base of the crustal plutonic section.
AGE OF MINERALIZATION: Mesozoic and
younger.
HOST/ASSOCIATED ROCK TYPES: Variably
serpentinized peridotite; residual mantle harzburgite; cumulate dunite.
DEPOSIT FORM: Podiform, tabular
lenses, irregular masses, cumulate layers. Pods and lenses typically occur
in clusters of variable size.
TEXTURE/STRUCTURE: Massive to
disseminated, nodular (syn. leopard, grape, bean or shot ore),
chromite net, occluded silicate, orbicular.
ORE MINERALOGY: Chromite.
GANGUE MINERALOGY (Principal and
subordinate): Variably serpentinized olivine and orthopyroxene,
magnetite, iddingsite.
WEATHERING: Black, no noticeable
affects resulting from surface oxidation.
ORE CONTROLS: Proximity to the
crust-mantle transition zone. Restricted to dunite bodies in tectonized
harzburgite below this transition, or lower dunitic portions of ultramafic
cumulate section above it.
GENETIC MODEL: Early fractional
crystallization of chromite from a basaltic liquid either (1) just below
the crust-mantle transition (syn. petrological MOHO) in small magma
pockets or possibly conduits within the residual mantle harzburgite; or
(2) immediately above the crust-mantle transition as cumulate layers
within dunite at the base of the axial magma chamber. Pods and lenses in
harzburgite obtain their diagonistic shape as a result subsolidus to
hypersolidus ductile deformation due to mantle convection.
COMMENTS: Ophiolites of
suprasubduction zone affinity with harzburgite mantle sections appear to
be the only ophiolite type to host economic deposits of podiform chromite.
A lack of any sizable chromite occurrence in British Columbia may reflect
the fact that most ophiolitic complexes in the province are of mid- ocean
ridge affinity. Occurrences of podiform chromite are found in ophiolitic
ultramafic rocks in the Slide Mountain, Cache Creek and Bridge River
terranes. Most of these known occurrences have been reviewed by Hancock
(1990).
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Cr
GEOPHYSICAL SIGNATURE: Gravity
anomaly.
OTHER EXPLORATION GUIDES: Found in
rocks formed near or within the ophiolitic crust- mantle transition zone.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE: Grades
range from 20 to 60% Cr2O3 and are a function of the texture of the
chromite; i.e. amount of chromite relative to gangue serpentinite.
Tonnages are variable, ranging from several thousand tonnes to several
million tonnes.
ECONOMIC LIMITATIONS: The complex
structure and irregular distribution make exploration and development
difficult.
END USES: Chromium has a wide range
of uses in the iron and steel industry which accounts for over 75% of its
use. Chromite is also used in making refractory bricks for furnace linings.
IMPORTANCE: An important source of
metallurgical-type chromite ores (45-60% Cr2O3: Cr/Fe = 2.8-4.3). Podiform
chromite is the only source of refractory-type ore (min. 25% Al2O3: min.
60% Cr2O3 + Al2O3: max. 15% FeO). Historically podiform-type ore fields
account for 57% of all chromite produced.
REFERENCES
Albers, J. P. (1986): Descriptive
Model of Podiform Chromite; in Mineral Deposit Models, Cox, D.P. and
Singer, D.A., Editors, U.S. Geological Survey, Bulletin 1693, page
34.
Christiansen, F.G. (1986):
Structural Classification of Ophiolitic Chromite Deposits; in Metallogeny
of Basic and Ultrabasic Rocks, Gallagher, M.J., Ixer, R.A., Neary, C.R.
and Pichard, H.M., Editors; The Institution of Mining and Metallurgy,
pages 279-289.
Duke, J.M. (1983): Ore Deposit
Models 7. Magmatic Segregation Deposits of Chromite; Geoscience Canada,
Volume 10, Number 1, pages 15-24.
Hancock, K.D. (1990): Ultramafic
Associated Chrome and Nickel Occurrences in British Columbia; B.C.
Ministry of Energy, Mines and Petroleum Resources, Open File 1990- 27,
62 pages.
Roberts, S. (1988): Ophiolitic
Chromitite Formation: A Marginal Basin Phenomenon?; Economic Geology,
Volume 83, pages 1034-1036.
Singer, D.A., Page, N.J. and Lipin, B.R.
(1986): Grade and Tonnage Model of Major Podiform Chromite; in Mineral
Deposit Models, Cox, D.P. and Singer, D.A., Editors, U.S. Geological
Survey, Bulletin 1693, pages 38-44.
Singer, D.A. and Page, N.J. (1986):
Grade and Tonnage Model of Minor Podiform Chromite; in Mineral Deposit
Models, Cox, D.P. and Singer, D.A., Editors, U.S. Geological Survey,
Bulletin 1693, pages 34-38.
Stowe, C.W. (1987): Evolution of
Chromium Ore Fields; Van Nostrund Reinhold Co., New York, 340
pages.
Thayer, T.P. (1964): Principal
Features and Origin of Podiform Chromite Deposits, and Some Observations
on the Guleman-Soridag District, Turkey; Economic Geology, Volume
59, pages 1497- 1524. |
