Torshab Mine Stone

Mineral Deposit Models
DENNIS P. COX and DONALD A. SINGER, Editors

U.S. DEPARTMENT OF THE INTERIOR
MANUEL LUJAN, JR., Secretary
U.S. GEOLOGICAL SURVEY
Dallas L. Peck, Director
Any use of trade, product, or firm names
in this publication is for descriptive purposes only
and does not imply endorsement by the U.S. Government
First printing 1986
Second printing 1987
Third printing 1992

DESCRIPTIVE MODEL OF PODIFORM CHROMITE
By John P. Albers
APPROXIMATE SYNONYM Alpine type chromite (Thayer, 1964).
DESCRIPTION Podlike masses of chromitite in ultramafic parts of ophiolite
complexes (see fig. 20).
GENERAL REFERENCE Dickey (1975).
GEOLOGICAL ENVIRONMENT
Rock Types Highly deformed dunite and harzburgite of ophiolite complexes;
commonly serpentinized.
Textures Nodular, orbicular, gneissic, cumulate, pull-apart; most relict
textures are modified or destroyed by flowage at magmatic temperatures.
Age Range Phanerozoic.
Depositional Environment Lower part of oceanic lithosphere.
Tectonic Setting(s) Magmatic cumulates in elongate magma pockets along spreading plate
boundaries. Subsequently exposed in accreted terranes as part of ophiolite assemblage.
Associated Deposit Types Limassol Forest Co-Ni-S-As.
DEPOSIT DESCRIPTION
Mineralogy Chromite ± ferrichromite ± magnetite ± Ru-Os-Ir alloys ± laurite.
Texture/Structure Massive coarse-grained to finely disseminated.
Alteration None related to ore.
Ore Controls Restricted to dunite bodies in tectonized harzburgite or lower portions of ultramafic
cumulate (see fig. 99).
Weathering Highly resistant to weathering and oxidation.
Geochemical Signature None recognized.
EXAMPLES
High Plateau, Del Norte Cty, USCA (Wells and others, 1946)
Coto Mine, Luzon, PLPN
(LeBlanc and Violette, 1983)
GRADE AND TONNAGE MODEL OF MINOR PODIFORM CHROMITE
By Donald A. Singer and Norman J Page
DATA REFERENCES Singer and others (1980); Calkins and others (1978); Carlson and others (1985).
COMMENTS All deposits in this grade-tonnage compilation are from California and Oregon. The two
largest tonnage deposits are actually districts rather than individual deposits. The majority of
the grades represent shipping grades. Grades less than 35 percent typically represent in-place
"ore". The mixture of shipping grades and in-place grades may explain the significant negative
correlation (r = -0.25) between grade and tonnage. Rh, Ir, Ru, Pd, and Pt grades are based on
reported analyses of samples from the deposits. Unreported PGE grades are probably similar to
those presented here. Rhodium is correlated with chromite (r = 0.35,
n = 69), platinum (r= 0.69, n = 31), iridium (r = 0.47, n = 35), ruthenium (r = 0.56, n = 28).
Ruthenium is correlated with palladium (r = 0.72, n = 21) and iridium (r = 0.59, n = 29). See
figs. 21-23.






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