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12 Cards in this Set
- Front
- Back
2 deposit subtypes |
1. Stratiform (monominerallic horizons in layered intrusions) 2. Podiform (lenses, pods in ophiolites) |
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Grade and tonnage |
100-1000 Mt @ 45-50% Cr2O3 |
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Source of Cr |
- partitions only in chromite - can form solid solution with magnetite - always oxide, never natural |
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Ore textures |
- host rocks with cumulate textures (compaction of cumulate pile and main chamber composition still able to enter the system, additional growth around grains)
- growth boundary between initial and subsequent is distinct (poikilitic texture - interstitual minerals join to form large, background matrix)
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Sequence of ultrabasic cumulates |
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Tectonic settings |
- continental rifts (stratiform) - subduction/collision zones; thrusted oceanic crust - ophiolites (podiform)
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Genesis |
- crystallisation in a mafic intrusion by gravitational settling - earliest freactional crystallisation produced pure chromite
- arguments against: - can't explain lateral continuity across whole chamber - cant explain 99% chromite, normal crystallisation produces layers too small |
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1. Increase FO2 (oxygen fugacity) |
- oxidising conditions, favourable to oxide precipitation - mechanism of increase: decomposition of siderite ( siderite - hematite, carbon monoxide and O could potentially flood chamber with O2) |
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2. Magma Mixing (+arguments against) |
- early magma fractionates in chamber - injection of primitive basaltic magma into earlier fractionated magma - mechanism of chromite precpitation can be explained through phase diagram (ol crystallises first, gets used up reaches boundary and precipitates both, triple point crystallises all 3) - reinjection of more felsic magma drives the reaction into the chromite field, allows for precipitation of pure oxide long enough to produce a thin seam
Arguments against - injection of magma is localised, should produce zonation around intrusion and does not explain lateral continuity |
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3. Assimilation of country rock (+arguments against) |
- Hot basaltic magmas (>1300C) intrude upper crustal rocks with low melting points - causes melting of country rock and mixing of melt with primitive basalt
Arguments against - would produce downward plumes or chromite not long stratiform layers |
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4. Pressure change in chamber |
- at low pressures, reaction is driven from the phase boundary into the chromite field |
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Exploration (both Stratiform and Podiform) |
Stratiform - identification of well layered mafic-ultrabasic intrusion - prospect below cumulate portion of intrusion
Podiform - carefully prospect within all dunite portions of Alpine-type peridotites |