Model Answer
0 min readIntroduction
Chromite (FeCr₂O₄) is a primary ore of chromium, an essential metal in stainless steel production. Platinum Group Elements (PGEs) – platinum, palladium, rhodium, ruthenium, iridium, and osmium – are highly valued for their catalytic and industrial applications. Both chromite and PGEs are frequently associated with layered mafic and ultramafic igneous intrusions. The Bushveld Igneous Complex in South Africa is the world’s largest layered igneous intrusion and a major source of both chromite and PGEs. The Sukinda chromite deposit in Odisha, India, is one of the largest chromite deposits globally, though PGE concentrations are generally lower than in the Bushveld Complex. Understanding their modes of occurrence and mineralization characteristics is crucial for exploration and resource management.
Bushveld Igneous Complex: Chromite and PGE Mineralization
The Bushveld Complex is a Proterozoic layered intrusion consisting of a basal zone, a chromitite layer, and an upper group. Chromite mineralization is concentrated within the Chromitite Layer, a series of discontinuous layers within the Lower Group. PGE mineralization is most significant in the Merensky Reef and UG2 Reef, located within the Upper and Lower Groups respectively.
Mode of Occurrence – Chromite
- Layered Intrusion: Chromite occurs in layers, often associated with olivine-rich cumulate rocks.
- Cumulate Textures: Chromite crystals are typically found as euhedral to subhedral grains within cumulate textures, formed by the settling of crystals from a magma.
- Association with Silicates: Chromite is commonly associated with olivine, pyroxene, and plagioclase.
Nature of Chromite Mineralization
- Composition: Bushveld chromite is typically ferrochromite (high iron content).
- Layering: The chromitite layers exhibit rhythmic layering, reflecting changes in magma composition and flow.
- Alteration: Chromite can be altered to magnesiochromite or spinel due to metamorphic processes.
Mode of Occurrence – PGEs
- Merensky Reef: PGEs occur in a thin, discontinuous reef consisting of chromitite, sulfides (pyrrhotite, pentlandite, chalcopyrite), and silicate minerals.
- UG2 Reef: PGEs are concentrated in a broader, more continuous layer containing chromitite and sulfides.
- Magmatic Segregation: PGEs are believed to have been concentrated by liquid immiscibility, forming sulfide melts that segregated from the silicate magma.
Nature of PGE Mineralization
- Sulfide Association: PGEs are primarily found as alloys within sulfide minerals, particularly pyrrhotite and pentlandite.
- High Tenor: The Bushveld Complex is known for its high PGE tenor (PGE concentration per ton of ore).
- Fractional Crystallization: PGE enrichment is attributed to fractional crystallization and the preferential partitioning of PGEs into sulfide melts.
Sukinda Chromite Deposit: Mode of Occurrence and Nature
The Sukinda chromite deposit is located within the Eastern Ghats Mobile Belt of Odisha, India. It is associated with ultramafic intrusions emplaced during the Precambrian period. The deposit is characterized by massive chromitite lenses and pods within a serpentinized peridotite host rock.
Mode of Occurrence – Chromite
- Disseminated and Massive: Chromite occurs as both disseminated grains and massive lenses within serpentinized peridotite.
- Podiform Bodies: The chromitite bodies are typically podiform, lacking the extensive layering seen in the Bushveld Complex.
- Tectonic Control: The formation of the chromitite lenses is thought to be influenced by tectonic activity and faulting.
Nature of Chromite Mineralization
- Composition: Sukinda chromite is generally aluminiferous chromite (high aluminum content).
- Serpentinization: The host peridotite has undergone extensive serpentinization, altering the surrounding rocks.
- Lower PGE Content: PGE concentrations are significantly lower compared to the Bushveld Complex.
PGE Mineralization in Sukinda
- Limited Occurrence: PGE mineralization is less prominent and occurs in association with sulfide minerals (pyrrhotite, pentlandite) within the chromitite lenses.
- Hydrothermal Influence: Hydrothermal alteration may have played a role in PGE mobilization and concentration.
- Lower Tenor: The PGE tenor is considerably lower than in the Bushveld Complex, making economic recovery more challenging.
Comparative Table: Bushveld vs. Sukinda
| Feature | Bushveld Igneous Complex | Sukinda Chromite Deposit |
|---|---|---|
| Geological Setting | Layered Mafic-Ultramafic Intrusion | Ultramafic Intrusion within Eastern Ghats Belt |
| Chromite Occurrence | Layered, Rhythmic | Disseminated & Podiform |
| PGE Occurrence | Reef-like zones (Merensky, UG2) | Associated with chromitite lenses |
| PGE Tenor | High | Low |
| Alteration | Moderate | Intense Serpentinization |
Conclusion
In conclusion, both the Bushveld Igneous Complex and the Sukinda chromite deposit represent significant sources of chromite, but differ substantially in their PGE endowment. The Bushveld Complex’s layered structure and magmatic processes facilitated the concentration of both chromite and PGEs to economic levels. Sukinda, while a major chromite deposit, exhibits lower PGE concentrations due to different geological conditions and a greater influence of hydrothermal alteration. Further research into the specific geological controls on PGE mineralization in Sukinda could potentially unlock its PGE potential.
Answer Length
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