Model Answer
0 min readIntroduction
The Aravalli region, primarily spanning Rajasthan, is one of India's most significant metallogenic provinces, renowned for its rich lead-zinc (Pb-Zn) deposits. These deposits are predominantly associated with the Proterozoic Aravalli Supergroup and Bhilwara Supergroup, representing some of the oldest carbonate-hosted Pb-Zn orefields globally. The region's complex geological history, marked by multiple phases of sedimentation, deformation, and metamorphism, has created diverse settings conducive to the formation and concentration of these economically vital base metals. Understanding the geological context, modes of occurrence, genesis, and distribution of these deposits is crucial for mineral exploration and resource management in India.
Geology of the Aravalli Region and Pb-Zn Deposits
The Aravalli region is characterized by a complex tectono-stratigraphic framework, comprising Archaean basement gneisses (Banded Gneissic Complex or Mewar Gneiss) overlain by Proterozoic supracrustal sequences, primarily the Bhilwara Supergroup and the Aravalli Supergroup. The Pb-Zn mineralization is intimately associated with these Proterozoic metasedimentary and metavolcanic rocks.
- Bhilwara Supergroup: This Archaean to Paleoproterozoic basement complex consists of heterogeneous metamorphics, migmatites, granitoids, and mafic rocks. Significant Pb-Zn deposits like Rampura Agucha are hosted within the graphite-mica-sillimanite gneiss/schist of the Bhilwara Supergroup, specifically at the sheared transition between the Mangalwar and Sandmata Complexes.
- Aravalli Supergroup: This Paleoproterozoic metasedimentary succession unconformably overlies the Archaean basement. It comprises a varied sequence of quartzites, phyllites, dolomites, and mica schists, deposited in environments ranging from shallow marine shelves to deeper basinal settings. Major deposits like Zawar and Rajpura-Dariba are hosted within the carbonate sequences (dolomites) and associated carbonaceous phyllites of the Aravalli Supergroup.
Mode of Occurrence
The Pb-Zn deposits in the Aravalli region exhibit various modes of occurrence, largely influenced by the host rock lithology and the degree of deformation and metamorphism.
- Stratabound Deposits: Many deposits, particularly in the Zawar belt, are dominantly stratabound, meaning the ore bodies are confined to specific stratigraphic horizons. The mineralization often occurs as finely bedded layers within carbonaceous phyllites and dolomites, suggesting a syngenetic or early diagenetic origin.
- Disseminated Ores: Fine-grained sulphides (sphalerite and galena) are disseminated throughout the host rocks, particularly in quartzites and schists, as seen in deposits like Kayad.
- Vein and Massive Ores: Epigenetic mineralization is significant, especially in structurally controlled settings. Ore bodies occur as massive sulphide lenses, veins, and lodes along shear zones, fold hinges, and fractures. This is prominent in deposits like Rampura Agucha and parts of Zawar, where later deformation events remobilized and concentrated the earlier stratabound mineralization.
- Laminated Ores: In certain schistose rocks, ores can occur as laminations conforming to the regional schistosity and folding.
Genesis of Pb-Zn Deposits
The genesis of Pb-Zn deposits in the Aravalli region is complex and often polygenetic, involving initial syngenetic deposition followed by significant remobilization and enrichment during later tectonothermal events.
The primary models include:
- Sedimentary Exhalative (SEDEX) Type: Many deposits, including Rampura Agucha, Rajpura-Dariba, and initially Zawar, are interpreted to have originated as SEDEX deposits. This model suggests that metal-rich hydrothermal fluids discharged into anoxic seafloor basins, leading to the precipitation of sulphides alongside sediments.
- Evidence includes stratabound nature, association with carbonaceous sediments and volcanic rocks, and isotopic signatures indicative of marine environments and biogenic processes.
- For example, Rampura Agucha is considered to have been originally a SEDEX deposit before experiencing high-grade metamorphism.
- Carbonate-Hosted (Mississippi Valley Type - MVT) and Hybrid Deposits: The Zawar deposits, hosted primarily in dolomites of the Aravalli Supergroup, exhibit characteristics of both SedEx and MVT deposits. While initial stratabound proto-ore may have formed in a marine setting, subsequent deformation and fluid flow led to significant epigenetic mineralization.
- Remobilization of sulphides during folding and faulting (Aravalli Orogeny) created vein and massive ores. Isotopic studies suggest involvement of both marine and magmatic fluids, later mixing with meteoric water.
- Polygenetic Evolution: The intricate deformational history of the Aravalli Supergroup implies that initial syngenetic sulphide mineralization occurred during early basin development. These stratiform ore bodies were subsequently affected by successive deformation events (D1 and D2 phases of the Aravalli orogeny), which remobilized and concentrated the mineralization along axial-planar cleavages, fold hinges, and shear zones. Later tectonothermal overprints further recrystallized and upgraded these deposits.
Distribution of Pb-Zn Deposits
The major Pb-Zn deposits in the Aravalli region are concentrated in specific belts within Rajasthan. Hindustan Zinc Limited (HZL) operates most of these significant mines.
Key Mineralized Belts and Deposits:
| Mineralized Belt | Major Deposits/Mines | Key Geological Features/Host Rocks | Remarks |
|---|---|---|---|
| Bhilwara Belt (e.g., Rampura Agucha) | Rampura Agucha Mine | Graphite-mica-sillimanite gneiss/schist of the Bhilwara Supergroup (Pre-Aravalli Banded Gneissic Complex). High-grade metamorphosed sedimentary exhalative deposit. | World's largest zinc mine by reserves, known for high-grade Zn-Pb-Ag ore. Owned by HZL. |
| Zawar-Jahazpur Belt | Zawar Mines (Mochia, Baroi, Balaria, Zawarmala) | Carbonate-hosted (dolomite) and carbonaceous phyllites of the Aravalli Supergroup. Stratabound with significant epigenetic remobilization. | One of the oldest carbonate-hosted zinc-lead orefields, with historical mining activities dating back 2100 years. Owned by HZL. |
| Rajpura-Dariba-Sindesar Khurd Belt | Rajpura Dariba Mine, Sindesar Khurd Mine | Metasedimentary sequences (recrystallized dolomite, graphite-mica schist) of the Aravalli Supergroup. Considered SEDEX type, modified by deformation and metamorphism. | Known for polymetallic assemblage of Pb, Zn, Ag, Cd, and Cu. Owned by HZL. |
| Kayad Mine | Kayad Mine | Disseminated ores in quartzite and quartz-mica schist; massive ores at fold hinges. Occurs within the Proterozoic Aravalli-Delhi fold belt. | Younger discovery with significant reserves. Owned by HZL. |
The distribution of these deposits is closely linked to the tectonic evolution of the Aravalli-Delhi Orogenic Belt, with mineralized zones often occurring along major lineaments, shear zones, and within specific lithostratigraphic units that provided favorable conditions for ore formation and preservation.
Conclusion
The Aravalli region stands as a cornerstone of India's non-ferrous metal resources, primarily due to its extensive and rich Pb-Zn deposits. These deposits, deeply rooted in the geological history of the Proterozoic Aravalli and Bhilwara Supergroups, showcase diverse modes of occurrence ranging from stratabound to epigenetic vein systems. Their genesis reflects a complex interplay of sedimentary exhalative processes, followed by significant remobilization and enrichment during multiple phases of deformation and metamorphism. The continued exploration and sustainable exploitation of these world-class deposits, particularly by entities like Hindustan Zinc Limited, are vital for India's industrial growth and self-reliance in base metals.
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