(a) Explain the processes by which sediment hosted Pb-Zn deposits are formed. Describe the geological setting of Agucha and Zawar…

India’s economic mineral wealth includes sediment-hosted Pb-Zn, diamond-bearing kimberlites and Tertiary coal/lignite, each controlled by distinct basin, mantle and sedimentary processes.

Sediment-hosted Pb-Zn and the Aravalli craton Sediment-hosted Pb-Zn deposits form when metal-rich fluids are expelled from sedimentary basins and precipitate sulfides in carbonate or shale hosts. The causal sequence is source, transport and trap: burial and heating generate brines, faults provide conduits, and carbonate-shale interfaces provide chemical traps. In the SEDEX model, extensional or back-arc basins generate high-temperature, metal-rich brines that rise through faults and precipitate at the seafloor as massive sulfide exhalations; rapid cooling and mixing with seawater cause sulfide precipitation, often producing thick, massive orebodies. SEDEX deposits are commonly associated with volcanic-sedimentary successions. In the MVT model, basin dewatering and brine expulsion from evaporite or shale-rich sequences drive low-temperature, chloride-rich brines along faults; when they meet carbonate rocks, redox and carbonate dissolution reactions replace limestone and dolomite with galena and sphalerite, commonly in vein and replacement forms. MVT deposits are typically hosted in carbonate platforms. In both models, sulfide precipitation is triggered by changes in temperature, pH, redox, or sulfate reduction. The Aravalli craton provides the Proterozoic Delhi Supergroup carbonate-shale host rocks, faulted and folded, that focus these fluids. The Delhi Supergroup includes dolomitic limestones, shales and sandstones, providing varied host lithologies. Later deformation created fault-controlled pathways and structural traps. Agucha, in the Bhilwara belt, is a world-class SEDEX deposit where exhalative sulfides are hosted in carbonate-shale sequences. Zawar, in Udaipur district, is an MVT-type deposit in carbonate rocks, formed by basin-brine replacement and vein filling.

Diamond-bearing kimberlites Diamond-bearing kimberlites form when small-degree mantle melting occurs at depths greater than 150 km in the lithospheric mantle. CO2-H2O volatile fluxing lowers the solidus and creates highly volatile-rich, buoyant melts. These melts ascend rapidly as diatremes, carrying diamonds and lithospheric mantle xenoliths; rapid ascent and decompression prevent diamond destruction and preserve them in the kimberlite. Kimberlites are among the fastest ascending magmas, so they can transport diamonds from mantle depths without prolonged residence at lower pressures. Diamonds crystallize at high pressure in the mantle and are transported as xenocrysts, while mantle xenoliths record the pressure-temperature history of the source. Majhgawan, in Panna district, is India’s only diamond-producing mine; it is a Cretaceous kimberlite pipe whose structure and mantle xenoliths record deep mantle sampling. Wajrakarur, in Khammam district, Andhra Pradesh, is a multiple-pipe kimberlite field, representing a cluster of kimberlite pipes rather than a single producing pipe. This shows that diamond exploration in India must consider regional mapping of multiple pipes.

Tertiary coal in NE India and lignite in Tamil Nadu Tertiary coal in NE India occurs in the foreland basin fill of the Assam-Arakan basin and in the Eocene and Oligocene sequences of the region. In Meghalaya, the Garo, Khasi and Jaintia hills coalfields are hosted mainly in the Eocene Jaintia Group, whereas in Assam coal occurs in the Oligocene Barail Group, including the Tikak Parbat Formation. The deposits form a discontinuous belt from Makum in Assam to Dilli-Jeypore in Meghalaya, reflecting deltaic, fluvial and shallow marine sedimentation in a tectonically active basin. Structural control by basin margins and faults makes the coal seams discontinuous. Many seams are high-sulfur and oil-prone because of organic matter deposited under reducing, marine-influenced conditions. In Tamil Nadu, lignite is concentrated in the Cuddalore Formation, of Miocene-Pliocene age, in a shallow marine-lagoonal environment. The Cuddalore Formation records transgressive-regressive marine cycles, with lignite in lagoonal swamps. The Neyveli deposit is the largest lignite field in India, and the broader Ariyalur-Pondicherry belt contains important shallow lignite seams. Neyveli is exploited by Neyveli Lignite Corporation. Together, these deposits show how sedimentary basins, mantle processes and Tertiary sedimentation control India’s mineral security.