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

(a) Sediment-hosted Pb-Zn deposits, Agucha and Zawar

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 typically hosted in fine-grained clastic–carbonate successions with little direct volcanic association. 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 Aravalli and Bhilwara Supergroup carbonate-shale host rocks, faulted and folded, that focus these fluids. Rampura-Agucha, in Bhilwara district, is a world-class metamorphosed SEDEX deposit and India’s largest zinc mine (Hindustan Zinc), where sphalerite-galena ore with pyrite and pyrrhotite lies in graphite-mica-sillimanite schists and gneisses of the Bhilwara Supergroup, folded and metamorphosed to upper amphibolite facies. Zawar, south of Udaipur, is carbonate-hosted: sphalerite and galena occur in dolomites of the Aravalli Supergroup as stratabound lodes and veins along fold hinges and faults at Mochia, Balaria, Zawar Mala and Baroi; its genesis is debated between syngenetic-diagenetic SEDEX and epigenetic MVT-type replacement. Zawar is also known for ancient zinc smelting dating back over two thousand years.

(b) Diamond-bearing kimberlites, Majhgawan and Wajrakarur

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. By Clifford’s rule, diamondiferous pipes lie on old, thick, cold cratons with deep lithospheric roots. A pipe has a root zone of hypabyssal kimberlite, a carrot-shaped diatreme of tuffisitic breccia and a crater facies at the top.

Majhgawan, in Panna district of Madhya Pradesh, is India’s only mechanised diamond mine, run by NMDC; it is a Mesoproterozoic (~1.1 Ga) pipe intruding Vindhyan sediments on the northern Bundelkhand craton, now usually classed as a lamproite rather than a true kimberlite. Wajrakarur, in Anantapur district, Andhra Pradesh, is a field of more than forty kimberlite pipes of about 1.1 Ga intruding the granite-greenstone terrain of the Eastern Dharwar craton; several pipes are diamondiferous, and the region has long yielded alluvial diamonds, but none is in regular production.

(c) Tertiary coal of NE India and lignite of 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 to Dilli-Jeypore in Upper Assam, 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 further deposits occur at Jayamkondam and Mannargudi in the Cauvery basin. Neyveli is exploited by NLC India Limited (formerly Neyveli Lignite Corporation) mainly for power generation.