(a) Give an account of geology, mode of occurrence, genesis and distribution of Pb-Zn deposits of Aravalli region. (15…

(a) Aravalli Pb-Zn deposits

The Aravalli Pb-Zn province of Rajasthan is a classic Proterozoic sediment-hosted metallogenic belt. The deposits are largely stratiform or semi-stratiform, developed in Precambrian metasediments of the Aravalli and Bhilwara supergroups, with mineralization controlled by dolomite and calc-silicate horizons, graphitic (carbonaceous) schists and folded or faulted contacts. Rampura-Agucha (in graphite–mica–sillimanite schist and gneiss, among the world's largest zinc deposits), the Rajpura–Dariba–Bethumni belt and Sindesar Khurd (now the largest producing mine), all in the Bhilwara Supergroup, and the Zawar belt (Mochia, Balaria, Zawarmala, Baroi) in Aravalli dolomites, are type examples; Kayad near Ajmer is a newer find. Ore minerals are sphalerite and galena with pyrite, pyrrhotite and silver, and the host successions are Palaeoproterozoic (about 1.8 Ga). Genesis is debated between syngenetic-diagenetic models, where metals were deposited from basinal brines or seawater during sedimentation, and epigenetic models, where later hydrothermal fluids remobilized and concentrated metals along faults and bedding. In practice, most deposits show a mixed history: primary sedimentary mineralization followed by tectono-thermal reworking during Aravalli orogeny. Distribution is concentrated in the southern Aravalli, especially the Zawar belt and Bhilwara district, making Rajasthan India’s principal Zn-Pb province and the source of nearly all of India's zinc (Hindustan Zinc).

(b) Petroliferous basins and Cambay Basin

India’s petroliferous basins are distributed across onshore and offshore sedimentary provinces. Geographically, they span the northeast, western, central, southern and offshore margins. Onshore, the major producing basins include Assam-Arakan in the northeast, Kutch, Saurashtra, Cambay, Rajasthan (Barmer), Mahanadi, Krishna-Godavari and Cauvery; important exploration or potential basins include Ganga, Vindhyan, Bengal and Jaisalmer. Offshore, the Mumbai offshore (Mumbai High), Krishna-Godavari offshore and Cauvery offshore are the key hydrocarbon provinces. Category-wise, Assam Shelf, Assam-Arakan, Cambay, Rajasthan, Krishna-Godavari, Cauvery and Mumbai offshore are generally treated as producing Category-I basins, while Kutch and Mahanadi are Category-II and Saurashtra, Ganga and Vindhyan remain exploration targets; the Bengal basin became India's eighth producing basin with ONGC's Ashoknagar discovery (2020). Category assignments are periodically revised by the Directorate General of Hydrocarbons. The Cambay Basin is a narrow intracratonic rift (graben) of latest Cretaceous–Tertiary age in Gujarat, following the NNW–SSE Dharwar trend and floored by Deccan Trap basalt. Transverse faults divide it into the Patan–Tharad, Mehsana–Ahmedabad, Tarapur, Jambusar–Broach and Narmada blocks. The fill begins with the Palaeocene Olpad Formation of trap-derived conglomerates and volcaniclastics, followed by the organic-rich Early Eocene Cambay Shale (the principal source rock), the Middle Eocene Kalol Formation and the Hazad–Ankleshwar sandstones (the main reservoirs), and younger Oligocene–Miocene cover. Fields include Ankleshwar (1960, India's first major onshore find after Digboi's era), Kalol, Mehsana, Gandhar and Nawagam. Hydrocarbons are trapped in fault-block, anticlinal and stratigraphic traps, and the basin illustrates a rift-related, thermogenic petroleum system.

(c) VMS deposits

Volcanogenic massive sulphide deposits are formed by seafloor hydrothermal systems in submarine volcanic settings. They are commonly associated with bimodal volcanics, where intermediate to felsic magmatism drives metal-rich fluids that precipitate sulphides on the seafloor. The classic types differ in tectonic setting and metal endowment: Cyprus type (Cu–Zn in ophiolitic basalts at spreading ridges), Kuroko type (Zn–Pb–Cu–Ag–Au in felsic volcanics of arcs and back-arc rifts) and Besshi type (Cu–Zn in mixed mafic volcanics and pelites), but all share a concordant massive sulphide lens above a discordant stringer (stockwork) zone with chloritic–sericitic footwall alteration, metal zoning from Cu-rich base to Zn–Pb-rich top, and exhalative cherts or iron formations on the flanks, the ancient equivalents of modern black smokers. In India, VMS occurrences include the Ambaji–Deri–Basantgarh belt in the South Delhi Fold Belt, the Betul belt in Madhya Pradesh, Ingaldhal in the Chitradurga schist belt of Karnataka, and Sargipalli in the Gangpur Group of Odisha (which some workers class as SEDEX). The Ambaji-Deri belt lies in the South Delhi Fold Belt and is Mesoproterozoic, thus younger than the Paleoproterozoic Aravalli SEDEX-type Zn-Pb deposits; Sargipalli is hosted by Palaeoproterozoic Gangpur Group metasediments-volcanics, while Betul is another Indian volcanogenic province. These deposits are important for copper and zinc, which are now on India's critical-minerals list, and their exploration relies on electromagnetic and gravity surveys over conductive, dense sulphide lenses.