Answer the following in about 150 words each: (a) What are the major changes in the process of formation of uranium deposits…

(a) Evolution of Uranium Deposits through Geological Time

The temporal evolution of uranium deposits is intimately linked to the oxygenation of Earth's atmosphere and hydrosphere. During the anoxic Archean and early Paleoproterozoic (>2.4 Ga), uranium remained in its reduced, insoluble tetravalent state (U⁴⁺). It was transported mechanically as detrital grains, forming quartz-pebble conglomerate placer deposits, exemplified by the Witwatersrand Basin in South Africa and Elliot Lake in Canada.

Following the Great Oxidation Event (GOE, ~2.4–2.2 Ga), the rise of atmospheric oxygen converted uranium into the highly soluble hexavalent uranyl ion (UO₂²⁺). This enabled widespread chemical transport by oxidizing groundwaters and hydrothermal fluids, leading to high-grade Proterozoic unconformity-related deposits at the interface of basement rocks and oxidized cover sediments, such as the Athabasca Basin (Canada) and comparable settings in India's Cuddapah Basin.

In the Phanerozoic, the proliferation of land plants supplied abundant organic reductants in terrestrial sedimentary basins. This promoted the formation of low-temperature, sandstone-hosted roll-front and tabular deposits (e.g., the Colorado Plateau and Mahadek Basin in Meghalaya), marking a transition from physical placer concentrations to redox-controlled geochemical traps.

(b) Geological Setting of Singhbhum and Khetri Copper Deposits

The Singhbhum Shear Zone (SSZ) in eastern India is a 200-km-long, arcuate ductile shear zone situated within an Archean granite-greenstone terrane. Mineralization is localized along thrust contacts within deformed chlorite-biotite-quartz schists and metavolcanics of the Chaibasa and Dhanjori formations. It exhibits Iron-Oxide-Copper-Gold (IOCG) affinities, where chalcopyrite occurs intimately associated with apatite-magnetite and uraninite. Hydrothermal fluids and thermal gradients were driven by the intrusive Singhbhum Granite, with mineralization strictly controlled by structural shear fabric.

In contrast, the Khetri Copper Belt in Rajasthan occupies a NE–SW-trending zone within the Proterozoic North Delhi Fold Belt. Mineralization is hosted in multiply-deformed, amphibolite-facies metasediments of the Delhi Supergroup, primarily garnetiferous chlorite schists, quartzites, and phyllites of the Alwar and Ajabgarh Groups. Chalcopyrite and pyrrhotite occur as stratabound disseminations, stringers, and vein fillings within reverse faults, hinge zones of tight folds, and brittle-ductile shear zones, representing a structurally controlled, metamorphosed sediment-hosted deposit.

(c) Mass Balance Calculation for Copper Beneficiation

The daily tonnage of copper ore concentrate produced is determined through metallurgical mass balance:

1. Copper mass contained in the daily feed: Cu_feed = 12000 tons × 0.8/100 = 96.0 tons

2. Copper mass recovered into the concentrate at 80% recovery: Cu_recovered = 96.0 tons × 80/100 = 76.8 tons

3. Total mass of concentrate produced at 25 wt.% Cu grade: Mass of concentrate = (76.8 tons)/0.25 = 307.2 tons

The beneficiation plant produces 307.2 tons of copper ore concentrate per day.

(d) Thermodynamic Equilibrium, Entropy, Enthalpy, and Gibbs Free Energy

Thermodynamic Equilibrium is the state of a system in which all intensive properties (temperature, pressure, chemical potential) are uniform and no spontaneous driving force or net energy transfer occurs, defined by a minimum in Gibbs free energy (Δ G = 0) at constant T and P.

Enthalpy (H = U + PV) denotes the total heat content of a system at constant pressure. In geological systems, the change in enthalpy (Δ H) dictates whether metamorphic reactions or magma crystallization processes are exothermic (Δ H < 0) or endothermic (Δ H > 0).

Entropy (S) quantifies the degree of microscopic randomness or energy dispersion in a system. Spontaneous natural transformations lead to an increase in total entropy (Δ S > 0).

Gibbs Free Energy (G = H - TS) combines enthalpy, entropy, and absolute temperature into a single thermodynamic potential. The sign of Δ G serves as the fundamental criterion for reaction spontaneity (Δ G < 0) and governs mineral phase stability boundaries and reaction univariant curves on petrogenetic grids.

(e) Environmental Hazards Caused by Mining

Mining activities induce severe multi-dimensional environmental degradations across physical, biological, and human spheres.

Acid Mine Drainage (AMD) represents a major geochemical hazard, caused by the atmospheric oxidation of exposed sulfide minerals (mainly pyrite), producing low-pH sulfuric acid leachates enriched in heavy metals such as cadmium, lead, and arsenic at base-metal mining sites like Zawar and Khetri.

Underground extraction leads to critical land subsidence and long-burning subsurface fires, prominently seen in the Jharia Coalfield, whereas open-cast mining triggers massive deforestation, topsoil erosion, and loss of biodiversity.

Airborne fugitive dust containing respirable crystalline silica causes chronic occupational diseases such as silicosis among workers in Rajasthan's quartz and sandstone belts. Furthermore, structural failures of tailings dams and unlined disposal ponds result in heavy-metal siltation of river basins.

Mitigating these hazards demands strict implementation of Progressive and Final Mine Closure Plans under Indian Bureau of Mines (IBM) guidelines, lined tailings management, concurrent afforestation, and zero-effluent discharge protocols.