Hydrothermal alteration and elemental mass balance for the Surda Copper Deposit, Singhbhum Shear Zone, Eastern India: implications for both copper and magnetite-apatite mineralization

Abstract: This paper explores the geological relationships, mineralogical, and geochemical characteristics, and quantitative elemental mass balance constraints of the hydrothermal alteration zones within the uraniferous Surda copper deposit in the Singhbhum Shear Zone in eastern India, which is affiliated with an iron oxide copper-gold system. Five distinct alteration zones are identified: a proximal potassic (biotitization) zone containing magnetite- apatite; a central chloritization zone containing copper, uranium, gold, and magnetite; distal silicification (quartz) and albitization zones; and a transitional chlorite-sericitization zone. Evidence suggests these zones originated from mafic precursors linked to a subduction zone. Magnetite mineralization shows both magmatic (δ¹⁸O +2.0 to +4.8‰) and hydrothermal (δ¹⁸O +0.4 to +0.6‰) signatures. Hydrothermal magnetite-apatite formed at temperatures of 440° to 550°C, with continued crystallization at lower temperatures (280° to 360°C) concurrent with copper sulfide and uranium precipitation from an Fe-rich, isotopically heavy fluid (δ³⁴S + 4.55 to + 9.66‰). Major elements (Ca, Na, Mg) and some minor and rare earth elements decrease from weakly-altered mafic rocks to late alteration zones, reflecting the breakdown of Ca-bearing hornblende, biotite and plagioclase. Potassium decreases from early to late alteration zones due to mineral transformations. Mass-balanced geochemical data align with hydrothermal alteration mineral assemblages, indicating processes like mineral dissolution, element absorption, and sulfide behavior. Overall, the study highlights the complex hydrothermal alteration processes shaping the Surda copper deposit and provides insights into ore formation mechanisms.