Magmatic-hydrothermal nature of copper prospect of Masayapeta area, Nellore Schist Belt, Southern India: Constraints from petrography, geochemistry, fluid inclusion and sulphur isotope

Abstract

The present study discusses the geochemical, mineral composition, sulphur isotope and fluid inclusion studies of the Masayapeta copper prospect, Nellore Schist Belt, Southern India. The detailed integrated study reveals that surface samples have 0.13%–0.52% Cu, whereas subsurface drill core samples have 0.95%–1% Cu in quartz-chlorite-sericite schist, making it an interesting area for further investigation to understand the nature and genesis of the ore-forming processes. Ore petrographic study reveals that chalcopyrite and pyrite occur as dominant phases, followed by bornite and magnetite in different textural associations. These phases show strong late fluid alteration, i.e., conversion into chalcopyrite → covellite → digenite, indicating a replacement fluid path.

Masayapeta pyrite is characterized by elevated Co/Ni ratios (> 10), high core Co contents (0.06–2.1 wt%), low initial As concentrations (~ 0.25 wt%) and minimal incompatible element enrichment in cores, reflecting precipitation from high-temperature, reduced magmatic fluids exsolved directly from magmatic source. In contrast, it exhibits progressive As rimward enrichment (up to 0.99 wt%), oscillatory zoning with rising incompatible metals (Au to 0.13 wt%, Pb to 0.16 wt%, Ag to 0.23 wt%) and low Co/Ni ratios (1–10), indicative of evolved, cooler hydrothermal fluids influenced by boiling, mixing and meteoric dilution, which enhance metal mobility and copper fertility. The compositions of chalcopyrite (CuFeS₂) and oxidized magnetite (high FeOᵀ, low TiO₂, variable Al₂O₃) confirm high-T magmatic fluids under stable fS₂–fO₂, with bimodal Fe–Ti oxides indicating mafic-intermediate igneous precursors overprinted by felsic-hydrothermal alteration. This indicated the role of regional granitic magmatism for the evolution of an ore system, akin to the granite-greenstone belt, elsewhere. Fluid inclusions (Th 160–330 °C, salinity 0.66–9.32 wt% NaCl equiv., CO₂-rich, densities 0.63–0.93 g/cm³) document hypersaline magmatic brines mixed with meteoric influx, driving copper sulphide precipitation. Furthermore, the δ³⁴S values at Masayapeta (− 0.2‰ to − 12.3‰) reveal a hybrid sulphur budget blending magmatic-hydrothermal fluids with reduced sedimentary/biogenic sources. This is further amplified by isotopic fractionation via boiling, fluid mixing or disequilibrium sulphide precipitation in magmatic-hydrothermal systems. The above comparative study using various proxies indicates mixed signatures, i.e., a dominant magmatic fluid source, locally modified by meteoric input during late-stage hydrothermal evolution, signifies a magmatic-hydrothermal type of copper mineralization.