Suprasolidus and hydrothermal controls on tungsten mineralization in granite-greisen systems: insights from Degana and Balda deposits, northwest India

Abstract

Peraluminous granitic rocks are widely recognized as sources of hydrothermal fluids responsible for tungsten (W) mineralization, though the anatectic origin of these granites remains underexplored. While the evolution of hydrothermal fluids and mechanisms of mineralization have been extensively studied, the source of iron (Fe) for wolframite precipitation, and the exact mechanism behind it, remain topics of debate. Some researchers argue that fluid-rock interactions supply the necessary Fe for wolframite formation, while others suggest that the fluid already contains sufficient Fe, with boiling and phase separation acting as the primary drivers of wolframite precipitation. To investigate the anatectic origin of granite associated with W(±Sn) deposits and the mechanism of wolframite precipitation, this study focuses on two types of tungsten mineralization in NW India. The first is linked to an endogreisen system at Degana, where mineralization is primarily confined to veins and disseminated forms within greisen pervasive in granite. The second involves an exogreisen system at Balda, where mineralization occurs as vein-type deposits within greisenized schist rocks. This study examines the origins and formation of these deposits by analyzing both supra-solidus and sub-solidus processes, using an integrated approach that includes phase equilibria modeling, trace element analysis, mineral chemistry (major and trace elements), whole-rock geochemistry, and fluid inclusion studies. The study highlights the importance of supra-solidus processes, such as source rock composition, melting models, and fractional crystallization, alongside sub-solidus processes, including fluid chemistry and fluid-rock interaction, in shaping tungsten deposits in the region. For the W-Sn ore localities in NW India, the metapelitic country rock composition serves as the basis for open-system phase equilibria modeling, which evaluates batch melting, accumulated fractional melting, and fractional crystallization to assess their impact on the W Sn budget of the granitic melt. Partial melting (~30-35%) of the metapelitic source rock, involving muscovite and biotite dehydration, plays a key role in determining the major element composition and the W and Sn content of the resulting melt. Fractional crystallization significantly enhances ore-metal concentrations, with accumulated fractional melting producing a more metal-enriched granitic melt (W: 141 ppm, Sn: 455 ppm) compared to batch melting (W: 92 ppm, Sn: 355 ppm). This underscores the potential of chemically mature metasediments to generate metal-fertile granitic melts, highlighting the importance of recycled sediments pre-enriched in W and Sn for peraluminous granite formation. The study also examines B-poor and B-rich peraluminous intrusions (Degana and Balda granites) in NW India, with a focus on their alteration patterns and the evolution of pre-ore and ore-stage fluids to better understand the role of hydrothermal fluid chemistry and fluid-rock interactions in tungsten precipitation. Greisenization, driven by moderate- to high-salinity H2O-CO2 fluids, resulted in the formation of quartz veins, wolframite-bearing greisenized granitic wall rocks, and stockwork greisen veins in Degana, along with mineralized quartz veins and greisenized metapelites in the Sirohi region. Both tri- and dioctahedral micas played essential roles in W enrichment by influencing fluid composition and tungsten solubility. The study constrains the pressure-temperature conditions of W enrichment (380–450°C and 1.2 1.8 kbar) and highlights post-magmatic potassic alteration as a critical pre-conditioning process for ore formation. The key mechanism for wolframite precipitation is boiling, which caused phase separation within the fluid system, leading to wolframite deposition in veins.