Fluid inclusion, stable isotope and tourmaline chemistry at Gadag Gold Field, Dharwar Craton, India: Implications for metamorphic origin of auriferous fluid

The sources of auriferous fluid and metal in orogenic gold deposits have long been a subject of debate. This paper investigates the Gadag Gold Field (GGF) in the western Dharwar Craton, India—a representative greenstone-hosted orogenic gold system—to understand the gold forming process. The litho associations (metabasalt, meta-argillite and BIF) and spatial distribution of GGF demands a deposit specific study, while other orogenic gold deposits are in the eastern Dharwar Craton. The nature and sources of the auriferous ore fluid in GGF are still arguable and require further constraints. The GGF comprises three auriferous lodes (western, central, and eastern), hosted in metabasalt, argillite, and banded iron formation (BIF), respectively. Despite lithological differences in host rock, the western and central lodes show similar alteration assemblages (quartz + carbonate + muscovite + chlorite + sericite ± tourmaline + arsenopyrite + pyrite + chalcopyrite ± sphalerite).

Petrography and microstructures indicate two stages of mineralization corresponds to progressive D₃ dextral shearing. Where stage-1 is followed by stage-2 during progressive deformation. Although visible native gold grains are rare, electron microprobe analysis of arsenopyrite reveals the presence of structure-bound invisible refractory gold in the GGF. Detailed fluid inclusion (FI) studies on quartz-carbonate veins reveal a low- to medium-saline (0.04–9.6 wt% NaCl equiv.), reduced H₂O-NaCl-CO₂ ± CH₄ fluid, with evidence for fluid immiscibility. Estimated mineralization conditions (1.2–2.0 kbar, 275–333 °C) from fluid inclusion and chlorite thermometry is consistent with greenschist-facies alteration. Tourmaline geochemistry (Na_avg ∼ 0.68 apfu, low Fe³⁺/Fe²⁺, Fe-Mg ratio) and coeval sulfides from the mineralized zone reflect the characteristics of low saline, reduced fluid and partitioning of Fe into chlorite and sulfides during fluid-rock interaction with Fe-rich metabasalt is also inferred.

The similar and overlapping ranges of δ³⁴S isotopic data between mineralized sulfide grains (+1.5 to +4.5 ‰) and host rock pyrites (–1 to +3.8 ‰), suggest the dissolution of preexisting magmatic and metasedimentary pyrites of the host rocks during high fluid flux contributed most of the sulfur in the hydrothermal system. While carbon isotope values (δ¹³C_fluid: −6.0 to +1.5 ‰) indicate that the carbon was sourced from a well-buffered and isotopically homogeneous fluid reservoir, most likely maintained by interaction with carbonate-bearing lithologies present in the study area and/or a deep-seated metamorphic fluid system. These isotopic signatures, combined with tourmaline and FI data, predominantly support metamorphogenic origin of the auriferous fluid at GGF, though influences of magmatic fluid could not be completely ruled out. Where gold was likely transported as Au(HS)₂⁻ and precipitated due to pressure drop-induced phase separation and wall-rock interaction during shear zone reactivation.