The possibility of a ZnS-bearing sulfide melt at 600 °C: Evidence from the Rajpura–Dariba deposit, India, supported by laboratory melting experiment

• Melting galena + sphalerite + chalcopyrite + pyrite assemblage possible at 600 °C.
• High S-fugacity and addition of Cu to PbS–ZnS–FeS system are essential for melting.
• Differentiation of melt results in refractory and low-melting vein assemblages.
• Tendency of pyrite and sphalerite to be S-rich causes S-poor galena and melt.
• S-deficient and metallic phases, paradoxically possible under high S-fugacity.

Inclusions of euhedral pyrite crystals are seen within sphalerite occupying veins and fractures in the Rajpura–Dariba Zn–Pb–Cu sulfide deposit in Rajasthan, India. Sphalerite in the deposit shows wide variation in composition and contains high sulfur, with Fe/(S–Zn) varying from 0.49 to 1.06, indicating that a portion of the Fe in sphalerite could be expressed as a pyrite (FeS2) component and that the pyrite inclusions within sphalerite in the vein ore could have formed due to exsolution. We have conducted experiments by the evacuated silica tube method at 600 °C, pertinent to the peak metamorphic conditions of the Rajpura–Dariba deposit. Our experiments were aimed at exploring the possibility of a ZnS-bearing sulfide partial melt that could precipitate sufficiently S-rich sphalerite, which in turn might exsolve pyrite. The starting mixture comprised synthetic FeS, ZnS, PbS, Cu2S and S in the molar proportions of 40, 33, 9, 9 and 9%, respectively, which approximately corresponded to molar proportions of 27% FeS, 40% ZnS, 11% PbS and 22% CuFeS2. Electron probe micro analysis (EPMA) of the resulting run product showed the presence of a Cu–Fe–Zn–Pb–S melt containing 4.61 mol% ZnS. In the coexisting sphalerite the estimated FeS2 component was 9.07 mol%. The eutectic temperature was determined to be 595 °C with a melt containing 22.44 mol% FeS, 31.24 mol% Cu2S, 5.37 mol% ZnS, 35.81 mol% PbS and 5.14 mol% FeS2. The sphalerite that was in equilibrium with the eutectic melt contained 6.72 mol% FeS2. Our experimental results suggest that a ZnS-bearing sulfide melt containing as high as 5 mol% ZnS was produced during metamorphism at Rajpura–Dariba by partial melting of the preexisting ore. Further experiments confirmed that a minimum sulfur fugacity of − 2.46 log units is required for initiating melting. Pyrite and sphalerite (containing Fe and stoichiometrically excess S) are the solidus phases present in all melt-bearing experimental run products except one with much higher ZnS, suggesting that these two minerals at Rajpura–Dariba could have simultaneously crystallized from a sulfide partial melt giving rise to the observed poikilitic textural relation between the two.