Metamorphism, graphite crystallinity, and sulfide anatexis of the Rampura-Agucha massive sulfide deposit, northwestern India

Located adjacent to the Banded Gneissic Complex, Rampura-Agucha is the only sulfide ore deposit discovered to date within the Precambrian basement gneisses of Rajasthan. The massive Zn-(Pb) sulfide orebody occurs within graphite-biotite-sillimanite schist along with garnet-biotite-sillimanite gneiss, calc-silicate gneisses, amphibolites, and garnet-bearing leucosomes. Plagioclase-hornblende thermometry in amphibolites yielded a peak metamorphic temperature of 720-780A degrees C, whereas temperatures obtained from Fe-Mg exchange between garnet and biotite (580-610A degrees C) in the pelites correspond to postpeak resetting. Thermodynamic considerations of pertinent silicate equilibria, coupled with sphalerite geobarometry, furnished part of a clockwise P-T-t path with peak P-T of 6.2 kbar and 780A degrees C, attained during granulite grade metamorphism of the major Zn-rich stratiform sedimentary exhalative deposits orebody and its host rocks. Arsenopyrite composition in the metamorphosed ore yielded a temperature [and log f(S (2))] range of 352A degrees C (-8.2) to 490A degrees C (-4.64), thus indicating its retrograde nature. Contrary to earlier research on the retrogressed nature of graphite, Raman spectroscopic studies on graphite in the metamorphosed ore reveal variable degree of preservation of prograde graphite crystals (490 A +/- 43A degrees C with a maximum at 593A degrees C). The main orebody is mineralogically simple (sphalerite, pyrite, pyrrhotite, arsenopyrite, galena), deformed and metamorphosed while the Pb-Ag-rich sulfosalt-bearing veins and pods that are irregularly distributed within the hanging wall calc-silicate gneisses show no evidence of deformation and metamorphism. The sulfosalt minerals identified include freibergite, boulangerite, pyrargyrite, stephanite, diaphorite, Mn-jamesonite, Cu-free meneghinite, and semseyite; the last three are reported from Agucha for the first time. Stability relations of Cu-free meneghinite and semseyite in the Pb-Ag-rich ores constrain temperatures at > 550A degrees C and < 300A degrees C, respectively. Features such as (1) low galena-sphalerite interfacial angles, (2) presence of multiphase sulfide-sulfosalt inclusions, (3) microcracks filled with galena (+/- pyrargyrite) without any hydrothermal alteration, and (4) high contents of Zn, Ag (and Sb) in galena, indicate partial melting in the PbS-Fe0.96S-ZnS-(1% Ag2S +/- CuFeS2) system, which was critical for metamorphic remobilization of the Rampura-Agucha deposit.