Ore-forming fluids in Archean gold-bearing quartz veins at the Sigma Mine, Abitibi greenstone belt, Quebec, Canada

Abstract

Archean gold-bearing quartz veins at the Sigma mine occur in deformed metavolcanics intruded by two generations of preore porphyries. The veins were emplaced during a late compressional tectonic event and they consist primarily of quartz, tourmaline, carbonates, pyrite, and scheelite. Vein minerals have been deformed and are also cut by abundant healed fractures outlined by fluid inclusions. In the veins, gold typically occurs in late fractures and in recrystallized quartz. Microthermometric, SEM-EDA, leachate analysis, and gas chromatographic techniques were used to study fluid inclusions in quartz.Most fluid inclusions at Sigma occur in late fractures and are secondary. The fluid inclusions are commonly found in fractures parallel to, or less commonly within, gold-bearing fractures and are genetically related to the gold. Three types of fluid inclusions are identified: (1) high-salinity Na-Ca-Cl aqueous inclusions (25-34 wt % salt) with small amounts of CO 2 , some containing halite, calcite, and nahcolite (?) as daughter minerals; (2) CO 2 -rich inclusions containing small amounts of H 2 O and CH 4 ; and (3) low-salinity H 2 O-CO 2 inclusions (<10 equiv wt % NaCl) with CO 2 contents which are variable but commonly in the range of 15 to 30 mole percent CO 2 . Leachate analyses show that, in addition to Na and Ca, the aqueous fluids contain minor amounts of K, Fe, Mg, and Li. Homogenization temperatures of H 2 O-CO 2 inclusions with constant phase ratios range from 285 degrees to 395 degrees C; those of aqueous inclusions range from 60 degrees to 295 degrees C.The aqueous and CO 2 -rich inclusions are interpreted to have trapped two coexisting fluids resulting from the unmixing of a homogeneous H 2 O-CO 2 parent fluid, itself occasionally trapped in late fractures. The ore-bearing fluid is interpreted to be a low-salinity (<10 equiv wt % NaCl) fluid containing 15 to 30 mole percent CO 2 , at minimum temperatures in the range of 285 degrees to 395 degrees C. The relatively low homogenization temperatures of aqueous inclusions are interpreted to be the result of CO 2 effervescence from a CO 2 -saturated aqueous fluid due to pressure fluctuations at the time of entrapment. It is also proposed that the onset of gold deposition is related to the onset of fluid unmixing.