Abstract

This study was undertaken to characterize fluid inclusions associated with high-grade, vein-hosted gold at the Pamour mine in the Porcupine camp of Ontario and to compare them with fluid inclusions in quartz veins associated with wall rock-hosted gold, which is much more common. The Pamour mine contains two principal types of orebodies; metasediment-hosted fracture zones and larger metavolcanic or metasediment-hosted quartz-carbonate veins. Gold in the veins is distributed erratically, attains very high grades locally, and increases in grade at higher levels. Zoned alteration around the veins is dominated by addition of K 2 O, CO 2 , As, S, and CaO and depletion of Na 2 O, K 2 O, CO 2 , and S are concentrated in the hanging wall and the intensity of CO 2 and H (super +) metasomatism increases upward in the mine, roughly parallel to the trend seen in gold values. Gold was introduced, along with sphalerite, galena, calcite, chlorite, and chalcopyrite, late in the paragenetic history after brittle deformation of earlier vein-hosted quartz, tourmaline, feldspar, and pyrite. Sphalerite that is directly associated with high-grade gold contains two types of primary inclusions: a CO 2 -CH 4 fluid (20-45 mole % CH 4 ), and an H 2 O-rich liquid with a carbonaceous phase (CO 2 -CH 4 ) as well as muscovite and graphite daughter minerals. Aqueous phase salinity estimates range from 2 to 9 equiv wt percent NaCl and SEM analyses of evaporitic residues of thermally decrepitated inclusions contain NaCl and CaCl 2 , with lesser amounts of KCl, MgCl 2 , S, Zn, and Fe. These two inclusion types appear to represent immiscible fluids trapped along the solvus in the H 2 O-CO 2 -CH 4 -salt system at temperatures of 325 degrees + or - 25 degrees C and pressures of 1 to 2 kbars.These fluids are similar to those observed in vein quartz at the McIntyre and Hollinger deposits, where most gold is hosted by wall rocks, except that CO 2 immiscibility appears to have been more widespread at Pamour. The separation of immiscible fluids of the type identified in this study would increase the f (sub O 2 ) and pH of the residual fluid owing to the partitioning of H 2 S, CH 4 , and CO 2 into the carbonaceous phase. The net effect of these changes would be to decrease the solubility of bisulfide-complexed gold, suggesting that CO 2 immiscibility is an important contributor to gold deposition within veins where wall-rock interaction is limited. It is not clear whether all vein-hosted gold formed from such unmixing or only the higher grade ore shoots.

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