Hydrothermal evolution of auriferous shear zones, Wawa, Ontario

Abstract

Deformation zones in the southwestern portion of the Archean Michipicoten greenstone belt host a large number of quartz vein systems, some of which contain small gold deposits. Quartz vein development occurred in three stages: (1) a predeformation set of quartz-carbonate-chlorite-tourmaline veins that contains gold in the deformation zones, (2) late-stage en echelon extension quartz-tourmaline veins, and (3) postdeformation quartz-carbonate veins. Hydrothermal alteration accompanied deformation and produced mineralogically diverse schists containing variable amounts of muscovite, chlorite, biotite, carbonate, quartz, sulfides, tourmaline, and gold. Gold mineralization predates the formation of vein sets 2 and 3, and most likely accompanied deformation, but does not appear to have been associated with vein formation. A variety of postdeformation alteration events has also affected the schists including riebeckite, chlorite, K feldspar, and carbonate alteration. A wide variety of fluid inclusion types is present within the quartz veins. Aqueous and aqueous-carbonic inclusions occur in all vein sets. The former have salinities ranging from 0 to 23 wt percent NaCl equiv, and the latter have X CO2 values ranging from 0.05 to 0.26 (mode approximately 0.1) and CO 2 /CH 4 ratios of between 0.68 and 1. Nahcolite-bearing aqueous and aqueous-carbonic inclusions with high CO 2 /CH 4 ratios and moderate salinities (5-18 wt %) are only present in vein sets 1 and 2, as are carbonic inclusions (with little or no H 2 O) with CO 2 /CO 2 +CH 4 ratios ranging from 0 to 1. Data from fluid inclusion distributions, microthermometry, Raman spectroscopy, and quadrupole mass spectrometry indicate that fluid salinities decreased and that the CO 2 /CH 4 ratios of the fluids increased with time. Of the fluids identified, the earliest were high-salinity (>18 wt %) aqueous fluids and CH 4 -rich aqueous-carbonic and carbonic fluids. These fluids are restricted to vein set 1 in the best mineralized system studied, the Surluga deposit, and are the most likely candidate for the mineralizing fluids. Local water-rock interaction cannot explain the presence of CH 4 and requires initially low f O2 or water-carbon reactions at some depth below the deposit. Methane-rich fluids appear to be absent from poorly mineralized or barren veins. Moderate salinity (avg approximately 11 wt %) HCO (super -) 3 -rich, CH 4 -poor aqueous and aqueous-carbonic fluids infiltrated the deformation zones after the formation of vein set 2 and are probably related to Proterozoic carbonatitic-lamprophyric magmatism. Stage 3 veins were formed from low-salinity aqueous-carbonic fluids. There is evidence of fluid immiscibility in all stages except the last stage of hydrothermal activity. Carbonate delta 13 C values (-6.1 to -2 per mil) are typical of Archean mesothermal gold deposits, and although delta 18 O values (5.2-10.8 per mil) are light compared to most such deposits, they are comparable to those of carbonates from the Renabie deposit, which occurs at the eastern margin of the Michipicoten greenstone belt. Carbonate delta 13 C values increase with time and reflect differences in the isotopic composition of the source reservoir or different sources, rather than localized or reservoir effects. Late-stage carbonate values are consistent with derivation from carbonatite or from leaching of regional carbonate.