Alteration pattern and fluid inclusions of gold-bearing quartz veins in Archean trondhjemite near Wawa, Ontario, Canada

Abstract

Systems of auriferous quartz veins occur in Archean granitic rocks within 1.6 km of the Wawa greenstone belt, 80 km northeast of Wawa, Ontario. The vein system at the Renabie mine strikes west for approximately 850 m, dips steeply to a depth of at least 950 m, and hosts orebodies up to 30 m wide and 210 m long. The six vein systems at the Braminco prospect strike north to northwest, dip steeply, and are up to 20 m wide. Alteration of feldspar to quartz + calcite + white mica + albite, and of biotite + amphibole to chlorite, intensifies within 2 to 4 m of quartz vein systems bearing native gold. Mass balance calculations suggest that the alteration of granitic rocks added K, CO2 + H20, Rb, and Ba and removed Na. Three types of primary fluid inclusions coexist in quartz from the auriferous vein systems. Type I, the most abundant type, is an aqueous solution with 10 to 25 vol percent CO2 with density of 0.75 g/cm3; homogenization temperatures range from 220 o to 360oC. Type II is nearly 100 vol percent liquid CO2, with a density of 0.95 g/cm 3. Type III, the least abundant type, is an aqueous liquid with 10 to 50 vol percent H20 gas; homogenization temperatures range from 120 o to 220oC. Apparent salinities of types I, II, and III, based on clathrate and ice melting points, are estimated to be 4 to 8, 0, and 11 to 14 equiv wt percent NaCI, respectively. The preferred interpretation is that the gold-bearing quartz veins formed from CO2-1aden fluids of less than 10 equiv wt percent NaCI during regional greenschist metamorphism of granitic rocks. The three types of fluid inclusions may represent two or three different generations of fluids trapped over a range of temperature and pressure. Another possibility is that the fluid inclusions represent samples of a fluid comprised of immiscible CO2 and NaCIH20 phases, entrapped during quartz crystallization. The source of the gold is not known.