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Economic Geology; April 1987; v. 82; no. 2; p. 429-439; DOI: 10.2113/gsecongeo.82.2.429
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Alteration pattern and fluid inclusions of gold-bearing quartz veins in Archean trondhjemite near Wawa, Ontario, Canada

Paul A. Studemeister, and Stephanos Kilias

Portland Cement Assoc., Skokie, IL, United States
Geol. Centralinst., Denmark

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, CO 2 + H 2 O, 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 CO 2 with density of 0.75 g/cm 3 ; homogenization temperatures range from 220 degrees to 360 degrees C. Type II is nearly 100 vol percent liquid CO 2 , 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 H 2 O gas; homogenization temperatures range from 120 degrees to 220 degrees C. 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 NaCl, respectively.The preferred interpretation is that the gold-bearing quartz veins formed from CO 2 -laden fluids of less than 10 equiv wt percent NaCl 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 CO 2 and NaCl-H 2 O phases, entrapped during quartz crystallization. The source of the gold is not known.

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