THE PTARMIGAN MINE, YELLOWKNIFE, CANADA: IS IT A MESOTHERMAL GOLD DEPOSIT OF MAGMATIC ORIGIN?

Wallrock petrography, stable isotope thermometry on quartz-tourmaline pairs and electron microprobe analyses of arsenopyrite in Ptarmigan vein samples support that the Ptarmigan gold deposit was formed by 550°C ± 50°C mineralizing fluids. These high fluid temperature estimates suggest that the Ptarmigan deposit was formed by either a high-temperature metamorphic fluid or possibly a magmatic fluid, associated with emplacement of the nearby Prosperous granite. To test whether the Ptarmigan fluid is of magmatic or non-magmatic origin, aqueous fluid inclusions and the salts they contain were extracted from both gold-bearing and barren Ptarmigan quartz vein samples using a crush-leach technique (Bottrell, 1988). The leachates were analyzed for Li, Na, NH4, K, Mg, Ca, F, Cl, Br, NO3, PO4 and SO4 using an Ion Chromatograph. Preliminary petrographic and microthermometric analysis suggests that the quartz samples contain a saline H2O-CO2 primary fluid as well as several types of secondary fluids that may contribute variably to the analyzed composition of the bulk-fluid. Charge balance calculations confirm the completeness of most leachate analyses.

The composition of leachates extracted from Ptarmigan vein quartz samples are variable but well constrained along a mixing line between a "granitic" end-member, represented by leachate analyses of the Prosperous, Fern, and Pancho Villa pegmatite quartz samples, and a "non-granitic" end-member. The potential non-granitic end-member, represented by a metabasalt-hosted gold-bearing Negus vein sample, is similar compositionally to modern seawater, 3.23 Ga seawater (Channer et al., 1997), and present day deep-seated Canadian Shield groundwaters (Con mine, Bottomley et al., 1999). An average Ptarmigan Li/Cl atomic ratio of 0.00198 is ~17 times higher than that in Con mine "Shield" water and ~44 times higher than that in modern seawater. The enrichment in Li suggests a granitic affiliation and possibly a magmatic origin for the Ptarmigan gold deposit.