Abstract

The Proterozoic Star Lake pluton in northern Saskatchewan, Canada, hosts several auriferous quartz veins that were deposited along an array of northeasterly striking shear zones. Gold mineralization was fracture controlled, and the main stage of deposition occurred during late fracturing episodes within the shear zones. The majority of the fluid inclusions trapped within the vein quartz are of secondary origin but are compositionally similar to those assigned as primary inclusions. The aqueous and CO 2 -bearing fluids (5-60 mole % CO 2 ) are of low to moderate salinity (1-17 wt % NaCl equiv) and low to moderate bulk density (< or =0.8- approximately 1.03 g/cc). The fluids in the inclusions have a wide range of homogenization temperatures from 200 degrees to 500 degrees C and, in conjunction with oxygen isotope equilibration temperatures of 400 degrees to 500 degrees C for most of the vein minerals, indicate trapping pressures for most fluids of 3 to 5.5 kbars. Fluid unmixing occurred within the veins, particularly among secondary fluid inclusions along microfractures. Fluid immiscibility facilitated through pressure release within shear zones may have been a major factor in gold precipitation wherein CO 2 -phase separation acted to destabilize gold complexes and subsequently led to gold precipitation.Quartz from auriferous veins has delta 18 O values ranging from 10 to 11.6 per mil, whereas barren quartz veins yield higher delta 18 O values of 12.9 to 13.4 per mil, indicating derivation from a distinct source. Calculated delta 18 O and delta D values for the mineralizing fluids range from approximately 6 to 8, and approximately -80 to -40 per mil, respectively, suggesting derivation from a metamorphic source or, at least, a system involving low water/rock ratios. The delta 34 S values of sulfides from the veins have a restricted range of 2 to 3.3 per mil, consonant with derivation from a uniform source. The delta 13 C values of calcite range from -11.2 to -5.6 per mil and thus are less distinctive as to the origin of carbon. Rb-Sr isotope systematics of tourmaline and feldspar in the veins indicate that the Au veins were deposited at 5 to 100 Ma after crystallization of the Star Lake pluton and contemporaneous with the waning stages of Hudsonian metamorphism.

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