| Popis: |
The Early Proterozoic Star Lake pluton in northern Saskatchewan, Canada, hosts several auriferous quartz veins that were deposited along an array of subparallel 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, and are compositionally similar to primary inclusions. The aqueous and CO2-bearing fluids (2) are of low to moderate salinity (1- 17 wt % NaCl equivalent) and low to moderate bulk density (~0.8-1.05 g/cc). Fluid inclusion microthermometry indicates that gold precipitation took place over a temperature range of 200 to 500ºC. Estimates for pressure of trapping range from ~3 to 5.5 kb. Fluid unmixing was common within the different veins, particularly among secondary fluid inclusions along healed microfractures. Quartz from auriferous veins has δ18O values ranging from 10 to 11.6 per mil, similar to δ18O values of quartz in the pluton, whereas barren quartz veins yield higher δ18O values of 12.9 to 13.4 per mil indicating derivation from a distinct source or formation at lower temperatures. Fractionation of 180 between vein-quartz and coexisting minerals such as biotite, potassium feldspar, and magnetite yield veinformation temperatures near 500ºC. Calculated δ18O and δD values for the mineralizing fluids range from 6-8.7, and -80 to -40 per mil, respectively, suggesting derivation from a metamorphic source and low water/rock ratios. Sulfur isotope compositions of the vein sulfides show a restricted range of values of 2-3.3 per mil, consistent with derivation of sulfur from a uniform source. δ13C values of calcite range from -11.2 to -5.6 per mil and thus, are less distinctive as to the origin of carbon in the fluid. Rb-Sr isotope systematics of tourmaline and feldspar in the veins indicate that the auriferous veins were deposited about 50-100 Ma after the termination of plutonism at Star Lake. Deformation continued for 100 Ma or more after deposition of the auriferous veins, contemporaneous with the waning stages of Hudsonian dynamothermal metamorphism in northern Saskatchewan. Fluid immiscibility was probably a major factor in promoting gold deposition in the Star Lake area. H2O-NaCl-CO2 fluid unmixing and related changes in the chemical composition of the mineralizing fluid acted to destabilize gold complexes that subsequently led to gold precipitation in dilational sites along shear zones. Available geological data suggest that movement along major structures such as the McLennan Lake Tectonic Zone during the Hudsonian Orogeny may have contributed to creating a subparallel array of shear zones in the Star Lake pluton and the surrounding supracrustal rocks along which mafic dikes were intruded. The shear zones, which are the loci for gold mineralization at Star Lake, are characterized by penetrative foliation, polygonization, ribboning, transgranular fracturing of the mineral grains, and quartz veining signifying brittle-ductile behavior during mineralization. |
