The fluid history of the Seabee mesothermal gold deposit Northern Saskatchewan
Schultz, Dale Joel
The Seabee deposit is a mesothermal, gold-bearinq vein system hosted within mylonitic shear structures in the Glennie Domain of the Proterozoic Trans-Hudson Orogen, northern Saskatchewan, Canada. Field and underground observations indicate that shear structures nucleated at rheological contrasts at the contacts between felsic intrusive dykes, and mafic intrusive lithologies. High-grade ore is located where shear structures intersect to form zones of transtension or dilational jogs. Geochemical patterns for altered mylonites are similar to those of the primary lithologies. Mafic host rocks proximal to the Seabee quartz veins have been altered to biotite, actinolite, epidote assemblages as the result of hydrothermal activity. Two stages of mineralization are present within quartz veins of the Seabee deposit. Stage I consists of quartz, tourmaline, K feldspar, and pyrite. Stage II is confined within microfractures that cut the Stage I assemblage. Stage II is dominated by carbonate, pyrrhotite, chalcopyrite, tellurides, and gold. Quartz adjacent to microfractures was locally recrystallised during the Stage II event. Gold is sited where Stage II microfractures intersect pyrite, whereas the microfractures that transect quartz contain only carbonate with trace quantities of chalcopyrite and tellurides. The occurrence of gold indicates that interaction of an auriferous fluid with Stage I pyrite resulted in gold deposition. In addition, gold blebs locally occur as linear arrays within pyrite proximal to Stage II microfractures, and are interpreted to represent Stage II gold grains about which Stage I pyrite recrystallized. No primary fluid inclusions have been preserved in Stage I quartz veins, although numerous secondary inclusions are present. Secondary inclusions consist dominantly of H₂O(g)-H₂O (1) or CO₂(g)-CO₂ (1); the occurrence of 3-phase CO₂(g)-H₂O(g)-H₂O(1) is limited. Microthermometric analysis indicates that H₂O(g)-H₂O(1) inclusions were trapped at 212 °C and at pressures of approximately 2.0 kb, suggesting that the Seabee vein system was emplaced at depths of 7.0 km. Stage I barren quartz has δ¹⁸O values of 8.4 -8.9‰, whereas auriferous Stage II quartz has higher values of 10.0-10.6‰. Tourmaline is variably altered, particularly adjacent to Stage II microfractures. This alteration is reflected in the O-isotope compositions; in unaltered tourmaline the δ¹⁸O is approximately 6.6‰, whereas in altered tourmaline δ¹⁸O ranges from 7.2 to 7.8‰ The O-isotopic compositions of barren quartz and unaltered tourmaline are assumed to preserve the primary isotopic composition of Stage I fluid. The average isotopic fractionation temperature for Stage I quartz tourmaline pairs is 430 °C. The O-isotope composition of Stage II fluid is preserved by microfracture quartz and altered tourmaline. The average calculated isotopic fractionation temperatures for Stage II is 360 °C. Although the Stage I and II fluid events differed by 70 °C in temperature, the calculated δ¹⁸Oh₂₀ values for both mineral pairs are in the 5.2 to 5.4‰ range. The mineralization associated with Stage I and II crosscut all lithologies so that the ²⁰⁷Pb/²⁰⁶Pb zircon evaporation age of 1877 ± 10 Ma obtained from a late feldspar porphyry dyke provides an upper limit on the timing of hydrothermal activity. ⁴⁰Ar/³⁹Ar age spectra for hornblende, obtained from mafic rock lithologies, have disturbed patterns signifying that the minerals were reset, likely during the hydrothermalactivity. Biotite from the alteration assemblage of the Seabee deposit has two different ⁴⁰Ar/³⁹Ar plateau ages of 1728 ± 5 Ma and 1769 ± 7 Ma. The 1769 Ma age is correlated with the Stage I event, whereas the 1728 Ma age may date the Stage II event. Younger step ages in both biotite spectra are associated with the distinctly younger fluid event. The initial ⁸⁷Sr/⁸⁶Sr ratio of hydrothermal fluids from Stage I tourmaline is 0.702171 ± 23, whereas ⁸⁷Sr/⁸⁶Sr initial = 0.711093 ± 87 to 0.7078 ± 13 for Stage II sulphides and gold. These results imply that the Stage II gold bearing fluid was either tapping an older Archean source, or selectively leaching Rb-rich minerals of Paleoproterozoic age. Collectively the field, petrographic, and geochemical results are consistent with the interpretation that the Seabee deposit is a structurally hosted mesothermal lode gold deposit, but not a sedimentary exhalative, or a copper-gold porphyry-type system.