Elizabeth Anne Jennings MSc thesis abstract
The Thunder Bay Silver District is composed of two curvilinear groups of veins, the Mainland and Island Belts. The veins can be mineralogically divided into the barren, silver-bearing, and 5-element association veins.
During the present study, fluid inclusion, sphalerite composition, and sulfur isotopic composition were investigated, as well as some aspects of the mineralogy of the veins. Fluid inclusions in quartz, calcite, and fluorite indicate temperatures ranging from 70E-450EC during vein deposition could be found in veins of both geographical and all mineralogical groups. Fluid inclusions in sphalerite indicate ore minerals were deposited between 80E-120EC at all locations, 1-30 equiv. wt. % NaCl. The dominant salt in solution was CaCl2.
Fe content in sphalerite indicates a log a82 of -18 to -25 during ore deposition at all deposits except Spar Island . At Spar Island copper sulfide mineralogy suggests log a82 of -14 to -18. Sphalerite often contained trace quantities of Cd.
Sulfur isotopic composition of sulfides and barite ranged from -9.7 to +12.2%, with two anomalous values (near +30%) from supergene samples. Paired data indicated isotopic non-equilibrium during deposition sulfides.
The consistency of fluid inclusion, sulfur activity, and sulfur isotopic data suggest both depositional environment and hydrothermal fluid source were similar for all veins in the Thunder Bay district. Heterogeneity of the deposits, particularly the presence or absence of Ag, Ni, Co and As may be due to the evolution of the fluids between their inception and entry into the depositional environment.
The source of the fluids is unknown, although their original sulfur isotopic composition is inferred to be fairly low (+4.7%), suggesting a magmatic source of sulfur is possible.
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