Survey of the mainland and island belts, Thunder Bay Silver District, Ontario : fluid inclusions, mineralogy, and sulfur isotopes

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, silverbearing, 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 70-450 degrees C 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 80-120 degrees C at all locations. Salinity of the hydrothermal solutions was highly variable, 1-30 equiv. wt. % NaCl. The dominant salt in solution was CaCla. Fe content in sphalerite indicates a log [a** ]of -18 to -25 during ore deposition at all deposits except Spar Island. At Spar Island copper sulfide mineralogy suggests log [a**] 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 indicate isotopic nonequilibrium during deposition of 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.