Abstract

The Tom Ba-Pb-Zn deposit, Yukon Territory, Canada, is a stratiform, exhalative, massive sulfide deposit, which is hosted by carbonaceous mudstones, turbiditic sand-banded mudstones, and debris-flow, chert-pebble conglomerates of the Upper Devonian Earn Group. The deposit consists of a lens of massive ore (pyrrhotite-galena-sphalerite-ankerite-chalcopyrite) that passes vertically upward and laterally into laminated ore (sphalerite-galena-pyrite-barite-chert). The footwall units host an area of ankerite and quartz veining, which is interpreted to represent the feeder zone for the mineralizing fluids. The ankerite veins are believed to be related to mineralization, whereas fluid inclusion and stable isotope data suggest that the quartz veins formed during one of the postmineralization deformation events.Homogenization temperatures of fluid inclusions from ankerite and quartz range from 157 degrees to 335 degrees C. This range is interpreted to be due to variable leakage from inclusions during deformation and metamorphism and is not representative of temperatures of original entrapment. However, fluid inclusion type and salinity data can be used to distinguish between the mineralizing fluid and the fluid present during later deformation. Ankerite was deposited from an aqueous solution with a salinity of 9.1 + or - 3.4 equiv wt percent NaCl; tectonic quartz was deposited from fluids with high and variable CO 2 (+ or -CH 4 , N 2 ) contents and salinities of 4.8 + or - 1.3 equiv wt percent NaCl. There are distinctive intergrain and intersample variations in the CO 2 contents of inclusions.A lack of variation in the oxygen isotope compositions of mudstones and <2-mu m mineral fractions, plus the absence of an alteration halo around the mineralization, implies that the extent of interaction between the mineralizing fluid and the country rock was small. This may have been the result of a low water/rock ratio, a high fluid-flow rate, and/or a low temperature (<150 degrees C) for the mineralizing fluid.The mineral assemblages and paragenesis suggest that early mineral deposition (fine-grained quartz and ankerite) occurred in a sulfur-deficient environment. Local production of reduced sulfur eventually allowed the precipitation of sulfides. The mineral zonation exhibited by the massive and laminated ore is typical of hot, reduced hydrothermal fluids mixing with cooler, more oxygenated water.

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