Abstract

The St.-Fabien deposit occurs in the Taconic thrust belt of the Quebec Appalachians. It consists of intimately associated veins and disseminations of barite, galena, and sphalerite hosted in interdigitated sandstones and limestone conglomerates comprising a proximal, deep submarine facies of the Upper Cambrian St.-Damase Formation.At a late stage in the tectonic evolution of the host rocks, hydraulic fracturing driven by the buildup of a geopressured zone took place in a dilatant stress field without mechanical relationships to folding. Episodic opening of the fractures enhanced corrosive fluid flow and led to the development of a secondary dissolution porosity followed by pervasive dolomitization of large volumes of host rocks and partial sealing of the fractures and pores by dolomite, followed by ferroan dolomite, in a reducing environment of low sulfur activity.We propose that an interconnected network of incompletely cemented fracture and dissolution porosities channeled oxidizing, sulfate-bearing fluids into this reducing environment. These oxidizing fluids mixed with deep orogenic metalliferous fluids, set in motion by the Taconic orogeny, and were partially reduced to aqueous sulfide. At this stage, barite, galena, and sphalerite precipitated in the remaining open spaces of varying redox potential and partially replaced precursor cements and framework constituents. Remaining open spaces were cemented by calcite.Rare earth element patterns for the various generations of gangue dolomite and late calcite are characterized by positive europium anomalies ascribed to the partial dissolution of detrital potassium feldspars before, during, and after mineralization. Detrital feldspars are suggested as a major source of lead and barium in the St.-Fabien deposit. Lead isotopes support this crustal source for the lead, and their model age is consistent with the precipitation of galena during the late stages of, or slightly after, the Taconic orogeny. Homogeneity of the lead isotope ratios of both vein and disseminated galenas throughout the St.-Fabien deposit suggests that the lead isotopes were efficiently averaged throughout this part of the Taconic belt by a large volume of fluid.

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