A model for epigenetic Ba-Pb-Zn mineralization in the Appalachian thrust belt, Quebec; evidence from fluid inclusions and isotopes

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Ba-Pb-Zn vein and disseminated deposits occur within a 125-km section of the Taconic thrust belt of the lower Saint Lawrence region of Quebec, Canada. The deposits are hosted by Cambrian sandstone and conglomerate and are not obviously associated with igneous rocks.There are two main fluid inclusion populations, one with homogenization temperatures between 60 degrees and 190 degrees C and salinities between 15 and 32 wt percent NaCl equiv, and another with homogenization temperatures and salinities of 220 degrees to 390 degrees C and 2 to 32 wt percent NaCl equiv, respectively. Primary fluid inclusions and all but one of the fluid inclusions in sphalerite are restricted to the low-temperature-high-salinity fluid inclusion population. The data for sphalerite define a weak trend of decreasing salinity with homogenization temperature. The high-temperature inclusions are interpreted to reflect the influx of a postmineralization fluid and stretching of earlier trapped inclusions.The delta 34 S values for galena and sphalerite in the Saint Fabien deposit range from 8.2 to 10.7 per mil and from 13.0 to 14.1 per mil, respectively. In the other occurrences delta 34 S values for galena are as low as 1.5 per mil. Temperatures obtained from four sphalerite-galena pairs were 86 degrees , 96 degrees , 163 degrees , and 276 degrees C. The delta 34 S values for barite range from 17.5 to 27.5 per mil. There is no correlation between the sulfur isotope ratios of galena and sphalerite and those of barite. The data suggest that there were two sources of sulfur, one from diagenetic pyrite or organic matter and the other from sulfate.delta 18 O (sub V-SMOW) values for carbonate minerals in the Saint Fabien deposit range from 15.1 to 18.8 per mil. If a temperature of 110 degrees C is assumed, the delta 18 O (sub V-SMOW) of the corresponding water was -2.4 to +3.2 per mil which suggests a seawater source. In the other occurrences delta 18 O (sub V-SMOW) are, on average, 2 per mil lower. The delta 13 C values in the Saint Fabien deposit range between -1.9 and +0.9 per mil, but in the other occurrences are as low as -11.4 per mil. These data indicate variable contributions of carbon from seawater and organic matter.Strontium isotope ratios for dolomite and barite range from 0.71195 to 0.71646; ratios for coexisting calcite are consistently higher. All values are significantly more radiogenic than Cambro-Ordovician seawater (0.7092-0.7078). The high values are similar to those interpreted for the Cambro-Ordovician sedimentary succession. However, the wide range of values for barite suggests that this strontium was mixed with strontium from a source with a seawater signature.The geochemical data are consistent with the transport of metals and reduced sulfur in a basinal brine. A model is proposed in which metalliferous fluids were expelled from the Cambro-Ordovician sedimentary succession near the end of, or immediately after, the Taconic orogeny (Late Ordovician to earliest Silurian), migrated under gravity to zones of extension, and mixed with ground waters that had interacted with evaporites. Sulfide deposition is attributed to an increase in pH and barite deposition to an increase in sulfate activity, both of which occurred as a result of fluid mixing. In terms of timing, geologic parameters, and depositional processes, the Ba-Pb-Zn mineralization discussed in this paper is similar to certain Mississippi-Valley-type Pb-Zn deposits and may record a major lower Paleozoic fluid expulsion event in the Appalachian orogen.