Isotope geochemistry of the Prairie Creek carbonate-hosted zinc-lead-silver deposit, southern Mackenzie Mountains, Northwest Territories

Abstract

The Prairie Creek Zn-Pb-Ag deposit comprises three distinct types of carbonate-hosted sulphide mineralization that includes stratabound massive sulphide, classic Mississippi Valley-type (MVT), and quartz-carbonate-sulphide veins. The stratabound massive sulphides containing pyrite, sphalerite, and galena preferentially replaced the "Mottled Dolostone unit" of the Ordovician-Silurian Upper Whittaker Formation, but also formed within the "Lower and Upper Spar units" of the Upper Whittaker Formation. The MVT sulphides occur as open-space fillings of pyrite, sphalerite, and rare galena in a series of showings within the Silurian Root River dolostone. The quartz-carbonate-sulphide veins containing sphalerite, galena, chalcopyrite and tennantite-tetrahedrite fill fractures parallel to regional-scale faults crosscutting the Upper Whittaker Formation, Ordovician-Silurian Road River Formation, Silurian-Devonian Cadillac Formation, and Devonian Arnica Formation. The stratabound mineralization occurs over several stratigraphic levels for a strike length of 3 km and a thickness of ?28 m. The vein-type sulphides are found along a 16 km long section of the north-south trending Prairie Creek Fault structure in twelve showings. Spatially, the stratabound sulphides are closely associated with the vein-type mineralization, fault structures, and regional north-south trending fold axis. However, the isotope data do not favour a genetic link between the stratabound sulphides and the vein sulphides. Values of (delta)18OPDB for the mineralized dolomite associated with the stratabound sulphides range from - 6.9 to -14.9 per thousand and values of (delta)13CPDB range from -2.3 to -4.7 per thousand, similar to mineralized values of the Manetoe Facies in the Mackenzie Mountains of Yukon and Northwest Territories. The stratabound sulphides have similar (delta)18OPDB values but lower (delta)13CPDB values than the non-mineralized host dolostone of the Upper Whittaker Formation. (delta)18OPDB and (delta)13CPDB values for the quartz-carbonate-sulphide veins are slightly lower than the stratabound sulphides. The MVT showings have (delta)18OPDB values ranging from -17.6 to -10.7 per thousand and (delta)13CPDB values ranging from -7.2 to -1.0 per thousand, which are similar to values for the Presqu'ile barrier of Alberta and Northwest Territories, and the Manetoe Facies. 87Sr/86Sr values of mineralized dolomite from the stratabound sulphides range from 0.7138 to 0.7239, which are similar to non-mineralized dolostone of the Upper Whittaker Formation. 87Sr/86Sr values for the quartz-carbonate-sulphide veins are variable and range from 0.70963 to 0.72848. These values are some of the most radiogenic values recorded in the northern Canadian Cordillera. They suggest that deep-circulating hydrothermal fluids were interacting with continental crust or derived siliciclastic rocks. 87Sr/86Sr values of mineralized dolomite from the Mississippi Valley-type showings range between 0.7105 and 0.7127, similar to Manetoe Facies, and overlapping the lighter values for the Robb Lake deposit in northeastern British Columbia. Lead isotope values for the stratabound sulphides plot in a tight cluster below the "shale curve" with 206Pb/204Pb values ranging from 18.664 to 18.801 and 207Pb/204Pb values ranging from 15.640 to 15.663. These values overlap with the values of the Ordovician-Silurian SEDEX (sedimentary exhalative) deposits and the Devonian SEDEX deposits of the Selwyn Basin in Yukon. The quartz-carbonate-sulphide veins have 206Pb/204Pb values of 18.978 to 19.132 and 207Pb/204Pb values of 15.656 to 15.731 that form a linear array crosscutting the shale curve at 250 Ma. These data suggest that the stratabound sulphides and the vein sulphides are isotopically different and not genetically related. Lead isotope values from the Mississippi Valley-type sulphide occurrences plot in proximity to the stratabound sulphides, similar to values of the Silurian-Devonian MVT deposits of the Mackenzie Mountains and the northern Rocky Mountains. Preliminary results on the sulphur isotopes of the Prairie Creek mineralization show their similarity with sulphides of the Selwyn Basin in Yukon and the Kechika Trough in northeastern British Columbia. The source of reduced sulphur in both cases was most likely bacterially reduced seawater sulphate. The metallogenic model invoked for the stratabound sulphide mineralization at Prairie Creek is similar to the SEDEX deposits of the Selwyn Basin. The mineralization formed in a secondary rift-controlled sedimentary basin, the Prairie Creek Embayment, along the margin of the carbonate platform, which is adjacent to the Selwyn Basin. It is suggested that the stratabound sulphides formed by replacement of carbonates at the time or slightly later to the genesis of SEDEX deposits in the Selwyn Basin. Sulphides precipitated as a consequence of fluid mixing within dolomitic units. This process involved relatively high temperature (110-240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cu) and cooler, saline (~10-23 wt.% NaCl) formation waters rich in bacteriogenic H2S.