Oxygen-isotope geochemistry of metamorphosed, massive sulfide deposits of the Flin Flon ? Snow Lake belt, Manitoba

Abstract

Oxygen-isotope compositions have been measured for whole-rock and mineral samples of host and hydrothermally altered rocks from three massive sulfide deposits, Centennial (CL), Spruce Point (SP), and Anderson Lake (AL), in the Flin Flon — Snow Lake belt, Manitoba. Wholerock δ 18O values of felsic metavolcanic, host rocks (+8.5 to +16.1‰) are higher than those of altered rocks from the three deposits. The δ 18O values of altered rocks are lower in the chlorite zone and muscovite zone-I (CL=+ 5.3‰; SP=+5.4 to +8.3‰; AL= +3.7 to +5.9‰) than in the gradational zone (CL= +9.9 to +11.7‰; SP= +8.4 to +9.8‰; AL= + 6.6 to +7.7‰). Muscovite schist (Muscovite Zone-II) enveloping the Anderson Lake ore body has δ 18O values of +7.2 to +8.3‰. Quartz, biotite, muscovite, and chlorite separated from the altered rocks have lower δ 18O values compared to the same minerals separated from the host rocks. However, isotopic fractionation between mineral-pairs is generally similar in both host and altered rocks. It is interpreted that differences in the oxygen-isotope compositions of the altered and host rocks were produced prior to metamorphism, during hydrothermal alteration related to ore-deposition. Isotopic homogenization during metamorphism occurred on a grain-to-grain scale, over no more than a few meters. The whole-rock δ 18O values did not change significantly during metamorphism. The generally lower δ 18O values of altered rocks, the Cu-rich nature of the ore and the occurrence of the muscovite zone-II at Anderson Lake are consistent with the presence of higher temperature hydrothermal fluids at Anderson Lake than at the Centennial and Spruce Point deposits.