Application of sphalerite geobarometry and sulfur isotope geothermometry to ores of the Quemont Mine, Noranda, Quebec

Abstract

The Quemont Mine comprises massive stratiform Cu-Zn bodies in Archean volcanics (3.3 to 3.1 b.y.) of the Noranda-Matagami greenstone belt. The deposit appears to be volcanogenic and has a complex geological history. At least three modifying events are recognizable, including low-grade regional metamorphism and silicic dike intrusion during the Kenoran orogeny (2.5 b.y.), intrusion by diabase dikes 1.7 to 1.2(?) b.y. ago, and postdike remobilization of sulfides.Application of sphalerite geobarometry to the ores is problematic because of the low regional metamorphic temperature inferred from sulfur isotope geothermometry. Accordingly, indicated pressures of 6 or 8 kb appear to be too high for a suggested isotopic temperature around 300 degrees C. Isotopic temperatures of 300 degrees to >500 degrees are indicated for ores metamorphosed by later dikes. In general, these values show reasonable agreement with equilibrium conditions expected for low temperature greenschist metamorphism and subsequent contact metamorphism by diabase and felsic dikes. Sulfur isotope values for the deposit appear compatible with a volcanic-exhalative origin 3 b.y. ago.This study demonstrates that phase equilibrium was achieved by the Zn-Fe-S system at a uniform pressure corresponding to low-grade regional metamorphism, and that the corresponding equilibrium FeS content of spahlerite was preserved in ores that underwent subsequent contact metamorphism and accompanying recrystallization. On the other hand, sulfur isotope partitioning between coexisting sulfides responded to contact metamorphism by the dikes and approximately reflects the temperatures expected. Remobilization of sulfides by plastic flow may be responsible for some apparently anomalous isotopic results.