Genesis of the Rockstone graphite showing, Shebandowan Greenstone Belt, Western Superior Province, Canada

Abstract

Beyond its economic significance as a critical mineral, graphite provides valuable insights into the early carbon cycle and the environmental conditions of early Earth. The origin of graphite in Precambrian rocks has been a longstanding subject of interest in geological studies, owing to the possibility that it could represent remnants of early life on Earth. At the Rockstone property in Thunder Bay, Ontario, graphite occurs as very fine-grained particles (~10 to 80 μm) within late-stage chlorite–sulfide–quartz veins hosted by Neoarchean metasedimentary and metavolcanic rocks. Transmitted Electron Microscopy and Raman studies confirm that graphite is predominantly poorly crystalline and exhibits a turbostratic structure, which is indicative of deformation. Ti-in-biotite geothermometry yields peak amphibolite facies temperatures between ~500 and 565°C (±50°C). Raman temperatures of graphite are lower, varying from ~350 to 465°C (±50°C). Chlorite, interpreted to be in textural equilibrium with graphite, yields temperatures ranging from ~110 to 280°C (±20°C). In situ δ13CVPDB values of vein graphite ranges between -41.1 to -33.3‰ with a median of -36.7‰ (n=28) and indicates that the carbon is biogenic. In situ, sulfur isotope values (δ34SVCDT) of pyrite, sphalerite and pyrrhotite, vary from -0.5 to +6.7‰ with a median of +2.4‰ (n=55), which fall within the range characteristic of both metamorphic and igneous origins and are comparable to other sulfur isotope values of Archean VMS deposits in the Superior Province. Overall, our data indicates that organic matter was impacted by graphitization processes during regional metamorphism. Subsequently, vein graphite, highly depleted in 13C, might have formed from two main processes: (1) devolatilization of organic matter during low-grade metamorphism, and/or (2) migration from an existing graphite body at depth, during regional metamorphism. Alternatively, The Fischer-Tropsch (FT) reactions may have involved in the deposition of graphite at Rockstone. The occurrence of graphite within veins, closely associated with hydrous minerals such as chlorite and muscovite, as well as sulfide minerals, supports this interpretation.