Deposition of highly crystalline graphite from moderate-temperature fluids

Abstract

Recognized large occurrences of fl uid-deposited graphite displaying high crystallinity were previously restricted to high-temperature environments (mainly granulite facies terranes). However, in the extensively mined Borrowdale deposit (UK), the mineralogical assemblage, notably the graphite-epidote intergrowths, shows that fully ordered graphite precipitated during the propylitic hydrothermal alteration of the volcanic host rocks. Fluids responsible for graphite deposition had an average XCO2/(XCO2 + XCH4) ratio of 0.69, thus indicating temperatures of ~500 °C at the fayalite-magnetite-quartz buffered conditions. Therefore, this is the fi rst reported evidence indicating that huge concentrations of highly crystalline graphite can precipitate from moderate-temperature fluids.