Nickel, cobalt and copper are critical metals for green energy technology. They can be extracted from base metal sulfide minerals. This project seeks to understand fundamental limitations of their supply in compressional tectonic settings. The Moxie mafic intrusion in northern Maine, USA hosts low-tenor Cu-Ni-(Co) sulfide bodies and was formed in a compressional tectonic setting during the Acadian orogeny. This case-study focuses on sulfur exchange and redox reactions between crustal metasedimentary rocks and the mafic magmas parental to the Moxie intrusion to better understand the processes that govern how critical metal deposits form.