Tourmalinites, or fine-grained sedimentary rocks containing 15–20% or more tourmaline, occur within the Papaskwasati Formation of the Aphebian Mistassini Group in central Quebec. The formation, which is largely composed of fluvial sand-stones and conglomerates with minor argillite beds, was deposited in a restricted embayment close to the transition into marine carbonates. Concentrations of tourmaline are limited to fine-grained sediments and intraclasts, and fine sedimentary structures are preserved within the resulting tourmalinites. This suggests that the tourmalinites formed from preexisting boron-rich muds. It is unlikely that the muds were formed from sea-floor hydrothermal emanations, as no hydrothermal manifestations or spenecontemporaneous vulcanism is known to be associated with this formation. An evaporitic origin for the tourmalinites is consistent with both the sabkha conditions postulated for the origin of nearby carbonates and the limited areal extent of the sedimentary basin.