Many pyrrhotite and pyrrhotite-pyrite bodies, known to prospectors and geologists as "dikes", are being discovered in the Canadian Shield as a result of intensive geophysical prospecting. The Samreid Lake pyrrhotite-pyrite body [about 28 mi. NNE. of Blind River] was chosen to serve as a type example for this kind of deposit. The area in which the sulfide deposit occurs was mapped in detail to determine the lithological relationship between the sulfide and the country rock. In the northern part of the area, the rocks are mostly amphibole-bearing and include amphibolites and amphibolitic tuffs with sporadic metatuffs and quartz siltites. These rocks are interbedded with minor flows. The southern part of the area is mostly underlain by metatuffs, quartz siltites, and quartzites in which amphibole is sporadic or absent. The amphibolites, quartz siltites, and quartzites have probably been formed from shale and argillaceous siltstone and sandstone. The location of the mineralized quartzite, the host rock of the sulfide body, roughly coincides with the contact between the amphibolitic and non-amphibole-bearing rocks. The mineralized quartzite is recrystallized chert. Lava flows are interbedded with the sulfide-bearing quartzite. Diabase dikes intruded the rocks of the area. The essential minerals making up the sulfide body are pyrrhotite and pyrite. Abundant magnetite is intergrown with the sulfides; marcasite is rare. Chalcopyrite and cubanite were noted in minor amounts. The sulfide body is essentially barren of base-metal mineralization. Along part of the mineralized zone pyrite predominates, elsewhere pyrrhotite predominates or is the sole sulfide mineral. As a rule, however, pyrite and pyrrhotite occur together. Magnetite is considered to have been precipitated in a marine environment. Deposition took place in an area of volcanic activity in which the clastic sediments were permeated by a gaseous network of H 2 S. Locally precipitated iron hydroxide gels, and perhaps some magnetite, were converted to pyrite. At a post-geosynclinal orogenic stage in the history of the area, pyrrhotite is considered to have formed at the expense of pyrite. Geological thermometry points to a temperature of formation of 420 to 6000 C. for these rocks. This temperature reflects post-depositional metamorphic conditions to which these rocks were exposed.