The country rocks in this part of Revillagigedo Island are marine, pelitic phyllite and schist that are intruded by Cretaceous stocks, sills, and dikes of feldspar-porphyritic granodiorite, and by a stock and probably related plugs of Tertiary gabbro (Berg and others, 1988). The strata and some of the granodiorite were regionally metamorphosed to greenschist grade in Late Cretaceous time. These regionally metamorphosed rocks subsequently were locally contact metamorphosed to hornblende hornfels near the contacts of Cretaceous granodiorite plutons that were emplaced after the regional metamorphism, and then more widely remetamorphosed to hornblende hornfels near the contacts of the Tertiary gabbro. The premetamorphic age range of the pelitic strata is uncertain. Berg and others (1988) assign them a Mesozoic or (Late) Paleozoic age; Brew and Ford (1998) and Crawford and others (2000) assign them to the Gravina belt, of Late Jurassic or Cretaceous age.

Wright and Wright (1908, p. 150) described the principal Peterson deposit as a fault-gouge-bounded, sulfide-bearing, quartz-calcite-graphite fissure vein that averages 15 feet wide and has been traced along strike for 1000 feet. The vein strikes NW, parallel to the foliation of pelitic schist host rock, and dips steeply NE to vertical, which intersects the SW dip of the foliation. The sulfide minerals in the vein are pyrite, galena, sphalerite, and pyrrhotite, along with reported values of Au and Ag. Pyrite is also sparsely disseminated in the wallrocks adjacent to the vein.