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.

The Londevan deposit consists of sulfide-bearing quartz-calcite(-albite?)-fissure veins in dark gray, graphitic, pelitic phyllite and argillite. The country rocks locally are intruded by dikes or sills of feldspar-porphyritic granodiorite (Wright and Wright, 1988, p. 150; Cobb and Elliott, 1980, p. 68). The main vein and several subparallel, smaller veins dip steeply and strike NW, parallel to the foliation of the country rocks. Many of these veins are adjacent to the granodiorite intrusions. The main vein is about 3 feet thick; the other veins are up to about a foot thick. These veins generally contain 5% or less of pyrite, sphalerite, and galena, and traces of chalcopyrite, silver, and gold. Slickensides and sulfide minerals are common along the footwall portions of these veins. Small amounts of pyrite also are disseminated in the metapelite country rocks. The veins have been traced on the surface for several hundred feet along strike, and the main vein was followed in underground workings for about 2100 feet. Another set of quartz veins that is older than the main vein system follows an anastomosing shear zone that strikes east and dips steeply south (Maas and others, 1995, p. 203). Brooks (1902, p. 63) describes a set of east-west fissure veins, the largest of which is 12 feet thick. These veins contain pyrite, and, locally, considerable argentiferous galena.