Geologic description
The country rocks in this part of Revillagigedo Island are marine, interbedded, andesitic and basaltic metavolcanic rocks and subordinate pelitic metasedimentary rocks that are intruded by stocks, sills, and dikes of Cretaceous feldspar-porphyritic granodiorite (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 remetamorphosed to hornblende hornfels near the contacts of Cretaceous granodiorite plutons that were emplaced after the regional metamorphism. The premetamorphic age of the strata is uncertain. Berg and others (1988) assign them a Mesozoic or (late) Paleozoic age. Berg (1982) and Crawford and others (2000) assign them to the Gravina belt, of Late Jurassic or Cretaceous age, or to the Taku terrane, of late Paleozoic to Late Triassic age. The metamorphic and intrusive rocks locally are overlain by basalt and andesite lava flows of Quaternary or Tertiary age. The Baltic Star and Queen claims are on a quartz fissure vein that strikes generally eastward and dips steeply to vertically, across the foliation of the enclosing schist, which strikes N20W and dips 70NE (Wright and Wright, 1908, p. 148). The vein is 1-6 feet thick and contains small amounts of pyrite, sphalerite, and [probably] galena, along with low values in gold. The deposit was explored in the early 1900s by opencuts, two short prospect tunnels, and an inclined shaft 40 feet deep (Wright and Wright, 1908, p. 148). Maas and others (1995, p. 215) note that the quartz in the veins in this area is not recrystallized; the veins thus are probably younger than most or all of the Late Cretaceous regional metamorphism. |
