Geologic description
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 Alaska Lead and Silver deposit consists of a sulfide-bearing quartz fissure vein in hornfelsed biotite schist (Maas and others, 1995, p. 203 and figs. 52, 53). The vein is up to three feet thick and contains 1-4% combined sphalerite, galena, chalcopyrite, and pyrite. It strikes NE and dips 73 SE, and has been traced along strike for about 300 feet. The deposit was discovered in 1967. By 1968, the operators had extended drifts in two separate adits (37 and 260 feet long), stoped 88 feet along the vein, constructed a flotation mill at the beach, and mined about 500 metric tons of ore of unknown grade. Samples along about 210 feet of the vein contained a weighted average of 21.6 ppm Ag, 685 ppm Pb, and 2731 ppm Zn across an average width of two feet (Maas and others, 1995, p. 209). A high-grade sample of this vein contained 63.1 ppm Ag, 0.36% Pb, and 0.29% Zn across three feet. Some of the samples also contained a significant amount of Cd. |
