Location: The Londevan prospect is at an elevation of about 200-1000 feet, from near the west shore of George Inlet to about a mile inland, and about 1.2 miles north of the mouth of Beaver Falls Creek. The site is in section 6, T. 75 S., R. 92 E., of the Copper River Meridian. It corresponds to loc. 78 in Elliott and others (1978), and to loc. 294 in Maas and others (1995). The location is accurate within 0.1 mile.
Geology: 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 remetamorposed 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. The deposit was discovered in the early 1900s and originally referred to as the Telegraph group. By 1913, a 4350-foot-long adit had been abandoned, apparently due to oxygen-poor air (Maas and others, 1995, p. 202). Efforts as recent as the 1980s to examine or reopen the mine also have been hampered or abandoned because of bad air. Some ore was mined in the early 1900s and stockpiled at the water's edge, but not shipped (Cobb and Elliott, 1980, p. 68). Sampling of the main vein system by Maas and others (1995, p. 209-210) was limited by bad air and caving. A chip sample of a subparallel vein about 650 feet into the adit contained 237.7 ppm Ag, 4300 ppm Pb, and 1.84% Zn across 3.5 feet; and a selected sample of the same vein contained 936 ppm Ag and 6.43% Zn. Two samples across a 12-foot-thick portion of quartz in the main drift averaged 5 ppm Ag and 68 ppb Au, along with minor Pb and Zn.
Workings: The deposit was discovered in the early 1900s and originally referred to as the Telegraph group. By 1913, a 4350-foot-long adit had been abandoned, apparently due to oxygen-poor air (Maas and others, 1995, p. 202). Efforts as recent as the 1980s to examine or reopen the mine also have been hampered or abandoned because of bad air. Sampling of the main vein system by Maas and others (1995, p. 209-210) was limited by bad air and caving. A chip sample of a subparallel vein about 650 feet into the adit contained 237.7 ppm Ag, 4300 ppm Pb, and 1.84% Zn across 3.5 feet; and a selected sample of the same vein contained 936 ppm Ag and 6.43% Zn. Two samples across a 12-foot-thick portion of quartz in the main drift averaged 5 ppm Ag and 68 ppb Au, along with minor Pb and Zn.
Age: The fissure veins probably are Cretaceous or younger in age.
Alteration: Small amounts of pyrite are disseminated in the country rocks adjacent to the veins.
Production: Some ore was mined in the early 1900s and stockpiled at the water's edge, but not shipped (Cobb and Elliott, 1980, p. 68).

Commodities (Major) - Ag, Pb, Zn; (Minor) - Au, Cu
Development Status: Yes; small
Deposit Model: Polymetallic veins (Cox and Singer, 1986; model 22c)

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Mineral List

6 valid minerals.