Londevan

Prospect, Undetermined

Alternative names

Lon-De-Van
Telegraph group

Commodities and mineralogy

Main commodities Ag; Pb; Zn
Other commodities Au; Cu
Ore minerals chalcopyrite; galena; pyrite
Gangue minerals albite; calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale B-5
Latitude 55.399
Longitude -131.481
Nearby scientific data Find additional scientific data near this location
Location and accuracy 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.

Geologic setting

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 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.
Geologic map unit (-131.482688600851, 55.398652631709)
Mineral deposit model Polymetallic veins (Cox and Singer, 1986; model 22c)
Mineral deposit model number 22c
Age of mineralization The fissure veins probably are Cretaceous or younger in age.
Alteration of deposit Small amounts of pyrite are disseminated in the country rocks adjacent to the veins.

Production and reserves

Workings or exploration 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.
Indication of production Yes; small
Production notes Some ore was mined in the early 1900s and stockpiled at the water's edge, but not shipped (Cobb and Elliott, 1980, p. 68).

References

MRDS Number A012334

References

Reporters H.C. Berg, USGS
Last report date 7/3/1999