Prospect, Undetermined

Commodities and mineralogy

Main commodities Au
Other commodities Pb; Zn
Ore minerals galena; gold; pyrite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale B-4
Latitude 55.383
Longitude -131.191
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Baltic prospect is at an elevation of about 150-200 feet, 0.3 mile east of the shore of Thorne Arm, and 0.5 mile west of Star Lake. The site is in section 7, T. 75 S., R. 94 E., of the Copper River Meridian. It corresponds to loc. 87 in Elliott and others (1978), and to loc. 299 in Maas and others (1995). The location is accurate within 0.1 mile. Elliott and others, following an error in Wright and Wright (1908, p. 148), erroneously called the site 'Baltic Star.'

Geologic setting

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.
According to Wright and Wright (1908, p. 148), the Baltic prospect (which they erroneously called 'Baltic Star') consists of a 1.5-foot-thick quartz fissure vein in mineralized schist. The vein, which was traced on the surface for 300 feet, strikes NE and dips 75 SE, and contains small amounts of pyrite, sphalerite, and galena, in addition to low values in gold. The northeast end of the Baltic claim abuts against a Quaternary basalt lava flow.
Maas and others (1995, p. 215) interpret the 'mineralized schist' in the Sea Level mine area as hydrothermally altered mafic metavolcanic rock (see Alteration, below; also see KC095). They also 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.
Geologic map unit (-131.192694264218, 55.38265970811)
Mineral deposit model Low-sulfide gold-quartz veins (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization 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.
Alteration of deposit According to Maas and others (1995, p. 215), the wallrocks of the auriferous veins in the Sea Level mine area are hydrothermally altered to bluish-gray carbonate-sericite-pyrite rock, called 'blue porphyry' in some early reports (for example, Brooks, 1902; Wright and Wright, 1908) (also see KC095).

Production and reserves

Indication of production None

Additional comments

The Baltic prospect was erroneously called 'Baltic Star' by Wright and Wright (1908, p. 148) and by Elliott and others (1978, loc. 87).