Massachusetts

Prospect, Undetermined

Alternative names

Keystone

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 55.387
Longitude -131.169
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is at an elevation of 500 feet in the south wall of the unnamed creek that flows west into Granite Lake. The site is about 0.3 mile east of Granite Lake, in section 8, T. 75 S., R. 94 E., of the Copper River Meridian. It corresponds to loc. 86 in Elliott and others (1978), and to loc. 298 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, 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 deposit consists of a sulfide-bearing quartz fissure vein in intercalated metapelitic and metavolcanic (greenstone) schists that are cut by a granodiorite dike (Wright and Wright, 1908, p. 148-9). The vein at the surface is five feet wide, but at depth it decreases to six inches and divides into stringers. The sulfides are pyrite, galena, and sphalerite, accompanied by favorable gold values near the surface. According to Maas and others (1995, p. 215), the Massachusetts auriferous veins strike NE and are hosted in hydrothermally altered mafic metavolcanic rocks that crop out on both sides of the granodiorite dike (see Alteration). The quartz in these veins is not recrystallized, so the veins are probably younger than most or all of the Late Cretaceous regional metamorphism.
The prospect was explored in the early 1900s by several opencuts, an 80-foot adit, a 30-foot shaft, and a 50-foot drift off the shaft. Gold values of $12/ton (Au at $20.67/oz.) were reported by the operators at that time (Brooks, 1902, p. 68).
Maas and others (1995, p. 217, 218) report a mean value of 2071 ppb Au in 12 samples of the Massachusetts deposit; their best assay showed 11.1 ppm Au and 134 ppm Ag in a 3-foot sample across the main vein.
Geologic map unit (-131.170694515102, 55.3866599286921)
Mineral deposit model Low-sulfide gold-quartz veins (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Late Cretaceous or younger.
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

Workings or exploration The prospect was explored in the early 1900s by several opencuts, an 80-foot adit, a 30-foot shaft, and a 50-foot drift off the shaft. Gold values of $12/ton (Au at $20.67/oz.) were reported by the operators at that time (Brooks, 1902, p. 68). Maas and others (1995, p. 217, 218) report a mean value of 2071 ppb Au in 12 samples of the Massachusetts deposit; their best assay showed 11.1 ppm Au and 134 ppm Ag in a 3-foot sample across the main vein.
Indication of production None

Additional comments

Wright and Wright (1908, p. 148) refer to this property as the Massachusetts claims nos. 1 and 2, and note that they were originally known as the Keystone claims (Brooks, 1902, p. 68).

References

MRDS Number A012339

References

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