Massachusetts; Keystone Prospect, Ketchikan District, Ketchikan Gateway Borough, Alaska, USA
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Location: 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.
Geology: 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.
Workings: 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.
Age: Late Cretaceous or younger.
Alteration: 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).
Commodities (Major) - Au; (Minor) - Pb, Zn
Development Status: None
Deposit Model: Low-sulfide gold-quartz veins (Cox and Singer, 1986; model 36a)
Mineral List
5 entries listed. 5 valid minerals.
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References
Berg, H.C., 1982, The Alaska Mineral Resource Assessment Program; guide to information about the geology and mineral resources of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Circular 855, 24 p. Berg, H.C., Elliott, R.L., and Koch, R.D., 1988, Geologic map of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Mineral Investigations Series Map I-1807, 27 p., scale 1:250,000. Brooks, A.H., 1902, Preliminary report on the Ketchikan mining district, Alaska, with an introductory sketch of the geology of southeastern Alaska: U.S. Geological Survey Professional Paper 1, 120 p. Crawford, M.L., Crawford, W.A., and Gehrels, G.E., 2000, Terrane assembly and structural relationships in the eastern Prince Rupert quadrangle, British Columbia, in H.H. Stowell and W.C.McClelland, eds., Tectonics of the Coast Mountains, southeastern Alaska and British Columbia: Geological Society of America Special Paper 343, p. 1-21. Elliott, R.L., Berg, H.C., and Karl, S.M.,1978, Map and table describing metalliferous and selected non-metalliferous mineral deposits in the Ketchikan and Prince Rupert quadrangles, Alaska: U.S. Geological Survey Open-File Report 78-73B, 17 p., 1 sheet, scale 1:250,000. Maas, K.M., Bittenbender, P E., and Still, J.C., 1995, Mineral investigations in the Ketchikan mining district, southeastern Alaska: U.S. Bureau of Mines Open-File Report 11-95, 606 p. Wright, F.E., and Wright, C.W., 1908, The Ketchikan and Wrangell mining districts, Alaska: U.S. Geological Survey Bulletin 347, 210 p.
