Sea Breeze Prospect, Ketchikan District, Ketchikan Gateway Borough, Alaska, USA

Location: The Sea Breeze prospect is at an elevation of about 200 feet, approximately 0.2 mile south-southwest of the mouth of Beaver Lake. The site is in section 18, T. 75 S., R. 94 E., of the Copper River Meridian. It corresponds to loc. 91 in Elliott and others (1978), and to loc. 300 (1-5) in Maas and others (1995). The location is accurate within a few hundred feet.
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 Sea Breeze claim adjoins the Sea Level claim (KC095) on the northeast, and is located on the extension of its mineralized belt (Brooks, 1902, p. 67; Wright and Wright, 1908, p. 146; Maas and others, 1995, fig. 55). The deposit consists of quartz fissure veins 1-8 feet thick in mafic metavolcanic (greenstone) country rock. The veins strike NE and dip steeply SE, and are sharply discordant to the NW foliation of the metavolcanic country rocks. The veins are characterized by intense fracturing, brecciation, and abrupt variations in mineralization. The ore minerals include [auriferous] pyrite, galena, sphalerite, and an occasional speck of native gold. Auriferous pyrite also is sparsely disseminated in the wallrocks adjacent to the veins (Maas and others, 1995, p. 215). The walls of some of the veins are bluish-gray, hydrothermally altered rock that was interpreted by the early miners as a dike, or dikes, of intrusive porphyry ('blue porphyry' of Brooks, 1902, p. 65-67; and Wright and Wright, 1908, p. 144-146). Maas and others (1995, p. 215), however, interpret it as a zone of hydrothermally altered mafic metavolcanic rock. Maas and others (1995) also report that the quartz in the veins is not recrystallized. The veins thus probably are younger than most or all of the Late Cretaceous regional metamorphism, but are older than a period of intense local faulting and brecciation. The prospect was explored in the early 1900s by two short tunnels, and by numerous opencuts that exposed the veins for several hundred feet along strike. Maas and others (1995, p. 218) report two adits, one 69 feet long, and one caved. Maas and others (1995, p. 217-218) report a mean value of 661 ppb Au in 12 samples of the Sea Breeze vein. Their best assay showed 2.0 ppm Au in an 8.5-foot sample across the vein.
Workings: The prospect was explored in the early 1900s by two short tunnels, and by numerous opencuts that exposed the veins for several hundred feet along strike. Maas and others (1995, p. 218) report two adits, one 69 feet long, and one caved. Maas and others (1995, p. 217-218) report a mean value of 661 ppb Au in 12 samples of the Sea Breeze vein. Their best assay showed 2.0 ppm Au in an 8.5-foot sample across the vein.
Age: Maas and others (1995) report that the quartz in the veins is not recrystallized. The veins thus probably are younger than most or all of the Late Cretaceous regional metamorphism, but are older than a period of intense local faulting and brecciation.
Alteration: The walls of some of the veins are bluish-gray, hydrothermally altered rock that was interpreted by the early miners as a dike, or dikes, of intrusive porphyry ('blue porphyry' of Brooks, 1902, p. 65-67; and Wright and Wright, 1908, p. 144-146). Maas and others (1995, p. 215), however, interpret it as a zone of hydrothermally altered mafic metavolcanic rock.

Commodities (Major) - Au; (Minor) - Pb, Zn
Development Status: Undetermined.
Deposit Model: Low-sulfide Au-quartz veins (Cox and Singer, 1986; model 36a)

Mineral List

5 entries listed. 5 valid minerals.

References