|Quadrangle map, 1:250,000-scale||DE|
|Quadrangle map, 1:63,360-scale||D-2|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||This site represents six, geologically similar occurrences of massive-sulfide mineralization. They are scattered along a broad, east-west band about 7 miles long that extends through the middle of the Barrier Islands. The site is about at the center of the area of occurrences, near the center of section 33, T. 81 S., R. 86 E. The occurrences are described individually by Gehrels and others (1983).|
As first identified by Gehrels and others (1983), the six massive sulfide occurrences represented by this site are virtually identical in hostrock, mineralogy, and genesis. Gehrels and others described the hostrocks as two volcanic units of the Silurian or Ordovician Descon Formation: one basalt or andesite; the other dacite or rhyolite. They distinguished the hostrocks from Cambrian or older Wales Group rocks to the north by the absence of a penetrative metamorphic fabric. S.M. Karl (oral communication, 2003), however, believes that the rocks are more metamorphosed than the Descon Formation and that they are a distinct Silurian or Ordovician metamorphic unit on southern Prince of Wales Island.
Most of the deposits are associated with layers several meters thick of fragmental, intermediate to silicic volcanic rocks. The sulfide minerals generally form thin layers less than 5 centimeters (cm) thick in the volcanic rocks, or form rinds around 5- to 20-cm-long volcanic fragments that may be flattened pillows. Sulfide minerals also occur in layers less than 5 cm thick in siliceous black slate and silicic volcanic rocks. The dominant sulfide is pyrite; the presence of arsenopyrite, galena, and sphalerite is indicated by spectrographic analysis of samples. Selected samples contain up to 30 parts per million (ppm) silver, more than 5,000 ppm barium, 30 ppm copper, 1,500 ppm lead, and 3,000 ppm zinc. The occurrences are probably syngenetic, submarine, and exhalative in origin. Later low-grade metamorphism resulted in local remobilization of the the metals.Maas and others (1991) collected samples of iron-stained schist, volcanic rocks, tuff, slate, and graywacke at several of these occurrences. Chip and representative samples across iron-stained bands 0.1 to 20 feet wide contain up to 62 parts per billion gold, 10.8 ppm silver, 570 ppm copper, 0.15 percent lead, 0.66 percent barium, and 1.34 percent copper.
|Geologic map unit||(-132.42386314164, 54.802756535502)|
|Mineral deposit model||Kuroko massive sulfide (Cox and Singer, 1986; model 28a).|
|Mineral deposit model number||28a|
|Age of mineralization||Silurian or Ordovician.|
|Alteration of deposit||Not specified, but the hostrocks probably are propylitically altered, as is common in submarine, exhalative massive-sulfide deposits in intermediate to felsic volcanic rocks.|
|Indication of production||None|
Additional commentsAll of these occurrences are in the South Prince of Wales Wilderness, which is closed to prospecting and mineral exploration.
Gehrels, G. E., 1991, Geologic map of Long Island and southern and central Dall Island, southeastern Alaska: U.S. Geological Survey Map MF-2146, 1 sheet, scale 1:63,360.
Gehrels, G. E., Berg, H. C., and Saleeby, J. B., 1983, Ordovician-Silurian volcanogenic marine sulfide deposits on southern Prince of Wales Island and the Barrier Islands, southeastern Alaska: U.S. Geological Survey Open-File Report 83-318, 10 p., 1 sheet.
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.
Maas, K.M., Still, J.C., Clough, A.H., and Oliver, L.K., 1991, Mineral investigations in the Ketchikan mining district, Alaska, 1990: southern Prince of Wales Island and vicinity: U.S. Bureau of Mines Open-File Report 33-91, 139 p., 12 sheets, scale 1:63,360.
|Reporters||D.J. Grybeck (Applied Geology)|
|Last report date||9/1/2003|