|Quadrangle map, 1:250,000-scale||KC|
|Quadrangle map, 1:63,360-scale||B-5|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||This site is in section 8, T. 76 S., R. 91 E., of the Copper River Meridian. It includes two, closely-spaced, deposits at sea level on southeastern Gravina Island opposite Pennock Island. They are the Heckman mine, which is about 0.3 mile from Gravina Point, and an unnamed prospect about 0.3 mile northwest of the Heckman. The coordinates of the map site are for the Heckman prospect. The site corresponds to loc. 71 in Elliott and others (1978), and to locs. 287 and 288 in Maas and others (1995). The location of the Heckman mine is accurate within 0.1 mile.|
The country rocks on southeastern Gravina Island are Upper Jurassic or Cretaceous marine andesitic or basaltic volcanic rocks that are gradationally interbedded with subordinate flyschlike sedimentary rocks (Berg, 1973; Berg and others, 1988). The strata were regionally metamorphosed to greenschist-grade phyllite and semischist in Late Cretaceous time. The country rocks are cut by a high-angle fault along Tongass Narrows that displays about 4 miles of right-lateral offset.
Maas and others (1995, p. 194, 199, 201-202, and fig. 51) describe the Heckman prospect, and a similar unnamed lode about 0.3 mile to the northwest, as stratiform, sulfide-bearing, northwest-trending layers of greenschist or quartz-chlorite schist, locally cut by sulfide-bearing quartz fissure veins. At the Heckman prospect, pyrite, sphalerite, and galena occur as bands of massive sulfides, and as disseminations, in several layers of quartz-chlorite schist, separated by several layers of relatively barren schist.
The sulfides also occur in quartz stringers parallel to the foliation of the schist. The stratiform deposit, as exposed in old trenches, is at least 25 feet wide and 60 feet long. Samples of this deposit indicated a zone up to about 10 feet wide containing 6.2 ppm Au and 1.6% Zn. The other prospect, at sea level about 0.3 mile northwest of the Heckman prospect, consists of talc-chlorite schist that contains pyrite, sphalerite, and galena disseminated in quartz-rich layers parallel to the foliation of the schist.
Brooks (1902, p. 62) interpreted the Heckman deposit as the southeast continuation of the deposit at the Goldstream (Bell) mine (KC072). Maas and others (1995, p. 194, 201) suggest that the stratiform deposits are of volcanogenic origin. If so, the Heckman probably can be classified as a metamorphosed Besshi massive sulfide deposit of Late Jurassic age. The sulfide-bearing quartz fissure veins that cut the foliation probably formed by remobilization, during or following Upper Cretaceous regional metamorphism.These prospects were discovered in the early 1900s (Brooks, 1902, p. 62; Wright and Wright, 1908, 179). The Heckman was explored at that time by a 60-foot shaft, and drifts totalling 32 feet. The other prospect was explored by a 50-foot shaft.
|Geologic map unit||(-131.618683982729, 55.292653958619)|
|Mineral deposit model||Besshi massive sulfide?; Polymetallic veins (Cox and Singer, 1986; models 24b, 22c)|
|Mineral deposit model number||24b?, 22c|
|Age of mineralization||Late Jurassic?; Late Cretaceous or younger.|
|Workings or exploration||These prospects were discovered in the early 1900s (Brooks, 1902, p. 62; Wright and Wright, 1908, 179). The Heckman was explored at that time by a 60-foot shaft, and drifts totalling 32 feet. The other prospect was explored by a 50-foot shaft. Samples collected by Maas and others (1995) of the Heckman deposit indicated a zone up to about 10 feet wide containing 6.2 ppm Au and 1.6% Zn.|
|Indication of production||Undetermined|
|Production notes||There is no public record of any production from the Heckman mine, but the extent of the underground workings suggests that at least a small amount of gold was recovered in the early 1900s.|
Brooks (1902, p. 62), noting their similarity, interpreted the Heckman deposit as the southeast continuation of the deposit at the Goldstream (Bell) mine (KC072).Also see Production notes.
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.
Elliott, R.L., Berg, H.C., and Karl, S.M., 1978, map and table describing metalliferous and selected non-metalliferous mineral deposits, Ketchikan and Prince Rupert quadrangles, Alaska: U.S. Geological Survey Open-File Report 78-73-B, 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.
|Reporters||H.C. Berg, USGS|
|Last report date||7/3/1999|