|Quadrangle map, 1:250,000-scale||MF|
|Quadrangle map, 1:63,360-scale||D-3|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||
The Orange Point deposit is on a south-facing point on the north side of Johns Hopkins Inlet in section 13, T. 34 S., R. 50 E., of the Copper River Meridian. The main mineral occurrence is about 290 feet above sealevel on Hill No. 2 (Kimball and others, 1978, fig. C36), and this is the location of the coordinates given. Mineralization on Hill No. 3 is at about 490 feet elevation; it probably extends into the Mount Fairweather D-4 quadrangle. Discontinuous sulfide layers also occur on Hill No. 1 at an elevation of about 120 feet above the Inlet.The location of the main deposit at Hill No. 2 is accurate to within about 100 feet. The Orange Point deposit location is essentially that of number 64 of MacKevett and others (1971) and number 11 of Cobb (1972).
The deposit is a semi-condordant disseminated and massive sulfide deposit contained in a mixed volcanic-sedimentary rock package of Permian and Permia(?) age (Brew and others, 1978). The volcanic units include greenstone and meta-andesite and basalt. The strata strike north-northwest and dip steeply, with stratigraphic top to the east (Kimball and others, 1978, p. C132). A sill-like diorite intrusion of Cretaceous age, semi-concordant to the Permian package, crops out about 400 feet west of the deposit. Igneous dikes locally parallel, but also cut across the deposit.
The deposit consists of disseminated and massive sulfide zones approximately concordant to the metavolcanic-volcaniclastic layers. Disseminated sulfide zones in andesitic strata appear to grade into massive sulfide layers. On Hill No. 2, a zone 80 feet across, that contains both massive and disseminated sulfides, is interrupted by an east-striking fault but can be followed on strike for about 500 feet.
Resources have been estimated for a 400-foot long zone. The maximum grades are about 19 percent zinc, 5. 2 percent copper, 2 ounces per ton of silver and 0.1 ounce per ton gold. The average composition of the best zone is 5.2 percent zinc, 2.7 percent copper, 1 ounce per ton silver and 0.03 ounce per ton gold.
The origin of the deposit is uncertain. Kimball and others (1978), based on detailed work done by Mary Ann Parke, argued for a volcanogenic (syngenetic) origin. Bases of the argument were mainly concordancy, copper concentrated near the stratigraphic base of the deposit, and trace elements, both present in and absent from the deposit. The apparent absence of arsenic, antimony, bismuth, tin, and tungsten was a criterion cited for a volcanogenic origin. The common presence of molybdenum, cadmium, and relative enrichment in barium were also thought to favor the syngenetic hypothesis.The gradation between disseminated and massive sulfides, local discordancy, and relatively coarse size of sulfides and the regional setting, in a mixed intrusive-metamorphic series with widespread weak epigenetic mineralization, argues for a replacement origin. Absence of certain components, such as arsenic and bismuth, could be a function of relatively high analytical detection limits rather than real absence.
|Geologic map unit||(-137.001082781331, 58.922492933729)|
|Mineral deposit model||The choice is either epigenetic skarn or syngenetic volcanogenic massive sulfide, that is polymetallic replacement or Besshi massive sulfide (Cox and Singer, 1986; model 19a or 24b). The compiler favors an epigenetic origin (also see Additional comments).|
|Mineral deposit model number||19a or 24b|
|Age of mineralization||Permo-Triassic or Cretaceous.|
|Alteration of deposit||Country rocks are extensively sulfidized between the diorite and the occurrence. Sericite, epidote, and actinolite also occur as possible alteration products in metavolcanic rocks.|
|Workings or exploration||There are no workings. Mineralization in the area was first reported by Reed (1938). MacKevett and others (1971) found copper in excess of one percent at or near the site in 1966. The massive sulfide mineralization was discovered in 1976 by a field party of the U.S. Bureau of Mines, who then began detailed mapping and surface sampling of area (Kimball and others, 1978).|
|Indication of production||None|
The tonnage of the main deposit at Hill No. 2 body has been estimated at 800,000 tons on assumptions of average width of 65 feet, length of 400 feet, depth of 300 feet and average density of 9.5 cubic feet per ton. The richest part of this resource consists of about 270,000 tons having an estimated average grade of 2.7 percent copper, 5.2 percent zinc, and byproduct gold and silver.The approximate size of the resource of the Hill No. 3 zone is estimated at about 160,000 tons based on an assumed volume of 560 x 280 x 10 feet and a tonnage factor of 10. The No. 3 zone contains approximately 1.6 percent copper and 0.4 percent zinc.
A strong case for a syngenetic volcanogenic origin was made by Kimball and others (1978, p. 129-147), with considerable input from Mary Ann Parke (Kimball and others, 1978, p. 129-147) based on the concordant or nearly concordant nature of the deposit, and its occurrence in a mixed marine volcanic-sedimentary package. The authors also emphasized the absence of trace elements arsenic, antimony, bismuth, tin and tungsten. An alternative origin as a polymetallic igneous-affiliated epigenetic replacement deposit is the irregular and poddy character of the deposit and lack of good stratigraphic definition. Furthermore, the limits used in analysis are not sufficiently low to prove that arsenic, antimony, and bismuth are absent, and tungsten and tin content arguably are appropriate to distinguish between volcanogenic and replacement deposits. Molybdenum is widespread in the area and probably also is not diagnostic either way.
As presently defined, the deposit is subeconomic because of its small size and because recovery of copper and zinc would require a complex flow sheet not justified by the small size of the deposit. The deposit is worth drilling and more geologic study; it is within a belt that could contain other massive sulfide deposits as well as porphyry-type copper-molybdenum deposits, as at Margerie Glacier, which is only a few miles north of Orange Point in the Skagway quadrangle.The deposit is in Glacier Bay National Park and Preserve and is inactive.
Brew, D.A., Johnson, B.R., Grybeck, D., Griscom, A., Barnes, D.F., Kimball, A.L., Still, J.C., and Rataj, J.L., 1978, Mineral resources of the Glacier Bay National Monument Wilderness Study Area, Alaska: U.S. Geological Survey Open-File Report 78-494, 670 p., 7 sheets.
Cobb, E.H., 1972, Metallic mineral resources map of the Mount Fairweather quadrangle, AK: U.S. Geological Survey Miscellaneous Field Study Map MF-436, 1 sheet, scale 1:250,000.
Cobb, E. H., 1981, Summaries of data and lists of references to metallic and selected non-metallic mineral occurrences in the Mt. Fairweather quadrangle, Alaska, Supplement to Open-file Report 78-316: U. S. Geological Survey Open-File Report 81-249B, 15 p.
Cobb, E.H., 1981, Summaries of data on and lists of references to metallic and selected nonmetallic mineral occurrences in the Mt. Fairweather quadrangle, Alaska; Supplement to Open-file Report 78-316; Part A, Summaries of data to January 1, 1980: U.S. Geological Survey Open-File Report 81-249-A, 20 p.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Kimball, A.L., Still, J.C., and Rataj, J.L., 1978, Mineral resources, in Brew, D. A., and others, Mineral resources of the Glacier Bay National Monument wilderness study area, Alaska: U.S. Geological Survey Open-File Report 78-494, p. C1-C375.
MacKevett, E.M., Jr., Brew, D.A., Hawley, C.C., Huff, L.C., and Smith, J.G., 1971, Mineral resources of Glacier Bay National Monument, Alaska: U.S. Geological Survey Professional Paper 632, 90 p., 12 plates, scale 1:250,000.
Reed, J.C., 1938, Some mineral deposits of Glacier Bay and vicinity, Alaska: Economic Geology, v. 33, p. 52-80.
|Reporters||C.C. Hawley (Hawley Resource Group)|
|Last report date||4/8/1999|