Rainbow (ARDF #MF027) Au

Rainbow

Mine, Inactive

Commodities and mineralogy

Main commodities	Au
Other commodities	Ag; As; Pb; Zn
Ore minerals	arsenopyrite; galena; native gold; pyrite; sphalerite
Gangue minerals	calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale	MF
Quadrangle map, 1:63,360-scale	D-3
Latitude	58.8781
Longitude	-136.8355
Nearby scientific data	Find additional scientific data near this location
Location and accuracy	The Rainbow adit is essentially at tidewater, west of the mouth of Reid Inlet on Glacier Bay. The upper Rainbow prospect is at an elevation of about 1000 feet; it is almost on trend with the bearing of the lower Rainbow adit. The upper prospect has a latitude of about 58.8748 and longitude of 136.8343 The locations are accurate within 0.1 mile.

Geologic setting

Geologic description The Rainbow quartz (-calcite) fissure vein occurs in the part of the Reid Inlet gold area (Kimball and others, 1978) underlain by granodiorite of Cretaceous age (Rossman, 1959, B 1058-B; Brew and others, 1978). Minor amounts of alaskite are present near the mine (MacKevett and others, 1971, p. 59). The Rainbow vein-bearing fault zone strikes about N 30 E, and dips 70 southeast to nearly vertical. The vein is narrow, generally 1 foot or less in thickness, but it is well mineralized. Arsenopyrite is common; galena and locally sphalerite are present up to a few percent each. Free gold is locally present. The vein was traced upslope for at least several hundred feet, and the vein-fault zone, nearly free of quartz, could be traced an additional 2000 feet up hill by subcropping bright red (hematitic?) altered rocks (Rossman, 1959, p. 52). The mineralization is similar to the LeRoy mine (MF022), moderately abundant auriferous sulfides in a quartz-calcite rich vein matrix. Locally the vein was rich. MacKevett found some rich parts of the vein when he sampled the Rainbow adit in 1966 (MacKevett and others, 1971, p. 59-60, table 11). Three samples collected below overhand-stoped quartz veins in the adit contained from 1.518 to 10.208 ounces per ton gold. The richest sample also contained about 2 ounces per ton silver, 1000 ppm arsenic, 100 ppm copper, 15 ppm molybdenum, 500 ppm lead, and 2000 ppm zinc. All the mineralized samples had trace amounts of molybdenum. (Bismuth, antimony, and tungsten were looked for but not found). Major elements contained in the veins include calcium up to 2 percent and magnesium up to 0.8 percent, along with iron, suggesting the possiblity of ankerite or dolomite in the gangue.
Geologic map unit	(-136.837368312156, 58.8776988720544)
Mineral deposit model	Low-sulfide gold-quartz vein (Cox and Singer, 1986; model 36a).
Mineral deposit model number	36a
Age of mineralization	Late Cretaceous or younger.
Alteration of deposit	Alteration was not described specifically at the Rainbow. In general for the Reid Inlet area, Rossman (1959, p. 43) says that the vein walls are altered for up to 10 to 15 feet from the veins. Oxidation formed a rust-red color, consistent with either sulfides or ankerite disseminated in the altered zone. MacKevett and others (1971, p. 59) noted abundant secondary iron minerals, gouge, and scattered free gold in the altered vein zones.

Production and reserves

Workings or exploration	An adit was driven on the vein just above tide level for an estimated 190 feet. The vein was further developed by two raises and an overhand stope above adit level. Mining was in conjunction with the LeRoy vein (MF022) by the LeRoy Mining Co. The ore was hauled by barge and truck to the LeRoy mill. Shallow surface workings on the Upper Rainbow are at an elevation of about 1000 feet. Rossman mapped several veins of various strikes close to the Rainbow (Rossman, 1959, pl. 4).
Indication of production	Yes; small
Reserve estimates	There is a resource present in the floor of the adit that could be developed with a winze. The winze would be below sea level. Inflows are uncertain but might be manageable with an adequate pillar. Reserves could likely be developed by continuing the drift. Rossman's (1959) mapping suggests the existence of vein intersections that could be targets for wider mining zones.
Production notes	Based on narrow vein, tonnage small, but likely multiounce per ton gold.
Additional comments The mine is in Glacier Bay National Park and Preserve.

References

MRDS Number	A013153
References 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. 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. Rossman, Darwin, 1959, Geology and ore deposits in the Reid Inlet area, Glacier Bay, Alaska: U.S. Geological Survey Bulletin 1058-B, p. 33-58.
Reporters	C.C. Hawley (Hawley Resource Group)
Last report date	4/8/1999