Sentinel

Mine, Inactive

Commodities and mineralogy

Main commodities Au
Other commodities Ag; Pb
Ore minerals galena; hematite; native gold; plumbojarosite
Gangue minerals quartz; alteration minerals of granodiorite

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale D-3
Latitude 58.8718
Longitude -136.8339
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Sentinel mine is at an elevation of about 900 feet 0.4 mile south of the Rainbow (MF027) adit on the northeast-sloping sidehill above the mouth of Reid Inlet. The location is accurate within 100 feet. The Sentinel is location 21 of Cobb (1972).

Geologic setting

Geologic description

The Sentinel vein is hosted by granodiorite of Cretaceous age; it is locally covered by surface alluvium (Rossman, 1959, B 1058-B. pl. 4). The vein is similar in strike and characteristics to the Rainbow (MF027). The vein is steep and strikes northeast. The vein, six to 10 inches wide, is within an altered envelope about 1-foot thick. Rossman (1959, p. 54) noted that vein was quartz poor, and consisted mainly of altered granodiorite containing disseminated galena, other sulfides, and occasionally gold. The vein was discovered by Joe Ibach; it had a cavity a few feet long at the discovery cut. The floor of the cavity contained a red oxide rich in gold. The oxide has been inferred to be lead oxide (plumbojarosite?), but it probably included iron oxides, possibly hematite. The high-grade ore was mined by Ibach.
The Sentinel is at the head of a swarm of veins, also of northeast to east- northeast strike, which extend over about 1600 feet of beach front below the mine. Apparently most of the veins have not been sampled.
MacKevett and others, (1971, p. 60, also table 11, loc. D) found about 0.1 oz/ton gold in a nearby vein parallel to the Sentinel.
Geologic map unit (-136.835767739091, 58.8713990829503)
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 Intensely altered haloes in granodiorite around veins.

Production and reserves

Workings or exploration The Sentinel vein was discovered and mined in shallow open cuts by Joe Ibach shortly after Glacier Bay National Monument was opened to mining in 1936. Shallow surface cuts are now obscured by overburden and vegetation.
Indication of production Yes; small
Reserve estimates None: Some potential especially if nearby vein swarm is auriferous.
Production notes Joe Ibach is reported to have transported high-grade oxidized gold-bearing ore to Lemesurier Island where it was milled.

Additional comments

Ore was from a nearly quartz-free shear zone in granodiorite. Rossman (1959, p. 54) reports galena and other sulfides disseminated along vein-bearing fault. The Sentinel and Rainbow (MF027) were discovered in 1936 after the National Monument was reopened for mining. The vein is now in Glacier Bay National Park and Preserve.

References