Incas

Mine, Inactive

Commodities and mineralogy

Main commodities Au
Other commodities As
Ore minerals arsenopyrite; native gold; pyrite
Gangue minerals calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale D-3
Latitude 58.86
Longitude -136.833
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Incas vein crops out on the steep side-hill west of Reid Inlet at an elevation of about 1000 feet. Incas is about 0.75 mile south of the Sentinel vein (MF028) and 0.6 mile north-northwest of the Galena (MF031). The Incas is a vein-mineralized fault zone that can be traced for nearly 2000 feet. The coordinates given are the approximate location of workings on the vein; they are probably correct within 0.15 mile of the workings.

Geologic setting

Geologic description

The Reid Inlet gold area is mainly underlain by granitic rocks of Cretaceous age (Brew and others, 1978). The area was mapped in detail by Rossman (1959, B 1058-B).
The Incas is a productive north-striking, steeply-dipping vein fissure 1-3-feet thick with lenses of quartz and calcite in a shear zone. Rossman (1959) traced the vein for about 2000 feet and found free gold in all quartz lenses. Locally, gold may have been concentrated by supergene enrichment. Workings driven below the surface outcrops found only low grade quartz veins. The Incas structure is subparallel to and similar geologically to the Monarch vein fissure (MF026). Rossman (1959, plate 4) mapped another subparallel vein about 400-feet east of the Incas vein; the vein was partly covered by colluvium.
Geologic map unit (-136.834867322462, 58.8595997953638)
Mineral deposit model Low-sulfide gold-quartz vein (Cox and Singer, 1986; model 36a).
Mineral deposit model number 36a
Age of mineralization Tertiary.
Alteration of deposit Rossman (1959) notes alteration (bleaching?) along the 2000-foot strike exposure of the vein fissure.

Production and reserves

Workings or exploration Incas was discovered in 1924 by Joe Ibach and is one of the earliest discoveries in the district. Claims were located by Ibach and famous Alaska novelist, Rex Beach, in 1936 after Glacier Bay National Monument was opened for mining (Kimball and others, 1978, p. C217). Gold, possibly enriched by supergene processes, was mined along the exposed vein fissure for about 60 feet near an elevation of 1100 feet. A crosscut tunnel was driven below the stoped area for about 170 feet. Near the face of the crosscut, the adit drifts for about 40 feet along a fissure reasonably inferred to be on the same vein mined at the surface. MacKevett found only about 0.029 oz/ton gold in narrow samples taken from the underground workings. One sample contains 20,000 ppm arsenic (MacKevett and others, table 11, location G).
Indication of production Yes; small

Additional comments

Small pods of gold-bearing quartz exist along an altered but weakly mineralized shear zone. Best chances for ore are believed to be in the surface pods of quartz or in secondary fissures opened near the strong north-trending vein fissure. The vein is not well explored at depth. The Incas vein is in Glacier Bay National Park and Preserve.

References