Big Boulder

Occurrence, Probably inactive

Commodities and mineralogy

Main commodities Au
Ore minerals gold
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale SK
Quadrangle map, 1:63,360-scale B-4
Latitude 59.44
Longitude -136.23
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is at an elevation of 1,500 feet west of Big Boulder Creek and north of the the Klehini River across from the old Porcupine town site. It is in the south-central part of the section 16, T. 28 S., R. 54 E of the Copper River Meridian. It is shown as number 4 of Still and others (1991). MacKevett and others (1974) show a 'Au?' notation (without further explanation or reference) at the same elevation but approximately 1/2 mile farther to the west on the south facing slope above the Klehini River.

Geologic setting

Geologic description

Wright (1904 [B 225 and B 236]) describes this occurrence as a quartz ledge. The brief description of Still and others (1991) is a little unclear, but the prospect appears to be quartz segregations and veins in felsic dikes within slate. The dikes strike north-northwest, dip steeply, are up to 1.3 feet thick, and crop out for up to 60 feet along strike. An adit was driven through a felsic dike for 18 feet where is cuts a quartz band for about 5 feet. None of the samples of quartz and dike rock contained anomalous gold, but they had up to 1.8 ppm silver, 308 ppm lead, 100 ppm tin, 500 ppm arsenic, 700 ppm bismuth, and 900 ppm antimony.
Bedded rocks in the area are metasediments and metavolcanics of the Cheetdeekahyu Group of Redman and others (1985). Their metamorphic grade and average grain size increases to the north and northeast towards Four Winds Mountain. They have been included in the Four Winds metamorphic complex (Forbes and others, 1987; Gilbert and others, 1987). The probable Devonian or Late Triassic age of the Cheetdeekahyu Group (Gilbert and others, 1987) establishes a maximum age for the veins at this prospect. However, if the dike is Cretaceous as are nearby intrusives, then the veins are Cretaceous or younger in age.
Geologic map unit (-136.231854681959, 59.4396756777453)
Mineral deposit model Auriferous quartz vein (Cox and Singer, 1986; model 22c or 36a).
Mineral deposit model number 22c or 36a
Age of mineralization Probably Cretaceous or younger based on the age of nearby intrusives (MacKevett and others, 1974; Gilbert and others, 1987).

Production and reserves

Workings or exploration Still and others (1991) report a shallow adit that, '..was likely driven about 80 years ago.'
Indication of production None

References

References

Forbes, R.B., Gilbert, W.G., and Redman, E.C., 1987, The Four Winds Complex; a newly recognized Paleozoic metamorphic terrane in southeastern Alaska [abs.]: Geological Society of America, Abstracts with Program, v. 19, no. 6, p. 378.
Reporters T.C. Crafford (T. Crafford & Associates, Anchorage)
Last report date 2/4/2001