Sweetheart Ridge

Prospect, Undetermined

Commodities and mineralogy

Ore minerals chalcopyrite; galena; pyrite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale SD
Quadrangle map, 1:63,360-scale D-5
Latitude 57.91917
Longitude -133.62737
Nearby scientific data Find additional scientific data near this location
Location and accuracy The center of the Sweetheart Ridge prospect is at an elevation of about 2,800 feet about 1.9 miles south of the southwest end of Lower Sweetheart Lake. The mineralization extends for more than 2,000 feet in a north-northwest direction. The center is about 0.4 mile south of the center of section 32, T. 45 S., R. 73 E. Figure D-222 of Redman and others (1989) is a map of the deposit.

Geologic setting

Geologic description

Claims were staked on or near the Sweetheart Ridge prospect in the 1890s but little was known about the deposit until it was rediscovered during a Wilderness study by the U.S. Bureau of Mines and U.S. Geological Survey in 1974 (Kimble and others, 1984; Redman and others, 1989). The deposit consists of stratiform mineralization in a zone about 200 feet wide and 10,000 feet long that strikes about N20W and dips about 65-85W. The host rocks have been variously described. Kimble and others (1984) describe them as iron-stained schist and gneiss with gray chlorite schist, part of the Cretaceous hornblende schist unit of Brew and Grybeck (1984). Redman and others (1989) describe the host rocks as mainly greenstone interlayered with felsic schist, quartz-sericite schist, and biotite-feldspar schist. Newberry and others (1997) suggest that the deposit is Devonian but query that age.
Redman and others (1989) defined four types of mineralization: 1) lenses of massive sulfides, mainly chalcopyrite and sphalerite with subordinate galena; 2) disseminated pyrite and chalcopyrite outward from the massive sulfide lenses; 3) chalcopyrite in concordant quartz veins and veinlets; and 4) stringer and blebs of chalcopyrite. The massive sulfide lenses occur in a distinct horizon near the top of a muscovite-schist unit. The massive sulfides occur in lenses, the largest of which is about 0.75 by 6 feet in section that can be traced for 200 feet. Drilling indicates that the massive-sulfide mineralization increases at depth. For instance, the best mineralized zone sampled by Kimble and others (1984) is 5.5 feet thick at the the surface; it contained 0.93 percent copper, 0.52 percent zinc, 0.23 percent lead, and 10.3 parts per million gold. Later drilling by Mapco Exploration cut the same lens at a depth of 150 feet where it contained 1.1 percent copper, 1.58 percent zinc, 0.63 percent lead, and 9.3 ppm gold. The chalcopyrite-quartz veins are thin, typically less than 1 inch thick, and usually are associated with chalcopyrite stringers. The chalcopyrite stringers are extensive through the length and width of the zone.
The Sweetheart Ridge prospect was staked by Mapco Exploration in 1978 and they explored the deposit until 1982. They drilled several thousand feet of hole in the belt. (Redman and others (1989) show the location of two holes; Newberry and others (1997) refer to 14 holes.) As a result of the Mapco drilling and surface sampling, Redman and others (1989) estimate an inferred resource for the prospect of 45,000 tons of material that contain an average of 0.22 ounce of gold per ton, 0.9 percent zinc, 0.9 percent copper, and 0.45 percent lead.
Geologic map unit (-133.629099987761, 57.9188372742572)
Mineral deposit model Metamorphosed volcanogenic massive-sulfide deposit.
Age of mineralization Brew and Grybeck (1984) place the deposit in Cretaceous metamorphic rocks; Newberry and others (1997) suggest that the deposit and the age of the protolith of the host rocks is 'Devonian?'

Production and reserves

Workings or exploration Some open cuts and pits; several thousand feet of hole were diamond drilled from 1978 to 1982.
Indication of production None
Reserve estimates As a result of the Mapco drilling and surface sampling, Redman and others (1989) estimate an inferred resource for the prospect as 45,000 tons of material that contain an average of 0.22 ounce of gold per ton, 0.9 percent zinc, 0.9 percent copper, and 0.45 percent lead.

Additional comments

The prospect is along the boundary of the Tracy Arm-Ford Terror Wilderness Area which is closed to mineral exploration and mining.

References

MRDS Number A013325

References

Reporters Donald Grybeck (U.S. Geological Survey)
Last report date 10/8/2004