Geological Survey Bulletin 1359 Geology and Mineral Resources of the Northern Part of the North Cascades National Park, Washington

By MORTIMER H. STAATZ, ROWLAND W. TABOR, PAUL L. WEIS, and JACQUES F. ROBERTSON, U.S. Geological Survey, and RONALD M. VAN Nov and ELDON C. PATTEE, U.S. Bureau of Mines

SETTING

The northern part of the North Cascade Mountains, which contains the North Cascades National Park, has numerous mineral occurrences of copper and molybdenum, but production has been relatively small except in the now-dormant Holden mine in Chelan County, about 40 miles southeast of the present study area. This property, the largest producer of copper in the history of the State of Washington during the 20 years it was in operation, produced about 106,000 tons of copper, or more than 83 percent of the total copper mined in Washington to 1963 (Vhay and Weissenborn, 1966, p. 72, 74). In addition, about 600,000 ounces of gold were recovered from this property (Mac Laren and others, 1966, p. 86). About 12 miles west-northwest of the Holden mine, on Miners Ridge in the Glacier Peak Wilderness, is a large low-grade ore body consisting of a stockwork of quartz-pyrite chalcopyrite-molybdenite veinlets (Vhay and Weissenborn, 1966, p. 79). Although this property has been extensively explored, it had not been mined by 1971.

Copper deposits larger than those in Washington are known in the North Cascade Mountains in Canada. The Copper Mountain mine, 34 miles northeast of the park, produced about 340,000 tons of copper, from a porphyry-type deposit averaging 1 percent copper, before it was shut down in 1957 (McTaggart and others, 1960, p. 38). At the Craigmont mine, about 82 miles north of the northeast corner of the park, mined ore and reserves of several ore bodies average 1.77 percent copper in grade and have a total copper content of 513,300 tons (Carr, 1966, p. 323). On the Bethlehem property in Highland Valley, 20 miles north of the Craigmont mine, four porphyry ore bodies have an average copper content of 0.70 percent and a total copper content of more than 490,000 tons (Carr, 1966, p. 321).

Gold has been recovered from several places in the North Cascade Mountains : the Mount Baker mining district, which lies adjacent to the northwest corner of the park, and the Slate Creek—Azurite mining district, which lies several miles east of the south end of Ross Lake. Ruby Creek drains the Slate Creek—Azurite district, and gold has been recovered from placers on this stream as far west as the part now flooded by Ruby Arm of Ross Lake. Production from the Mount Baker mining district was 43,300 ounces, and from the Slate Creek—Azurite district, 149,700 ounces (MacLaren and others, 1966, p. 86). The value of gold produced from these two dormant mining districts is approximately $5 million.

Because the geologic environment of some of the previously mentioned mines and mining districts is similar in part to that found in the northern part of the North Cascades National Park, we made a careful study of all prospects as well as a search for other mineralized localities. No ore has ever been shipped from any mining property in the area now occupied by the northern part of the park. Many mineralized deposits crop out in the northern part of the park, and according to Whatcom County records, 234 lode and 10 placer claims have been located there. Some of these claims may be relocations of older claims. Many of the claims were not found because of dense vegetation, lack of workings, or vaguely described locations. Commonly, relatively large iron oxide-stained zones are present in the general area of the lode claims.

The claims are in six principal areas : Silver Creek, Sulphide basin, headwaters of Chilliwack River, Whatcom Pass, Indian Creek, and Stetattle Creek. In the Silver Creek drainage, in the northeast corner of the study area, claims are on deposits that contain molybdenite and chalcopyrite. In the Sulphide basin area, claims are on molybdenite occurrences and on a large zone that contains disseminated iron sulfide. In the Chilliwack River area, lode claims were apparently staked on dike rocks that contain hematite and limonite stain resulting from decomposition of minor amounts of iron sulfides, and placer claims were also located in the same area along the river. In the Whatcom Pass area, the claims are on granitic rocks, pegmatites, and quartz veins that contain minor amounts of sulfides. The locations on Indian Creek were placer claims. In the Stetattle Creek area, we did not find the claims reported nor any likely areas that had workings, veins, or significant altered rocks.

Mineral occurrences in both the northern part of the North Cascades National Park and in the mines in adjacent parts of Washington and British Columbia are related to granitic intrusions. Within the park the Chilliwack composite batholith is interpreted as the source of anomalous copper, molybdenum, zinc, and lead. Almost all rock samples that contain anomalous amounts of these metals are either granitic rocks or country rock within 2 miles of exposures of the batholith. Mineralization is associated with several different plutons within the composite batholith, which is late Tertiary in age. The southeast fifth of the area, where the Chilliwack is not present, produced few anomalous samples.