| Basis for focus area |
Area delineated by large mapped areas of alunite-rich advanced argillic and residual quartz alteration (John and others, 2003; Blakely and others, 2007) underlain by partly drilled porphyry Cu mineralization; 1 km buffer around mapped alteration. |
| Identified resources |
Identified resources for aluminum (alunite). Historical production of aluminum (alunite) and silica. |
| Production |
Significant silica produced from 1987 to present; minor production from 1930s to 1948. |
| Status |
Past mining. Active silica mining; past exploration for high sulfidation Au-Ag, lithocap alunite, and porphyry Cu mineralization. |
| Estimated resources |
Early 1940s resource estimate of 1.14 Mt rock containing >20% alunite; 20-year resource at Scatter Creek quarry when it opened in 1999. |
| Geologic maps |
Tabor and others (2000), scale 1:100,000; John and others (2003), scale ~1:40,000; Blakely and others (2007), scale ~1:65,000; McCulla (1986), scale unknown. |
| Geophysical data |
Adequate aeromagnetic coverage. Inadequate aeroradiometric coverage. Blakely and others (1999, 2007) provide detailed regional aeromagnetics and gravity data. |
| Favorable rocks and structures |
Silicified and altered volcanic rocks of the early Miocene Fifes Peak Formation. These rocks host a structurally controlled zone of intense acid-sulfate alteration leaching primary elements leaving a residuum Si/Al caprock. This deposit bears striking similarity to the Goldfield deposit of Nevada, however a polymetallic system at depth has yet to be identified. |
| Deposits |
Enumclaw alunite prospect (White River alunite deposits) (MRDS dep_id: 10135527). |
| Evidence from mineral occurrences |
MRDS; John and others (2003); Blakely and others (2007). |
| Geochemical evidence |
Geochemical summary in John and others (2003). |
| Geophysical evidence |
See Blakely and others (2007) for interpretation of detailed aeromagnetic and gravity data. |
| Evidence from other sources |
Drilling by Kennecott in mid-2000s intersected porphyry copper mineralization at shallow depths but drilling was not sufficient to define potential resources. |
| Comments |
Area is heavily forested tree farm with several active/recently active silica quarries; exposures are mostly residual quartz alteration or manmade. Mostly private and some state (mineral lease) ownership. Future mineral development may be possible but anticipate significant opposition as it is in a watershed of a major population center. |
| Cover thickness and description |
Exposed at the surface. |
| Authors |
Daniel W. Eungard. |
| New data needs |
High resolution geophysics; develop deposit model, lidar. |
| Geologic mapping and modeling needs |
Mapping complete; deposit model. |
| Geophysical survey and modeling needs |
High resolution aeromagnetic and aeroradiometric survey. Gravity also inadequate, a state-led ground based study is possible, low density deposits at surface and suspected high density deposits at depth would be well resolved with gravity. |
| Digital elevation data needs |
Lidar planned/funded in northern part of focus area; incomplete in southern portion. |
