Explained by Roger P. Ashley
On the choice of deposit models
Granitic stocks and batholiths intrude the Tertiary volcanic rocks of the western Cascades in southern Washington, and batholiths of both Mesozoic and Tertiaty age intrude the pre-Tertiary rocks of the northern Cascades. Seven known porphyry copper deposits are spatially associated with these subvolcanic intrusions, and there are about two dozen porphyry-type prospects (Peterson, 1991), indicating that magmatism in the Cascade arc produced a favorable environment for these deposits. Mesozoic magmatism produced many porphyry copper deposits in the Canadian Cordillera and accounts for one of the seven known deposits in Washington. The model for porphyry copper deposits in British Columbia and Alaska (Menzie and Singer, 1993) is used here, rather than the worldwide model (Singer and others, 1986), because the lower copper grades of the known Washington deposits fit this model better. Although some porphyry copper deposits of the Cascade Range have significant gold and molybdenum resources, not enough information is available to determine whether the Cu-Au or Cu-Mo porphyry models would be more appropriate than the British Columbia-Alaska model.
On the delineation of permissive tracts
The southern boundary of the tract is the northwest-trending Olympic-Wallowa lineament, which approximately marks the northern limit of extensive Tertiary volcanic rocks in the Cascade Range. The tract includes all significant exposures of intrusive rocks in the Cascades from the Olympic-Wallowa lineament northward to the Canadian border (Smith, 1993). It also includes concealed magnetic plutons shown on an interpretive geophysical map prepared from aeromagnetic and gravity data (Blakely and Plouff, 1991). Parts of the western boundary represent an approximate projection of plutonic or metamorphic rocks under 1 km of Quaternary cover. Locally, the west boundary is defined by exposed plutons; a buffer zone about 3 km wide has been drawn around these exposures. The southeast boundary represents the projection of plutonic, metamorphic, and Tertiary volcanic rocks under flows of the Miocene Columbia River Basalt Group (Drost and Whiteman, 1986), where they are <1 km thick.
Important examples of this type of deposit
The Glacier Peak deposit is the largest known porphyry copper deposit in this tract, having a reserve of 1.7 billion metric tons of ore containing 0.334 percent copper (Derkey and others, 1990). Other known deposits in the tract include Mazama, Gold Mountain, Sunrise, North Fork, Clipper-Three Brothers, and Condor-Hemlock (Peterson, 1991; Derkey and others, 1990). The tract also contains three copper-bearing breccia pipes that contain substantial known ore reserves. These are not included here as known porphyry copper deposits because their grades are too high and tonnages too low to fit the published porphyry copper models. However, because several of the known porphyry deposits include breccia pipes in addition to stockwork mineralization, the breccia pipes are regarded as porphyry prospects.
On the numerical estimates made
For the 90th, 50th, and 10th percentiles, the team estimated respectively, 3, 6, and 15 deposits consistent with the grade and tonnage model of Menzie and Singer (1993); (Mark3 index 89). This estimate expresses the belief that additional exploration of known prospects would disclose porphyry deposits. Note that at the 10th percentile, more than half of the known prospects are expected to yield deposits. The lack of additional deposits at lower percentiles reflects the perception that exploration of this tract for this deposit type has been relatively thorough, and there is a very low probability that completely undiscovered prospects remain.
Blakely, R.J., and Plouff, Donald, 1991, Geophysical interpretations, in Diggles, M.F., ed., Assessment of undiscovered porphyry copper deposits within the range of the Northern Spotted Owl, northwestern California, western Oregon, and western Washington: U.S. Geological Survey Open-File Report 91-377, p. 21-30.
Derkey, R.E., Joseph, N.L., and Lasmanis, Raymond, 1990, Metal mines of Washington—Preliminary report: Washington Division of Geology and Earth Resources Open-File Report 90-18, 577 p.
Drost, B.W., and Whiteman, K.J., 1986, Surficial geology, structure, and thickness of selected geohydrologic units in the Columbia Plateau, Washington: U.S. Geological Survey Water Resources Investigations Report 84-4326, 1 text sheet, 10 map sheets, scale 1:500,000.
Menzie, W.D., and Singer, D.A., 1993, Grade and tonnage model of porphyry Cu deposits in British Columbia, Canada, and Alaska, USA: U.S. Geological Survey Open-File Report 93-275, 8 p.
Peterson, J.A., 1991, Known copper deposits, in Diggles, M.F., ed., Assessment of undiscovered porphyry copper deposits within the range of the Northern Spotted Owl, northwestern California, western Oregon, and western Washington: U.S. Geological Survey Open-File Report 91-377, p. 17-21.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu, in Cox, D.P., and Singer, D.A., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.
Smith, J.G., 1993, Geologic map of upper Eocene to Holocene volcanic and related rocks in the Cascade Range, Washington: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-2005, scale 1:500,000, 19 p.