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 Oregon and southern Washington. One known porphyry copper deposit is associated with these subvolcanic intrusions, and there are about a dozen porphyry-type prospects (Peterson, 1991), indicating that magmatism in the Cascade arc produced a favorable environment for these deposits. The porphyry copper grade and tonnage model for British Columbia and Alaska (Menzie and Singer, 1993) is used here because known deposits in the north Cascades of Washington fit this model, and deposits in the southern Cascades are expected to be similar.
On the delineation of permissive tracts
The tract includes all significant exposures of intrusive rocks in the Cascades south of the Olympic-Wallawa lineament. In the eastern part of the Cascades in southern Washington and throughout the Cascades of Oregon, exposures of intrusive rocks are scattered throughout the extensive Tertiary volcanic rocks, and regional maps for Oregon (Sherrod and Smith, 1989; Walker and MacLeod, 1991) do not distinguish phaneritic intrusions from finer-grained bodies. Therefore, in Oregon and in much of the area in Washington, the tract includes both volcanic and intrusive rocks of Tertiary age. The tract also includes concealed magnetic plutons shown on an interpretive geophysical map prepared from aeromagnetic and gravity data (Blakely and Plouff, 1991).
In detail, the western boundary of the tract in Washington is drawn to include all exposed intrusions, both coarse-grained and fine-grained, shown on the regional geologic maps of Smith (1993) and Walsh and others (1987). A buffer zone about 3 km wide has been drawn around individual exposed intrusions. Tertiary rocks without intrusions that are likely older than the Cascades arc have been excluded. Outlines of inferred concealed plutons lie east of this boundary. The western boundary in Oregon is drawn to include all Cascades volcanic, volcaniclastic, and intrusive rocks, but in most places is located at the approximate projection of these rocks under either the Columbia River Basalt Group or Quaternary deposits, where they are <1 km thick. North of Eugene, the tract has been expanded to include an inferred concealed intrusion.
The southern boundary of the tract is the southern edge of an area where concealed intrusions can be inferred. Although intrusions exist in the Tertiary Cascades farther south in Oregon and northern California, they are dikes, sills, and small plugs, mostly fine-grained, and thus are unlikely sources for porphyry copper deposits. Also, there are no known base-metal vein occurrences or other signs of porphyry systems in Tertiary rocks south of the tract, so we conclude that porphyry mineralization, if it exists, is deeper than 1 km.
The eastern boundary in Washington and northern Oregon represents the approximate projection of Tertiary volcanic rocks under flows of the Miocene Columbia River Basalt Group (Drost and Whiteman, 1986) and Quaternary volcanic rocks, where they are <1 km thick. Inliers of pre-Cascades rocks are included in the tract east of Mt. Rainier because many Tertiary intrusions are exposed there. In Oregon, the eastern boundary represents the approximate projection of Tertiary volcanic rocks under Quaternary High Cascades volcanic rocks, where they are <1 km thick.
Important examples of this type of deposit
The Margaret deposit of southwestern Washington is the only known porphyry copper deposit in this tract. It contains an estimated ore reserve of at least 523 million metric tons grading 0.36 percent copper, and has significant amounts of molybdenum, gold, and silver (Derkey and others, 1990). The tract also contains two copper-bearing breccia pipes that contain substantial known reserves of copper, molybdenum, gold, and silver. 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 in the northern Cascades include breccia pipes in addition to stockwork mineralization, the breccia pipes are regarded as porphyry prospects.
On the numerical estimates made
Our estimate reflects the belief that additional exploration of known prospects would disclose deposits. Note that at the 10th percentile, almost all known prospects are expected to yield deposits. The lack of additional deposits below the 10th percentile reflects the perception that, although exploration of this tract for this deposit type has not been as thorough as it has in the northern Cascades, the potential for new deposits lies almost entirely within known polymetallic vein districts, which constitute or contain the known porphyry prospects in this tract. Thus, there is a very low probability that completely undiscovered prospects remain. For the 90th, 50th, and 10th percentiles, the team estimated 1, 3, and 10 or more porphyry-copper deposits consistent with the grade and tonnage model of Menzie and Singer (1986).
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.
Sherrod, D.R., and Smith, J.G., 1989, Preliminary map of upper Eocene to Holocene volcanic and related rocks of the Cascade Range, Oregon: U.S. Geological Survey Open-File Report 89-14, 1:500,000 scale, text 20 p.
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.
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.
Walsh, T.J., Korosec, M.A., Phillips, W.M., Logan, R.L., and Schasse, H.W., 1987, Geologic map of Washington–southwest quadrant: 1:250,000 scale, 28 p.