Explained by Roger P. Ashley
On the choice of deposit models
Comstock-type deposits, mostly in the Wenatchee district, occur in Eocene sandstones on the east side of the Cascade Range in central Washington. Mineralization is localized near Eocene and Oligocene domes, which are related to a magmatic episode older than the Cascade arc. The sandstones overlie a basement dominated by metasedimentary and plutonic rocks mostly of Cretaceous age. Such basement rocks are favorable for Comstock deposits (Mosier and others, 1986a, b).
On the delineation of permissive tracts
The tract includes upper Eocene and Oligocene sandstones of the Chiwaukum graben, because mineralization in and near the Wenatchee district is localized where Eocene and Oligocene domes cut these rocks (Tabor and others, 1982; Gresens, 1983; Margolis, 1989). It also includes early Eocene sandstones that host epithermal veins of the Swauk district, west of Wenatchee. Some pre-Tertiary rocks are included in a 3-km-wide buffer zone around small Tertiary intrusions immediately east of the Chiwaukum graben (Tabor and others, 1987). Where flows of the Columbia River Basalt Group cover the lower Tertiary sandstones on the southeast, the boundary is drawn where the younger basalt covers the sandstone to a depth of approximately 1 km (Drost and Whiteman, 1986).
Important examples of this type of deposit
The most prominent example is the Wenatchee district (Cannon and Lovitt mines), Chelan County, Washington, with total resources (production and reserves) of nearly 50 metric tons of gold and more than 100 metric tons of silver (Margolis, 1989). The Swauk district, in Kittitas County, has produced more than 0.25 metric tons of gold from lode mines, and considerably more from placers (Huntting, 1956).
On the numerical estimates made
For the 90th, 50th, and 10th percentiles, the team estimated 1, 3, and 4 or more Comstock epithermal-vein districts consistent with the grade and tonnage model of Mosier and others (1986b) (Mark3 index 16). The single district estimated at the 90th percentile reflects reports that exploration continues at several mineralized sites near Wenatchee (Derkey, 1993). The estimate of three districts at the 50th percentile and an additional district at the 10th percentile expresses a relatively high perceived probability that additional districts may exist elsewhere in the tract, particularly in the Chiwaukum graben in a geologic setting similar to Wenatchee.
Derkey, R.E., 1993, Washington’s mineral industry—1992, metallic mineral deposits: Washington Geology, v. 21, p. 4-25.
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.
Gresens, R.L., 1983, Geology of the Wenatchee and Monitor quadrangles, Chelan and Douglas Counties, Washington: Washington Division of Geology and Earth Resources Bulletin 75, 75 p.
Huntting, M.T., 1956, Inventory of Washington minerals—Part II, Metallic minerals: Washington Division of Mines and Geology Bulletin 37, v. 1, 428 p./ v. 2, 67 p.
Margolis, Jacob, 1989, Arkose-hosted, aquifer-controlled, epithermal Au-Ag mineralization, Wenatchee, Washington: Economic Geology, v. 84, no. 7, p. 1891-1902.
Mosier, D.W., Singer, D.A., Sato, T., and Page, N.J, 1986a, Relationship of grade, tonnage, and basement lithology in volcanic-hosted epithermal precious- and base-metal quartz-adularia-type districts: Mining Geology, v. 36, p. 245-264.
Mosier, D.W., Singer, D.A., and Berger, B.R., 1986b, Descriptive model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 150.
Tabor, R.W., Frizzell, V.A., Jr., Whetten, J.T., Waitt, R.B., Swanson, D.A., Byerly, G.R., Booth, D.B., Hetherington, M.J., and Zartman, R.E., 1987, Geologic map of the Chelan 30-minute by 60-minute quadrangle, Washington: U.S. Geological Survey Miscellaneous Investigations Map I-166 1, scale 1:100,000, text 33 p.
Tabor, R.W., Waitt, R.B., Jr., Frizzell, V.A., Jr., Swanson, D.A., Byerly, G.R., and Bentley, R.D., 1982, Geologic map of the Wenatchee 1:100,000 quadrangle, central Washington: U.S. Geological Survey Miscellaneous Investigations Map I-1311, scale 1:100,000, text 26 p.