Explained by Roger P. Ashley
On the choice of deposit models
Central Oregon and northeastern California, including the east side of the Cascade Range and the northwestern part of the Basin and Range province, are dominated by volcanic rocks of Tertiary and Quaternary age. The magmatism that produced these extensive volcanic sequences could generate ore-depositing hydrothermal systems, and the region contains many structures that could be sites for resulting epithermal ores, including through-going fracture systems, major normal faults, and fractures related to local doming (Rytuba, 1988; 1989). Several small districts with Comstock-type epithermal gold-silver deposits are present in the tract in Oregon.
Geophysical and isotopic data indicate that pre-Tertiary basement rocks exist beneath the area, with the possible exception of the northwest corner (Couch and Riddihough, 1989; Church and others, 1986) suggesting that this volcanic terrane is favorable for Comstock-type deposits rather than Sado-type deposits (Mosier and others, 1986a, b).
On the delineation of permissive tracts
Calc-alkaline volcanic rocks of the Cascade volcanic arc, mainly late Miocene to Quaternary in age, occupy the western part of the tract (Sherrod and Smith, 1989; Walker and MacLeod, 1991; Jennings, 1977). The tract includes calc-alkaline rocks of the Eocene and Oligocene(?) Clarno Formation in north-central Oregon (Walker and MacLeod, 1991). South-central Oregon and northeastern California have back-arc Cascades andesites, and basalts and rhyolites of the bimodal suite of the Basin and Range Province (Walker and MacLeod, 1991; Jennings, 1977). High-level intrusions are locally associated with all volcanic sequences. All volcanic rocks in the tract are considered permissive for Comstock-type deposits.
In Oregon and northernmost California the west side of the tract is the boundary between Oligocene and Miocene rocks of the western Cascades and predominantly Quaternary volcanic rocks of the high Cascades. Ridge-capping flows of late Miocene and Pliocene age are included in the tract. In most of northern California, the tract includes both Tertiary and Quaternary rocks of the Cascades; here the west side of the tract is the boundary between Tertiary rocks and pre-Tertiary rocks of the Klamath Mountains province.
East of this tract is an area recognized in a mineral resource assessment of the Malheur-Jordan BLM Resource Area(work in progress) as favorable for epithermal precious metal deposits (Tract PC21). The location of the boundary with the latter tract, in which hot-spring Au-Ag occurrences are relatively common, is somewhat arbitrary. The southeast edge of the tract, which closely follows the Oregon-Nevada and California-Nevada borders, is the boundary of the Great Basin region, which has abundant epithermal precious metal occurrences. The eastern and southeastern boundaries do not represent a geologic discontinuity or a distinct change in geologic features.
In north-central Oregon, the boundary is drawn where flows of the Columbia River Basalt Group cover the Cascade rocks to a depth of approximately 1 km. An isolated area of rocks of the Columbia River Basalt Group near the northeast corner of the tract is also excluded because it is underlain by pre-Tertiary rocks; favorable volcanic rocks of the Basin and Range bimodal suite and the Clarno Formation are missing. Areas covered with more than 1 km of Quaternary alluvium in the Goose Lake, Summer Lake, and Klamath grabens are also excluded.
Important examples of this type of deposit
The largest deposit of Comstock type in the tract is the Oregon King mine, Jefferson County, Oregon, which produced 0.075 metric tons of gold, 0.72 metric tons of silver, and small amounts of copper and lead (Libby and Corcoran, 1962). It is not large enough to fit the Comstock grade-tonnage model. Parts of the tract in northeastern California have been prospected for epithermal occurrences but there are no known Comstock-type deposits with production (Diggles and others, 1988; Munts and Peters, 1987).
On the numerical estimates made
For the 90th, 50th, and 10th and 5th percentiles, the team estimated 0, 1, 3 and 5 or more Comstock epithermal-vein districts consistent with the grade and tonnage model of Mosier and others (1986b) (Mark3 index 16). The estimates of one deposit at the 50th percentile and three deposits at the 10th percentile together express a the perceived probability that exploration of known districts and prospects could yield deposits. Two additional districts are included at the 5th percentile because there is extensive favorable ground in the tract, but the density of known deposits is low, so there is a low probability that other districts large enough to fit the grade-tonnage model may exist.
Church, S.E., LeHuray, A.P., Grant, A.R., Delevaux, M.H., and Gray, J.E., 1986, Lead-isotopic data from sulfide minerals from the Cascade Range, Oregon and Washington: Geochimica et Cosmochimica Acta, v. 50, p. 317-328.
Couch, R.W., and Riddihough, R.P., 1989, The crustal structure of the western continental margin of North America, in Pakiser, L.C., and Mooney, W.D., eds., Geophysical framework of the continental United States: Geological Society of America Memoir 172, p. 103-128.
Diggles, M.F., Frisken, J.G., Plouff, Donald, Munts, S.R., and Peters, T.J., 1988, Mineral resources of the Skedaddle Mountain Wilderness Study Area, Lassen County, California, and Washoe County, Nevada: U.S. Geological Survey Bulletin 1706-C, 27 p.
Jennings, C.W., 1977, Geologic map of California: California Division of Mines and Geology, scale 750,000.
Libby, F.W., and Corcoran, R.E., 1962, The Oregon King mine, Jefferson County, Oregon: Oregon Department of Geology and Mineral Industries Short Paper 23, 49 p.
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.
Munts, S.R., and Peters, T.J., 1987, Mineral resources of the Skedaddle study area, Lassen County, California and Washoe County, Nevada: U.S. Bureau of Mines Open-File Report MLA 022-87, 52 p.
Rytuba, J.J., 1988, Volcanism, extensional tectonics, and epithermal systems in the northern Basin and Range, CA, NV, OR, and ID [abs.]: Geological Society of Nevada Newsletter, May, 1988.
Rytuba, J.J., 1989, Volcanism, extensional tectonics, and epithermal mineralization in the northern Basin and Range Province, California, Nevada, Oregon, and Idaho, in Schindler, K.S., ed., U.S. Geological Survey Eesearch on Mineral Resources—1989 Program and Abstracts, Fifth Annual V.E. McKelvey Forum on Mineral and Energy Resources [abs.]: U.S. Geological Survey Circular 1035, p. 59-61.
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.
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.