Explained by Roger P. Ashley
On the choice of deposit models
Hot-spring precious metal deposits are found in volcanic-dominated sequences containing subaerial, intermediate to felsic volcanic and volcaniclastic rocks (Berger, 1986). The volcanic sequences of the Basin and Range Province and the Clarno Formation are entirely continental, and those of the Cascades arc are mostly subaerial, so these volcanic-dominated terranes of western Washington, western and central Oregon, and northeastern California are permissive for hot-spring Au-Ag deposits. They are also permissive for Sado- or Comstock-type deposits. A few widely-scattered hot-spring-type deposits and prospects are recognized in the tract. Some deposits in the tract known only through brief examinations and descriptions by early investigators, and assumed to be veins, could be hot-spring deposits.
On the delineation of permissive tracts
Criteria for tract delineation are similar to those for Sado- and Comstock-type vein deposits, and this tract is coextensive with the combined Sado and Comstock tracts that cover the Cascade Range and central Oregon. All areas containing intermediate to silicic volcanic rocks are considered permissive for hot-spring Au-Ag deposits. Because these deposits usually have a very limited vertical extent, they could be present within 1 km of the surface and have little surface manifestation.
The tract includes all volcanic and volcaniclastic rocks and associated small plutonic bodies of the Cascades arc (Oligocene to Holocene in age), as shown on maps of Smith (1993), Walker and MacLeod (1991), Sherrod and Smith (1989), and Jennings (1977). On the west side of the tract in both Oregon and Washington, Eocene marine sandstone units and pre-Cascades volcanic rocks, predominantly of basaltic composition, are excluded. Where the Cascades-related rocks are covered by non-volcanic Quaternary deposits, the boundary is drawn where the younger rocks are approximately 1 km thick. Batholiths of Tertiary age in the northern Cascades of Washington are excluded because they indicate an environment too deep for the presence of epithermal deposits. Volcanic rocks of Eocene and Oligocene age in the northern Cascades are included, however, although some may predate Cascade arc volcanism.
In central Washington, Eocene and Oligocene sandstones in and west of the Chiwaukum graben are included because they host significant Comstock-type deposits. A pre-Tertiary inlier east of Mt. Rainier is also included.
The basaltic lavas of the Columbia Plateau are excluded. In north-central Oregon and east-central Washington, the boundary is drawn where flows of the Columbia River Basalt Group cover the Cascade rocks to a depth of approximately 1 km (Drost and Whiteman, 1986).
The tract includes calc-alkaline rocks of the Eocene and Oligocene(?) Clarno Formation in north-central Oregon. In south-central Oregon and northeastern California it includes Tertiary basalts and rhyolites of the bimodal suite of the Basin and Range Province and Tertiary back-arc Cascade rocks of intermediate composition (Walker and MacLeod, 1991; Sherrod and Smith, 1989; Jennings, 1977). The eastern boundary in eastern Oregon and northeastern California separates this tract from tracts in southeastern Oregon and the Great Basin that have numerous epithermal occurrences and are more favorable for undiscovered deposits.
An isolated area of rocks of the Columbia River Basalt Group in east-central Oregon is excluded because it is underlain by pre-Tertiary rocks; favorable volcanic rocks of the Basin and Range bimodal suite and 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 hot-spring Au-Ag deposit in the tract is Quartz Mountain, in Lake County, south-central Oregon (Sawlan and Russell, 1991). It is associated with rhyolite domes related to bimodal volcanism. Although subeconomic at this time, it is credited with more than 60 metric tons of gold reserves (Wiley, 1991).
Several isolated precious-metal occurrences in the southern Oregon Cascades may be hot-spring deposits, and other poorly-known precious-metal occurrences in the western Cascades districts of Oregon could be hot-spring deposits as well. The recently-explored Mashel River prospect, in the western Cascades of Washington (Pierce County), may be a hot-spring deposit.
Hayden Hill, in California, is another prominent example, in northern California.
On the numerical estimates made
For the 90th, 50th, and 10th percentiles, the team estimated 0, 1, and 3 or more hot-spring Au-Ag deposits consistent with the grade and tonnage model of Berger and Singer (1992) (Mark3 index 45). This estimate reflects the perception that additional exploration in the vicinity of Quartz Mountain and in several western Cascades districts could yield deposits. The additional deposits included at the 5th and 1st percentiles reflect the probability that other districts large enough to fit the grade-tonnage model may exist, particularly in the southern Cascade Range of Oregon and Ochoco Mountains of north-central Oregon, where there are many hot-springs mercury deposits, or where through-going fracture systems cut late Tertiary rocks of the bimodal suite in south-central Oregon and northeastern California (Rytuba, 1988, 1989).
Berger, B.R., 1986, Descriptive model of hot-spring Au-Ag, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 143-144.
Berger, B.R., and Singer, D.A., 1992, Grade and tonnage model of hot-spring Au-Ag, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23-25.
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.
Jennings, C.W., 1977, Geologic map of California: California Division of Mines and Geology, scale 750,000.
Sawlan, M.G., and Russell, K.D., 1991, Rhyolite domes, NW-trending faults, gold deposits, and high-alumina basalt in the Devils Garden–NW Basin and Range—Rapid volcanic and structural evolution from 8-6 Ma at Quartz Mountain, south-central Oregon [abs.]: Geological Society of America Abstracts with Programs, v. 23, p. 94.
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 Research 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.
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 Investigations Series Map I-2005, scale 1:500,000, text, 19 p.
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.
Wiley, T.J., 1991, Mining and exploration in Oregon during 1990: Oregon Geology, v. 53, p. 63-69.