National mineral assessment tract PC27 (Epithermal vein, Sado)

Tract PC27
Geographic region Pacific Coast
Tract area 56,200sq km
Deposit type Epithermal vein, Sado
Deposit age Tertiary

Deposit model

Model code 25d
Model type descriptive
Title Descriptive model of Sado epithermal veins
Authors Dan L. Mosier, Byron R. Berger and Donald A. Singer


Confidence Number of
90% 0
50% 2
10% 5
5% 8
1% 8

Estimators: DCox, Albino, Church, Ashley, Diggles, Rytuba, Kleinkopf, Sawlan, Peterson


Explained by Roger P. Ashley
On the choice of deposit models
Calc-alkaline volcanic rocks of Tertiary and Quaternary age related to the Cascade arc are widespread in western Oregon and Washington. Arc volcanic sequences include volcanic centers in which magmatic events can generate ore-depositing hydrothermal systems. Several Sado-type deposits are present in the western Cascade Range, where the Tertiary arc rocks are mainly exposed. Geophysical and isotopic data indicate that pre-Tertiary basement rocks are thin or missing beneath the western Cascade Range of west-central Oregon and southwestern Washington (Couch and Riddihough, 1989; Church and others, 1986). Oceanic crust of Eocene age is inferred to lie beneath the Cascades rocks and extends to depths greater than 10 km. This basement geology is favorable for Sado type deposits rather than Comstock type deposits (Mosier and others, 1986).
On the delineation of permissive tracts
All areas of calc-alkaline volcanic rocks in the western Cascades are considered permissive for Sado-type deposits. Because these deposits usually have limited vertical extent (a few hundred meters), they could be present within the 1 km depth limit while having little surface manifestation.
The tract includes mainly volcanic and volcaniclastic rocks and associated plutonic bodies (dikes, sills, stocks, and small batholiths) of the Cascades arc (Oligocene and Miocene in age), as shown on maps of Smith (1993), Walker and MacLeod (1991), and Sherrod and Smith (1989). 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 deposits 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.
In Oregon and northern California, the east 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 excluded from the tract. In Washington, on the east, the boundary is drawn where flows of the Columbia River Basalt Group cover the Oligocene and Miocene Cascade rocks to a depth of approximately 1 km (Drost and Whiteman, 1986). Quaternary volcanic rocks of Mt. Rainier, Mount St. Helens, Mt. Adams, and the Indian Heaven volcanic fields, which locally cover the Tertiary Cascades rocks in Washington, are included. A pre-Tertiary inlier east of Mt. Rainier is also included. Along the northeast edge of the tract, a small area of Eocene sandstone is included. North of Mt. Rainier, volcanic rocks of Eocene and Oligocene age are included, some of which predate Cascade arc volcanism.
Important examples of this type of deposit
Sado-type deposits with recorded precious metal production, all relatively small, include the Quartzville district, the Blue River district, and the Al Sarena mine in Oregon, and the Wind River mine in Washington. The Quartzville district has the largest production that can definitely be attributed to Sado-type veins, with 0.27 metric tons gold and 0.09 metric tons silver recorded (Brooks and Ramp, 1968), and possibly considerably more that is unrecorded (Munts, 1978).
On the numerical estimates made
For the 90th, 50th, 10th, and 5th percentiles, the team estimated 0, 2, 5, and 8 or more Sado deposits consistent with the grade and tonnage model of Mosier and Sato (1986) (Mark3 index 28). Known Sado-type prospects and occurrences, although mostly small and not numerous, are widely distributed throughout the tract. Hydrothermally-altered areas, many of which are favorable for epithermal deposits as well as porphyry deposits, are common in the Tertiary volcanic rocks of the western Cascades (Peck and others, 1964; Power, 1984). The estimates of two deposits at the 50th percentile and five deposits at the 10th percentile together express a relatively high perceived probability that exploration of known districts and prospects could yield additional deposits. Recent exploration drilling in several districts has produced ore-grade intercepts. The three additional deposits estimated at the 5th percentile reflects the perception that extensive favorable ground exists, but the density of known deposits is low, thus some, but probably not many, new Sado-type districts could be discovered in the tract.
Brooks, H.C. , and Ramp, Len, 1968, Gold and silver in Oregon: Oregon Department of Geology and Mineral Industries Bulletin 61, 337 p.
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.
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.
Mosier, D.L., and Sato, Takeo, 1986, Grade and tonnage model of Sado epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 155-157.
Mosier, D.W., Singer, D.A., Sato, T., and Page, N.J, 1986, 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.
Munts, S.R., 1978, Geology and mineral deposits of the Quartzville mining district, Linn County, Oregon: University of Oregon, unpub. M.S. thesis, 212 p.
Peck, D.L., Griggs, A.B., Schlicker, H.G., Wells, F.G., and Dole, H.M., 1964, Geology of the central and northern parts of the western Cascade Range in Oregon: U.S. Geological Survey Professional Paper 449, 56 p.
Power, S.G., 1984, The “tops” of porphyry copper deposits—Mineralization and plutonism in the western Cascades, Oregon: Corvallis, Oregon State University, Ph.D. thesis, 234 p.
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.

Geographic coverage

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