Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
This deposit type consists of massive quartz-gold veins that typically are low in sulfide minerals (<5 percent), are vertically and horizontally persistent, and are deformed into pinch-and-swell structures due to compressive deformation (Berger, 1986). These mesothermal veins occur in belts of regionally metamorphosed, low- to moderate-grade, marine sedimentary and volcanic rocks, penetratively deformed, and cut by high-angle regional-scale faults . Pre-Cenozoic rocks of northeastern Oregon and adjacent west-central Idaho and southeasternmost Washington fit this description. Four deposits considered to be examples of this type occur in northeastern Oregon.
On the delineation of permissive tracts
Pre-Cenozoic rocks of northeastern Oregon and adjacent west-central Idaho and southeasternmost Washington consist of andesitic and basaltic volcanic rocks, marine clastic and pelagic sedimentary rocks, dismembered ophiolitic sequences, and cross-cutting granitic rocks (Walker and MacLeod, 1991). Except for the granitic rocks, these rocks are moderately to strongly deformed and metamorphosed to greenschist facies. Because the area of these rocks in west-central Idaho is currently under study for its mineral resource potential (Bruce Johnson, U.S. Geological Survey, personal commun., 1994), we have separated that part of the area as a separate tract. In northeast Oregon all rocks of the Wallowa, Baker, and Olds Ferry terranes are included as part of the permissive tract (Brooks and Vallier, 1978). Major cross-cutting, post-metamorphic Jurassic and Cretaceous plutons are excluded from the tract.
Important examples of this type of deposit
Four mining districts in northeastern Oregon are considered here to be examples of this type (Mormon Basin, Connor Creek, Sanger and Virtue districts), all in the Baker terrane. The tonnage, Au grade and Ag grade of these deposits overlap the median on the grade and tonnage model for low-sulfide Au-quartz vein deposits of Bliss (1986). Other, apparently related, Au-bearing quartz veins in northeastern Oregon differ from the above districts in their higher sulfide content and higher silver grade. Recently Bliss (1994) suggested that all of the mesothermal vein deposits in northeastern Oregon represent a distinct deposit-type, although he noted the similarity of some of the above deposits to the low-sulfide Au-quartz vein deposit type. For this assessment, we consider only the above-named districts as representative of the low-sulfide Au-quartz vein deposit type.
On the numerical estimates made
Extensive exploration for this deposit type occurred in the late 1800s, and most of the known deposits and prospects were developed at that time. We presume that any exposed deposits have been discovered and thoroughly explored. However areas covered by younger rocks have received little exploration. Because of the relatively small unexplored area and the small size of the known deposits within the tract, the team made a relatively low estimate with a high uncertainty. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 3, and 8 or more undiscovered deposits consistent with the grade and tonnage model of Bliss (1986).
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239-243.
Bliss, J. D., 1994, Grade, tonnage, and other models of Blue Mountain-type Au-Ag polymetallic veins, Blue Mountains, Oregon, for use in resource and environmental assessment: U.S. Geological Survey Open-File Report 94-677, 33 p.
Brooks, H.C., and Vallier, T.L., 1978, Mesozoic rocks and tectonic evolution of eastern Oregon and western Idaho, in Howell, D.G., and McDougall, K.A., eds., Mesozoic paleogeography of the western United States: Pacific section, Society of Economic Paleontologists and Mineralogists, Pacific Coast Paleogeography Symposium 2, p. 133-146
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.