National mineral assessment tract PC29 (Hot-spring Au-Ag)

Tract PC29
Geographic region Pacific Coast
Tract area 31,200sq km
Deposit type Hot-spring Au-Ag
Deposit age Tertiary

Deposit model

Model code 25a
Model type descriptive
Title Descriptive model of hot-spring Au-Ag
Authors Byron R Berger
URL https://pubs.usgs.gov/bul/b1693/html/bull0g4n.htm
Source https://pubs.er.usgs.gov/publication/b1693

Estimates

Confidence Number of
deposits
90% 2
50% 6
10% 10
5% 16
1% 30

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

Rationale

Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Hot-spring Au-Ag deposits consist of precious metal-bearing siliceous sinters and silicified rocks cut by breccias and stockworks of veins and veinlets, which were deposited at or near the paleo-ground surface in and around felsic volcanic fields, usually associated with normal faults (Berger, 1986). In Southeastern Oregon, widespread Neogene felsic volcanic rocks, along with considerable Neogene normal faulting, provide a compositionally and structurally appropriate environment for hot-spring Au-Ag deposits (Berger, 1986).
On the delineation of permissive tracts
The tract encircles essentially all Neogene volcanic and associated sedimentary rocks in southeastern Oregon (Peters and others, 1994), exclusive of the Ore-Ida graben. The most prospective area in southeastern Oregon is the Ore-Ida graben, which is not included in this tract but is included in a separate tract. Some basaltic volcanic rocks and/or thick Quaternary alluvial cover are included in this tract which may not be appropriate for the occurrence of hot-spring deposits. However insufficient data are available to exclude these unfavorable parts of the tract.
Important examples of this type of deposit
No significant hot-spring Au-Ag deposits are known from this tract, although the Grassy Mountain deposit occurs in contemporary rocks in the adjacent tract to the east (Wheeler, 1988). Evidence for a number of hot-spring type geothermal systems are known within this tract, but none have had significant production or have known reserves.
On the numerical estimates made
The area has lesser potential for undiscovered hot-spring deposits than the Ore-Ida graben to the east. In this tract, there is less support for the existence of widespread geothermal activity. Surface erosion has been minimal, so most deposits were probably buried rather than eroded away. However the accumulated Neogene section is thinner and the opportunity for stacking deposits is more limited than in the adjacent Ore-Ida graben. For this reason the team expressed less confidence in its estimate for this tract. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 2, 6, 10, 16, and 30 or more deposits (Spanski, 1994) consistent with the Au-Ag grade and tonnage model for hot-spring deposits of Berger and Singer (1992).
References
Berger, B.R., 1986, Descriptive model of hot-spring Au-Ag deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 143.
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.
Peters, S.G., Albino, George, Brooks, H.C., Evans, J.G., Carlson, R.R., Lee, G.K., Griscom, Andrew, and Halvorson, P.F., 1994, Deposit models and tracts for undiscovered metallic resources in the Malheur-Jordan-Andrews Resource Area, southeastern Oregon, in Smith, C.L., ed., Mineral and energy resources of the BLM Malheur-Jordan Resource Area, southeastern Oregon: U.S. Geological Survey Administrative Report, p. B1-B21.
Spanski, G.T., 1994, Quantitative resource assessment for locatable minerals, in Smith, C.L., ed., Mineral and energy resources of the BLM Malheur-Jordan Resource Area, southeastern Oregon: U.S. Geological Survey Administrative Report, p. C1-C12.
Wheeler, G.R., 1988, Epithermal gold deposits in eastern Oregon: Mining Engineering, v. 40, no. 11, p. 1033-1035.

Geographic coverage

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