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 surface in and around felsic volcanic fields, usually associated with normal faults (Berger, 1986). This tract encompasses the Ore-Ida graben, a north-south structural feature controlling the distribution of Miocene bimodal volcanic and sedimentary rocks. Within this tract, widespread occurrences of hydrothermal alteration, silica sinter, and anomalous concentrations of gold, silver, arsenic, antimony, and mercury in rocks, soil, and stream sediments indicates the presence of numerous extinct geothermal systems with precious metal contents.
On the delineation of permissive tracts
The permissive tract was delineated to include all the area of Neogene rocks within the Ore-Ida graben (Peters and others, 1994). Since much of the Neogene section is buried under younger Neogene rocks, much of the prospective ground is buried. Pervasive north-south Neogene faulting, recurring volcanism, evidence of numerous extinct hot springs, and overall subsidence within the Ore-Ida graben suggest the entire upper km of the tract is favorable for this deposit type.
Important examples of this type of deposit
The Grassy Mountain deposit is a large Au-Ag hot-spring deposit (31 metric tons Au, 77 metric tons Ag) within the tract (Wheeler, 1988). The deposit is capped by an extensive zone of silica sinter.
On the numerical estimates made
The widespread occurrence of silica sinter confirms the existence of recent geothermal activity throughout the graben area. Based on his investigations in the area, J. Rytuba (U.S. Geological Survey, personal commun., 1993) infers that geothermal activity has persisted during subsidence and filling of the graben from the Miocene to the present day, suggesting that deposits of this type may be found associated with deeper stratigraphic horizons, representing earlier ground surfaces in the graben. This suggests there is a uniform likelihood for deposit occurrence throughout the km below the present surface. Combined with an exploration history dating back only to the early 1980s for this type of deposit in this area, and the shallow depth targets for this exploration for bulk mineable deposits (Spanski, 1994), the team estimated 6, 12, 18,2 4, and 30 or more deposits at the 90th, 50th, 10th, 5th, and 1st percentiles. These deposits would be consistent with the Au-Ag grade and tonnage model for hot-spring deposits of Berger and Singer (1992).
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.