National mineral assessment tract NR32 (Sediment-hosted Au)

Tract NR32
Geographic region Northern Rocky Mountains
Tract area 2,420sq km
Deposit type Sediment-hosted Au
Deposit age Phanerozoic

Deposit model

Model code 26a
Model type descriptive
Title Descriptive model of carbonate-hosted Au-Ag
Authors Byron R. Berger


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

P(none): 0.99

Estimators: Box, DCox, Bookstrom


Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
These deposits consist of very fine-grained gold and sulfide minerals disseminated in carbonaceous calcareous sedimentary rocks and associated jasperoids (Berger, 1986). This model was considered to include disseminated gold deposits hosted in carbonaceous, silty, and shaley carbonate sequences along the continental margin. The host rock lithology of carbonaceous, silty, and shaley carbonate sequences was considered to be the primary criteria for inclusion in a permissive tract for this deposit type. Paleozoic miogeoclinal strata in northeastern Washington include this lithology.
On the delineation of permissive tracts
Strictly on the basis of host rock lithology, the Cambrian through Devonian miogeoclinal rocks of northeastern Washington (Stoffel and others, 1991) are considered permissive for Carlin-type, sediment-hosted gold deposits. However, there was disagreement within the team as to whether the host lithology is the most significant factor in the localization of these deposits, and whether the model is appropriate to northeastern Washington.
Significant examples of deposit type
No deposits or prospects of this deposit type are known from the permissive area or from the contiguous correlative area across the Canadian border in southern British Columbia.
On the numerical estimates made
Since a large number of gold exploration companies (including several with major producing sediment-hosted gold deposits in Nevada) have been actively exploring for gold deposits of several other types in the general vicinity, we presume that this area has been explored to some degree for deposits of this type. Given this presumed exploration history and the lack of known prospects, the assessment team judged the possibility of undiscovered deposits to be very low. However exposure in the area is poor, and forest cover is extensive, so that undiscovered deposits might still occur in the upper kilometer of the tract. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more for deposits consistent with the grade and tonnage model of Mosier and others (1992).
Berger, B.R., 1986, Descriptive model of carbonate-hosted Au-Ag, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 175.
Mosier, D.L., Singer, D.A., Bagby, W.C., and Menzie, W.D., 1992, Grade and tonnage model of sediment-hosted Au, in Bliss, J.D., ed. Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 26–28.
Stoffel, K.L., Joseph, N.L., Waggoner, S.Z., Gulick, S.W., Korosec, M.A., and Bunning, B.B., 1991, Geologic map of Washington-Northeast quadrant: Washington Division of Geology and Earth Resources, Geologic Map GM-39, scale 1:250,000.

Geographic coverage

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