National mineral assessment tract NR48 (Homestake stratiform Au)

Tract NR48
Geographic region Northern Rocky Mountains
Tract area 6,180sq km
Deposit type Homestake stratiform Au
Deposit age Archean

Deposit model

Model code ofr94250
Model type descriptive
Title Descriptive and grade-tonnage models of Archean low-sulfide Au-quartz veins and a revised grade-tonnage model of Homestake Au
Authors Klein, T.L., and Day, W.C.
URL https://pubs.usgs.gov/of/1994/ofr-94-0250/
Source https://pubs.er.usgs.gov/publication/ofr94250

Estimates

Confidence Number of
deposits
90% 0
50% 2
10% 4
5% 5
1% 6

Estimators: Elliott, Frishman, Ludington, Wallace, Nash, Berger, Spanski

Rationale

Explained by James E. Elliott
On the choice of deposit models
These deposits are stratiform, and are found within predominantly carbonate and silicate facies of iron formation that is the product of submarine hot-spring activity related to volcanism. Metamorphism and deformation may have caused minor to significant remobilization of metals, sulfides, and gangue. Iron formations of this composition are exposed in several parts of Montana, and two deposits of this type are known.
On the delineation of permissive tracts
Iron formations and related sedimentary exhalative deposits are exposed in several areas in Montana and nearby parts of Wyoming and Idaho, including the Gravelly and Ruby Ranges and the Tobacco Root and Beartooth Mountains (Bayley and James, 1973). These areas were deemed permissive for Homestake stratiform gold deposits due to the association of this deposit type with iron formations. It should be noted that, at the scale of this assessment, the permissive areas do not take into account how thin (10–70 m) the host iron formations actually are. The tracts define general areas where iron formations are exposed, not the much smaller area that they actually occupy.
Important examples of this type of deposit
The Jardine (Mineral Hill) deposit in the western Beartooth Mountains, east of Gardiner, is a known Homestake deposit. In addition, a new deposit named Crevice Mountain has been recently discovered nearby. Jardine is somewhat different from the typical Homestake deposit in that both arsenic and tungsten were produced in the earlier part of this century (Seager, 1944).
On the numerical estimates made
The permissive tract is relatively large, these deposits may have a minor surface expression, and there are two known deposits in the Beartooth Mountains. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 2, 4, 5, and 6 or more deposits consistent with the grade and tonnage model for Homestake stratiform gold deposits of Klein and Day (1994).
References
Bayley, R.W., and James, H.L., 1973, Precambrian iron-formations of the United States: Economic Geology, v. 68, no. 7, p. 934-959.
Klein, T.L., and Day, W.C., 1994, Descriptive and grade-tonnage models of Archean low-sulfide Au-quartz and a revised grade-tonnage model of Homestake Au: U.S. Geological Survey Open-File Report 94-250.
Seager, G.F., 1944, Gold, arsenic, and tungsten deposits of the Jardine-Crevasse Mountain district, Park County, Montana: Montana Bureau of Mines and Geology Memoir 23, 111 p.

Geographic coverage

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