National mineral assessment tract GB16 (Polymetallic replacement)

Tract GB16
Geographic region Great Basin
Tract area 43,400sq km
Deposit type Polymetallic replacement
Deposit age Tertiary

Deposit model

Model code 19a
Model type descriptive
Title Descriptive model of polymetallic replacement deposits
Authors Hal T. Morris
URL https://pubs.usgs.gov/bul/b1693/html/bull7q0k.htm
Source https://pubs.er.usgs.gov/publication/b1693

Estimates

Confidence Number of
deposits
90% 3
50% 7
10% 11
5% 13
1% 20

Estimators: Stoeser, Nutt, Albino, Ludington, Wallace, Nash, Berger, Spanski

Rationale

Explained by Douglas B. Stoeser
On the choice of deposit models
Polymetallic replacement deposits are common in western Utah, and are found where Tertiary felsic to intermediate intrusive rocks were emplaced into predominantly Paleozoic carbonate rocks.
On the delineation of permissive tracts
The permissive tract for this deposit type is defined primarily by the distribution of intrusive rocks and suitable reactive host rocks, especially carbonate-bearing sedimentary rocks. It is made up of three east-trending belts in western Utah. The southernmost of the three is less deeply eroded than the other two, and any undiscovered replacement deposits in the south would most likely be concealed by the volcanic cover and alluvial cover. Aeromagnetic maps were employed to define areas of buried intrusive bodies.
Important examples of this type of deposit
Other than the Bingham Canyon porphyry copper deposit, polymetallic replacement (and associated vein deposits) comprise by far the most important type of metallic mineral deposits in Utah. Major districts are all in the north-central area of the permissive tract and are Park City, Little and Big Cottonwood, Bingham Canyon (Lark and U.S. mines), Ophir, American Fork, Stockton (Rush Valley), Tintic and East Tintic. Minor deposits within the north-central area include West Tintic, Detroit, Fish Springs, Lucin, Silver Island, and Gold Hill, and in the southern area, Preuss, Deer Trail, Bradshaw, Pine Grove, and Star.
Many polymetallic replacement districts contained polymetallic veins as well, and production figures for some, if not most, of these districts reflects combined production from replacement and vein orebodies.
On the numerical estimates made
Although there has been extensive exploration for this deposit type since the late 1880s, major discoveries within the exposed parts of the permissive areas are unlikely. However, because of the extensive cover within the permissive areas, the widespread and numerous examples of this deposit type, the certainty of many concealed plutons within the covered areas, and the fact that the permissive tract is extensively underlain by carbonate sedimentary rocks, it seems likely that a number of undiscovered replacement deposits are present. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 3, 7, 11, 13, and 20 or more deposits consistent with the grade and tonnage model of Mosier, Morris and Singer (1986) (Mark3 index 47). The group noted that the estimate requires a somewhat anomalous distribution with probability falling off rapidly above 11 deposits, but felt the size of polymetallic replacement districts restricts their maximum numbers.
References
Mosier, D.L., Morris, H.T., and Singer, D.A., 1986, Grade and tonnage model of polymetallic replacement deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 101-104.

Geographic coverage

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