National mineral assessment tract SB21 (Polymetallic replacement)

Tract SB21
Geographic region Southern Basin and Range
Tract area 20,900sq km
Deposit type Polymetallic replacement
Deposit age Laramide

Deposit model

Model code 19a
Model type descriptive
Title Descriptive model of polymetallic replacement deposits
Authors Hal T. Morris


Confidence Number of
90% 4
50% 5
10% 6
5% 8
1% 10

Estimators: Bartsch-Winkler, McLemore, Ludington, Wallace, Nash, Berger, Spanski


Explained by Alan R. Wallace and Virginia McLemore
On the choice of deposit models
Southwestern New Mexico contains known Laramide polymetallic replacement districts, characterized by the replacement of carbonate rocks by base-metal sulfides, precious metals, and related gangue minerals.
On the delineation of permissive tracts
Polymetallic replacement deposits form where intermediate to felsic intrusive rocks, which are the principal sources of the metals, are emplaced into carbonate rocks. The general rule for delineation of the permissive tract was to exclude all areas that show no evidence of Laramide plutonism. In addition, areas where erosion has left Precambrian igneous and metamorphic rocks exposed at the surface were excluded, as were areas inferred to be covered by surficial deposits thicker than 1 km.
Important examples of this type of deposit
Three Laramide polymetallic replacement districts are known—Chloride Flat, Georgetown, and Fierro-Hanover—and four other prospective areas.
On the numerical estimates made
Whereas the permissive area is relatively small, the number of known prospects is substantial. The team also placed a major emphasis on the productive nature of exposed areas and the substantial area (>50 percent of the permissive terrain) of covered permissive rocks. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 4, 5, 6, 8, and 10 or more undiscovered districts consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier and others (1986).
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

Show this information as XML or JSON