National mineral assessment tract SB20 (Polymetallic replacement + skarn Zn-Pb)

Tract SB20
Geographic region Southern Basin and Range
Tract area 61,700sq km
Deposit type Polymetallic replacement + skarn Zn-Pb
Deposit age Mesozoic - Tertiary

Deposit model

Model code 19a
Model type descriptive
Title Descriptive model of polymetallic replacement deposits
Authors Hal T. Morris
Model code 18c
Model type descriptive
Title Descriptive model of Zn-Pb skarn deposits
Authors Dennis P. Cox


Confidence Number of
90% 1
50% 3
10% 5
5% 7
1% 10

Estimators: Church, DCox, LCox, Diggles, Force, Titley


Explained by Leslie J. Cox
On the choice of deposit models
Polymetallic replacement and skarn Zn-Pb deposits are formed in similar environments. Both are due to metasomatism that results from the intrusion of calcareous rocks by magma. In Arizona, the ore in replacement deposits is rich in sulfur, pyrite is the dominant sulfide, and silica occurs as jasperoid, rather than in calc-silicate minerals. In skarn deposits, ore minerals are disseminated, not massive, and iron is fixed in calc-silicate minerals rather than in pyrite. These differences are difficult to predict on a regional scale, and we estimated these deposits together, using a grade and tonnage model that combines the two types (Mark3 index 92).
On the delineation of permissive tracts
The permissive tract was delineated by modifying the tracts for porphyry Cu deposits, and including Late Cretaceous to early Tertiary muscovite-garnet-bearing peraluminous granite and associated pegmatite (unit TKgm, Reynolds, 1988). We then outlined areas within that modified tract that are known to contain limestones and other carbonate rocks. The tract also includes all pertinent known mineral districts.
Important examples of this type of deposit
For polymetallic replacement deposits, there are 6 to 9 Arizona examples with tonnages distributed about the median of the grade-tonnage model of Mosier and others (1986); of the 3 above the median in tonnage, two are very large, the Superior deposit in the Pioneer district (Hammer and Peterson, 1968) and the Copper Queen deposit in the Warren district at Bisbee. Two skarn Zn-Pb deposits in Arizona, Aravaipa, and Washington Camp are listed as examples in the grade and tonnage model of Mosier (1986), but there are no world-class skarn deposits of this type.
On the numerical estimates made
The quantitative estimate was made considering the distribution and size of the known deposits of each type. The Pearce, Pioneer (Superior and Belmont deposits), Oro Blanco (American Mine), Turquoise, Waterman, Slate, Swisshelm, Harshaw (French Flux deposit), and Copper Queen-Bisbee districts are all consistent with the worldwide grade tonnage curves. In addition, there are a dozen or so smaller deposits about which we do not know a great deal. To arrive at estimates, we considered first the probability of there being zero deposits, the maximum number likely, and the most likely number. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 3, 5, 7, and 10 or more deposits consistent with the combined grade and tonnage models of Mosier (1986) and Mosier and others (1986).
Hammer, D.F., and Peterson, D.W., 1968, Geology of the Magma mine area, Arizona, in Ridge, J.D., ed., Ore deposits of the United States, 1933-1967 (Graton-Sales volume): New York, American Institute of Mining, Metallurgy and Petroleum Engineers, v. 2, p. 1282-1310.
Mosier, D.L., 1986, Grade and tonnage model of Zn-Pb skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90-93.
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.
Reynolds, S.J., 1988, Geologic map of Arizona: Arizona Geological Survey Map 26, scale 1:1,000,000.

Geographic coverage

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