National mineral assessment tract SB24 (Distal disseminated Ag-Au)

Tract SB24
Geographic region Southern Basin and Range
Tract area 61,700sq km
Deposit type Distal disseminated Ag-Au
Deposit age Mesozoic - Tertiary

Deposit model

Model code 19c
Model type descriptive
Title Descriptive model of distal disseminated Ag-Au
Authors Dennis P. Cox


Confidence Number of
90% 1
50% 2
10% 5
5% 7
1% 10

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


Explained by Leslie J. Cox
On the choice of deposit models
Distal disseminated Ag-Au deposits form in sedimentary rocks distal to plutons. The deposits are somewhat similar to sediment-hosted Au deposits, but have significantly higher Ag grades and base-metal contents. They are associated with hypabyssal or subvolcanic intrusions. Arizona has at least two known distal disseminated Ag-Au deposits.
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
Two deposits in Arizona, Hardshell and Tombstone, are part of the grade and tonnage models (Cox and Singer, 1992), but both are below the median tonnage. Tombstone is classified as a distal disseminated Ag-Au deposit rather than as a polymetallic replacement deposit because it contains no massive replacement bodies and lacks a clear relationship to a plutonic contact. The Vekol district might be classified as distal disseminated Ag-Au, but it produced less than 100,000 metric tons of ore and is not a significant deposit.
On the numerical estimates made
Estimators thought that deposits of this type might be both commonly formed and readily concealed in Arizona, because they are low in sulfide content and relatively inconspicuous. This type of mineralization was not widely recognized as ore until the 1970s, because of its low-grade, disseminated nature. For the 50th percentile, estimators noted that the number of known distal-disseminated deposits is near that of polymetallic-replacement deposits: about 2 to 7. Therefore, some argued that there should be perhaps 4 to 10 total known plus undiscovered (the estimate for undiscovered polymetallic-replacement deposits at the 50th percentile is 3). For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 2, 5, 7, and 10 or more deposits consistent with the grade and tonnage model of Cox and Singer (1992).
Cox, D.P., 1992, Descriptive model of distal disseminated Ag-Au, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 19.
Cox, D.P., and Singer, D.A., 1992, Grade and tonnage model of distal disseminated Ag-Au, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 20-22.
Reynolds, S.J., 1988, Geologic map of Arizona: Arizona Geological Survey Map 26, scale 1:1,000,000.

Geographic coverage

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