Explained by Steve Ludington

On the choice of deposit models

Southwestern New Mexico contains an important Laramide skarn Zn-Pb deposit, characterized by the replacement of carbonate rocks by calc-silicate minerals and base-metal sulfides.

On the delineation of permissive tracts

Skarn Zn-Pb 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 for 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

The Groundhog mine (Meinert, 1987) is an important zinc-lead skarn deposit, and other Laramide plutonic systems could have given rise to concealed skarn deposits.

On the numerical estimates made

Whereas the permissive area is relatively small, and has only one known deposit, the team placed a major emphasis on productive related deposits in the area and the substantial proportion (>50 percent) of covered permissive rocks. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, and 3 or more deposits consistent with the grade and tonnage model of Mosier (1986).

References

Meinert, L.D., 1987, Skarn zonation and fluid evolution in the Groundhog Mine, Central mining district, New Mexico: Economic Geology, v. 82, no. 3, p. 523-545.

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.