Explained by Steve Ludington

On the choice of deposit models

In New Mexico, middle Tertiary felsic to intermediate intrusive rocks are emplaced into Paleozoic carbonate rocks. The model used here follows Einaudi and others (1981) and Cox and Singer (1986) that distinguish two type of copper skarns, those related to porphyry copper deposits and those not. Known examples of this deposit type are related to alkaline intrusions; this type of occurrence is accounted for by the assessment for alkaline porphyry Cu-Au deposits.

On the delineation of permissive tracts

In New Mexico, known copper skarn deposits formed where alkaline intrusive rocks, which are the principal sources of the metals, and are related to the alkaline porphyry copper-gold deposit type, are emplaced into carbonate rocks. But because such deposits may also form during calc-alkaline igneous activity, the permissive tract for copper skarn deposits in New Mexico is that area where carbonate rocks are present at or within a kilometer of the surface in the area of all middle Tertiary and younger igneous rocks. Areas of exposed carbonate rocks outside of the area of igneous activity were judged to have no potential for a copper skarn deposit because they are beyond the thermal and chemical influence of middle Tertiary and younger igneous systems.

Important examples of this type of deposit

Gold-bearing copper skarn deposits in the Orogrande and New Placers (San Pedro) districts are associated with the middle Tertiary intrusive systems that may also host Cu-Au porphyry deposits or alkaline Au-Te deposits. No examples are known that are related to calc-alkaline intrusions.

On the numerical estimates made

In addition to the known deposits, we judged that more may exist based primarily on the wide extent of carbonate rocks. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 2 or more deposits consistent with the copper skarn model of Jones and Menzie (1986).

References

Cox, D.P., and Theodore, T.G., 1986, Descriptive model of Cu skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86-89.

Einaudi, M.T., Meinert, L.D., and Newberry, R.J., 1981, Skarn deposits, in Skinner, B.J., ed., Economic Geology Seventy-Fifth Anniversary Volume, 1905–1980: Lancaster, Pennsylvania, Economic Geology Publishing Company, p. 317-392.

Jones, G.M., and Menzie, W.D., 1986, Grade and tonnage model of Cu skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86-89.