Explained by Alan R. Wallace and Virginia McLemore
On the choice of deposit models
Polymetallic replacement deposits consist of hydrothermal, epigenetic, Ag, Pb, Zn, and Cu sulfide minerals in massive lenses, pipes, and veins in limestone, dolomite, or other reactive rocks near contacts with intrusive rocks. New Mexico contains some of the classic examples of polymetallic replacement deposits.
On the delineation of permissive tracts
These deposits form where intermediate to felsic intrusive rocks, which are the principal sources of the metals, are emplaced into or near carbonate rocks. Therefore, the permissive tract for polymetallic replacement deposits in New Mexico, exclusive of the southwestern part of the State, is that area where carbonate rocks are present at or within a kilometer of the surface and middle Tertiary and younger igneous rocks are also present. Areas of exposed carbonate rocks outside of the area of igneous activity were judged to have no potential for a polymetallic replacement deposit because they are beyond the thermal and chemical influence of middle Tertiary and younger igneous systems. Undiscovered districts postulated in this tract are only those associated with calc-alkaline intrusions. Areas characterized by alkaline rocks were assessed for alkaline Au-Te deposits, which may include some replacement ore.
Important examples of this type of deposit
New Mexico hosts an impressive number of middle Tertiary polymetallic replacement deposits and prospects. Magdalena (Loughlin and Koschman, 1935) is a known polymetallic replacement deposit, and we identified 4 incompletely-explored areas (Hermosa, Organ, Chloride, Water Canyon) where rocks mineralized in this style are known, but where recorded production is unknown, incomplete, or small, and further exploration could lead to discovery of a significant district.
On the numerical estimates made
We believe that the known prospects are not fully explored, and further exploration could result in at least one of them advancing to the status of known districts consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier, Morris, and Singer (1986). The team evaluated the four known prospects and arrived at an expected value of about 1.25 deposits consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier, Morris, and Singer (1986). This estimate was then combined with consideration of a large covered area to arrive at the overall assessment. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 2, 4, 6, 8, and 11 or more districts consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier, Morris, and Singer (1986).
Loughlin, G.F., and Koschman, A.H., 1935, Geology and ore deposits of the Magdalena mining district, New Mexico: U.S. Geological Survey Professional Paper 200, 168 p.
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.