Explained by Sandra H.B. Clark and Michael P. Foose, with advice and consultation from C. Ervin Brown
On the choice of deposit models
Major deposits of probable sedimentary exhalative origin are known in the Balmat-Edwards-Pierrepont district of northern New York. The polydeformed and highly metamorphosed nature of host rocks and the ore bodies have led to different interpretations about the origin of the mineralization. The deposits have been interpreted as Mississippi Valley type because of the carbonate host rocks and simple nature of the sulfides. However, later interpretations are that the Balmat-Edwards-Pierrepont deposits may be carbonate-hosted sedimentary-exhalative deposits (deLorraine and Dill, 1982; Whelan and others, 1984).
The deposits have a unique combination of characteristics that do not clearly fit either model. The main features of these deposits that were used to select a model for this assessment were the occurrence of a thin pyritic schist unit in the sequence, the stromatolite-bearing shelf environment of deposition of the carbonate strata, association of evaporites with the deposits, and the association with a paragneiss in the district that was probably derived from a dacitic volcanic rock. No clear exhalites have been identified, and any original alteration has been obscured by metamorphism . The host rocks are metamorphosed carbonate rocks, rather than black shales, as is characteristic of many sedimentary-exhalative deposits. The assessment team concluded that the deposits of the Balmat-Edwards-Pierrepont deposits more closely fit a distal variety of the sedimentary exhalative model of Menzie and Mosier (1986).
Grades and tonnages of production and reserves are poorly known. Estimated production figures suggest that the major production has been from the Balmat mines, but a significant proportion of ore came from the Edwards mine, which is no longer being mined, in the past along with a much smaller production from the Hyatt mine. The Pierrepont deposit, which was discovered in 1979, was estimated to contain 2.3 million metric tons of 16 percent Zn ore, which would be about 360,000 metric tons of contained Zn (Burger, 1983). Even if more accurate data for the district were available, the relationship to original grades would be uncertain because present grades may have been altered by metamorphism.
Estimated tonnages, which are based mainly on production, put the Balmat deposit well above the median on the tonnage curve of Menzie and Mosier (1986); Edwards and Pierrepont are below the median, but above the 90th percentile. A minimum estimate for the Hyatt deposit puts it at the low end of the tonnage distribution. The zinc grade for these deposits is above the median on the grade curve of Menzie and Mosier (1986). Comparison with zinc grades in the deposit model is complicated because these deposits have been strongly metamorphosed, and it is likely that the original zinc grades have been increased. The lead and silver grades of the deposits are well below the 90th percentile on the grade curves of Menzie and Mosier (1986). The grade of lead is one of the lowest on the curve, and silver is reported only from the Edwards mine. No geologic reason for the low lead and silver contents of Balmat ores is known. The Balmat ores may be part of a low lead-silver subtype of sedimentary exhalative ores, but no consistent geologic basis for separating low lead-silver deposits from other deposits in the worldwide model has been identified.
On the delineation of permissive tracts
All Middle Proterozoic marbles in the Adirondack Mountains and surrounding lowlands are considered to be permissive for the occurrence of sedimentary exhalative zinc-lead deposits. Contacts were extended to 1 km depth based on dips. In most of the areas, dips are about 45° and some tract boundaries are high angle faults. The permissive tract includes the Middle Proterozoic marble that is the host rock in the Balmat-Edwards-Pierrepont district.
On the numerical estimates made
The grade and tonnage models for sedimentary exhalative deposits (Menzie and Mosier, 1986) are based on ore districts and not on occurrences of individual concentrations or lenses of ore. This fact constrains numerical estimates to be an expression of the probability that new districts remain undiscovered. The likelihood that significant undiscovered ore exists in the tract is high, but it is probable that all ore bodies would be a part of the existing Balmat-Edwards-Pierrepont district. Therefore, the probability that a new sedimentary exhalative ore district exists in this tract is rated as less than 0.01. This estimate reflects the extensive exploration that this area has received, as well as its small size.
References
Burger, John R., 1983, St. Joe's Pierrepont mine: Engineering and Mining Journal, v. 184, no. 2, p. 58-60.
deLorraine, W.F., and Dill, D.B., 1982, Structure, stratigraphic controls, and genesis of the Balmat zinc deposits, northwest Adirondacks, New York: Geological Association of Canada, Special Paper No. 25, p. 571-596.
Lea E.R., and Dill, D.B., Jr., 1968, Ore deposits of the Balmat-Edwards district, New York, in Ridge, J.D., ed., Ore deposits of the United States, 1933-1967, v. 1: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, v. 1, p. 95-107.
Menzie, W.D., and Mosier, D.L., 1986, Grade and tonnage model of sedimentary exhalative Zn-Pb, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 212-215.
Whelan, J.F., Rye, R.O., and deLorraine, W.F., 1984, The Balmat-Edwards zinc-lead deposits—Synsedimentary ore from Mississippi Valley-type fluid: Economic Geology, v. 79, no. 2, p. 239–265.
