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Geochemical studies of the orebodies have been published mostly as subordinate parts of other subjects. No modern, systematic overview exists and much remains to be done. The studies of Ridge (1952), Squiller (1976), and Squiller and Sclar (1976) are particularly meritorious. Many geochemical data were gathered by the New Jersey Zinc Company but were not published. Isotopic data were given by Valley and O’Neil (1981), Johnson (1990), Moore and Swihart (1990), and Davis (1993).
A few very specific geochemical studies examined many minerals from here, but used specimens with unknown orebody relations. These include those of Papish and Stilson (1930) on gallium; Keevil (1943) on helium indexes, which gave generally poor results; Keevil (1950) on the radioactivity of many local minerals; Frondel (1970) on the scandium content of some minerals; and Li et al. (1994) on xenon and krypton in barite.
The great enigma of Franklin and Sterling Hill, aside from genesis but related to it, is the very uncommon and unique chemical composition of the ores. Although well-known as zinc deposits, the fact that they are predominantly iron-rich is often overlooked. An undifferentiated Zn-Fe-Mn oxide ore mineral (franklinite) in intimate association with zinc oxide (zincite) and zinc silicate (willemite), occurring in a carbonatian-siliceous gangue, and noted for the absence of both appreciable lead and sulfur, is a remarkable chemical combination not found elsewhere.
The ore deposits, in gross terms, consist of Fe3+-Mn-Zn-Si-O ore units generally partitioned from silicate assemblages hosting Ca together with much Al, Mn, Mg, Fe3+, and large cations such as K and Na, the whole being hosted by the calcium carbonate of the Franklin Marble. A primary geochemical characteristic of the deposits, noted at Sterling Hill, is that the ores “are not chemically equilibrated with the surrounding Franklin Marble, but are higher in oxidation state and/or lower in sulfidation state” and that they “contrast with the surrounding marble in isotopic composition” (Johnson, 1990).
Both orebodies have a halo of Pb in the surrounding marble, and the marble is graphite-free in a zone some 5-6 feet (~1.5 meters) surrounding the orebodies. There is also an asymmetrical halo, depleted in 18O, which erratically surrounds the Sterling Hill orebody, and which is coincident with the graphite-free zone (Johnson, 1990). Following earlier studies by Metsger et al. (1953, 1954b), Buis et al. (1986, 1987) examined the dispersion of trace and minor elements (Mn, Fe, Zn, Mg, Cd, and Pb) in the Franklin Marble at Sterling Hill and found Mn and Fe concentrations of 1-2 (wt.?) % in the Franklin Marble within 50 feet (15 meters) of the orebody, which drop off abruptly at a distance of over 100 feet (30 meters). They found Mn in the carbonate to be “the most beneficial [aid?] in the location of more Mn-Fe-Zn silicate-oxide ores.” Buis (1983) also studied a fluorite band at Sterling Hill. Germine (1986) studied Cd and Zn concentrations in the Franklin Quarry and in the Lime Crest Quarry near Sparta; both elements were higher in concentration at the latter. The Mn and Zn contents of some regional amphibolites were studied by Collins (1971).
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| Copyright © 1995 by Pete J. Dunn |
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by Herb Yeates
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