21. Sulfides, arsenides, antimonides, and sulfosalts

Sulfides are moderately rare at both Franklin and Sterling Hill and are not economically useful. The relative absence of sulfide ores is one of the prime criteria which differentiate Franklin and Sterling Hill from many other zinc orebodies. Local sulfides have not been extensively studied.

The sulfides, in general, occur in veins averaging 10-15 cm or less in thickness within both orebodies and in the surrounding wall rocks. The veins are relatively recent, perhaps Paleozoic in age, and are likely related to similar Paleozoic sulfide veins in the region (Cummings, 1993). Dolomite commonly accompanies such veins, which strike east-west and dip gently to the north; quartz and calcite are common gangue minerals. The predominant sulfides in such veins are sphalerite, galena, pyrite, and chalcopyrite. A partial list of sulfides is given by Frondel (1972); it includes minerals not shown to occur in the aforementioned veins.

Excepting sphalerite, the local manganese and zinc minerals, many of which are common in the orebodies, are not found in these sulfide veins. Some of the sulfide veins, when hot, reacted locally with willemite/franklinite  ore; the reactants are sphalerite and magnetite, which form a black, very fine-grained, intimately-mixed rind around the intruding sulfides.

The dominant sulfide at Franklin and Sterling Hill is sphalerite. Although not an ore mineral locally, it is not uncommon in small amounts at Franklin and is locally abundant at Sterling Hill. The sphalerite which occurs within the orebodies has been little studied. Some occurrences in the deposits, together with occurrences in the dolomite-bearing veins, were described by Baum (1986a). Arsenopyrite is common in the Franklin Marble, but not in the orebodies, and pyrrhotite is not uncommon in the Franklin Marble.     

The geologic distribution of sulfides at Sterling Hill was discussed by Pinger (in Yeatman, 1933). He described them as occurring in “small veins and fractures; in vein networks or stockworks; in brecciated limestone [marble] and ore; and locally disseminated in the limestone near the hanging-wall or foot-wall of the ore, generally in the vicinity of sulfide-carrying fractures or breccia zones.” He provided descriptions of sulfide concentrations in the southernmost keel, the keel of the crossmember, the west limb, the northern extension of the east branch of the west limb, and the east limb. Jenkins and Misiur (1994) described a complex, oxidized sulfide area at Sterling Hill, and Johnson et al. (1990) discussed the observation by R. W. Metsger of primary sulfides in a laterally continuous phlogopite-pyrrhotite band unrelated to fractures or faults. The occurrence of umangite mentioned by Ramdohr (1980) could neither be verified by the writer nor found in the literature.

Arsenides are rare, except for loellingite which is locally abundant in the black-willemite zone at Sterling Hill. Two anomalous arsenide occurrences are known, one at each deposit. A superb suite of nickel arsenides occurred once, highly localized, at Franklin. A zone of realgar-rich calcite exists in the Franklin Marble at Sterling Hill; the rarest sulfosalts occur in this unit. Domeykite, a copper arsenide, and cuprostibite, a copper antimonide, have been found in willemite and may have been locally abundant in tiny grains.  

Sulfosalt occurrences are mostly sporadic and exceedingly rare; some species, notably tennantite, commonly accompany sulfides in veins.

The locally occurring sulfides, arsenides, antimonides, and sulfosalts are listed below.

Sulfides
Acanthite Arsenopyrite Bornite Carrollite Chalcocite Chalcopyrite Covellite Digenite Djurleite Galena		Gersdorffite Greenockite Hawleyite Marcasite Molybdenite Pyrite Pyrrhotite Sphalerite Stibnite Wurtzite
Arsenides and antimonides
Breithauptite Cuprostibite Domeykite Loellingite Nickeline		Pararammelsbergite Rammelsbergite Realgar Safflorite Skutterudite
Sulfosalts
Baumhauerite Berthierite Seligmannite		Tennantite Tetrahedrite Zinkenite

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