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The orebodies at Franklin and Sterling Hill are host to a number of special and mostly unique mineral assemblages. Few of these assemblages have been studied even petrographically. Some of the assemblages are widespread while others are rare and of narrow interest; some are synchemical with the deposit while others are chemically anomalous. The partial selection provided here is admittedly biased, both by the numerous biases discussed in detail in the section entitled “The specimen base” and by the writer’s biases.
These assemblages are organized here by geologic provenance; thus they are presented, with those from the Franklin deposit given first, in order of their occurrence: (1) from the ore units, (2) from the calcium-silicate units, (3) from veins, (4) from weathering or alteration environments, and (5) from an anomalous geologic niche. There is some redundancy here, with repetition of discussion of a few assemblages mentioned above in other contexts.
The common ore minerals, willemite, franklinite, and zincite occur in various combinations as discussed on preceding pages. Among these ore minerals are silicates and carbonate minerals, giving rise to a great variety of ore-silicate-carbonate assemblages. There are many other such combinations of these minerals. A very few such common assemblages are:
tephroite-zincite-franklinite-willemite
franklinite-willemite-zincite-calcite
franklinite-willemite-calcite
franklinite-calcite
sonolite-zincite-franklinite-calcite
Pervasive at the Buckwheat/Taylor Mine at Franklin, and less so at Sterling Hill, are secondary carbonate veins containing willemite, serpentine, rhodochrosite, sphalerite, talc, serpentine, and dolomite and/or calcite (Figure 12-45). Most of the veins have a high magnesium content and, at Franklin, are known for radiating willemite. Additional discussion is provided in the section on willemite. They have not been studied in detail.
Found only at Franklin, this coarse-grained mutual exsolution of zincite and manganosite occurred on the 900 level at the extreme northern end of the mine. It was described by Frondel (1940) and McSween (1976), but has not been investigated in great detail. Sonolite and leucophoenicite are commonly present; tephroite is less common. Much material was preserved in systematic collections.
Massive tephroite from both deposits contains exsolution lamellae of willemite. The relation was first studied by Hurlbut (1961) and later in more detail by Francis (1980, 1985b), who suggested the original high-temperature tephroite solid solution held about 20 mole % Zn2SiO4. Additional information is given under tephroite.
The occurrence of chalcocite on the 1050 level at Franklin has been little studied. Magnetite octahedra occur within massive chalcocite. Silver and, rarely, gold, occur interstitially to magnetite and chalcocite.
Large fist-sized masses of hematite with octahedral magnetite as inclusions were found associated with andradite at Franklin. A pronounced parting yields pseudocubic fragments of hematite. Much material was preserved.
Sterling Hill tephroite, particularly that from the east limb of the orebody, is rimmed by dark brown sonolite, which formed by hydroxylation of tephroite. Such composite crystals range in size from 5 mm to 15 cm in diameter (Figures 12-33 and 12-34).
In the cross-member of the Sterling Hill orebody is an ore unit, the black willemite zone, characterized by black willemite, fayalite, loellingite, sphalerite, calcite, fluorite, and other minerals (Figure 12-12). The assemblage is in need of additional study; Makovicky and Skinner (1990) examined some specimens, and Davis (1993) provided much information.
The occurrence of abundant calcium carbonate and silicon under greatly varying conditions has generated a large number of calcium-silicate assemblages. Calcium-silicate species are abundant at Franklin; there are markedly less calcium-silicate species at Sterling Hill.
Some of these assemblages are notable and are described below. A few others, not containing calcium and silicon, are included here for convenience inasmuch as they are assumed to have formed in the non-ore units. The incompleteness of this listing must be emphasized; the large number of calcium-bearing clinopyroxenes, amphiboles, and pyroxenoids, together with feldspars, micas, and a vast array of other minerals, prohibit more than a partial listing. Willemite-bearing assemblages are pervasive, but are not specifically listed here; assemblages for the preponderance of mineral species, including willemite, are presented within their respective descriptive sections.
Found only at Franklin, the assemblage of brown glaucochroite, hardystonite, willemite, and esperite forms massive coarse-grained material, with or without esperite and zincite. This was studied in part by Leavens et al. (1987) and is discussed in more detail in the section on glaucochroite. Although a silicate assemblage, it is found only in proximity to ore, and is important as a high-temperature host for Pb (hosted by hardystonite and esperite). Blue glaucochroite occurs with willemite, franklinite, and leucophoenicite in veins; these may have resulted from recrystallization and mobilization of the above assemblage.
This assemblage, from the 300 level at Franklin, consists of secondary silicates; the primary mineral may have been aegirine. The dominant silicates are magnesioriebeckite, which forms fibrous, blue-gray masses, and lennilenapeite, which forms light yellow-green druses and aggregates on dolomite and other minerals. Superb crystals of hematite and sphalerite are found associated, together with rounded fragments of sphalerite, and angular, colorless shards of end-member willemite.
Among the more esthetic Franklin assemblages is massive light blue vesuvianite (cyprine), light pink lath-like crystals of bustamite, white masses of hyalophane, and calcite, garnet, and mica (Figure 12-22). A previously existing mica has been replaced by vesuvianite in part.
The assemblage of andradite, rhodonite, and willemite is pervasive at Franklin and accommodates all of the principal chemical constituents of the deposit; franklinite may be present or absent. Andradite is an early, high-temperature host for Fe3+, Si, and Ca; rhodonite for Mn2+, Si, and Ca; and willemite for Zn and Si. Solid solutions in these minerals accommodate many minor and trace elements. Diopside (hosting Ca, Mg, and Si) and calcite are commonly associated. This was a volumetrically significant assemblage.
The intimate association of gahnite and rhodonite with andradite and microcline from the Trotter Mine at Franklin remains unstudied. Much material has been preserved.
The Franklin replacement assemblage of minehillite associated with microcline, allanite, margarosanite, and wollastonite has been little studied. Native lead occurs at the replacement boundary and commonly indicates the position of minehillite aggregates within hand-specimens.
Material recovered from the weathered pits and selected areas at Sterling Hill consists in part of specimens with very large, well-formed crystals of spessartine, gahnite, fayalite, and augite (jeffersonite). The individual minerals have been analyzed, but the assemblage remains unstudied as a unit.
This assemblage occurs most spectacularly in the Franklin Marble, accessed on the 900 level at Sterling Hill, and to some extent as deep as the 1100 level. Realgar varies from bright-red micro-crystals to orange powder; arsenopyrite crystals are superb and well-formed. This assemblage is host to a substantial number of uncommon-to-rare sulfosalt minerals, among them zinkenite, tennantite, baumhauerite, seligmannite, and others.
The mixture known as “caswellite” was common on the Parker dump. It was a mica, now replaced by varying amounts of chlorite, grossular, andradite, and vesuvianite (Figure 12-23). The associated minerals vary widely; andradite and willemite are common associates.
This assemblage of calcium silicates, illustrated in figure 12-20, is a primary and important one in which bustamite hosts Ca and Mn, diopside hosts Ca and Mg, andradite hosts Ca and Fe, and microcline hosts K and Al.
General observations concerning secondary veins have been given in preceding pages. Sterling Hill is host to vein assemblages of rare manganese arsenates, as well as uncommon vein assemblages of zinc- and iron-arsenates. Willemite veins are pervasive; some are discussed under the willemite heading. Listed below are a few of the more noteworthy vein assemblages.
Among the most colorful of vein assemblages at Franklin is that of the enigmatic leucophoenicite with willemite. The veins vary widely in the relative proportions of these minerals, and these minerals may occur in varying grain-sizes.
Sodium-rich, secondary, vein assemblages are rare at these deposits, and the colorful Franklin assemblage of ganophyllite and marsturite on rhodonite and manganaxinite is particularly anomalous.
Quartz is an uncommon mineral at Franklin. The assemblage of bannisterite, quartz, and richterite was an isolated one.
Within the north orebody at Sterling Hill are great masses of sussexite, associated with much dolomite, serpentine, pyrochroite, and hematite, in varying proportions. Torreyite and lawsonbauerite are found in vuggy areas among these minerals. The assemblage is wholly unstudied.
The mud zone at Sterling Hill, a collapsed saprolite in part, consists of severely weathered minerals. Iron has been removed or remains as goethite; zinc and manganese have been retained in chalcophanite and hetaerolite, and some residual minerals such as woodruffite, todorokite, cryptomelane, and others, contain higher oxides of manganese. The assemblage has not been studied using modern methods. Deeply weathered crystals of pyroxene, fayalite and other minerals also occur in this zone, together with hemimorphite and trivial, secondary, lead minerals such as mimetite, descloizite, cerussite, etc.
The pits at Sterling Hill were noted for voluminous quantities of hemimorphite; the underside of such specimens contains much clay, sauconite, and sphalerite.
The Buckwheat Dolomite, described by Palache (1935) as a “veinlike mass of gray dolomite” in the west wall of the Buckwheat Open Cut, is included here with uncertainty. The Buckwheat Dolomite hosts superb sphalerite crystals and a great many accessory minerals in microcrystals (Peters et al., 1983). These varied minerals, however, have no evident relation to the Zn-Mn-Fe character of the orebodies. Almost nothing is known of this gray-brown dolomite; there has been no published, comprehensive work on it, but Cummings (1988) has proposed an idealized genetic model.
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| Copyright © 1995 by Pete J. Dunn |
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