ORE MINERALS Chapter 12. Mineral assemblages

The ore minerals

General relations and mineralogy

Early estimates of the average ore content of the orebodies were skewed by numerous factors, among which were the separate deeding of franklinite and zincite mineral-rights in early years, the observations of apparent “separate beds” of ore, the selective mining of zincite, and the inducements to write favorable or unfavorable, but generally biased, reports during the 1857-1897 period of litigation. The New Jersey Zinc Company was not overly generous with the publication of such data, a wise business practice inasmuch as such information could influence investors, bankers, and others.




	Figure 12-1. Franklin specimens showing gneissic ore textures. Left (Franklin Mineral Museum, unnumbered) is franklinite (black) with willemite (gray), tephroite (gray), and calcite (white). Right (privately owned) is franklinite (black), and willemite (gray); note the lenticular franklinite layer and compare with large-scale features in figure 9-5. Large specimen is 13 cm in maximum dimension. Photo by Vic Krantz.

Published data on the composition of the ores at Franklin and Sterling Hill is given below. Additionally, Frondel and Ito (1966a) provided the average chemical composition of the ore units and “skarn” units, presented below, and Johnson (1990) provided bulk compositions for 16 Sterling Hill specimens, based on microprobe and modal analyses.

Pinger (1948) noted that, in general, “The distribution of ore minerals is remarkably uniform throughout the orebodies. Similarly, the distribution and ratios of iron, zinc, and manganese are surprisingly constant. The ore minerals: franklinite, willemite, and zincite are present at both mines in approximately the same proportions.”

The relative concentrations of the major ore minerals are difficult to state with precision. Few data have been published regarding Sterling Hill; those of Hague et al. (1956), given below, suggest that the Franklin deposit was much richer in franklinite and willemite than was that at Sterling Hill. Some contradictions in various reports remain unreconciled. Sterling Hill ore is anomalously much richer in sulfur; sphalerite is a constituent of the common ore in some areas.

However, compositions of the average mill concentrates calculated for a ten-year period (see below) show that both deposits yielded comparable proportions of zinc, with Franklin richer in manganese by twofold and Sterling Hill slightly richer in iron. In gross economic terms, however, Franklin was by far the richer of the two deposits, in light of the volume of ore; the abundance of calcite-free, high-tenor ore; and the density of its natural concentration.

Composition of the ores at Franklin and Sterling Hill.



	Franklinite	Willemite	Zincite	Carbonates	Silicates	Total
1.	51.92%	31.58%	0.52%	12.67%	3.3	100.00%
2.	48.20	28.10	2.70	11.32	9.50	99.82
3.	51.50	20.23	6.40	10.0	11.13	99.26
4.	43	26	1	25	5	100
5.	40	23	<1	25	11	n.g.
6.	33	16	1	rem.	rem.	n.g.

Abbreviations: rem. = remainder; n.g. = not given.

1. Franklin deposit; Spencer et al. (1908).

2.   Franklin deposit; Spencer et al. (1908).

3.   Franklin deposit; Spencer et al. (1908).

4.   Specific deposit unknown. Data supplied by The New Jersey Zinc Company and published by Palache (1935).

5.   Franklin deposit; Pinger (1950).

6.   Sterling Hill deposit; Hague et al. (1956).

Average chemical composition of
normal ore (1) and of skarn (2) from Franklin.


	SiO₂	Al₂O₃	Fe₂O₃	FeO	ZnO	MnO	MgO	CaO	BaO	Na₂O	K₂O	Rem.
1.	7.5	0.1	25.8	24.5	10.0	0.4	14.4					17.3
2.	40.6	4.9	11.2	1.2	4.4	13.3	1.2	17.6	0.7	0.22	1.75	2.93

Rem. = Remainder; Totals = 100 wt. %

(1) Normal ore. Remainder is CO₂ and 5 percent miscellaneous minerals. Total Fe and Mn arbitrarily calculated to trivalent and divalent states, respectively; the bulk of the Fe is trivalent and that of Mn divalent, but accurate analytical data are lacking. Mineral composition in weight percent: franklinite 39, willemite 24, zincite 1, tephroite 1, andradite 1, rhodonite 1, calcite 28, miscellaneous 5.

(2) Skarn [referred to herein as “calcium silicate units”]. Calcite and other components of the normal ore, usually present in more or less amount, have been excluded. Remainder is H₂O, Sr, Pb, F, SO₄, and various rare minerals in part containing B, Be, As, etc. Mineral composition in weight percent: andradite 42, rhodonite 28, feldspars 13, hendricksite 4, pyroxenes and amphiboles 4, hardystonite 5, fluorite 1, hancockite 1, miscellaneous 2.

Comments:

The ore data are based on a composite sample of crude ore representing mill feed over a five-year period, on analyses of mill concentrates averaged over a ten-year period, and on averaged analyses of individual ore minerals. The average mineral composition of the skarns [calcium silicate units] is less well known. The available quantitative data are based on mineral separations made of a composite sample of mill feed containing a mixture of skarn and ore. The averaged mineralogical and chemical data presented here represent only the relative amounts of the skarn silicates and do not include calcite, franklinite, and other constituents of the normal ore that are present in more or less amount in the skarn. (Data and full caption from Frondel and Ito (1966a)).

Mill concentrates for franklinite.


	Zn	Mn	Fe	Ca	SiO₂
Franklin	17.30	13.52	36.04	1.57	3.0
Sterling Hill	17.09	6.53	41.40	2.72	2.42

Average composition of franklinite mill concentrates over a ten-year period. Some willemite and gangue is admixed. Taken from Frondel and Baum (1974).

Frondel and Baum (1974) plotted the compositions of 132 franklinite analyses, noting that local “franklinite” varies from highly manganoan jacobsite compositions to common franklinite compositions to highly ferroan near-magnetite compositions. Squiller (1976), in a superb and extensive study at Sterling Hill, found an average composition of 394 Sterling Hill franklinites to be Fe₆₇Zn₂₂Mn₁₁, which is very close to the ratio Fe₆₆Zn₂₃Mn₁₁ reported for Sterling Hill material by Frondel and Baum (1974). Although never stated explicitly in the zinc-preoccupied literature, these proportions do demonstrate that Franklin and Sterling Hill were iron deposits as well as zinc deposits.

The geological features of the ores are discussed in part in the section entitled “The geology and structure of the zinc deposits,” and more specific features are discussed for Franklin ore by Frondel and Baum (1974), and for Sterling Hill ore by Squiller (1976) and Metsger et al. (1958). The ores do not yet fit into a neat framework classification of types, as noted by Frondel and Baum (1974), and those attempted have been “forced.”

The ore minerals are dominantly franklinite and willemite, with more or less zincite, associated with widely varying amounts of calcite; tephroite and manganese-humites may be present or absent. Only franklinite occurs in relatively large units, and it is nearly ubiquitous. The variation in mineral content and relative proportions is indeed great, both among and within discrete ore units. Ore units which are predominantly zincite with minor franklinite or calcite and units which are very rich in willemite and calcite are known (Frondel and Baum, 1974), but these are much less common than ore containing much variety in mineral composition. Ore may be calcite-free or may be composed of much calcite; the latter is particularly true at Sterling Hill. Ries and Bowen estimated the calcite content of the ores to vary from 0 to 50%. Zincite, in general, is much less abundant (only about 1% of the mill-feed) than are willemite and franklinite, but this too can vary locally. Zincite-dominant ore is known and, indeed, was very obvious at the original outcrops at Franklin and Sterling Hill, as described in the sections entitled “Historical perspective of local zinc mining” and “The geology and structure of the zinc deposits.”




	Figure 12-2. Rich Franklin zinc ore consisting of franklinite (black), willemite (gray), and sparse, indiscernible zincite. Gneissic banding is thin. Specimen is 17 cm in maximum dimension. Franklin Mineral Museum, #5139. Photo by the author.

In general, putting tenor (which is affected by calcite-content, silicate-content, and other impurity factors) aside, the ore at the two deposits is similar. Principal physical differences are the color of willemite, in general being green to yellow and rarely red or brown at Franklin and brown to brick-red and black at Sterling Hill, and the disseminated nature of much Sterling Hill ore. Black willemite ore, apparently sparse at Franklin, is found in great quantities at Sterling Hill.

Sonolite and leucophoenicite are the principal, hydrogen-containing, silicate primary minerals of the ore units at Franklin; sonolite and alleghanyite play this role at Sterling Hill.

Textural aspects

Although massive monomineralic franklinite ore is known, the common ores are predominantly polymineralic granular aggregates. The ores may have gneissic or disseminated textures; see figures 12-1 through 12-4 and 12-11 through 12-18.




	Figure 12-3. Coarse-grained, thick-layered Franklin ore consisting of franklinite (black), willemite (gray), and indiscernible zincite. Specimen is 8 cm in maximum dimension. Privately owned. Photo by the author.		Figure 12-4. Tephroite (gray) with willemite (white) and franklinite (black), from Franklin. In this specimen the gneissic texture is almost indiscernible. Specimen is 10 cm in maximum dimension. Privately owned. Photo by the author.

The individual gneissic layers generally vary from 1-10 mm in thickness; many exceptions are known. Disseminated ores (Figure 12-11) consist of the same ore minerals distributed in generous amounts of calcite, mostly as isolated crystalloblastic grains, but also as clusters of varying ore-mineral grains, xenoliths of gneissic ores, and in other textures. Metsger (1977, 1980) noted that “A variety of textures, especially at Sterling Hill, reveal that ore and adjacent calc-silicates were originally carbonate-free granulites” and that these friable masses “became disaggregated within an extremely plastic or, in part, even fluid carbonate.”




	Figure 12-5. Fragments of gneissic, granular Franklin ore, consisting of franklinite (black) and willemite (gray), are cemented together by rhodonite to form an ore-breccia. Specimen is 11 cm in maximum dimension. Privately owned. Photo by the author.		Figure 12-6. Franklin specimen showing gneissic ore, consisting of willemite (gray) and franklinite (black), cut by veins of younger material consisting of willemite and a carbonate mineral, likely rhodochrosite. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #143545. Photo by the author.

Although the gneissic ore is abundant, there are many specimens in which banded texture is not evident, or is defined by contrasting and differing mineral content rather than gneissic texture. The grain size of the ore minerals in gneissic ore is generally on the order of 2-3 mm. Smaller grain size is rarely encountered; larger grain size is common.




	Figure 12-7. Four Franklin specimens from the Smithsonian Institution showing ore textures. Top left (#17637) is zincite (black) with a tephroite band (gray) bordering sheared franklinite-willemite-calcite ore. Top right (#78558) is zincite in a flow structure within white calcite; irregular, isolated grains are franklinite on left and tephroite in bottom center. Bottom left (#17638) is franklinite (black) surrounded by tephroite (gray) in contact with light-colored willemite; zincite is present (dark areas on right and top). Bottom right (#R3522-4) is zincite in a flow structure within white calcite; these minerals are in contact with tephroite (gray), which is dominant in the upper part of the franklinite-zincite-calcite ore at the very bottom. Large specimen (upper right) is 11 cm in maximum dimension. Photo by Vic Krantz.

Most mineral grains are equant and anhedral; many have typical, metamorphic, equigranular, triple-point, interlocking textures. Both crystal faces and crystal sizes of the ore minerals are increasingly developed with increasing calcite content of the ore; this is more true for franklinite than for willemite. Large, unstudied, apparently recrystallized areas were common at both deposits, and locally the grain size is appreciably increased, in many cases to the multi-centimeter level and greater. Movement along fault-planes has created local slickensides of franklinite and willemite. Some slickensides contain serpentine; rhodonite, amphiboles and other minerals have been reported. Breccias, deformed ores, and flow-textures all occur here; see figures 12-5 through 12-10.

Ries and Bowen (1922) reported from thin-section studies that in general, for the ore minerals in primary ores, willemite and tephroite predate franklinite, which in turn predates zincite. They found that some franklinite may have formed before tephroite and willemite were completely formed, giving rise to some co-crystallization of silicate-phases and oxide-phases and temporal overlap in some cases, but that the formation of zincite was clearly last and separate in time sequence. This order of crystallization was largely accepted by Spurr and Lewis (1925), but these writers ascribed only trivial significance to it. Palache (1929a, 1935) adopted the views of Ries and Bowen; it is not clear, however, if he did independent thin-section investigations. Metsger et al. (1953, 1958), studying Sterling Hill ores, agreed that willemite formed first, but did not propose a paragenetical order for the other minerals.

Most recent writers have been silent on this topic. Ries and Bowen also noted some petrographic relations and many replacement textures; some of these are mentioned below in subsequent sections.




	Figure 12-8. Vein of Franklin zincite in calcite, showing intense fracturing of much of the zincite (lighter color is due to microscopic grain size). Flow structure is evident in the zincite aggregate. Grains of franklinite (black) and willemite, leucophoenicite, and tephroite (all gray) are seen at bottom center; the visible surface is polished. Specimen is 8 cm in maximum dimension. Privately owned. Photo by the author.		Figure 12-9. Franklin ore specimen composed of lamellar bands of franklinite (black), and willemite and calcite (both white). Note that the originally equant mineral grains have been sheared and are smeared-out. The visible surface is polished. Specimen is 13 cm in maximum dimension. Privately owned. Photo by the author.

Notwithstanding the above and other descriptions of the primary ores, it should be strongly emphasized that the ores from Franklin and Sterling Hill, primary and otherwise, have been very inadequately studied; indeed, to date they have been only trivially described. Aside from the work of Ries and Bowen (1922), which needs some modern re-examination, the literature is exceedingly sparse. There is truly great variation in the mineral composition, chemical composition, and textural aspects of these ores. The textural features present the greatest challenges; their study is substantially hindered by the gross grain-size of some primary ores. Moreover, the grain-size of many recrystallized ores approaches that of pegmatites. Indeed, the large grain-size makes some petrographic observations and interpretations quite difficult, if not impossible. The ores have great textural variability, in some instances over small intervals, and the ores seem to mock and frustrate the petrologist’s fundamental need for representative samples.




	Figure 12-10. Franklin specimen showing part of a likely vein assemblage. The original gneissic ore (top right) consists of franklinite (black), indiscernible zincite, willemite (gray), and tephroite (gray). The rest of the specimen exhibits a strong flow structure (flaser structure) in which most of the minerals have been pulverized, partially disaggregated, and smeared-out, but many franklinite crystals are intact. Note the larger size franklinite crystals in the center of the specimen, which may illustrate material introduced along the vein. The visible surface is polished. Specimen is 15 cm in maximum dimension. Smithsonian Institution, #83938-1. Photo by the author.		Figure 12-11. Evenly distributed, granular ore comprised of willemite (gray), franklinite (black), and calcite (white), from Sterling Hill. This sample was chosen as typical of ore from the 800 stope, 180 level, just above the zincite band in the west limb. Specimen is 14 cm in maximum dimension. Privately owned. Photo by the author.

The definitive recent studies are those done only on Sterling Hill ore assemblages under Dr. Charles Sclar at Lehigh University by Squiller (1976), Carvalho (1978), and Valentino (1983), and at Yale University by Johnson (1990). No comparable recent work has been done on the Franklin deposit; the geologic work which remains to be done at both deposits is greater than that done so far.

Alterations and replacements

There has been no systematic or broad study of the alterations of the ore minerals; most observations have been limited. The mud zone, a special, unique, localized alteration-and-enrichment saprolite at Sterling Hill discussed previously in this work, provides a superb in situ “laboratory” for part of such a study; its genesis has been briefly discussed by Moore (1875) and Palache (1935). Aside from the mud zone, the alterations of the primary ore minerals have received scant attention. A description of the gross physical relief resulting from surficial weathering of the ores by Alger (1845), quoted previously in this text, is the best we have. It suggests that, as a rock unit, massive zincite or zincite-rich, calcite-poor ore was more resistant to weathering than was calcite-bearing franklinite ore, the franklinite being composed in part of isolated grains easily released from their rock unit by the weathering and dissolution of surrounding calcite. These isolated franklinite grains then were used as buckshot by boys shooting at birds, providing the local term “buckshot ore” (Silliman, 1822). In general, aside from the special conditions of the Sterling Hill mud zone, franklinite is apparently relatively resistant to alteration, perhaps to the degree magnetite is; goethite or hematite may form from franklinite alteration in areas where alteration products are not naturally removed.




	Figure 12-12. Dense, black ore from Sterling Hill. This sample was chosen as typical of ore from the 1540 pillar, on the 1500 level, in the black willemite zone. Black ore is essentially a mixture of fayalite, willemite, franklinite-magnetite, and calcite in varying proportions. Loellingite and sphalerite, not evident here, are commonly associated. Specimen is 11 cm in maximum dimension. Privately owned. Photo by the author.

The focus of the extant studies of alterations and replacements has been on willemite. Ries and Bowen (1922) gave much attention to this mineral, discussing its solution, redeposition, alterations, and replacements. Zincite may be coated on parting planes with a whitish film composed predominantly of hydrozincite, which can be of geological or post-mining origin. Other references, including the work of Metsger et al. (1958), are cited in the descriptive sections for each specific mineral.

Silicates in ore

Silicates are in general much more common in the ore than is reflected in the literature and are more diverse in the ore units at Franklin than in those at Sterling Hill. At Franklin, they are chiefly silicates of manganese and zinc; olivine-group minerals predominate; hardystonite was volumetrically significant; and andradite was common. In hydrogen-free ore assemblages at Franklin, tephroite is the most common manganese silicate; glaucochroite may have been more restricted in occurrence, occurring principally in recrystallized calcium-rich material, but much may have passed unrecognized. In (OH)-and H₂O-bearing assemblages at Franklin, sonolite is common in calcium-poor ore units, and leucophoenicite is present in veins in some calcium-rich ore units. Overall, willemite is the most common silicate in ore.




	Figure 12-13. Coarsely crystallized zincite- franklinite-calcite ore from Sterling Hill. In this photograph zincite is very dark gray; franklinite is black. This sample was chosen as typical of ore from the 1200 stope, on the 600 level, in the east limb. Note the occurrence of zincite and franklinite as individual crystals. Willemite is present only as thin films on fractures. White calcite is abundant. Specimen is 11 cm in maximum dimension. Privately owned. Photo by the author.		Figure 12-14. Irregular segregations and large curved crystals of zincite (black) with franklinite (small black irregular patches at right center and top center) in calcite (white) from Sterling Hill. Specimen is 13 cm in maximum dimension. Smithsonian Institution, #118066. Photo by the author.




	Figure 12-15. Low-grade ore from Sterling Hill containing much calcite (white), moderate amounts of willemite (gray), and less franklinite (black, center and top-center). Specimen is 13 cm in maximum dimension. Smithsonian Institution, #162065. Photo by the author.		Figure 12-16. Coarsely-crystallized ore from Sterling Hill consisting of willemite (very dark gray), franklinite (black), calcite (white), and minor sphalerite. Specimen is 11 cm in maximum dimension. Smithsonian Institution, #C6075. Photo by the author.

At Sterling Hill, hardystonite is absent, and olivine-and Mn-humite-group minerals are the common silicates in the ore units; in the east limb, tephroite is not uncommonly rimmed by sonolite which was formed by hydration. The New Jersey Zinc Company, however, in its mapping, used “tephroite” as a broad term for tephroite-like silicates at Sterling Hill, without any species differentiation. Much fayalite in the black willemite zone was so-described. Johnson (1990) contributed additional observations.




	Figure 12-17. Curved, finger-like segregations of zincite in white calcite from Sterling Hill. Most of visible surface is polished. Specimen is 16 cm in maximum visible dimension. Franklin Mineral Museum, unnumbered. Photo by the author.		Figure 12-18. Rough crystals of willemite (gray) with franklinite (black) in calcite (white), from the 430 level, Sterling Hill. Note the distributions of franklinite. The visible surface is sawn flat. Specimen is 18 cm in maximum dimension. Privately owned. Photo by the author.

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