ORIGIN OF DEPOSITS Chapter 9. The geology and structure of the zinc deposits

The origins of the deposits

Introduction

The historical interpretation of ore deposits in general has relied in large part on comparative studies; ore deposits are compared with each other in search of features in common. For this reason, the extraordinary uniqueness of the deposits at Franklin and Sterling Hill has complicated matters considerably: such comparisons with other orebodies could not be made and still cannot be. Two of the principal ore minerals, franklinite and zincite, remain exceedingly rare elsewhere, and the third, willemite, is not found in this association as ore elsewhere and was rare in historical times. These and other difficulties arising from the uniqueness of the ores precluded any definitive argument from gaining more than a temporary toehold of credibility as “the” final solution. Some generalizations could be made, but they were few and tenuous in the early years.

Retrospectively, one must be mindful that economic geology, the science of ore genesis, was in its infancy when geological studies were started here. Many of the early studies, although they seem primitive now, are deserving of much respect. Considering what they did with what was available, some investigators were very good for their time, especially considering that the complete outlines and features of both orebodies were not known until well into the twentieth century, that the historical surface exposures and apparent ore-bed differentiations were misleading in part, and that commercial interests may not have permitted scientists full access to all exposed areas.

As a result, there is an extensive literature, extending well over 150 years, regarding the complex origins of these ores. Some of it is contradictory, some of it is muddled, and much of it is inconclusive, but most of it is sincere.

Although most of the extant studies were well done, others were quick-shot “solutions” based on very limited observations which were not in all cases accurate. Many men have visted the deposits and published discussions of the origins of the orebodies, “the printed length of which often varied inversely with the duration of the visit” as noted insightfully by Ridge (1952). Additionally, charlatans and arm-wavers have been active and, having done no work at all or exceedingly little, have brought forth their supposed “theories.” These are sometimes decorated with “insights,” which are commonly erroneous, usually without data, and largely arrived at by the adoption of and reworking of different combinations of earlier, but often uncited hypotheses, some also unsupported by data. The lack of comparative ore deposits elsewhere on earth left each Franklin-Sterling Hill genesis-theory standing almost alone. In such an environment, windy, irresponsible wordsingings, substituting motion, commotion, and verbiage for solid research and hard work, are almost invited because precise rebuttal is exceedingly difficult and in itself speculative. Unfortunately, such temptations are strong and such efforts continue.

To all who made serious investigations, we are deeply indebted; among the best of these studies are those of Kemp (1893a), Spencer et al. (1908), Ries and Bowen (1922), Spurr and Lewis (1925), Palache (1929a, 1935), Pinger (1948, 1974), Hague et al. (1956), Frondel and Baum (1974), Squiller and Sclar (1980), and Johnson et al. (1990b).

The studies of Amstutz (1957) on the roasting of zinc pellets, Brown (1948) on the reduction of zinc silicate, and Worner et al. (1990) on microwave-oven methods are interesting investigations.

To some extent, the historical evolution of genesis proposals followed the emergence of new ideas on ore genesis in general, and most represented reasonable attempts to fit the local genetical problems into new or extant frameworks; some attempts were forced, a few absurdly so. Most were adoptions of various interpretations of syngenetic and epigenetic theories.

Another factor in such theory-evolution was the progressive development of the orebodies, which provided new exposures of the geology, thereby discrediting some theories, modifying others, giving birth to new ones, and stimulating more investigations. This was not a trivial factor, nor was the introduction of the geologists’ ultraviolet lamps underground; such insights were invaluable and were not available earlier.

The summary given below of the historical evolution and exposition of such serious studies is abbreviated and generalized for the most part. Additional summaries were given by Pinger, Ries and Bowen, Palache, and others, as cited above, and especially by Bowen (1935); they provided approaches to step-by-step argument and are must readings for the seriously interested. A more extensive discussion of recent work is given at the end of this section.

Theories of origin

Early observations and hypotheses

Although many men had speculated weakly, one of the first scientists to state a specific case for one particular mode of genesis was Henry D. Rogers (1836, 1840). He envisioned the deposits as having formed by igneous injection of mineral matter into the blue limestone (Kittatinny Limestone). Alger (1845) offered some speculations, but they were not scientifically based. Kitchell (1855) proposed that the deposits were sedimentary features enclosed in the limestone, which was the precursor of the Franklin Marble, and that the deposits were metamorphosed with it (the first, and very insightful, exposition of the syngenetic theory). Cook (1868) provided good orebody descriptions but did not specifically address the origin of the zinc ores. He did, however, consider the local magnetite ores to have been sedimentary at one time; one might assume he held the same opinion of the zinc deposits (Bowen, 1935).

Frank L. Nason was a geologist who studied these deposits for a long time, but much of his later work was under contract as a private consultant to the New Jersey Zinc Company. Such studies done at the behest of the company were considered proprietary, and thus much of his work, and that of superb New Jersey Zinc Company geologists, such as Hague, Pinger, Baum, and Metsger, was unpublished. Nason had studied the local limestones and marbles most intensely (1890a, 1891a, 1891b, 1894b, 1894c) and had much expertise in local geology. Pinger (1973-74) said that Nason favored contact metamorphism, in part, as an origin for the deposits.

In one of the most careful studies, Kemp (1893a, 1900) suggested that the protore for the deposits might have formed originally by the replacement of favorable beds in the limestone by solutions derived from igneous activity; he agreed that these beds, of unknown mineralogic composition, were later metamorphosed. His presentation was carefully made; indeed, it was done superby and stands high as one of the better studies.

Blake (1894a) considered the zinc ores not to be veins but to be beds and sedimentary in origin, having later been metamorphosed; he also proposed a linkage between the two deposits. Blake (1894b) indirectly suggested desulfurization of primary sulfides, based in part on his observations of deposits in New Mexico; his argument is slightly “forced.”

Wolff (1898b, 1903) considered the ores to have acquired their mineralogical composition, form, and structure contemporaneously with the limestone. He suggested there may have been pre-existing sulfides later oxidized to carbonates, but admitted there was little evidence upon which to speculate about the pre-metamorphic nature of the manganese and zinc minerals in such strata.

Spencer (1908) and Spencer et al. (1908), who provided the best descriptions of the orebodies until then, admitted that no direct evidence was found for the origin of the Franklin orebody, but supported the possibility of injection of the ores in the manner in which igneous rocks are injected. He was particularly concerned about the genetical implications of the cross-member in the Sterling Hill orebody and was the first to consider this important feature in detail, demanding that it be considered. Lewis and Kümmel (1915) also adopted the concept of igneous injection.

Among the very best studies of these deposits was that of Ries and Bowen (1922), who provided a good summary of the previous theories; this is also discussed by Bowen (1935). Ries and Bowen suggested in part that the host limestone was first metamorphosed, and then the orebodies were emplaced by replacement of favorable beds in the limestone, followed by the folding which deformed the orebodies. They provided a chronicle of possible genesis.

Spurr (1923) and a more detailed study by Spurr and Lewis (1925) advocated a Zn-Fe-Mn sulfide precursor, mixed with carbonates of Mn, Zn, Fe, and Ca, which was magmatically locally injected as replacement veins or dikes. They argued for the alteration of sulfides to oxides and silicates with the increasing temperatures of metamorphism, at the peak of which they considered sulfides to be unstable. Their study is detailed, lengthy, very well-illustrated, and well-written. The earlier text by Spurr (1923), although presenting a briefer argument, is important and very pivotal in this writer’s estimation, inasmuch as he bluntly demanded that scientists see the formation of these complex deposits as possibly the result of compound processes, rather than as the result of one simple, known and understood, ore-forming process. Nonetheless, these two studies do represent an attempt to gently “force” these deposits into an igneous injection theory.

In 1923, Rastall published a brief, hinting theory of the metamorphism of already-emplaced, replacement-derived, precursor ores in limestone and coupled this with reactions similar to those of dedolomitization, but he did not develop his proposal at length.

Gregory (1927) suggested that a protore of sphalerite and galena was eroded and produced an alluvium, which was subject to selective transport of sphalerite to an area where Fe and Mn existed; his theory was not mentioned again, except by Fitch (1928) who dismissed it.

Fitch suggested that the zinc orebodies and the magnetite deposits were magmatically intruded as oxides by magma from the Byram Gneiss and were subsequently metamorphosed; in this connection and in others, the studies of Zn and Mn in regional amphibolites by Collins (1971) is interesting. Fitch stated without proofs that the sulfide precursor theories were untenable, but he gave no indication of having seen the work of Rastall, nor the published work of a good many other investigators. Fitch’s work was held in quite low esteem by the eminently knowledgeable Ward Bowen (1935), who noted that Fitch had not even visited the deposit.

The metasomatic replacement theory was adopted by Palache (1929a, 1935), who had worked with Spencer et al. (1908); he also apparently was unaware of Rastall’s paper. Palache advocated a metasomatic emplacement of hemimorphite, hydrous oxides of manganese and zinc, and perhaps carbonates of the same elements, in the limestone near the surface in Precambrian time; these minerals being later subjected to intense metamorphism, resulting in the present orebodies. In the same year, Tarr (1929) presented a partially parallel discussion of the same theory, while retaining the idea of a sulfide precursor, and presented a very interesting and energetic attempt to estimate the exact precursor minerals prior to their metamorphism; it is a truly fine effort, worthy of a very careful reading.

Thus, by the time Palache wrote his monograph (1935), many principal theories for the origin of the deposits had been set out with interesting variants on them; these are noted below.

a) Igneous injection, with and without later metamorphism.

b) Sedimentary deposition, with later metamorphism.

c) Contact metamorphism.

d) Replacement by magmatic solutions, with and without later metamorphism.

e) Metasomatic emplacement, with later metamorphism.

The best review paper of the extant theories was the superb and insightful study of Ward Bowen (1935), who boldly declared that the whole genetical question remained open; it is unfortunate that modern writers have lacked his perspective and frank, blunt humility.

Although by 1935 there had been many recent efforts and publications, the genesis of these complicated orebodies was not to be settled so early; discussion and contention continued. Pinger (1948) provided much information and a fine critical review of the extant theories. His report is superb and is the result of 17 years of mapping and investigating the ore deposits. He was a good and careful scientist, not given to speculation he could not strongly support. Given the brashness and carelessness of some writers and the creative nonsense of others, Pinger’s conservative paper, which prudently sought to be careful and accurate, is of much value. In addition to criticizing Palache’s proposal, he provided his observation that the evidence was consistent with replacement of favorable horizons in the folded structure by a primary oxide ore. This finding was also adopted by Hague et al. (1956) when they investigated and described the geology of the overall Franklin-Sterling Hill region in detail.

Ridge (1952) proposed that the deposits are pyrometasomatic in origin, with the source of the ore-forming fluid at some distance from the deposits. He also developed further some of the ideas of Ries and Bowen (1922) and provided much detailed and elegant argument; later (Ridge, 1972) he revisited the subject. Ridge’s work was followed by an unpublished private investigation for the New Jersey Zinc Company by Stockwell (1953). King (1958) systematically reviewed the extant theories and compared Franklin and Sterling Hill with other deposits, particularly that at Broken Hill in Australia. More so than others, King made extensive comparisons; his work is in part a fine and thoughtful contribution. Takahashi and Myers (1962, 1963) did not propose a specific mode of occurrence, but provided a thermochemical interpretation of the Sterling Hill assemblages, based on the calculated temperatures and partial pressures of water, carbon dioxide, oxygen, and sulfur, by assuming a closed system at equilibrium. In 1966, Callahan resurrected the syngenetic arguments for the deposits, in this case suggesting they were volcanogenic in origin, akin perhaps to the sediments formed in the Red Sea hot brines (Bischoff, 1969; Degens and Ross, 1969).

In 1969, using Sterling Hill as a model, Metsger et al., in an abstract, suggested a “sinking hypothesis” whereby the dense orebody (by implication, both orebodies) sank through the less dense Franklin Marble. This concept was additionally discussed in a broad comparison with the external morphologies of other ore deposits by Skinner and Johnson (1987).

The definitive review paper on the Franklin deposit was published by Frondel and Baum (1974). They supported the early syngenetic arguments, suggesting that the protore was sedimentary in nature, formed in part by volcanogenically-derived material, intercalated with more argillaceous layers. They suggested that this protore was largely carbonate in composition, with the bulk of the metals provided by hydrous oxides of Mn, Zn, and Fe. They suggested, quite clearly, that decarbonation of smithsonite or hydrozincite may have played a central role here; see Harker and Hutta (1956). There is no parallel, definitive, published study of the Sterling Hill deposit.

Recent studies

Sterling Hill, often ignored or subordinate in early studies, has been more intensely investigated in recent times. This is in part because the Sterling Hill deposit represents a simpler and more general case, of which the Franklin deposit (closed and economically exhausted in 1954 and flooded by 1956), with its pegmatite intrusives and substantial chemical differences, is a complicated variant with a strong hydrothermal overprint. Recently, Sterling Hill was also studied more intensely in part because of its accessibility until 1986 and again, privately, in 1989-1995.

It is now clear that the metal content of the protore was in place prior to metamorphism. However, few investigators have intensely pursued studies of decarbonation or oxidation during the primary metamorphic event.

Among the more extensive recent investigations have been those done at Lehigh University, including the studies of Squiller (1976) and Squiller and Sclar (1976, 1980); supporting observations were given by Carvalho (1978) and Valentino (1982). Squiller and Sclar adopted the theory of syngenetic origin, suggesting in an elegant argument that the progenitor of the present mineralogy was zincian-ferroan-manganoan dolomite intimately mixed as a mud with iron- and manganese-oxides and silica gel. They noted, as did Callahan (1966), the possiblity of sediments like those of the Red Sea as precursors. Additionally, they explicitly rejected the older theories of transport of heavy metals and the involvement of aqueous solutions. They suggested that, under high-rank metamorphism and dedolomitization, the above constituents would give rise to a calcite solid-solution and an oxide solid-solution; the latter resulting in either zincite or franklinite locally. Willemite was suggested to have been formed by the reaction of zincite and local concentrations of silica. These writers suggested that “franklinite occurs where oxides with a high Fe/Mn ratio were present and silica gel was absent; willemite occurs where silica gel was present and iron and manganese were virtually absent; zincite occurs where both silica gel and iron and manganese oxides were absent.” These Lehigh efforts were intellectually honest.

Johnson et al. (1990a, 1990b) and Johnson (1990) presented the results of petrological and oxygen-isotopic-composition studies done on Sterling Hill ores and the Franklin Marble. These efforts resulted in a number of observations which contrasted with the theory of Squiller and Sclar (1976, 1980). Johnson (1990) found a lack of isotopic correlations, which ruled out a metal-rich carbonate sediment that underwent decarbonation during metamorphism to give Zn-Fe-Mn oxides and silicates. He also found that whole-rock isotopic ¹⁸O values are depleted up to 9% relative to normal Proterozoic marine sediments of equivalent carbonate content. Johnson argued that (1) depletion took place by exchange with a fluid prior to the beginnings of metamorphism at a time when the protolith contained carbonate, clay, and possibly oxy-hydroxide phases, and (2) the rocks behaved largely as closed systems throughout Grenvillian metamorphism and were not substantially altered by infiltrating metamorphic fluids. He further argued that the Sterling Hill deposit is the product of hydrothermal metal deposition in an environment of unusually high fO₂/fS₂, and that the isotopic composition of the ores is distinct from that of the enclosing marble and is a feature inherited from the protore. Johnson et al. (1990a, 1990b) addressed the old weathering arguments by observing that the isotopic ¹⁸O values of the zinc-rich ore layers appear to preclude formation by weathering and suggest that the metal-carrying solutions were on the order of 150^o C and had isotopic ¹⁸O values larger than those of Proterozoic seawater. He addressed the “removal of sulfur” arguments by calculating the amount of water which would be required by this process, 4.8 km³, and suggesting, as Bowen (1935) did previously, that it was an unlikely hypothesis.

Johnson (1990) provided two models for the Sterling Hill protolith: (a) metal deposition on the ocean floor or in well-irrigated, shallowly-buried sediments where redox conditions were controlled by seawater, or (b) oxidation of an already-deposited sulfide body by hot seawater. He favored the former model and argued for a possible deposition like that in the Red Sea hot-brine pools (Bischoff, 1969; Degens and Ross, 1970; Callahan, 1966). Boron isotopic data for Sterling Hill sussexite and Franklin datolite given by Moore and Swihart (1990) are supportive of seawater-host suggestions, as are those of Davis (1993) who provided isotopic data for minerals from the black willemite zone at Sterling Hill. Leavens (1988, 1990) and Leavens and Nelen (1990) provided some arguments concerning the origins of the deposits.

In spite of much good work by the research teams at Lehigh University (by Sclar, Squiller, Carvalho, and Valentino) and at Yale University (by Johnson, Skinner, Rye, and Tracy), there is still much work to be done on the genesis of Franklin and Sterling Hill. The more lengthy discussion given herein of recent studies does not in any way imply this writer’s endorsement of them; it only recognizes a responsibility to set out the less well-known ideas in much more detail than those which have been in the literature for many decades to a century and a half. Progess toward a final, agreed, and comprehensive solution to these problems will likely be very incremental. Nonetheless, the duration will assuredly be punctuated by periodic, emphatic cries of “victory.”

The lack of any thermodynamic data for many of the local minerals is a great shortcoming. This, together with the ancient problem of the lack of a comparable ore deposit (even slightly comparable) to use as a basis for comparison, directly and greatly limits the scope of supportable theoretical models for the formation of the deposits. The great puzzles of the past are not alone; recent studies, too, leave great unanswered questions.

In light of some historically excellent scholarship, it should be noted and considered that some recent works have contained very little open or comprehensive dialogue regarding the vast legacy of previous work on these deposits; there have been too many grossly inadequate reviews and considerations of the extant literature. The essay by Macdonald (1993), entitled “What’s wrong with these publications? or, some pathologies of scientific authorship,” or other similar essays, should be strictly required reading for anyone investigating these orebodies or reading the genetical arguments.

At the end of his critical review, Pinger (1948) noted that “The foregoing criticism leaves a rather mutilated picture,” and his observation remains true today. This writer’s viewpoints on the genesis of these closely related deposits are still under development.




Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates

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