Geochemistry of the Sterling Hill Zinc Deposit, Sussex County, New Jersey

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 69, PP. 775-788. 2 FIGS. JUNE 1968

GEOCHEMISTRY OF THE STERLING HILL ZINC DEPOSIT, SUSSEX COUNTY, NEW JERSEY

BY R. W. METSGER, C. B. TENNANT, AND J. L. RODDA

ABSTRACT The Sterling Hill ore body is mineralogically similar to its neighbor to the north in Franklin, N. J. It is an intricately folded, steeply plunging body in Precambrian graphitic, sparsely silicated, coarsely crystalline white marble. The marble is graphite-free within 5 feet of ore. The ore is wrapped around a central core of graphitic marble and an annular gneissic cylinder of mica, feldspar, hornblende, pyroxene, and garnet composition. Ore minerals are willemite, franklinite, and zincite. Franklinite, tephroite, pyroxene, and biotite compose the leanly mineralized areas. The mineral distribution is zoned. Black willemite with magnetic franklinite forms a band of varying thickness that follows closely the convolutions of the upper surface of the brown willemite-franklinite body. Ore textures are identical with those of the areas of lean mineralization, and the transi- tion is not sharply defined. A thin band of rhodonite is found at this boundary between pyroxenes and willemite. Paragenetically, it appears that of the ore minerals, willemite formed first in the sequence, with tephroite, zincite, and franklinite following in undetermined order; tephroite and zincite may have been emplaced at the same time. There is abundant evidence of later generations of willemite, franklinite and zincite. Primary zincite has been observed only in the presence of tephroite. There is evidence that tephroite has replaced willemite. Manganese solutions possibly replaced the zinc in willemite to form tephroite and may have resulted in the formation of zincite. The rock core of the ore body is altered to zinciferous clayey mud, locally containing hemimorphite. This alteration extends from the surface to a depth of 680 feet. The variation in willemite color is due to color differences in <10 micron franklinite inclusions. Red franklinite is nonmagnetic with a unit cell dimension of nearly 8.51 A. Black franklinite is magnetic with a unit cell dimension of about 8.42 A. Macro- scopic franklinite cell dimensions are intermediate. Origin of franklinite inclusions is attributed to willemite serpentinization, similar to that commonly observed in magnetite formation by serpentinization of olivine.

CONTENTS

TEXT Page Spinel (franklinite) lattice measurements 782 Pasc Results of lattice measurements 784 Introduction and acknowledgments 776 Paragenesis of the macroscopic ore minerals. . . 785 Description of the Sterling Hill ore deposit 776 Genesis of inclusions 785 Geological environment 776 References cited 787 Outer zincite zone 776 Central zincite zone 777 Black willemite zone 778 ILLUSTRATIONS Brown willemite zone 778 Pyroxene zones 780 Figure Page Franklinite zone 780 1. Geologic map of the Sterling Hill ore body... 779 Gneiss zone 780 2. Ferrite lattice dimensions 783 Calamine-mud zone 780 Causes of variation in willemite physical properties 781 TABLE Color differences 781 Fluorescence 781 Table Page Microscopic franklinite inclusions 781 1. Chemical compositions of minerals dis- Serpentine inclusions 782 cussed 777 775

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INTRODUCTION AND ACKNOWLEDGMENTS which joins the middle of the west limb with the middle of the east limb. The ends of the The Sterling Hill zinc deposit is one of a two limbs, the keel of the syncline, the thick- geologically unique pair of ore occurrences in ened part of the cross member, and the latter's Franklin and Ogdensburg boroughs of Sussex junctions with the two limbs plunge roughly County, New Jersey. Innumerable papers have parallel at 45°, N.70E. been written on their vast suite of minerals. The ore distribution in plan is similar to its The Franklin mine was recently (1954) shut pattern in section, owing to the 45-degree down because of exhaustion of ore. plunge of the ore body. Most of the published works on these de- The ore body occurs in a coarse crystalline posits describe individual minerals rather than white marble generally well banded with mica, their relationships to each other and to their amphibole, and chondrodite. Graphite is host rock. The few papers concerned with the ubiquitous in the limestone except in and near genesis of the ore bodies are, for the most ore and, in some cases, adjacent to pegmatite part, speculative and are based on brief field and gneiss. A marble zone 5 feet thick which observations. Ries and Bowen (1922) pub- envelopes the ore body is graphite-free but lished the most detailed study of the ore bodies. otherwise apparently identical with the country Finger (1950), Chief Geologist at Franklin rock. The banding of the limestone appears to from 1929 to 1950, summarized the important parallel the foliation of the adjacent gneisses hypotheses of origin. He described the ore and has been attributed to bedding. Locally, bodies only briefly. Ridge (1952) discussed at large masses of the limestone have been dolo- length solutions that may have formed the mitized. The dolomitized masses in the mine mineral deposits, on the assumption that they appear to be related to joints, faults, and are a pyrometasomatic deposit. Hague et al. breccia zones which are known to be post- (1956) described the areal geology and structure Algonkian. of the area with particular emphasis on the The Sterling Hill ore body lies within the gneisses. White (Franklin) Limestone about 600 feet The present paper is the result of a continu- away from its contact with the Cork Hill ing study undertaken by the writers early in gneiss. The Franklin ore body 3 miles to the 1953. Recent work on the Sterling Mine is re- north is only a few feet from the same contact. ported. It is hoped that continuation of the The well-annotated paper by Hague et al. present work will find application in mining (1956) should be referred to for specific geo- and exploration and in furnishing better geo- graphical and stratigraphic location. logical understanding of processes associated The ore body proper is distinctly zoned with ore formation. mineralogically, but, owing to extreme struc- The work has been carried out co-operatively tural complexity, the distribution of the zones between the Exploration and Research de- is imperfectly understood. partments of The New Jersey Zinc Company. A number of people from each department Outer Zincite Zone have contributed to the work in various ways. The authors are pleased to acknowledge their The outer zincite zone comprises the east assistance. Also, the authors wish to express and west limbs of the ore structure. The ore is appreciation to The New Jersey Zinc Com- composed typically of pale flesh to red wil- 1 2 pany for permission to publish this informa- lemite and tephroite (or a related olivine), tion. nonmagnetic franklinite, and sparsely disseminated to locally abundant zincite (Table DESCRIPTION OF THE STERLING HILL 1). These minerals are found in varying propor- ORE DEPOSIT tions, disseminated pepper-and-salt fashion in white limestone which generally displays a Geological Environment pink fluorescence, or as sheets or lenses with

The Sterling Hill ore body is structurally 1 These willemite grains are fluorescent along complex. It occupies an isoclinal syncline in fractures and in irregular zones, although to the white crystalline limestone. Its long east limb naked eye they appear to fluoresce uniformly. and shorter west limb dip eastward at 55°, 2 Tephroite as used here is not a specific mineral This structure is complicated by a cross mem- name but refers to a series of olivine minerals not ber, greatly thickened in its middle portion, yet differentiated.

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gneissic texture and essentially free of included of limestone present. Where limestone occurs limestone. In general, the franklinite comprises within the gneissic bands, it appears as augen- 40-60 per cent of the ore minerals in all zones. like lenses 1-3 inches long oriented in the plane The limestone is everywhere coarsely crys- of the gneissic foliation, or as larger irregular talline and highly twinned. masses. An unusual feature of the ore is visible primarily in the west limb. In the hanging-wall TABLE 1.—CHEMICAL COMPOSITIONS half of the limb, the ore is of the normal dis- OF MINERALS DISCUSSED* seminated variety. About midway between the hanging and foot walls, this pepper-and-salt Mineral Composition ore contains sparsely distributed, generally rounded nodules of calcite-free willemite and Franklinite (Zn,Fe,Mn)O-Fe203 franklinite, gneissic in texture. A few of these Willemite Zn2Si04 nodules on close inspection have revealed Zincite ZnO hexagonal outlines, suggesting that they are Tephroite Mn2SiO4t possibly "ghosts" of some pre-existing crystal. Serpentine H4Mg3Si20., Toward the foot wall, these nodules become Friedelite H7(Mn,Cl)Mn4Si4Oi6 more and more abundant, until they coalesce Pyroxenes Diopside— CaMgSi206 to form a thick band of the gneissic ore de- Augite— Ca(MgFe)Si2O6 scribed above. Rhodonite MnSiOs The contacts between ore and rock are Garnet (var. Ca3Fe2Si3Oi2 normally gradational. The ore minerals become Andradite) coarser and more disseminated over distances Gahnite ZnAl2O4 of from 6 inches to a foot. Where they are very Calcite CaCO3 sparse (as in the hanging wall of the east Dolomite CaCO,-MgCO, limb) the willemite and franklinite crystals in many cases have diameters ranging from 1 * These formulae disregard elements that may inch to more than 1 inches. The crystals locally be present in minor concentrations. are well formed. This coarseness of texture is t As used in this report Mg can proxy for Mn. also observed where the minerals are sparsely disseminated in the local masses of limestone In the disseminated ore the grain size of the within the ore. willemite and franklinite are roughly com- Within the ore band, generally within less parable and are of the order of a quarter of an than a foot of the hanging-wall contact in the inch in diameter. In general, the grains are case of the east limb, and the foot-wall contact rounded and equidimensional but locally are in the case of the west limb, is a band of con- euhedral. The tephroite, which closely re- centrated zincite roughly 6 inches thick. In sembles the willemite and occurs locally to the many places the zincite occurs as irregularly exclusion of willemite, appears in general as a shaped single crystals as much as 3 inches in peripheral replacement of individual willemite diameter. These are invariably associated with grains. The zincite is relatively fine-grained similarly coarse crystals of silicate minerals, and sparsely disseminated through much of the which until recently were erroneously called ore but has not been observed in normal ore tephroite but are actually three or more phases except in the presence of tephroite. of an unidentified olivine. The gneissic type of ore, which in general The ore band of the outer zincite zone varies occurs as sheets or lenses within the dissemi- considerably in thickness (Fig. 1) and feathers nated ore, is composed of willemite and frank- out by increasing dissemination at the north linite grains slightly smaller in diameter than end of the east limb. those in the disseminated variety. Zincite, as before, is generally sparsely disseminated. In Central Zincite Zone this ore the willemite grains are elongated parallel to the crystallographic axis. L. A. Midway between the keel and the northern Herrmann (Unpublished report) has shown termination of the west limb, the ore branches that these willemite grains, as well as those off to the northeast to form a cross member from other zones, parallel the major structural which is greatly thickened in the middle and elements of the ore body. The degree of ori- either joins or abuts (the structure is not yet entation is inversely proportional to the amount clear) the east limb approximately midway Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/69/6/775/3431868/i0016-7606-69-6-775.pdf by guest on 01 October 2021 778 METSGER, TENNANT, AND RODDA—STERLING HILL ZINC DEPOSIT

between its northernmost extremity and the type is probably also gneissic, but foliation is keel. obscured by the similarity in color of the fer- Although it is continuous with and not rite and silicate minerals. The willemite grains greatly different mineralogically from the are probably oriented parallel with the struc- outer zincite zone, the western half of the cross ture of the ore body, but no work has been member has been designated as the central done to prove this. zincite zone. The black ore, until recently, was thought to The zincite band in the outer zone has not occur only in the thick part of the cross mem- been traced in the central zone, although dis- ber, as local masses or lenses in the midst of seminated zincite is present. The distribution of the pyroxene zone, and near the junction of the magnetic franklinite is not yet well defined, the west limb and cross member. It has now but for the most part it appears to be confined been traced (Fig. 1) as a relatively thick mass to the eastern side of the zone near its contact at the north end of the west limb; it thins to a with the black willemite zone. ±3-inch band of disseminated gray willemite A considerable amount of dull brownish- and magnetic franklinite, which parallels the green, glassy, essentially nonfluorescent wil- west limb about 2 feet above its hanging wall lemite is present in the middle of the thickened to another thick mass at the junction with the part of the central zincite zone, but it has not cross member. From there it again thins to been observed in the outer zone. ±3 inches and extends parallel with and 2 feet Except for these differences, which may be beneath the foot wall of the cross member to more apparent than real, the ore of the central another thick mass north of the thick part of zone is similar to that of the outer zincite zone. the central zincite zone. Its course from there to the main body of black ore has not been Black Willemite Zone traced precisely and is indicated in Figure 1 as a dashed line. The writers' examination has Black willemite ore is radically different in not been extensive enough to determine appearance from all the other ore in the mine. whether or not this connection exists. Most of this ore is in the eastern half of the At all places where the black ore adjoins the thickened part of the cross member and, where red ore, there is a gradational zone of dark-brown massive with little limestone, appears as a ore. jet-black rock. Mineralogically it is not very different from Brown Willemite Zone the red and brown willemite ores of the re- mainder of the ore body. The major constitu- About halfway between the thick part of the ents are willemite and franklinite in grain black willemite zone and the junction of the sizes comparable with those described for the cross member and the east limb, the black ore outer zincite zone. In this zone, however, the thins and grades into a dark-brown to red- willemite is jet black to grayish black to gray brown willemite-franklinite ore containing no and in many cases is difficult to distinguish zincite. The franklinite is magnetic and in from franklinite in unpolished hand specimens. general black in finest powdered form. Tex- The metallic luster of the franklinite differ- turally, the ore is similar to all ore previously entiates the two. As with all willemite, this described. Also included in this zone are lenses material is transparent in thin section. The of ore lying parallel with and close under the associated tephroite (roepperite) likewise is foot wall of the east limb. These lenses may be black. In polished section and thin section its connected through thin low-grade bands with appearance is similar to that of willemite. The the ore of the cross member, and the latter franklinite differs from that of the outer zincite may not actually join the east limb. More field zone in that it is highly magnetic and black study will be needed to prove or disprove this. even in powdered form. The black willemite is The margins of the brown willemite ore sufficiently magnetic to make its separation bands are typically of the pepper-and-salt from franklinite difficult. variety, whereas their inner parts have a Zincite has never been found in the black generally gneissic texture. There is a mineralogi- willemite zone. cal gradation between this disseminated ore As in the outer zone, the ore is disseminated, and the surrounding mineralization of the with a pepper-and-salt texture, and also in pyroxene zone, although the texture remains massive form with little limestone. This latter the same,

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ZONES Outer Zincite- flesh to red willemite and tephrpite, zincite, nonmagnetic franklinite, limestone. Central Zincite-similar to outer zone, but with magnetic franklinite in eastern part. Black Willemite- black willemite and tephroite, magnetic fronklinite Brown Willemite-brown to dark brown willemite and tephroite, magnetic franklinite, limestone. Pyroxene- limestone,green and brown pyroxene, magnetic franklinite, biotite, garnet. Franklinite- limestone and magnetic franklinite.

Gneiss-( pyroxene, feldspar, biotite, hornblende.)

155'

Graphitic White Limestone

FIGURE 1.—GEOLOGIC MAP OF THE STERLING HILL ORE BODY

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Pyroxene Zones limestone, in the east limb near the keel of the ore body. Its plunge is somewhat less than There are two pyroxene zones, mincralogi- that of the ore body, so that near the surface cally and texturally identical, but occupying it wraps around the keel and appears, in part, different structural positions in the ore body. in the west limb. In depth this zone is entirely Because they are not considered to be of ore within the east limb (Fig. 1). More work is grade, there are few openings in them, and necessary to determine its relationship to the they are therefore the least-known parts of the rest of the ore. ore body. One of these zones extends northward from Gneiss Zone the thick part of the cross member beneath the foot wall of the east limb and pinches out This zone is a band of pyroxene and/or near the northern end of that limb. It is com- hornblende-feldspar-mica gneiss, locally in- posed of fluorescent limestone with green truded with pegmatite, which lies between the pyroxene (optically near diopside), brown pyroxene zone and the central core of the ore pyroxene (optically near augite), magnetic body. Because it is of no economic importance, franklinite of low zinc content, biotite, and it has not been studied in detail. The core of relatively sparse garnet. These minerals are the ore body is normal graphitic white lime- distributed in bands and closely resemble the stone. banded, disseminated ore in texture. Data are insufficient to reveal whether or not these Calamine-Mud Zone bands are tightly folded in the same manner as the black willemite ore. Replacing part of the limestone core as well This zone appears to be an important and as parts of the surrounding gneiss and pyroxene integral part of the mineral deposit. The high zones in the upper levels of the ore body is a magnesian content of the pyroxene phase (see huge cavity filled with mud. The cavity paral- Palache, 1935 for typical analysis) seems to lels the hanging wall of the west limb and that contradict Ridge's (1952) suggestion that the of the cross member south of its thickened ore fluids were low in magnesium content. part. It extends 680 feet beneath the surface. At the contact between this limestone pyrox- At the surface in plan it looks much like a ene-franklinite rock and the ore of the brown pair of spectacles. The material filling the willemite zones, particularly noticeable in the cavity appears to be transported in part and in hanging wall of the lenses beneath the east part residual. It appears to be zinc bearing in limb, is a narrow zone (±1 inch thick) of some degree. rhodonite and franklinite. When seen from a In the mine the roof of a cross cut appeared distance underground, it is difficult to see to be brown-stained, very coarsely crystalline where ore ends and rock begins. On close in- marble containing disseminated franklinite. spection it is obvious that the texture and the Blocks 3 feet in diameter and more, bounded amount of franklinite remain the same, and by intersecting joints such as are common the silicates range from willemite in the ore, throughout the Franklin Limestone, displayed through rhodonite, to pyroxene. excellent diffusion rings, obviously controlled The second pyroxene zone forms a rough by the joints. Cleavage rhombs and twinning cylinder between the ore and the gneiss zone. traces were clearly visible. On close inspection Except at its contact with the gneiss, it is the "marble" proved to be claylike in consis- mineralogically the same as its counterpart to tency and, on drying, crumbled to dust. Chemi- the north. At that contact, coarse garnet, cal analysis of the material after removal of a pyroxene, biotite, and gahnite crystals are de- small amount of franklinite showed only 0.54 veloped, products of the reaction between the per cent CaO and 1.71 per cent MgO. In the limestone and adjacent sedimentary gneiss at same cross cut a layer of hemimorphite several the time of raetamorphism. Such "skarn" minerals are typical of gneiss-limestone con- feet thick rests on the footwall of the cavity. tact zones throughout the area. The hemimorphite crystals contain mud inclusions, and mud is found interstitially in the porous mass. In some places the banding in the Franklinite Zone marble is retained in the mud, in others the This zone is composed of highly magnetic, mud appears to have been transported. low-zinc franklinite sparsely disseminated in Other minerals composing the mud as identi- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/69/6/775/3431868/i0016-7606-69-6-775.pdf by guest on 01 October 2021 DESCRIPTION OF THE STERLING HILL ORE DEPOSIT 781

fied by X ray are goethite, kaolinite, sericite gradually returns, and then the green color type, and nontronitic type. returns. As yet there are few mine openings in the mud, and the distribution of the various min- Microscopic Franklinile Inclusions erals is not known well. The black willemite and tephroite and red The problems posed by the existence of the willemite and tephroite are colored by minute mud zone are numerous, and too little is known 1-10 micron inclusions. These inclusions have to speculate on its origin. It is surrounded by not been found in any of the pyroxenes, calcite, mine workings in solid rock, and, until it was penetrated by diamond drilling from beneath, or other gangue minerals of the ore body. Several sets of thin sections were prepared it appeared to be completely water tight. The from samples collected in series across the zone bottoms about SO feet below sea level. Circulating ground water is therefore an im- contact zone between the black and brown to red willemite in the thickened part of the cross probable agency of weathering. member. Although Ries and Bowen (1922, p. Finger (1950) suggested that sulfuric acid derived from blende acted in the formation of 543) attributed the color of black willemite to inclusions of franklinite, and that of red wil- what was then known of the mud zone, i.e., lemite to zincite, the authors found that both two pits at the surface outcrop of the ore. However, the authors have studied the un- varieties were colored by franklinite inclusions. Willemite, regardless of its apparent color in altered marble directly beneath the zone on the 700 level, as well as in other places in the mine, the hand specimen, is transparent and colorless in thin section. Black willemite, in thin and cannot find evidence of sufficient sulfides section, is crowded with opaque black l-to-10- to account for such a massive corrosive attack. micron inclusions, many of which display good octahedral form (although most are anhedral). CAUSES OF VARIATION IN WILLEMITE These are thickly scattered in planes and, PHYSICAL PROPERTIES locally, in what appear to be irregular or curved, apparently healed cracks. At first, this distri- Color Differences bution pattern was thought to indicate forma- There are striking differences in the color tion by exsolution from iron-bearing willemite. and magnetic properties of the willemite. Be- Accordingly, samples of coarse-grained Sterling cause of the zoning of these properties in the black willemite (received from L. H. Bauer in ore body and the possible genetic significance 1947) were annealed in nitrogen as follows: of the variations, the writers attempted to Thin section discover their causes. number Treatment Willemite occurs in three colors, green, red, 2196 Untreated and black. The green variety predominated in 2197-1 5 hours at 985°C. the Franklin ore body, although considerable 2197-2 5 hours at 985°C. red willemite was taken from the upper parts 2226 100 hours at 1050°C. of the mine near the keel in the early years of 2378 63 hours at 1300°-1350°C. the mining operation. Black willemite, if Some slight change in the color of the inclu- present, was rare. At Sterling Hill the primary sions may have taken place at the lower willemite is either red or black, and the only temperatures, but even at 1300°-1350°C., the green willemite is secondary. About one-third inclusions had not dissolved but had grown. of the willemite in the ore body is black. Exsolution, then, is probably not the source of the inclusions in black willemite. Fluorescence Red willemite in thin section is seen to contain spinel (franklinite) inclusions distributed The typical green color of willemite from the in the same fashion as those in the black. These Franklin mine is apparently a green fluorescence spinels, however, are transparent and ruby, and induced by ultraviolet wave lengths in day- although generally rounded, many display good light and common artificial sources. When octahedral form. These apparently are the in- heated to a temperature just below incandes- clusions mistaken by Ries and Bowen for cence, green willemite becomes grayish white, zincite on the basis of color. Where zincite has and there is no ultraviolet fluorescence. On been observed in such fine particles, it is yellow cooling, the ultraviolet-induced fluorescence rather than red.

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The brown willemitc typical of regions of The friedelite patterns were identical with gradational contact contains a mixture of red patterns obtained from known samples. The and black spinel inclusions. Its shade depends serpentinelike inclusions show one diffraction on the relative proportions of black and red pattern, with recurring minor differences which particles. indicate three members of the serpentine The color variations of tephroite (roepperite) family. One of these patterns closely resembles are also due to these spinel inclusions dis- the pattern obtained from a specimen of tributed in fractures typical of the olivine chrysotile from Thetford, Quebec. The re- group. A few slides showed concentrations of maining two patterns are like the first but less inclusions in the tephroite, whereas the asso- like the chrysotile. ciated willemite was free of inclusions. Such The inclusions most like chrysotile were inclusion-free willemite in hand specimens ap- from one of the large pale-flesh willemite pears as a glassy, dirty, brownish-green mineral. crystals. These inclusions were sufficiently Willemite and tephroite also occur in paler large to afford optical data. These data, as re- shades of gray and flesh pink. These shades are ported by L. A. Herrmann, are as follows: due to the sparse scattering of inclusions. The foregoing description has considered only the equidimensional, rounded to octahedral Biaxial positive Uniaxial or biaxial posi- inclusions. Other thin sections, from large tive with small 2V single crystals of red and flesh-colored wille- Length slow Length slow mite, revealed isotropic transparent red rod- Fibrous Fibrous like inclusions 1-10 microns thick and of n/3 Slightly greater n/3 Very slightly less variable length. These are continuous with rods than 1.56 than 1.54 of a colorless mineral of lower index than the willemite. The two minerals contact in a The optical properties of chrysotile (Rogers featherlike manner. Further study proved that and Kerr, 1942) follow: the red material is franklinite and the colorless mineral is chrysotile. Biaxial positive Thin-section work indicated that the in- 2V = 0-50° clusions were spinels; more specific identifica- Length slow tion necessitated the separation and concentra- Fibrous tion of the inclusions from the willemite. This n|3 1.504-1.550 was accomplished by dissolving the willemite in 5 per cent HN03 and centrifuging the residue. These colorless inclusions have not been After repeated washing with distilled water identified definitely because the authors do and additional centrifuging, a residue clean not have access to exactly comparable refer- enough for X-ray-diffraction studies was ob- ence patterns, and the particles are, for the tained. most part, too minute to permit optical de- In developing the method for separation and terminations or to permit separation for concentration, it was found that black inclu- chemical analysis. It is safe to say, however, sions suspended in liquid could be attracted by that they are all members of the serpentine an Alnico hand magnet. The rounded and family. octahedral red inclusions were not attracted by In addition to the serpentine inclusions, in- the magnet. The rodlike red inclusions, how- clusions of friedelite (a hydrous chlorosilicate ever, possessed polarity, and, although they of manganese) were identified in many of the were not attracted by the magnet, their orienta- franklinite concentrates from willemite. In tions could be altered by movement of the some cases, this was present to the exclusion of magnet. serpentine varieties, and in others it was pres- Powder X-ray studies of the concentrated ent with them. colored inclusions (all from willemite) proved that all, regardless of color or form, are frank- SPINEL (FRANKLINITE) LATTICE linite. MEASUREMENTS

Serpentine Inclusions The method of determining the cell size of the ferrite samples was not elaborate. The d In the course of the powder X-ray work on values for the lines the inclusions, it was found that all were asso- (511) (731) (333), (440), (533), and (553) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/69/6/775/3431868/i0016-7606-69-6-775.pdfciated with serpentinelike minerals or friedelite. by guest on 01 October 2021 SPINEL (FRANKLINITE) LATTICE MEASUREMENTS 783

were measured from a metal scale made up for cedures in some cases were not of high pre Cu radiation. No correction was made for cision. film-length variation. Each of these d values A synthetic 1-to-l zinc-manganese ferrite was used to calculate an ao value using ao = sample (ZnO-MnO-(Fe2O3)2) gave a sharper FERRITE LATTICE DIMENSIONS

MNO • FE O3 AT 8.589 DONNAY AND NOWACKI 0954)

RED INCLUSIONS 8.50 A

GROSS FRANKUNITE (GC-19}

ZnO • MNO CFE203)2 (HYLAND W> I.

FR-27 GROSS FRANKLINITE FR-6 GC-21, I 2 NO • FE203 - 8.45 A

BLACK INCLUSIONS

GROSS FRANKLINITE (GC-18) = BLACK INCLUSIONS

MAGNETITE 8.40 A

FIGURE 2.—FERRITE LATTICE DIMENSIONS

+k2 +12. The arithmetic mean of the X-ray pattern than the others, and the lattice four ao values thus determined was used as the constant was determined by the extrapolation cell size. It was found that the standard devia- procedure described by Henry, Lipson, and Wooster (1951, p. 191). This result for ZnO- tion of this determination was 0.01 lA. This MnO-(Fe2O3)2 (Fig. 2) compares satisfactorily value was determined from data on four with the value determined by the simpler pro- samples, two run in duplicate and two in cedure described above and used for routine triplicate. More elaborate determination of the cell-size determinations in this work. cell size was not warranted, since the quality of Figure 2 lists the values for the lattice con- the X-ray patterns of these iron-bearing stants of several ferrite materials. The measured Downloaded frommaterial http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/69/6/775/3431868/i0016-7606-69-6-775.pdfs was poor, and the sampling pro- lattice dimensions have been plotted on a by guest on 01 October 2021 784 METSGER, TENNANT, AND RODDA—STERLING HILL ZINC DEPOSIT

linear scale in angstrom units. Multiple listings will be determined by the interaction of all on the plot are for different samples of the three ions, Fe++, Mn++, and Zn++. same type. Values shown on the plot include: The transparency and redness of the inclusions with the largest cell sizes might be (1) Red inclusions recovered from willemite samples expected to correlate with increased zinc and (2) Black inclusions recovered from willemite manganese contents, since zinc chemicals are samples typically transparent, and manganese chemi- (3) Gross franklinite I—Large franklinite particles cals are typically pink or red. Increased trans- occurring adjacent to either red or black parency observed microscopically correlates willemite. The inclusions from these wille- with increasing zinc content of franklinite mite samples were recovered and measured. samples. (4) Gross franklinite GC or FR—Mine location of The ionic radii of these ions are Fe++ = these samples not known. They are ore Zn++ = 0.74 and Mn++ = 0.80 (Green, 1953). franklinite samples on which analytical data The large value of Mn++ is consistent with the were available. indication that it results in larger cell sizes. ++ (5) ZnOFe2O3 (synthetic zinc ferrite) The indicated increase by Zn substitution ++ ZnO-MnO-(Fe2O3)2 (Synthetic 1-to-l zinc- for Fe must be due to a larger effective ionic manganese ferrite) size of zinc in the spinel environment than that Fe3O4 (Magnetite sample from the bottom of indicated by its listed equivalence to iron. This the north shaft, Sterling) seems possible since the Zn++ and Fe++ occupy (6) MnO-Fe203 Value obtained from Donnay and different lattice sites in the spinel in accordance Nowacki (1954) with the inverse structure for FeO-Fe2O3 and the normal structure observed for ZnO-Fe203. RESULTS OF LATTICE MEASUREMENTS Analytical data and lattice dimensions were obtained for a series of gross franklinite samples The plotted cell-size data (Fig. 2) show a (Fig- 2). striking distribution. The magnetite standard Calculations based on various assumptions has the smallest lattice dimensions. Somewhat have been made involving possible valence larger, yet still close to magnetite, are the states, ionic radii, and structural considerations black microscopic inclusions from black wil- in spinel-type materials in an attempt to cor- lemite. The macroscopic franklinites are next relate composition and lattice dimensions. The in increasing size. The largest lattice dimensions proximity of Mn, Fe, and Zn in the periodic are exhibited by the microscopic red inclusions table and the resulting similarity in properties from brown or red willemite. reduces the effectiveness of calculations, as The small spread in the cell size of the gross does the lack of precision of the lattice-dimen- franklinite samples is remarkable when con- sion measurements. trasted with the variations of the microscopic The microscopic inclusions recovered from ferrite inclusions. The average dimension for willemite crystals were not pure enough or in gross franklinite is fixed in the range near sufficient quantity for chemical analysis. Hence, 8.46A. sampling was made of gross franklinite, on The color and magnetic properties of the which analysis would be possible immediately ferrite apparently correlate with the lattice adjacent to both red and black willemite. changes. The darker, low-spaced samples are Lattice dimensions were determined for the more magnetic, and the red, high-spaced gross franklinite samples and for the red and materials are less magnetic. black inclusions from the willemite, with the A progressive increase in lattice dimension hope that correlation of cell size of correspond- is observed in the measured values for the ing samples could be established, and hence some information might be indirectly obtained samples FeO-Fe2O3, ZnO-Fe^s, ZnO-MnO- (Fe20 ) , and MnO-Fe O (Fig. 2). This sug- on inclusion composition. These values are 3 2 2 3 designated as / on Figure 2. No correlation gests that the same composition variations exists between lattice dimensions of such in- cause lattice expansion in the natural materials clusions and adjacent large franklinite particles. from the mine. The lattice dimension probably These materials are in proximity in the mine, increases as manganese or zinc progressively —that is, a gross franklinite grain might con- replaces iron in Fe3O4. The increase is greater tact a willemite grain which could include the for manganese substitution than for zinc. It microscopic ferrite inclusions. The dimension follows that the lattice dimensions of samples variations observed are probably due to varia-

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tions in chemical composition of the various would have been thrown down as the silicate ferrite samples. fayalite. If the ferrous iron had actually ex- These variations support the conclusion that solved as an oxide, the annealing experiments the ferrite inclusions in willemite are a product should have resulted in its resolution into the of post-ore alteration, probably serpentiniza- willemite lattice. tion. It seems significant that the franklinite inclusions are confined primarily to the ortho- PARAGENESIS OF THE MACROSCOPIC silicate minerals, willemite and tephroite, and ORE MINERALS are not found in the pyroxenes. Also, they are missing in the calcite associated with those The determination of paragenesis of the ore silicates. The distribution of the inclusions in minerals is complicated by the wide distribu- fractures and cleavage planes of the silicates tion of secondary occurrences. Thin sections of indicates that they are of more recent origin the massive or gneissic type of ore, in which all than the silicates. As the willemite is considered the ore minerals are in contact, have indicated to have replaced calcite, if the inclusions existed that willemite precedes franklinite, tephroite, before willemite, they should also be present in and zincite. This is in disagreement with Ries the calcite. and Bowen (1922, p. 546) who say, The apparently universal co-occurrence of the franklinite inclusions with inclusions of the "In the very few sections where the two silicates hydrous silicates serpentine (H4Mg3Si2O9) and (willemite and tephroite) were found in association, friedelite H7(Mn, Cl) Mn4SiAe suggests that they occurred as granular intergrowths which indi- these minerals are genetically related. Such a cated contemporaneous formation, though it is relationship is easily visualized. possible that the tephroite slightly antedated the Iron-bearing olivines when partially ser- willemite". pentinized are altered along their cleavage planes and fractures, and microscopic grains of In the sections studied, the interstices of the magnetite-type material have been found dis- rounded willemite grains are filled with frank- seminated in the serpentine within those zones. linite and/or tephroite. The former tends to In some completely serpentinized minerals, it is wedge in between the willemite grains; the possible to infer the nature of the original latter, in many cases, peripherally replaces the mineral by the pattern of distribution of the willemite and may replace all but a few small magnetite-type grains. The pattern of distribu- remnants of a grain. Zincite, too, appears to be tion of franklinite inclusions in tephroite is interstitial and has been observed thus far only similar to the pattern of magnetite-type dis- in thin sections containing tephroite. tribution in olivine. The pattern in willemite is This association of zincite with tephroite, less similar because of its crystallographic dif- and the latter's relation to willemite suggests ference from olivine. the possibility that manganese solutions, in Bowen and Tuttle (1949) reported on ex- replacing the zinc in willemite to form tephroite, perimental serpentinization of iron-bearing resulted in the formation of zinc oxide which olivines. Some black, magnetic iron oxide was has been thrown down as zincite. found. They concluded that this was not pure This paragenetic sequence applies only to iron oxide but had the composition (FeMg) primary ore minerals. There is at least one O-Fe203. later generation of willemite, zincite, and This serpentinization (or a similar process franklinite. A hypothesis involving the forma- producing friedelite) then seems to be a possible tion of secondary zincite and franklinite follows. mechanism for the production of franklinite inclusions. GENESIS OF INCLUSIONS The authors believe the silicate ore minerals were formed at great depth and at high tem- Although the distribution of inclusions in peratures (whether deposited from hydrother- willemite suggests origin by exsolution from mal solutions, or by metamorphism of a pre- the silicate lattice, this has been ruled out on existing zinc-manganese-iron deposit). Phase both theoretical and experimental grounds. studies on the serpentinization reaction re- Theoretically, it is improbable that iron would ported in the literature indicate that serpentini- exsolve from a silicate lattice as an oxide phase zation occurs only at relatively low tempera- such as franklinite. Had ferrous iron exsolved tures. Also, since serpentine is a hydrate, the from the willemite lattice, it more logically period of mineralization that produced the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/69/6/775/3431868/i0016-7606-69-6-775.pdf by guest on 01 October 2021 786 METSGER, TENNANT, AND RODDA—STERLING HILL ZINC DEPOSIT

inclusions of franklinite and serpentine and/or products is far in excess of the volume of the friedelite must be comparatively recent and initial minerals. This increase must be ex- must have occurred at temperatures and pres- plained if the serpentinization has taken place sures below those prevailing during ore forma- on a large scale. tion. The temperature and pressure must have In spite of the probable complexity of the remained low, or the serpentine would have reactions in nature, a reaction similar to that been destroyed. This would place the period of described in the above equation may have inclusion formation in the late Precambrian or taken place. In the Sterling Hill mine, there is post-Algonkian, long after the folding of the a rather large mass of rock having the appear- ore body. A post-Algonkian period of magnes- ance and texture of normal red willemite- ium mineralization which resulted in the dolo- franklinite ore disseminated in limestone. mitization of the limestone may also have been However, in this case the red mineral is anti- responsible for the formation of the inclusions. gorite, colored by a more than normal abun- The inclusion-forming solutions were either dance of red franklinite inclusions. The asso- simple magnesian solutions, or they contained ciated macroscopic franklinite is normal in magnesium, iron, manganese, and zinc. appearance, and the limestone matrix is made The black willemite has been described as up of interlocking grains of calcite and dolomite. composing a band that varies greatly in thick- At a distance of the order of 15-20 feet from the ness and is apparently intricately folded paral- serpentinous mass is an unusually large ir- lel with the structure of the west limb and the regular mass of fine granular zincite with cross member. In all cases it lies within a few franklinite, sparse willemite, and almost no inches of, or in gradational contact with, the carbonate. The rock is generally brecciated. red or brown willemite of those structures. It is therefore suggested that the antigorite If the inclusion-forming solutions contained was originally willemite containing iron and all the metal ions necessary to produce frank- manganese, which was acted upon by magnesian linite, serpentine, and friedelite by reaction solutions. In the initial stages of the alteration, with the silicate minerals, it seems inconceivable manganese and iron oxides and zinc were re- that color variations in the ore would coincide leased. These combined to form the highly with the folded structure of the ore body. It insoluble franklinite inclusions that remained seems more likely, therefore, that the mineral- within the grains. When all the manganese and izing solutions were of simple composition, iron in the willemite lattice had been removed, perhaps containing only magnesium as the the remaining zinc, by far the most abundant active metallic ion, and that the other metal metallic ion of the three, was released as ZnO ions were derived from the silicates themselves. (zincite), a stable form. Because it is much A reaction to produce serpentine from wil- more soluble than franklinite in a carbonate lemite would require simple magnesian solu- environment, it was transported from the tions. The reaction might be written: immediate vicinity and precipitated some distance away. 2Zn2SiO4 + 3Mg++ + SH2O Total serpentinization of the willemite, ac- 4ZnO companied by dolomitization of the limestone, implies that the circulation of magnesian solu- The hydrogen ion would react with the carbo- tions was sufficient to alter the silicates com- nate of the host rock. pletely. It is not presumed that the reactions were The hypothesis suggested to explain the as simple as stated above. For example, wil- formation of franklinite by the action of lemite from Sterling Hill in general contains magnesian solution on willemite may be ex- appreciable manganese in solid solution. It has tended to explain the differences in the in- been established experimentally from other clusions and the co-occurrence of black and red unpublished work that willemite can also con- inclusions in the same crystal. tain ferrous iron in solid solution. The presence Magnesian solutions acting on willemite con- of admixed franklinite, however, has made it taining considerable iron and manganese in impossible to determine whether or not iron is solid solution would, according to the proposed present in natural willemite. These impurities hypothesis, produce serpentine and franklinite. could complicate the reaction and result in If the iron and manganese in the willemite vary formation of a mixed ferrite as the oxide in a regular fashion areally, the franklinite product. product of the serpentinization reaction would In the equation given, the volume of the also vary in composition. As the serpentiniza- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/69/6/775/3431868/i0016-7606-69-6-775.pdf by guest on 01 October 2021 GENESIS OF INCLUSIONS 787

tion proceeds, willemite crystals with a certain further explanation. Either the manganese initial balance of manganese and iron could at content of the inclusions is more or less uniform first react to give ferrite crystal inclusions with in all franklinites and the valence state differs, a composition producing a dark color. As the or there are varying concentrations of man- reaction continues, the initial balance of iron ganese. Impurity of the inclusion concentrates and manganese in the willemite crystals may has prevented checking the latter. As to whether alter. This change in balance might then result the valence of manganese varies in different in the production of ferrite inclusions of simi- franklinites, it seems impossible to check be- larly altered composition of different color. cause of the co-occurrence of manganese and This may explain the willemite peculiar to the iron in the same mineral and the oxidation- brown-black contact zone where ferrite in- reduction reaction that takes place between clusions of both color types occur mixed in them when the mineral is dissolved in acid. single crystals. It follows that, if such a compo- Friedelite has been detected as a common sition variation did exist in the unaltered associate of franklinite inclusions, in places willemite in a gradational fashion across the almost to the exclusion of serpentine. A reac- willemite body, inclusions of only color, black tion such as that described previously for the or red, should be found on either side of the serpentinization of willemite could be written brown-black contact, as is the case. with manganese substituted for magnesium. If the serpentinizing process continues to a The reaction would perhaps take place under stage in which manganese and iron are no similar environmental conditions, and the longer available in the willemite, then zincite metallic oxide inclusion products would be the rather than the ferrites begins to form in addi- same. It is improbable, however, that both tion to the serpentine. Such zincite inclusions processes could have continued simultaneously. have been observed, but only in the pale flesh- Thus far, insufficient facts are available to colored to red willemites. reconcile the two processes. The variation in cell size of the inclusions The foregoing generalized hypothesis can with color as determined by powder X ray explain certain features of the ore that seemed qualitatively supports this suggestion of vary- irreconcilable. If the microscopic inclusions ing composition. were considered to be emplaced in fractures of The writers suggest that, in the case of wil- the silicate minerals at the time the macroscopic lemite containing inclusions of franklinite and franklinite was formed and thereby to be serpentine, the solutions were present in directly related to it genetically, it was difficult quantities and concentration sufficient only for partial serpentinization. In such a case, the to account for the discrepancies in lattice reaction did not progress past the stage of dimensions between gross franklinite and franklinite formation and left the willemite microscopic franklinite from the same hand essentially intact. If this is true, some masses specimen. It was even more difficult to explain of willemite should be unaltered. Such masses the existence, side by side, of black and red do exist, notably in the thickened part of the franklinite, and in some cases, zincite, in the cross member. The willemite in these portions same willemite grain. The serpentine hypothe- of the ore body is a dull brownish green. sis seems to clarify the problem somewhat. The X-ray data on franklinite indicate a wide variation in lattice dimensions. The black REFERENCES CITED magnetic inclusions have cell dimensions in the vicinity of 8.42-8.44A and are only slightly Bowen, N. L., and Tuttle, P. F., 1949, The system larger than those of magnetite 8.40A. The red MgO-SiO2H20: Geol. Soc. America Bull. v. 60, p. 439-460 nonmagnetic inclusions have cell dimensions in Donnay, J. D. H., and Nowacki, W., 1954, Crystal the vicinity of S.SOA. Gross franklinite has been data: Geol. Soc. America Mem. 60, 719 p. found with cell dimensions ranging from those Green, J., 19S3, Geochemical table of the elements of the black inclusions through about 8.47A for 1953: Geol. Soc. America Bull. v. 64, p. 1001-1012 but this far have not been found to match the Hague, J. M., Baum, J. L., Herrmann, L. A., and red inclusions. Pickering, R. J., 1956, Geology and structure The experimental measurement of the ferrites of the Franklin-Sterling area, New Jersey: indicates that manganese, and zinc to a lesser Geol. Soc. America Bull. v. 67, p. 435-474 Henry, N. F. M., Lipson, H., and Wooster, VV. A., extent, control the ferrite lattice variations. 1951, The interpretation of the X-ray diffrac- Accepting bivalent manganese as the control- tion photographs: New York, Macmillan and ling factor, there are two alternatives for Company, 258 p. 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Palache, C., 1935, The minerals of Franklin and Ries, H., and Bowen, W. C., 1922, Origin of the Sterling Hill, Sussex County, New Jersey: U. zinc ores of Sussex County, New Jersey: S. Geol. Survey Prof.'Paper 180, p. 1-135 Econ. Geol., v. XVII, p. 517-571 Finger, A. W., 1950, Geology of the Franklin- Rogers, A. F., and Kerr, P. F., 1942, Optical Sterling area, Sussex County, New Jersey, p. mineralogy: 2d. ed., New York, McGraw- 77-87 in Dunham, K.C., Editor, Symposium Hill, 390 p. on the geology, paragenesis, and reserves of the ores of lead and zinc: 18th Internal. Geol. THE NEW JERSEY ZINC COMPANY, FRANKLIN, Congr., 1948, Great Britain, Rept. pt. 7, NEW JERSEY; THE NEW JERSEY ZINC COMPANY, 400 p. PALMERTON, PENNSYLVANIA; THE NEW JERSEY Ridge, J. D., 1952, Geochemistry of the ores of ZINC COMPANY, PALMERTON, PENNSYLVANIA Franklin, N. J.: Econ. Geology, v. 47, p. 180- MANUSCRIPT RECEIVED BY THE SECRETARY OF 192 THE SOCIETY APRIL 29, 1957

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