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The best of the older studies are those of Kemp (1893a) and Spencer et al. (1908), which also summarize earlier works.
In an early description of the spectacular zincite outcrop of the Sterling Hill orebody, Alger (1845) observed that “the red oxide of zinc forms a prominent ridge, or wall, along the side of the hill, ... considerably above the adjoining bed of pure franklinite, appearing thus to have resisted the disintegrating process by which immense quantities of the franklinite have been crumbled into a loose gravel, which so covers the ground as to destroy vegetation.” Alger, a generous writer, illustrates an adit driven into Sterling Hill by John Hitz on behalf of interested French investors and goes on to describe the ore beds, stating “that there is, in fact, only one bed - this being made up of three continuous parallel divisions. When we have penetrated the outcrop of limestone at Sterling, on the east side of the hill, we first come to an outer belt of from three to seven feet [1-2 meters] in thickness, consisting of nearly equal parts in bulk, of red oxide of zinc and franklinite. Second, in close contact with the mixed ore, but no where blended with it, we meet a seam of brownish-black ferruginous limestone, very heavy, but easily disintegrating, being a coarse crystalline variety; this is from two to six inches [5-15 centimeters] thick. Third, we reach a bed of pure franklinite, more or less crystallized, sometimes perfectly so, ... this bed is ten to twenty feet [3-6 meters] in thickness.”
In 1854, J. D. Whitney noted that “A portion of the rocks covering the ore-bed on the face of the hill has been removed, to a depth of about 70 feet [21 meters]...The outer bed, thus exposed, is a mixture of the red oxide of zinc with franklinite; its width at the surface was about 3 feet [1 meter], but it widens out to 8½ feet [2.6 meters] in descending. Next to this is the bed of franklinite, which is from 20-30 feet [6-9 meters] in width” (Whitney, 1854b).
The Sterling Hill deposit is wholly enclosed in the Franklin Marble, without the close proximity relations to magnetite bodies, the Cork Hill Gneiss, and the Cambrian-Ordovician rocks as were noted for the Franklin deposit. The Sterling Hill deposit is distant from the Cork Hill Gneiss, about 600 feet (183 meters) away according to Metsger et al. (1958) or about 1000 feet (305 meters) away according to Pinger (1950) (Figures 8-1, 8-2, and 8-3). Another sharp distinction is the relative absence of extensive pegmatite, such as was found at Franklin, although a few small ones were noted here. The north end of the deposit is about 2 miles (3.2 kilimeters) south of the south end of that at Franklin. The distances reported in the literature vary considerably because of a lack of agreed reference points in early times and the substantial underground northeastward extension of the Sterling Hill deposit. The only modern published studies of the geology are ones by Metsger et al. (1958, 1969), Metsger (1962, 1977, 1979, 1980, 1985, 1990), and Stephens et al. (1988), all from which much of this section is drawn; unpublished studies by Metsger et al. are listed in the bibliography. Compared with the Franklin orebody, the subject of a vast literature, relatively little geologic detail has been published concerning the Sterling Hill orebody, partly as a result of New Jersey Zinc Company policies. The written record is spotty at best.
According to Squiller and Sclar (1980), the Sterling Hill deposit, like that at Franklin, was probably exposed in the Precambrian; no proof is known at Sterling Hill. Mineralogically and chemically, the Sterling Hill deposit is markedly less diverse than that at Franklin. Sterling Hill is the general case, and mineralogically and chemically it is the simpler of the two deposits. This is discussed below, in part, and in the introduction to the silicate minerals given later. As at Franklin, the host marble has a 5-6 foot (1.5-1.8 meters) graphite-free zone surrounding the orebody.
The Sterling Hill orebody, like that at Franklin, is incomplete; the known orebody is the uneroded remnant of a larger, original ore mass and, additionally and importantly at Sterling Hill, an eastern portion has been faulted off at depth and lost. This portion is known in local parlance as “The Lost Orebody.” The New Jersey Zinc Company invested heavily in exploration drilling to find this part of the Sterling Hill orebody for general prospecting purposes (Baum, 1947; Stockwell, 1952a) and to define other geologic relations; these efforts are in part described by Johnson (1931), Baum (1950, 1954a, 1954b), Stockwell (1952a, 1952b), and others.
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| Figure 9-7. Idealized geologic sketch map of the Sterling Hill orebody in plan section. The N-arrow is for “Franklin North.” Scale not given, but Johnson et al. provided one. Illustration from Metsger et al. (1958). | ||
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Unlike the relatively linear, planar, and simply folded Franklin ore deposit, that at Sterling Hill, although having some similarities in structure and ore minerals and a hook-like form in plan and cross-section, is much more complex. It has been very extensively folded, fractured, and deformed. According to Metsger et al. (1958), the deposit “is an intricately folded, steeply plunging body in Precambrian, graphitic, sparsely silicated, coarsely crystalline, white marble. The ore is wrapped around a central core of graphitic marble and an annular gneissic cylinder of mica, feldspar, hornblende, pyroxene, and garnet composition.” Metsger et al. (1969) described the configuration of the orebody as a “complex series of attenuated isoclinal folds which modify the west limb of a larger, but similarly attenuated, isoclinal syncline. Structures in both the ore and the infolded wallrock demonstrate extreme plastic deformation during folding. The structural complexity of the orebody is much greater than that of the surrounding region...” They also noted that “the ore distribution [pattern] in plan is similar to its pattern in section, owing to the 45-degree plunge of the orebody.” Johnson (1990) employed the term “synform” instead of syncline.
The structural complexity of the deposit precludes any simple description. An idealized, geological, plan-projection map of the orebody, given by Metsger et al. (1958), is shown in figure 9-7. Like the Franklin deposit, that at Sterling Hill is an isoclinal syncline conformable with the banding in the marble. There are two major limbs, the longer of which is the east limb (front limb) and the shorter the west limb (back limb), both of which dip east at 55o. Ries and Bowen (1922) noted that the west limb is much longer at depth than suggested by its short surface exposure. They also noted that the limbs are somewhat buckled, as if from compression; this was observed at depth. The limbs are connected; in addition to a keel connection as at Franklin, they are connected through a cross-member, rich in iron on the east side and thicker in its central portion, which joins the middle parts of the east and west limbs. A discussion of the principal structural elements of the orebody follows.
Metsger et al. (1958) reported that “the ends of the two limbs, the keel of the syncline, the thickened part of the cross-member, and the latter’s junctions with the two limbs plunge roughly parallel at 45o, N. 70o E.” Ries and Bowen (1922) reported that the keel is far from being smoothly descending, but pitches steeply in places and is nearly flat in others. Spurr and Lewis (1925) plotted the position of the keel and provided several diagrams. In such discussions, the word “keel” refers to the main fold at the southern end of the orebody, as traced at depth (Figure 9-7). This distinction is necessary because of numerous other features within the orebody, which locally resemble keels and have been called such by miners and geologists. These include the bifurcations of the west limb, the north-sloping surface of the cross-member, and the several fold-backs in the east limb on the 1850 and lower levels.
The long east limb is the principal linear feature of the orebody (Figure 9-7). In addition to its extensive linear outcrop, it extends to great depth; remnants of its northern extent are found on the 2550 level. It is first intersected and truncated by the Zero fault (discussed below under “Faulting”) at about the 1500 level. Below the 1680 level only the portion north of the cross member remains; the rest was faulted off. The east limb, with increasing depth, is continuously shortened in length by this fault until, near the 1850 level, it folds back on itself, forming a false keel. This reverse fold is continued, compressing with depth, to its termination. The concentration of zincite, the “zincite band,” on the outer (eastern) side of the limb is an important feature.
At the extreme northern end of the orebody is an area of the east limb cut off from the rest of the orebody by the Zero fault. It is a physically distinct area, located between the 1850 and 2550 levels, and is referred to as the “north orebody.” The Zero fault truncated the orebody to this depth, permitting much replacement of primary minerals by Mg-bearing solutions. Much sussexite, serpentine, sphalerite, and hematite also were found here.
The west limb varies in length from approximately 500-900 feet (~152-274 meters). The northern half of the west limb is bifurcated into several branches; this split occurs near the cross-member, and the bifurcation, athough abrupt, forms another false keel (Figure 9-7). It is first truncated by the Zero fault near the 1600 level, and this truncation continues until the west limb is but a small remnant on the 1950 level. The concentrated zincite band is on the outer (western) side of the limb.
A cross-member, formerly known as the east branch of the west limb, extends from just south of the mid-point of the principal west limb to the mid-point of the east limb (Figure 9-7). It is much thicker in its central part. On the west side, it is comprised of normal ore (see “central zincite zone” below); on the east side it is composed mostly of black willemite ore (see “black willemite zone” below). The cross-member, highly irregular in shape throughout its vertical extent, extends from the surface to approximately the 1850 level, at which point it has been completely faulted off by the Zero fault. In addition, some informal older nomenclature was confusing. Until the early 1950’s, the term “east limb” was sometimes used for the east limb only south of the cross-member, while the term “east branch of the west limb” was applied to all of the cross-member and to all of the orebody north and east of the cross-member (R. W. Metsger, personal communication).
The outcrop at Sterling Hill was known to Kemp (1893a) to extend 1000 feet (305 meters) on the east limb, curving 300 feet (91 meters) through the elbow of the fold, and continuing 475 feet (145 meters) on the west limb. Kemp provides a very good description of the known 1893 relations of the Sterling Hill orebody, few though they be. By 1908, the exposures were noted as 1500 feet (457 meters) on the east limb and 600 feet (183 meters) on the west limb, with a total outcropping of 2200 feet (670 meters) including the join of the limbs at the south end (Spencer et al., 1908). Pinger (1973-74) noted that the orebody outcrop was 1700 feet (508 meters) in length with a total orebody length of 2600 feet (792 meters), and that it extended to a depth of 2500 feet (762 meters), twice that of the Franklin deposit.
As at Franklin, erosion, dating perhaps from the Precambrian in part, and the effects of more recent glaciation, preclude any clear estimate of the original size of the orebody. The sketch map published by Metsger et al. (1958) is without scale; Johnson et al. (1990) added a scale to this figure. Spencer (1909) reported the width of the limbs at 10-30 feet. The specific overall dimensions of the Sterling Hill orebody are formally unpublished as of this writing. Exploratory drifts were developed to search for extensions of the orebody and to provide drilling platforms for further exploration. One of these was on the 1850 level and was driven 1500 feet northward from the area above the north orebody; it reached into the Borough of Franklin by 300 feet. Another such drift was driven southward on the 1680 level from the keel of the orebody for a length of 500 feet, trending along and then near the Zero fault (R. W. Metsger, personal communication).
Very little has been published concerning the internal structure of the Sterling Hill deposit, aside from the general description by Metsger et al. (1958). The structure is exceedingly complex, a consequence in part of severe deformation. Available published information, very limited in scope, suggests that the Sterling Hill deposit does not have the lenticular, subparallel, conformable layers of ore and calcium silicates found at the Franklin deposit. It is markedly different. The relative paucity of calcium silicate species at Sterling Hill in general, as noted in the section entitled “The silicate minerals,” indirectly supports these limited observations. In the gangue, tephroite and Mn-humites are common, but rhodonite is generally sparse, and hardystonite is absent, relative to Franklin.
Metsger (1977, 1980) noted that “Certain distinctive graphite-phlogopite-chondrodite-pyrrhotite bands in the marble wall rock and a fluorite band at the contact between ore and marble as well as recognizable bands within the ore itself suggest that the ore was at one time stratiform.” Johnson (1990) discussed more specific studies by R. W. Metsger which established “the lateral continuity of the strata (1) by tracing distinctive bands, including a thin fluorite-bearing band, some 1968 feet (600 meters) down the plunge of the synform, and (2) by establishing that the deposit has a macroscopic stratigraphy which continues through both limbs of the synform.” Johnson (1990) further noted that modal abundances and mineral compositions vary substantially from layer to layer within the deposit, but are relatively uniform within individual layers.
Although of minor importance at Franklin, faulting has played a major role at Sterling Hill, where at least two significant faults have affected the orebody. The larger of these is the Zero fault.
The Zero fault is a major fault trending N. 35 E. through the Wallkill Valley and is nearly vertical to a depth of at least 4000 feet (1.22 kilometers) (Hague et al., 1956; Stephens et al., 1988; Metsger, 1990). It is located approximately 1100 feet (335 meters) to the east of the orebody at the upper levels, initially truncates the orebody at approximately the 1500 level, and then truncates the orebody more severely at greater depths. It was named the Zero fault because of its proximity to the zero E-W coordinate of the Sterling Hill coordinate system. The Zero fault was described by Metsger (1990) as being marked in the mine “by a vertical or nearly vertical zone of intense shearing which ranges up to as much as 25 feet [7.6 meters] in thickness. Its shistose layering is lubricated by heavy black smears of graphitic gouge which sharply delineate the fault against the light-gray to white rocks on either side.” The character of the grain-orientation within this shear zone was described by Stephens et al. (1988). Metsger (1990) also noted that “a zone of intense crackle-brecciation along the west side of the shear zone is bounded on its west side by a much less prominent serpentine shear at a distance of from 50 to 100 feet (15-30 meters).”
The Zero fault was intensively studied by the New Jersey Zinc Company in a search for the eastern portion of the orebody which was faulted off. The fault locally is likely east side down, suggesting that the faulted-off part of the orebody, “The Lost Orebody,” might lie at depth, but it was never found in spite of deep drilling (Metsger, 1990). Hague et al. (1956) have noted that some shear patterns in the two sides of the Zero fault are incompatible with the “east side down” fault movement, and have suggested that there might have been several periods of movement. Additional studies of the specific problem of the missing ore segment were made, one of which resulted in a unpublished study by Stockwell (1952b). He used the Median Gneiss as a horizon marker, did not prove or disprove the existence of the missing segment, and suggested further exploration in the vicinity of the 4300N coordinate at a depth of 4550 feet or along a zone extending southward.
Within the orebody, the Zero fault is accompanied locally by extensive replacement of pre-existing minerals by magnesium-bearing solutions, which entered the orebody utilizing the Zero fault as a channelway. Dolomitization of the Franklin Marble and calcite in the orebody is very common in proximity to the Zero fault; willemite and tephroite are commonly serpentinized, especially below the 1850 level in the north orebody, where they may be accompanied by sussexite, hematite, and sphalerite.
The Sterling Hill orebody is also displaced by approximately 40 feet (12 meters) by a lesser fault, the Nason fault, which lies barely east of the orebody at the surface and “migrates” westward through the orebody with increasing depth. Its effects are evident to at least the 1680 level.
There is a pervasive cross-strata fracturing at Sterling Hill, and Johnson (1990) has attributed this to a period some 900-933 million years ago, supported by K-Ar dating of biotites. Johnson noted several replacement textures associated with these fracture systems, such as willemite after tephroite, and hematite+spessartine after willemite. He also noted the removal of zincite from strata located near fractures, especially in the outer zincite zone.
Metsger et al. (1969) suggested that “following initial folding of an original Zn-Fe-Mn-rich sediment, the orebody with entrained wallrock (48 x 106 metric tons; density of 3.02 g/cm3) moved downward through the marble (density of 2.71 g/cc3) as an inverted diapir, at the peak of metamorphism, producing the observed folds.” This was the first explicit suggestion of a sinking hypothesis, although many writers had previously discussed movements of the orebody. Skinner and Johnson (1987) cite an unpublished study by Metsger suggesting that the Sterling Hill orebody sank through the marble at a rate between 102 and 104 cm per million years. They also discussed the morphological features which numerous high-grade metamorphic deposits have in common. By extension, some of the same arguments can be applied to the Franklin deposit.
As described by Metsger et al. (1958), the Sterling Hill ore deposit is wrapped around a gneissic cylinder which, in turn, surrounds a core of graphite-bearing marble. They describe the orebody in terms of zones of particular mineralization as illustrated in figure 9-7; these are:
Outer zincite zone
Central zincite zone
Black willemite zone
Brown willemite zone
Pyroxene zones
Franklinite zone
Gneiss zone (“black rock”)
Johnson (1990) noted that “the lithological sequence established by Metsger et al. (1958) corresponds to a chemical sequence of (1) silica-poor ores with Zn>Fe>Mn (outer zincite band), (2) ores richer in silica (willemite + franklinite + calcite band), and (3) silica-rich carbonates with Fe>Mn@ Zn (calc-silicate band).”
The outer zincite zone, one of two spatially distinct zincite-bearing zones, comprises the outer part of the major east and west limbs and the keel of the orebody, and it varies in thickness. The general metal concentrations are Zn>Fe>Mn, according to Johnson (1990). The ore, consisting of franklinite, willemite, zincite, tephroite, and calcite, varying in relative proportions, is primarily of two types: disseminated grains in calcite and gneissic sheets or lenticular areas without calcite. This zone becomes very disseminated at its northern extent until it has no economic value.
Disseminated ore is composed of equant coarse-grained (~4 mm diameter) crystals. Zincite is present but sparse and disseminated; it resembles shards in the ore and is found only with a tephroite-group mineral which appears to replace willemite (Metsger et al., 1958). The contacts with marble are sharp.
Gneissic ore, some occurring within the disseminated variety, has equigranular willemite and franklinite with grain size less than that noted above. Zincite is sparsely disseminated, and calcite occurs as augen-like 2-8 cm conformable inclusions.
A band of concentrated zincite, intimately associated with high-Zn franklinite and 6-8 inches (15-20 cm) thick, occurs in the footwall of the west limb and within a foot of the hanging-wall contact of the east limb. This is the remarkable and visually striking feature referred to in historical reports of separate beds. It has exceptionally high tenor, and it was mined as the first zinc ore.
The central zincite zone is midway between the southern and northern ends of the orebody. It forms the western part of the cross-member of the orebody, where it branches off the west limb to the northeast and connects in a greatly thickened middle part with the black willemite zone (Figure 9-7). The contact with the west limb is blunt and not gradational, and the zone is continuous to the Zero fault. The ore of the central zincite zone is mineralogically similar to that of the outer zincite zone, with but subtle differences. There is no zincite band. A limited amount of dull green or colorless nonfluorescent willemite, in gneissic ore nearly identical to that from Franklin, occurs here.
The black willemite zone comprises the eastern part of the cross-member (Figure 9-7). To the eye, the black ore of the black willemite zone is grossly unlike all other Sterling Hill (or Franklin) ores. In addition to apparently black willemite (with franklinite inclusions which color it black), the ore contains black fayalite (roepperite), loellingite, franklinite, and some sphalerite as associated minerals. Zincite is absent; graphite is present. Gradations of brown willemite are found wherever the black willemite zone contacts common red willemite. The black willemite ore has also been found as lenses in the pyroxene zone, and near the junction of the cross-member and the west limb, as a thick mass at the northern end of the west limb, and in several other areas as indicated by Metsger et al. (1958) (Figure 9-7). The zone is continuous to the Zero fault and has folds on a scale of hundreds of feet. Both fresh and altered material from this zone were studied by Makovicky and Skinner (1990). Sphalerite and associated minerals from this zone were studied by Davis (1993), who proposed that the zone had been affected by a post-metamorphic deformational event.
The brown willemite zone was described by Metsger et al. (1958) as follows: “About halfway between the thick part of the black willemite zone and the junction of the cross-member and the east limb, the black ore thins and grades into a dark-brown to red-brown willemite-franklinite ore containing no zincite.” This ore has a texture similar to that in other zones. Brown willemite ore is also found in lenses parallel to the east limb and under its footwall (Figure 9-7). In general, the brown willemite ore is gneissic internally, with disseminated ore at the margins.
The term “pyroxene zones” was used by Metsger et al. (1958) to describe the calcium silicate units at Sterling Hill. There are two such zones, largely unstudied (Figure 9-7). They are similar in texture and mineralogy, but occupy different spatial positions in the orebody.
The first of these zones is located beneath the footwall of the east limb, extending north from the cross-member, and terminating at the north end of the east limb. The second pyroxene zone forms a crude cylindrical shape surrounding the gneiss zone which in turn surrounds the marble core of the orebody. Within this second pyrozene zone, at the contact with the gneiss, coarse-grained gahnite, garnet, pyroxene, and mica occur. These pyroxene zones are scientifically important, but uneconomic parts of the orebody. The pyroxene zones include much calcite associated with diopsidic to augitic pyroxene, biotite, little garnet, and low-Zn franklinite. This zone is banded, similar in texture to the ore. Additional discussion is given by Reilly (1983) and Johnson (1990).
The franklinite zone is in the east limb, near the keel, and wraps around the keel near the surface, entering the west limb in part. This zone is wholly in the east limb at greater depths. It contains the highly magnetic, low-Zn “franklinite” used as an iron ore in the 1876-1882 period.
The gneiss zone is composed of a fragmented feldspar-mica gneiss containing pyroxene or hornblende and some pegmatite intrusions (Metsger et al., 1958). It lies between one of the pyroxene zones and the graphitic marble core of the orebody (Figure 9-7), but has been little studied; the gneiss fragments are rotated relative to each other. Johnson (1990) found the gneiss to be Zn-free (<0.1 wt. % Zn), but noted that zinc occurs in the micas, amphiboles, and spinels present in what are assumed to be metamorphic reaction zones; he also obtained a K-Ar date of 883 (± 35) million years for the gneiss. Although not mentioned by Metsger et al. (1958), the large curving hornblende pegmatite described by Spencer (1908) is very likely part of the gneiss zone, which is quite variable locally.
Metsger (1977, 1979, 1980, 1990) described a karstification-produced rubble-breccia which overprints the ore in part, as well as part of the host marble. This breccia zone extends from the hanging wall of the east limb above the 700 level downward through the 900 level, and within the west limb and the east branch of the west limb from above the 1400 level downward to the 1850 level. It could not be traced deeper due to a lack of rock exposures (Metsger, 1990). In the host marble, the breccia consists of fragments of Franklin Marble cemented by dissolved calcite; in many places the breccia cement is similar to the Kittatinny Dolomite. In the ore units, the breccia consists of ore fragments identifiable as being from different parts of the deposit, cemented by a solidified residue of granular willemite, franklinite, and zincite in various proportions. Sphalerite forms part of the cementing media in the deeper parts of the orebody. The rubble breccia is found in close proximity to the Nason fault, which might have provided a mechanism for the fracturing (Metsger, 1990). Fossil shell fragments in the breccia date this to Phanerozoic time (Metsger, 1990; Johnson, 1990), in keeping with observations of a breccia in the Franklin deposit (Ries and Bowen, 1922).
The mud zone at Sterling Hill was described by Metsger et al. (1958) and Metsger (1979, 1990). It consists of a deeply weathered, large, columnar cavity, penetrating from the surface to a depth of 680 feet (207 meters). At depth, between the 180 and 430 levels, the mud zone was oval in plan section, more than 300 feet (91 meters) long and 200 feet (61 meters) wide. On the 500 and 600 levels, the mud zone was seen as a dumbbell-shaped opening, narrower, but as long as observed on the higher levels. The mud zone parallels the hanging wall of the cross-member south of the thick part, and the hanging wall of the west limb. According to Metsger, it has formed by the replacement of parts of the gneiss zone, the pyroxene zone, and the marble core and contains materials both residual and locally transported. It consists of a black-to-brown zinc-bearing mud; material from the 500 level was found to contain 35 wt. % Zn (Metsger, 1990). The mud was studied by Metsger et al. (1958) and found to be comprised of hemimorphite, goethite, kaolinite, and nontronite in part. The mud zone is also host to a number of manganese oxides, such as chalcophanite, hetaerolite, woodruffite, birnessite, and others. A halo of copper mineralization surrounds the mud zone at a distance of 15 feet (4.5 meters), in unaltered rock (Metsger, 1990).
According to Metsger (1990), the surficial manifestation of the Sterling Hill mud zone is the presence of two pits, the Noble and Passaic pits, also known generally as the “calamine pits.” These pits are well-known sources for fine hemimorphite specimens and giant crystals of franklinite, pyroxene, spinel, and spessartine. Near the surface these famous pits are filled with mud and contain much natural debris and mineral matter. Pinger (1950) noted the presence of sphalerite within the mine in the keel areas where these pits formed and postulated that the hemimorphite may have been formed from locally abundant sphalerite, rather than from willemite-franklinite-zincite ore. The present physical extent of the open cut is larger than the area enclosed by the limbs of the orebody; the Passaic pit was enlarged in the twentieth century, particularly on the east side, to obtain fill-rock for use in the mine.
The pits were described by Darton (1883) as “...two high basins, two hundred feet [61 meters] in diameter, and about 80 feet [24 meters] in depth.” (These were later referred to as the North Basin and the South Basin within the New Jersey Zinc Company’s twentieth century operations.) Darton went on to say that “...Both [pits] were half filled with dirt when found, but under this was a thick bed of franklinite; and under this the calamine.” Darton described one of the hemimorphite occurrences as “a cylinder forty feet long, two to three feet in diameter, and with walls about two to four inches in thickness of a pure white color, lying upon an incline up the basin, evidently at one time a water course.” He also described augite (jeffersonite) crystals a foot long. However, Darton’s report is of limited usefulness. Kemp (1893a) reported the great pit to be an open cut 50 feet (15 meters) deep, 100 feet (30 meters) on an E-W axis, and 75 feet (23 meters) on a N-S axis.
Spencer (1908) noted, “In the natural state of the ground at Sterling Hill, just where the large open pits of the Passaic Mine are now, the ore limbs were crossed by a broad swale 20-30 feet deep draining to the Wallkill Valley. Under this basin-like depression was found a deposit consisting in part of loose franklinite gravel, and in part more or less decomposed franklinite and willemite crystals cemented by calamine and smithsonite. The manner of occurrence indicates that this material was derived from the breaking down of the surficial portion of the main orebody, the minerals of which were freed by solution of the calcite in which they were embedded, and at the same time were partially dissolved to furnish solutions from which the secondary zinc minerals were precipitated.”
Palache (1935) gave a (1906?) description of these pits provided by O. J. Conley, who was superintendent of the Noble Mine in 1878; it is reprinted here.
“The calamine [hemimorphite] formed a layer 6 to 12 inches think, lying directly on the limestone. The principal filling of the excavated mass was more or less fragmental, consisting of sand, clay, limestone fragments, and loose and broken crystals of franklinite, willemite, garnet, and the like, all stained by oxides of iron and manganese. Separating this loose material from the calamine layer on the north side of the pit was a layer, as much as 4 inches thick, of greasy black mud, rich in manganese, which was the cause of dangerous slides in the pit. On the south side, in a similar relation to the calamine, were found the deposits of chalcophanite and hydrohetaerolite characteristic of this locality. Excellent specimens of the calamine are preserved in collections, and nearly all those examined showed considerable harsh brown or yellow clay adhering to their lower surfaces.”
Because very little has been formally published about the minerals of the mud zone, a report by Baum (1962b) is here given partially:
”The mud zone has an irregularly oval pattern, and is perhaps most contorted in shape on the surface, where it forms somewhat of a dumbbell in outline, one knob representing the southern or Noble pit of the open cut, and the other the Passaic pit close by. The Noble pit...produced a small amount of calamine [hemimorphite]... The Passaic pit and mine...was the source of most of the showy calamine. The old timers failed to follow the mud zone to any depth, and their penetrations of it underground apparently did not disclose the same type of calamine found at the surface, if indeed they recognized any at all.
“At the present time [1962] there are about six mineral [collecting] localities associated with the mud zone, four of them on the surface. In the Passaic pit is a remnant of the original mud, the source for much of the chalcophanite and hydrohetaerolite in modern collections. Here were collected the reported todorokite and woodruffite, and here also originated specimens of goethite (limonite) containing 42.7% iron and 0.3% manganese, with virtually no zinc. These, occurring close to sparkling white to clear calamine, show how completely the processes of alteration responsible for the mud acted to separate the elements of the original ore.
“Between the Passaic and Noble pits is a smaller pit, almost a cave, in which the contact between the gneissic “black rock” and the white marble shows remnants of the noted coarse crystals of jeffersonite [augite] and feldspar. From here and similar situations close by came king-sized crystals shown in Palache [1935] including large garnets, apatites, hornblendes and the like. These were the residual contact minerals, most of them found floating in the mud.
“The Noble pit is less productive, but small jeffersonite [augite] crystals are found, and coarse pyroxene masses contain galena and its alteration products.
“Underground, the 500 foot level is noted for its clay-encrusted calamine, which is vastly inferior to the old surface specimens, but attractive nonetheless. The mineral occurs as porous layers on the inclined floor of what may have been an open fissure now clay-filled and overlain by a thick sequence of mud apparently produced in place and fairly high in zinc.
“On the 420 foot level the mud zone contains areas of relatively unaltered rock, so that a passageway encounters repeated rotten zones, one of them in part open, and on the wall of which a giant franklinite crystal teases the visitor. Here one zone contained weathered ore, the calcite removed and the franklinite and willemite grains cemented in a cindery mass by velvety iron and manganese oxides, mainly chalcophanite. Other zones show where alteration had attacked coarse franklinite and tephroite to produce cryptomelane as a dull, black, dense, and very hard mineral, and chalcophanite as a shining, feathery- appearing but fairly compact mass. These have been identified at the Zinc Company research facilities, as has a faintly greenish-yellow powdery coating found to be nontronite..., derived apparently from the alteration of tephroite, which releases manganese and leaves a porous siliceous residue, some of it yet retaining the delicate willemite inclusion [exsolution] pattern characteristic of tephroite.”
A second saprolite zone was described by Metsger (1956, 1979, 1990) as overlying the orebody to the east and “occupying a basin, about twelve hundred feet (366 meters) deep in the bed rock below the floor of the Wallkill Valley.”
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| Copyright © 1995 by Pete J. Dunn |
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