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In an early recording of the Franklin outcrop, Fowler (1836) stated “The [franklinite] bed here is about one hundred feet [30 meters] above the adjoining land, on the west side of it, and from ten to forty feet wide.” The bed was informally described by “A Jerseyman” in 1849 as “being of great thickness and standing almost like a wall on the side of the hill, upwards of two hundred feet [~60 meters] high” (newspaper article reprinted in The Picking Table, 36, #1, 30-31). These estimates of the outcrop of the ore suggests it stood out boldly, as did that at Sterling Hill. Alger (1845) describes the then-visible outcrop by stating “the outer bed nearest to the valley, is the immense bed of pure franklinite, the inner one being the mixed red oxide [zincite], thereby rendering the mining operations less favorable here than at Sterling.” Alger goes on to note that “the most singular feature in these beds...is the beautiful and perfectly distinct separation observed between them, along the whole line of their contact.” The east limb was not discovered until 1852 (Palache, 1935).
There was a long period of confusion regarding the separation of beds. The historical “separate beds” viewpoint, although now discredited, must have been based on some persistent and thoroughly misleading exposures; the viewpoint was pervasive in the early literature.
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| Figure 9-1. Projections of deposits at Franklin and Sterling Hill from Baker (1881) who adapted them from Cook (1868). Top and center figures are of Franklin (plan and longitudinal sections, respectively). Bottom figures give plan-, longitudinal-, and cross-sections at Sterling Hill. Scale given in the original, for the Franklin deposit, shows this exposure of the west limb to be 1560 feet (475 meters); for the Sterling Hill deposit sections, a different scale given in the original shows the middle section to be 500 feet (152 meters) long from left edge to right edge. Arrows are for true north. | ||
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Spencer (1908) addressed the subject by describing the zincite zone at Sterling Hill as occurring in the hanging wall of the east limb, which was 2-10 feet (0.6-3 meters) thick and then mined for 700 feet (213 meters) on the dip of the limb, and occurring on the footwall of the west limb, which was mined for 200 feet (61 meters) on the strike and 100 feet (30 meters) on the dip of the limb. At Franklin he noted the occurrence of zincite at shallow northeast workings on the west limb and said that “these are the only instances exhibited [in 1908] in the mines of so definite and persistent a separation of the ore minerals.” Early maps of the Franklin and Sterling Hill orebodies are shown in figure 9-1.
The Franklin orebody is wholly encased in the Franklin Marble. The stratigraphically younger Cork Hill Gneiss, which does not have folds like those in the orebody, underlies the west limb in the upper part of the deposit by 30 feet (9 meters). The Furnace Magnetite Bed occurs within marble in this interval; it is from 3 to 8 feet (~1-2.5 meters) in average thickness and is described above in more detail under magnetite deposits. The gneiss, the magnetite bed, and the host marble with the included zinc orebody are overturned in the Franklin and Ogdensburg areas; their present relations are shown in figure 9-2.
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| Figure 9-2. Plan section (top), longitudinal section (middle), and vertical sections (bottom) of the Franklin orebody. Map coordinates north-south and east-west are on the Franklin Mine coordinate system; the zero-zero-zero coordinate is at the Parker Shaft. Scale is in feet. Longitudinal projection is projected on an arbitrary north-south vertical plane. Adapted from maps by A. W. Pinger and C. H. Stockwell, New Jersey Zinc Company, 1951. Illustration taken from Frondel and Baum (1974) and reproduced from Economic Geology, 1974, Vol. 69, p. 161. | ||
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The north end of the Franklin orebody was exhumed in the late Precambrian and is unconformably covered with Paleozoic rocks. These overlying younger rocks, the Hardyston Quartzite and the Kittatinny Limestone, are found in abundance more to the west of the orebody. Although there are no large saprolitic units such as exist at Sterling Hill, there were buried weathering profiles from the 300S to the 500N coordinates (Figures 3-21 and 9-2). Gossans were observed at this previously exhumed ore surface by J. L. Baum and J. M. Hague, geologists of the New Jersey Zinc Company, but there is no published information on this weathered area.
The Franklin orebody has been cross-cut by enormous, black, nearly vertical dikes at its southern end. The dikes are massive, striking features, in bold and stark contrast to the white marble. These dikes, now exposed in the Buckwheat Open Cut, were first described by Emerson (1882) as a micaceous diabase. He described the largest of these, 20-22 feet (~6.5 meters) in thickness, by study of thin-sections, and he reported it having in part a pseudo-amygdaloidal texture, the “amygdaloids” being what he called xenoliths of willemite melted from the ore by the intrusion, with franklinite, zincite, and calcite locally abundant. There are no other reports of the contact effects between the dikes and the ores. His samples may have been of highly localized material, but no studied specimens are known. Other eruptive rocks, mostly from the western part of the county, were described by Wolff (1896).
The largest of these dikes completely cut both limbs of the orebody at about 600 feet (182 meters) north of the big fold at the elbow of the deposit, and at 2100 feet (640 meters) south of the Parker Shaft (Figures 3-4, 3-6, 3-8, and 9-2). Although the east limb flexes where the dike crosses the orebody, and passes underground on the north side of the dike, this flexure is unrelated to the subsequent geological activities that generated the dikes; it is coincidental. Wolff (in Spencer et al., 1908) mentioned that there were 18 other such dikes nearby.
The rock of this dike was described and referred to as a minette lamprophyre by J. P. Iddings in Diller et al. (1898), as camptonite by Wolff in Spencer et al. (1908) and by Palache (1935), as a kugel minette by Milton (1952), and as a mica-diabase rock by Frondel and Baum (1974). It is considered to be post-Ordovician, perhaps Silurian or Triassic in age, but has not been recently studied. It is a dull-black rock, fine-grained, with chilled margins; xenoliths of gneissic ore occur within sections of the dike.
Other dikes in the general region, perhaps Silurian in age, were described by Milton (1947); these are markedly different from the dikes which cut the Franklin deposit. Ries and Bowen (1922) reported some post-Ordovician dikes in the Sterling Mine, and Spurr and Lewis (1925) also noted a basic dike, said to be identical with those at Franklin, in the open cut at Sterling Hill; this might have been an orebody feature.
Pegmatite bodies, varying in size from stringers 1 x 24 inches (2.5 x 60 centimeters) to larger masses of 15 x 400 feet (4.5 x 122 meters), occur in the orebody, but have no significance for the genesis of the ores; they postdate the folding of the orebody. Those in the ore are considered to be late Precambrian in age; they may be genetically related to a period of regional pegmatite crystallization about 965 million years ago (Grauch and Aleinikoff, 1985). In the early literature they are generally referred to as syenite or granite. Venuto (1953) and Hague et al. (1956) classified these pegmatites as potassic and sodic. Others have been found, fragmented and folded, within the Franklin Marble (Spurr and Lewis, 1925); their age is unknown.
The potassic pegmatites in general are younger, discordant, and form larger units. The main concentration of such pegmatites is in the footwall of the west limb, toward the northern end of the deposit, especially north of the Palmer Shaft pillar. Pegmatites occur here and in the keel and hanging wall as numerous small and irregular bodies. Such pegmatites invade the Cork Hill Gneiss, the Furnace Magnetite Bed, and the Franklin Marble, and penetrate the footwall of the orebody (Venuto, 1953). Where they intersect ore units, they do so at 5 to 10o and generally follow the synclinal structure of the orebody.
These potassic pegmatites are chiefly composed of microcline and quartz, and also contain allanite, zircon, biotite, muscovite, epidote, apatite, plagioclase, magnetite, titanite, and rarely thorite. They vary substantially in color from white to gray, brown, red, or green.
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| Figure 9-3. Double Rock, a big pegmatite body, is shown in the center of an excavated portion of the west limb at Franklin. Photograph courtesy of the Franklin Mineral Museum. | ||
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An enormous, green-microcline-bearing potassic pegmatite, several hundred feet on strike with the orebody, with vertical attitude, and a maximum width of 100 feet (30 meters), occurred 400 feet (122 meters) south of the Trotter Shaft, near the southern limit of the Trotter Mine. Mining men called it a “horse-in-the-ore.” At the surface, it was located from 1697S-1747S and approximately 800W-850W; on the 400 level, it was located from 1535S-1660S and 563W-572W. The prominent outcrop of this pegmatite (Figures 9-3 and 3-17) was known as “Double Rock.” Material from this pegmatite body is high in rare-earth elements, most of which are present in allanite. Microcline crystals from here may attain 7 cm in size and some are equant. The occurrence was described by Ries and Bowen (1922).
Contacts of pegmatite with ore and/or marble produce reaction-products locally. The pegmatite- marble-gneiss contacts were discussed in detail by Venuto (1953), who noted that the reactants at marble contacts include scapolite, grossular, pyroxene, and other minerals. Many early descriptions mentioned substantial amounts of fluorite near such contacts.
Pegmatite may invade fracture zones in the ore, and foliation and banding in the ore may be affected by distortion and bowing for a distance of several feet from the contacts. Various workers, including Kemp (1893a) and Wolff (1903), have reported a “hardening” of the ore in the vicinity of pegmatite contacts; this has also been reported to this writer by Franklin miners. At such contacts, the pegmatite may contain xenoliths of ore, garnet, or magnetite, and disseminated blebs of rhodonite and franklinite. Farrington (1851b) noted that franklinite was more highly magnetic near contacts with syenite [probably pegmatite]. Reactants at franklinite ore contacts include andradite, commonly with rhodonite and other minerals. Ries and Bowen (1922) reported ore-pegmatite reactants to be indicated by the presence of hardystonite, rhodonite, leucophoenicite, jeffersonite [augite], gahnite, and garnet [andradite]. Most importantly, they noted these effects to be limited to the “vicinity of the pegmatite-ore contact, and not in other parts of the Mine Hill orebody.” Many writers have reported magnetite in the pegmatites; it is likely, but not certain, that some of these magnetite occurrences are xenoliths brought up from the underlying Furnace Magnetite Bed, which was similarly invaded in its northern extent. These relations are unstudied in detail.
Venuto (1953) provided a very specific description of the general case: “As the ore contact is approached from a pegmatite, rhodonite and blebs of franklinite rimmed with garnet and sometimes quartz became numerous. Often these occur in long streaks and give a gneissic appearance to the rock. The microcline becomes progressively greener as the ore contact is approached. At the contact with the ore a garnet [andradite] - red willemite skarn almost inevitably occurs. When the intruded ore is composed of willemite and franklinite, a concentration of franklinite occurs along the borders of the rhodonite-garnet [andradite] zone. It is succeeded by a zone rich in willemite and then by typical ore.” Locally, the willemite of the ore is commonly changed to a red color, perhaps colored by thermal effects (Frondel and Baum, 1974). Some specific examples and reactants were discussed by Frondel and Baum (1974), but much direct in situ evidence is unpublished.
Frondel and Baum (1974) commented on the theories, including Palache’s (1935), that invasive pegmatites had supplied B, Be, Pb, Cu, As, F, Cl and other elements to the orebody; they reported that there is “neither geological or mineralogical support” for this hypothesis, and that “transfer of material at the pegmatite contacts has been a minor and local effect, chiefly involving Si, Al, Fe, and Ca.”
Frondel and Baum’s statement is very likely accurate for most of the intrusive pegmatites of small to moderate size, but both Spencer et al. (1908) and Palache (1935) attributed much recrystallization and metasomatism to the enormous pegmatite near the Trotter Shaft (described above). Spencer noted “that the silicate minerals containing essential proportions of manganese and zinc were formed by [contact, implied] metamorphism due to the pegmatite is shown by the fact that they do not occur throughout the ore mass, but only along or near contacts with the pegmatite.” In this writer’s opinion, based only on examined specimens, there is a strong possibility that such effects were indeed present in the orebody under admittedly very localized conditions, contributing anomalous assemblages like the nickel arsenides. Similarly, in the north end of the Franklin Mine, near the Parker and Palmer Shafts, where pegmatites were the most abundant, there is an extensively recrystallized and hydrated area; here the lead silicates and numerous other anomalous minerals were found. For at least these two specific areas, this writer holds out the possibility of direct and extensive infusion of pegmatitic solutions. Other explanations have not been proposed.
The older sodic pegmatites occur mostly in the gneiss and the magnetite bed, in lit-par-lit formations, and as an interface between the gneiss and the potassic pegmatites; only two were found in the orebody (Venuto, 1953). They are composed of albite-oligoclase and quartz, with accessory biotite and hornblende. The two in the orebody also contained mica and garnet.
The evaluation of New Jersey Zinc Company (NJZC) maps requires special caution: many of the abundant, primary, conformable, light-colored, and feldspar-containing calcium-silicate units of the orebody were classified, as were also the locally abundant true pegmatites, as pegmatite (Pt. in NJZC abbreviation notation) on such internal maps. Distinctions between true pegmatite and calcium-silicate units were not generally made. Only after the closing of the Franklin Mine, less than a year after Venuto’s study, and after the detailed studies of hyalophane and hendricksite by Frondel and others in the 1966-1968 period, was this problem generally illuminated, explaining perhaps Pinger’s (1948) ambiguity in discussing the relative age of the pegmatites. Thus, some such mapped units are of uncertain composition and/or origin. Clearly, of the two possibilities for such “pegmatite” designations, calcium-silicate units were predominant. True pegmatite was less abundant volumetrically and was generally concentrated at the footwall zone in the northern end. However, these invasive pegmatite masses were very numerous, some were massive, and their effects are not to be dismissed easily.
Other mapping designations used by the NJZC geologists are similarly open to varying interpretation. The term ore was used to designate economically recoverable parts of the orebody, and this varied greatly, not only between Franklin and Sterling Hill, but temporally as economic conditions varied and as the technology of mining and processing changed over the years. Such mapping did not use the word “ore” in the purely mineralogical sense.
The term tephroite (Tft) was used to designate much material which visually resembled willemite but did not fluoresce in ultraviolet, especially at Sterling Hill. Tephroite so mapped was not necessarily the species tephroite; some was likely sonolite or alleghanyite. Similarly, the term chondrodite (Cdt) was used for all humite-group minerals in the marble, not specifically for the species chondrodite; mica (Mc) was used for all dark-colored micas; and skarn (Skn) was used for many rock units containing much garnet and other calcium-silicate minerals.
The Franklin deposit is the uneroded remnant of a larger orebody. It is a trough-like open syncline, the unequal limbs of which form the two major surface exposures historically referred to as the front vein (the west limb) and the back vein (the east limb). The two limbs of the orebody are connected through a common fold, which is seen as the nose of the syncline and the keel; Spencer (1908) noted the length of the keel to be greater than 3500 feet (1067 meters). The greatest contiguous mass of ore was found in the keel. The axis of the synclinal fold plunges 25o NE (Hague et al., 1956). It is an elongate syncline, and its outcrop is hook-shaped in plan section (Figure 9-2); this hook-like aspect is also seen in cross-section due to folding around the keel, but is less well developed. According to Frondel and Baum (1974), “toward the northern end of the mine the plunge of the keel flattens and then reverses slightly to give a basin-like structure;” see also Spencer (1908). The join of the two limbs of the orebody at its southern end was not naturally exposed and was not discovered until the end of the 19th century.
The matter of the east limb being overturned on itself, forming a compressed pitching anticline, was first reported by Bemis (1885). Although his thesis was reported by Kemp (1893a) to be lost, it was found in 1988 and read by the writer. Bemis premised his argument on numerous factors, some invalid, and obviously did not investigate the matter in detail. Although his experience, observations, and logic were limited, he deserves full credit for this report of the morphology of the east limb. There are few other similar records of this part of the deposit; evidence was either not gathered or it was lost. Extensive mining of this exceedingly rich ore obliterated all the areas needed for confirmatory studies. Bemis’s idea was accepted by Nason (1894a, 1894d), who compared relative bed thickness in the limbs. Kemp offered the following points, among others, in support of the existence of the east limb anticline.
a) The ore here is twice the thickness observed in the west limb.
b) As it pitches underground toward the north, the crest of the limb shows a strong arch.
c) Sections of the orebody by Kemp show that this anticlinal feature extends far to the north underground.
Spencer et al. (1908) largely ignored the matter, and Ries and Bowen (1922) expressed some mild skepticism, but did not refute it; they might have misinterpreted a part of Kemp’s presentation of Bemis’s imprecise argument, which concerned radial cracks filled with willemite at the crest of the limb.
According to Frondel and Baum (1974), the crest of the east limb, “as it passed underground, plunged essentially parallel to the keel of the fold. At the nose of the syncline, to the south, the fold was relatively open and the upper part of the eastern limb was overturned and dipped steeply to the east. In the northern part of the mine, the eastern limb stood nearly vertical and the fold [between the two limbs] became tight, the two limbs being separated by only a few feet of marble and ultimately apparently came into direct contact. The west limb of the fold rather uniformly dips 55o to the east.” They also reported that “the crest of the east limb as it plunged underground was smoothly rounded, conformable with both the banding in the adjacent marble and, it is said, with the ore banding within the limb.” This fold was illustrated by Kemp (1893a) and suggested by Nason’s (1894a) drawings. The footwall contact of the orebody with the Franklin Marble is sharp; the hanging wall contact, although also sharp, is banded. The nature of these large concordant bands was discussed by Frondel and Baum (1974); one of them was of lean franklinite- calcite-willemite ore, 85 feet (26 meters) from the orebody, and traced for 150 feet (46 meters) along the orebody, from 730S to 880S coordinates.
The Franklin orebody, unlike that at Sterling Hill, was predominantly in one contiguous unit. Frondel and Baum (1974), however, mentioned that a “nearly separate lens of ore approximately 800 feet (244 meters) long on strike, 350 feet (107 meters) on dip, and up to 80 feet (24 meters) thick occurred adjacent to the hanging wall of the west limb.” It was “connected to the west limb by a thin band of lean mineralization” and is shown on figure 9-2 (longitudinal section) as the area northward from 735N coordinate. They indicated that it “may be due to folding, but the geologic relations are obscure.”
Faults, many with slickensides, were common in and near the Franklin deposit, but were relatively minor in scope. Joints, principally at N. 30o E., are often host to secondary minerals, such as common calcite and serpentine, sphalerite, willemite, pyrite, dolomite, galena, and others. Ries and Bowen (1922) described several areas where movement took place on faults. One is 400 feet (122 meters) north of the Palmer Shaft on the 130 level, where coarse breccias have resulted and dense ore is broken into fragments and mixed with blocks of Kittatinny Limestone; one such ore fragment was a block 9 feet (2.7 meters) square.
Pinger (1973-74) noted that the total outcrop was 2500 feet (762 meters) long and the total orebody length 5000 feet (~1.5 kilometers). The long west limb of the Franklin orebody was 1215 feet (370 meters) deep and mineralogically, but not economically, about one mile in length. The original surface exposure of the west limb was approximately a half-mile (800 meters). As measured by Palmer in 1903, the west limb exposure was 2970 feet (905 meters). Spencer, in 1908, indicated it was 2800 feet (853 meters), beyond which it was overlain by the Cambrian-Ordovician rocks. There is some ambiguity in these earlier measurements.
The west limb was not mined for its full extent. As is generally common in mining operations, the end of what was called “ore” was determined not by the geological occurrence of ore minerals but by economic factors, because the orebody became lean and poor in grade as it thinned out to the north; contacts in all other directions were sharp. The “economic terminus” of the Franklin orebody was at about 900N, nearly under the present intersection of Hudson and Sterling Streets in Franklin. However, thin stringers of franklinite, willemite, and manganoan calcite were found further north. The extent of this mineral-bearing area was determined by an exploratory drift driven north on the 750 level; it encountered ore minerals at least as far north as 2480N (Figure 3-21). This is in part supported by Stockwell’s (1951) observation that the Furnace Magnetite Bed extended to at least 2420N.
The west limb extended to its maximum depth toward the north end of the orebody; see figures 9-4 and 9-5 for a more detailed view of the irregular nature of the base of this limb. The west limb varied in an undulating and uneven manner from 10 to 100 feet (3-30 meters) in thickness, at right angles to the dip. Ries and Bowen (1922) reported a thickness of 200 feet (61 meters) near the keel, north of the large camptonite dike, on the 800 level at 1020S coordinate. This is consistent with all other reports of greatly increased thickness in the keel. Earlier reports, in particular by Nason (1894a), gave many measurements and calculations of bed-width, but these were in part inconsistent. However, in general, now that the mine has been fully exploited, it can be seen that the great variation in thickness precludes any accurate general estimation of overall average width.
The shorter east limb of the orebody extended for approximately 3100 feet, tapering from the surface, where it was exposed for only about 600 feet (Figure 9-2). The overall thickness varied considerably, as in the west limb. In the southern area, the east limb was up to 80 feet thick; this was likely due to the fact that the southernmost part of the east limb was, as noted above, a doubled layer, having folded back on itself. The thickness of the east limb in the more northern part of the mine was roughly comparable to that of the west limb, or less in places.
The Franklin orebody is layered internally. These layers are conformable and of two general types: (1) ore units composed of willemite, franklinite, and zincite in varying proportions, with or without calcite, and (2) calcium silicate units composed of andradite, rhodonite, feldspars, micas, pyroxenes, calcite, and a great many other minerals. The classification into two types is forced in small part. Not all units were neat and of one type; there was much variance.
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| Figure 9-4. Vertical cross section of the keel of the Franklin orebody at coordinate 1000S in the Palmer Shaft support pillar, showing the severely crumpled and distorted structure in the keel area. Key to symbols: black areas are calcsilicate bodies; A = franklinite, willemite, and calcite, locally with zincite; B = franklinite, willemite, and zincite, locally without zincite; H = calcite, franklinite, and willemite; K = massive franklinite with sparse calcite; L = massive calcite; M = calcite with sparse franklinite; P = intrusive pegmatite body? The underlying bed, labeled Mt, is the Furnace Magnetite Bed. Scale is in feet, on the Franklin Mine coordinate system. Illustration from Frondel and Baum (1974) and reproduced from Economic Geology, 1974, Vol. 69, p.168. | ||
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The gross texture of the orebody is that of an irregular sequence of lenticular or lamellar, highly irregular, discontinuous, tabular bodies arranged with their long dimensions roughly parallel to the hanging wall and footwall of the orebody (Figures 9-4 and 9-6). These tabular bodies, ore units or calcium silicate units, are laminated and extend for hundreds of feet along the strike or dip of the orebody, but no one unit occurs along its whole length; one was reported to be over 2000 feet in length, but most were considerably smaller. Full dimensions are difficult to estimate but such layers vary substantially in thickness, from less than 1 foot to 30 feet (0.3-9 meters). Frondel and Baum (1974) reported that “the bodies are elongate along the strike and plunge gently to the northeast. The smaller ore units range down to pods or lenses a foot (0.3 meters) or less in thickness and 10 feet (3 meters) or so in length on the dip, with a northeasterly plunge.” These bodies are thickened near the keel, folded around it, and deformed into bulbous and irregular, rounded masses there (Figure 9-4). Some units are nonparallel to the orebody walls near the keel; they extend across the orebody from hanging wall to footwall. These lenticular units are sometimes found intermixed. Conformable units of calcium silicates occur within the ore units; the reverse situation, with ore as included lenses within the calcium silicates, is even more common. Boundaries and deformation effects were discussed by Frondel and Baum (1974).
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| Figure 9-5. An exposure of franklinite-willemite ore on the 300 level in the Franklin Mine, with pick and shovel for scale. Note the irregular, lenticular shape of some black, franklinite ore layers. Photograph courtesy of the Franklin Mineral Museum. | Figure 9-6. Vertical cross-section of the Franklin orebody at coordinate 970S within the Palmer Shaft support pillar. Calcium-silicate units are shown in black within the orebody; only the boundaries of the ore units are indicated. The layer under the orebody, labeled “magnetite,” is the Furnace Magnetite Bed. Scale is in feet on the Franklin Mine coordinate system. Illustration taken from Frondel and Baum (1974) and reproduced from Economic Geology, 1974, Vol. 69, p.162. | |||
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The geological mapping was done almost wholly after 1930 and mostly in the northern end of the mine; earlier mining had obliterated many geologic features, especially in the mine’s southern end. The most detailed mapping was of the Palmer Shaft pillar (Figures 9-4 and 9-6), a solid part of the orebody left in place over the Palmer Shaft to provide support and prevent cave-ins; it was removed in the late 1940’s and early 1950’s in preparation for the closing of the Franklin Mine. The best detailed sections of the Palmer Shaft Pillar are from 730S to 1030S on the Franklin Mine coordinate system. Copies of these maps are at Harvard University; the originals are at the Franklin Mineral Museum.
The ore, of very high economic grade, is gneissic in texture for the most part; the banding follows the external morphology of the orebody and also conforms with the highly folded and bulbous bodies deep near the keel (Figures 9-4 and 9-6). In general, within the ore units, franklinite and willemite predominate, with calcite varying in amount and zincite subordinate or absent. Much ore occurs as sequential layers of these minerals, with regular and mostly irregular repeats. However, the mineral composition of the ore units is not uniform; it varies in composition and texture where calcite variation is particularly noticeable, both along the length of the orebody and across it. In the literature, ore units have been described as franklinite-rich or willemite-rich, but such classifications are “forced” and useful only for general discussion purposes. Frondel and Baum (1974) have reported one ore unit 5-20 feet (1.5-6 meters) thick, 2000 feet (610 meters) long, and extending several hundred feet (61 meters) down dip. They also reported an ore unit 2-10 feet (0.6-3 meters) thick by hundreds of feet long almost wholly comprised of rounded zincite crystals, several inches in size, in calcite. Although they reported the absence of any characteristic sequence of the ore units, they noted, separately, that “in the early workings of the mine, however, near the nose of the syncline and in the general area of the present Buckwheat open pit, a persistent body of ore rich in zincite was present along the hanging wall of the eastern limb. This body extended into the foot wall of the west limb.” They also provided descriptions of other units. Although uncommon ore units may be predominantly willemite and calcite, or mostly zincite and calcite, only franklinite occurs as discrete monomineralic ore units. Ore units composed of varying amounts of franklinite, willemite, and calcite are the most common.
Silicates of manganese and of zinc, occurring as accessory minerals, are common in the ores. Olivine-group minerals and hardystonite (a mineral in the melilite group) predominate. These and the specific ores are discussed in greater detail in the section entitled “The mineral assemblages.”
Intercalated with the ore units are similarly-shaped and similarly-sized lenticular, irregular, conformable units composed mostly of calcium-, zinc-, and manganese-silicate minerals, with minor amounts of willemite and franklinite. These were called “skarn” by Palache and some other writers; the use of the term “calcium silicate units” here is adopted from Frondel and Baum (1974). In general, such calcium-silicate units behaved much like the ore units, but were more competent during deformation and folding than ore units containing much calcite. Like the ore units, those silicate units in which the tabular nature is undisturbed parallel the limbs and folds of the orebody. Those near the keel are deformed (Figures 9-4 and 9-6). Such units formed approximately a fourth of the orebody in the northern end (Frondel and Baum, 1974); such data was not preserved for the earlier-mined southern end of the orebody. As noted above, the extant maps of the Franklin Mine sometimes indicate, as pegmatite, both true intrusive pegmatite and these indigenous calcium silicate units, so some mapped details are confusing. As do the ore units, these calcium-silicate bodies may vary substantially in composition both on strike with and across the orebody. Such variation may be reflected in feldspar-rich and garnet-rich zones, but this is a forced categorization. These units are relatively coarse-grained for the most part and are generally unsegregated as to species, but at least some of these bodies were in part internally banded. The minerals in these units are discussed in greater detail in the section entitled “The mineral assemblages.”
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