FRANKLIN AND STERLING HILL NEW JERSEY: THE WORLD'S MOST MAGNIFICENT MINERAL DEPOSITS
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ELEMENTS SULFIDES ARSENIDES ANTIMONIDES AND SULFOSALTS OXIDES AND HYDROXIDES HALIDES AND CARBONATES
SULFATES BORATES TUNGSTATES AND MOLYBDATES ARSENATRES ARSENIDES PHOSPHATES AND VANADATES UNNAMED MINERALS


Overview

 

The ore minerals

 

The calcium-silicate minerals

 

Recrystallization of minerals

 

Special features

 

Special chemically-distinct mineral groups

 

Special mineral assemblages

 

Special features

Hydrothermal vein minerals

 
 
 
  Figure 12-41. A crack in banded, primary, gneissic willemite-franklinite ore from Franklin has been filled by a vein of secondary willemite. Note that the primary ore has been displaced, causing two different ore units to be in juxtaposition. Specimen is 13 cm in maximum dimension. Franklin Mineral Museum, #SG-674. Photo by the author.  
   

Cracks in both the ore units and calcium silicate units are commonly healed or coated, partially or completely, by secondary minerals deposited in part hydrothermally as veins. Frondel and Baum (1974) suggested the veins are “genetically associated with hydrothermal activity accompanying a regional metamorphism that was later and of lower grade than that which affected the region in Precambrian time. Most veins are thin (x mm to 10 cm wide) and are volumetrically insignificant relative to the mass of the orebodies; none is of economic significance. Some veins offer bright contrast with the host minerals, particularly when the veins are brightly colored in pink and green hues; such veins are called “ribbons” by some local collectors (Figures 12-41 and 12-48).

 
 
 
  Figure 12-42. Black vein of altered pyrochroite in Sterling Hill ore composed of calcite (white), willemite (gray), and franklinite (black). Specimen is 9 cm in maximum dimension. Smithsonian Institution, #R19107. Photo by the author.  
   

Open veins, those with open spaces between their sidewalls, may be either simple or complex, the latter containing up to a dozen species locally, although most have less than six megascopically visible minerals (Figure 12-43). The primary ore minerals, zincite, willemite, and franklinite, also occur in such open veins as superb, euhedral, secondary crystals. Willemite in this context, although well-studied morphologically, has been little studied otherwise; some data are given by Makovicky and Skinner (1990). Secondary zincite has been studied by Peretsman (1981) and secondary franklinite by the author, as described herein under franklinite.

 
 
 
  Figure 12-43. Typical druse of various mineral crystals on the sidewalls of an open seam in franklinite-willemite ore from Franklin. This assemblage is of crystals of tephroite (black), barite (white), and hodgkinsonite and pyrobelonite (both indiscernible) on an andradite druse. Such vein occurrences host many of the rare and unusual species. Specimen is 8 cm in maximum dimension. Smithsonian Institution, #C6167. Photo by the author.  
   

The rarest minerals have been found in the open veins and seams; they occur, however, in exceedingly small amounts. Solutions rich in zinc silicate and/or manganese silicate are very mobile; these result from the hydrothermal alteration of both primary and secondary minerals. As with the closed veins, willemite and carbonates are nearly ubiquitous.  The vast preponderance of the well-crystallized secondary minerals in vein assemblages have formed from such hydrothermal activity. Such veins at Sterling Hill host the basic manganese arsenates (Parker and Troy, 1982); these were found at Franklin, too, but were not noted in situ.

Common closed veins are apparently of simple mineralogy but have not been studied in detail; they appear to contain but three or four species (Figures  12-41, 12-44, and 12-48). The veins are quite varied in thickness and extent. Most observations are from hand-specimens without known geologic relations. The observation of only hand-specimens of veins can induce a subconscious bias: the observer is often not consciously aware that such specimens are but small, fragmented parts of planar features. Such veins may be parallel to, but more commonly crosscut, the gneissic banding of the ores (Figure 12-41). Contact effects in the host ore or calcium silicates may be sharply defined, subtle, or absent. Such effects may include discoloration of surrounding ore or rock, recrystallization of certain minerals, removal or leaching of certain minerals, apparent and/or real diffusion-effects, and others (Figure 12-45). In many instances, observation of such effects is facilitated by the use of ultraviolet radiation; the fluorescence of altered willemite or calcite is commonly changed in appearance relative to that of primary material (Figures 12-48 and 12-49). Grain-size in veins generally varies from coarse-granular to fine-grained, is often indiscernible, and may be affected by chill borders. Xenoliths of matrix minerals are commonly included. At Sterling Hill, Johnson (1990) found willemite in veins to be more pure, and calcite to be less pure, than that of the host rocks.

     
 
 
 
 
 
  Figure 12-44. Multi-layered vein in franklinite-willemite ore from Franklin. Within the vein, sussexite (medium gray) is interlayered with willemite (light gray, and white). Note the non-symmetrical deposition of willemite and sussexite relative to the vein-sidewall contacts. The visible surface is polished. Specimen is 12 cm in maximum dimension. Smithsonian Institution, #R6606. Photo by the author.   Figure 12-45. Typical carbonate-bearing vein from Franklin. Willemite-franklinite ore at very bottom is unaltered; the rest of the ore has been altered and partially recrystallized by the thermal and/or solution effects of the vein. Within the vein, light- and medium-gray areas in contact with ore are secondary willemite; calcite (white) and serpentine (black) occupy the central areas of the vein. Specimen is 44 cm in maximum dimension. Smithsonian Institution, #108829. Photo by the author.  
         

The vein minerals may be layered; if so, some such layering may be symmetrical or non-symmetrical relative to the vein sidewalls. Mineralization within veins may be continuous or discontinuous; such discontinuous or brecciated mineralization was apparently less abundant at Franklin than at Sterling Hill. Locally, such brecciation may include fragments of previously layered veins resulting in extremely varied textural relations (Figure 12-46).

     
 
 
 
 
 
  Figure 12-46. Franklin ore composed of black franklinite and gray willemite is cut by a composite vein comprised of veinlets of willemite (light gray, and white), rhodochrosite (white at lower left), and an impure mixture containing carbonate minerals (dark gray). The visible surface is polished, Specimen is 10 cm in maximum dimension. Smithsonian Institution, #C6603. Photo by the author.   Figure 12-47. Veins of willemite (both light gray and white zones) cut Franklin ore consisting of willemite (gray) and franklinite (black). The visible surface is polished. Specimen is 12 cm in maximum dimension. Smithsonian Institution, #R6601. Photo by the author.  
         

At Franklin, the principal minerals in such veins are willemite, commonly green to faint green in color, and calcite, varying from white to faint pink in color. Such willemite veins, composed of recrystallized material low in Fe and Mn, commonly have a markedly brighter-green phosphorescence than much primary willemite. Such vein fillings may be monomineralic, but excepting calcite, serpentine, willemite, and some friedelite, this is uncommon. Carbonates and manganese silicates commonly accompany willemite and have contact relations with the ore; willemite and carbonates occur in some zoned veins.

     
 
 
 
 
 
  Figure 12-48. A vein of rhodonite (gray) bordered by willemite (white) in Franklin ore composed of black franklinite and gray granular willemite. Alteration effects are barely discernible in visible light (left photograph). In ultraviolet radiation (right photograph), the bright zones, left and right, in the host ore are primary, unaltered ore; the darker zone shows the effects of the reaction of vein material and primary ore. The specimen is 13 cm in maximum dimension. Smithsonian Institution, #R9387. Left photo by the author; right photo by Henry Van Lenten.   Figure 12-49. Viewed in both visible light (left) and ultraviolet radiation (right), willemite-calcite-franklinite ore from Franklin has been transected by two veins of willemite. In visible light the vein and its effects are barely discernible. In ultraviolet light, the outermost visible fluorescing area (left, right, and bottom) is the primary ore. The thin, bright areas intersecting at nearly a right-angle are the willemite veins, and the dark area is the zone affected by the reaction of the vein material and primary ore. The specimen is 12 cm in maximum dimension. Smithsonian Institution, #C6311. Photos by Henry Van Lenten.  
         

The silicates which commonly accompany willemite in Franklin veins are extremely varied, but leucophoenicite, rhodonite, and some friedelite occur in high-temperature veins, and serpentine and some layer silicates occur in those formed at lower temperatures (Figure 12-45). Carbonates, such as calcite, manganoan calcite, dolomite, kutnahorite (at Sterling Hill), and rhodochrosite, are common, both as monomineralic veins or ones associated with willemite. Sphalerite, manganpyrosmalite (at Sterling Hill), bementite, pyrochroite (Figure 12-42), and many other species occur less commonly in veins, except in localized concentrations. High-temperature minerals, such as amphiboles, pyroxenes, feldspars, and micas are generally absent. The Buckwheat Dolomite (Peters et al., 1983; Cummings, 1988) is possibly a large-scale vein occurrence.

 
 
 
  Figure 12-50. Folded willemite band in franklinite-willemite granular Franklin ore. Note the coarseness of the grains (crystals) in the band. The white and gray areas in the folded band are both willemite, and minor white calcite is present. Specimen is 11 cm in maximum dimension. Privately owned. Photo by the author.  
   

             

Exsolution mineral textures

Both deposits have had a slow cooling history, and exsolution intergrowths are common here. The original oxide, spinel-structure phase had the composition franklinite80-gahnite20  (Carvalho, 1978; Carvalho and Sclar, 1988), and the original silicate olivine-structure phase had the composition tephroite80willemite20  (Francis, 1985b). Both have unmixed. Exsolution textures occur in ores, calcium silicate units, and pegmatites. The best known are those with gross textures. Aside from studies of willemite, very few microscopic silicate intergrowths have been studied in detail.

In the ore, the most common exsolutions are of hetaerolite from franklinite (Mason, 1946); hetaerolite, hematite, and gahnite from franklinite (Frondel and Klein, 1965; Carvalho, 1978); magnetite from franklinite (Sclar and Leonard, 1992); and magnetite and pyrophanite from franklinite (Valentino, 1978).

A most spectacular and notable exsolution relationship was found at Franklin in 1914 at the then-extreme north end of the 900 level. Here was found the grossly-crystallized mutual exsolution of zincite in manganosite and manganosite in zincite (Frondel, 1940).

Largely unrecognized but widespread is the exsolution of hetaerolite in zincite, giving rise to an apparent, but false “cleavage.” Such material is very common, but has been little studied. Some few lamellae are jacobsite or manganosite; most are hetaerolite.

Gross exsolutions are less abundant in the silicate bodies, but are most common in tephroite which has relations to the ore assemblages. The exsolution of willemite from tephroite was studied by Hurlbut (1961) and Francis (1985b); it is very common. Exsolution textures are also found in some Franklin bustamite which occurs in large, pink-orange masses; the exsolved minerals are abundant willemite and a reddish-brown, highly sodic johannsenite. On a small scale, franklinite has been found as thin elongate platelets in a number of minerals and is not uncommon as such in willemite and hardystonite. Many of the silicate minerals exhibit both opaque and transparent inclusions with preferred orientations, but these are unstudied. The massive pegmatite at the Trotter Mine is perthitic.

Weathered and oxidized minerals

The mineralogical products of weathering are incompletely studied. Detrital willemite was found in the bed of the Wallkill River, at least as far north as Hamburg. Palache (1935) mentions minor local gossans on veins of pyrite and of sphalerite, but these were likely of trivial significance.

The Franklin orebody and possibly both orebodies were exposed during a very lengthy period in the Precambrian. The surficial extent of this exposure is uncertain, but part of the west limb of the Franklin deposit was weathered, resulting in the formation of goethite, hemimorphite, and other minerals. It is not known if manganese minerals, such as chalcophanite, hetaerolite, and others, occurred in this zone. Detrital franklinite was found in the younger, overlying, Cambrian age Hardyston Quartzite. At Sterling Hill, local effects, perhaps drainage anomalies, contributed to the formation of the previously described hemimorphite (calamine) pits and the underlying deep penetration (680 feet; 207 meters) of the mud zone. This is the greatest local manifestation of weathering, but has been little studied. Much black mud was found here, together with extensive deposits of iron- and manganese-oxide minerals (see “Special features” herein, in the section entitled “The Sterling Hill zinc deposit.” Clays from the alteration of feldspars are found in but minor amounts; they might not have been generated in quantity or may have been removed by erosional and glacial forces. Clays are of minor importance in the orebodies; nontronite occurs sporadically in weathered areas at Sterling Hill. The existence of brick-making yards in Franklin in the 1850’s, however, suggests clay deposits were available not far from the orebodies.

Surficial weathering of ores and other minerals occurs here, but exposures are of generally limited extent: franklinite may be weathered to goethite or hematite, zincite to hydrozincite, and copper sulfides to films of copper sulfates and carbonates; others are listed by Palache (1935). The surface of the commonest gangue mineral, manganoan calcite, weathers readily to a gray-to-black surface; such material is locally abundant. Other Mn-minerals, such as rhodonite and bustamite, weather similarly. Most such surficial effects are unstudied.

Post-mining minerals

These deposits, worked for three centuries, also have been exposed to substantially varying man-made conditions of oxidation, hydration, and atmospheric exposure.

A small number of minerals have occurred in the mines as a result of leaching by supergene waters and by liquids introduced in the mining process. Precipitates of some of these solutions have formed dripstones, concretions, mine-pearls, and the like; most of these formations are formed of calcite. Monohydrocalcite has also formed as a dripstone, and hydrozincite, a zinc carbonate-hydroxide, has formed as thin films and crack fillings near and on zincite. Similarly, some arsenates of Ca and Mg (uranospinite, picropharmacolite, and guerinite) have formed on the walls of old adits and drifts. A few sulfates (hexahydrite, starkeyite, and mcallisterite) have formed as efflorescences in part, and others (epsomite, bassanite, and some gypsum) have formed under varying conditions. The few “minerals” which may have formed only after removal from the mine, such as “melanterite” from the decomposition of iron sulfide, are not included here.

 

 

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CHAPTER 12. MINERAL ASSEMBLAGES