Research Needs at Franklin and Sterling Hill
Most of the research needs listed below are mineralogical in nature, but the last is geological as well, and should be undertaken before the exposures become increasingly obscured by vegetation. This list is incomplete, especially as regards isotopic and experimental geochemical studies; these too would be welcomed.
Frondel (1972) stated that “caswellite” refers to fine-grained andradite pseudomorphous after mica, but Dunn (1995) emphasized that caswellite is an impure mixture of minerals and reported grossular as the dominant component. In actual use the term refers to any conspicuously altered and/or replaced mica from Franklin. A recent XRD scan of one specimen revealed no garnet but was consistent with clinochlore. The mineralogical composition of caswellite has never been investigated in any detail, nor has the nature of the precursor micas (hendricksite? phlogopite? biotite?) been much studied. Caswellite from Franklin is generally tan to medium brown, but some is dark red (hematitic?), some is of palest ivory color, and some is green (cyprine?) to brown. Specimens range from barely to fully altered, sometimes in the same piece, offering opportunity to study both the original and final phases.
Chlorite and stilpnomelane groups
Minerals of these groups are widely present at both Franklin and Sterling Hill, both as components of replacement assemblages and as hydrothermal fracture fillings and coatings. Not much descriptive or analytical work has been done on them.
The feldspars are both volumetrically and scientifically significant at Franklin and Sterling Hill but rank among the least-studied of the local rock-forming minerals. Compositionally they are quite variable, and some are perthitic. As noted by Frondel, they include microcline, orthoclase, hyalophane, celsian, anorthoclase, albite, oligoclase, and anorthite. They are difficult to tell apart visually, but informal guidelines have arisen over the years. Sky blue to chalky white feldspars are generally called “celsian,” the brownish red ones (as described by Palache) conform to his “hyalophane”, ivory-colored feldspars in calc-silicate layers are commonly called hyalophane as well, those in pale green granular masses are assumed to be microcline, small crystals in dolomite vugs are regarded as albite, etc. Some of these guidelines may have merit, but we suspect that others, particularly as regards the barian feldspars (celsian-orthoclase series, and also barian microclines) probably do not. Much work is needed to correlate feldspar compositions and structural states to the numerous and varied assemblages in which they occur.
Garnet from Franklin and Sterling Hill includes andradite, grossular, spessartine, almandine, and goldmanite, the latter two rare. Some are titanian, some are hydrated, and a wide range of compositional impurities and intermediate members of solid-solution series have been noted. Dunn (1995) mentioned that garnet is “associated with a large number of species in a variety of assemblages” and is one of the most common of the local silicates, but the group has not been much studied from either deposit. Members of the garnet group are known from peak metamorphic assemblages, retrograde assemblages, and hydrothermal vein and alteration assemblages. They are both petrologically and volumetrically significant.
Norbergite and chondrodite in Franklin Marble
These are the two dominant humite-group minerals in the Franklin Marble and are common in the local marble quarries, as well as both Sterling Hill and Franklin. These minerals range in color from palest yellow to dark brown and are difficult to tell apart visually. Loosely following Dunn (1995), collectors have taken to calling light-colored humites in the marble “norbergite” and darker-colored ones “chondrodite,” but this is probably not a reliable criterion. Both minerals fluoresce, and in each there seems to be a correlation with the daylight color: the darker the mineral, the less intense the fluorescence. A study of the major and trace-element geochemistry of local chondrodites and norbergites, correlating their compositions both to daylight color and to observed brightness of fluorescence, (paying particular attention to iron content) would be a good senior thesis topic unless unwelcome complications arise.
These are perennial mysteries. The mine passages at Sterling Hill are in many places encrusted with various post-mining minerals, some of which fluoresce brightly in various colors (blue, turquoise, green, white….). Almost no research has been done on these. Some of the coatings on and near ore have a blue component to their fluorescence, which possibly is due to hydrozincite, but coatings distant from the ore fluoresce gray to white instead. Similarly, coatings showing green- (monohydrocalcite?) and turquoise-colored fluorescence are present only near or on ore; they are lacking elsewhere. Certainly this would be an interesting study, but possible disadvantages include analytical hurdles (possible mixed phases in thin coatings) and low scientific importance compared to other mineral groups.
These are of minor importance locally, but members of this series are widely distributed in small amounts from both mines and several of the local marble quarries. Some are known from vein assemblages, and others are in retrograde metamorphic assemblages. Not much is known about the compositional range exhibited by local members of this series, though some are known to fall near the midpoint, and little is known of their paragenesis. Probably a good topic for a senior thesis.
Secondary manganese minerals
Because these are secondary rather than primary minerals they have not yet received the attention they deserve, but there are quite a few of them, including chalcophanite, hetaerolite, hydrohetaerolite, birnessite, hollandite, woodruffite, todorokite, and cryptomelane. They were formed by thorough weathering of the primary ores such that much of the iron was removed, but manganese remained. As noted by Dunn (1995), they have not been studied by modern methods. Numerous specimens are available, both from collections and from exposures in place at Sterling Hill. Little information is available to tell these species apart visually. There is some interesting geochemistry here, to document the alteration of primary unweathered ore to the compositionally much different weathering products left in its place, and to determine under what conditions such alteration occurred. Some is preglacial and some is postglacial – the deposits were exposed at or near the ground surface during two widely separated geological periods.
The serpentines are both abundant and widespread at Franklin and Sterling Hill and are known from diverse assemblages. Much is manganoan. As noted by Dunn (1995), no major studies of the serpentine group have been undertaken since the time of Palache’s (1935) review. We know little about their compositions or manner of formation, other than that many of them are late-stage alteration phases and occur as replacements of earlier minerals, components of vein assemblages, breccia cements, etc.
Tephroite and olivines
Not much is yet known about the compositional limits of tephroite at Franklin and Sterling Hill, or the extent to which a solid solution series extends to the olivines. Among the latter, fayalite is fairly common at Sterling Hill, and forsterite has been found (rarely) at Franklin.
Apart from the secondary manganese minerals (see above), a host of other little-studied minerals occur in the weathering zones of both Franklin and especially Sterling Hill. These include such species as cerussite, mimetite, and sulfur in vugs in corroded galena, and various unidentified secondary copper minerals due to alteration of chalcopyrite (typically) and chalcocite. The secondary copper minerals, in particular, offer opportunity to identify species new to the locality (though probably not new to science). In addition there exist fracture coatings of various minerals that no one has yet analyzed, so almost nothing is known about them. Some of the fracture coatings appear to be secondary carbonates, but others are black dendritic coatings of the type formerly (but often mistakenly) assumed to be pyrolusite. Possibly they are romanechite, not yet on the local species list. Moreover, some of the feldspars in the local gneisses have been thoroughly altered to clays of uncertain affinity. No one has yet done a comprehensive inventory, let alone a good description, of the weathering-zone minerals at these localities. The weathering-zone minerals, properly studied, might provide insights into the temperatures and compositions of the fluids that so thoroughly altered the rocks over time.
Footwall rocks of East limb of ore
The layered succession of rocks in the footwall of the East limb of ore at Sterling Hill is quite well exposed on the high ground between the Passaic and Noble pits. Exposure is close to 100% due to stripping operations in recent years. The footwall gneisses contain a complex succession of peak-metamorphic and retrograde minerals, including members of the pyroxene, amphibole, spinel, and mica groups, plus such exotic phases as graeserite and genthelvite. These footwall rocks contain a strong exhalative signature: The pyroxenes and amphiboles, especially, are zincian, manganoan, and ferroan, and depart significantly from their pure, hypothetical end-member compositions. This succession of rocks is begging for a good, thorough petrologic study. This could be done at the M.S. level as a descriptive study, but at the Ph.D. level if a more ambitious interpretive study is to be done. Reference: Leavens, P.B., Zullo, J., and Verbeek, E.R., 2009, A complex, genthelvite-bearing skarn from the Passaic Pit, Sterling Hill Mine, Ogdensburg, New Jersey: Axis, vol. 5, no. 1, p. 1-26. [Axis is the online journal of The Mineralogical Record]