COMPARISON DEPOSITS Chapter 9. The geology and structure of the zinc deposits

Comparisons of the Franklin-Sterling Hill deposits

Similarities

The two proximate orebodies share many features in common. The most apparent of these are their common age, their synclinal shape pitching northeast, their hook-like plan-section and cross-section, their occurrence wholly enclosed in the Franklin Marble, and their unique ores which have similar mineralogical compositions. Additionally, both deposits in part have been subjected to hydrothermal activity on a large scale, by pegmatites at the north end of the Franklin orebody, and by Mg-silicate infusion through the Zero fault on the east side of the Sterling Hill orebody. The unique association of Mn, Zn, and Fe, unpartitioned and broadly distributed in the gneissic and disseminated oxide-silicate ores, is the most noteworthy chemical feature. These great similarities strongly override the differences noted below.

Differences

The differences between the deposits are numerous if trivial, beginning with the physical outcrop morphologies. The Franklin deposit has a long west limb and short east limb; the Sterling Hill deposit exhibits the reverse relation. These features, however, are but artifacts of erosion and weathering. The Franklin deposit apparently has a simple primary deformation resulting in one major fold and keel (itself deformed), whereas the Sterling Hill deposit is severely deformed. The reported internal structure of the two deposits is also extremely dissimilar. Likewise, the substantial amount of pegmatite at Franklin contrasts with its near-absence at Sterling Hill. Structurally, Franklin may be the general case, whereas mineralogically and chemically, Sterling Hill is. Rhodonite and andradite are common in Franklin calcium silicate units, but less common at Sterling Hill. The overall geochemical characteristics are discussed in the section entitled “Geochemistry.”

Relations to nearby formations

Although nearly every investigator has pointed out the proximity of the Franklin deposit to the underlying (but like the Franklin Marble, overturned) Cork Hill Gneiss, none has made a convincing argument that this closeness is more than geologic happenstance. The occurrence of the Franklin deposit 30 feet (9 meters) from a large-scale gneiss contact, and the Sterling Hill deposit 1000 feet (305 meters) from it (Pinger, 1948), is not yet shown to be of significance. If the “sinking hypothesis” is true, indicating great mobility of at least one and possibly both of the ore deposits within the marble, this possibly further diminishes the significance of both the orebody-gneiss distances and the distance of separation of the individual orebodies.

The Furnace Magnetite, bed, however, probably has as yet unshown relations to the Franklin orebody. The matter remains largely unstudied; outcrops are available.

Physical linkage hypotheses

The commonality of these unique but separated gneissic ore deposits has tempted some who have studied here to attempt to link the deposits physically by various theories. The challenge posed early-on was and remains: were these orebodies originally parts of a common unit, or did they evolve separately under extremely similar unique conditions with minor-to-moderate local variations?

Early efforts emphasized faults in the marble as the mechanism which separated the purportedly previously connected orebodies. There is no direct geological evidence for this theory; its credence lies only in its simplicity. Nineteenth-century geologists, and some writing more recently, thought this idea was supported in part by the obvious hook-like aspects of both orebodies, which slightly resemble drag folds, as if the orebodies were peeled back in part from a body with a planar, lenticular morphology (perhaps linear in plan-section). Of late, the hook-shape of the deposits has been attributed in part to the “sinking hypothesis” (Skinner and Johnson, 1987).

One of the more creative early approaches was taken by Blake (1894a). He called attention to the apparent physical separation of beds of franklinite and zincite in the exposed limbs at both Franklin and Sterling Hill, and noted that the order of the beds was apparently reversed. Blake noted that, in depth, zincite underlay franklinite at both deposits, more consistently at Sterling Hill, but only locally at Franklin. Blake advanced a theory that the two deposits were originally connected through an anticline now eroded away. The sketch he gave was projected as if looking nearly north-south. It is not to scale and projects both deposits onto an east-west plane, omitting the several-mile N-S displacement of the orebodies. Thus, Blake was able to postulate a connection which satisfied the apparent relations in the exposed beds.

Although a tempting puzzle, the possible historical linkage of these orebodies remains unproven. The paucity of much structural data for Sterling Hill precludes modern attempts at correlation. The deposits likely have formed while separated spatially, although under similar conditions. Intrusion of probably late Precambrian pegmatites at Franklin overprinted that deposit.

The Långban Mine near Filipstad, Sweden

All discussions of the uniqueness of the mineral deposits at Franklin and Sterling Hill involve some comparison with the deposit at Långban, Sweden. The geology of Långban was described by Sjögren (1892, 1910), Aminoff (1918), Magnusson (1924, 1930), and Geijer and Magnusson (1944). A superb overview of Långban’s geology, mineralogy, and history was given in English by Moore (1970a). Lists of the minerals found at Långban have been published by Flink (1921, 1922, 1923a, 1923b, 1924, 1926, 1927, 1928), Mason (1943), and Moore (1970a). Moore et al. (1971) provided a status report on all of Flink’s published unknown minerals. Bibliographies of publications on Långban have been published by Pilström (1965) and Strand (1987).

Although the vernacular of the mineral collector relates Långban to Franklin and Sterling Hill, this linkage is commonly overstated. As shown by Palache (1929b), Vreeland (1963), and Moore (1970a), these deposits have much less in common than might be implied by casual observation of lists of rare minerals.

The principal differences are strong ones. Although both deposits contain oxides of Mn and Fe, at Franklin and Sterling Hill they are in one mineral, franklinite. At Långban, Fe and Mn are segregated both mineralogically and spatially, with Fe in hematite and magnetite and Mn in braunite and hausmannite. The most singular difference between the deposits is the presence of Zn, rare at Långban, but in abundance at Franklin and Sterling Hill. Thus, the ores are fundamentally different.

The oft-mentioned similarities arise from suites of uncommon minerals which occur at both deposits. Perhaps foremost among these are the lead silicates, many of which occur at both localities. However, even in this case there are differences, inasmuch as some of the Långban lead silicates, kentrolite and melanotekite for example, occur within the ores and others within the skarns. At Franklin, most of the lead silicates occur in the calcium silicate units. Other similarities include the mutual occurrences of manganosite, native lead, Ca-Mn silicates, basic manganese arsenates, Långban Mn-As minerals such as dixenite and hematolite which are related to kraisslite and mcgovernite of Sterling Hill, and others. A third locality, the Kombat Mine in Namibia, has produced additional suites of some of these minerals (Dunn et al., 1988; Dunn, 1991).

Foremost among the distinctions of Långban, as of Franklin and Sterling Hill, is the substantial number of new minerals which have been initially discovered from here. These minerals result from the uncommon association of Fe and Mn, with Be, Ba, Pb, As, Sb, and B, and, at Franklin and Sterling Hill, with abundant Zn. The number of species first-described from Franklin and Sterling Hill is greater than that from any other locality in the world and is a source of community pride. However, this trivial distinction may be relinquished to Långban eventually. For some mineral species found first at one deposit, the second occurrence of the species was from the other deposit. Considering the relative size of the deposits, Långban, while much smaller than Franklin and Sterling Hill, is richer in proportion in the rare elements.




Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates

Link to homepage
This page created: January 11, 2001