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THE FRANKLIN MINING DISTRICT
The skarn minerals, as shown in the second column of the table opposite page 20 [Web Ed. note: to be added], are chiefly characterized by containing zinc, iron, or manganese, or all three. Rhodonite is particularly abundant at Franklin, in great masses crystallized against calcite. At Sterling Hill the skarn was composed chiefly of jeffersonite and gahnite, but roepperite was abundant in places, as were large crystals of manganese hornblende. The cyprine variety of vesuvianite occurs at Franklin, intergrown with pegmatite and with rhodonite. Hardystonite, found only in Franklin, seems to be transitional to the next subgroup, as it invariably contains traces of lead.
The primary ore minerals, in coarse aggregates and large well-formed crystals, are found with skarn in many places and also in independent masses. Franklinite in octahedrons and dodecahedrons as much as six inches in diameter; willemite, particularly the manganiferous variety troostite, in stout hexagonal prisms a foot or more long; and zincite in broad plates as much as six inches across, are examples of recrystallization. Their association with skarn in some places indicates that they are related to the pegmatite intrusions, in some places they have no such visible relation. The most notable occurrence of the latter sort was found in the early workings at Sterling Hill, where pegmatite is less abundant than Mine Hill. At such places local accumulations of water may have become an active solvent through heating by intrusions at some distance.
In a few places recrystallization seems to have been accompanied by some breaking down of the original minerals. This is indicated by the occurrence of manganosite, although the MnO molecule is usually present only as a constituent of franklinite or in solid solution in zincite. Another example of it is the development of masses of hematite with a remarkable cubelike parting, intergrown was franklinite. These masses may have been formed by the separation of some of the iron of franklinite during the recrystallization.
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© by Herb Yeates 1997-2001.
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page created: January 12, 2001 6:54 PM
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