Minerals resulting from surface oxidation and other alteration

With one notable exception the minerals formed by surface alteration are of minor interest in the Franklin district. Manganese-bearing serpentine forms pseudomorphs after rhodonite, and desaulesite has replaced niccolite and chloanthite at the Trotter mine. Quartz, hematite, and limonite form gossans on a few pyritic veins at Franklin, and small amounts of calamine and hydrozincite are found in the gossans of veins of sphalerite. The ordinary copper oxidation minerals form about the rare copper sulphides, and descloizite, cerusite, and anglesite are the alteration products of galena in a pyroxene skarn at Sterling Hill.

The exception noted above was so remarkable that it requires special description. During the seventies the noble and Passaic mines, open pits in two great bodies of calamine lying in the angle between the two legs of the ore body (see plate 1), were the chief sources of zinc ore mined Sterling Hill. When mining was begun there that area was a shallow watercourse, and the pits were carried to a depth of more than 40 feet below the original surface. In 1906 the sites of the ore bodies were occupied by two great excavations, having roughly the shape of inverted cones, separated by wall of barren pegmatite. The bare limestone walls of the ore bodies then remained as they were left by the stripping and were clearly seen to be solution surfaces, probably the result of long-continued action by ground water. These exposures have long since been caved in by the later mining Sterling Hill.

Mr. O. J. Conley, Superintendent of the Noble mine in 1878, kindly went over the ground with the author and described the deposit, of which no contemporary account was published. According to Mr. Conley, and calamine formed a layer 6 to 12 inches thick, lying directly on the limestone. The principal filling of the excavated mass was more or less fragmental, consisting of sand, clay, limestone fragments, and loose and broken crystals of franklinite, willemite, garnet and the like, all stained by oxides of iron and manganese. Separating this loose material from the calamine layer on the north side of the pit was a layer, as much as 4 inches thick, of greasy black mud, rich in manganese, which was the cause of dangerous slides in the pit. On the south side, in a similar relation to the calamine, were found the deposits of chalcophanite and hydrohetaerolite characteristic of this locality. Excellent specimens of the calamine are preserved in collections, and nearly all those examined showed considerable harsh ground or yellow clay adhering to their lower services. This clay is rich in zinc and has been called vanuxemite.

The relations of the calamine deposits to the main ore bed, as described by Mr. Conley, fully establish their secondary nature. There is, however, other direct evidence in the presence, in the fragmental material from the pit, of crystals of willemite still retaining their form but wholly covered with needles of calamine stained with manganese and deeply corroded franklinite crystals enclosed by and embedded in calamine. These zinc and manganese ore deposits resulted from the weathering of a part of the outcropping ore body, the products of solution being carried to a lower position, where they replaced limestone with zinc silicate and hydrous oxides of zinc, iron, and manganese. The reason for describing this deposit at such length is that it seems to throw some light on the origin of the major deposits, as shown in the next section.


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