Minerals of the hydrothermal veins

The mineralogy of the hydrothermal veins is scarcely less complex than that of the pneumatolytic veins. Many of the minerals, however, were obviously formed at lower temperatures than the pneumatolytic minerals and either farther from the pegmatitic intrusions or during later fissuring. The fissures are either wholly filled or are open and crystal-lined, and the veins are of characteristic form with clean-cut walls. They are generally short and small, as a rule but a few inches thick, and many are mere seams less than an inch thick. Thus, although they are fairly numerous, their total bulk is insignificant compared to the great mass of the ore bodies. Their most notable characteristic is their great variety of mineral contents, as may be gathered from the following selected examples of associations found in the collections.

A typical example is the paragenesis garnet, willemite, zincite, leucophoenicite, gageite, chlorophoenicite, pyrochroite, calcite. Another is the combination hodgkinsonite, willemite, hetaerolite, calcite. Rhodonite, friedelite, schallerite, and mcgovernite are each found as the sole filling of numerous fissures. Friedelite is ordinarily followed by barite and calcite. Willemite, especially the light-green or white fibrous variety, low in manganese and highly phosphorescent, is a common vein filling. Zincite is known in crystals only from vugs in calcite veins, and the form called "calcozincite" is really a mixture of granular zincite with fibrous calcite, generally coating slickensides in ore.

The arsenates hedyphane, holdenite, and allactite are found in veins with calcite, whereas chlorophoenicite is found with gageite and acicular willemite. Ganophyllite and heulandite are found in veins with rhodonite or sulphides. The other zeolites of the list are minor vein minerals. Sussexite is typically a vein mineral, its asbestiform fibers filling narrow cracks or coating slickensides in ore. It is closely simulated by veins of fibrous tremolite mixed with calcite.

Another type of vein common at Franklin consists commonly of some carbonate, as a rule strikingly layered parallel to the walls. In some places the filling is calcite with or without fibrous willemite, in other places it is siderite or an intermediate calcium-manganesian-iron carbonate, and in still others it is rhodochrosite or smithsonite. Dolomite constitutes the filling of a few veins with open vugs lined with crystals of dolomite, calcite, albite, quartz, or sphalerite, and more rarely with millerite, marcasite, or the oxides goethite, manganite, and hematite. Pyrochroite and chalcophanite are also found in calcite veins, and aragonite, in radiating needles, coats cracks.

Compact quartz, alone or with massive sphalerite or pyrite, forms thin clean-cut veins and in places shows crystals on free surfaces. On the whole, quartz is rare Franklin. The paragenesis calcite, sphalerite, and calcite, sphalerite, quartz, willemite, crocidolite are common and conspicuous vein formations. In fact, is only in these veins that sphalerite occurs in any noteworthy amount at Franklin, except in the mass at the Trotter mine.

Bementite and the closely related manganese-bearing serpentine are not uncommon as vein fillings, alone or more commonly with carbonate such as rhodochrosite or smithsonite.

It seems highly probable but the carbonate and quartz veins containing sulphides were formed during the much later post-Paleozoic deformation, as suggested in an earlier paragraph.


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