Minerals in the pegmatite contact zones

Pneumatolytic products

The subgroup composed of pneumatolytic products, which is practically confined to Franklin, includes a great variety of minerals, chiefly silicates, containing some element not normally present in either the ore or the pegmatite, such as lead, chlorine, fluorine, boron, arsenic, and sulfur, with or without hydroxl. A number of metallic sulphides are placed in this subgroup with considerable doubt.

The mode of occurrence of these minerals is various. Some of them form irregular masses mingled with skarn minerals, which they seem to replace; others fill definite veins in ore or skarn, along whose walls more or less replacement of the older minerals has occurred. The detailed paragenesis is almost infinitely variable, and only a few examples are mentioned.

The early development of the Trotter mine brought to light a pegmatite which was notable for the presence of green microcline and allanite. At the surface about the mine there was an abundance of garnet skarn, and throughout the mine rhodonite and manganese axinite were plentiful. Niccolite and chloanthite were found in considerable masses mingled with sphalerite and purple fluorite.

The ore body in the vicinity of the Parker shaft contained great masses of garnet-rhodonite-axinite skarn, associated with which were many lead silicates. Here were first found nasonite, margarosanite, roeblingite, and hancockite, and with them the hydroxl silicates leucophoenicite, clinohedrite, pectolite, and prehnite. At this locality fluorine is a constituent of fluorite, cuspidine, and apatite, and copper occurs native and in cyprine. Much the same suite of minerals was found in the replacement veins containing the recently found lead silicates, larsenite and calcium larsenite. Cahnite, a calcium boroarsenate, was found in cavities in axinite, in veins with willemite, hedyphane, and datolite, and in open vugs in rhodonite veins.

Veins with the succession garnet, tephroite, willemite, barylite, calcite seem to belong in this subgroup. Hodgkinsonite and leucophoenicite, the latter especially likely to be associated with sussexite, are widespread both as replacement and as vein minerals. Willemite in particularly fine, complex crystals is characteristic of the subgroup, and franklinite, where developed in open veins, has a rare cubical habit.

The metallic sulphides of this subgroup occur mostly in ill-defined masses that apparently have replaced normal ore. They rarely show any relation to the pegmatites and may be of much later introduction. As typical examples of their grouping may be mentioned chalcocite surrounding octahedrons of magnetite and separated from them by films of native silver; films of native copper and native lead in axinite-willemite-barite veins; and granular aggregates of galena, chalcopyrite, bornite, pyrite, lollingite, and sphalerite, alone or variously mingled. All these occurrences of sulphides are small and rare.

There is no sharp delimitation between the pneumatolytic veins and those of the next or hydrothermal group. Some species of minerals are found in both, but in the hydrothermal group there is less evidence of replacement in the walls, the veins being in general clearly fissure veins.


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