THE FRANKLIN MINING DISTRICT
Masses of pegmatite, ranging from rather large lenses to small dikes, form a considerable part of the pre-Cambrian complex. They cut both the gneiss and the limestone, and some of them at least are younger than the zinc ore deposits, hence their intrusion probably continued through a long period. Spurr and Lewis (234) pointed out that in some places the pegmatite dikes have been squeezed into lenses or even sheared into blocks that have reacted on their whole surface with the enclosing limestone. A shearing movement sufficient to dissect the pegmatite dikes so effectively could hardly have failed similarly to affect the nearby ore bodies had they been in existence. As they are not so dislocated, it seems more reasonable to suppose that they were formed after the intrusion and deformation of the older pegmatites. The later pegmatites, on the contrary, cut the ore deposits and are not extensively deformed.
The pegmatites are all of granitic composition and consist essentially of dominant microcline and oligoclase, subordinate quartz, and accessory apatite, muscovite, titanite, epidote, and allanite and rarely thorite and zircon. Magnetite is rarely absent and may be dominant, as described in the next section. Galena and sphalerite in minute grains are common and are believed to be original constituents. Zoisite occurs as an alteration product of microcline. At the contacts of the pegmatite with the limestone and with the zinc ore bodies, groups of reaction minerals and of pneumatolytic minerals have generally been developed. They are described on the next few pages.
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© by Herb Yeates 1997-2006.
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