Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

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KYANITE DEPOSITS 77 quantities. At Henry Knob and other localities pyrite constitutes as much as 8 percent of the rock. Chemical and spectrochemieal analyses of kyanite quartzite ore from Henry Knob and kyanitechloritoid quartzite from a small ridge about one mile south of Crowders Mountain village are given in table 3. The ore at Henry Knob consists almost entirely of quartz, kyanite, and pyrite but contains minor amounts of white mica and rutile. The kyanite-chloritoid quartzite is a light-gray rock consisting of radial aggregates of chloritoid and coarse parallel prisms of kyanite in a groundmass of fine-grained quartz (fig. 14). This rock is interlayered with kyanite quartzite. Although staurolite and chloritoid occur in kyanite quartzite along the northern part of the east limb of the Sherrars Gap syncline, the rock represented by the chemical analyses occurs over a distance of only 200 feet. This rock has the following mode: Quartz 40 percent, kyanite 40 percent, chloritoid 17 percent, magnetite 3 percent, and minor amounts of andalusite, staurolite, rutile, and zircon. FIGURE 48. Foliated kyanite quartzite at south end of The Pinnacle, Gaston County, N.C., looking southeast along: base of cliff. or sprays and flattened radial aggregates along the foliation planes. Individual blades are from 3 to 10 mm long; some radial sprays are as much as 8 inches in diameter, but these are rare. In thin section the principal constituents are seen to be quartz, kyanite, and, locally, white mica. Common accessory minerals are white mica, rutile, pyrite, magnetite, and zircon. Rare accessory minerals include apatite, barite, lazulite, tourmaline, pyrophyllite, and dolomite. The groundmass consists principally of fine-grained (0.05 to 0.5 mm) quartz (fig. 14). The habit and appearance of kyanite vary from well-formed blades with few or no inclusions and regular crystal boundaries (rare) to typical irregular elongate crystals with varying amounts of inclusions (fig. 2). In quartzite from Henry Knob and some parts of Clubb Mountain, kyanite occurs as well-formed blades holding numerous quartz inclusions (fig. 8). Some kyanite crystals show the obvious effects of deformation: crystals are bent, fractured, and faulted. Irregular quartz embayments and fracture fillings characterize these deformed kyanite crystals. Rutile occurs as very small crystals generally evenly dispersed through the rock. Pyrite content is variable, but the mineral generally is present in at least trace FIGURE 49. Complex folding in kyanite auartzite, Shelton property, Gaston County, N.C., looking: southwest along trend of outcrop. Photograph by W. C. Overstreet.
78 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES Replacement of kyanite quartzite by clay is widespread in the central part of Clubb Mountain (pi. 8). A large volume of kyanite quartzite on the northeast side of Crowders Mountain (pi. 7) is replaced by dickite. The mineralogy and general microscopic character of the clay at Clubb Mountain resemble those at Crowders Mountain. Much minor shearing and faulting have occurred in the area of replacement at Crowders Mountain. In both areas replacement has taken place along a complex ramifying system of small veins; dickite has replaced quartz as well as kyanite. Locally the quartzite is completely altered to dickite. Andalusite is associated with the kyanite in these areas, and pyrite and goyazite (a strontium-aluminum phosphate) occur locally; the unaltered nature of pyrite and the close association of pyrite and goyazite with clay suggest that pyrite and goyazite formed at the time of alteration to clay. Local pseudomorphic replacement of kyanite by clay occurs at the north end of the Shelton property. Several thin sections of kyanite quartzite from other parts of the district indicate incipient replacement of kyanite by clay. Quartz veins and irregular masses of coarse milky quartz are common in many kyanite quartzite deposits. Some of these veins apparently were introduced into cross fractures in the quartzite; some masses of quartz are conformable lenses and segregations in the foliated quartzite. In general these veins and masses of quartz carry little or no kyanite or other minerals. At a few places, as The Pinnacle, Crowders Mountain, and Clubb Mountain, kyanite occurs along the margins of the vein (fig. 25) or as radial aggregates in the quartz. Other minerals occurring sparingly in quartz veins at Clubb Mountain are lazulite, ilmenite, and the calcium aluminum phosphate crandallite. Because more quartz veins occur in quartzite than in schist, and because all kyanite-bearing quartz veins are restricted to kyanite quartzite, it is thought that these veins and irregular masses of quartz were produced by metamorphic differentiation; hydrothermal fluids perhaps aided in mobilizing the materials. SILLIMANITE QUARTZITE Sillimanite quartzite occurs as thin lenses and beds within about 2,000 feet of the Yorkville quartz monzonite contact in the Crowders Mountain-Henry Knob area (pi. 7). Figure 42 shows the location of the sillimanite quartzite beds in this area; locally, these beds are gradational along strike with kyanite quartzite. A few short septa of sillimanite quartzite occur in the Yorkville quartz monzonite in this area. In the Reese Mountain-Clubb Mountain area (pi. 8), sillimanite quartzite occurs as thin beds and lenses, principally at Reese Mountain and southwest of Machpelah Church. Most sillimanite quartzite beds are from 10 to 30 feet thick. In highly folded areas, as at the Will Knox property, these beds are lenticular; in less deformed areas the beds are continuous along strike for as much as 3,000 feet. The beds are generally vertical and are surrounded by white mica schist that carries small and varying amounts of sillimanite, andalusite, and garnet. Sillimanite quartzite is more resistant to weathering than the surrounding schist and crops out as low narrow ledges. Reese Mountain, about 150 feet high, is the most prominent hill underlain by sillimanite quartzite. At many localities sillimanite quartzite is interlayered with, or in contact with, coarse-grained granular magnetite-bearing quartzite which carries little or no sillimanite. This rock is probably a metamorphosed sandstone or siltstone poor in clay. Sillimanite quartzite is typically a light-gray rock. Sillimanite occurs as thin bone-white mats or flattened pencillike aggregates, varying from a few millimeters to about S 1/^ cm in length, evenly distributed through the quartz groundmass. These mats generally define a strong foliation in the quartzite, but in some places a massive structure is produced by the randomly oriented mats and anastomosing stringers of sillimanite. Each mat consists of extremely fine grained fibers of sillimanite (fibrolite). In general, sillimanite quartzite has less iron than kyanite quartzite. Thin-section study of sillimanite quartzite indicates little compositional variation between the different beds or deposits. The principal minerals are quartz, sillimanite, and, in several places, andalusite. Kyanite and diaspore are major constituents locally. Common accessory minerals are rutile, white mica, magnetite, and pyrite. Rare accessory minerals include lazulite, topaz, zircon, chloritoid, and apatite; lazulite is locally abundant at a few places. The mats and aggregates of sillimanite are seen in thin section to consist of radiating aggregates of fibrolite, each prism being 0.04 to 0.2 mm long and less than 0.004 mm wide (fig. 15). Coarse prisms of sillimanite, about 2 mm long and from 0.05 to 0.09 mm wide, are intergrown with the fibrolite. Both varieties of sillimanite are generally inti-
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UNITED STATES DEPARTMENT OF THE INT
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IV Page Kyanite deposits Continued
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VI CONTENTS TABLES TABLE 1. Major d
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Type of mica Muscovite . . . Do....
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Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

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