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

More documents

Recommendations

Info

KYANITE DEPOSITS 39 KYANITE QUARTZITE The kyanite quartzite is typically a medium- to coarse-grained light-gray gneissic rock made up mainly of quartz and kyanite. The kyanite crystals generally lie within the foliation planes, and commonly have a linear orientation parallel to fold axes. In rocks with very pronounced gneissic structure, kyanite and quartz are segregated into thin layers. The kyanite crystals are commonly several centimeters long and white to light gray, though brightblue kyanite occurs at Baker Mountain and vicinity. The kyanite content varies considerably, but is generally between 15 and 30 percent. Chemical and mineral analyses of a composite sample of kyanite quartzite taken from drill core at Willis Mountain are given in table 3. A few percent of mica and pyrite are commonly present. Muscovite is so abundant in places that the rock becomes a kyanite-quartz-muscovite schist. Two varieties of muscovite occur: a common white variety found at Willis Mountain and most of the other localities, and a less abundant bright-green chromian muscovite that is found at Baker Mountain and near Madisonville (table 4). Small amounts of the sodium mica paragonite and the calcium mica margarite are found together in an outcrop along Willis River to the southwest of Willis Mountain (table 4). Disseminated pyrite occurs in more than half of the samples of unweathered rock that were examined; much of the weathered rock has leached cavities and limonite stains that indicate the former presence of pyrite. Rutile is one of the most persistent accessory minerals, generally occurring in amounts of less than 1 percent as small grains disseminated through kyanite and quartz. The occurrence of rutile with kyanite and the alteration of kyanite to mica in the kyanite quartzite deposits near Madisonville have been described by Watson and Watkins (1911); see also figure 10. Less common accessory minerals (found in 10 to 20 percent of the thin sections examined) are topaz, apatite, and zircon. Topaz, in small rounded grains about 0.2 mm across, is the most abundant of these, but it seldom exceeds 1 percent of the rock. Rare accessories (found in very small amounts in less than 5 percent of the thin sections examined) are diaspore, corundum, barite, spinel, garnet, and magnetite. Very small amounts of clay of probable hydrothermal origin are present in some of the kyanite quartzite from Willis Mountain. The clay is generally visible only by microscopic examination but is apparent to the naked eye in a few sections of drill core. Clay in stringers a few millimeters thick has been identified as dickite, whereas clay that replaced kyanite crystals adjacent to the dickite string ers, was found to be kaolinite. 5 In the highly weathered kyanite quartzite at Baker Mountain, clay of probable supergene origin is abundant. It has been identified as kaolinite and halloysite and also mixtures of these two minerals. Coarse, massive kyanite occurs in small elongated bodies that range from a few inches to 90 feet in length in a zone about 1,000 feet long in the kyanite quartzite at Willis Mountain. Kyanite makes up 90 percent or more of the rock; quartz and muscovite are the principal accessory minerals (fig. 7). Smaller aggregates of similar coarse kyanite are scattered through some of the other kyanite quartzite deposits. The principal deposits of kyanite quartzite in the region occur at Willis Mountain and vicinity and at Woods Mountain in Buckingham County, at Baker and Leigh Mountains in Prince Edward County, and near Madisonville in Charlotte County (fig. 27, pis. 2, 3). Thickness of the kyanite quartzite layers ranges from a few feet to several hundred feet; the layers are thickest where highly folded, as at Willis Mountain. Barren granular quartz forms layers that are 10 to 20 feet thick in kyanite quartzite in some localities, particularly at Woods Mountain. The kyanite quartzite probably is in the same stratigraphic position throughout the region in the upper part of the thick series of biotite gneiss and in rather close association with hornblende gneiss. The parent rock of the kyanite quartzite probably was a clay-bearing sandstone. HORNBLENDE GNEISS AND SCHIST Hornblende gneiss and schist exhibit considerable variety, ranging from very fine grained dark greenish-black well-foliated rock to extremely coarse massive amphibolite; it is generally fine to medium grained, and has good foliation and lineation. Hornblende and plagioclase (whose composition is generally in the range An15_40 ) are the essential constituents; quartz is generally present also. Biotite, epidote, and garnet are commonly present. Minor partial alteration of hornblende to chlorite is rather widespread; hornblende is partly altered to palegreen or colorless amphibole (probably tremolite or actinolite) at a few places. Accessory minerals include the opaques (probably ilmenite in most places), apatite, rutile, sphene, and calcite. Scapo- 3 The clay minerals have been identified from X-ray determinations by F. A. Hildebrand and electron microscope examinations by C. P. Da vis and E. J. Dwornik.
40 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES lite was found in hornblende-bearing rocks at two localities just east of Woods Mountain and at the copper prospect on Round Mountain. Fine-grained bright-green chlorite-actinolite schist with talc(?) was found in a belt about 1 mile long extending southwest from Willis Mountain and also at 2 localities near Baker Mountain. Layers rich in epidote and quartz occur at various places, especially in hornblende gneiss several miles east of Woods Mountain. In the Baker Mountain-Madisonville area the thickest series of hornblende gneiss occurs between the thick series of biotite gneiss and muscovitequartz schist and with kyanite quartzite at the same stratigraphic position (pi. 3). The hornblende gneiss and schist in this area are typically very fine grained, and have strong lineation formed by alinement of the hornblende needles. Anthophyllite is the major amphibole in coarse garnet-amphibole gneiss exposed about iy% miles northeast of Darlington Heights. Much of the area between Willis Mountain and Woods Mountain is underlain by hornblende gneiss, which appears to occur above the horizon of the kyanite quartzite; thin layers of hornblende gneiss are also associated with biotite gneiss in the center of the Whispering Creek anticline (pi. 2). The hornblende gneiss is particularly well exposed along the stream valleys. One of the best exposures is on Willis River just below the old dam at Arcanum. Here layers of very fine grained garnetiferous hornblende gneiss alternate with layers of medium-grained gneiss in crumpled folds plunging 10° to 15° to the northeast. Chemical and spectrochemical analyses of a sample of medium-grained hornblende gneiss and a sample of fine-grained gneiss from this locality are given in table 6. Plagioclase has a composition of about An20 in VB 111 and nearly An30 in VB 112. Epidote is abundant in VB 112 and absent in VB 111; a little calcite replaces the hornblende in VB 111. Very coarse massive amphibolite is well exposed in a road-metal quarry operated by the State about 0.9 mile north of U.S. Highway 60 at the east edge of this area (pi. 2). Hornblende and plagioclase (An25 ) are the principal constituents; considerable pale-green chlorite is present, and partly replaces both hornblende and feldspar. A little anthophyllite, garnet, and calcite are found locally. OTHER QUARTZOSE ROCKS Thin short layers of barren quartzite occur at the south end of the Whispering Creek anticline and at several other places in the district. Dense blue-gray ferruginous quartz rock which has disseminations and stringers a few inches thick of magnetite and specular hematite and in some places fine-grained pink to brown manganiferous garnet forms layers as much as 10 feet thick. Most of these thin layers of iron-manganese-quartz rock are in hornblende gneiss; originally they probably were ferruginous and manganiferous chert beds of the type associated with basaltic lavas in many regions. Coarse crystals of dark-brown staurolite are found in a thin layer of spinel-bearing quartz rock at the north end of the Madisonville kyanite deposit. Quartzose rock banded with layers of coarse garnet, actinolite, and anthophyllite, containing a few percent of kyanite, sillimanite, and staurolite, occurs at various localities at the north end of the Whispering Creek anticline (pi. 2). The largest deposit underlies Tower Hill, where the rock is exposed over widths of 50 to 75 feet; red garnets, 1A t° % of an inch across, are extremely abundant. The mineral composition indicates that the rock contains considerable lime and magnesia; possibly it was originally a dolomite that contained sand and clay. GRAPHITIC SCHIST Fine-grained bluish-gray schist having tightly crinkled cleavage is exposed in the headwaters of Whispering Creek to the west of biotite gneiss (pi. 2). The rock is made up of quartz, fine shreds of muscovite, and pale-brown biotite and is thoroughly permeated with specks and tiny grains of opaque material that is probably graphite. This schist appears less metamorphosed than the nearby biotite and hornblende gneisses because it is fine grained here and has relict bedding where exposed along the road about 1 mile southeast of Dillwyn (locality not shown on pi. 2). Since it lies south of the trend of the Arvonia syncline, it is probably equivalent to the Arvonia slate of Ordovician age. A few miles north of Dillwyn (and just west of the mapped area), fine-grained fissile quartz-chlorite-muscovite schist, containing less graphite and a little kyanite, is exposed for a distance of several miles; this schist also seems to be in the Arvonia syncline. The bluegray siliceous graphite schist closely resembles the siliceous graphite schist member of the Archer Creek formation of probable Paleozoic age in the James River Valley, about 25 miles to the west (Espenshade, 1954). It could not be determined within the mapped area whether the graphitic schist is in depositional
Page 1 and 2: Kyanite, Sillimanite, and Andalusit
Page 3 and 4: UNITED STATES DEPARTMENT OF THE INT
Page 5 and 6: IV Page Kyanite deposits Continued
Page 7 and 8: VI CONTENTS TABLES TABLE 1. Major d
Page 9 and 10: KYANITE, SILLIMANITE, AND ANDALUSIT
Page 15 and 16: 8 KYANITE, SILLIMANITE, AND ANDALUS
Page 17 and 18: Type of mica Muscovite . . . Do....
Page 19 and 20: 12 KYANITE, SILLIMANITE, AND ANDALU
Page 45: 38 KYANITE, SILLIMANITE, AND ANDALU
Page 97 and 98:
90 KYANITE, SILLIMANITE, AND ANDALU
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
100 KYANITE, SILLIMANITE, AND ANDAL
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
120 KYANITE, SILLIMANITE. AND ANDAL
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?