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

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ORIGIN 27 510205 O - 60 - 3 springs and solfataras has resulted in end products that may consist of porous masses of silica, or silica, alumina, and titania. It must be realized that considerable amounts of silicon and aluminum may actually be removed in the leaching process; on the other hand, silicon and (or) aluminum may be deposited by the altering solutions under some circumstances. This type of rock alteration may have been active in the formation of some of the Southeastern deposits of aluminous minerals in quartzose rocks. Similar deposits of pyrophyllite, alunite, and quartz in Mesozoic volcanic rocks at Kyuquot Sound, British Columbia, were thought by Clapp (1915) to have been formed by solfataric processes. Stuckey (1928) states that hydrothermal activity may have taken place at shallow depths in the Southeastern deposits; however, he favors the view that pyrophyllite was formed by hydrothermal solutions from granitic magmas at intermediate temperatures and pressures after metamorphism of the volcanic rocks. D. E. White (written communication, 1956) is doubtful that this type of alumina-silica deposit could be formed through surficial leaching by acid spring waters, with acid derived by oxidation of H 2S with atmospheric oxygen. He points out that deposits formed in this way are generally small, are restricted to, or near, the water table, have a different paragenesis (aluminous minerals formed before silica), and are not known to have undergone appreciable fluorine fixation. White considers that the deeper alumina-silica deposits of the type described by Burbank (which contains the fluorine-bearing aluminum silicate zunyite) and Levering are formed by hypogene acid leaching that probably does not involve atmospheric oxygen; he agrees that it may be possible that the aluminasilica replacement deposits of the Southeast have formed by similar processes. The environment of formation of these replacement deposits of quartz, pyrophyllite, kyanite, andalusite, and topaz in the Southeast remains a problem. Some kyanite-quartz deposits may be masses of volcanic rock that were first altered by solfataric activity and then metamorphosed to the present mineral assemblage. Other deposits, such as some andalusite-pyrophyllite-quartz deposits, for example, might represent the deeper zones of extensive solfataric centers. Possibly in some places the rock alteration was contamporaneous with mild metamorphism of the volcanic rocks. Deposits of kyanite-quartz rock and sillimanitequartz rock that seem to be metamorphosed sediments might have several possible relationships to a volcanic environment. These deposits probably were formed by the metamorphism of beds of sandy clay or clayey sand, which might have been formed in several ways. Some volcanic rocks are readily weathered to very clayey soils under certain conditions; some volcanic soils from Java, Sumatra, and Hawaii have been found to carry free alumina (Clarke, 1924, p. 502), and the ultimate product of thorough weathering can be clay or bauxite. This material then might be eroded and transported by streams to a site of deposition, such as a lake basin. The parent sediment of kyanite quartzite may have been a mixture of (a) quartz sand derived from siliceous residue of hydrothermal origin or from some quartz-bearing rock, and (b) transported clay that has been derived from weathered basalt (Farmville district) or from more silicic volcanic rocks (Kings Mountain district). High-alumina clays of probable sedimentary origin are associated with volcanic rocks at several places in the Pacific Coast States. Some geologists have regarded these clays as being of volcanic origin, but Alien (1946) has given evidence for the sedimentary origin of the following clay deposits: lone clay, California; Cowlitz clay deposit, Washington; and the Hobart Butte and Molalla clay deposits in Oregon. Sedimentary deposits of clay and quartz sand in a volcanic terrane might also be formed by the erosion of hydrothermal silica and clay from a solfataric center and deposition of the material in a restricted basin that was uncontaminated by other sediments. In this way, alumina-silica deposits of sedimentary origin might occur in the same region as alumina-silica deposits of hydrothermal origin. The basic feature of both these processes hydrothermal leaching and weathering is the removal of practically all constituents except alumina and silica; a certain proportion of alumina and silica might be removed, too. Titania is also a final product of both processes, which doubtless explains the widespread occurrence of small amounts of rutile in the deposits of Al2Si0 5 minerals in quartzose rocks. ORIGIN OF HIGH-ALUMINA QUARTZOSE DEPOSITS IN OTHER REGIONS Various modes of origin have been ascribed to quartzose deposits of high-alumina minerals in other parts of the world. In the Picuris Range, N. Mex. (Montgomery, 1953), beds of kyanite quartzite and sillimanite quartzite, ranging from a few feet to
28 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES about 25 feet in thickness and extending for distances of 1 mile or more, occur in a series of barren quartzite beds that is over 2,500 feet thick. The beds of kyanite and sillimanite quartzites were evidently formed by regional metamorphism of clayrich beds. Hydrothermal activity associated with later quartz veins caused reworking and some segregation of the high-alumina minerals along the quartz veins. Kyanite quartzite deposits in Kenya (Temperley, 1953) seem to be metamorphosed sedimentary rocks very similar to those in the Farmville district, Virginia, and are likewise associated with biotite and hornblende gneisses. Temperley points out, however, that they could not have been pure clays because their molecular ratio of Si02 to A1203 is too high. He finds that loesses from different regions have molecular ratios of Si02 to A1203 corresponding to the ratio in the kyanite quartzites and suggests as a possible mode of origin that the rocks were originally loesses whose compositions were modified by metamorphic differentiation. The Sid2-Al203 molecular ratio of some bentonites and river silts also fall in this range. Conversion of any of these materials into alumina-silica rock would require the removal of large amounts of sodium, potassium, calcium, magnesium, and iron during metamorphism. As an alternate form of origin, Temperley proposes that the original rock was sandstone, and that aluminum was introduced into the sandstone by migration from aluminous sedimentary rocks during metamorphism. He regards this mode of origin as more probable but admits that this theory is not altogether satisfactory. Kyanite quartzite and kyanite-quartz schist deposits at Lapsa Buru, India (Dunn, 1929) are closely associated with hornblende schist, and Dunn has suggested that the kyanite rocks are metamorphosed clays that had been formed originally from the weathering of basalt. Dunn (1933) later suggested that aluminum may have been reworked or segregated from aluminous sedimentary rocks during metamorphism. He has modified this view further (1937), and now thinks that magmatic waters may have been active in segregating aluminum, or in removing other constituents to leave the rock enriched in aluminum. At White Mountain in the northern Inyo Range, Calif. (Kerr, 1932; Lemmon4), lenticular bodies of high-alumina minerals occur in quartzite that is associated with metamorphosed volcanic rocks; 4 Lemmon, D. M., 1937, Geology of the andalusite deposit in the northern Inyo Range ,Calif.: Stanford University, Doctor of Philosophy dissertation. these rocks are intruded by quartz monzonite. Andalusite is the most abundant aluminous mineral and is commonly accompanied by corundum, quartz, muscovite, pyrophyllite, diaspore, alunite, rutile, and lazulite; topaz, tourmaline, barite, and other minerals are also present in lesser amounts. The deposit was apparently formed in two stages: thermal metamorphism of aluminous sedimentary or volcanic rocks, followed by hydrothermal activity that caused formation of such minerals as pyrophyllite, muscovite, alunite, and lazulite. A somewhat similar deposit occurs at Semis Bugu, Kazakstan, U.S.S.R. (Ozerov, 1933). Lenticular bodies of high-alumina rock occur in fracture zones in silicified volcanic flows and tuffs (called secondary quartzite by Ozerov) that are intruded by granite and granodiorite. The lenses have strong mineral zoning, andalusite being the principal mineral in the outer portions and corundum predominating in the center. Quartz, muscovite, hematite, and minor amounts of diaspore, pyrophyllite, alunite, rutile, pyrite, barite, and zircon are also present. Ozerov concludes that the deposit was formed by the action of magmatic fluids during stages of declining temperature and changing composition. The volcanic rocks were silicified at an early stage; decomposition of feldspar provided A1203 and Si02 for the formation of andalusite, which was later replaced by corundum. The dumortierite-andalusite deposit at Oreana, Nev. (Kerr and Jenney, 1935), occurs in quartzsericite schist (originally rhyolitic and trachytic tuffs) that is intruded by quartz monzonite and granite porphyry. Mineralization seems to have taken place in pneumatolytic and hydrothermal stages by the action of magmatic fluids on aluminarich zones in the tuffs. Bodies of quartz-sillimanite rock occur in kyanite schist and gneiss at Mount Crawford near Williamstown, South Australia (Alderman, 1942, 1950). Kyanite-bearing pegmatites and quartz veins are also present. The sillimanite is extensively altered to clay (mainly kaolinite and minor amounts of dickite), and kyanite is altered to damourite. Alderman concludes that sillimanite and kyanite were formed through metasomatism of schists by high-alumina fluids of magmatic origin at high temperatures. During the last phase of mineralization, lower temperature hydrothermal solutions altered sillimanite to clay and kyanite to mica.
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