ASBESTOS 



67 



ous plane of slippage; or as mass-fiber, which is an 

 aggregation of variously oriented fibers or stellate 

 groups of radially arranged fibers. Most cross-fiber 

 veins are split by one or more partings about paral- 

 lel to vein walls ; as a result, most of the fibers of 

 this mode of occurrence are considerably shorter 

 than vein width. Asbestos of the cross-fiber mode is 

 the type most readily separated from enclosing rock 

 and prepared for use. Slip-fibers may be of consid- 

 erable length and flexibility, but commonly present 

 difficulties in preparation. Chrysotile, crocidolite, 

 and amosite occur mainly as the cross-fiber type. 

 Some slip-fiber is found in most chrysotile deposits, 

 and from a few it is the principal type recovered. 

 The remaining asbestiform amphiboles — anthophyl- 

 lite, tremolite, and actinolite — occur mainly as mass- 

 fiber or slip-fiber, are least amenable to separation, 

 and commonly are brittle or weak and therefore 

 limited in use. The rare deposits that do include 

 these minerals as strong flexible fibers in cross-fiber 

 veins contain them in very small amounts. 



GENERAL SETTINGS 



Chrysotile asbestos the world over occurs in two 

 geologic settings of diff'erent type. Chrysotile is 

 won mainly from large stockworks of veins in ser- 

 pentinized peridotite, pyroxenite, and dunite, col- 

 lectively termed "serpentinites." From some of their 

 prime occurrences these deposits can be termed the 

 "Quebec type." Chrysotile asbestos is mined sec- 

 ondarily from deposits of the Arizona type, in which 

 the veins are confined to thin serpentine layers in 

 limestone. 



Anthophyllite and tremolite asbestos deposits oc- 

 cur in ultramafic intrusions and in associated green- 

 stones and amphibolites ; thus, their gross geologic 

 setting parallels that of the principal chrysotile de- 

 posits. The deposits are small and erratic in distri- 

 bution and in character of fibers. 



Crocidolite and amosite occur only in certain fine- 

 grained cherty ferruginous metasediments of the 

 sort commonly designated "banded ironstones" or 

 "banded iron formation." The banded iron forma- 

 tions that host crocidolite occurrences are addition- 

 ally distinctive in that aggregates of sodium-rich 

 minerals occur in certain layers. Such geologic set- 

 tings are known in only a few localities worldwide, 

 and in still fewer places have geologic processes 

 coincided in such a way as to bring about the forma- 

 tion of crocidolite and amosite (Cilliers and Genis, 

 1964). Locally in the northern Transvaal, amosite 

 and crocidolite occur together (Cilliers, 1964). The 

 only exploited deposit of crocidolite in the Western 

 Hemisphere, and the only recognized potential 



source other than the South African and Australian 

 occurrences previously mentioned, is in a remote 

 part of east-central Bolivia. Reportedly, blue asbes- 

 tos from Bolivia lacks tensile strength inherent in 

 the crocidolite from other sources. 



CHRYSOTILE DEPOSITS IN MASSIVE SERPENTINITE 



REGIONAL SETTINGS OF PRINCIPAL 

 NORTH AMERICAN DEPOSITS 



The zone of ultramafic rocks in the Appalachian 

 belt of eastern North America is a prime example 

 of the regional setting found wherever chrysotile 

 deposits of the Quebec type exist. Hundreds of ultra- 

 mafic intrusive masses, ranging from essentially 

 concordant lenses and sills a few tens of feet wide 

 and of no great length to thick irregular tabular or 

 bulbous masses with maximum dimensions measura- 

 ble in miles, are confined to a belt that is commonly 

 less than 5 miles wide, that is only in a few places 

 more than 25 miles wide, and that stretches north- 

 east from Alabama to Nevirfoundland along a sinu- 

 ous trace of more than 2,000 miles. This trace marks 

 the site of an ancient fold mountain belt, which 

 began as a eugeosynclinal trough that — starting in 

 Late Ordovician time — accumulated a thick sequence 

 of sediments, which were indurated, were invaded 

 by intrusive masses, and were complexly folded. 

 The belt was terminated as mountain terrane in 

 Middle Devonian time. 



The ultramafic rocks of this and other asbestos- 

 bearing belts are dominated by peridotite, but in- 

 clude much dunite and pyroxenite. These ultramafic 

 rocks are commonly much altered to form serpen- 

 tinites, and are intricately juxtaposed with basaltic 

 volcanics, diabase, and gabbro to make up complexes 

 that have been termed "ophiolites." The ophiolites 

 of orogenic (mountain-forming) belts are generally 

 designated as the "alpine type" (Thayer, 1967). 

 The genesis of the alpine-type ophiolites, and espe- 

 cially the mode of transport and emplacement of the 

 serpentinites, long has been the subject of spirited 

 debate ; the debate has been accelerated particularly 

 during the last decade with the rapid evolution of 

 concepts of global tectonics. Early workers regarded 

 peridotite, pyroxenite, and dunite as rock types di- 

 rectly crystallized from a magma; the lack of asso- 

 ciated thermal metamorphism and other magmatic 

 features ultimately forced the conclusion that alpine- 

 type serpentinites were emplaced in their present 

 host rocks at low temperatures and probably in the 

 solid state (for review, see Chidester, 1962). Recent 

 hypotheses — in various versions — propose that ser- 

 pentinites represent slabs of ultramfic mantle ripped 



