68 



UNITED STATES MINERAL RESOURCES 



from the base of the thin ocean crust and trans- 

 ported tectonically to be emplaced along margins 

 of the continents. Coleman (1971) reviews such con- 

 cepts; Bailey, Blake, and Jones (1970) discus per- 

 tinent ophiolites in California. Certain large alpine- 

 type serpentinite masses are almost certainly sedi- 

 mentary accumulations derived fi-om tectonically 

 transported mantle material (Lockwood, 1971). 



The restriction of asbestos-bearing terranes to 

 short increments of the Appalachian belt is typical 

 of most ophiolite belts. Veinlets of chrysotile may 

 be seen sporadically in many ultramafic masses of 

 the Appalachians. Until recently, the only minable 

 concentrations vi^ere found along a 55-mile segment 

 that trends northeast in the Eastern Townships of 

 southeastern Quebec. (See Cooke, 1937; Riordon, 

 1957.) Farther northeast, noteworthy prospects have 

 been found; but only one major deposit, near Bale 

 Verte on the north coast of Newfoundland, has been 

 opened — and that only in the last decade. To the 

 south, the Belvidere Mountain deposits of northern 

 Vermont are the only minable deposits to lie outside 

 the Eastern Townships, Quebec, interval. South in 

 the Appalachian belt beyond Staten Island, N.Y., 

 no chrysotile occurrances that would warrant more 

 than cursory exploration seem to be present. 



A wholly separate ophiolite terrane of Precam- 

 brian age is exposed widely in eastern Ontario and 

 extends eastward into western Quebec. Large-scale 

 chrysotile asbestos production in Ontario began 

 with opening of the Munro mine in 1950. Since then, 

 widely separate serpentinite masses in this older 

 terrane have been targets of much exploration and 

 of mine development which is still current. 



A longer, much broader, and structurally more 

 complex belt of ultramafic bodies, emplaced during 

 Mesozoic time, crudely parallels the Pacific Coast 

 of North America. The best known interval is in 

 California where numerous small to very large ser- 

 pentinite masses occur (1) in the Coast Ranges from 

 southern part of the State to southwestern Oregon, 

 (2) to the east of the Coast Ranges across the Great 

 Valley of central California in the western foothills 

 of the Sierra Nevada, and (3) and in especially wide 

 exposures in the Klamath Mountains of northwest- 

 ern California (Bailey and others, 1964). Serpen- 

 tinites of a presumed offset extension of the belt 

 occur in east-central Oregon. Next seen in scanty 

 occurrences east of Puget Sound along the Cascades 

 of northern Washington, the belt continues north- 

 northwest through central British Columbia and 

 Yukon Territory to the vicinity of Dawson. Sub 

 parallel to the British Columbia portion and 80-160 

 miles west, along the panhandle of southeastern 



Alaska, is a separate noteworthy belt, 30 miles wide 

 and almost 400 miles long, of zoned ultramafic bodies 

 (Taylor and Noble, 1960). These bodies are little 

 serpentinized and apparently lack chrysotile de- 

 posits. The continuation of the main belt in an arcu- 

 ate trend northwest and perhaps west an undeter- 

 mined distance into central Alaska is poorly known. 

 As yet, only a few large deposits of asbestos have 

 been recognized in the Pacific belt. 



LOCALIZING FACTORS 



The succession of geologic events (Cooke, 1937; 

 Riordon, 1957) that culminated in the formation of 

 chrysotile, deposits in southeastern Quebec is typical 

 of events wherever large deposits occur. Two stages 

 of serpentinization are recognized. The first stage 

 was pervasive but incomplete and apparently oc- 

 curred during initial cooling of magma or while 

 solid ultramfic masses were being emplaced. Peri- 

 dotite, which is the principal host for asbestos veins, 

 and pyroxenite were only partly altered to serpen- 

 tine ; commonly, dunite segregations were converted 

 rather thoroughly to serpentine. Much later, after 

 or near the end of an episode of deformation that 

 modified the structural disposition of the intrusive 

 bodies, all ultramafic rocks in the vicinity of shear 

 zones and associated faults and fractures were 

 altered almost completely to serpentine. The stock- 

 works of chrysotile veins that make up the deposits 

 were formed during this second episode. A final epi- 

 sode of hydrothermal activity resulted in the forma- 

 tion of talc schist, steatite, and massive quartz- 

 carbonate bodies. These materials are largely con- 

 fined to major shear zones that transect the ser- 

 pentinites and to sheared margins of the masses. 

 In places, the asbestos was deleteriously altered by 

 the talc-forming process. 



Most of the Quebec deposits border or contain 

 small acidic intrusions which range from granite 

 to albite syenite in composition. The parts of the 

 ultramafic masses that border the granitic masses 

 may be intricately fractured and thoroughly ser- 

 pentinized. A few instances of chrysotile veining 

 granite have been noted. These relations cause many 

 investigators to believe that acid intrusions had a 

 part in the genesis of chrysotile concentrations. This 

 hypothesis is not universally upheld, however, prob- 

 ably because many chrysotile deposits exist that do 

 not show obvious association to granite. 



The location, form, and size of individual chryso- 

 tile deposits in great part have been determined by 

 the configuration of major faults and by the volume 

 of peridotite affected by subsidiary faults and frac- 

 tures ; both of which in turn reflect the relative com- 



