ASBESTOS 



69 



petencies of contiguous rock units. Riordon and 

 others (1957) provide descriptions that permit com- 

 parisons of individual deposits. Major persistent 

 shear zones in serpentinite are largely barren of 

 asbestos; but stockworks of asbestos veins occupy 

 intricately fractured rocks that border these shear 

 zones. Similarly, thoroughly serpentinized aureoles 

 adjacent to acid intrusions and completely altered 

 dunite bodies tend to fail by shearing and flow^; in 

 response to the resultant stresses the more compe- 

 tent peridotites adjacent to these masses developed 

 and sustaind complex networks of open fractures in 

 which chrysotile was formed. Where peridotite 

 masses have considerable breadth, the chrysotile 

 deposits commonly hug a margin of the peridotite 

 mass, reflecting again the tendency for optimum 

 fracturing to occur where contrasts in rock com- 

 petency exist. Several of the larger deposits occur 

 where an irregularity in the plane between peri- 

 dotite and host rock influenced the trend and distri- 

 bution of faults. Others occur in "wedges" of frac- 

 tured peridotite where throughgoing shear zones 

 converge along strike or dip. 



SIZE, SHAPE, AND ASBESTOS CONTENT 



Ore bodies are irregular in outline, may be erratic 

 in asbestos content so that margins of ore bodies 

 are sharp to ill defined, and range considerably in 

 size. In Quebec, the minimum dimension of rudely 

 eUipsoidal bodies is about 400 feet. Lenticular bodies 

 are 200-2,500 feet thick and have maximum dimen- 

 sions of 1,000-5,000 feet. Most of these bodies bot- 

 tom at depths of 400-1,000 feet, but one major ore 

 shoot extends to a depth of at least 1,800 feet. In- 

 dividual deposits currently mined in Quebec origin- 

 ally contained 10-200 million tons of ore. A very 

 exceptional deposit, the site of the Jeffrey mine 

 with production capacity since the mid-1950's in 

 excess of 500,000 tons of fiber per year, still has ore 

 reserves on the order of 500 million tons. 



Asbestos content and quality vary. Most ore bodies 

 are stockworks of cross-fiber veins ; in some tabular 

 bodies, veins, disposed in parallel, ribbon large parts 

 of the serpentinized peridotite. Some cross-fiber 

 veins are 6 inches or more thick, but all except a 

 small proportion are less than 1/2 inch wide ; ribbon- 

 vein occurrences in particular may average much 

 less. Thus, one deposit may consistently yield short 

 fibers ; whereas another yields various lengths. Simi- 

 larly, some deposits are rather uniform in quality of 

 fiber; others contain both harsh and soft fibers. 



Deposits with an overall asbestos content of IV2-2 

 percent have been mined ; some deposits locally have 

 contents as high as 25 percent; the typical occur- 



rence has an overall content of 5-7 percent. Thus 

 the deposits are of sizes and proportions amenable 

 to large-scale open-pit or underground mining meth- 

 ods, and in some instances only large-scale opera- 

 tions would permit the economic recovery of a 

 range of variable ores. 



CHRYSOTILE DEPOSITS IN LIMESTONE 



The Arizona type of chrysotile deposit is made up 

 of one or more tabular asbestos-bearing zones 6-8 

 inches thick and of small areal extent. These tabular 

 lenses were localized where cherty or otherwise sili- 

 ceous magnesian limestones were metamorphosed 

 adjacent to igneous intrusions — in most instances 

 sill-form intrusions of gabbro or diabase. The 

 Arizona-type deposits are mostly of small tonnage 

 and are amenable to mining only by small-scale, 

 high-cost methods; but the asbestos content of the 

 ore is high, long fiber predominates, and low iron 

 content is characteristic. Thus asbestos from the 

 Arizona-type deposits can complement the short 

 magnetite-rich chrysotile characteristic of most 

 Quebec-type deposits. 



ASBESTOS RESOURCES IN NORTH AMERICA 



A review of exploration activities provides a 

 basis for an optimistic view that many geologically 

 favorable sites remain to be prospected and that 

 known occurrences have not everywhere been widely 

 or deeply enough explored. No attempt is made here 

 to appraise asbestos resources outside North 

 America. The following summation of the resources 

 in the United States and contiguous areas (table 

 11) is probably conservative. 



Table 11. — Chrysotile asbestos resources of North America, 

 in short tons of fiber 



Identified Hypothetical 



resources ^ resources ^ 



Canada 70,000,000 35,000,000 



United States '3,700,000 2,000,000 



Mexico 1,500,000 



^ Identified resources : Specific, identified mineral deposits that may or 

 may not be evaluated as to extent and grade, and whose contained min- 

 erals may or may not be profitably recoverable with existing technology 

 and economic conditions. 



- Hypothetical resources: Undiscovered mineral deposits, whether of re- 

 coverable or subeconomic grade, that are geologically predictable as exist- 

 ing in known districts. 



3 Exclusive of Group 7 resources in California, see under "United 

 States." 



CANADA 



Canadian production prior to 1950 was almost 

 exclusively from the Eastern Townships of Quebec. 

 Since 1950 the volume of reserves outlined in On- 

 tario, British Columbia, Yukon Territory, New- 

 foundland, and other parts of Quebec has increased 

 steadily, and about 15 percent of Canadian produc- 

 tion is from these sources. Considerable additions 

 also have been made to the known reserves in the 



