THE GEOLOGY AND GEOGRAPHIC DISTRIBUTION OF NATURAL ASBESTOS OCCURRENCES IN THE CONTERMINOUS UNITED STATES

Through an extensive search of the geologic literature, this study has located and described about 800 natural asbestos occurrences in the conterminous United States. Of these sites, 143 represent former asbestos mines, 218 are former asbestos exploration prospects, and the others are simply sites where a natural occurrence is described. A natural asbestos occurrence has been reported in 35 of the 50 U.S. States, including Alaska (Alaskan occurrences not yet compiled by this study; no occurrences known in Hawaii). The asbestos occurrences range widely in size and character, including large, concentrated bodies of asbestos minerals; vein deposits from millimeters to meters in width; asbestos minerals that occur as a trace constituent of a larger mineral deposit or bedrock; and asbestos transported by alluvial processes (landslide, alluvial fan). The largest U.S. asbestos deposits are massive replacement or vein deposits of chrysotile in serpentinized ultramafic rocks, once mined in Vermont and California, and chrysotile veins replacing contact metamorphosed dolostone, once mined in Arizona. A number of anthophyllite asbestos mines once operated in the ultramafic belt of the eastern U.S.; they were relatively small operations supplying to local specialty markets and products. A dozen small tremolite asbestos producers also supplied to niche markets. No mine in the U.S. produced a product described as actinolite asbestos, amosite, or crocidolite; however, many small U.S. occurrences are known.

Asbestos mineralization occurs in Mg-rich host rocks altered by silica-rich fluids under conditions of moderate temperatures and moderate pressures. Specifically, asbestos deposits in the U.S. are hosted by metamorphosed and metasomatized ultramafic rocks (particularly serpentinite) and some mafic rocks (metabasalt, metagabbro), metadolostones, and contact metamorphosed iron formation. Asbestiform amphiboles also occur as accessory minerals in some hydrothermally altered, mafic to ultramafic types of alkaline igneous intrusions. In all of the typical asbestos-forming geologic environments, asbestos occurrences are still relatively rare and occur where ideal geologic conditions were present (microfracturing, siliceous fluids, specific temperature and pressure conditions, and subsequent preservation).