Asbestos-Bearing Talc Deposits, Southern Death Valley Region, California

摘要

Beginning in 1910, at least 43 talc deposits have been prospected and mined from occurrences scattered throughout several mountain ranges that surround the southern part of Death Valley, California. Until 1988, talc was mined from deposits that are now part of Death Valley National Park. Talc mining is still being conducted in the Kingston Range, east of the Park. Death Valley talc has been used in wall tile, as an extender in paints, and in cosmetics. The southern Death Valley talcs are talc-tremolite deposits that are geologically similar across the region; they are consistently associated with a carbonate horizon of the Crystal Spring Formation of Proterozoic age. In this interval, thick regionally persistent gabbroic sills intruded dolomite during the Mesoproterozoic, and formed laminated talc-tremolite-rich rock along the sill-dolomite contacts. Metasomatic reactions during sill emplacement caused the massive replacement of dolomite by talc-tremolite-rich bodies, which are approximately 500 to 5,000 feet long and 10 to 100 feet thick. Relative proportions of talc versus tremolite vary across the deposits and either mineral can predominate within any particular deposit. Our petrographic examinations of the ore suggest the talc and tremolite were co-genetic. Samples of talc-tremolite rock were collected by the lead author in the Alexander Hills-Kingston Range region, located east of southern Death Valley National Park. The samples were analyzed by X-ray diffraction and examined by scanning electron microscopy accompanied by energy-dispersive x-ray analyses. Our examination found asbestiform tremolite, including bundles of fibers and loose fibers, to be common in talcose rock from all of the sample sites. Asbestiform winchite and/or asbestiform richterite were identified in 5 of 13 talcose samples examined by this study. These observations suggest.that mining and milling of the southern Death Valley talc deposits could produce asbestos-bearing dusts.