A USGS Study of Talc Deposits and Associated Amphibole Asbestos Within Mined Deposits of the Southern Death Valley Region, California

Van Gosen, Bradley; Lowers, Heather; Sutley, Stephen

The study described in this report was conducted under National Park Service Scientific Research and Collecting Permit # DEVA-2003-SCI-0014 (with park-assigned study identification # DEVA-00075). The study investigated the mineralogy of talc deposits mined during the 20th century in several mountain ranges that surround the southern part of Death Valley, California (fig. 1). The permit authorized the collection and analyses of talc-rich and related rocks from former talc mining properties within Death Valley National Park (DVNP). The permit stipulated that, “In addition to the above permitted sample collections, brick-sized samples of talc will be taken from each mine area for the park museum study collection”. This report describes the mineralogy and chemical composition of eight brick-sized rock samples that were contributed to the DVNP museum collection, which were collected by the lead investigator (Bradley Van Gosen) in June of 2003. Possibly the best examples in the U.S. of talc ore bodies formed by contact metamorphism are 45 deposits that have been mined and prospected in the southern Death Valley region, California (fig. 1). Twenty of these mines and prospects are now inside the boundaries of DVNP. The talc deposits are consistently associated with a carbonate horizon of the Crystal Spring Formation where it was intruded by regionally persistent gabbro sills. This sill-carbonate rock contact zone contains talc-tremolite rock, which has been mined from 1910 to present. Talc mining ended in 1988 in the mining claims that are now within DVNP. Active mining of talc appears to continue on a small scale in the Kingston Range, east of DVNP. The Death Valley talc has been used mainly in the manufacture of ceramic wall tiles and as an extender in paints (Wright, 1957; Evans and others, 1976). The talc-tremolite mines of the region lie in mountain ranges that surround southern Death Valley; the talc-bearing belt is about 120 km long by 24 km wide. The geology of the region, the mine sites, and their early mining history are well described by Wright (1968). These deposits are geologically rather similar across the region (Wright, 1957, 1968; Evans and others, 1976). Thick mafic (gabbroic) sills, mostly medium-grained, intruded a siliceous (cherty) dolomitic member of the Proterozoic-age Crystal Spring Formation, forming friable talc tremolite-rich rock along the sill-carbonate contact zones. The gabbro sills were emplaced during the Mesoproterozoic, based on age determinations by the U-Pb method from baddeleyite crystals in two sills (1087 ±3 and 1069 ±3 Ma, Heaman and Grotzinger, 1992). These ages indicate that the associated talc-tremolite deposits are Mesoproterozoic in age.