Unusual occurrence of asbestiform libby-type sodium amphibole near populated areas of southern Nevada, USA

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Asbestiform amphibole minerals are known human carcinogens that can cause mesothelioma and other serious health problems. Asbestiform NaFe+3-amphiboles (winchite, richterite and magnesoioriebeckite) have been implicated in high rates of death and disease at Libby, Montana, and form primarily by alkali metasomatism from magmatic fluids expelled from carbonatite or alkaline silicate intrusions. Asbestiform actinolite, which is a regulated asbestos mineral, primarily forms during metamorphism of mafic extrusive (metabasalt) and intrusive igneous rocks (metagabbro) or metamorphism of ultramafic bodies. Here we report on unusual occurrences of asbestiform NaFe+3-amphibole and actinolite associated with faulted Miocene plutons that crop out within and adjacent to urban areas of southern NV. These plutons are dominated by sub alkaline biotite, hornblende-bearing quartz monzonite with minor monzodiorite and diorite. The asbestiform amphiboles are found as fracture-fill veins and replacement of magmatic hornblende, and formed from high fO2 hydrothermal fluids which circulated through active fault zones as plutons cooled through sub solidus temperatures. Asbestiform NaFe+3-amphibole is found exclusively in the Wilson Ridge pluton in AZ, where Na-rich hydrothermal fluids produced hyper sodic conditions; halite deposits in adjacent Miocene sedimentary basins are the likely source of Na. Asbestiform amphiboles associated with Miocene plutons in southern Nevada (Boulder City, Black Hill, McCullough Mtns), where hydrothermal fluids lacked Na enrichment, are dominantly actinolite (plus magnesiohornblende). SEM images document that that amphibole fibers from all plutons are <0.5 µm in diameter (typically 0.15-0.4µm) and vary from tens to hundreds of microns in length with length-to-width aspect ratios of 20:1 to over 100:1, capable of dust transport and human inhalation. Transport and deposition in alluvial fans of sediment eroded from primary pluton sources increased the areal distribution of asbestiform amphiboles. Mitigation strategies require an understanding of the geologic settings where hazardous geologic materials are found. Our results suggest that asbestiform amphibole may be present in areas not generally considered at risk for naturally occurring asbestos.