-amphiboles 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 peralkaline silicate magmas. Fibrous actinolite, a regulated asbestos mineral, forms primarily during dynamothermal metamorphism of mafic to ultramafic igneous protoliths. Here we report on unusual occurrences of fibrous NaFe+3
-amphibole (winchite, magnesoioriebeckite and richterite) and fibrous actinolite found as fracture-fill veins and replacement of magmatic hornblende within faulted Miocene plutons that crop out near urban areas of southern NV. These plutons are dominated by metaluminous biotite, hornblende-bearing quartz monzonite with lesser monzodiorite and diorite. The asbestiform amphiboles formed soon after pluton solidification from high fO2
hydrothermal brines which circulated through active fault zones as plutons where uplifted and cooled through subsolidus temperatures. Fibrous NaFe+3
-amphibole is found in the Wilson Ridge pluton in AZ; fibrous actinolite is found in Miocene plutons in southern Nevada (Boulder City, McCullough Mtns). Differences in amphibole composition in the plutons likely reflect variations in Na+
activity and/or fO2
in hydrothermal fluids; halite deposits in adjacent Miocene sedimentary basins are the likely source of Na. Erosion and transport of bedrock redistributed the fibrous amphiboles into Miocene-Pliocene alluvial fans and Quaternary alluvium.
SEM imaging revels asbestiform habits with fibers <0.5 µm in diameter (typically 0.15-0.4µm) and length-to-width aspect ratios 20:1 capable of dust transport and human inhalation. Based on morphology and composition, these fibers should be considered pathogenic and hazardous. The most likely pathway for human exposure in the arid region is through dust emissions, particularly from alluvial deposits that include public lands popular for outdoor recreation, urban areas of southern Nevada and the path of a planned Boulder City bypass highway project. Our results suggest that asbestiform amphibole may be present in areas not generally considered at risk for naturally occurring asbestos.