Fibrous NaFe3+-amphiboles (winchite, richterite, and magnesioriebeckite) form primarily by alkali metasomatism from magmatic fluids expelled from carbonatite or peralkaline silicate magmas, and have been implicated in high rates of death and disease at Libby, Montana (USA). Fibrous NaFe3+-amphiboles, principally winchite and magnesioriebeckite, are found as fracture-fill veins and as replacement of magmatic hornblende in faulted margins of the dominantly subalkaline, metaluminous Miocene Wilson Ridge pluton, Mohave County, Arizona (USA). Here, the fibrous NaFe3+-amphiboles formed from hypersodic, high- hydrothermal fluids, which circulated through active faults as the pluton cooled through subsolidus temperatures. Halite deposits in adjacent Miocene sedimentary basins are the likely source of Na in the hydrothermal fluid. Amphibole fibers are <1 µm in diameter (typically <0.5 µm), vary from tens to hundreds of microns in length with length-to-width aspect ratios of 20:1 to over 100:1, are capable of dust transport and human inhalation, and should be considered hazardous. Transport and deposition of sediment eroded from primary pluton sources significantly increase the areal distribution of the fibrous amphiboles. Mitigation strategies require an understanding of the geologic settings where hazardous geologic materials are found. Our results suggest that fibrous NaFe3+-amphibole may be present in areas not previously considered at risk for naturally occurring asbestos.