METASOMATIC MICROTEXTURES AND THE PETROGENESIS OF AMPHIBOLE ASBESTOS

Amphiboles exhibit a variety of habits including non-fibrous (e.g. massive, prismatic, acicular) fibrous (asbestiform) morphologies. Fibrous amphibole is a known health hazard and is classified as asbestos. An understanding of the processes forming, and geologic settings hosting, amphibole asbestos (AA) is necessary to mitigate non-occupational exposure to AA.

AA has been found in hydrothermally altered and metasomatized Miocene age hornblende-bearing granitoid plutons along the Nevada-Arizona (NV-AZ) border near Hoover Dam. In the 13.9 Ma Boulder City pluton (BCP) in NV, amphibole asbestos is actinolite and is associated with secondary epidote, quartz and chlorite. In the 13.7 Ma Wilson Ridge pluton (WRP), AZ, amphibole asbestos is a NaFe³⁺-amphibole solid solution, winchite – mg-riebeckite, and is associated with extensive albitization of magmatic plagioclase (An_20-45) and K-feldspar. In both plutons hydrothermal systems were active within 2 million years of pluton solidification during faulting and uplift.

A similar suite of metasomatic microtextures are recognized in both plutons and are divided into primary texture and secondary textures. In the BCP actinolite asbestos is found as (1) mats of fibers in monomineralic fracture fill veins (primary); (2) splayed bundles and cross-fibers intergrown with quartz and epidote in polymineralic fracture-fill veins (primary); and (3) partial to complete pseudomorphic replacement of magmatic Mg-hornblende (± clinopyroxene) by fibrous actinolite (secondary). In the WRP winchite asbestos is found (1) as slip-fibers in monomineralic fracture fill veins (primary), (2) as splayed bundles intergrown with albite in veins (primary), and (3) as partial to complete pseudomorphic replacement of magmatic Mg-hornblende by fibrous winchite.

We interpret AA in fracture-fill veins as the result of primary growth of fibrous amphibole by direct precipitation from hydrothermal fluids, and replacement AA as secondary growth via a grain-scale dissolution-reprecipitation process. These results suggest that (1) both primary and secondary growth processes can produce AA, (2) fibrous habit in amphibole is largely independent of amphibole composition, and (3) any hydrothermally altered amphibole-bearing igneous rock may be a source of amphibole asbestos.