GROWTH OF FIBROUS AMPHIBOLE DURING HYDROTHERMAL ALTERATION: IMPLICATIONS FOR NON-OCCUPATIONAL EXPOSURE TO AMPHIBOLE ASBESTOS

Mitigation strategies aimed at reducing non-occupational exposure to amphibole asbestos (AA) require an understanding of AA petrogenesis and the geologic settings where they can form. Recently fibrous NaFe³⁺-amphiboles, winchite + Mg-riebeckite, have been reported from the Miocene (13.7 Ma) Wilson Ridge pluton in NW AZ, and are similar to AA implicated in high rates of death and disease at the EPA Superfund site at Libby, Montana. In the WRP, winchite formed from hypersodic, high-ƒ_O2 hydrothermal/metasomatic fluids, which circulated through active faults as the pluton cooled through subsolidus temperatures.

WRP amphibole asbestos is present as (1) monomineralic fracture fill veins which we interpret as primary precipitation of fibrous winchite from hydrothermal solution, and (2) partial to complete secondary pseudomorphic replacement of magmatic Mg-hornblende by fibrous winchite. To test whether these two growth mechanisms produce similar fiber morphologies, we used dental tools to micro-sample primary and secondary winchite, and imaged fibers on the SEM. Imaged particles were classified as fiber, fiber bundle, or prismatic crystals (cleavage). Length (L), width (W), and aspect ratio (AR) of primary (L = 12, W = 0.69, AR = 20.9) and secondary (L = 11.7, W = 0.67, AR = 20.1) are nearly identical. Based on morphology such fibers are respirable, and should be considered hazardous. The secondary growth of fibrous winchite demonstrates that growth during local shearing is not required to form AA.

Pseudomorphic replacement of non-fibrous Mg-hornblende by fibrous winchite requires mineral dissolution-reprecipitation and exchange of ions between amphibole and fluid. We used EPMA data collected from partially replaced magmatic Mg-hornblende to evaluate the ionic composition of hydrothermal fluids attending the growth of the secondary winchite amphibole. The reaction Mg-hornblende → winchite, removed Na¹⁺, Fe³⁺, and Si⁴⁺ from the fluid, and Mg²⁺, Fe²⁺, Al³⁺, and Ti⁴⁺ were released to the fluid. This process drove the fluid to super-saturation with respect winchite resulting in fluids capable of direct precipitation of fibrous winchite in fracture fill veins. Hydrothermal alteration of common hornblende-bearing plutonic rocks should be consider a potential source of non-occupational exposure to AA.