Controls of amphibole formation in chrysotile deposits: Evidence from the Jeffrey Mine, Asbestos, Quebec

Epidemiological studies have demonstrated strong correlations between long- term exposure to asbestos fibers and a variety of pulmonary diseases, inclu ding fibrosis, lung carcinoma and a type of pleural cancer known as mesothe lioma. Moreover, studies of chrysotile mine and mill workers in Quebec have shown that the principal fiber in their lungs tissues is not, as might be expected, chrysotile, but rather the amphibole-group mineral tremolite. In view of these findings, it is essential to establish the distribution of tr emolite in the mines and, in particular, to determine whether or not the ch rysotile ores are tremolite-bearing. Detailed investigations of the Jeffrey mine, in Asbestos, Quebec, have reve aled the presence of the following amphibole-group minerals: anthophyllite, cummingtonite, hornblende and tremolite-actinolite. The bulk of the amphib ole, however, is in the form of tremolite and actinolite, and is found main ly in serpentinite adjacent to or included within felsic dykes. Appreciable quantities of amphibole also are present in pyroxenite (tremolite) and sla te (actinolite) in contact with serpentinite distal to the ore zones. Signi ficantly, the chrysotile ores are essentially amphibole-free. Most of the a mphibole is fibrous, but a small proportion is asbestiform according to cri teria established by the U.S. Occupational Safety and Health Administration . The principal control on the formation of tremolite and actinolite in the serpentinite was an increase in aSiO(2) associated with felsic dykes. Incr eased aCa(2+) favors the crystallization of tremolite-actinolite, but is no t a prerequisite. The formation of anthophyllite and cummingtonite required both an increase in aSiO2 and high temperatures (> 600 degreesC), which we re associated with the intrusion of felsic dykes. Amphibole is easily detected by conventional (>2.5 wt.%) and digestion-enha nced (>0.1 wt.%) powder X-ray-diffraction analysis. Lithogeochemical analys es afford an indirect and less expensive method for screening samples for t he amphiboles. Amphibole-free samples contain < 15 ppm CaO, <0.6 wt.% Al2O3 , and >2,350 ppm Ni. In conjunction with conventional geological mapping, i t should be possible, using these methods of detection, to identify potenti al amphibole-rich zones and to design methods to mine chrysotile ores with minimal contamination.