Controls of amphibole formation in chrysotile deposits: Evidence from the Jeffrey Mine, Asbestos, Quebec
Williams-Jones, AE; Normand, C; Clark, JR; Vali, H; Martin, RF; Dufresne, A; Nayebzadeh, A
Epidemiological studies have demonstrated strong correlations between long-term exposure to asbestos fibers and a variety of pulmonary diseases, including fibrosis, lung carcinoma and a type of pleural cancer known as mesothelioma. 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 tremolite in the mines and, in particular, to determine whether or not the chrysotile ores are tremolite-bearing.
Detailed investigations of the Jeffrey mine, in Asbestos, Quebec, have revealed the presence of the following amphibole-group minerals: anthophyllite, cummingtonite, hornblende and tremolite-actinolite. The bulk of the amphibole, however, is in the form of tremolite and actinolite, and is found mainly in serpentinite adjacent to or included within felsic dykes. Appreciable quantities of amphibole also are present in pyroxenite (tremolite) and slate (actinolite) in contact with serpentinite distal to the ore zones. Significantly, the chrysotile ores are essentially amphibole-free. Most of the amphibole is fibrous, but a small proportion is asbestiform according to criteria 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. Increased aCa(2+) favors the crystallization of tremolite-actinolite, but is not a prerequisite. The formation of anthophyllite and cummingtonite required both an increase in aSiO2 and high temperatures (> 600 degreesC), which were associated with the intrusion of felsic dykes.
Amphibole is easily detected by conventional (>2.5 wt.%) and digestion-enhanced (>0.1 wt.%) powder X-ray-diffraction analysis. Lithogeochemical analyses afford an indirect and less expensive method for screening samples for the amphiboles. Amphibole-free samples contain < 15 ppm CaO, <0.6 wt.% Al(2)O(3), and >2,350 ppm Ni. In conjunction with conventional geological mapping, it should be possible, using these methods of detection, to identify potential amphibole-rich zones and to design methods to mine chrysotile ores with minimal contamination.