CONDITIONS OF FORMATION OF LIZARDITE, CHRYSOTILE AND ANTIGORITE, CASSIAR,
BRITISH COLUMBIA
DAVID S. O'HANLEY* and FREDERICK J. WICKS
Department of Mineralogy, Royal Ontario Museum, Toronto, Ontario M5S 2C6
*Present address: Trinity School at River Ridge, 2300 East 88th Street, Bloomington, Minnesota 55425-2187, U.S.A.
Abstract
Observations at the Cassiar (British Columbia) chrysotile asbestos deposit have defined a continuous series of serpentine textures, between pseudomorphic and nonpseudomorphic. These minerals and textures are distributed with respect to shear zones in the interior of the serpentinite such that the degree of recrystallization and replacement increases as the shear zones are approached. Patterns of spatial distribution suggest that recrystallization and replacement were caused by infiltration- driven metamorphism as the serpentinite equilibrated with an externally derived fluid. The shear zones served as conduits for the fluid. The recrystallization and replacement of lizardite formed after olivine and of that formed after enstatite proceed independently early in the process, such that composition of the various serpentine minerals was controlled by the bulk composition of the precursor. Later in the process, Fe2+/Mg, Cr/Al and Fe3+/(Mg + Si) values, and the distribution of boron, between serpentine after olivine and that formed after enstatite indicate that equilibrium was closely approached. The formation of a chrysotile + antigorite assemblage marks this transition. The recrystallization of lizardite and its replacement by chrysotile + antigorite occurred at 250 ± 25 C and a P(H2O) of less than 1 kbar. To estimate the temperatures and pressures of serpentinization can be modeled in the system MgO-SiO2-H2O (MSH) because: 1) clinochlore is stabilized by the breakdown of chromite, rather than a serpentine mineral; and 2) magnetite is both a product and a reactant in the conversion of lizardite to antigorite, indicating that it is not an essential compound in the conversion of lizardite to antigorite. The mineralogy, textures and compositions of lizardite and chrysotile indicate that they are polymorphs in the MSH system.
Keywords: serpentinite, serpentine, recrystallization, replacement, infiltration metamorphism, mineral stability, Cassiar, British Columbia.