The chrysotile asbestos deposits of southeastern Quebec occur in serpentinized peridotites of Ordovician ophiolites. The asbestos is localized in veins within blocks of partly serpentinized peridotite bounded by zones containing schistose serpentinite and granitic masses. It is thought that these zones localized shear and provided the channelway s by which water entered the peridotite. Different generations of chrysotile veins can be identified locally by cross-cutting relationships, but these observations cannot be generalized regionally. The asbestos was produced late during the deformation of the peridotite, filling extensional fractures in the peridotite induced by stresses resulting from motion along the shear zones.The factors that control the formation of the chrysotile veins are the degree of serpentinization of the peridotite and the size of the peridotite mass. Other factors being equal, the smaller the peridotite mass is, the shorter the time necessary to serpentinize it. Smaller masses of peridotite, once serpentinized, weaken, develop a foliation, and will not develop cross-fiber veins. Larger peridotite masses will be less serpentinized and stronger and will develop cross-fiber veins.