Considerações sobre a origem dos grãos "olhos-de-quartzo" nas rochas porfiroidais do distrito de Western Mines, Ilha de Vancouver, Canadá

Abstract

Quartz-eye bearing porphyroidal occur in straight association with the vent ore, in the vent zone, of the base metal massive sulphide deposits of the Western Mines District. Microscopic observations on these indicated two stages of development, which are part of a continuous process of rock alternation. In the earlier stage (first generation quartz) there was a total or quasi-total replacement of chloritecalcite spherulites or a filling of vesicles by quartz. It was accompanied by development of strong vertical foliation which superimposed to the primary bedding. In the final stage (second generation quartz) the quartz overprinted the vertical schistosity, and deformation was minimum. Quartz-eye typical of the earlier stage, termed first generation quartz, is surrounded by a bowed micaceous matrix; this is not the case when it is examined in a section perpendicular to the foliation. Composite grains and recrystallized single round grains may be in association with deformed or broken grains of original quartz and with latter or second generation grains. A preserved nucleus of calcite is occasionally observed. Hypidiomorphic and idiomorphic coarse grains of quartz are typical of the final fase of the vent alteration process, they are termed second generation quartz. Overgrowth of second generation quartz without optical character of continuity upon first generation quartz is commonly observed. In addition most of the round rims of quartz represent the outer zone of the crystallized second generation quartz. It becomes distinguishable from the rest of the quartz grain because of the abundance of microinclusions. Since both stages of quartz generation affected either mafic-intermediate or intermediate-acidic rocks, it is suggested that the term quartz-eye bearing porphyroidal rocks might be better regarded as a textural terminology than a compositional one.