Arlene C. Beisswenger, Michael
D. Higgins and Larry Hoy
2001, Canadian Journal of Earth
Sciences, 38: 1129-1140
The Canton St. Onge wollastonite deposit is situated near the centre of the Lac-St-Jean anorthosite complex, in a small patch of granites and metasediments, associated with a major NE-SW lineament that stretches for over 200 km. Magmatic alignment of plagioclase in the host indicates that the lineament was active when the anorthosite was emplaced. A linear granite (Du Bras) adjacent to the anorthosite, and parallel to the lineament, has been interpreted to a granophyre, produced by partial melting of the host sediments by the anorthosite. The different bands of metasediments are generally aligned parallel to the granite, and the lineament. They mostly comprise dolomitic marble, quartzite, diopside-rich calc-silicates and wollastonite-diopside rock. An alkali granite (Astra) was also emplaced into the lineament and a pegmatite dyke invaded a metalliferous part of the metasediments and produced a deposit of amazonite. The lineament must have been active at some later time to drop down this block and protect it from subsequent erosion.
The wollastonite-bearing layer is in contact with dolomitic marble on one side. The other side is concealed, but wollastonite-free calc-silicates occur on the other side of a small valley. Within the deposit the wollastonite is interlayered with diopside on a mm to cm scale and is commonly folded. Few other minerals are present. Although the dolomitic marbles contain silica, there was not enough to form the calc-silicate rocks. Therefore, either the protolith of the calc-silicate rocks was heterogeneous, or silica was imported into the rocks via a fluid.
Whole-rock oxygen isotope ratios were measured to determine the origin of the fluids involved in the formation of the deposit. Both granites and the Lac-St-Jean anorthosite have del18O values of 8 to 10, consistent with values in similar intrusions elsewhere in the Grenville Province. Dolomitic marbles have much higher del18O from 23 to 28, again similar to other Grenville marbles. Wollastonite-free calc-silicates have del18O values from 14 to 18, with one sample at 8. Wollastonite-bearing rocks mostly have del18O values from 8 to 13, but two samples lie at 22 and 23.
These data clearly indicate that pristine meteoric water was not involved in the formation of the calc-silicate rocks. The fluid may have been derived from one or other of the intrusions, or it may have been meteoric water that re-equilibriated with the intrusions. The generally lower values of the wollastonite-bearing calc-silc8iates suggests that more fluid was involved in their formation, compared to the wollastonite-free rocks, if both came from protolith with similar del18O values. If Ca and Mg were relatively immobile, then chemical compositions suggest that the wollastonite-bearing rocks were derived from a protolith with approximately equal amounts of calcite and dolomite, whereas the barren calc-silicates were made from almost pure dolomite