MINERALOGICAL AND TEXTURAL INVESTIGATION OF A NIPIGON DIABASE SILL

The Nipigon diabase sills are olivine tholeiites associated with the 1100-Ma (Keweenawan) Midcontinent Rift system. We have sampled the lowermost 30 meters of a section of the sill located at Kama Hill, approximately 25 km east of Nipigon on Highway 17, where the sill is intruded into the Sibley Group, which consists of interbedded dolostone and hematite-bearing siltstones. Mineralogical and textural analyses are being used to evaluate various injection models that may explain mineralogical and textural variations in the sill.

Point count data shows an increase in plagioclase content from ~50% to 65% over the lowermost 3 meters, then a steady decrease over the next 15 meters to a constant value of ~55%. Olivine abundance remains relatively constant at ~2-4% throughout the lowermost 20 meters, but increases to ~9% between 20 and 30 meters above the basal contact. The modal olivine content returns to ~2-4% above this layer. Along with this mineralogical anomaly, the zone from 20 to 30 meters above the base also exhibits a textural anomaly, as the mean length of the plagioclase crystals is greater than expected based on the coarsening-inwards trend developed below this zone.

We are investigating three models for the magmatic history of this sill. In the first model, an initially homogeneous magma body cooled uniformly via convection. Olivine crystallized throughout the sill when the magma reached its liquidus, and settled out to form the olivine-rich layer. In the second model, olivine and plagioclase crystals were intruded along with the magma, but were segregated into the center of the sill via flow differentiation. In the third model, the olivine layer and the textural anomaly reflect a second injection of magma into the sill after an initial cooling period. These three models will be further tested through additional sampling and more detailed mineralogical, textural, and chemical analysis.