Genetic Model

It is understood that a polymetamorphic event was necessary to form the extensive wollastonite skarns at the St. Lawrence Deposit. Although certain temperature and pressure regimes existed throughout, the sequence of these events is unknown.

The current theory for wollastonite formation is as follows:

1.		Deposition of a shallow inter-tidal sequence of interbedded sandstones and impure limestones in a small oceanic or inter-arc basin between 1200 and 1300 Ma.
2.		Grenville erogenic event, 1200-1000 Ma with regional metamorphic temperatures reaching 800 degrees Celsius and pressures of 2-3 kilobars. Calcite and quartz remain stable at this t and p range.
3.		Ballooning diapirism between 1180-1073, early syenitic phases developed into flat lensoid shapes within thick pelitic sequences. Subsequent monzonite, diorite, gabbro and pyroxenite phases(pyroxenite indicative of shallow depths (<7km) and high temperatures (600-700 °C).
4.		A reduction in pressure, (late in the erogenic event) to an optimal pressure window (approximately 2km/0.5 kb) and continued high temperature (pervasive over large area) due to multiple intrusive pulses subsequent to the syenitic phase resulted in the formation of wollastonite in a specific stratigraphic thickness (horizon) which had the necessary chemical and physical properties required for complete recrystallization. It is probable that silica was added to the system at some stage. Approximate age date for formation: 1000 -1100 Ma. (1-1.1Billion Years)