Regional Geology

The St. Lawrence Wollastonite deposit  is located in a portion of the Precambrian Grenville geologic province known as the Frontenac Arch. This narrow segment of predominantly carbonate and siliclastic rocks extends from south-eastern Ontario to the Adirondack Mountains of New York State. The area has undergone granulite facies metamorphism and has been subjected to multiple phase intrusions during the Grenville orogeny, uplift during the latter stages of the orogeny resulted in the formation of the arch.

The area is dominated by two main rock units: the "Leeds Marble Belt" and the Taylor syenitic pluton. The Leeds marble belt consists of a wide variety of metasedimentary rocks covering a 300km , roughly triangular shaped area. These metasedimentary rocks represent the deposition of interbedded sandstones and impure limestones in a small oceanic or inter-arc basin approximately 1200-1300Ma (Moore). The Central Metasedimentary Belt was subjected to a period of regional metamorphism and diapiric plutonism between 1200 and 950Ma, ie. the Grenville Orogenic period. This plutonism was a multi-phase event with the emplacement of syenite through pyroxenite beginning during the orogenic event and continuing through to the waning stages. The Taylor syenitic pluton is an example of this plutonism with all phases of intrusion represented in the southern portion of the project area.

The metasedimentary rocks generally trend northeast and dip steeply southeast. Structural deformation has resulted in the formation of a series of southwest plunging anticlines and synclines. The Saint Lawrence deposit is set in the centre of a synclinorium open to the east. This synclinorium has been truncated on its southern limb by the late mafic stages of the Taylor syenitic intrusion which imposed a contact metamorphic overprint.

The marble belt is characterized by relatively narrow alternating layers of marble and siliclastic rocks (quartzite, paragneiss and calc-silicates) with the latter predominating as one moves laterally from the centre of the belt. This sequence is indicative of a basin/margin transition zone separating dominantly carbonate rocks from clastic rocks.

Wollastonite skarns have been identified in eight separate locations of the Leeds marble belt, all in the transition zone. These occurrences are not always in close proximity to any single intrusive rock type and in some cases no intrusive is evident. The Wollastonite skarn bands resemble one another in mineralogy and thickness. In the St. Lawrence Deposit area these bands have been folded repeatedly, resulting in a greater combined thickness particularly in the noses of folds.

Diabase dykes trend north-northwest through the region and congregate (swarm) in the South Lake and Gananoque areas. They are believed to have occupied tension fractures along the axis of the Frontenac Arch which occurred during the height of the Grenville Orogeny.