LAKE SKOOTAMATTA

LAKE SKOOTAMATTA

  • Type: Syenite/Pluton
  • Diameter: ~5.5 Km
  • Location: South of Algonquin Park, Ontario, Canada N 44° 50.1’ W 77° 16.7’
  • Skootamatta is a pluton* in the southwestern Grenville Province

*Pluton: A large igneous intrusion formed at great depth in the crust. Any body of igneous rock that solidified below the earth’s surface.

LAKE SKOOTAMATTA – Google image.
LAKE SKOOTAMATTA – east (looking west – courtesy Jon Buchannan sitting in the right seat of GOZooM).
Compare this image of the confirmed impact structure at Presqu’ile Lake with Lake Skootamatta (above). There is a striking resemblance between the two structures. This is a classic example that a circular geologic shape is not firm evidence of an impact event, but it is worth investigating. It took ground explorations of the structures to confirm (in the case of Presqu’ile) or refute (in the case of Skootamatta) an impact event.
Illustrated left to right, MARMORA MINE – Mt MORIAH – LAKE SKOOTAMATTA – may be part of a approximately 1085 Ma suite of syenite intrusions (plutons) that stretch from Madoc in Ontario to Mount Laurier in Quebec*** (Easton, personal communication). – Google image.

SKOOTAMATTA SYENITE**

The Skootamata Syenite is a large, roundish body centred in Anglesea Township. The body covers an area of approximately 30 km2 , and underlies a large part of Skootamatta and Sheldrake Lakes. Access is provided by county roads east from Highway 41 and the town of Cloyne.

Geology

The Skootamatta Syenite is centrally located in the Elzevir Terrane. The body is slightly elongate to the south-southwest with a length of 7.5 km and a width of 4 to 6 km. The Skootamatta Syenite has not been studied in detail. Mapping of Anglesea Township was undertaken by Meen (1944) and was later followed up by the work of Moore and Morton (1986) who cover only the far eastern part of the pluton. These two sources provide all the known information on the Skootamatta body.

The syenite is a light grey to pale pink, massive equigranular rock. There is a weak foliation in places. The syenite is coarse- to medium-grained for the most part with a minor decrease in grain size towards the contacts. Mineralogy consists of microcline, albite and biotite with accessory sphene, apatite and magnetite. The very low colour index is suggestive of a mafic content of 5% or less. Quartz is present in very minor amounts, varying from trace to 5%, and can only be detected in thin section (Moore and Morton 1986).


Elzevir Terrane Syenites

The alkalic rocks of the Elzevir Terrane typically occur as large (10 to 40 km2), round plutonic bodies. A northeast trending group of syenites, the Gawley Creek Syenite in the south, followed by the Mount Moriah and Skootamatta Syenites are situated to the east and northeast of the Blue Mountain Syenite.

The Gawley Creek, Mount Moriah, Skootamatta and Mount St. Patrick Syenites may be part of a approximately 1085 Ma suite of syenite intrusions that stretch from Madoc in Ontario to Mount Laurier in Quebec*** (Easton, personal communication). The equivalent plutons have recently been studied in detail by Corriveau (personal communication). These bodies are younger than the peak of Grenville metamorphism, having distinct pluton shapes and little affected by Grenville tectonism (Easton, personal communication) unlike the syenites of the Bancroft Terrane. These bodies are therefore attractive targets since they are relatively uniform, massive, coarse grained homogeneous bodies.

Ground explorations failed to identify any impact related features at Skootamatta.


**SYENITE: is a plutonic rock defined as an igneous, intrusive rock composed of at least 65% alkali feldspar, and less than 5% quartz. Plagioclase and mafic minerals make up the remaining mineralogy. A leucocratic syenite can contain up to 30% mafic minerals, and a mesocratic syenite will contain more than 30% mafic minerals (e.g., mesocratic nepheline syenite of the Callander Bay Complex). The term alkali syenite is often used to describe a rock where alkali feldspars constitute 90% or more of the rock, and plagioclase compositions are Ano-40, where plagioclase is present. Syenites such as the Otto Stock, Lebel Stock, McElroy Stock of the Superior Province and the Gawley Creek, Mount Moriah, Skootamatta, Burns Lake and Mount St. Patrick Syenites of the Grenville Province containing neither quartz nor nepheline is considered saturated. Oversaturated rocks contain modal quartz, incompatible with the presence of nepheline and therefore representing less attractive targets since AhOa content is limited. Undersaturated rocks contain modal nepheline, and these bodies, where they contain less than 10% mafic minerals, represent more prospective A12O3 and CaO+NazO+KzO targets (Dolan et al, 1991).

***The SYENITE intrusions that stretch from Madoc, Ontario to Mount Laurier, Quebec include: Skootamatta Lake, Mount Moriah and the Sainte-Véronique structure.


Prior to 2008, all younger granitoid intrusions (1065 to 1090 million-years-old) within the Central Metasedimentary Belt were considered to be part of the ultrapotassic Kensington–Skootamatta suite (see references in Easton 2008), even though some of these intrusions consist mainly of granite rather than diorite to monzonite. A renewed look at the data on these intrusions by Easton (2008) indicates that there are actually 2 types of late felsic intrusions within the Central Metasedimentary Belt. The slightly older, alkalic plutons of the classic Kensington–Skootamatta suite and the slightly younger, anorogenic granites of the Catchacoma intrusive suite. The former have limited uranium potential, whereas the later have potential for Rössing-style granite-hosted uranium mineralization.

Plutons of the ultrapotassic Kensington–Skootamatta suite are alkalic, shoshonitic to ultrapotassic, and generally have low to moderate silica contents (45 to 60 weight %). They are characterized by TiO2 >0.8 and P2O5 >0.21 (Corriveau 1990). They are typically associated with intense aeromagnetic highs, and generally have low to moderate eU and eTh contents on detailed airborne gamma ray spectrometric maps of the area. Monzonite to syenite plutons of the Kensington–Skootamatta suite in Ontario have U/Pb zircon age determinations ranging from 1077 to 1090 Ma, but most ages cluster around 1088 Ma (Easton et al, 2009).

Reference

Andrews, Oluf E.G., Apatite in the Kensington and Sainte-Véronique Plutons: An Indicator of Magmatic and Hydrothermal Processes. Department of Earth and Planetary Sciences, Mcgill University, December 1996.

Corriveau, L. 1990. Proterozoic subduction and terrane amalgamation in the southwestern Grenville Province, Canada:evidence from ultrapotassic to shoshonitic plutonism; Geology, v.18, p.614-617.

Davidson, A., van Breemen, O., Late Grenvillian granit plutons in the Central metasedimentary belt, grenville Province, southeastern Ontario. Radiogenic age and isotopic studies: report 13, Geological Survey of Canada, 2000.

M.L. Dolan, D.H. Hains and D.R. Ash, High-Alumina Rocks in Ontario: Resources and Process Technology. Mines and Minerals Division, Ministry of Northern Development and Mines, 1991.

Easton, M., Sangster, P., LeBaron, P., Rössing-style (Granite-Hosted) Uranium Mineralization in the Central Metasedimentary Belt of the Grenville Province, 2009.

The Geology of Skootamatta Lake