Geology of the Southeastern Ontario District

Bedrock Geology

Southeastern Ontario District encompasses some of the most varied and enigmatic geology in the province. There is limited geological mapping for large parts of the Grenville Province of the Canadian Shield. There is no modern regional airborne magnetic or electromagnetic coverage for the Southeastern District. Regional airborne gamma-ray spectrometric coverage is available for all of the Grenville Province, and high-resolution coverage is available for the Central Metasedimentary Belt. In addition, regional lake sediment geochemical survey coverage is available for the Central Metasedimentary Belt.

Grenville Province

Roughly 60% (35,000km2) of the area of Southeastern Ontario District is within the Grenville Province of the Canadian Shield. Within in the District, the Grenville Province is subdivided into the Central Gneiss Belt to the northwest and the Central Metasedimentary Belt to the southeast. The Central Metasedimentary Belt Boundary Zone is a major shear zone several kilometers wide which separates the Central Gneiss Belt from the Central Metasedimentary Belt.

Central Gneiss Belt (CGB)

Understanding of the CGB in Southeastern District is hampered by a lack of detailed mapping and modern geophysical coverage. Much of the area has been consumed by Algonquin Park and exploration has been further limited by a perception that there is little mineral potential in high-grade gneiss terranes.

The CGB is divided into several lithotectonic terranes based on rock type, geologic and structural history, and metamorphic grade. In Southeastern District, the Algonquin Terrane, primarily quartzofeldspathic gneisses of plutonic and supracrustal origin, may be further divided into the Algonquin and Muskoka domains based on structural trends and metamorphic grades. (Easton 1992). Past and current mineral extraction has included graphite and dimension and flagstone quarries.

Central Metasedimentary Belt (CMB)

The CMB is a major Mesoproterozoic accumulation of marble, volcanic rocks and metasedimentary rocks. Compositionally diverse syntectonic, late tectonic and post tectonic plutonic rocks have invaded the supracrustal rocks and; the entire succession has been metamorphosed at grades varying from greenschist to granulite facies. (Easton 1992)

From northwest to southeast the CMB is divided into the Bancroft terrane, Elzevir terrane – Mazinaw terrane, Sharbot Lake terrane, and Frontenac terrane. These lithotectonic divisions are based on rock type, geologic and structural history, and ages of metamorphism and plutonism. Most of the documented mineral deposits in the Grenville Province of Ontario are in CMB.

Reference:
Easton, R.M. 1992. The Grenville Province and the Proterozoic history of central and southern Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 2, p.714-904

Paleozoic

St. Lawrence Platform

Sedimentary rocks of Paleozoic Age, 450 – 500my, occur in the eastern portion of the District. Bounded by the St. Lawrence River to the south, the Ottawa River to the east, and the Grenville Province of the Canadian Shield to the west, sedimentary rocks of the Ottawa Embayment cover approximately 10 000km2 and range in age from Early Cambrian to Late Ordovician. The oldest Paleozoic rocks in Ontario, clastic sedimentary rocks of the Covey Hill Formation (Potsdam Group), were deposited in an alluvial/fluvial environment. The overlying Nepean Formation (Potsdam Group) is a thick succession of sandstones and minor conglomerates.

All rock units are flat-lying, of uniform composition and relatively undeformed with the exception of Ordovician rocks along the Ottawa-Bonnechere Graben.
Recent research has documented arthropod-produced trackways in basal terrestrial deposits in the Nepean Formation north of Kingston, that extend the record of the first arthropod landfall back by as much as 40 my.

In the southwestern portion of the District, west of the Frontenac Arch and south of the Canadian Shield, Ordovician sedimentary rocks cover approximately 10 000km2 . These rocks were deposited in shallow seas, are generally gently dipping, of uniform composition and relatively undeformed. The basal formation, the Shadow Lake Formation, consists of red and green, dolomitic and sandy shale, generally 2 to 3 m in thickness, unconformably resting on Precambrian basement. The overlying succession of limestones, carbonate mudstones, calcareous shales and non-calcareous shales of the Gull River, Bobcaygeon, Verulum and Lindsay Formations represent relatively continuous deposition. These units comprise the Simcoe Group. Two exposed hypabyssal kimberlite dykes of Jurassic age intrude Simcoe Group sedimentary rocks near Picton and Varty Lake.

Economic Geology

In 2008, there were over 30 operating mines and quarries in the Southeastern Ontario District. In the southern and eastern extremities of the District, Ordovician limestone, dolostone and shale are extracted to produce cement, building stone aggregate and clay brick respectively.
Deposits in the Central Gneiss Belt are quarried to produce a variety of dimension and landscaping stone. The Central Metasedimentary Belt hosts the world’s largest nepheline syenite mine. Other significant commodities currently produced in the CMB include: high purity mineral fillers, talc, granite, marble and sandstone dimension stones, vermiculite, traprock and other high quality crushed stone aggregate. Previously produced commodities include feldspar, graphite and muscovite.

Although there are currently no metal mines in southeastern Ontario district there has been significant production in the past from the following deposits: the Marmoraton contact metasomatic iron deposit, the Kingdon lead-bearing vein deposit, the Cordova and Deloro gold deposits, the Long Lake carbonate-hosted stratiform zinc deposit, several pyrite deposit types and the Madawaska (Faraday) uranium deposit. (Easton 1992).

In addition, unconsolidated aggregate from various glacial deposits is extracted from numerous pits throughout the district.

There is ongoing exploration and development within the District for gold, copper, nickel, platinum group elements, iron, molybdenum and uranium. Non-metallic mineral exploration and development projects are evaluating dimension stone, muscovite, vermiculite, wollastonite, nepheline and gemstone projects.