Calciocarbonatite in the Precambrian of Ontario, Canada.

Carbonatites are rare igneous rocks - they are composed of carbonate minerals, typically calcite (CaCO3 - calcium carbonate). Carbonatites are essentially igneous limestones.

The four categories of carbonatites are: 1) calciocarbonatite (calcite-dominated) 2) magnesiocarbonatite (dolomite-dominated) 3) ferrocarbonatite (siderite-dominated) 4) natrocarbonatite (sodium carbonate-dominated)

Calciocarbonatites are the most common type. Natrocarbonatites are extremely rare, and are only found at Ol Doinyo Lengai Volcano in eastern Africa.

The orangish brown-weathering rocks are part of a calciocarbonatite dike. Freshly broken surfaces are dark gray or mottled light gray to light brown. The surrounding host rocks are greenstones (phyllitic in part) of the Michipicoten Greenstone Belt. The metamorphic rocks in the greenstone belt experienced greenschist to amphibolite facies regional metamorphism at 2.65 to 2.7 billion years ago. Black-colored, finely-crystalline contact metamorphic zones occur next to the dikes.

The calciocarbonatites here are slightly radioactive. The dikes are probably a northern extension of the Firesand River Carbonatite Complex (also known as Firesand Creek), which occurs due south of this roadcut.

Geologic unit: Firesand River Carbonatite Complex, late Mesoproterozoic, 1.078 Ga