The Pegmatites of Bancroft Ontario


Maine Mineral & Gem Museum | June 1 | 6:00pm - 7:00pm

The Pegmatites of Bancroft Ontario (known as the mineral capital of Canada) are within the Grenville Geologic Province (about 1.1 billion years old) a high-grade metamorphic terrane with extensive deformation. The main economic pegmatite was the Madawaska uranium mine. These are simple, unzoned granitic pegmatites related to granitic gneiss domes that were generated late in the Grenville Orogeny under granulite to upper amphibolite conditions. They are deep seated anatectic NYF* pegmatites. Pegmatites intruded largely by replacement and assimilation of the host metagabbro.  The reduced iron in the host rocks is responsible for the precipitation of the uranium thus making this economic versus pegmatites hosted by other rocks.

Bancroft is also within a subterrane that is very rich in alkali’s (sodium and potassium rich relative to aluminum). These provinces often have carbonatites (igneous carbonate rocks) which can be rich in rare earths and rare minerals. Alkalic Provinces can also have silica undersaturated rocks such as nepheline. Bancroft has carbonatites, nepheline, and sodalite pegmatites.

The talk will highlight the differences between the Maine pegmatites that are zoned LCT** types generated by remelting of sediments at more shallow levels than the ones in Bancroft. These are generated from S type granites which are rich in aluminum and quartz. Whereas the pegmatites in Bancroft are poorly to un-zoned NYF type, generated very deep in the crust from A type granites (anorogenic or anhydrous) that occur in rift zones.

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Under the new classification by Wise, et. al. (2022) the Madawaska Mines are Group 2 uraninite-magnetite as a direct product of anatexis (DPA).

The varied lithological and chemical source rocks are responsible for the remarkable mineral richness of the region.  The Bancroft pegmatites have been producing a wealth of mineral species for collectors since many decades.  Fine crystals of rock-forming minerals such as pyroxenes, amphiboles, feldspars, nepheline as well as rare species in well-formed crystals such as betafite, allanite, zircon, apatite, and many more have been found regularly.  In addition, some lapidary material has also been extracted, for example sodalite, albite peristerite and others.

*Niobium Yttrium Fluorine

**Lithium Cesium Tantalum

Reference:

Wise, M. A., A. Müller, & W. B. Simmons (2022).  A proposed new mineralogical classification system for granitic pegmatites.  The Canadian Mineralogist, 60, 229-248.

About Richard Bedell

Richard is a recognized expert in remote sensing and geo-spatial technologies and was lead editor for a recent Society of Economic Geologists publication on remote-sensing. Richard is a Certified Professional Geologist (CPG) with the Society for Mining, Metallurgy, and Exploration. He is a graduate from The School of Natural Sciences & Mathematics, Hampshire College with a Bachelor of Arts degree in 1979, earned a Master of Science in geology from the University of Toronto in Ontario in 1985, and a Master of Science in Remote Sensing and GIS from the University of London, United Kingdom in 1986. He is a Fellow with the Society of Economic Geologists, a member of the American Exploration and Mining Association, the Geologic Remote Sensing Group, and a member of the Geological Society of Nevada. Richard is also on the board of the Society of Economic Geologists Foundation.

Now he is a technical advisor to multiple exploration companies, chairman of TerraCore, which uses spectral data to map drill core, and an advisor to the Jet Propulsion Lab for the next generation of Earth Observing satellites.