The Lac Conviac region was recently resurveyed during the summer of 2021, resulting in the production of a new geological map on a scale of 1:50,000 (SNRC sheets 33B02 and 33B07). A significant outcome of the mapping survey was redefining boundary between the Opinaca and La Grande subprovinces in this sector, by moving the contact about thirty kilometers to the NE.

The 2021 survey also resulted in the recognition of a new unit, the Pacific Intrusive Suite (nApcf), which represents a series of post-tectonic pegmatitic granite intrusions trending NE-SW. Near the Natel and Auclair formations, in particular, these intrusions are particularly rich in tourmaline (nApcf2), with contents varying between 1 and 25%.

As a result of the survey and these important new observations and conclusions, the following recommendation was made with respect to the potential for rare-element mineralization in the Lac Conviac region:

The SW sector of the study area is dotted with numerous muscovite-garnet-tourmaline S-type pegmatitic granite intrusions likely to contain Li, Be, Ta and Nb mineralization. Among other things, the intrusive rocks of the Pacific Intrusive Suite (Pacific favorable zone) are distinguished by their significant content of tourmaline, a mineral commonly enriched in incompatible elements. This unit contains on average >1% centimetric tourmaline, and up to 25% (outcrop 21-WM-2067). Beryl has also been observed in places (Esnault and Corriveau, 2020). No economic value could be associated with these intrusions, but significant values ​​of 14 ppm Be and 5.25 ppm Ta were recorded in a paragneiss host (outcrop 21-SG-4227). The Pacific Intrusive Suite is part of a swarm of NE-SW trending dykes hosted in the paragneisses of the Prosper Formation (nAprp1) and in the volcanic rocks of the eastern part of the Natel Formation (nAnt), within the Middle and Lower Eastmain Greenstone Belt (CRVMBE). The geological context of this suite is thus similar to that of several LTC-type pegmatite deposits in the La Grande Subprovince, such as the Cyr-Lithium deposit also located in the CRVMBE, to the west of the region east of study.

Modified from:

Myriam Côté-Roberge, William Chartier-Montreuil, Maxym-Karl Hamel-Hébert et Daniel Bandyayera, 2022. BG 2022-05 Geologie de la Region du Lac Conviac, Sous-Provinces D’Opinaca et de La Grande, Eeyou Istchee Baie-James, Quebec, Canada