Fertile and peraluminous granites and related rare-element mineralization in pegmatites, north-central and northeastern Superior Province, Ontario
Breaks, FW; Selway, JB; Tindle, AG
This study marks the second regional investigation of mineralization associated with rare-element pegmatites and related S-type, peraluminous granites in the Superior Province of Ontario. The area examined in north-central and northeastern Ontario has had no previous, systematic geological and mineralogical work directed at this deposit-type. The mineral and petrochemical database developed in this study includes 10 155 electron microprobe compositions of important pegmatite minerals, 196 bulk rock and 118 bulk mineral (potassium feldspar, muscovite and beryl) compositions. Rare-element pegmatites and chemically evolved, S-type, peraluminous granite and pegmatitic granite in the Superior Province are most commonly found in the boundary zones between high-grade metasedimentary-rich subprovinces, such as the Quetico and the English, and adjoining greenstone-belt-rich subprovinces, such as the Uchi, Wabigoon and Abitibi. Several lithium-rich, complex-type rare-element pegmatites occur in the study area and include the North and South Aubry, Swole Lake, Superb Lake, Lowther Township and Case pegmatites. Beryl-type rare-element pegmatites were discovered during the present survey at 4 localities: North Jackfish Road and Ketchican Road occurrences in the Armstrong pegmatite field, and the Clavet Township and Shetland Township pegmatites in the Hearst area. S-type, peraluminous pegmatitic granites are widespread in the Hearst area of the Quetico Subprovince and several exposures demonstrated a degree of chemical evolution similar to fertile granites elsewhere in Ontario. Such pegmatitic granites were found in the Pagwachuan River area, and in McCrea, Pelletier and Shetland townships. Exploration for rare-element pegmatite deposits is accordingly recommended for the following areas: 1. English River-Wabigoon subprovincial boundary zone: - area around the Seymour Lake pegmatite group (Aubry spodumene-subtype pegmatites); - area around Swole Lake pegmatite boulder field (lepidolite-subtype pegmatite); - area around North Jackfish and Ketchican Road beryl-type pegmatites; - area adjacent to the Superb Lake spodumene-subtype pegmatite and Abamasagi Lake beryl-type pegmatitic granites; - Maytham-Queenston lakes peraluminous granite pluton. 2. Wabigoon-Quetico subprovincial boundary zone: - Pagwachuan River area (beryl-type pegmatites). 3. Quetico Subprovince interior; - Pelletier Township pegmatitic granites; - McCrea-Shetland pegmatitic granite zone. 4. Opatica-Abitibi subprovincial boundary zone 8 area around Case pegmatite group. Recommended exploration techniques to utilize in the above target zones include mineralogical and petrochemical detection of fertile, peraluminous parent granites coupled with regional sampling aimed at discerning exomorphic dispersion in the host rocks adjacent to rare-element pegmatites. Detection of fertile granite and pegmatitic granite, which potentially spawned rare-element pegmatite swarms, is effectively undertaken by evaluation of indicator mineral chemistry (potassium feldspar, muscovite, garnet and tourmaline).