The Georgia Lake rare-element pegmatite field and related S-type, peraluminous granites, Quetico Subprovince, north-central Ontario

Breaks, F. W.; Selway, J. B. and Tindle, A. G. (2008). The Georgia Lake rare-element pegmatite field and related S-type, peraluminous granites, Quetico Subprovince, north-central Ontario. Open File Report (6199). Sudbury, Canada: Ontario Geological Survey.



This study marks the third consecutive regional investigation of mineralization associated with rare-element pegmatites and related S-type, peraluminous granites in the Superior Province of Ontario. Although the Georgia Lake pegmatite field within the Quetico Subprovince has witnessed a long period of exploration for various rare-elements and most notably lithium since its discovery in the mid-1950s, peripheral areas in the Glacier Lake batholith have not received any systematic examination and chemical characterization. Furthermore, a mineral chemistry database of Nb-Ta- and Sn-bearing oxide minerals was almost completely lacking previous to the present investigation. The mineral and petrochemical databases developed in this study include 2331 electron microprobe compositions of important pegmatite minerals, 237 bulk rock and 126 bulk mineral (potassium feldspar, muscovite and beryl) compositions.

The S-type, peraluminous pegmatitic granites, which comprise the Glacier Lake batholith, are widespread over a 40 km by >110 km zone in the Quetico Subprovince immediately south and northeast of a medium-grade, clastic metasedimentary belt that comprises the northern part of the Quetico Subprovince. Rock types of the Glacier Lake batholith are identical to those of other areas of the Superior Province of Ontario such as in the Dryden and Separation Lake areas. The main rock types include biotite and two-mica granite, pegmatitic leucogranite, potassic and sodic pegmatites, sodic aplite and rare finegrained leucogranite. Beryl-type rare-element pegmatites were discovered during the present survey in the following areas: MNW stock (2 localities), Cosgrave Lake pegmatite group (1 locality), Barbara Lake stock (3 localities), Gathering Lake pegmatite group (5 localities) and in the northeastern part of the Glacier Lake batholith (one locality). Tantalum-niobium– (Ta-Nb-) and Sn-bearing oxide minerals were discovered at 12 new localities and mainly within the Gathering Lake pegmatite group delineated by this study. These minerals include ferrocolumbite, manganocolumbite, ferrotantalite, manganotantalite, ferrotapiolite, cassiterite and wodginite. The S-type, peraluminous granites were generated by extensive partial melting of clastic metasedimentary rocks at 2670±2 Ma. Fractionation of residual pegmatitic granite melts produced fertile granite masses and related rare-element pegmatites that are distributed along the northeastern contact zone of the Glacier Lake batholith contact between Barbara Lake and Gathering Lake area. Fertile granites were also generated along a similar contact zone associated with the southwestern part of the Glacier Lake batholith as exemplified by the MNW stock and Pine Portage pegmatitic granite. Exploration for rare-element pegmatite deposits is accordingly recommended for the following areas:

1. Contact zone between northeastern part of Glacier Lake batholith and clastic metasedimentary rocks of northern Quetico Subprovince
• Gathering Lake area
• Parks Lake area
• Barbara Lake stock

2. Contact zone between southwestern part of the Glacier Lake batholith and clastic metasedimentary rocks of the northern Quetico Subprovince
• Cosgrave Lake–Hanson Lake area

3. Clastic metasedimentary rocks of northern Quetico Subprovince
• area around the Brink and Southwest complex-type, spodumene-subtype pegmatites
• area around the McVittie and Foster albite-spodumene pegmatites
• MNW stock, especially area around the MNW complex-type, petalite-subtype pegmatite

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).

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