Accessory rare metal mineralization in the Coldwell Alkaline Complex, Northwest Ontario

Abstract

Accessory rare metal mineralization has been investigated in seven lithologies in intrusive Centres I and III of the Coldwell alkaline complex. All units contain minerals that are enriched in a suite of granitophlle elements, which typically include Nb, REE, Y, Th, U and Zr. Mineral abundances, composition, and mode of occurrence differ between units. Centre III is characterized by crystallization of subhedral-to-euhedral chevkinite, pyrochlore and monazite from late-stage melts or residual pore fluids in the more-evolved quartz and ferro-edenite syenites. These minerals are invariably altered to fluorocarbonate or recrystallized by later F‘ and C032-bearing deuteric fluids. The Centre I, ferroaugite syenite minerals exhibit similar morphological and replacement textures to those present in Centre III. In contrast, the Craddock Cove syenite is mildly K and Fe-metasomatised with incipient replacement of plagioclase and amphibole by K-feldspar, zircon, fluorocarbonate, Nb-rutile (?), allanite, and rare chevkinite. Fe-rich fluids under oxidizing conditions are believed to have precipitated Fe3+ bearing fluorocarbonate in which one third of the (REE)F layers are replaced by Fe3+ layers. Most Centre I rare earth minerals are enriched in the HREE relative to those from centre III, in particular pyrochlore, fluorocarbonate, allanite in the eastern contact pegmatites and the quartz syenite dykes. Compositional data for adjacent syntaxial intergrown domains of bastnaesite, synchysite, and parisite indicate that HREE-enrichment may, in part, be influenced by the Ca content of the species. The highest contents of Ce (4193 ppm), Zr (1613 ppm), V (650 ppm), Th (223 ppm) and U (428 ppm) were found in the quartz syenite dykes intruding the Craddock Cove syenite and Port Munroe megaxenolith. The emplacement of the quartz syenite dykes and the introduction of the metasomatizing fluids of the Craddock Cove syenite may be temporally related to the differentiation of residual fluids in the apical zone of the Centre I magma chamber. Complexing of F and CO32- with rare metals may have permitted their concentration, transportation and precipitation in structurally favourable settings. The megaxenoliths have been susceptible to brittle fracturing and should be considered primary targets for further exploration. The Craddock Cove syenites, although intruded by the dykes, may have been hot during dyke emplacement and therefore not as prone to brittle fracturing.