The mineralogy of NYF pegmatites from the Coldwell alkaline complex, Northwestern Ontario

Abstract

The Coldwell Complex, northwestern Ontario, is a multiphase alkaline intrusion that is host to rare earth element, actinide and other high field strength element mineralization. Preliminary studies have shown that these minerals are concentrated in pegmatites associated with Center One ferrorichterite-ferroaugite syenites and Center Three syenites. The Center One syenites differentiate to pegmatitic residua and are characterized by cumulus perthitic-to-cryptoperthitic alkali feldspar, hedenbergite-aegirine pyroxenes, and intercumulous quartz, calcite, and calcicto- sodic-calcic-to-sodic amphiboles. Center Three residua are similar, except that amphiboles are limited to calcic varieties (hastingsite) and precipitate before feldspar (as opposed to after). All pegmatitic residua are of the niobium-yttrium-fluorine (NYF) type. Back-scattered electron petrography has been used to characterize the mineral paragenesis. Pegmatitic syenitic residua emplaced in, but not derived from the border gabbro (Border Gabbro pegmatite), and residua within ferroaugite syenite units (Railway pegmatites) contain a wide range of rare element minerals which include britholite, chevkinite, fergusonite, monazite, allanite, kainosite, xenotime, REE tluorocarbonates bastnaesite, synchysite and parisite. Other rare element enriched minerals include apatite, thorite, zircon, zirconolite, niobium rutile and U-Th-Sipyrochlore. Early-formed rare element minerals such as allanite, britholite, chevkinite, kainosite, and pyrochlore are commonly replaced by complex aggregates of later-forming phases such as REE-tluorocarbonates. Other riebeckite-quartz (Upper Marathon Shore pegmatites), richteritequartz (Black pegmatites) and hastingsite-quartz (Center Three pegmatites) bearing pegmatitic residua contain a more restricted range of rare element minerals, which include zircon, xenotime, monazite and tluorocarbonates together with REE-bearing apatite, thorite and pyrochlore. The differences in rock forming and accessory mineralization suggest that most, if not all, residua are derived from different batches offerroaugite syenite and syenite magma. Intensive parameters have been estimated using the habit of perthites, the coexistence of zircon and baddeleyite, Fe-Ti-oxide compositions, amphibole mineralogy, and tluorocarbonate stability. These parameters indicate all pegmatitic units are similar, with initial silica activities of 1 o-0 · 75 , alkali-feldspar precipitation at approximately 750 °C, magnetite-ilmenite subsolidus equilibration temperatures of 531 to 633 oc and oxygen fugacities of 1 o-'6 · 5 to 1 o-22 · 9 bars, subsolidus quenching of magnetite occurs at approximately 450 °C, and subsequent 200 oc hydro- and carbothermal induced recrystallization of rare earth mineral and REB-bearing minerals.