A Detailed Study of the Mineralogy and Petrology of the Kipawa Rare Earth Elements Deposit (Canada)

Abstract The Kipawa Syenite Complex is composed of peralkaline leucosyenites, mesosyenites, mafic syenites, amphibole syenites, fenites, peralkaline granite, and monzonite, with intercalated lenses of marble and skarn. The main REE-HFSE (rare earth element-high field strength element) bearing mineralogy reflects an agpaitic character and the following assemblages are observed: (1) vlasovite + gittinsite ± eudialyte ± fluorapatite ± armstrongite, (2) eudialyte + rinkite group minerals + britholite group minerals ± fluorapatite ± vlasovite ± miserite, (3) eudialyte ± Th-, Ti-, and Nb-bearing minerals, (4) rinkite group minerals + britholite group minerals ± Th-, Ti-, and Nb-bearing minerals, (5) britholite group minerals ± parisite-(Ce) ± bastnäsite-(Ce) ± monazite-(Ce) ± Nb-bearing minerals, (6) britholite group minerals + chevkinite group minerals ± bastnäsite-(Ce), and (7) xenotime-(Y) + monazite-(Ce). The occurrence of zircon, baddeleyite, and zirconolite locally in both agpaitic britholite-bearing marble and skarn may indicate a transition to a miaskitic character. Two miaskitic assemblages are observed: (1) zircon + titanite + Th-bearing minerals + Nb-bearing minerals ± baddeleyite and (2) allanite + zircon + REE-bearing/Nb-bearing minerals ± Th-bearing minerals. The crystallization of REE-bearing minerals after allanite and zircon also supports the transition. Thirty-one mineral species were analyzed, including 26 REE- and HFSE-bearing species: eudialyte, fluorapatite, britholite-(Ce), britholite-(Y), fluorbritholite-(Ce), fluorbritholite-(Y), fluorcalciobritholite, rinkite-(Ce), rinkite-(Y), mosandrite-(Ce), “mosandrite-(Y)”, vlasovite, gittinsite, zircon, miserite, armstrongite, parisite-(Ce), bastnäsite-(Ce), parisite-(Ce), xenotime-(Y), monazite-(Ce), zirconolite, and baddeleyite. Amphibole, pyroxene, and feldspar crystallized early to form syenitoid and granitoid lithologies. In the skarn and marble, the paragenetic sequence is calcite, pyroxene, micas, and amphibole. The crystallization order varies in the fenites. Vlasovite and chevkinite-(Ce) are late-magmatic or metamorphic phases. The majority of eudialyte is post-magmatic, but some euhedral crystals in syenite are magmatic to late-magmatic in origin. Eudialyte is paragenetically followed by rinkite and miserite, then mosandrite and REE-bearing carbonates, REE-bearing phosphates, britholite group minerals, Nb-, Th-, and U-bearing minerals, zirconolite, baddeleyite, pectolite, and fluorapatite. Vlasovite is partially replaced by gittinsite + armstrongite + zircon, and chevkinite-(Ce) crystals have rims composed of perrierite-(Ce), bastnäsite-(Ce), and other unidentified REE-bearing minerals. The final stage of formation involved development of fenites, which were produced by fluid-induced metasomatism and metamorphism of syenitoids and marbles. The REE and HFSE mineralization resulted from the modification of host syenites by the infiltration of an aqueous fluid rich in CO2, S, P, F, and Cl. During the tectonic juxtaposition of marble with syenitic rocks this fluid remobilized REE, HFSE, Na, Ca, Al, Mn, and Fe from magmatic rocks and mixed with Ca and CO2 from marble. The high F content favored the precipitation of F-bearing species (eudialyte, rinkite, fluorbritholite) and Ce-bearing species, followed by Y-bearing species and H2O-bearing species (e.g., mosandrite).