Britholite Group Minerals from REE-Rich Lithologies of Keivy Alkali Granite—Nepheline Syenite Complex, Kola Peninsula, NW Russia

Abstract

The Keivy alkali granite-nepheline syenite complex, Kola Peninsula, NW Russia, contains numerous associated Zr-REE-Y-Nb occurrences and deposits, formed by a complex sequence of magmatic, late-magmatic, and post-magmatic (including pegmatitic, hydrothermal, and metasomatic) processes. The REE-rich lithologies have abundant (some of economic importance) and diverse britholite group minerals. The REE and actinides distribution in host rocks indicates that the emanating fluids were alkaline, with significant amounts of F and CO2. From chemical studies (REE and F variations) of the britholites the possible fluid compositions in different lithologies are proposed. Fluorbritholite-(Y) and britholite-(Y) from products of alkali granite (mineralized granite, pegmatite, quartzolite) formed under relatively high F activity in fluids with low CO2/H2O ratio. The highest F and moderate CO2 contents are characteristic of fluid from a mineralized nepheline syenite, resulting in crystallization of fluorbritholite-(Ce). Britholite group minerals (mainly fluorcalciobritholite and ‘calciobritholite’) from a nepheline syenite pegmatite formed from a fluid with composition changing from low F and high CO2 to moderate F and CO2. An extremely high F content is revealed for metasomatizing fluids emanating from alkali granitic magma and which affected the basic country rocks. The dominant substitution scheme for Keivy britholites is REE3+ + Si4+ = Ca2+ + P5+, showing the full range of ‘britholite’ and ‘calciobritholite’ compositions up to theoretical apatite.

An Apatite-Group Praseodymium Carbonate Fluoroxybritholite: Hydrothermal Synthesis, Crystal Structure, and Implications for Natural and Synthetic Britholites

“…Although this mismatch can be often attributed to analytical uncertainties, our geologically reasonable conditions employed in the synthesis described above indicate that an oxybritholite component may contribute to the mismatch. Silica-deficient and low-totals britholites observed in some localities are potential hosts for undetected carbonates. − In some cases, apatite–britholite analyses plot consistently below the 1:1 line on a Ca+P–REE+Si plot, ,− which could be easily explained by the presence of carbonate substituting for silicate. We expect this to occur mostly in carbonatite-associated fluorapatite. , …”

Extreme Alteration of Chevkinite-(Ce) by Pb-CO2-Rich Fluids: Evidence from the White Tundra Pegmatite, Keivy Massif, Kola Peninsula

“…The loss of LREE relative to HREE in the chevkinite-(Ce) and aeschynite-(Y) as alteration proceeded is consistent with their greater solubility in CO 2 -rich fluids (Smith et al [15]). Earlier, Zozulya et al [16] suggested, from their study of the britholite-group minerals, that they had crystallized at relatively high F activity and low CO 2 /H 2 O ratios. Zozulya et al [5] found that fergusonite in the pegmatite has [La/Nd]cn = 0.03-1.2, indicating relatively high H 2 O/CO 2 in fluids.…”

REE Minerals as Geochemical Proxies of Late-Tertiary Alkalic Silicate ± Carbonatite Intrusions Beneath Carpathian Back-Arc Basin

“…Representative analyses of silicates dominated by light rare earth elements associated with chevkinite-Ce are listed in Table S13. Atomic proportions recalculated from formulae based on eight cations plot along the britholite-apatite exchange vector (Ca + P) =~7.86 − (Re + Si) [29]. The Re + Si values between 6.40 and 6.64 indicate 82-85.1 mol.% britholite-Ce component in the Ca-Ce-silicate from syenite HP-3 (Figure 9a).…”