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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.
Highlights
The britholite group contains relatively common accessory minerals that are considered to be among the main carriers of the rare-earth elements (REE) in certain igneous rocks
The minerals have been identified in felsic magmatic rocks, and in metasomatites and pegmatites associated with subalkali and alkali granites, alkali and nepheline syenites, alkali volcanics, and carbonatites [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
We provide textural and compositional data for britholite-group minerals (BGM)
Summary
The britholite group contains relatively common accessory minerals that are considered to be among the main carriers of the rare-earth elements (REE) in certain igneous rocks. The minerals have been identified in felsic magmatic rocks, and in metasomatites and pegmatites associated with subalkali and alkali granites, alkali and nepheline syenites, alkali volcanics, and carbonatites [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. Yttrium-rich britholites are typical of granites and their derivatives, whilst the cerium-dominant species occur in nepheline syenites and carbonatites.
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