Chemical and textural relations of britholite- and apatite-group minerals from hydrothermal REE mineralization at the Rodeo de los Molles deposit, Central Argentina

Abstract

Britholite group minerals (REE,Ca)5[(Si,P)O4]3(OH,F) are widespread rare-earth minerals in alkaline rocks and their associated metasomatic zones, where they usually are minor accessory phases. An exception is the REE deposit Rodeo de los Molles, Central Argentina, where fluorbritholite-(Ce) (FBri) is the main carrier of REE and is closely intergrown with fluorapatite (FAp). These minerals reach an abundance of locally up to 75 modal% (FBri) and 20 modal% (FAp) in the vein mineralizations. The Rodeo de los Molles deposit is hosted by a fenitized monzogranite of the Middle Devonian Las Chacras-Potrerillos batholith. The REE mineralization consists of fluorbritholite-(Ce), britholite-(Ce), fluorapatite, allanite-(Ce), and REE fluorcarbonates, and is associated with hydrothermal fluorite, quartz, albite, zircon, and titanite. The REE assemblage takes two forms: irregular patchy shaped REE-rich composites and discrete cross-cutting veins. The irregular composites are more common, but here fluorbritholite-(Ce) is mostly replaced by REE carbonates. The vein mineralization has more abundant and better-preserved britholite phases. The majority of britholite grains at Rodeo de los Molles are hydrothermally altered, and alteration is strongly enhanced by metamictization, which is indicated by darkening of the mineral, loss of birefringence, porosity, and volume changes leading to polygonal cracks in and around altered grains. A detailed electron microprobe study of apatite-britholite minerals from Rodeo de los Molles revealed compositional variations in fluorapatite and fluorbritholite-(Ce) consistent with the coupled substitution of REE3+ + Si4+ = Ca2+ + P5+ and a compositional gap of ~4 apfu between the two phases, which we interpret as a miscibility gap. Micrometer-scale intergrowths of fluorapatite in fluorbritholite-(Ce) minerals and vice versa are chemically characterized here for the first time and interpreted as exsolution textures that formed during cooling below the proposed solvus.

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

“…Nevertheless, the boundary between fluorcalciobritholite and fluorapatite from the nepheline syenite pegmatite in their intergrowths is always sharp (Figure 2). The same is found for coexisting britholite-(Y) and fluorapatite from Keivy mineralized granite (see Figure 7a in [38]) and for britholite-apatite intergrowths from Rodeo de los Molles [20]. This is consistent with the observation of [12] that fluorcalciobritholite forms a complete solid solution with 'true' REE-dominant britholite, but not with apatite.…”

“…Yttrium-rich britholites are typical of granites and their derivatives, whilst the cerium-dominant species occur in nepheline syenites and carbonatites. In addition, in some complex rare-metal deposits related to felsic and intermediate alkaline rocks (Mushugui [14]; Eden Lake [2]; West Keivy [17]; Azovskoe [18]; Rodeo de Los Molles [19,20]; Sakharjok [21]; Misery [22]; Weishan 2 of 23 and Maoniuping [23]; Pilanesberg and Ilimaussaq [24]), britholite is not a traditional ore mineral representing an economically important source of REE.…”

A solar optical hyperspectral library of rare-earth-bearing minerals, rare-earth oxide powders, copper-bearing minerals and Apliki mine surface samples

“…Some of the REMin (xenotime, bastnaesite, fluorapatite, synchysite and ilmenite) were additionally analyzed by using a JEOL JXA-8200 electron microprobe (EPMA) at the University of Potsdam based on a method developed by Lorenz et al (2019). The conditions used for the analysis were 20 kV acceleration voltage, 20 nA beam current and a beam size of 2 µm.…”

“…We aim to contribute to the already existing, accredited libraries, e.g., the USGS and the ECOSTRESS Spectral Library and various others (Baldridge et al, 2009;Clark et al, 2007;Hunt, 1977;Kokaly et al, 2017;Meerdink et al, 2019;Percival et al, 2016). The available reflectance spectral libraries are commonly based on powdered natural or synthetic samples that are spectrally pure.…”

The Syenite–Carbonatite Complex of Ihouhaouene (Western Hoggar, Algeria): Interplay Between Alkaline Magma Differentiation and Hybridization of Cumulus Crystal Mushes

“…Britholite is a REE mineral isostructural with apatite with general formula (Na,Ca, REE) 10 (Si,P) 6 O 24 (OH,F) 2 and Si > P (Oberti et al, 2001). They are present in one Si-rich carbonatites (plus one Si-poor sample) and white syenites as fine lamellar exsolutions (<10 um) or as irregular-shaped grains (10-200 um) in apatite Lorenz et al, 2019). The apatite exsolutions correspond to a britholite substitution of Ca 2+ and P 5+ for REE 3+ and Si 4+ , and the very low Na content indicates no belovite substitution.…”