TY  - JOUR
A1  - Lorenz, Melanie
A1  - Altenberger, Uwe
A1  - Trumbull, Robert B.
A1  - Lira, Raul
A1  - Lopez de Luchi, Monica Graciela
A1  - Günter, Christina
A1  - Eidner, Sascha
T1  - Chemical and textural relations of britholite- and apatite-group minerals from hydrothermal REE mineralization at the Rodeo de los Molles deposit, Central Argentina
JF  - American mineralogist : an international journal of earth and planetary materials
N2  - Britholite group minerals (REE,Ca)(5)[(Si,P)O-4](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 similar to 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.
KW  - Britholite
KW  - apatite
KW  - exsolution textures
KW  - miscibility gap
KW  - compositional gap
KW  - REE
KW  - fenite
KW  - alkaline granites
KW  - hydrothermal alteration
Y1  - 2019
U6  - https://doi.org/10.2138/am-2019-6969
SN  - 0003-004X
SN  - 1945-3027
VL  - 104
IS  - 12
SP  - 1840
EP  - 1850
PB  - Mineralogical Society of America
CY  - Chantilly
ER  -