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The first occurrence of fluocerite [(Ce,La)F3] in Argentina is reported, with a description of its mineral association and mineral chemistry. Preliminary data was acquired by Pilcic (2019) as part of his undergraduate thesis. Fluocerite-(Ce), partially replaced by bastnäsite-(Ce), is found as an accessory species in amazonitic, alkali feldspar A-type granite dikes, which are eventually crosscut by a genetically linked swarm of amazonite-, topaz-bearing NYF pegmatite dikes. Both the granite and pegmatite dikes were exposed by the open pit mining activities at ~ 4670 m.a.s.l., along the in-pit ramp of the so-called "Oriental Sur" pit of the El Aguilar world class SEDEX (or MVT-like) Zn-Pb-Ag deposit in Jujuy province, NW Argentina. The open pit is located in the central to southeastern sector of Sierra de Aguilar between coordinates 65º42´-65º43´ W and 23º11´-23º12´ S, at ~ 4 km NW of the El Aguilar town and is currently being operated by the CMASA mining company.The amazonite alkali feldspar granite dike (1.5 m average thickness) is emplaced along 120º strike and locally 32º SW dip with a sub-horizontal tendency; at present the dike is exposed for ~ 240 m, subjected to the pit development. The granite dike is notoriously displaced along strike by normal faults with < 1 m offset; it is frequently crosscut by amazonite-rich, topaz-bearing pegmatite dikes intruded along some of the normal faults (20º strike and 45º ESE dip), with variable thicknesses from 1 to 15 cm and extensions not longer than 10 m. Both granite and pegmatite dikes are locally emplaced in cordierite-andalucite hornfels, metaquartzites and skarns produced by the intrusion of the rift-related Aguilar composite granitic pluton (150.4 ± 0.91 Ma, Insel et al. 2012). The contact metamorphosed and metasomatized rocks are marine, barren and Zn-Pb-Ag mineralized Lower Ordovician sedimentary rocks of the Padrioc Formation of the Santa Victoria Group (see Martín et al. 1994 and previous references therein; Sureda 1999; Cristiani et al. 2005; Omarini et al. 2013).The studied alkali feldspar granite dike is a fine to medium grained equigranular rock composed of quartz (41.6 %), K-feldspar (34.2 %; 85.7 to 97.4 mol % Or), plagioclase (18.8 %; < 4 mol % An), micas (4.4 %; protholithionite, "zinnwaldite" and Li-bearing phengite) and primary accessory phases (~ 1 %). Magmatic accessory minerals are zircon, thorianite, unidentified species of the pyrochlore supergroup and fluocerite-(Ce). Late to post-magmatic albitization of primary magmatic feldspars is widespread and occurs in finer grained chessboard or patchy interstitial aggregates. Other postmagmatic phases are phengite and minor chlorite after primary micas, scarce tourmaline-supergroup phases replacing K-feldspar, bastnäsite-(Ce) and subordinate clay minerals. Late crosscutting pyrite veinlets and chalcopyrite grains altered to covellite hosted in micas may be present. Pyrite, pyrrhotite, titanite, wollastonite, clinozoisite, sphalerite, and calcite may be found as xenocrystals in other sites where the same dikes were emplaced into skarn or mineralized metaquartzites.Fluocerite-(Ce) occurs in irregular shaped grains that range in size from a few to about 80 µm, intergrown and variably replaced by bastnäsite-(Ce). The smallest grains occur as relics within bastnäsite-(Ce) whereas the largest grains only show partial and discontinuous inward growth of bastnäsite-(Ce) from the rims. A few, < 10 µm sized grains of bastnäsite-(Ce) found included in euhedral zircon are interpreted as replaced inclusions of fluocerite-(Ce).Fluocerite-(Ce) and bastnäsite-(Ce) were identified by means of EDS analyses and subsequently analyzed with a JEOL JXA 8230 electron probe micro-analyzer (EPMA) in the WDS mode, located in the LAMARX- CONICET-UNC facility. Natural and synthetic reference materials and standards were used for analyses. The average composition of 8 analyses of fluocerite-(Ce) shown in figure 1b is (element wt. %): La= 14.42, Ce= 40.42, Pr= 3.39, Nd= 8.00, Sm= 0.08, Gd= 0.24, Yb= 0.07, Y= 1.76, Th= 1.34, Si= 0.04, Mg= 0.01, Ca= 0.59, Na= 0.01, F= 29.64, Total= 100.03. The structural formula of fluocerite-(Ce) is: (La0.209Ce0.580Pr0.048Nd0.112Sm0.001Gd0.003Yb0.001Y0.040Ca0.045Th0.009)1.046F3. The occurrence of some fluocerite-(Ce) grains paragenetic with zircon (also included in primary micas) and the absence of fluorite suggest high F- and low Ca activities since the early stages of magmatic crystallization. A low aCa is also supported by An-poor plagioclase and the absence of fluorapatite. Considering that the studied rock is practically non weathered, the partial to almost complete replacement of fluocerite-(Ce) by bastnäsite-(Ce) suggests the availability of magmatic-hydrothermal CO2 along the cooling path of these dikes. Carbon dioxide likely fractionated from the dike itself and might have destabilized fluocerite-(Ce) shortly after crystallization. The alkali-feldspar granite dikes crystallized from silica-rich, subalkaline, sligthly peraluminous, P-deficient melts with a total REE content (average of 2 samples, in ppm) of 228.5 (LREE= 184.8, La/Lu= 15.38). These melts might have emplaced during the syn-rift magmatic stage proposed by Zappettini (2017), given that crosscut the hornfels, skarns and metaquartzites produced by the Upper Jurassic Aguilar intrusion during the pre-rift stage, but were thereafter faulted by still active extensional tectonics that controlled the NYF pegmatite dikes emplacement. The Nb-Ta-Ti-Th-, and REE-rich accessory minerals of both alkali feldspar granite and NYF pegmatite dikes (Pilcic 2019) are consistent with the suite of A-type subalkaline and alkaline intrusives emplaced along the Cobres lineament. To some extent are also akin to the HFSE accessory mineralogy of the miarolitic cavities of NYF affinity present in some granites of the Complejo Alcalino Sierra de Rangel in Salta province (Menegatti 2001) that are, accordingly, younger than the Aguilar intrusion (135-122 Ma, Zappettini, 2017).

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