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Françoisite-(Ce) American Mineralogist, Volume 95, pages 1527–1532, 2010 Françoisite-(Ce), a new mineral species from La Creusaz uranium deposit (Valais, Switzerland) and from Radium Ridge (Flinders Ranges, South Australia): Description and genesis NICOLAS MEISSER,1,* JOËL BRUGGER,2,3 STEFA N AN SER M ET ,1 PHILI pp E THÉLI N ,4 A N D FRA N ÇOIS BUSSY 5 1Musée de Géologie and Laboratoire des Rayons-X, Institut de Minéralogie et de Géochimie, UNIL, Anthropole, CH-1015 Lausanne-Dorigny, Switzerland 2South Australian Museum, North Terrace, 5000 Adelaide, Australia 3TRaX, School of Earth and Environmental Sciences, University of Adelaide, 5005 Adelaide, Australia 4Laboratoire des Rayons-X, Institut de Minéralogie et de Géochimie, UNIL, Anthropole, CH-1015 Lausanne-Dorigny, Switzerland 5Laboratoire de la microsonde électronique, Institut de Minéralogie et de Géochimie, UNIL, Anthropole, CH-1015 Lausanne-Dorigny, Switzerland AB STRACT The new mineral françoisite-(Ce), (Ce,Nd,Ca)[(UO2)3O(OH)(PO4)2]·6H2O is the Ce-analog of françoisite-(Nd). It has been discovered simultaneously at the La Creusaz uranium deposit near Les Marécottes in Valais, Switzerland, and at the Number 2 uranium Workings, Radium Ridge near Mt. Painter, Arkaroola area, Northern Flinders Ranges in South Australia. Françoisite-(Ce) is a uranyl- bearing supergene mineral that results from the alteration under oxidative conditions of REE- and U4+- bearing hypogene minerals: allanite-(Ce), monazite-(Ce), ±uraninite at Les Marécottes; monazite-(Ce), ishikawaite-samarskite, and an unknown primary U-mineral at Radium Ridge. The REE composition of françoisite-(Ce) results from a short aqueous transport of REE leached out of primary minerals [most likely monazite-(Ce) at Radium Ridge and allanite-(Ce) at La Creusaz], with fractionation among REE resulting mainly from aqueous transport, with only limited Ce loss due to oxidation to Ce4+ during transport. Keywords: Françoisite-(Ce), new mineral, rare earth elements, supergene IN TRODUCTIO N OCCURRE N CES A N D GEOLOGICAL SETTI N GS The phosphate françoisite-(Ce), (Ce,Nd,Ca)[(UO2)3O(OH) At both localities (La Creusaz and Radium Ridge), fran- (PO4)2]·6H2O, is a new mineral species discovered simultane- çoisite-(Ce) forms part of a varied assemblage of uranyl-bearing ously at the La Creusaz uranium deposits near Les Marécottes minerals resulting from the alteration of a primary uranium in Valais, Switzerland, and at the Number 2 uranium Workings, (±REE) mineralization. However, the metallogenesis, geochemi- Radium Ridge near Mt. Painter, Arkaroola area, Northern cal, and mineralogical characters of both occurrences are very Flinders Ranges of South Australia. At the La Creusaz deposit, different, as summarized below. françoisite-(Ce) was first found in 1981 but analyzed and identi- fied as a new mineral species only in 1999. La Creusaz uranium prospect Françoisite-(Ce) is the Ce-analog of françoisite-(Nd), a Françoisite-(Ce) was found in the “Gisiger” surface scratching mineral described by Piret et al. (1988) from Kamoto, Shaba, (~630 m2) at the La Creusaz uranium prospect near the village Democratic Republic of Congo (RDC). The new mineral is of “Les Marécottes” (canton Valais, Western Alps, Switzerland; named françoisite-(Ce) following rule of Levinson (1966) in Swiss federal coordinates: 566.208/107.809; altitude 1642 m). The accordance with the International Mineralogical Association uranium deposit of La Creusaz was discovered in 1973 (Gilliéron (IMA) recommendations for the nomenclature of rare-earth ele- 1988), and explored episodically between 1973 and 2008 using ment (REE)-bearing minerals. The new mineral was approved drill holes, surface scratching, and galleries. by the IMA Commission on New Minerals and Mineral Names The mineralization occurs in hydrothermal breccia veins at (vote no. 2004-029). Holotype specimens are deposited at Musée the contact between the pre-Variscan gneissic basement of the géologique cantonal, Lausanne, Switzerland, under the follow- Aiguilles Rouges Massif and the Carboniferous (Variscan) Val- ing catalog numbers: MGL58321 (La Creusaz, Les Marécottes, lorcine granite. According to Meisser (2003), the complex mineral Valais, Switzerland) and MGL79288 (Number 2 Workings, Ra- assemblages and ore textures found at La Creusaz result from a dium Ridge, Arkaroola, South Australia). Due to the abundance complex geological history. and quality of crystals found at La Creusaz, most analyses were (1) The primary mineralization event (probably Permo-Trias- performed on material from this locality. sic) resulted in the precipitation of uraninite and pyrite in breccia and veins. A second, later mineralization stage is characterized by intense brecciation and silicification with precipitation of * E-mail: [email protected] minor amounts of siderite, chalcopyrite, sphalerite, Se-bearing 0003-004X/10/0010–1527$05.00/DOI: 10.2138/am.2010.3413 1527 1528 MEISSER ET AL.: FRANÇOISITE-(Ce), A NEW MINERAL SPECIES galena, and laitakarite [Bi4(Se,S)3]. Laitakarite gave a Pb/Pb age al. 2003b; Coats and Blissett 1971). The workings are located in of 239 ± 7 My (Triassic). the Mesoproterozoic Mt. Painter Inlier in the Northern Flinders (2) The ores were partially remobilized during the Tertiary Ranges, South Australia (Drexel et al. 1993; Fanning et al. Alpine metamorphism under lowest Greenschist facies conditions. 2003). The Mt. Painter Inlier contains large volumes of granites Locally intense fluid circulation led to the formation of the rare min- and gneisses highly enriched in U and Th (several tens of parts erals wittite [Pb3Bi4(S,Se)9] and Se-rich weibullite [Pb5Bi8(Se,S)18] per million). The radiogenic heat released by these granites is by sulfidation of laitakarite. In late-stage Alpine vugs (~2.5 kbar and responsible for a long-lasting thermal anomaly (e.g., Sandiford 350 °C), well-shaped Dauphiné habit quartz crystals coexist with et al. 1998; Neumann et al. 2000), which resulted in large-scale albite, abundant clinochlore ± chamosite, coffinite, arsenopyrite, hydrothermal activity within and around the Mt. Painter area. chalcopyrite, minor Ag-poor lillianite (AgxPb3–3xBi2+2xS6, with x = Small-scale on-going hydrothermal activity—probably limited 0.12), Se-poor galena, anatase, and titanite. by the low precipitation levels since the last ice age—is docu- (3) Strong mechanical and chemical weathering occurred dur- mented by the Paralana Hot Springs (Brugger et al. 2005). ing the Quaternary, especially at the beginning of the interglacial The province contains many small Paleozoic hematite-U-Cu- Riss-Wurm period. At this time, the melting of the ice sheet Nb-REE deposits, and a large epithermal system characterized by produced rapid decompression, uplift, and fracturing of the rocks, complex quartz±fluorite veins and breccias (Coats and Blissett followed by intense fluid circulation. The oxidation of pyrite and 1971; Drexel and Major 1990; Elburg et al. 2003). The Number arsenopyrite produced acidic fluids, which reacted with uraninite, 2 Workings explored a small (~4 × 4 m) lens of coarse hematite- fluorapatite, and silicates from the host rock. This resulted in the quartz ore; the other primary minerals include monazite-(Ce), formation of a complex assemblage of uranyl-bearing minerals xenotime-(Y), a solid solution between ishikawaite and Fe-rich at La Creusaz, characterized by the coexistence of silicates, oxy- samarskite, an unidentified Ca-Fe-phosphate, and an unidentified hydroxides, arsenates, phosphates, and selenites. This event is U(Pb,Mn)-oxide primary mineral always fully replaced by a com- dated at ~140 000 years using the 238U-234U-230Th disequilibrium plex assemblage of secondary minerals (mainly metaschoepite method applied to the abundant uranophane characteristic of and a Mn-Pb oxide, probably cesarolite). The phosphate-REE this weathering stage. The new mineral species described here, mineralization can constitute >50 vol% of the ore locally. This françoisite-(Ce), crystallized during this stage. Fe-U-Cu-Nb-REE mineralization is overprinted by the epith- (4) Since the end of the underground exploration in 1981, ermal mineralization, consisting of quartz veins with abundant exposed veins and stockpiled U ore have been subjected to pseudomorphs after fluorite and “needle quartz” growing around acid mine drainage water and atmospheric oxygen in the (now usually dissolved) acicular crystals of laumontite. A similar abandoned galleries. Oxidation of the sulfides (mainly pyrite evolution from magmatic hydrothermal conditions (510 ± 20 °C) and chalcopyrite) in the presence of strong bacterial activ- to epithermal (100–140 °C) was described by Bakker and Elburg ity resulted in the production of acid (pH ~ 3.1), sulfate-rich (2006) from unusual diopside-titanite veins located ~3 km SSE waters. These waters reacted with uraninite, clinochlore, illite, of the Number 2 Workings. A distinct geochemical relationship calcite, and siderite to form a rich assemblage of neoformed between the hematite lens at Number 2 and the titanite-diposide uranyl minerals, including the new minerals marécottite veins is indicated by the presence of Nb-rich inclusions in the ti- Mg3(UO2)8(SO4)4O6(OH)2·28H2O (Brugger et al. 2003a) and tanite, as well as abundant fluorapatite (Bakker and Elburg 2006). pseudojohannite Cu5(UO2)6(SO4)3(OH)16·14H2O (Brugger et The titanite was dated at 440 Ma at the time of intrusion of the al. 2006b), as well as jáchymovite (UO2)(SO4)(OH)14·13H2O, British Empire Granite (Elburg et al. 2003), but paleomagnetic johannite Cu(UO2)2(SO4)2(OH)2·8H2O, magnesiozippeite data suggest that at least two major hydrothermal events affected Mg(UO2)2(SO4)O2·3.5H2O, natrozippeite Na(UO2)2(SO4) the Mt. Painter area in the Permo-Carboniferous (Idnurm and O(OH)·2H2O, rabejacite Ca(UO2)4(SO4)2(OH)6·6H2O, schoe- Heinrich 1993). pite (UO2)8O2(OH)12·12H2O, schröckingerite NaCa3(UO2) At the Number 2 Workings, secondary uranium minerals (CO3)3(SO4)F·10H2O, uranopilite (UO2)6(SO4)(OH)10·12H2O, occur mainly in the cavities of the epithermal quartz, and in zippeite K(UO2)2(SO4)O(OH)·2H2O, and an unnamed Al-equiv- cavities resulting from the dissolution of the unknown primary alent of coconinoite Al4(UO2)2(PO4)4(SO4)(OH)2·20H2O.
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