Synchysite

Abstract: We have studied two groups of overprinted (altered) carbonatites at Amba Dongar, India: weakly overprinted and strongly overprinted. The mineral association of quartz, florencite-(Ce), strontianite, barite and sparse bastnasite – parisite – synchysite typical of weakly overprinted carbonatites shows evidence of a high activity of OH−, (SO4)2−, Al and Si in the fluid. Later, the fluid was characterized by an increased activity of F−, (SO4)2−, REE and Si, reflected in the association of barite, fluorite, quartz and REE fluorcarbonates typical of strongly overprinted carbonatites. Hydrothermal minerals at the Amba Dongar were formed by re-equilibration and recrystallization of the primary minerals in the presence of OH−, (SO4)2−, F−, REE, Al and Si carried in solution by the hydrothermal fluid.

Abstract: A fully cored drillhole was drilled to 1596 m by the Czech Geological Survey in 1961–1963 in the central part of the Cinovec (Zinnwald) granite cupola. Two types of granite were intersected: zinnwaldite granite (ZG), observed down to a depth of 730 m, and protolithionite granite (PG), occurring to the end of the hole. The core was used to study the distribution and chemistry of: zircon, thorite, xenotime, monazite, bastnasite, synchysite, REE oxyfluorides and hydroxyfluorides. Zircon occurs throughout the drillcore; it is strongly hydrated and fluorinated with about 18.5 wt.% H2O content in the apical part of the cupola. Its F-content reaches 2.41 wt.%. Within the PG, the F concentration in zircon is low. Zircon is poor in Th and U and its HfO2 contents vary from 1.01 to 5.24 wt.%. Thorite is common in the PG, becoming rare in the ZG. It is strongly hydrated (up to 14 wt.% H2O) and fluorinated (up to 2.04 wt.% F). Extensive solid solution between ThSiO4 and YPO4 was observed. Xenotime is strongly hydrated (up to 16 wt.% H2O), but its F content is low (<0.31 wt.%). Two types of monazite were identified: Th-rich (up to 9.3 wt.% ThO2) in the ZG, and Th-poor (<2.5 wt.% ThO2) in the PG. Monazite remained stable during the hydration and fluorination process. Its REE chondrite-normalized distribution patterns show negative anomalies for La and Nd and a pronounced negative anomaly for Eu. Chemical compositions of several REE oxyfluorides and hydroxyfluorides were studied. REE fluorocarbonates are represented by bastnasite and synchysite. Bastnasite is abundant in the ZG. Its chondrite-normalized REE patterns are characterized by an important negative Eu anomaly and downward kinks at La and Nd. Synchysite-(Ce) and synchysite-(Y) are particularly well developed in the deeper parts of the cupola, and exhibit REE distribution patterns characterized by a weak negative Eu anomaly (synchysite-(Ce)), or a weak positive Eu anomaly (synchysite-(Y)).

Abstract: The 2060 Ma old Palabora Carbonatite Complex (PCC), South Africa, comprises diverse REE mineral assemblages formed during different stages and reflects an outstanding instance to understand the evolution of a carbonatite-related REE mineralization from orthomagmatic to late-magmatic stages and their secondary post-magmatic overprint The 10 rare earth element minerals monazite, REE-F-carbonates (bastnasite, parisite, synchysite), ancylite, britholite, cordylite, fergusonite, REE-Ti-betafite, and anzaite are texturally described and related to the evolutionary stages of the PCC The identification of the latter five REE minerals during this study represents their first described occurrences in the PCC as well as in a carbonatite complex in South Africa The variable REE mineral assemblages reflect a multi-stage origin: (1) fergusonite and REE-Ti-betafite occur as inclusions in primary magnetite Bastnasite is enclosed in primary calcite and dolomite These three REE minerals are interpreted as orthomagmatic crystallization products (2) The most common REE minerals are monazite replacing primary apatite, and britholite texturally related to the serpentinization of forsterite or the replacement of forsterite by chondrodite Textural relationships suggest that these two REE-minerals precipitated from internally derived late-magmatic to hydrothermal fluids Their presence seems to be locally controlled by favorable chemical conditions (eg, presence of precursor minerals that contributed the necessary anions and/or cations for their formation) (3) Late-stage (post-magmatic) REE minerals include ancylite and cordylite replacing primary magmatic REE-Sr-carbonates, anzaite associated with the dissolution of ilmenite, and secondary REE-F-carbonates The formation of these post-magmatic REE minerals depends on the local availability of a fluid, whose composition is at least partly controlled by the dissolution of primary minerals (eg, REE-fluorocarbonates) This multi-stage REE mineralization reflects the interplay of magmatic differentiation, destabilization of early magmatic minerals during subsequent evolutionary stages of the carbonatitic system, and late-stage fluid-induced remobilization and re-/precipitation of precursor REE minerals Based on our findings, the Palabora Carbonatite Complex experienced at least two successive stages of intense fluid–rock interaction