Cerite

Abstract: Nolans-type ore deposits contain REE and Th mineralization hosted in fluorapatite veins. These veins intrude granulite facies rocks and are surrounded by a diopside selvage. Nolans-type deposits are thought to form by REE, F and P-rich hydrothermal fluids derived from alkali or carbonatitic intrusions. However, REE are not effectively transported in F and P-rich systems. REE ore deposits are commonly hydrothermally overprinted, possibly obscuring the igneous nature of the primary mineralization. We conducted a series of piston cylinder “sandwich” experiments, testing the hydrothermal fluid hypothesis, and a newly suggested process of carbonatite metasomatism. Our results confirm theoretical predictions that REE are hydrothermally immobile in these systems and the experimental phase assemblage is not compatible with the natural rocks. Our results show that fluorapatite can only host several weight percent levels of REE at temperatures higher than ∼600 °C. Below that temperature, a miscibility gap exists between REE-poor fluorapatite and REE-rich silicates such as britholite or cerite. In contrast, experiments reacting P and REE-rich carbonatite with silicate rock above 700 °C closely resemble natural rocks from Nolans-type deposits. Selvage mineralogy is sensitive to the MgO content of the carbonatite. A diopside selvage formed at carbonatite MgO/(CaO+MgO) ≈ 0.2 while wollastonite and forsterite formed at lower and higher ratios, respectively. Phosphate solubility in carbonatites decreases with decreasing MgO contents. As diopside formed, REE-rich fluorapatite preferentially crystallized from the selvage inwards. Thus, carbonatites are effective at simultaneously mobilizing REE, F and P to the site of deposition. Nolans-type deposits are the cumulate residue of this reaction, with the carbonatite liquid migrating elsewhere. At temperatures below 700 °C the carbonatite–silicate reaction additionally formed monticellite, cuspidine and magnesioferrite, resembling a skarn assemblage. Whereas skarns form by infiltration of silicate magmas or related fluids to carbonate rocks, our experiments are the opposite: intrusion of carbonatite into silicate rock. These mid-crustal skarn-like rocks may host elevated ore elements of carbonatitic affinity, such as F, P, Y, REE, Th, Ba, Sr, and Nb. We propose the term “antiskarn” to describe such systems, and suggest they trace the migration of carbonatite liquids through the crust. Hydrothermal reworking, retrogression, or metamorphism of antiskarns may obscure the carbonatitic genesis of the rocks. These metasomatic zones are the crustal equivalent of wehrlites that form by peridotite–carbonatite reaction at mantle depths.

Abstract: Lovozero, the largest of the world’s layered peralkaline intrusions, includes gigantic deposits of Nb + REE-loparite ore. Loparite, (Na,Ce,Ca)2(Ti,Nb)2O6, became a cumulus phase after crystallisation of about 35% of the ‘Differentiated Complex’, and its compositional evolution has been investigated through a 2.35 km section of the intrusion. The composition of the cumulus loparite changes systematically upwards through the intrusion with an increase in Na, Sr, Nb and Th and decrease in REE and Ti. This main trend of loparite evolution records differentiation of the peralkaline magma through crystallisation of 1600 m of the intrusion. The formation of the loparite ores was the result of several factors including the chemical evolution of the highly alkaline magma and mechanical accumulation of loparite at the base of a convecting unit. At later stages of evolution, when concentrations of alkalis and volatiles reached very high levels, loparite reacted with the residual melt to form a variety of minerals including barytolamprophyllite, lomonosovite, steenstrupine-(Ce), vuonnemite, nordite, nenadkevichite, REE, Sr-rich apatite, vitusite-(Ce), mosandrite, monazite-(Ce), cerite and Ba, Si-rich belovite. The absence of loparite ore in the “Eudialyte complex” is likely to be a result of the wide crystallisation field of lamprophyllite, which here became a cumulus phase.

Abstract: A detailed survey of accessory minerals in the poorly to moderately differentiated F-poor biotite granites from Niederbobritzsch, Erzgebirge, Germany, reveals the presence of various, late magmatic to postmagmatic secondary rare-earth (REE) minerals, including cerite-(Ce), thorian synchysite-(Ce), synchysite-(Ce), and an unidentified Th-rich REE fluorocarbonate(?). Cerite-(Ce) is a REE silicate that, to date, has been found in only half a dozen occurrences worldwide. It had not previously been described from a granite. The composition of cerite-(Ce) from Niederbobritzsch is characterized by lower bulk REE contents but higher abundances of Si, Al, Ca, and F than that from other occurrences. Dissolution of thorian monazite(Ce) during interaction with a F-, CO 2- and Ca-bearing fluid gave rise to the formation of thorian synchysite-(Ce) containing up to 18.1 wt% ThO2. Previously reported contents of Th in synchysite-(Ce) did not exceed 1.6 wt% ThO 2. The spatial relations between the secondary REE minerals and their precursor attest to a differential mobility of the LREE and Th during fluid‐rock interaction. Under the prevailing PTX-conditions, the LREE were more soluble and, thus, mobilized further away from their site of removal relative to Th, which tended to be reprecipitated next to its precursor. Virtually unchanged whole-rock REE budgets and continuous, unfractionated chondrite-normalized LREE patterns of the secondary REE minerals, however, imply that the lanthanides were transported over distances of millimeters or centimeters only.

Abstract: The Virulundo carbonatite in Angola is one of the largest in the world and contains pyrochlore as an accessory mineral in all of the carbonatite units (calciocarbonatites, ferrocarbonatites, carbonatite breccias and trachytoids). The primary magmatic pyrochlore is fluorine dominant and typically contains about equal molar quantities of Ca and Na at the A site. High-temperature hydrothermal processes have resulted in the pseudomorphic replacement of the primary pyrochlore by a second generation of pyrochlore with less F and Na. Low-temperature hydrothermal replacement of the first and second generation pyrochlore, associated with quartz-carbonate-fluorite vein formation in the carbonatite, has produced a third generation of pyrochlore, with a high Sr content. The Sr appears to have been released by low-temperature hydrothermal replacement of the primary magmatic carbonates. Finally, supergene alteration processes have produced late-stage carbonates, goethite, hollandite and rare earth element (REE) minerals (mainly synchysite-(Ce), britholite-(Ce), britholite-(La), cerite(Ce)). Cerium separated from the other REEs in oxidizing conditions and Ce 4+ was incorporated into a late generation of supergene pyrochlore, which is strongly enriched in Ba and strongly depleted in Ca and Na.