Sulfur-bearing monazite-(Ce) from the Eureka carbonatite, Namibia: oxidation state, substitution mechanism, and formation conditions
Sam Broom-FendleyMartin SmithMarcelo B. AndradeSantanu RayDavid BanksEdward LoyeDaniel AtencioJonathan PicklesFrances Wall
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Abstract Sulfur-bearing monazite-(Ce) occurs in silicified carbonatite at Eureka, Namibia, forming rims up to ~0.5 mm thick on earlier-formed monazite-(Ce) megacrysts. We present X-ray photoelectron spectroscopy data demonstrating that sulfur is accommodated predominantly in monazite-(Ce) as sulfate, via a clino-anhydrite-type coupled substitution mechanism. Minor sulfide and sulfite peaks in the X-ray photoelectron spectra, however, also indicate that more complex substitution mechanisms incorporating S 2– and S 4+ are possible. Incorporation of S 6+ through clino-anhydrite-type substitution results in an excess of M 2+ cations, which previous workers have suggested is accommodated by auxiliary substitution of OH – for O 2– . However, Raman data show no indication of OH – , and instead we suggest charge imbalance is accommodated through F – substituting for O 2– . The accommodation of S in the monazite-(Ce) results in considerable structural distortion that may account for relatively high contents of ions with radii beyond those normally found in monazite-(Ce), such as the heavy rare earth elements, Mo, Zr and V. In contrast to S-bearing monazite-(Ce) in other carbonatites, S-bearing monazite-(Ce) at Eureka formed via a dissolution–precipitation mechanism during prolonged weathering, with S derived from an aeolian source. While large S-bearing monazite-(Ce) grains are likely to be rare in the geological record, formation of secondary S-bearing monazite-(Ce) in these conditions may be a feasible mineral for dating palaeo-weathering horizons.Keywords:
Carbonatite
Anhydrite
Ionic radius
Carbonatites 是通常与象 REE， Cu，和 P 那样的经济地珍贵的物质的累积有关。在到联系硅酸盐岩石的 carbonatites 和他们的关系的起源上的争论是进行中的大约 45 年了。全球，岩石表示特性地比沉积碳酸盐岩石在 Ba， Sr 和 REE 显示更多的 geochemical 丰富。然而，因为第二等的矿物质效果， carbonatites geochemical 特征被争论。从在在中国的 Qinling， Panxi 区域，和 Bayan Obo 的 carbonatites 的形成岩石的碳酸盐显示出从 LREE 丰富到扁平的模式的 REE 分发模式。他们被超过 10 比第二等的碳酸盐的高预定的一个 Sr 内容描绘。从 Bayan Obo H8 白云石云石的纹理粗糙、有细密纹理的白云石也显示出类似的高 Sr 丰富，显示他们具有火的起源。在中国 carbonatites 的一些碳酸盐显示出 REE (特别 HREE ) 整个岩石的类似于那些的内容和分布模式。这些恼人的 carbonatites 显示相对 high-Si 降低在磅和红外之间的铂组元素和更强壮的分别挤出的 carbonatite。这显示很恼人的 carbonatites 可以是碳酸盐堆积。在 Panxi 区域的 Maoniuping 和 Daluxiang 是大 REE 沉积物。热水的荧石矿石静脉发生在 carbonatite 身体外面并且是在 wallrock 的 emplaced 正长岩。在 Maoniuping 的荧石有类似于 carbonatite 的 Sr 和 Nd 同位素。Daluxiang 荧石在 Maoniuping 显示出与那些不同的 Sr 和 REE 作文。差别被 carbonatites 和形成岩石的碳酸盐反映，显示那个 REE 矿化作用与 carbonatites 有关。碳酸盐矿物质的 cumulate 进程从 LREE 由于为在碳酸盐和 carbonatite 之间的 REE 的系数融化的低分区和增加在 volatiles 和 LREE 使 fractionated 液体富有到 HREE。导出 carbonatite 的液体与 wallrock 交往了形成 REE 矿石静脉。发生在 Bayan Obo orebodies 附近的 carbonatite 女同性恋的数量可以支持一样的矿化作用模型，即液体从 carbonatite 女同性恋演变，这与 H8 白云石大理石反
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The Catalão alkaline carbonatite complex hosts a number of mineral resources including monazite. This mineral is a common accessory phase in two lithological units: carbonatite and silexite. Textural evidence suggest that monazite replaced carbonates in the carbonatite and crystallized simultaneously with quartz in the silexite. Monazite was resistant to the strong laterization that affected the massif, except for the incipient transformation into gorceixite or cerianite. In both carbonatite and silexite, monazite occurs as a complex aggregate of sub-micrometric crystals, showing unusual morphological and chemical characteristics. It contains Ca, Sr, and Ba in the A-site, and shows a certain degree of hydration indicated by ATD and IV data. Structural formulae calculated on the basis of sum of cations=1 show a moderate ionic deficiency in the anionic site. Rietveld reffinement indicated poor crystallinity. Notwihstanding these peculiar characteristics, cell dimensions are similar to those of standard monazite.
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The process of enrichment of middle and heavy rare earth elements (MREE and HREE = Sm-Lu plus Y) in magmatic-hydrothermal systems, especially the carbonatite system, remains unclear. Here, we performed in-situ monazite element and isotopic analysis to investigate the genesis of the Huayangchuan REE-Nb-U polymetallic deposit in central China and the enrichment process of MREE and HREE during the evolution of the magmatic hydrothermal system. The Huayangchuan deposit is spatially associated with HREE-rich calcite carbonatites, which exist as two mineralization stages: the ores of the first stage are hosted by carbonatite itself; in the second stage, the ores are present as veins, dominantly by wall rocks around the carbonatite. The results of secondary ion mass spectrometry (SIMS) monazite U-Pb dating on the thin sections yielded Tera-Wasserburg lower intercept ages of 207 ± 4 Ma and 206 ± 5 Ma for carbonatite-hosted monazite and vein-type monazite, respectively. However, some monazite analysis points provide a wide Yanshanian age range (112–182 Ma). The lower intercept age of 206–207 Ma is explained as the mineralization age of the deposit, whereas the wide younger age with higher 238U/206Pb and lower 208Pb/232Th ratios may be ascribed to the loss of radioactive Pb, as the varying degrees of reworking resulted from the later Yanshanian tectono-thermal event. The monazite laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Sr-Nd isotopes from both carbonatite-hosted and wall-rock-hosted vein-type ores indicate that the Huayangchuan deposit is closely related to enriched mantle-derived (EM1) calcite carbonatite but locally affected by late fluid activation. Combined with the integrated age spectra, such reworking processes associated with new weak mineralization widely exist in other carbonatite-related deposits in the Lesser Qinling, but are not important for carbonatite-related mineralization in this area. Controlled by element behaviors, MREE and HREE are more likely to dissolve in hydrothermal fluids, enabling their long-distance migration and deposition, than light REE (LREE) in some carbonatite systems, resulting in much higher MREE and HREE contents of the late monazite in the vein-type ores than those of the early carbonatite-hosted monazite. Therefore, it could be expected that there might be MREE and HREE enrichment and exploration potential in the periphery of some large carbonatite-related LREE-dominated deposits.
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Approximately >50% of global rare earth element (REE) resources are hosted by carbonatite related deposits, of which monazite is one of the most important REE minerals. Monazite dominates more than 30 carbonatite-related REE deposits around the world, including currently exploited mineralization at Bayan Obo and Mount Weld. These deposits are widely distributed across all continents, except Antarctica. Though rare, monazite occurs as the primary mineral in carbonatite, and mostly presents as a secondary mineral that has a strong association with apatite. It can partially or completely replace thin or thick overgrowth apatite, depending on the availability of REE. Other mineral phases that usually crystallize together with monazite include barite, fluorite, xenotime, sulfide, and quartz in a carbonate matrix (e.g., dolomite, calcite). This review of monazite geochemistry within carbonatite-related REE deposits aims to provide information regarding the use of monazite as a geochemical indicator to track the formation history of the REE deposits and also supply additional information for the beneficiation of monazite. The chemical compositions of monazite are highly variable, and Ce-monazite is the dominant solid solution in carbonatite related deposits. Most monazite displays steep fractionation from La to Lu, absent of either Eu or Ce anomalies in the chondrite normalized REE plot. The other significant components are huttonite and cheratite. Some rare sulfur-bearing monazite is also identified with an SO3 content up to 4 wt %. A 147Sm/144Nd ratio with an average ~0.071 for monazite within carbonatite-related ores is similar to that of their host rocks (~0.065), and is the lowest among all types of REE deposits. Sm/Nd variation of monazite from a single complex reflects the differentiation stage of magma, which decreases from early to late. Based on the differences of Nd and Sr abundances, Nd isotopic composition for monazite can be used to track the magma source, whereas Sr isotopic composition records the signatures of the fluid source. Th-(U)-Pb age determination of the secondary monazite records variable thermal or metasomatic disturbances, and careful geochronological interpretation should be brought forward combined with other lines of evidence. ThO2 is the most difficult contamination in the beneficiation of monazite, luckily, the ThO2 content of monazite within carbonatite is generally low (<2 wt %).
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<p>Table S1: The chemical compositions of allanite and monazite from the Huangjiagou carbonatite. Table S2: The U-Th-Pb date of monazite and allanite from the Huangjiagou carbonatite. Table S3: The Sr-Nd isotope data of monazite and allanite from the Huangjiagou carbonatite.</p>
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Abstract Apatite-dolomite carbonatite at Lesnaya Varaka, Kola Peninsula, Russia, hosts intricate mineral intergrowths composed of lueshite in the core and pyrochlore-group minerals in the rim. Lueshite is a primary Nb-bearing phase in the carbonatite and ranges in composition from cerian lueshite to almost pure NaNbO 3 . For comparison, the compositional variation of lueshite from the Kovdor and Sallanlatvi carbonatites is described. At Lesnaya Varaka, lueshite is replaced by nearly stoichiometric Na-Ca pyrochlore due to late-stage re-equilibration in the carbonatite system. X-ray powder diffraction data for both minerals are presented. Barian strontiopyrochlore, occurring as replacement mantles on Na-Ca pyrochlore, contains up to 43% Sr and 8–18% Ba at the A -site, and shows a high degree of hydration and strong ionic deficiency at the A - and Y -sites. This mineral is metamict and, upon heating, recrystallises to an aeschynite-type structure. Monazite-(Ce) found as minute crystals in fractures, represents the solid solution between monazite-(Ce) CePO 4 , brabantite CaTh(PO 4 ) 2 and SrTh(PO 4 ) 2 . Our data indicate the high capacity of the monazite structure for Th and accompanying divalent cations at low temperatures and pressures that has a direct relevance to solving the problem of long-term conservation of radioactive wastes. Monazite-(Ce) and barian strontiopyrochlore are products of low-temperature hydrothermal or secondary (hypergene) alteration of the primary mineral assemblage of the carbonatite.
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Monazite ages from carbonatites and high-grade assemblages exposed along a significant lineament within the Southern Granulite Terrane of India termed the Kambam fault were obtained in thin section (in situ) using an ion microprobe. X-ray maps for Ce and Th were acquired in larger monazites to decipher the significance of the ages of individual spots within grains. The Kambam carbonatite contains large (millimeter-sized) apatite rimmed by ~10 μm thick bands of monazite. Monazite commonly appears as a lower-Th, late-stage mineral in carbonatites, and bands surrounding apatite are interpreted as products of metasomatism, rather than exsolution. The age of a Kambam carbonatite monazite band is 715 ± 42 Ma (Th-Pb, ± 1σ), but monazite filling cracks within the apatite is ~300 m.y. younger (405 ± 5 Ma). The younger monazite grains are in contact with quartz, a mineral thought to be an indicator of subsolidus alteration in carbonatites. The age of the monazite rim is similar to ages of several carbonatites located 50-400 km further north, and chemical analyses show that this sample displays chemical trends similar to the other complexes (e.g., Y/Ho, Ce/Pb, REE, and HFSE patterns). The mid-Neoproterozoic event is recorded in garnet-bearing assemblages ~20 km west of the Kambam fault (733 ± 15 Ma) and garnet-bearing enclaves within Southern Granulite Terrane charnockites (701 ± 26 Ma; 786 ± 84 Ma). The results show that monazite can crystallize during metasomatism and be useful in deciphering fluid processes occurring at deeper crustal levels. The Kambam fault, which records over 300 million years of monazite growth, should be considered a major boundary in reconstructions of Gondwana.
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The Catalao alkaline carbonatite complex hosts a number of mineral resources including monazite. This mineral is a common accessory phase in two lithological units: carbonatite and silexite. Textural evidence suggest that monazite replaced carbonates in the carbonatite and crystallized simultaneously with quartz in the silexite. Monazite was resistant to the strong laterization that affected the massif, except for the incipient transformation into gorceixite or cerianite. In both carbonatite and silexite, monazite occurs as a complex aggregate of sub-micrometric crystals, showing unusual morphological and chemical characteristics. It contains Ca, Sr, and Ba in the A-site, and shows a certain degree of hydration indicated by ATD and IV data. Structural formulae calculated on the basis of sum of cations=1 show a moderate ionic deficiency in the anionic site. Rietveld reffinement indicated poor crystallinity. Notwithstanding these peculiar characteristics, cell dimensions are similar to those of standard monazite.
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<p>Table S1: The chemical compositions of allanite and monazite from the Huangjiagou carbonatite. Table S2: The U-Th-Pb dating of monazite and allanite from the Huangjiagou carbonatite. Table S3: The Sr-Nd isotope data of monazite and allanite from the Huangjiagou carbonatite. </p>
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