scholarly journals Light rare earth element redistribution during hydrothermal alteration at the Okorusu carbonatite complex, Namibia

2019 ◽  
Vol 84 (1) ◽  
pp. 49-64 ◽  
Author(s):  
Delia Cangelosi ◽  
Sam Broom-Fendley ◽  
David Banks ◽  
Daniel Morgan ◽  
Bruce Yardley
Keyword(s):  
Rare Earth ◽  
Hydrothermal Alteration ◽  
Iron Hydroxide ◽  
Chemical Change ◽  
Trace Element Content ◽  
External Source ◽  
Circulation System ◽  
Carbonatite Complex ◽  
Element Redistribution ◽  
Calcite Carbonatites

AbstractThe Cretaceous Okorusu carbonatite, Namibia, includes diopside-bearing and pegmatitic calcite carbonatites, both exhibiting hydrothermally altered mineral assemblages. In unaltered carbonatite, Sr, Ba and rare earth elements (REE) are hosted principally by calcite and fluorapatite. However, in hydrothermally altered carbonatites, small (<50 µm) parisite-(Ce) grains are the dominant REE host, while Ba and Sr are hosted in baryte, celestine, strontianite and witherite. Hydrothermal calcite has a much lower trace-element content than the original, magmatic calcite. Regardless of the low REE contents of the hydrothermal calcite, the REE patterns are similar to those of parisite-(Ce), magmatic minerals and mafic rocks associated with the carbonatites. These similarities suggest that hydrothermal alteration remobilised REE from magmatic minerals, predominantly calcite, without significant fractionation or addition from an external source. Barium and Sr released during alteration were mainly reprecipitated as sulfates. The breakdown of magmatic pyrite into iron hydroxide is inferred to be the main source of sulfate. The behaviour of sulfur suggests that the hydrothermal fluid was somewhat oxidising and it may have been part of a geothermal circulation system. Late hydrothermal massive fluorite replaced the calcite carbonatites at Okorusu and resulted in extensive chemical change, suggesting continued magmatic contributions to the fluid system.

scholarly journals Rare-earth mobility as a result of multiple phases of fluid activity in fenite around the Chilwa Island Carbonatite, Malawi

2017 ◽  
Vol 81 (6) ◽  
pp. 1367-1395 ◽  
Author(s):  
Emma Dowman ◽  
Frances Wall ◽  
Peter J. Treloar ◽  
Andrew H. Rankin
Keyword(s):  
Rare Earth ◽  
Inductively Coupled Plasma ◽  
Raw Materials ◽  
Schematic Model ◽  
Carbonatite Complex ◽  
Inductively Coupled ◽  
Rare Earth Minerals ◽  
Plasma Mass Spectrometry ◽  
Rare Earth Mineral ◽  
The Rare Earth

AbstractCarbonatites are enriched in critical raw materials such as the rare-earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and rare-earth minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study. In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, rare-earth minerals and pyrochlore in vein networks. Petrography using a scanning electron microscope in energy-dispersive spectroscopy mode showed that the rare-earth minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence ofREEmobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by laser ablation inductively coupled plasma mass spectrometry) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallization of plagioclase and co-crystallization with K-feldspar in the fenite. The fenite minerals have consistently higher midREE/lightREEratios (La/Sm ≈ 1.3 monazite, ≈ 1.9 bastnäsite, ≈ 1.2 parisite) than their counterparts in the carbonatites (La/Sm ≈ 2.5 monazite, ≈ 4.2 bastnäsite, ≈ 3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few micrometres in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and baryte were found in the inclusions. Decrepitation of inclusions occurred at ∼200°C before homogenization but melting-temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of ∼24 wt.% NaCl equivalent was determined. Among the trace elements in whole-rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, YandREE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct rare-earth mineral microassemblages in fenite at some distance from carbonatite could be developed as an exploration indicator ofREEenrichment.

scholarly journals The Saint-Honoré Carbonatite REE Zone, Québec, Canada: Combined Magmatic and Hydrothermal Processes

Minerals ◽  
2018 ◽  
Vol 8 (9) ◽  
pp. 397 ◽  
Author(s):  
Alexandre Néron ◽  
Léo Bédard ◽  
Damien Gaboury
Keyword(s):  
Rare Earth ◽  
Late Stage ◽  
High Volume ◽  
Primary Crystallization ◽  
Hydrothermal Activity ◽  
Hydrothermal Crystallization ◽  
Carbonatite Complex ◽  
Hydrothermal Processes ◽  
Rare Earth Minerals ◽  
New Evidence

The Saint-Honoré carbonatite complex hosts a rare earth element (REE) deposit traditionally interpreted as being produced by late-stage hydrothermal fluids that leached REE from apatite or dolomite found in the early units and concentrated the REE in the late-stage units. New evidence from deeper units suggest that the Fe-carbonatite was mineralized by a combination of both magmatic and hydrothermal crystallization of rare earth minerals. The upper Fe-carbonatite has characteristics typical of hydrothermal mineralization—polycrystalline clusters hosting bastnäsite-(Ce), which crystallized radially from carbonate or barite crystals, as well as the presence of halite and silicification within strongly brecciated units. However, bastnäsite-(Ce) inclusions in primary magmatic barite crystals have also been identified deeper in the Fe-carbonatite (below 1000 m), suggesting that primary crystallization of rare earth minerals occurred prior to hydrothermal leaching. Based on the intensity of hydrothermal brecciation, Cl depletion at depth and greater abundance of secondary fluid inclusions in carbonates in the upper levels, it is interpreted that hydrothermal activity was weaker in this deepest portion, thereby preserving the original magmatic textures. This early magmatic crystallization of rare earth minerals could be a significant factor in generating high-volume REE deposits. Crystallization of primary barite could be an important guide for REE exploration.

scholarly journals Magmatic-Hydrothermal Processes Associated with Rare Earth Element Enrichment in the Kangankunde Carbonatite Complex, Malawi

Minerals ◽  
2019 ◽  
Vol 9 (7) ◽  
pp. 442 ◽  
Author(s):  
Frances Chikanda ◽  
Tsubasa Otake ◽  
Yoko Ohtomo ◽  
Akane Ito ◽  
Takaomi D. Yokoyama ◽  
...  
Keyword(s):  
Rare Earth ◽  
Geochemical Data ◽  
Carbonatite Complex ◽  
Compositional Zoning ◽  
Ree Mineralization ◽  
Hydrothermal Processes ◽  
Magmatic Stage ◽  
Isotope Study ◽  
Late Magmatic Stage ◽  
Element Enrichment

Carbonatites undergo various magmatic-hydrothermal processes during their evolution that are important for the enrichment of rare earth elements (REE). This geochemical, petrographic, and multi-isotope study on the Kangankunde carbonatite, the largest light REE resource in the Chilwa Alkaline Province in Malawi, clarifies the critical stages of REE mineralization in this deposit. The δ56Fe values of most of the carbonatite lies within the magmatic field despite variations in the proportions of monazite, ankerite, and ferroan dolomite. Exsolution of a hydrothermal fluid from the carbonatite melts is evident based on the higher δ56Fe of the fenites, as well as the textural and compositional zoning in monazite. Field and petrographic observations, combined with geochemical data (REE patterns, and Fe, C, and O isotopes), suggest that the key stage of REE mineralization in the Kangankunde carbonatite was the late magmatic stage with an influence of carbothermal fluids i.e. magmatic–hydrothermal stage, when large (~200 µm), well-developed monazite crystals grew. The C and O isotope compositions of the carbonatite suggest a post-magmatic alteration by hydrothermal fluids, probably after the main REE mineralization stage, as the alteration occurs throughout the carbonatite but particularly in the dark carbonatites.

Manganoan fayalite and products of its alteration from the Strzegom pegmatites, Poland

1989 ◽  
Vol 53 (371) ◽  
pp. 315-325 ◽  
Author(s):  
J. Janeczek
Keyword(s):  
Hydrothermal Alteration ◽  
Fluid Inclusions ◽  
Iron Hydroxide ◽  
Silicate Phase ◽  
Second Stage ◽  
Multi Stage ◽  
Composite Mixture ◽  
Quartz Aggregates ◽  
Lower Silesia

AbstractNodules of manganoan fayalite occur in schlieren pegmatities in the vicinity of Strzegom, Lower Silesia. The fayalite, Na0.02(Fe1.812+Mn0.16Mg 0.03)Si0.99O4, is unzoned and non pleochroic. 2Va = 42° a 4.826(3), b 10.500(2), c 6.102(2) A, d130obs. = 2.83 Å, d130calc. = 2.833 Å, D = 4.35 g cm-3, Dcalc. = 4.353 g cm-3. The role of Na+ ions in the fayalite chemistry is discussed. The fayalite underwent multi-stage hydrothermal alteration beginning at the highest temperature (440°C) of homogenization of gaseous-fluid inclusions in quartz adjacent to the fayalite grains. Increase in fO2 and then in PH2O resulted in the formation of magnetite-quartz and Mn-grunerite-magnetite-quartz aggregates within the fayalite grains. The fayalite is mantled by a Mn-greenalite-magnetite rim, Mn-grunerite-magnetite-Mn-minnesotaite zone in a stilpnomelane or greenalite-rich groundmass. The minnesotaite is believed to have formed at the expense of grunerite. Stilpnomelane, the most abundant silicate phase in the rim, is the product of biotite and presumably greenalite alteration at the second stage of increasing Na activity (the crystallization of cleavelandite) in the pegmatites. The fayalite is also heavily altered to iddingsite—a composite mixture of amorphous FeOOH and silica. The iron-hydroxide recrystallized partially to poorly-crystalline goethite.

Multiple-stage hydrothermal alteration in porphyry copper systems in northern Turkey: the temporal interplay of potassic, propylitic, and phyllic fluids

1980 ◽  
Vol 17 (7) ◽  
pp. 901-926 ◽  
Author(s):  
R. P. Taylor ◽  
B. J. Fryer
Keyword(s):  
Rare Earth ◽  
Hydrothermal Alteration ◽  
Rare Earth Element ◽  
Earth Element ◽  
Root Zone ◽  
Porphyry Copper ◽  
Hydrothermal Fluids ◽  
New Mineral ◽  
Potassic Alteration ◽  
Different Levels

The Bakircay and Ulutas Cu–Mo prospects represent the first occurrences of porphyry mineralization to be described in Turkey.Differences observed in the two prospects in terms of hydrothermal alteration (in particular, alteration overprinting), igneous textures, abundance of xenoliths, breccia phenomena, and style and intensity of fracturing may relate to different levels of exposure within a model porphyry system, Bakircay representing the deep root zone of such a system and Ulutas reflecting much higher levels close to the apex of such a system, or may simply reflect different levels of emplacement.The alteration assemblages present at the Bakircay prospect lend themselves to a geochemical study of the temporal variations in the hydrothermal fluids responsible for single- and multiple-stage alteration–mineralization. The chemical changes involved during single-stage potassic alteration are related to amphibole breakdown and the deposition of hydrothermal biotite (and chalcopyrite). These changes are manifested in light rare-earth element (LREE) enrichment and heavy rare-earth element (HREE) depletion reflecting the high K+ and Cl− activity of the hydrothermal fluids. During propylitic overprinting of potassic alteration changes in whole-rock geochemistry relate to the destruction of biotite (both igneous and hydrothermal) and the formation of chlorite, epidote, calcite, and apatite. These changes result in the loss of ail rare-earth elements (REE) due to increasing fluid/rock ratios and further changes within the HREE relating to zircon stability and the deposition of new mineral phases, e.g., epidote. Conversion of preexisting alteration types lo the quartz–sericite–pyrite ± rutile, calcite assemblages, typical of phyllic alteration, results in the loss of all elements not accommodated in these phases. The high fluid/rock ratios and low pH of the fluids cause progressive leaching of all REE, particularly the lightest (La and Ce).

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