Abstract. Recent advances in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) open new perspectives for quantification of trace metals and metalloids in mineral-hosted fluid inclusions and glass-hosted gas bubbles. This work is devoted to a new method applied to quantify element concentrations (at parts-per-million and weight percent levels) in natural and synthetic fluid inclusions and gas bubbles by using only an external calibrator in cases where internal standardization is unavailable. For example, this method can be applied to calculate element (metal and metalloid) concentrations in carbonic (C–O–H) fluid inclusions and bubbles. The method is devoted to measuring incompatible (with the host mineral and glass) trace elements originally dissolved into the trapped fluid. The method requires precise estimation of the fluid density, the inclusion/bubble volume or average radius, and measurement of the laser ablation crater radius by independent microanalytical techniques as well as accurate data on the concentration of major/minor elements compatible with the host mineral (or host glass). This method, applicable for analyses of hydrous carbonic fluid inclusions and gas bubbles hosted in silicate minerals and glasses, relies on the absence of a matrix effect between fluid, host mineral and daughter phases (silicate, oxide or sulfide) and the external calibrator (e.g., reference silicate glasses) during the LA-ICP-MS analysis, an assumption validated by the use of femtosecond lasers.
Ce travail a pour objectif l'etude de la geologie du complexe annulaire alcalin d'Ambohimirahavavy, au nord-ouest de Madagascar, en mettant l'accent sur la comprehension des phenomenes a l'origine de la formation des mineralisations a metaux rares (Zr, Hf, Nb, Ta, Th, U et terres rares). Le complexe, mis en place a la limite oligo-miocene, serait issu de la differenciation magmatique d'un magma parent d'origine mantellique. En fin d'evolution, des phenomenes de contamination crustale expliqueraient l'association des series magmatiques sous- et sursaturees en silice dans le complexe. Les termes les plus differencies de ces deux series evolueraient vers des compositions hyperalcalines dans des conditions de fugacite en oxygene opposees. La mineralisation a metaux rares est essentiellement associee aux roches des facies sursatures, a savoir, des filons de granite et de pegmatite hyperalcalins. Deux types essentiels de mineralisation ont ete reconnus, une miaskitique caracterisee par la presence de plusieurs types de zircon et une agpaitique riche en eudialyte. La formation de ces mineralisations serait liee a la fois a des processus magmatiques et hydrothermaux. Les granites et les pegmatites riches en metaux rares se formeraient a partir de magmas hyperalcalins enrichis a l'extreme en elements fluants (F, Cl); la mineralisation est alors representee par un zirconosilicate riche en metaux rares, l'eudialyte. Les fluants vont diminuer considerablement la temperature du solidus et augmenter la solubilite de l'eau dans le magma. En fin de cristallisation, on pourra avoir des phenomenes de demixtions de fluides orthomagmatiques riches en Na, Si, F et Cl a l'origine des pseudomorphoses de l'eudialyte en zircon et des neoformations de mineraux a metaux rares. Des datations U-Pb sont en accord avec la formation de ces zircons a partir d'un fluide, donnant des âges autour de 21 Ma, soit 3 Ma plus jeunes que la mise en place des syenites a nepheline, datees egalement par la methode U-Pb sur zircon. Cette etude montre egalement que la composition de l'encaissant peut jouer un role essentiel en forcant la precipitation des mineraux a metaux rares dans des formations de type skarn, dans laquelle interviennent aussi des fluides meteoriques. Ce travail met en evidence la complexite des processus d'enrichissement et de fractionnement des metaux rares depuis le magma parent d'origine mantellique aux facies evolues pegmatitiques conduisant a la diversite des mineralisations rencontrees dans ces roches hyperalcalines. Enfin, cette etude propose egalement une nouvelle cible potentielle de mineralisation en relation avec les facies sous-satures
The Cenozoic Ambohimirahavavy alkaline complex in Madagascar consists of several syenitic to granitic intrusions (24.2 ± 0.6 Ma) the largest of which, the Ampasibitika intrusion, is characterized by the presence in its outer flanks of late peralkaline granitic dikes intruding mudstone and limestone of the Isalo Group. A network of dikelets and veinlets propagates from these dikes, intruding along bedding or obliquely to bedding. At the contact between the dikes and dikelets and a limestone, a reaction zone enriched in rare metals, dominated by calc-silicate minerals such diopside and andradite-grossular, forms a rare example of skarn resulting from peralkaline igneous activity.
Much of the rare-metal mineralization (REE, Zr, Nb, Th, Sn, and Ti) occurs as secondary phases in the dikelets and skarn. In the dikelets and endoskarn, high field strength element (HFSE)-rich phases consist mainly of zircon, bastnasite-(Ce), and Ca-REE-, and Ca-HFSE-rich phases in pseudomorphs after aegirine-augite. In the exoskarn, the main HFSE-rich phases are bastnasite-(Ce), zircon, pyrochlore, Nb-rich titanite, and an unidentified F-rich Ca-zirconosilicate finely disseminated in a matrix composed of calcite, diopside, andradite, phlogopite, quartz, fluorite, and fluorapatite. The secondary zircon is characterized by a low Zr content and by the presence of REE, Ca, Al, and Fe.
Three types of primary fluid inclusions were observed in dikelets and skarn in quartz, calcite, and diopside:
1. liquid-rich inclusions (L-V) (20 to 40 vol % vapor) occur in all three minerals and homogenize to liquid;
2. vapor-rich inclusions (V) (>90 vol % vapor) occur in diopside and quartz, and homogenize to vapor; and
3. halite-bearing L-V inclusions (L-V-H) occur in diopside and quartz, and homogenize either by disappearance of the vapor bubble or by halite dissolution. The L-V inclusions have low to intermediate salinity and homogenize at temperatures ranging from 200° to 380°C. The V inclusions have low salinity and homogenize at higher temperature (350°–395°C). The L-V-H inclusions mainly contain NaCl (35–45 wt % NaCl equiv), and homogenize by three modes, namely, bubble disappearance (mode A), halite dissolution (mode B), and simultaneous bubble and halite disappearance (mode C); the homogenization temperatures range from 260° to 380°C.
We propose a model in which rare metals were transported by a Cl−, F− and HFSE-rich orthomagmatic fluid exsolved at 400° to 450°C and about 20 MPa. At these conditions, the fluid was in the two-phase region and vapor dominated. The rare metals were deposited as a result of the interaction of this fluid with limestone and mixing with an external fluid. This interaction/mixing buffered the orthomagmatic fluid to higher pH and lower temperature, resulting in the destabilization of REE-chloride complexes and deposition of fluorocarbonate minerals in the limestone; Zr and Nb, which were likely transported as hydroxyl-fluoride complexes, precipitated as zircon and pyrochlore due to deposition of fluorite and a consequent decrease in fluoride activity.
Research Article| September 30, 2019 Economic mineralization in pegmatites: comparing and contrasting NYF and LCT examples Kathryn M. Goodenough; Kathryn M. Goodenough § British Geological Survey, Edinburgh, UK § Corresponding author e-mail address: kmgo@bgs.ac.uk Search for other works by this author on: GSW Google Scholar Richard A. Shaw; Richard A. Shaw British Geological Survey, Keyworth, Nottingham, UK Search for other works by this author on: GSW Google Scholar Martin Smith; Martin Smith School of Environment and Technology, University of Brighton, UK Search for other works by this author on: GSW Google Scholar Guillaume Estrade; Guillaume Estrade Geosciences Environnement Toulouse, France Search for other works by this author on: GSW Google Scholar Eva Marqu; Eva Marqu School of Environment and Technology, University of Brighton, UK Search for other works by this author on: GSW Google Scholar Cyrielle Bernard; Cyrielle Bernard Geosciences Environnement Toulouse, France Search for other works by this author on: GSW Google Scholar Paul Nex Paul Nex University of the Witwatersrand, South Africa Search for other works by this author on: GSW Google Scholar The Canadian Mineralogist (2019) 57 (5): 753–755. https://doi.org/10.3749/canmin.AB00013 Article history first online: 07 Oct 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kathryn M. Goodenough, Richard A. Shaw, Martin Smith, Guillaume Estrade, Eva Marqu, Cyrielle Bernard, Paul Nex; Economic mineralization in pegmatites: comparing and contrasting NYF and LCT examples. The Canadian Mineralogist 2019;; 57 (5): 753–755. doi: https://doi.org/10.3749/canmin.AB00013 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyThe Canadian Mineralogist Search Advanced Search Granitic pegmatites can represent superb deposits of rare metals, including some 'critical metals' that are important for many modern technologies (Linnen et al. 2012, London 2018). Notably, the majority of ore deposits associated with granitic pegmatites are found in LCT pegmatites, which represent many of the world's resources of lithium, tantalum, cesium and beryllium. In contrast, the NYF pegmatites may be enriched in the rare earth elements (REE) and niobium, but they rarely contain economic resources of these metals. Compared with LCT pegmatites, NYF pegmatites have seen relatively little research. This contribution will compare recent work... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Les terres rares (TR) sont un groupe de metaux utilises dans de nombreuses nouvelles technologies. Leur production est limitee, il est donc important de trouver de nouvelles sources d'approvisionnement. Les TR sont divisees en TR legeres et lourdes, ces-dernieres etant plus rares et ayant plus d'applications. Les granites et pegmatites alcalins sont des roches riches en TR lourdes. Une partie de cet enrichissement est d'origine magmatique, mais le role des fluides hydrothermaux (i.e. eau, CO2, methane surtout) pose question. Ce manuscrit se concentre sur l'etude de mineraux communs (amphiboles, pyroxene, zircon) et des inclusions fluides de six zones du monde pour comprendre l'impact des fluides hydrothermaux par rapport a celui des processus magmatiques dans les granites et pegmatites alcalins. Les resultats montrent que la concentration et le fractionnement des TR sont lies a la circulation de fluides hydrothermaux riches en Na et K ou Ca et Na a des temperatures inferieures a 400 °C.
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