Search Results (8)

Near the Segura pluton, hyper-differentiated magmas enriched in F, P, and Li migrated through shallowly dipping fractures, which were sub-perpendicular to the schistosity of the host Neoproterozoic to Lower Cambrian metasedimentary series, to form two swarms of low-plunging aplite–pegmatite dykes. The high enrichment factors for the fluxing elements (F, P, and Li) compared with peraluminous granites are of the order of 1.5 to 5 and are a consequence of the extraction of low-viscosity magma from the crystallising melt. With magmatic differentiation, increased P and Li activity yielded the crystallisation of the primary amblygonite–montebrasite series and Fe-Mn phosphates. The high activity of sodium during the formation of the albite–topaz assemblage in pegmatites led to the replacement of the primary phosphates by lacroixite. The influx of external, post-magmatic, and Ca-Sr-rich hydrothermal fluids replaced the initial Li-Na phosphates with phosphates of the goyazite–crandallite series and was followed by apatite formation. Dyke emplacement in metasediments took place nearby the main injection site of the muscovite granite, which plausibly occurred during a late major compression event. Full article

This paper reports the results from an investigation on the geochemistry and petrogenesis of the Aptian Li-F granites from the Omchikandya, Burgali, and Arga Ynnakh Khaya ore fields in the northern Verkhoyansk–Kolyma orogenic belt in eastern Russia. Li-F microcline–albite granites intrude the Late Jurassic to Early Cretaceous syn-collisional granitoids. According to their geochemical composition, they are close to A-type granites and can be subdivided into low-P and high-P varieties, differing in their geochemistry and genesis. The low-P microcline–albite granites (Omchikandya massif) intrude syn-collisional biotite granites. It is assumed that the formation of their parent melt occurred at deep levels in the same magma chamber that produced biotite granites. The high-P granites (Verkhne–Burgali ethmolith and Kester harpolith) are supposed to have been derived from melts originated from a high-grade metamorphosed lower crustal protolith under the influence of deep-seated fluid flows related to diapirs of alkaline-ultrabasic or alkaline-basic composition. It is supposed that their formation was related to post-collisional extension during the early stages of the evolution of the Aptian–Late Cretaceous Indigirka belt of crust extension. All studied Li-F granites are enriched with rare metals and have associated Li deposits with accompanying Sn, W, Ta, and Nb mineralization. In the low-P Li-F Omchikandya massif, mineralization tends to occur within greisenized granites and greisens in their apical parts. In the high-P granite massifs, mineralization is found throughout their volume, and, therefore, the Verkhne–Burgali ethmolith and Kester harpolith can be considered as large ore bodies. There is a direct dependence of the content and reserves of Li₂O on the content of P₂O₅. Minimum Li₂O reserves are established in low-P Li-F microcline–albite granites of the Polyarnoe deposit of the Omchikandya ore field, whereas in the high-P granites of the Verkhne–Burgali and Kester deposits, the Li₂O reserves are significantly higher. Full article

A dataset of >1190 published compositional analyses of muscovite from granitic pegmatites of varying mineralogical types was compiled to reevaluate the usefulness of K-Rb-Li systematics of muscovite as a tool for distinguishing mineralogically simple pegmatites from pegmatites with potential Li mineralization. Muscovite from (i) common, (ii) (Be-Nb-Ta-P)-enriched, (iii) Li-enriched, and (iv) REE- to F-enriched pegmatites contain Li contents that vary between 10 and 20,000 ppm depending on the degree of pegmatite fractionation. Common pegmatites are characterized by low degrees of fractionation as exhibited by K/Rb ratios ranging from 618 and 25 and Li contents generally being <200 ppm but infrequently as high as 743 ppm in muscovite. Moderately fractionated pegmatites with Be, Nb, Ta, and P enrichment contain muscovite having K/Rb ratios mostly between 45 and 7 plus Li contents between 5 to >1700 ppm. Muscovite from moderately to highly fractionated Li-rich pegmatites exhibit a wide range of K/Rb ratios and Li values: (i) K/Rb = 84 to 1.4 and Li = 35 to >18,100 ppm for spodumene pegmatites, (ii) K/Rb = 139 to 2 and Li = 139 to >18,500 ppm for petalite pegmatites, and (iii) K/Rb = 55 to 1.5 and Li = 743 to >17,800 ppm for lepidolite pegmatites. Pegmatites that host substantial REE- and F-rich minerals may carry muscovite with K/Rb ratios between 691 to 4 that has Li contents between 19 to 15,690 ppm. The K/Rb-Li behavior of muscovite can be useful in assessing the potential for Li mineralization in certain granitic pegmatite types. The proposed limits of K/Rb values and Li concentrations for identifying spodumene- or petalite-bearing pegmatites as part of an exploration program is reliable for Group 1 (LCT) pegmatite populations derived from S-type parental granites or anatectic melting of peraluminous metasedimentary rocks. However, it is not recommended for application to Group 2 (NYF) pegmatites affiliated with anorogenic to post-orogenic granitoids with A-type geochemical signatures or that derived by the anatexis of mafic rocks that generated REE- and F-rich melts. Full article

Apatite can be used as an archive of processes occurring during the evolution of granitic magmas and as a pegmatite exploration tool. With this aim, a detailed compositional study of apatite was performed on different Variscan granites, pegmatites and quartz veins from the Central Iberian Zone. Manganese in granitic apatite increases with increasing evolution degree. Such Mn increase would not be related to changes in the fO₂ during evolution but rather to a higher proportion of Mn in residual melts, joined to an increase in SiO₂ content and peraluminosity. In the case of pegmatitic apatite, the fO₂ and the polymerization degree of the melts seem not to have influenced the Mn and Fe contents but the higher availability of these transition elements and/or the lack of minerals competing for them. The subrounded Fe-Mn phosphate nodules, where apatite often occurs in P-rich pegmatites and P-rich quartz dykes, probably crystallized from a P-rich melt exsolved from the pegmatitic melt and where Fe, Mn and Cl would partition. The low Mn and Fe contents in the apatite from the quartz veins may be attributed either to the low availability of these elements in the late hydrothermal fluids derived from the granitic and pegmatitic melts, or to a high fO₂. The Rare Earth Elements, Sr and Y are the main trace elements of the studied apatites. The REE contents of apatite decrease with the evolution of their hosting rocks. The REE patterns show in general strong tetrad effects that are probably not related to the fluids’ activity in the system. On the contrary, the fluids likely drive the non-CHARAC behavior of apatite from the most evolved granitic and pegmatitic units. Low fO₂ conditions seem to be related to strong Eu anomalies observed for most of the apatites associated with different granitic units, barren and P-rich pegmatites. The positive Eu anomalies in some apatites from leucogranites and Li-rich pegmatites could reflect their early character, prior to the crystallization of feldspars. The increase in the Sr content in apatite from Li-rich pegmatites and B-P±F-rich leucogranites could be related to problems in accommodating this element in the albite structure, favoring its incorporation into apatite. The triangular plots ΣREE-Sr-Y and U–Th–Pb of apatites, as well as the Eu anomaly versus the TE_1,3 diagram, seem to be potentially good as petrogenetic indicators, mainly for pegmatites and, to a lesser extent, for granites from the CIZ. Full article

Highly fractionated granites and related magmatic-hydrothermal ore-forming processes can be traced by elemental ratios such as Nb/Ta, K/Rb, Y/Ho, Sr/Eu, Eu/Eu*, Zr/Hf, and Rb/Sr. The lanthanide “tetrad effect” parameter (TE_1,3) can also be a useful geochemical fingerprint of highly fractionated granites. This work assesses its application as an exploration vector for granite-related mineralization in the Central Iberian Zone by examining TE_1,3 variations with different elemental ratios and with the concentrations of rare metals and fluxing elements (such as F, P, and B). The multi-elemental whole-rock characterization of the main Cambrian–Ordovician and Carboniferous–Permian granite plutons and late aplite–pegmatite dykes exposed across the Segura–Panasqueira Sn-W-Li belt show that the increase in TE_1,3 values co-vary with magmatic differentiation and metal-enrichment, being the Carboniferous–Permian granite rocks the most differentiated, and metal specialized. The Argemela Li-Sn-bearing rare metal granite and the Segura Li-phosphate-bearing aplite–pegmatite dykes deviate from this geochemical trend, displaying TE_1,3 < 1.1, but high P₂O₅ contents. The results suggest that mineralized rocks related to peraluminous-high-phosphorus Li-Sn granite systems are typified by TE_1,3 < 1.1, whereas those associated with peraluminous-high-phosphorus Sn-W-Li (lepidolite) and peraluminous-low-phosphorus Sn-Ta-Nb granite systems display TE_1,3 > 1.1, reaching values as high as 1.4 and 2.1, respectively. Full article

The article discusses the composition of studied ore-forming solutions and the P-T conditions of molybdenum mineralization in the Pervomaisky stockwork deposit which is situated within the Dzhidinsky ore field (South-Western Transbaikalia, Russia). New geochronological data of zircons from granites, muscovite, and molybdenite from the ore zones indicates the association of the granite formation and ore deposition processes which occurred 119–128 million years ago. Quartz-molybdenite veins of the Pervomaisky deposit were formed at the temperature of ≥314–186 °C with some boiling periods. Fluid inclusions in these veins have total salt concentration of 6.3–12.7 wt. % NaCl equivalent (eq. NaCl). The salt solution is composed of chlorides of Na, Ca, K, and Fe. The gas phase contains CO₂, CH₄, and N₂. A series of elements were determined in fluid inclusions by laser ablation (LA)-ICP-MS: Li, Be, B, F, Na, Mg, Al, Cl, K, Ca, Mn, Fe, Cu, Zn, Nb, Mo, Ag, Sn, La, Ce, Ta, W, Au, Pb, Th, U. The Mo content reaches 559 ppm (average of 228 ± 190 ppm) in high-grade quartz-molybdenite veinlets, whereas Mo content is up to 212 ppm (average of 25 ± 29 ppm) in the low-grade veinlets. High-grade veinlets were formed by near-neutral solutions with a higher content of Mo, S, and F, while relatively low-grade veinlets were deposited from alkaline solutions. Our results demonstrate the pH of the solutions as one of the key factors for ore deposition. Full article

Oxide minerals (Nb–Ta-rich rutile, columbite-group minerals and W-bearing ixiolite) represent the most common host for Nb, Ta and Ti in high-F, high-P₂O₅ Li-mica granites and related rocks from the Geyersberg granite stock in the Krušné Hory/Erzgebirge Mts. batholith. This body forms a pipe like granite stock composed of fine- to middle-grained, porphyritic to equigranular high-F, high-P₂O₅ Li-mica granites, which contain up to 6 vol. % of topaz. Intrusive breccia’s on the NW margin of the granite stock are composed of mica schists and muscovite gneiss fragments enclosed in fine-grained aplitic and also topaz- and Li-mica-bearing granite. Columbite group minerals occur usually as euhedral to subhedral grains that display irregular or patched zoning. These minerals are represented by columbite-(Fe) with Mn/(Mn + Fe) ratio ranging from 0.07 to 0.15. The rare Fe-rich W-bearing ixiolite occurs as small needle-like crystals. The ixiolite is Fe-rich with relatively low Mn/(Mn + Fe) and Ta/(Ta + Nb) values (0.10–0.15 and 0.06–0.20, respectively). Owing to the high W content (19.8–34.9 wt. % WO₃, 0.11–0.20 apfu), the sum of Nb + Ta in the ixiolite does not exceed 0.43 apfu. The Ti content is 1.7–5.7 wt. % TiO₂ and Sn content is relatively low (0.3–4.1 wt. % SnO₂). Full article

The Krudum granite body comprises highly fractionated granitic rocks ranging from medium-F biotite granites to high-F, high-P₂O₅ Li-mica granites. This unique assemblage is an ideal site to continue recent efforts in petrology to characterize the role of zircon, monazite, and xenotime as hosts to rare earth elements (REEs). The granitic rocks of the Krudum body analyzed in this study were found to contain variable concentrations of monazite and zircon, while xenotime was only found in the high-F, high-P₂O₅ Li-mica granites and in the alkali-feldspar syenites of the Vysoký Kámen stock. Intermediate trends between cheralite and huttonite substitutions are characteristic for analyzed monazite grains from all magmatic suites. The highest concentration of cheralite was found in monazite from the alkali-feldspar syenites (up to 69.3 mol %). The proportion of YPO₄ in analyzed xenotime grains ranges from 71 to 84 mol %. Xenotime grains are commonly enriched in heavy rare earth elements (HREEs; 9.3–19.5 wt % HREE₂O₃) and thorite-coffinite and cheralite exchange was observed. Some xenotime analyses return low totals, suggesting their hydration during post-magmatic alterations. Analyzed zircon from granite suites of the Krudum granite body contains moderate Hf concentrations (1.0–4.7 wt % HfO₂; 0.010–0.047 apfu Hf). The highest concentrations of HfO₂ were found in zircon from the high-F, high-P₂O₅ Li-mica granites (1.2–4.7 wt % HfO₂). Analyzed zircon grains from the high-F, high-P₂O₅ Li-mica granites and alkali-feldspar syenites are enriched in P (up to 8.29 wt % P₂O₅; 0.24 apfu P), Al (0.02–2.0 wt % Al₂O₃; 0.00–0.08 apfu Al), Ca (up to 3.9 wt % CaO; 0.14 apfu Ca), Y (up to 5.5 wt % Y₂O₃; 0.10 apfu Y), and Sc (up to 1.17 wt % Sc₂O₃; 0.03 apfu Sc). Zircon grains from the high-F, high-P₂O₅ Li-mica granites were sometimes hydrated and fluorized. The concentrations of F in zircon from partly greisenised high-F, high-P₂O₅ Li-mica granites reached up to 1.2 wt % (0.26 apfu F). Full article