Julien Mercadier

Boron- and magnesium-rich marine brines at the origin of giant unconformity-related uranium deposits: δ11B evidence from Mg-tourmalines

In giant unconformity-related U deposits from the Athabasca Basin (Canada), the origin of the ore fluids and the source of elements typically associated with U ore, such as Mg and B, are highly controversial. This paper presents a petrographic and boron isotopic study of Mg-tourmalines (dravites) coeval with U oxides and of primary tourmalines from basement rocks. The heavy B isotopic compositions of dravites (δ 11 B = 19.6‰–36.5‰) contrast with the light composition of basement tourmalines (δ 11 B = −8.1‰–3.3‰) and clearly show that most of the B was brought to the ore system by basinal brines of marine origin. Because the latter are also typically Mg-rich, a similar origin for Mg is proposed. These results support the idea that marine-derived brines were the only fluids involved in the formation of the deposits, and therefore challenge the previous models in which the B and Mg were leached from sedimentary and/or basement rocks, or carried by a basement-derived fluid.

Capabilities of sequential and quasi-simultaneous LA-ICPMS for the multi-element analysis of small quantity of liquids (pl to nl): insights from fluid inclusion analysis

Three configurations of inductively coupled plasma mass spectrometers (ICPMS), namely: a quadrupole (QMS) and a sector-field (SFMS), both equipped with a standard cylindrical ablation cell, and an orthogonal time-of-flight (TOFMS), equipped with a fast washout ablation cell, were coupled with the same 193 nm Excimer laser ablation system in order to evaluate their capabilities for measurement of multiple minor and trace elements in small quantities of liquids (pl to nl), such as fluid inclusions. Analyses were performed with different objects: (i) multi-element solutions sealed in silica capillaries of internal diameter of 20 μm serving as synthetic analogues of natural fluid inclusions; (ii) natural two-phase (liquid + vapour) fluid inclusions with low salinity (ca. 4.8 wt% NaCl eq.) and homogeneous compositions, trapped in quartz crystals from the Alps; (iii) natural multi-phase (liquid + vapour + multiple solids) fluid inclusions with high salinity (ca. 13–15 wt% NaCl eq.) and homogeneous compositions, trapped in quartz crystals from the Zambian Copperbelt. This study demonstrates that the SFMS and TOFMS provide improvements, particularly in term of limits of detection (LODs) and precision, compared to the QMS traditionally used for the measurement of fluid inclusions. SFMS leads on average to lower LODs within one order of magnitude compared to QMS and TOFMS, but precision and accuracy are lower due to longer acquisition cycle times. TOFMS presents both advantages of having rapid and quasi-simultaneous acquisition for all isotopes from 6Li to 238U in a very short cycle time down to 30 μs, with higher precisions and lower LODs than for QMS for isotopes with m/Q > 11. Its use, coupled to a fast washout cell, leads to (i) the improvement in the analysis of small-size (<10 μm) and multi-phase fluid inclusions and (ii) detection of higher number of isotopes compared to QMS and SFMS, which are both limited by the number of measured isotopes from short transient signals of fluid inclusions. Consequently, the tested TOFMS, coupled with a fast washout ablation cell, appears to be a promising instrument for the analysis of natural fluid inclusions by LA-ICPMS, especially for small, multi-phase and/or low salinity fluid inclusions.

The ore-forming magmatic-hydrothermal system of the Piaotang W-Sn deposit (Jiangxi, China) as seen from Li-mica geochemistry

Abstract Many studies have proved the usefulness of Li-mica and chlorite geochemistry as indicators of the chemical and thermal evolution of magmatic systems. This study highlights the suitability of Li-micas as tracers of hydrothermal mineralizing events in world-class W-Sn deposits associated with Jurassic (190–150 Ma) granites in China through the complex magmatic–hydrothermal evolution of the Piaotang deposit (South Jiangxi). A paragenetic sequence has been established for the Piaotang deposit comprising (1) a first “silicate-oxide” stage that hosts abundant W-Sn mineralization (wolframite and cassiterite), (2) a “calcic” stage with scheelite and wolframite, (3) a “base metal sulfides” stage with cassiterite and wolframite, and (4) a late “sulfide” stage, involving for the first time a polyphase emplacement of the mineralization. Li-micas from the underlying granite, greisen, and the different stages represented in the veins, were studied. The chemistry of the micas (characterized by intermediate compositions between phlogopite-zinnwaldite-muscovite poles) demonstrates the presence of end-members representing three different fluids that were involved in the emplacement of the Piaotang deposit. These end-members can be linked to previous fluid inclusion studies conducted on this deposit. The three fluids are identified to be magmatic, meteoric (as previously reported in the literature), and also metamorphic, and are shown to have mixed throughout the different stages. Moreover, it appears that the magmatic fluids could not have been derived from the Piaotang biotite granite but instead must have originated from a more evolved rare metal granite that is presently unidentified. These fluids were responsible for the greisenization. Finally, chlorite geochemistry reveals the occurrence of a heating process (from 200 °C in stage II to 300 °C in stage III) during the post-mineralizing stages, which was responsible for the precipitation of new generations of ore-bearing minerals (cassiterite and wolframite) concomitant with a continuous gain of metals during the emplacement of the Piaotang deposit.

Geochemical Signature of Magmatic-Hydrothermal Fluids Exsolved from the Beauvoir Rare-Metal Granite (Massif Central, France): Insights from LA-ICPMS Analysis of Primary Fluid Inclusions

The Beauvoir granite (Massif Central, France) represents an exceptional case in the European Variscan belt of a peraluminous rare-metal granite crosscutting an early W stockwork. The latter was strongly overprinted by rare-metal magmatic-hydrothermal fluids derived from the Beauvoir granite, resulting in a massive topazification of the quartz-ferberite vein system. This work presents a complete study of primary fluid inclusions hosted in quartz and topaz from the Beauvoir granite and the metasomatized stockwork, in order to characterize the geochemical composition of the magmatic fluids exsolved during the crystallization of this evolved rare-metal peraluminous granite. Microthermometric and Raman spectrometry data show that the earliest fluid (L1) is of high temperature (500 to >600°C), high salinity (17–28 wt.% NaCl eq), and Li-rich ( 100 m) and interaction with external fluids.

A new style of rare metal granite with Nb-rich mica: The Early Cretaceous Huangshan rare-metal granite suite, northeast Jiangxi Province, southeast China

Abstract In rare-metal granites, niobium and tantalum are generally hosted by Nb–Ta oxides. However, in SE China, the Nb-specialized Huangshan granites are a unique occurrence in which Nb is essentially hosted by Li–Fe micas. The Huangshan granites are part of the Early Cretaceous (Late Yanshanian) Lingshan granite complex and belong to the A-type granite series, with two facies differing by their mica compositions: medium-grained “protolithionite” granite and medium-grained lithian (lithium-rich) annite granite. The granites are characterized by elevated whole-rock Nb contents (average 144 ppm in “protolithionite” granite and 158 ppm in annite granite), quite low Ta contents (average 9 and 4 ppm, respectively), leading to very high Nb/Ta ratios (average 15.3 and 31.2). Niobium is mainly hosted in the micas, with an average Nb content of 1347 ppm in the lithian annite and 884 ppm in the “protolithionite,” which is the highest ever reported in granitic mica. With an estimated endowment of ~80 kt Nb, the Huangshan granites represent a new style of potential Nb resource. Contrasting with the great rarity of columbite, there is abundant Hf-rich zircon, Y-rich fluorite, and Th-rich fluocerite included in the Huangshan micas. Such accessory minerals being typical of alkaline rhyolitic magmas and niobium enrichment in the Huangshan granites results from A-type melt. The extreme Nb enrichment in the micas results from the highly compatible behavior of Nb in this melt, combined with the high magma temperature (estimated at 790–800 °C) and possibly enhanced magma oxidation.

Chemical Composition and Age of Uraninite of the Zhovta Richka Uranium Deposit (Ukraine)

Two uraninite generations were recognized both from different chemical composition and chemical and U–Pb isotopic dating in the Zhovta Richka U deposit. Earlier uraninite occurs as disseminated in metasomatically altered iron quartzite. It is rich in Y (0.23–0.32) and Ca (0.67–1.21 wt%) and chemically dated from 1976 +41/–9.1 to 2019 +37/–48 Ma from time-related incorporation of Si (0.04–0.21), Fe (0.01–0.30) and Mn (0.00–0.19 wt%). U–Pb discordia age of 1751.2 ± 2.8 Ma was obtained for later uraninite generation. It occurs as veinlets and disseminated, and was doped by Zr (0.75–2.40), V (0.01–0.43) and Ti (0.00–0.60 wt%) during crystallization, but also contains Ca (1.05–3.20), Si (0.18–0.68), Fe (0.00–0.27) and Mn (0.01–0.48 wt%) which were dominantly incorporated into uraninite along with Pb loss. Diverse ages of uraninite in the deposit are well concordant with two episodes of regional scale granitization which involved the Ukrainian Shield for Paleoproterozoic.