Developing innovative methodologies has become essential for mining companies to optimize resource management and achieve economic sustainability. The spectroscopic techniques such as laser-induced breakdown spectroscopy (LIBS), used for on-site analyses, have great potential to provide reliable qualitative and quantitative geochemical data to support rapid decision-making. In this context, handheld LIBS was used to predict the concentrations of the main targeted elements lithium (Li), and rubidium (Rb), on unprepared core samples from the Beauvoir granite, a rare-metal granite currently studied to achieve lithium exploitation by 2028. The models were built using different spectral parameters and statistical approaches. A comprehensive comparative analysis was conducted to determine the optimal protocol for each element. Signal variation over successive measurements, as well as the minimum number of analyses required to maintain a representative LIBS signal of the whole rock composition, were investigated. The predicted concentrations are consistent with the reference geochemical analyses confirming the ability of handheld LIBS to provide relevant analytical results, in a short period, directly at the exploitation site.

Peraluminous rare-metal granites (PRMGs) represent highly differentiated crustal granites characterised by extreme enrichment in metals, such as Li, Sn, Nb, Ta, W, and Be. This geochemical specificity is considered to be the result of a succession of magmatic and hydrothermal processes, the importance and individual impact of which are still debated. In this study, we investigate the magmatic and hydrothermal evolution of the Beauvoir leucogranite, a world-class PRMG from the French Paleozoic Variscan belt, through extensive characterisation of suprasolidus to subsolidus apatite. We employ a multi-tracing approach, combining in-situ elemental composi- tions (major, trace, and halogen elements), oxygen isotopic systematics, and U-Pb geochronology. Four major magmatic and hydrothermal stages were identified through apatite petrography and U-Pb geochronology. Magmatic apatite crystallised at 314.6 f 4.7 (2s) Ma. Decrease in the amplitude of the Eu anomaly in magmatic apatite from the deeper to shallower granitic units record an increase in the oxygen fugacity (fO2) of the magma with differentiation, likely contributing to the crystallisation of a first and predominant cassiterite gener- ation. Magmatic apatite REE patterns show significant tetrad effects; they reflect the exsolution of magmatic fluids involved in the precipitation of early hydrothermal apatite replacing igneous minerals or precipitating within veins during greisenisation episode dated at 314.3 f 5.5 Ma and 311.7 f 8.1 Ma. Early hydrothermal apatite, charac- terised by enrichment in Sr or S and Mn-REE depletion along with variable Br/I ratios and delta 18 O compositions down to negative values, record mixing dynamics between two fluid end-members: (i) magmatic fluids, and (ii) oxidising meteoric fluids partially reequilibrated with country rocks. Meteoric fluids progressively invaded the Beauvoir PRMG at temperatures >= 450 degrees C and triggered precipitation of a second generation of cassiterite during mixing with magmatic fluids. Two late, non-magmatic, hydrothermal events are dated at 268.3 f 20.4 Ma and 148.5 f 26.6 Ma. Related apatite is marked by specific mineralogical and geochemical features such as high As contents and heavy oxygen isotope signatures. They are proposed to be linked to extension-related regional hydrothermal fluid cir- culation that contributed to metal endowment (U, F-Ba-Pb-Zn) in the crystalline basement and overlying sedi- mentary cover of Western Europe following the Variscan Orogeny. Our results demonstrate that apatite is a key mineral to decipher the role of magmatic and hydrothermal processes leading to ore deposition and remobilisation in PRMGs, in relation with geodynamic evolution. Apatite records of external fluid incursions at near-solidus conditions highlight the open system nature of the Beauvoir PRMG, which is crucial for developing fully integrated metallogenic models applicable to similar deposits.

Magmatic and hydrothermal apatite generations of the Beauvoir rare-metal (Li-Ta-Nb-Sn-Be) granite (Massif Central, France) were investigated through textural observations, in-situ major elements (EPMA) and oxygen isotopic (SIMS) compositions in order to trace the origin(s) of fluids having affected the intrusion as well as their geochemical evolution. Magmatic apatite shows relatively low delta O-18 values (similar to 4-8 %) corresponding to delta O-18(melt) values of similar to 5-10 % at 550 degrees C, i.e., lower than expected for typical peraluminous granitic melts. This could be explained either by percolation into the magma of meteoric water, or by isotopic re-equilibration of magmatic apatite during subsolidus interaction with external fluids at similar to 450-400 degrees C. Furthermore, cathodoluminescence imaging reveals complex textures for hydrothermal apatite highlighting their partial recrystallisation and chemical modification with the evolution of fluid chemistry. The wide range of delta O-18 values (-9 to +13%) and Mn contents of hydrothermal apatite suggest a progressive mixing of reduced magmatic and oxidized meteoric fluids during metasomatism. Moreover, an interaction of greisenrelated fluids with micaschist country-rocks is evidenced by relatively high delta O-18 values and S concentration in one apatite generation. Our results support protracted oxidized meteoric fluid incursions in the Beauvoir granite, possibly from supra- to sub-solidus conditions, which may have significantly affected metal deposition processes.

In this contribution, we present some of the first data on the elemental signature of deep crustal fluids in a basalt-hosted, low-chloride magmatic-hydrothermal system. Down-hole fluid samples (850–1600 m) from wells in the Theistareykir and Krafla geothermal fields in the Northern Volcanic Zone of Iceland were combined with well-head samples of condensed vapor, cuttings of altered rock, and fresh basalt (being some of the first concentration data for volatile and semi-volatile elements (Sb, Tl, Bi, Cd and As) for this area of Iceland). Results show that the deep fluids are relatively enriched in base metals and (semi)-volatile metals (in particular Te, Hg, Re and Tl) compared to local basalt. We interpret this enrichment in volatile metals to reflect a significant element input from magma degassing. Boiling of this deep fluid results in a well-head fluid composition that is significantly depleted in most elements. This well-head fluid has a distinct elemental signature, including a depletion in Sb that is mirrored in the altered rocks, and a depletion in the base metals that shows their selective sequestration in scale minerals, likely sulphides. As expected, the element content and patterns in surface fluids can thus not be interpreted to directly reflect that of the deep reservoir fluid. The behaviour of elements in Theistareykir and Krafla fluids is consistent, and largely agrees with similar data obtained for the Reykjanes geothermal system in SW Iceland. We therefore posit that our results are representative for this geological setting and indicate a significant magmatic degassing cation input to deep fluids, variably modified by water–rock interaction.

The source of metals in magmatic-hydrothermal systems is a matter of debate with opinions divided between those who favor magma degassing as the primary source, and those who see a principal role for water-rock interaction and metal leaching. Noble gases have the potential to provide key insights in this debate, because the elemental and isotopic signatures of the noble gases strongly differentiate these sources. Here, we present results of a study where we use correlations between noble gases and metals in deep geothermal fluids sampled down-well to elucidate metal sourcing. Samples are from the newly developed Theistareykir geothermal field of NE Iceland, as well as the adjacent Krafla system. Noble gas samples were collected at the well heads and analyzed for He, Ne, Ar, Kr and Xe concentrations, and their isotopes. Fluid samples were collected at the well heads, and in two wells at depths down to 1600 m using a down-hole fluid sampler. The noble gases indicate significant variations in fluid sources, including variations in magmatic gas input, over the geothermal field that correlate with selected metals. Thallium, in particular shows a strong correlation with 3 He. In comparison with their basaltic host rocks the deep fluids are enriched in alkali (Li, Na, K) and volatile metals (Ag, Cd, Sb, Tl, Pb, As, Hg) and are depleted in refractory (Ti, V, Cr) and rare earth elements. We therefore preliminarily

Chlorine (Cl) and bromine (Br) are rare elements when considering the whole Earth. However, being highly volatile elements, their delivery and retention processes during planetary accretion and chemical differentiation provide important clues about the formation of the Earth. Variations in Cl and Br isotopic systems (Cl-37/Cl-35 or delta Cl-37 and Br-81/Br-79 or delta Br-81) among terrestrial reservoirs could trace these processes. While the Br isotopic value of the mantle remains entirely unknown, the measurement of mantle Cl isotopic values is a controversial subject, with measured delta Cl-37 values ranging from -3 to 0 parts per thousand in midocean-ridge basalts (MORBs) and from -2 to +3 parts per thousand in oceanic island basalts (OIBs). Here, we report newly-determined delta Br-81 and delta Cl-37 values, together with noble gas He-3/He-4 (R) ratios, measured in geothermal fluids from production wells of three Mexican fields: Cerro Prieto, Las Tres Virgenes, and Los Azufres. Relationships between He-3/He-4 ratios and both delta Cl-37 and delta Br-81 suggest that geothermal fluid volatiles have three distinct sources: (1) a local crustal source, enriched in radiogenic He-4 (R = 1.7-1.9Ra, where Ra is the atmospheric He-3/He-4 ratio), and halogens from brines with delta Cl-37 and delta Br-81 of +0.1 and +0.3 parts per thousand respectively; (2) the mantle wedge, with He-3/He-4 ratios of 6-6.5Ra, typical of arc volcanism, and delta Cl-37 and delta Br-81 of -0.4 and -1.0 parts per thousand respectively, typical (for Cl) of fluids derived from the dehydration of serpentinite in the subducting slab; and (3) a mantle source, with He-3/He-4 ratios of 7.7-8.2Ra, typical of MORBs, and delta Cl-37 and delta Br-81 of +0.9 and +0.7 parts per thousand respectively. These results suggest that the primitive mantle Cl isotopic composition was positive - possibly >=+3 parts per thousand, as measured in some OIBs - and inherited during the Moon forming impact. The progressive subduction of isotopically lighter halogens over the last 2-3 Ga could have progressively lowered this initial value to those currently measured in the depleted mantle beneath Mexico. It is speculated that the different isotopic values measured in mantle rocks and fluids could reflect the heterogeneous regassing of subducted halogens and the inefficient homogenization of recycled material in the MORB source, as suggested in other studies by the heterogeneous isotopic compositions of the heavier Ar and Xe of the convective mantle. (C) 2020 Elsevier Ltd. All rights reserved.

Successful management of geothermal energy requires detailed understanding of physical and chemical condidons within the field prior to exploitation. It is thus crucial to identify fluids involved and their residence times, as well as the heat source, so as to assess the potential of the resource in terms of energy production. To this end, a geochemical study of relatively undisturbed fluids from the newly-developed Theistareykir geothermal field, Northern Volcanic Zone, Iceland was carried out on production wells, fumaroles, and mud pots. Noble gas (He, Ne, Ar, Kr, and Xe) elemental and isotopic abundances and stable isotopes delta O-18 and delta H-2 were measured to determine the system fluid sources and dynamics as exploitation proceeds. Results of this study, together with previously published data, show that four fluid sources are present: modem and local meteoric water (48.9%); sub-modem meteoric water from regional highlands precipitation (10.6%); pre-Holocene glaciated meteoric water (40.4%) with strongly depleted delta H-2 values of -127%., calculated K-40-Ar-40* fluid residence times from 57 +/- 20 ka to 92 +/- 30 ka and a (U/Th)-He-4 fluid residence times from 96 +/- 50 ka to 160 +/- 80 ka; and, finally, He-3-rich magmatic fluids. Concomitant enrichment in O-18 and radiogenic He-4 suggests that some fluids reside a long time in the reservoir, exchanging O and He with reservoir rocks. Maximum estimated helium isotopic ratios, He-3/He-4 (R), of 11.45 Ra (Ra = atmospheric ratio) show that the magma beneath Theistareykir is a depleted mid-ocean ridge basalt (MORB) mantle (DMM), with less influence (8.7 to 12.7%) of the Icelandic mantle plume source. Calculated heat (Q)/He-3 ratios plotted vs. R/Ra and He-4/Ar-36 ratios suggest that convective heat transport dominates the eastern part of the field where the magmatic heat source is located, while in other parts of the field, heat conduction seems to be dominant. Boiling and phase separation exists in the field, as indicated by delta O-18 values which fall to the left of the Global Meteoric Water Line in a delta O-18 vs. delta H-2 plot, but Q/He-3 ratios indicate that boiling affects only 1-10% of the fluid reservoir. With this obtained knowledge, any subsequent changes in the field conditions during the exploitation phase of Theistareykir can be better understood, helping to sustainably manage the resource. (C) 2020 Elsevier B.V. All rights reserved.