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Despite their well-known economic potential, the genesis of deposits associated with magmatic-hydrothermal fluids derived from granitic plutons is still not fully understood. The Cornubian batholith (SW England) represents an example of those: it was intensely mined for Sn and Cu, but also hosts W mineralizations and Li-rich granitic rocks. This study explores the use of halogens as tracers for the formation of such critical element enrichments, across the transition from magmatic to hydrothermal conditions in this well-studied natural laboratory of a Variscan S-type system. Given the high incompatibility of heavy halogens (Cl, Br, I) in most minerals, they are reliable indicators of fluid provenance and evolution and readily discern geofluid reservoirs, but questions remain as to halogen behaviour during crucial ore-forming processes such as fluid exsolution or phase separation. In situ LA-ICP-MS analysis of fluid inclusions and whole-rock combustion ion-chromatography will provide a detailed assessment of halogen behaviour across the magmatic-hydrothermal transition in samples ranging from granites and pegmatites, to greisens and mineralized Sn-W-Cu hydrothermal veins. Preliminary results show a progressive dilution of fluids with magmatic Br/Cl - I/Cl signatures and elevated concentrations of Cu, Sn and W as the main factor controlling mineralization.

This study presents the petrogenesis of intrusive rocks of the Iivaara alkaline complex (Finland) and associated enrichment of elements, which are mostly carried by titanite and apatite. Comprehensive mineralogical and textural observations were made and mineral chemical analyses were performed on apatite, titanite, clinopyroxene, and feldspathoid group minerals using EPMA and LA-ICP-MS. Intrusive rocks exposed in the Iivaara alkaline complex are mainly composed of ijolite group rocks, nepheline syenite, and pyroxenites. They generally show low mineralogical variability but large textural variety. Several textures indicate magmatic formation conditions, including special textures of mingling, schlieren, and Unidirectional Solidification Texture (UST). Some others show fluid-derived formation conditions like replacement texture of primary minerals, breccia as well as veins. The marked textural heterogeneity present at the margin of the magmatic intrusion and the innermost zone of the fenite aureole, reveals that the current erosion level of the Iivaara exposes the roof zone of an alkaline silicate intrusion. It is characterized by fast cooling, steep temperature gradients, association of small magma batches crystallizing at different conditions, and by expulsion of fluids. The high abundance of secondary feldspathoid group minerals, apatite, and titanite attests to the significant enrichment of halogens (F, Cl), CO2, SO4, P, and Ti in these rocks crystallized from melts or fluids, which in part have agpaitic character. Mineral chemical data reflect two main stages of fluid expulsion, a syn-magmatic Ca-dominated fluid causing marked fenitization of the wall rocks and a syn- to post- magmatic fluid forming feldspathoid minerals.

Heavy halogens are exceptional tracers of fluid provenance due to their largely conservative behaviour during most fluid-rock interaction processes. In particular, Br/Cl ratios have long since been established as a tracer of fluid origin in certain geological settings (e.g. unconformity-related ore systems), where they have significantly contributed to our understanding of ore forming processes. However, mounting evidence suggests that similar Br/Cl signatures can, in fact, be produced by very different geological processes, significantly impacting the applicability of Br/Cl as a fluid source tracer to other crustal environments. Iodine provides an additional proxy and constraint that makes it possible to discern between these different processes and is thus crucial in utilizing the full potential of halogens as fluid provenance tracers. The generally low concentrations of iodine in crustal fluids have posed significant analytical challenges, and iodine analysis was until very recently only possible using bulk analytical techniques. Recent advances in halogen fluid inclusion microanalysis have made it possible to determine Cl-Br-I systematics in individual fluid inclusions, providing novel opportunities for the study of complex ore forming systems with protracted and complex overprinting fluid signatures. We present a comprehensive triple-halogen fluid inclusion LA-ICP-MS dataset of various magmatic and metamorphic systems, showcasing the power of Cl-Br-I systematics in identifying and characterizing fluid sources, reconstructing processes such as fluid mixing, and the – rather surprising – temporal changes in metamorphic halogen signatures between the Archean and the Phanerozoic.

Post-Variscan hydrothermal vein deposits are widespread in central Europe and are associated with enrichments in Pb-Zn-Cu-(Ag), Mn-Fe or Co-Ni-Bi. Their formation is commonly explained by fluid mixing processes. However, direct evidence for the occurrence of such processes on the scale of individual ore veins is rare, and as a result their efficiency on these scales is still disputed. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS), multi-element analyses of individual fluid inclusions hosted in quartz crystals from a basement-hosted quartz-baryte-Pb-Zn vein mineralization in the Schwarzwald district, SW Germany, were performed. Negative correlations between fluid Cl/Br mass ratios and base metal (Pb and Zn) concentrations were identified in fluid inclusion assemblages (FIA) on subsequent growth zones of the same quartz crystal and even within the same FIA. This indicates transient and variable fluid mixing processes between a metal-depleted halite dissolution brine with high Cl/Br ratio derived from evaporite-bearing Triassic limestones (Muschelkalk) and a metal-rich basement-interacted brine with low Cl/Br ratio. The dataset provides the first direct evidence for fluid mixing processes between similarly saline fluids on the scale of individual gangue minerals. The hydrothermal Mn-Fe-As mineralization at Sailauf in the Spessart district, central Germany, is hosted by a Lower Permian rhyolite body and comprises a complex succession of Mn- and Fe-oxide minerals associated with several generations of carbonates of distinct major and trace element compositions. The paragenetic succession evidences a clear fractionation of Fe and Mn during discrete mineralization stages. The Mn-oxide mineralization stage comprises Mn-oxides and calcite, whereas the hematite stage is characterized by hematite and Mn-calcite. Thermodynamic modeling shows that these mineral associations evidence significant changes in the prevailing physicochemical conditions in terms of pH, fO2, fCO2 and aSiO2. Mixing processes between oxidizing and neutral to slightly acidic basement-interacted fluids with reducing and alkaline formation waters derived from Upper Permian (Zechstein) dolomites overlying the rhyolite body may explain these variations and are further evidenced by systematic changes in rare earth element distribution patterns of the different carbonate generations. LA-ICPMS microanalysis of petrographically well-constrained fluid inclusion assemblages within the carbonates reveals that chemically distinct fluid types can be related to the different mineralization stages. The fluids show systematic variation in their trace element compositions in terms of Li, B, K, Pb and Zn, whereas no differences exist in terms of their major element compositions or salinities. The fluids associated with the hematite stage are compositionally similar to fluids interacted with crystalline basement rocks, whereas the highly oxidizing Mn-oxide stage fluids show similarities to fluids derived from Lower Permian (Rotliegend) red bed sediments. These fluids were tapped during separate stages of hydrothermal activity under changed hydrological flow conditions and were mixed with Zechstein formation waters. Postvariszische, hydrothermale Gangmineralisationen sind in Zentraleuropa weitverbreitet und mit Elementanreicherungen wie Pb-Zn-Cu-(Ag), Mn-Fe und Co-Ni-Bi assoziiert. Ihre Bildung wird in der Regel durch Fluidmischungsprozesse erklärt, für deren Stattfinden auf der Skala einzelner Erzgänge aber meist nur indirekte Nachweise vorliegen bzw. deren Effizienz auf diesem Maßstab generell umstritten ist. Mit Hilfe von Laserablations-Massenspektrometrie (LA-ICPMS) wurden Multielementanalysen an einzelnen Fluideinschlüssen einer Quarz-Baryt-Pb-Zn Gangmineralisation im kristallinen Grundgebirge des Schwarzwalds durchgeführt. Die Fluide zeigen klare negative Korrelationen der Cl/Br Massenverhältnisse mit Konzentrationen von Pb und Zn, sowohl zwischen Gruppen kogenetischer Einschlüsse auf verschiedenen Wachstumszonen desselben Quarzkristalls, als auch innerhalb der Gruppen selbst. Dies belegt sehr kurzlebige und variable Mischungsvorgänge zwischen einem metallarmen Muschelkalk-Formationswasser, das seine hohe Salinität und hohes Cl/Br Verhältnis durch Halitlösung erhielt, mit einem metallreichen, hochsalinaren Grundgebirgsfluid während der Bildung der Mineralisation in Folge seismischer Aktivität. Diese Ergebnisse stellen den ersten direkten Nachweis für das Ablaufen von Fluidmischungsprozessen zwischen zwei hochsalinaren Fluiden auf dem Maßstab einzelner Gangartkristalle dar. Die hydrothermale Mn-Fe-As Gangmineralisation in Sailauf (Spessart) durchschlägt einen unterpermischen Rhyolithkörper und umfasst eine komplexe Abfolge von Mn- und Fe-Oxiden, die mit Karbonaten unterschiedlicher Zusammensetzung assoziiert sind. Die Mineralisationsfolge belegt klare Fraktionierungen von Fe und Mn während zeitlich voneinander abgrenzbarer Vererzungsphasen. Die Mn-Oxid-Vererzungsphase umfasst Mn-Oxide und Calcit, während die Hämatit-Vererzungsphase durch Hämatit und Mn-Calcit gekennzeichnet ist. Thermodynamische Modellierungen zeigen, dass die Paragenesen deutliche Fluktuationen physikochemischer Parameter wie pH, fO2, fCO2 und aSiO2 belegen. Mischungen von oxidierenden, neutralen Fluiden aus dem kristallinen Grundgebirge mit reduzierenden und alkalischen Fluiden aus den oberhalb der Gänge liegenden Zechsteindolomiten können diese Variationen erklären und werden auch durch Seltenerdelement-Verteilungsmustern der Karbonate angezeigt. Mit Hilfe von LA-ICPMS Analysen an Fluideinschlüssen mit petrographisch eindeutigen Altersbeziehungen können den paragenetisch klar abgrenzbaren Vererzungsphasen kompositionell unterschiedliche Fluide zugeordnet werden. Diese unterscheiden sich durch systematische Variationen ihrer Spurenelementgehalte von Li, B, K, Pb und Zn, während in Bezug auf Hauptelementchemie und Salinität praktisch keine Unterschiede bestehen. Die Fluide der Hämatit-Vererzungsphase zeigen Ähnlichkeiten zu Fluiden, die mit kristallinen Grundgebirgsgesteinen interagiert haben, während die hoch oxidierenden Fluide der Mn-Oxid Vererzungsphase Ähnlichkeiten zu Fluiden aus klastischen Sedimenten des Rotliegenden aufweisen. Diese Fluide mischten sich als Folge veränderter hydrologischer Rahmenbedingungen in zeitlich getrennten Phasen hydrothermaler Aktivität mit Formationswässern des Zechstein.

Ore geology reviews 112, 103006 (2019). doi:10.1016/j.oregeorev.2019.103006, Published by Elsevier, Amsterdam [u.a.]