Kalin Kouzmanov

This chapter overviews sulfur chemical speciation and behavior in different types of hydrothermal fluids across the lithosphere, spanning from active geothermal systems to basinal brines, to metamorphic and magmatic-hydrothermal fluids. The information on sulfur in these fluids stems from a wide range of recent analytical, experimental and computational methods whose advantages and limitations are discussed. A special emphasis is given to in situ approaches that have enabled unprecedented insights into the fascinating sulfur chemistry in hydrothermal fluids. These insights motivated a critical review of the chemical speciation of key base, precious, and critical metals whose behavior is intimately linked to sulfur. The results allow the role played by the different sulfur ligands on hydrothermal transport of metals and ore deposit formation to be evaluated. An outline of major challenges and emerging perspectives for sulfur-related research in geological fluids ends up this chapter.

This chapter overviews sulfur chemical speciation and behavior in different types of hydrothermal fluids across the lithosphere, spanning from active geothermal systems to basinal brines, to metamorphic and magmatic-hydrothermal fluids. The information on sulfur in these fluids stems from a wide range of recent analytical, experimental and computational methods whose advantages and limitations are discussed. A special emphasis is given to in situ approaches that have enabled unprecedented insights into the fascinating sulfur chemistry in hydrothermal fluids. These insights motivated a critical review of the chemical speciation of key base, precious, and critical metals whose behavior is intimately linked to sulfur. The results allow the role played by the different sulfur ligands on hydrothermal transport of metals and ore deposit formation to be evaluated. An outline of major challenges and emerging perspectives for sulfur-related research in geological fluids ends up this chapter.

Interpretation of ⁴⁰Ar/³⁹Ar dates of alkali feldspar and U-Pb dates of apatite depends on the dominant mechanism of isotopic transport in these minerals, which can be either diffusion or fluid-assisted dissolution-reprecipitation. To clarify the contributions of these processes, we have conducted a holistic study of alkali feldspar, apatite and other minerals from the Mt. Isa Inlier in NE Australia. Mineral characterisation by electron microscopy, optical cathodoluminescence imaging and element mapping reveal a complex interplay of textures resulting from magmatic crystallisation, deuteric recrystallisation, local deformation with subsequent higher-temperature alteration, and finally ubiquitous low-temperature alteration. U-Pb and Pb isotopic data for zircon, apatite, fluorite and alkali feldspar suggest that the latter event occurred at ~300 Ma and was associated with fluid-assisted exchange of Pb isotopes between minerals in the same rock, causing some apatite grains to have ²⁰⁷Pb-corrected U-Pb dates that exceed their crystallisation age. However, this event had no unequivocal effect on the ⁴⁰Ar/³⁹Ar or Rb-Sr systematics of the alkali feldspar, which were disturbed by higher-temperature alteration at ~1450 Ma. The age of the latter event is derived from Rb-Sr data. ⁴⁰Ar/³⁹Ar dates are very scattered and suggest that ⁴⁰Ar redistribution proceeded by diffusion in the presence of traps in some places and by dissolution-reprecipitation with variable amounts of recycling in other places. Our results demonstrate the complex effects that interaction with limited amounts of fluids can have on ⁴⁰Ar/³⁹Ar dates of alkali feldspar and U-Pb dates of apatite and thereby reinforce previous critique of their suitability for thermochronological reconstructions. We further identify and discuss potential implications for noble gas geochronology of groundwaters and fission track dating of apatite.

Orogenic processes encompass a complex interplay of deformation and metamorphic events, which can impact the formation of ore deposits to various degrees. However, distinguishing fluid signatures from orogenic versus post-orogenic events presents a significant challenge due to the scarcity of robust geochemical indicators that remain unaffected during multiple post-mineral reworking events. This study carefully examines the properties and chemistry of primary and secondary fluid inclusions (FIs), identifying distinct signatures of two fluid populations linked to different styles of Pb-Zn mineralization in the Pyrenean Axial Zone (PAZ) of Southern-France/Northern-Iberia: These included late-Carboniferous stratabound epigenetic Pb-Zn deposits and Mesozoic crosscutting Pb-Zn(-Ge) vein systems. Population (I) is identified in primary and secondary FIs in a few crosscutting Pb-Zn veins and constitutes a minor component in stratabound epigenetic bodies. It exhibits Na-dominated low to intermediate salinity (< 20 wt% NaCl eq.), intermediate temperatures (200–350 °C), abundant CO₂-rich FIs and shows low homogeneous Cl/Br molar ratios. These characteristics are consistent with a metamorphic origin of the fluids, associated with Late-Variscan metamorphism. Population (II) is commonly observed in the crosscutting vein systems where it occurs as primary and pseudosecondary FIs, as well as in stratabound epigenetic bodies where it represents the main fluid component of secondary FIs. Population (II) is Ca-dominated with intermediate to high salinity (15–35 wt% NaCl eq.), relatively low temperature (< 200 °C), and shows high Cl/Br molar ratios with significant variations. This last characteristic is typical of mixing of at least two fluids, one with a probable low Cl/Br molar ratio at shallow crustal levels and another with high Cl/Br molar ratio at deeper levels. Characteristics of population (II) are consistent with a fluid of basinal origin that interacted with the basement while circulating in the Pyrenees during the Mesozoic, although a Pyrenean-Alpine age cannot be excluded. Locally, in sphalerite-hosted secondary FIs that form trails in the crosscutting veins, we find evidence of high Ge concentrations (up to few 1000s ppm), which correlate with anomalous Pb and Tl concentrations. Very high metal concentrations (up to 1–2 wt% Pb, Zn), which are inversely proportional to Cl/Br molar ratios, are found in FIs mainly within veins hosted in deep-seated high-grade metamorphic rocks. Based on a compilation of fluid data from the literature, a first-order correlation can be deduced between the metamorphic grade of the rocks hosting the mineralization and the Pb and Zn content in the FIs. Early stratabound orebodies are considered likely sources of metal for the development of the late crosscutting vein mineralization. This study demonstrates the significance and complexity of orogen-scale fluid circulation and supports the importance of pre-existing metal enrichment in the crust, especially in high-grade metamorphic rocks as a prerequisite for the formation of Pb-Zn veins in complex multi-stage orogens.

The Elatsite–Chelopech ore field in the northern part of the Panagyurishte district in Central Bulgaria comprises numerous spatially associated porphyry copper and epithermal gold deposits and prospects. In addition to the mineralization and alteration features, trace elements, lead and sulfur isotope signatures of sulfide minerals from porphyry copper, base metal and gold-base metal deposits/prospects have been studied. LA-ICP-MS analyses of pyrite, arsenopyrite and sulfosalt minerals validate them as major carriers for Au, Ag, Sb, Se and Co. Pyrite from the three types of mineralization has specific geochemical characteristics. Pyrite from the porphyry copper deposits/prospects has generally lower total trace element content compared to pyrite from the epithermal prospects, except for Se, Co and Ni. Pyrite from the base metal and gold-base metal veins is enriched in As, Au, Ag, Sb and Pb. In pyrite from the base metal deposits, Co and Ni have contents comparable to the pyrite from the porphyry copper deposits, while pyrite from the gold-base metal veins shows lower Co and Ni. Arsenopyrite from these deposits shows similar features. Similarly, sphalerite from the gold-base metal veins also has lower Co content compared to sphalerite from the base metal veins but higher In and Cu contents. In addition to the close spatial relationships between the Elatsite and Gorna Kamenitsa porphyry Cu deposits and Negarstitsa-West and Dolna Kamenitsa base metal prospects, as well as similarities in the mineralization and alteration styles, the lead isotopic (²⁰⁶Pb/²⁰⁴Pb = 18.61–18.68, ²⁰⁷Pb/²⁰⁴Pb = 15.64–15.65 for porphyry and ²⁰⁶Pb/²⁰⁴Pb = 18.55–18.67, ²⁰⁷Pb/²⁰⁴Pb = 15.64–15.68 for base metal) and sulfur isotopic (δ³⁴S values of −3 to +1‰ for porphyry and δ³⁴S values of −1.7 to +3.5‰ for base metal) signatures of sulfides support the idea of a genetic link between these two types of deposits. The porphyry and base-metal mineralization result from a common major ore-forming event during the Late Cretaceous, corresponding to deep/higher-temperature and shallower/distal/lower-temperature environments, respectively. In particular, more radiogenic lead (²⁰⁶Pb/²⁰⁴Pb = 18.41–18.47, ²⁰⁷Pb/²⁰⁴Pb = 15.67–15.76) and slightly different sulfur isotopic compositions (δ³⁴S values of +3.5 to +10.6‰) of sulfides from the distal gold-base metal veins of Kordunsko Dere, Svishti Plaz and Shipkite might be a consequence of the interaction of the ore-forming fluids with an external older crustal and isotopically positive S source. Alternatively, a different fluid source/event for the formation of these gold-base metal veins may be suggested.

The studied Silurian nodules have hard light cores and dark shells. The cores consist of quartz with disseminated zonal carbonate mineral and pyrite. Zones in the carbonate have a variable composition from Mn-containing siderite, ankerite and dolomite to mixed Fe-Mn carbonate and Fe-containing rhodochrosite. The nodule shells are composed of quartz, chlorite, sericite, goethite, Mn-Fe oxyhydroxides and siderite. Fe-Mn oxyhydroxides in the nodule shells are formed due to an alteration of the carbonate minerals and inherited their composition. The formation of the nodules with Fe- and Mn-carbonates is related to diagenetic processes under reducing conditions of low Eh and high pH values. Later, carbonate minerals were altered into Fe-Mn oxyhydroxides. These alteration changes can be related to the influence of later magmatic activity in the area. An example of this activity is the presence of an igneous bodies established on both sides of the section.

The Panagyurishte ore district is characterised by a high abundance of copper and copper-gold deposits, related to Upper Cretaceous magmatic activity in the central part of the Banat-Srednogorie metallogenic belt. The Sr and Pb isotope study of the Elshitsa and Radka Au-Cu epithermal deposits, the Vlaikov Vrah porphyry-Cu deposit, and their host rocks from the southern part of the district, suggests a magmatic origin of the ore-forming components. This study shows that both types of deposits belong to the same metallogenic event.

The Passa Três granite is a 5 km² intrusion in southern Brazil with an NNE–SSW-elongated shape, hosting gold-bearing quartz veins with fluorite, carbonates, sulphides (pyrite, chalcopyrite, aikinite, molybdenite) and native gold. Orebodies are hosted by the pluton roof zone, which is marked by various textures indicating magmatic–hydrothermal transition processes. Mineralisation formed between 613 and 608 Ma in extensional pull-apart structures controlled by two conjugated N–S and E–W fault systems. We report results from petrography, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), scanning electron microscopy (SEM), electron probe microanalyses (EPMA), X-ray fluorescence (XRF), trace element analyses of pyrite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), sulphur isotope (δ³⁴S) analyses, and fluid inclusion microthermometry. Hydrothermal alteration is dominantly developed as phengite–quartz–carbonate and sericite–carbonate–chlorite assemblages along mineralised structures. Fluid inclusion study indicates mineralising fluids with H₂O-CO₂-NaCl composition, low to moderate salinity (0.2–12.84 wt % NaCl eq.), and temperatures from 400 to 150 °C. The sulphur isotopic composition of pyrite (−0.1 to 1.1‰) suggests magmatic origin. These data, in conjunction with structurally controlled mineralisation enriched in Au-Bi minerals shortly post-dating the granite emplacement, appoint towards similarities between the Passa Três deposit and intrusion-related gold systems. The specific location of the mineralisation in the core (and in the roof zone, regarding its vertical position) of the Passa Três granitic intrusion defines it as “granite-hosted” thus, it is representative of a specific model that can be used for exploration of other intrusion-related/hosted gold deposits near the studied area or in other locations.