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Hydrothermal ore deposits that are exploited for gold include both gold-only deposits, such as orogenic deposits, and gold-bearing examples of the common hydrothermal deposit types that are formed around upper-crustal magmatic centres, in particular porphyry and epithermal deposits. Fluid inclusion data have shown that ore fluids of gold-only deposits are compositionally distinct compared to fluids of other deposit types: Fluids of the latter deposit types are, in contrast, not distinctive, and recent research has been focussed on relating gold content to geochemical and petrological aspects of related magmatic systems and to fluid evolution on migration through the crust. This Special Publication includes an up-to-date summary of thermodynamic parameters of aqueous Au species at high temperatures and pressures, a dataset of fluid inclusion properties and compositions from orogenic deposits of different parts of the world, several comprehensive case studies of different types of gold deposits and their fluids from the USA, Brazil, Egypt, Slovakia and Bulgaria and numerical modelling aimed to define key parameters that affect fluid flow and gold deposition at a range of scales.

This study focuses on the geochemical features of the presently discharging thermal and cold springs and on paleofluids from the upstream portion of the Reno river basin (Alto Reno; central–northern Italy). The aim is investigating the primary sources of the modern and fossil fluids and the interactions between deep and shallow aquifers. Paleofluids are from fluid inclusions hosted within euhedral and hopper quartz crystals and consist of a two-phase, liquid–vapor aqueous fluid and a unary CH4 fluid. The aqueous inclusions have constant phase ratios and a calculated salinity of ~ 1.5 wt% NaCleq. They homogenize by bubble disappearance at 100–200 °C, whereas the estimated entrapment depth is ~ 3–5.5 km. The paleofluids likely represent the vestiges of the deep and hot, CH4-rich, Na+–Cl− fluids produced by the interaction between meteoric waters and Triassic and Miocene formations. The modern Na+–Cl−(HCO3−) thermal waters originate from meteoric waters infiltrating SW of the study area, at elevation > 800 m a.s.l., circulating within both the Triassic evaporites and the overlying Miocene turbiditic formations, where salt dissolution/precipitation, sulfate reduction, and production of thermogenic CH4 occur. The equilibrium temperature of the deep fluid source is ~ 170 °C, corresponding to > 5 km depth. Cold springs are Ca2+–HCO3− type and show low amounts of biogenic CO2 and CH4 with no inputs of deep-originated fluids excepting in the immediate surroundings of the thermal area, confirming the lack of significant hydraulic connection between shallow and deep aquifers. We propose a genetic link between the quartz-hosted paleofluid and the thermal waters present in the area.

Laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-ToF-MS) was used to generate quantitative elemental images of a mineralogically and texturally complex fault rock of the Northern Apennines of Italy (Zuccale Fault, eastern Island of Elba). Using LA-ICP-ToF-MS combined with a low-dispersion LA cell, we were able to generate large format (4 mm × 2 mm), high-resolution (5 μm), high dynamic range (1–106 μg g−1), and quantitative multi-elemental two-dimensional compositional maps within a data acquisition time of a few hours. To quantify element mass fractions across the heterogeneous sample of the Zuccale Fault, we used a 100% species-mass normalisation approach that took into account different mineral phases across the specimen. To assign mineral phase directly from LA-ICP-ToF-MS data, we exploited the segregation of sulfur and calcium between distinct phases to threshold element images and develop mineral-phase-specific masks. Moreover, we demonstrate agreement between elemental mass fractions determined by LA-ICP-ToF-MS imaging with 100% normalisation quantification and conventional LA-ICP-MS analysis with internal standard element-based quantification. Finally, we discuss how this elemental imaging provides unique insights into the genesis of the Zuccale Fault.

The dynamic evolution of fault zones at the seismogenic brittle–ductile transition zone (BDTZ) expresses the delicate interplay between numerous physical and chemical processes. Deformation and fluid flow at the BDTZ are closely related and mutually dependent during repeating and transient cycles of frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the exact role of fluids in facilitating broadly coeval brittle and ductile deformation at that structural level. We combine structural analysis, fluid inclusion, and mineral chemistry data from synkinematic and authigenic minerals to reconstruct the temporal variations in fluid pressure (Pf), temperature (T), and bulk composition (X) of the fluids that mediated deformation and steered strain localization along BFZ300, a strike–slip fault originally active at the BDTZ. BFZ300 deforms the Paleoproterozoic migmatitic basement of southwestern Finland and hosts in its core two laterally continuous quartz veins formed by two texturally distinct types of quartz – Qtz I and Qtz II, with Qtz I older than Qtz II. Veins within the damage zone are formed exclusively by Qtz I. Mesostructural and microstructural analysis combined with fluid compositional data indicate recurrent cycles of mutually overprinting brittle and ductile deformation triggered by oscillations of fluid pressure peaking at 210 MPa. Fluid inclusion microthermometry and mineral pair geothermometry indicate that the two documented quartz types precipitated from different fluid batches, with bulk salinities in the 1 wt % NaCleq–5 wt % NaCleq range for Qtz I and in the 6 wt % NaCleq–11 wt % NaCleq range for Qtz II. The temperature of the fluids involved with initial strain localization and later fault reactivation evolved through time from > 350 ∘C during Qtz I precipitation to ∘C at the time of Qtz II crystallization. The peak fluid pressure estimates constrain pore pressure oscillations between 80 and 210 MPa during the recorded faulting episodes. Our results suggest variability of the physico-chemical conditions of the fluids steering deformation (Pf, T, X), reflecting the ingress and effects of multiple batches of fluid in the fault zone. Initial fluid-mediated embrittlement generated a diffuse network of joints and/or hybrid–shear fractures in the damage zone; subsequent strain localization led to more localized deformation within the fault core. Localization was guided by cyclically increasing fluid pressure and transient embrittlement of a system that was otherwise under overall ductile conditions. Our analysis suggests that fluid overpressure at the BDTZ can play a key role in the initial embrittlement of the deforming rock and steer subsequent strain localization.

Geological Society, London, Special Publications, 402 (1), ISSN:0375-6440, ISSN:0305-8719, ISSN:2041-4927, Gold-transporting hydrothermal fluids in the earth's crust, ISBN:978-1-86239-657-9, ISBN:1-86239-657-4