The nature and genesis of gold-silver-tellurium mineralization in the Metaliferi Mountains of western Romania

Precious metal (Au, Ag) and base metal (Pb, Zn) deposits in the Metaliferi Mountains of western Romania occur in steeply clipping quartz-Ca/Mn carbonate veins, which are hosted by Miocene andesitic stocks and lava flows, and surrounding sedimentary rocks. The deposits consist predominantly of sulfides (pyrite, chalcopyrite, sphalerite, galena), sulfosalts of As and Sb, and a diverse range of Au-Ag tellurides. The igneous host rocks have undergone mild, pervasive propylitic alteration, whereas immediately adjacent to the veins the wall- rock alteration assemblages consist of quartz, sericite, K feldspar, calcite, and pyrite. Fluid inclusion, stable isotope, and thermodynamic data suggest that the majority of the mineralization and hydrothermal alteration in these deposits was caused by loci-salinity (0-5 wt % NaCl equiv), medium- temperature (200 degrees-300 degrees C), near-neutral (pH = 5- 6) fluids, which underwent occasional boiling. The fluid inclusion and stable isotope data support a model in which a metal-bearing, magmatic fluid was exsolved from a crystallizing calc-alkaline melt and ascended to higher levels in the crust, undergoing some isotopic exchange with surrounding sedimentary rocks but limited mixing with ground waters, Although the deposits in this part of the Romanian Carpathians exhibit many of the geologic characteristics of classic low-sulfidation, volcanic-hosted, Au-Ag, epithermal deposits, they seem to have formed from essentially magmatic waters, and there is little evidence for the incorporation of meteoric fluids into the hydrothermal system

A multistage origin for Kupferschiefer mineralization

New Re–Os age determinations on mineralized material from the Polish Kupferschiefer elucidate the timing of mineralization and thus the likely mechanisms of ore deposition. Three mineralization parageneses were analysed: (a) chalcocite as pore space filling in sandstone, (b) disseminated Cu–Mo mineralization in shale, and (c) massive, bedded copper sulphides. The resulting ages fall into two ranges: 245.2 (± 1.6)–264.7 (± 1.8) Ma and 162.3 (± 0.8)–184.3 (± 2.2) Ma. These results substantiate previous age determinations, although no Upper Triassic ages were found in this study. Some of the younger ages for the mineralization could represent alteration and recrystallization of existing sulphides. The results confirm that mineralization took place in several stages, from soon after Kupferschiefer sediment deposition in the Upper Permian and for at least 100 m.y. after, until at least the Cretaceous. The genesis of the mineralization can be explained by the episodic release of hydrothermal fluids from the subsiding adjacent Southern Permian sedimentary basin, although the relative importance of each successive mineralizing ‘event’ for introducing additional metals is as yet unknown

A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H2O-CO2-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V1) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ18O across the batholith (9.5 ± 0.4‰n = 12) suggests V1 precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V1 indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H2O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ18O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V2) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H2O-NaCl-CaCl2-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ18O (−3 to + 1.2‰), δ13CCO2 (−19 to 0‰) and δ34Ssulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V3). Correlations of salinity, temperature, δD and δ18O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ18O (1.2–2.5‰), high-δ13CCO2 (> −4‰), low-δ34Ssulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ18O (−5.4 to −3‰), low-δ13C (<−10‰), high-δ34Ssulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V3 veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated

A eukaryote assemblage intercalated with Marinoan glacial deposits in South Australia

Video of digital X-ray tomographs (µCT) in transverse plane through cylinder of siltstone 5.4 mm diameter (same plane as Fig 5e)