ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉The juxtaposition of a Triassic evaporite diapir with the organic matter-rich Fahdene Formation (Albian-Vra-conian) along major faults in the Slata ore district raises the question of the roles played by halokinesis, hydrocarbons, and tectonics in mineralization. The Slata mining district, located in the Tunisian salt diapiric zone, contains Ba-Pb-(± Zn) ore hosted in the Aptian carbonates. The mineralogical paragenetic sequence consists of barite (Ba-1)–galena ± sphalerite ± calcite (Ca-1)–barite (Ba-2) and finally, late calcites (Ca-2 and Ca-3). Fluid inclusions from early barite reveal that it was precipitated from a warm (134°–157°C), H〈sub〉2〈/sub〉O-NaCl-KCl-CaCl〈sub〉2〈/sub〉, moderately saline (13.3–24.6 wt % NaCl equiv) basinal brine. This fluid is thought to have resulted from the mixing of a deep-seated, hot, metal-bearing fluid with a cooler, dilute SO42−-rich fluid. Early calcite and cogenetic sulfides (galena and sphalerite) precipitated from fluids of similar salinities and temperatures as the barite-forming fluids, but with the additional involvement of hydrocarbons. Sulfur isotope data suggest that thermochemical sulfate reduction of Triassic gypsum was the main source of reduced sulfur for sulfides. Late barite precipitated as a result of the mixing between a Ba-rich, hot, ascending fluid with a cooler, dilute Triassic sulfate-rich fluid in the absence of hydrocarbons. The homogeneous Pb isotope compositions of galena along with the Sr isotope compositions of barite point to a Paleozoic reservoir as the main source of metals with a contribution from the Triassic-Cretaceous rocks. The emplacement of the ore occurred during the Eocene-Miocene Alpine compressional tectonic activity that triggered the circulation of Paleozoic-derived metal-bearing fluids.〈/span〉

Description: Stibnite was mined until the end of the twentieth century in the Schlaining ore district, Austria, near the easternmost border of the Eastern Alps where windows of Penninic ophiolites and metasediments are exposed below Austroalpine tectonic units. In Early Miocene, structurally controlled small vein and metasomatic stibnite-quartz deposits were formed in Penninic Mesozoic calcareous marbles and calcite schists. Fluid inclusion studies identified two fluids involved in the mineralization: (i) a low-salinity, low-CO2 metamorphic fluid that precipitated quartz at approximately 240 °C and (ii) a stibnite-forming ore fluid that had a meteoric origin. There is no evidence of boiling or that the fluids mixed during mineralization. The ore components Sb and H2S were leached by fluid/rock interaction from buried rock units. Stibnite mineralization occurred by cooling the ore fluid to below 300 °C, at less than 2000 m depth. Quartz precipitated at slightly lower temperatures, approximately contemporaneous with stibnite. Fluid migration and ore deposition are probably related to high heat flow during the exhumation of the Rechnitz Window in response to Neogene extension and/or shallow Early Miocene andesitic magmatism. The study emphasizes that data obtained from the analyses of gangue minerals alone cannot routinely be used to infer the origin and depositional conditions of the associated ore minerals.

Description: C–O–H–N–S-bearing fluids are known as one of the most challenging geochemical systems due to scarcity of available experimental data. H2S-rich fluid systems were recognized in a wide array of world-class mineral deposits and hydrocarbon reservoirs. Here we report on a nature of low-temperature (T ≥ −192 °C) phase transitions observed in natural CH4–H2S–CO2–N2–H2O fluid inclusions, which are modeled as closed thermodynamic systems and thus serve as natural micro-laboratories representative of the C–O–H–N–S system. For the first time, we document solid–solid H2S (α ↔ β ↔ γ) transitions, complex clathrates and structural transformations of solid state H2S in natural inclusion gas mixtures. The new data on Raman spectroscopic features and a complete sequence of phase transition temperatures in the gas mixtures contribute to scientific advancements in fluid geochemistry. Enhanced understanding of the phase equilibria in the C–O–H–N–S system is a prerequisite for conscientious estimation of P-T-V-X properties, necessary to model the geologic evolution of hydrocarbon and mineral systems. Our findings are a driver for the future research expeditions to extraterrestrial H2S-rich planetary systems owing to their low temperature environments.

Description: High CO2 contents were encountered by some exploration wells in the southern part of the upper Permian Zechstein-2-Carbonate (Ca2) fairway of the Lower Saxony Basin in northwestern Germany. The origin of high CO2 accumulations in this part of the Lower Saxony Basin is controversial. In a combined geochemical and structural study, we aimed to decipher the fluid evolution and trace the origin and migration paths of CO2 in the Ca2 reservoirs of the Lower Saxony Basin. The results of comprehensive analyses of fluid inclusions in fracture-filling minerals hosted by Ca2 and sub-Permian strata prove there were multiple stages of fluid and gas migration in the Lower Saxony Basin. Dry gas generated from upper Carboniferous coals charged the Ca2 reservoirs beginning in the Late Triassic and was partly altered in situ by thermochemical sulfate reduction. However, major amounts of CO2 were supplied by rising overpressured hot hydrothermal fluids in response to tectonic movements during Late Cretaceous inversion of the Lower Saxony Basin. Migration of CO2-rich fluids was recorded by fluid inclusions in minerals from fracture-fill mineralization in Devonian, upper Carboniferous, and Ca2 strata. Compelling fluid inclusion evidence indicates that CO2 released during high-temperature metamorphism of deeply buried Devonian platform carbonates ascended with hot hydrothermal fluids through deep-reaching faults into some Ca2 reservoirs. Locally, reservoirs that were charged with CO2 during Late Cretaceous basin inversion were significantly diluted by sweet gas that migrated laterally from the Pompeckj block southward into the Ca2 fairway beginning in the Paleogene.

Description: This paper reports microthermometric and noble gas isotope data for fluid inclusion assemblages (FIAs) with evidence of phase separation, i.e. coexisting vapor-rich and halite-saturated inclusions, hosted in the early-formed quartz stockwork veins and post-magmatic quartz eye crystals in two economic porphyry Cu deposits (PCDs; Sar Cheshmeh and Miduk) and two sub-economic prospects (Sar Kuh and Abdar) from the Kerman porphyry copper belt (KPCB), Iran. The multiphase halite-saturated inclusions (i.e., Type I) in all studied PCDs and prospects had the highest homogenization temperature (Th = 525–594 °C) and salinities (63–73 wt% NaClequiv), whereas vapor-rich inclusions (Type II) had lower Th (362–460 °C). Fluid inclusion data show that like economic PCDs, the sub-economic prospects were formed in a fertile hydrothermal system and benefited from a mineralizing fluid, which evolved from a primary hot (mostly 〉 400 °C), metal-rich and oxidized fluid (as evidenced by the presence of opaque- and hematite-bearing fluid inclusions) of unknown salinity, which underwent a phase separation process to form both brine and vapor phases in the early stage of mineralization. The helium abundance and its isotopic composition document a mantle-derived magmatic source for the primary ore fluid in the formation of the studied PCDs and prospects (3He/4He ratios ranging from 0.46 to 2.8 Ra, corresponding to a mantle He contribution in ore fluids between ~ 7 and 45%). However, subsequent hydrothermal processes, i.e., vapor–brine phase separation, fluid-rock interaction with crustal rocks, and mixing with meteoric pore water containing dissolved atmospheric (e.g., Ne and Xe) and some crustal noble gases (e.g., Ar), changed the initial noble gas composition of the magmatic ore fluid to predominantly atmospheric- and crustal-like compositions. A significant proportion of mantle-derived He (up to 45%) in high-temperature (513–594 °C) and high-salinity (61.5–73 wt% NaClequiv) FIAs may indicate the existence of buried, economic, porphyry Cu mineralization in the Abdar prospect; therefore, it is suggested to be a possible target for further exploration. Comparing the He and Ar noble gas isotope composition in porphyry copper systems of different size and economic importance in this study showed that the ore-forming fluids of the outsized PCD (i.e., Sar Cheshmeh) have higher contributions of crustal-derived fluids characterized by predominantly radiogenic noble gas signatures (4He and 40Ar) than the smaller PCDs. This could have been achieved by a prolonged hydrothermal circulation in a large volume of crustal rocks containing radiogenic noble gases under a long-lived heat regime resulting from a deeply emplaced and slowly cooled composite intrusive body.