Infiltration of basinal fluids into high-grade basement, South Norway: sources and behaviour of waters and brines

Quartz veins hosted by the high-grade crystalline rocks of the Modum complex, Southern Norway, formed when basinal fluids from an overlying Palaeozoic foreland basin infiltrated the basement at temperatures of c.220degreesC (higher in the southernmost part of the area). This infiltration resulted in the formation of veins containing both two-phase and halite-bearing aqueous fluid inclusions, sometimes with bitumen and hydrocarbon inclusions. Microthermometric results demonstrate a very wide range of salinities of aqueous fluids preserved in these veins, ranging from c. 0 to 40 wt% NaCl equivalent. The range in homogenization temperatures is also very large (99-322degreesC for the entire dataset) and shows little or no correlation with salinity. A combination of aqueous fluid microthermometry, halogen geochemistry and oxygen isotope studies suggest that fluids from a range of separate aquifers were responsible for the quartz growth, but all have chemistries comparable to sedimentary formation waters. The bulk of the quartz grew from relatively low delta(18)O fluids derived directly from the basin or equilibrated in the upper part of the basement (T< 200degreesC). Nevertheless, some fluids acquired higher salinities due to deep wall-rock hydration reactions leading to salt saturation at high temperatures (>300degreesC). The range in fluid inclusion homogenization temperatures and densities, combined with estimates of the ambient temperature of the basement rocks suggests that at different times veins acted as conduits for influx of both hotter and colder fluids, as well as experiencing fluctuations in fluid pressure. This is interpreted to reflect episodic flow linked to seismicity, with hotter dry basement rocks acting as a sink for cooler fluids from the overlying basin, while detailed flow paths reflected local effects of opening and closing of individual fractures as well as reaction with wall rocks. Thermal considerations suggest that the duration of some flow events was very short, possibly in the order of days. As a result of the complex pattern of fracturing and flow in the Modum basement, it was possible for shallow fluids to penetrate basement rocks at significantly higher temperatures, and this demonstrates the potential for hydrolytic weakening of continental crust by sedimentary fluids

Evidence for an impact-induced biosphere from the δ34S signature of sulphides in the Rochechouart impact structure, France

The highly eroded 23 km diameter Rochechouart impact structure, France, has extensive evidence for post-impact hydrothermal alteration and sulphide mineralization. The sulphides can be divided into four types on the basis of their mineralogy and host rock. They range from pyrites and chalcopyrite in the underlying coherent crystalline basement to pyrites hosted in the impactites. Sulphur isotopic results show that δ34S values vary over a wide range, from -35.8‰ to +0.4‰. The highest values, δ34S -3.7‰ to +0.4‰, are recorded in the coherent basement, and likely represent a primary terrestrial sulphur reservoir. Sulphides with the lowest values, δ34S -35.8‰ to -5.2‰, are hosted within locally brecciated and displaced parautochthonous and autochthonous impactites. Intermediate δ34S values of -10.7‰ to -1.2‰ are recorded in the semi-continuous monomict lithic breccia unit, differing between carbonate-hosted sulphides and intraclastic and clastic matrix-hosted sulphides. Such variable isotope values are consistent with a biological origin, via bacterial sulphate reduction, for sulphides in the parautochthonous and autochthonous units; these minerals formed in the shallow subsurface and are probably related to the post impact hydrothermal system. The source of the sulphate is likely to have been seawater, penecontemporaneous to the impact, as inferred from the marginal marine paleogeography of the structure. In other eroded impact craters that show evidence for impact-induced hydrothermal circulation, indirect evidence for life may be sought isotopically within late-stage (≤120°C) secondary sulphides and within the shocked and brecciated basement immediately beneath the transient crater floor

Origin of fluids in the shallow geothermal environment of Savo, Solomon Islands.

Savo is a recently emergent volcano. An active geothermal system has been present for at least 50 years, expressed at the surface by numerous hot springs, fumaroles and steaming ground. Samples of water and steam were collected from geothermal features and non-thermal springs and wells, and representative samples of altered rocks and precipitates were collected from geothermal areas. Analysis of the waters for anion, cation and stable isotope composition shows that the waters discharging at the surface fall into two groups Reoka type fluids have the high sulfate, low pH, and enriched δ18O and δD values typical of steam heated acid sulfate waters, where shallow groundwater is heated by rising steam and gas. Isotopically light H2S is oxidised in the near surface environment to produce the sulfate content. Rembokola type fluids have chemistry distinct from the Reoka type fluids, despite the two being found within close proximity (<10 m). Rembokola Type fluids produce a carbonate sinter, so are assumed to be saturated with bicarbonate. The aqueous sulfate has heavy δ34S, suggesting that it is not exclusively produced by the oxidation of H2S in the near surface environment. We suggest that condensation of volcanic gases (including CO2 and isotopically heavy SO2) into meteoric-derived groundwater in the upper levels of the volcanic edifice produces these carbonate–sulfate waters. The presence of SO2 suggests that there is a degassing magma at depth, and potentially a high sulfidation-type epithermal system beneath the steam heated zone

Sulphur isotope geochemistry of black shale-hosted antimony mineralization, Arnsberg, northern Rhenish Massif, Germany

Vein-type and bedding-concordant mesothermal (180–410 °C) stibnite–sulphosalt mineralization at Arnsberg, NE Rhenish Massif, Germany, is hosted by Carboniferous pyrite-rich black shales and siliceous limestones. A detailed sulphur isotope study of the stibnite–sulphosalt mineralization and pyrite from a variety of regional host-rock lithologies has been carried out using an in situ laser combustion technique. The 34S values of stibnite of various textural types are distinctly negative and lie in a narrow range between -23.9 and -17.1 (mean -20.1). In contrast, regional sedimentary–diagenetic pyrites display a large variation of their 34S values between -45.4 and +9.3. There is little evidence for significant modification of the hydrothermal fluid during deposition and the S isotope signatures suggest that the sulphur of the stibnite mineralization was not locally derived. The 34S values of pyrite in Givetian shales display a significantly narrower range of -28.2 to -7.5 and their mean composition (-17.1) is close to the 34S values of the Arnsberg stibnite deposits. Considering the temperature-dependent isotopic fractionation between stibnite and reduced sulfur species, the 34S values of the mineralizing fluid (-16.8; 200 °C) and the Givetian rock source are essentially identical. Therefore, we propose a model of leaching and isotopic homogenization of sulphur from the Middle Devonian shales and a subsequent northward migration of these fluids. The fluids were trapped in permeability-controlled positions within anticlinal zones, where fluid cooling induced deposition of stibnite and sulphosalts

Indicators of hot fluid migration in sedimentary basins: evidence from the UK Atlantic Margin

Microthermometric, petrographic and isotopic methods have been used to detect evidence for hot fluid flow in Mesozoic and Tertiary sediments from the NW UK continental margin, West of Shetland. New data presented here show that temperatures are hotter by c. 40°C in Tertiary samples than in the underlying Jurassic and Cretaceous sediments in wells 204/28-1, 206/5-2, 208/27-1, especially in cements from samples as young as mid–upper Eocene in age. Paleocene samples can be discriminated from older (Jurassic and Cretaceous) and younger (Eocene) sandstones on the basis of silica cement morphology and cathodoluminescence zonation. Jurassic, Cretaceous and Eocene quartz cements show oscillatory zoning as a consequence of relatively slow burial cementation. In direct contrast, rapid precipitation of silica cements from the cooling of hot fluids has produced unzoned cements in all but one Paleocene sample. No evidence for unzoned quartz cements was noted in any pre-Paleocene or Eocene samples. The restriction of hot fluid inclusions and unzoned cements to the Paleocene and post-Paleocene is consistent with lateral focusing of hot fluids. Isotopic data from kaolinites indicate that these fluids are best represented by mixtures of Mesozoic or Tertiary meteoric waters and marine porewaters that have undergone isotopic alteration through interaction with volcanic material. Our results indicate that hot fluid flow occurred over a relatively long time-scale (i.e. several million years), which may have important consequences for the degradation of reservoired hydrocarbons in West of Shetland Paleocene plays

Geochemistry and metallogeny of Neoproterozoic pyrite in oxic and anoxic sediments

The Neoproterozoic Dalradian Supergroup contains widespread diagenetic sulphides present as pyrite. The sulphides occur in both carbonaceous shales and glacial diamictites, that were deposited in relatively reducing and oxidising conditions respectively. The trace element compositions of the pyrite, and consequently the whole rock compositions, contrast between the two lithologies. The highest concentrations of selenium, tellurium and gold are all found in diamictite-hosted pyrite. The data suggest that increased mobility of these elements in oxidising conditions led to greater uptake when pyrite was precipitated. As one model for the formation of orogenic gold ore deposits assumes a sulphide-rich protolith, pyrite ultimately formed during relatively oxidising conditions could make a contribution, including the widespread pyrite precipitated during the Neoproterozoic ‘Snowball Earth’ glaciations

Stable Isotope Studies of the Rochechouart Impact Structure: Sources of Secondary Carbonates and Sulphides within Allochthonous and Parautochthonous Impactites

Hypervelocity impacts are among the most ubiquitous processes to affect solid bodies within our solar system [1, 2]. Although they are notoriously devastating, citing responsibility for mass extinction events and global climate perturbations, impacts can also create temporary environments which are favorable for life to thrive, if there is enough water present in the target, and sufficient energy is released as heat [1, 2]. One-third of impact structures on Earth contain fossil impact-initiated hydrothermal systems, and they are therefore being explored as potential “cradles of life” on other solid planets and satellites in our solar system [1].&lt;p&gt;&lt;/p&gt; We are presenting a case for the evaluation of the Mesozoic Rochechouart impact structure in France as a once-habitable environment. Initial δ 13C, δ18O and δ 34S isotope data collected in 2014 from hydrothermal carbonates and sulphides within monomict lithic impact breccia, collected from a site located 7.5km from the center of the structure at Champagnac quarry, supports our hypothesis of a warm, wet environment; we also found evidence for metabolically reduced sulphate [3]. Similar mineral assemblages can be found throughout the structure, including allochthonous breccias and low to unshocked target material. In order to explore our hypothesis further, a larger sample set was collected from various lithologies within the Champagnac site containing sulphide and carbonate mineralization for δ 13C, δ18O and δ34S isotope analysis in January 2015. These results will allow us to determine the relationships between the many hydrothermal mineral assemblages within this area of the structure, and ask whether the isotopic compositions recorded in secondary sulphides and carbonates of the impactites are inherited from the target, or possibly represent colonization by thermophilic microbes during the post-impact hydrothermal period.&lt;p&gt;&lt;/p&gt

A black shale protolith for gold-tellurium mineralisation in the Dalradian Supergroup (Neoproterozoic) of Britain and Ireland

The Dalradian Supergroup of Britain and Ireland is mineralised by gold-tellurium vein deposits. The host succession includes carbonaceous, pyritic shales (pelites) which were a source of trace elements, including gold and tellurium. LA-ICP-MS mapping of pyrite crystals shows that late stages are enriched in gold, tellurium and lead, representing concentration of these elements during metamorphism and related hydrothermal activity. The sulphur isotope composition of the pyrite varies with stratigraphic position, reflecting an origin for the pyrite in the depositional environment through microbial sulphate reduction. Where pyrite was converted to pyrrhotite, trace element contents are much lower, indicating element liberation during metamorphism. These observations are consistent with a model of black shale protoliths for orogenic gold deposits

The geology and genesis of the iron skarns of the Turgai belt, northwestern Kazakhstan

The magnetite deposits of the Turgai belt (Kachar, Sarbai and Sokolov), in the Valerianovskoe zone of the southern Urals, Kazakhstan, contain a combined resource of over 3 Gt of iron oxide ore. The deposits are hosted by carbonate sediments and volcaniclastic rocks of the Carboniferous Valerianovka Supergroup, and are spatially related to the gabbroic to granitoid composition intrusive rocks of the Sarbai–Sokolov intrusive series. The magnetite deposits are developed dominantly as metasomatic replacement of limestone, but also, to a lesser extent, of volcanic rocks. Pre-mineralisation metamorphism and alteration resulted in the formation of wollastonite and the silicification of limestone. Magnetite mineralisation is associated with the development of a high temperature skarn assemblage of diopside, grossular–andradite garnet, actinolite, epidote and apatite. Sub-economic copper-bearing sulphide mineralisation overprints the magnetite mineralisation and is associated with deposition of hydrothermal calcite and the formation of an extensive sodium alteration halo dominated by albite and scapolite. Chlorite formation accompanies this stage and further later stage hydrothermal overprints. The replacement has in places resulted in preservation of primary features of the limestone, including fossils and sedimentary structures in magnetite, skarn calc-silicates and sulphides. Analysis of Re–Os isotopes in molybdenite indicates formation of the sulphide mineral assemblage at 336.2 ± 1.3 Ma, whilst U–Pb analyses of titanite from the skarn alteration assemblage suggests skarn alteration at 326.6 ± 4.5 Ma with re-equilibration of isotope systematics down to ~ 270 Ma. Analyses of mineral assemblages, fluid inclusion microthermometry, O and S isotopes suggest initial mineralisation temperatures in excess of 600 °C from hypersaline brines (45–50 wt.% NaCl eq.), with subsequent cooling and dilution of fluids to around 150 °C and 20 wt.% NaCl eq. by the time of calcite deposition in late stage sulphide-bearing veins. δ18O in magnetite (− 1.5 to + 3.5‰) and skarn forming silicates (+ 5 to + 9‰), δ18O and δ13C in limestone and skarn calcite (δ18O + 5.4 to + 26.2‰; δ13C − 12.1 to + 0.9‰) and δ34S in sulphides (− 3.3 to + 6.6‰) and sulphates (+ 4.9 to + 12.9‰) are all consistent with the interaction of a magmatic-equilibrated fluid with limestone, and a dominantly magmatic source for S. All these data imply skarn formation and mineralisation in a magmatic–hydrothermal system that maintained high salinity to relatively late stages resulting in the formation of the large Na-alteration halo. Despite the reported presence of evaporites in the area there is no evidence for evaporitic sulphur in the mineralising system. These skarns show similarities to some members of the iron oxide–apatite and iron oxide–copper gold deposit classes and the model presented here may have implications for their genesis. The similarity in age between the Turgai deposits and the deposits of the Magnitogorsk zone in the western Urals suggests that they may be linked to similar magmatism, developed during post-orogenic collapse and extension following the continent–continent collision, which has resulted in the assembly of Laurussian terranes with the Uralide orogen and the Kazakh collage of the Altaids or Central Asian Orogenic Belt. This model is preferred to the model of simultaneous formation of very similar deposits in arc settings at either side of an open tract of oceanic crust forming part of the Uralian ocean

The role of post-Variscan extensional tectonics and mantle melting in the generation of the Lower Permian granites and the giant W-As-Sn-Cu-Zn-Pb orefield of SW England

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