Chloride waters of Great Britain revisited: from subsea formation waters to onshore geothermal fluids

It has long been known that chloride-dominated saline ground waters occur at depth in the UK, not only beneath the sea but also onshore at depths of a few hundred metres. In a few places in northern England, these saline waters discharge naturally at surface in the form of springs. In recent years, however, these saline ground waters have come to be regarded as resources: as potential geothermal fluids intercepted in deep boreholes. Comparisons of the major ions and stable isotopes (δ2H, δ18O and δ34S) of these saline ground waters with North Sea oilfield formation waters, and with brines encountered in former subsea workings of coastal collieries, reveal that they are quite distinct from those found in North Sea oilfields, in that their as δ2H/δ18O signatures are distinctly “meteoric”. δ34S data preclude a significant input from evaporite dissolution – another contrast with many North Sea brines and some colliery waters. Yet, enigmatically, their total dissolved solids contents are far higher than typical meteoric waters. It is tentatively suggested that these paradoxical hydrogeochemical properties might be explained by recharge during Cenozoic uplift episodes, with high concentrations of solutes being derived by a combination of high-temperature rock–water interaction in the radiothermal granites and/or ‘freeze out’ from overlying permafrost that surely formed in this region during cold periods. Geothermometric calculations suggest these saline waters may well be representative of potentially valuable geothermal reservoirs

Preliminary investigation on temperature, chemistry and isotopes of mine water pumped in Bytom geological basin (USCB,Southern Poland) as a potential geothermal energy source

Mine water from both operating and abandoned mines can be used for individual space heating projects, district heating/cooling systems or for preheating air for mine ventilation. Examples of such applications are already known from Canada, US, Netherlands, UK, and Spain. The Upper Silesian Coal Basin (USCB) in Poland, where 34 of 65 hard coal mines have been abandoned since 1989, represents a potentially large opportunity for mine water heating schemes. This paper describes the mines from Bytom (northern USCB) as a potential location for ground source heat extraction projects. Hydrogeological and hydrogeochemical studies of pumped waters have been carried out in order to better understand the potential of the Bytom heat resource. The monitoring program is still ongoing, but initial results compare favorably with existing mine water geothermal source systems where water temperatures are comparable or lower than those found at Bytom. Initial hydrochemical and isotope data demonstrate stability in water composition at most of the monitoring points. These data elucidate the hydrogeological cycle during active dewatering and provide a baseline for understanding the geothermal behavior of the system, which is crucial for optimizing heat extraction. Preliminary results also reveal very stable mine water temperatures in the pumped, and hydrologically connected, Szombierki system and suggest remarkable stability in the characteristics of the main hydrothermal reservoirs

Effective crustal permeability controls fault evolution: An integrated structural, mineralogical and isotopic study in granitic gneiss, Monte Rosa, Northern Italy

Two dextral faults within granitic gneiss in the Monte Rosa nappe, northern Italy reveal key differences in their evolution controlled by evolving permeability and water/rock reactions. The comparison reveals that identical host rock lithologies develop radically different mineralogies within the fault zones, resulting in fundamentally different deformation histories. Oxygen and hydrogen isotope analyses coupled to microstructural characterisation show that infiltration of meteoric water occurred into both fault zones. The smaller Virgin Fault shows evidence of periodic closed system behaviour, which promoted the growth of hydrothermal K-feldspar, whilst the more open system behaviour of the adjacent Ciao Ciao Fault generated a weaker muscovite-rich fault core, which promoted a step change in fault evolution. Effective crustal permeability is a vital control on fault evolution and, coupled to the temperature (i.e. depth) at which key mineral transformations occur, is probably a more significant factor than host rock strength in controlling fault development. The study suggests that whether a fault in granitic basement grows into a large structure may be largely controlled by the initial hydrological properties of the host rocks. Small faults exposed at the surface may therefore be evolutionary “dead-ends” that typically do not represent the early stages in the development of larger faults

The sulfur isotope evolution of magmatic-hydrothermal fluids : insights into ore-forming processes

This project was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 689909. W.H. also acknowledges support from a UKRI Future Leaders Fellowship (MR/S033505/1). A.J.B. is funded by the NERC National Environment Isotope Facility award (NE/S011587/1) and the Scottish Universities Environmental Research Centre.Metal-rich fluids that circulate in magmatic-hydrothermal environments form a wide array of economically significant ore deposits. Unravelling the origins and evolution of these fluids is crucial for understanding how Earth’s metal resources form and one of the most widely used tools for tracking these processes is sulfur isotopes. It is well established that S isotopes record valuable information about the source of the fluid, as well as its physical and chemical evolution (i.e. changing pH, redox and temperature), but it is often challenging to unravel which of these competing processes drives isotopic variability. Here we use thermodynamic models to predict S isotope fractionation for geologically realistic hydrothermal fluids and attempt to disentangle the effects of fluid sources, physico-chemical evolution and S mineral disequilibrium. By modelling a range of fluid compositions, we show that S isotope fingerprints are controlled by the ratio of oxidised to reduced S species (SO42−/H2S), and this is most strongly affected by changing temperature, fO2 and pH. We show that SO42−/H2S can change dramatically during cooling and our key insight is that S isotopes of individual sulfide or sulfate minerals can show large fractionations (up to 20 ‰) even when pH is constant and fO2 fixed to a specific mineral redox buffer. Importantly, while it is commonly assumed that SO42−/H2S is constant throughout fluid evolution, our analysis shows that this is unlikely to hold for most natural systems. We then compare our model predictions to S isotope data from porphyry and epithermal deposits, seafloor hydrothermal vents and alkaline igneous bodies. We find that our models accurately reproduce the S isotope evolution of porphyry and high sulfidation epithermal fluids, and that most require magmatic S sources between 0 and 5 ‰. The S isotopes of low sulfidation epithermal fluids and seafloor hydrothermal vents do not fit our model predictions and reflect disequilibrium between the reduced and oxidised S species and, for the latter, significant S input from seawater and biogenic sources. Alkaline igneous fluids match model predictions and confirm magmatic S sources and a wide range of temperature and redox conditions. Of all these different ore deposits, porphyry and alkaline igneous systems are particularly well-suited to S isotope investigation because they show relationships between redox, alteration and ore mineralogy that could be useful for exploration and prospecting. Ultimately, our examples demonstrate that S isotope forward models are powerful tools for identifying S sources, flagging disequilibrium processes, and validating hypotheses of magmatic fluid evolution.Publisher PDFPeer reviewe

The application of S isotopes and S/Se ratios in determining ore-forming processes of magmatic Ni–Cu–PGE sulfide deposits: a cautionary case study from the northern Bushveld Complex

The application of S/Se ratios and S isotopes in the study of magmatic Ni–Cu–PGE sulfide deposits has long been used to trace the source of S and to constrain the role of crustal contamination in triggering sulfide saturation. However, both S/Se ratios and S isotopes are subject to syn- and post-magmatic processes that may alter their initial signatures. We present in situ mineral δ34S signatures and S/Se ratios combined with bulk S/Se ratios to investigate and assess their utility in constraining ore-forming processes and the source of S within magmatic sulfide deposits. Magmatic Ni–Cu–PGE sulfide mineralization in the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex was used as a case study based on well-defined constraints of sulfide paragenesis and local S isotope signatures. A crustal δ34S component is evident in the most primary sulfide assemblage regardless of footwall lithology, and is inferred that the parental magma(s) of the GNPA member was crustally contaminated and sulfide saturated at the time of emplacement. However, S/Se ratios of both the primary and in particular secondary sulfide assemblages record values within or below the mantle range, rather than high crustal S/Se ratios. In addition, there is a wide range of S/Se ratio for each sulfide mineral within individual assemblages that is not necessarily consistent with the bulk ratio. The initial crustal S/Se ratio is interpreted to have been significantly modified by syn-magmatic lowering of S/Se ratio by sulfide dissolution, and post-magmatic lowering of the S/Se ratio from hydrothermal S-loss, which also increases the PGE tenor of the sulfides. Trace element signatures and variations in Th/Yb and Nb/Th ratios support both an early pre-emplacement contamination event as seen by the S isotopes and S/Se ratios, but also a second contamination event resulting from the interaction of the GNPA magma with the local footwall country rocks at the time of emplacement; though this did not add any additional S. We are able to present an integrated emplacement and contamination model for the northern limb of the Bushveld Complex. Although the multitude of processes that affect variations in the δ34S signature and in particular S/Se ratio may be problematic in interpreting ore genesis, they can reveal a wealth of additional detail on a number of processes involved in the genetic history of a Ni–Cu–PGE deposit in addition to crustal contamination. However, a prerequisite for being able to do this is to utilize other independent petrological and mineralogical techniques that provide constraints on both the timing and effect of various ore-forming and modifying processes. Utilizing both bulk and in situ methods in concert to determine the S/Se ratio allows for the assessment of multiple sulfide populations, the partitioning behaviour of Se during sulfide liquid fractionation and also the effects of low temperature fluid alteration. In comparison, S isotopes are relatively more robust and represent a more reliable indicator of the role of crustal S contamination. The addition of trace element data to the above makes for an incredibly powerful approach in assessing the role of crustal contamination in magmatic sulfide systems

Tracing organic matter composition and distribution and its role on arsenic release in shallow Cambodian groundwaters

Biogeochemical processes that utilize dissolved organic carbon are widely thought to be responsible for the liberation of arsenic from sediments to shallow groundwater in south and southeast Asia. The accumulation of this known carcinogen to hazardously high concentrations has occurred in the primary source of drinking water in large parts of densely populated countries in this region. Both surface and sedimentary sources of organic matter have been suggested to contribute dissolved organic carbon in these aquifers. However, identification of the source of organic carbon responsible for driving arsenic release remains enigmatic and even controversial. Here, we provide the most extensive interrogation to date of the isotopic signature of ground and surface waters at a known arsenic hotspot in Cambodia. We present tritium and radiocarbon data that demonstrates that recharge through ponds and/or clay windows can transport young, surface derived organic matter in to groundwater to depths of 44 m under natural flow conditions. Young organic matter dominates the dissolved organic carbon pool in groundwater that is in close proximity to these surface water sources and we suggest this is likely a regional relationship. In locations distal to surface water contact, dissolved organic carbon represents a mixture of both young surface and older sedimentary derived organic matter. Ground-surface water interaction therefore strongly influences the average dissolved organic carbon age and how this is distributed spatially across the field site. Arsenic mobilization rates appear to be controlled by the age of dissolved organic matter present in these groundwaters. Arsenic concentrations in shallow groundwaters (< 20 m) increase by 1 μg/l for every year increase in dissolved organic carbon age compared to only 0.25 μg/l for every year increase in dissolved organic carbon age in deeper (> 20 m) groundwaters. We suggest that, while the rate of arsenic release is greatest in shallow aquifer sediments, arsenic release also occurs in deeper aquifer sediments and as such remains an important process in controlling the spatial distribution of arsenic in the groundwaters of SE Asia. Our findings suggest that any anthropogenic activities that alter the source of groundwater recharge or the timescales over which recharge takes place may also drive changes in the natural composition of dissolved organic carbon in these groundwaters. Such changes have the potential to influence both the spatial and temporal evolution of the current groundwater arsenic hazard in this region

Enhanced microbial activity in carbon-rich pillow lavas, Ordovician, Great Britain and Ireland

Date of acceptance: 09/07/2015 ACKNOWLEDGEMENTS A. Sandison and C. Taylor provided skilled technical support. Boyce is funded by Natural Environment Research Council (NERC) support of the Isotope Community Support Facility at the Scottish Universities Environmental Research Centre. NERC supported the project through facility grant IP-1235- 0511. The Raman spectroscopy facility at the University of Aberdeen is funded by the Biotechnology and Biological Sciences Research Council. We are grateful to M. Feely, G. Purvis, and an anonymous reviewer for helpful criticism.Peer reviewedPostprin

Sequence stratigraphical and palaeoenvironmental implications of Cenomanian–Santonian dinocyst assemblages from the Trans¬-Sahara epicontinental seaway : a multivariate statistical approach.

ACKNOWLEDGEMENTS M.B. Usman thanks the Petroleum Technology Development Fund (PTDF) for funding this research at the University of Aberdeen. The editor and reviewer are also thanked for their corrections which improved the manuscript. We also acknowledge Stephen Ingram, Adamu Kimayim Gaduwang and Solomon Abafras for their contributions.Peer reviewedPublisher PD

Stable isotope (δ18O, δD) composition of magmatic fluids exsolved from an active alkaline magma chamber—the case of the AD 79 magma chamber of Vesuvius

This work documents, for the first time, the calculated oxygen and hydrogen isotope composition of the brines exsolved from the peripheral margin of the active magma alkaline chamber that fed the AD 79 (“Pompei”) eruption of Vesuvius. The exsolved hydrosaline fluids had a constant δ18O composition and a variable δD composition, showing a general lowering of δD at nearly constant δ18O content. We argue that the progressive fluid exsolution at the upper peripheral parts of the AD 79 magma chamber may explain this isotopic path. The modeling of the evolution of the hydrogen isotope composition of water remaining dissolved in the melt, and of the exsolved fluid as a consequence of progressive degassing, would favor multiple fluid-release events from the magmatic reservoir. A schematic model on the pulsed accumulation and release of fluids at the top of the magmatic reservoir prior to the eruption is thus proposed. The assessment of the stable isotope composition of the fluids exsolved from the AD 79 magma chamber of Vesuvius may be particularly relevant for the geochemical surveillance of the volcano as it may help to interpret the isotopic composition of fumarolic gases and its variations

Fluid and metal sources in the Fäboliden hypozonal orogenic gold deposit, Sweden

To model the formation of orogenic gold deposits, in a global perspective, it is important to understand the ore-forming conditions not only for deposits hosted in greenschist facies rocks but also in amphibolite facies. The Paleoproterozoic Fäboliden deposit in northern Sweden belongs to the globally rare hypozonal group of orogenic gold deposits and, as such, constitutes a key addition to the understanding of amphibolite facies orogenic gold deposits. The Fäboliden deposit is characterized by auriferous arsenopyrite-rich quartz veins, hosted by amphibolite facies supracrustal rocks and controlled by a roughly N-striking shear zone. Gold is closely associated with arsenopyrite-löllingite and stibnite, and commonly found in fractures and as inclusions in the arsenopyrite-löllingite grains. The timing of mineralization is estimated from geothermometric data and field relations at c. 1.8 Ga. In order to constrain the origin of gold-bearing fluids in the Fäboliden deposit, oxygen, hydrogen, and sulfur isotope studies were undertaken. δ18O from quartz in veins shows a narrow range of + 10.6 to + 13.1‰. δD from biotite ranges between − 120 and − 67‰, with most data between − 95 and − 67‰. δ34S in arsenopyrite and pyrrhotite ranges from − 0.9 and + 3.6‰ and from − 1.5 and + 1.9‰, respectively. These stable isotope data, interpreted in the context of the regional and local geology and the estimated timing of mineralization, suggest that the sulfur- and gold-bearing fluid was generated from deep-crustal sedimentary rocks during decompressional uplift, late in the orogenic evolution of the area. At the site of gold ore formation, an 18O-enriched magmatic fluid possibly interacted with the auriferous fluid, causing precipitation of Au and the formation of the Fäboliden hypozonal orogenic gold deposit