<p>Volcanoes in island arcs can undergo edifice evolution that includes submarine a... more <p>Volcanoes in island arcs can undergo edifice evolution that includes submarine and subaerial volcanism, providing a dynamic environment of magmatic heat and volatiles that drives hydrothermal fluid flow with potential inputs from sea and/or meteoric water. This, in turn, can generate significant hydrothermal alteration that can result in economic deposits of industrial minerals. One example includes bentonite, a smectitic rock composed dominantly of montmorillonite.</p><p>Economically viable bentonite deposits are typically only 0.5 – 5 meters thick and<strong> </strong>although Wyoming-type bentonites comprise 70% of the world’s known deposits, they are commonly no thicker than 8 m. The island of Milos is Europe’s largest and actively mined calcium bentonite resource from volcanic piles exceeding 80 m thickness. Here, we use the Milos island example to understand how magmatism, volcanic edifice evolution and hydrothermal activity interact. We integrate field relationships of volcanic stratigraphy and alteration zones, with clay mineralogy (XRD), stable (S, O and H) isotope analysis and high precision geochronology (CA-ID-TIMS zircon U-Pb, and alunite Ar-Ar) to elucidate the timescales, thermal drivers and fluid components that lead to the development of a globally important bentonite resource.</p><p>A vertical transect through bentonite-altered volcanic stratigraphy indicates multiple magmatic pulses ca. 2.8 Ma with a submarine andesitic cryptodome and accompanying pepperitic hyaloclastite. Cumulative volcanic and sub-volcanic processes occurred over ca. 170 kyrs, resulting in a vertically and laterally extensive volcanic pile overlain by an episode of magmatic quiescence and brackish-water diatomaceous sediments. It is overlain by a silicic pyroclastic flow host to pervasive silica-alunite-kaolinite alteration. Stable isotopic analyses of bentonite indicate a hydrothermal origin at around 70°C with the fluid being sourced from sea and meteoric waters. The timing of formation is defined by a maximum duration of ca. 170 kyrs, with clear geological evidence that a significant period of alteration occurred within < 20 kyrs at ~ 2.64 Ma. Alunite sulfur isotope compositions reflect steaming ground activity that could be interpreted as the oxidised, shallower level counterpart to a boiling geothermal system linked to development of extensive bentonite. However, the timing of alunite can be clearly resolved to > 1.5 myrs after bentonite formation to ~ 1.0 Ma, supporting a later overprint origin due to relatively recent steam heating of groundwater after emergence of the submarine system.</p><p>This study identifies key parameters that have resulted in the formation of an economic-scale bentonite resource on the emergent island of Milos. We conclude that the hydrology needed to form a bentonite deposit is not constrained to the marine environment and can be connected to emergent parts of the volcanic edifice. High precision geochronology indicates bentonite development happens on volcanic timescales (10 to 100 kyrs). A cumulative volcanic and sub-volcanic pile coeval with the formation of bentonite suggests multiple magmatic episodes over narrow timeframes provide and sustain the thermal driver for significant bentonite development. After emergence and development of a groundwater system, the subsequent steam heating is deleterious to grade and results in the development of alunite-kaolinite overburden.</p>
The livelihood of inhabitants from rural agricultural valleys in the arid Arica and Parinacota Re... more The livelihood of inhabitants from rural agricultural valleys in the arid Arica and Parinacota Region, northernmost Chile, strongly depends on water from high altitude rainfall and runoff to lower elevation areas. However, elevated arsenic, boron, and other potentially harmful elements compromise water quality, especially in rural areas. Samples (n = 90) of surface, underground, cold, geothermal springs, and treated and raw tap water were studied to assess water quality and to determine the main geochemical controls on water composition, origin, and geochemical evolution along dominant flowpaths. Water from major river basins across the region (Lluta, San Jose, Codpa-Chaca, Camarones and Altiplanicas) were collected for hydrogeochemical analysis of a suite of major and trace elements, δD and δ 18 O. Our new dataset was supplemented by hydrochemical data (n > 1500 data points) from secondary sources. Results show that 72% of the collected samples had As >10 μg/L (WHO drinking water provisional guideline) and affected 44% of the studied waters used for drinking (n = 32). Based on Chilean irrigation guidelines, elevated salinity (EC > 0.75 mS/cm) affected 80% of sampled waters, which were also impacted by high B (89% > 0.75 mg/L), and As (31% > 50 μg/L). Water composition was strongly controlled by geothermal water and freshwater mixing in high altitude areas. Magnitude and fate of As and B concentration was determined by the geothermal input type. Highest As (~21 mg/L) was associated with circum-neutral Na-Cl waters in Camarones basin, while lower As (~5 mg/L) with acid SO 4 waters in Lluta basin. Additionally, evaporative concentration and sediment-water interactions were shown to control the
In porphyry ore deposit models, the propylitic alteration facies is widely interpreted to be caus... more In porphyry ore deposit models, the propylitic alteration facies is widely interpreted to be caused by convective circulation of meteoric waters. However, recent field-based and geochemical data suggest that magmatic-derived fluids are likely to contribute to development of the propylitic assemblage. In order to test this hypothesis, we determined the oxygen and hydrogen isotope compositions of propylitic mineral separates (epidote, chlorite, and quartz), selected potassic mineral separates (quartz and magnetite), and quartz-hosted fluid inclusions from around the E48 and E26 deposits in the Northparkes porphyry Cu-Au district, New South Wales, Australia. In addition, the strontium isotope composition of epidote was determined to test for the potential contribution of seawater in the Northparkes system given the postulated island-arc setting and submarine character of some country rocks.Oxygen isotope geothermometry calculations indicate potassic alteration occurred between ~600° an...
The fluid sources of granite-related Sn-quartz vein deposits are commonly obscured by fluid mixin... more The fluid sources of granite-related Sn-quartz vein deposits are commonly obscured by fluid mixing or fluid-rock interactions. As a result, fluid inclusions, minerals and isotopes in these veins indicate an intermediate composition between magmatic and metamorphic, but the degree of mixing between Highlights Peribatholitic Sn-quartz veins often show secondary overprinting signatures. Fluid mixing was quantified by muscovite and fluid inclusion geochemistry Fluid mixing plays an important role in the genesis of Sn-quartz veins.
Highlights Putative stromatolites of the Mesoproterozoic Stoer Group show no evidence for bioge... more Highlights Putative stromatolites of the Mesoproterozoic Stoer Group show no evidence for biogenicity Some are laminated clastic sediment draped over an ancient land surface Clastic carbonate is calcite, probably derived from weathering of older rocks Other possible stromatolites result from soft-sediment deformation Crystalline calcite in these is a result of syn-depositional to burial diagenesis Detecting ancient life: investigating the nature and origin of possible stromatolites and associated calcite from a one billion year old lake
The Klaipeda Geothermal Demonstration Plant (KGDP), Lithuania, exploits a hypersaline sodium-chlo... more The Klaipeda Geothermal Demonstration Plant (KGDP), Lithuania, exploits a hypersaline sodium-chloride (salinity c. 90 g/L) groundwater from a 1100 m deep Devonian sandstone/siltstone reservoir. The hydrogen and oxygen stable isotope composition is relatively undepleted (δ18O=c. -4.5‰), while the δ34S is relatively “heavy” at +18.9‰. Hydrochemical and isotopic data support the existing hypothesis that the groundwater is dominated by a hypersaline brine derived from evapoconcentrated seawater, modified by water-rock interaction and admixed with smaller quantities of more recent glacial meltwater and/or interglacial recharge. The injectivity of the two injection boreholes has declined dramatically during the operational lifetime of the KGDP. Initially, precipitation of crystalline gypsum led to a program of rehabilitation and the introduction of sodium polyphosphonate dosing of the abstracted brine, which has prevented visible gypsum precipitation but has failed to halt the injectivity...
The Kisenge-Kamata manganese deposit (Katanga, D.R. Congo) belongs to a group of Paleoproterozoic... more The Kisenge-Kamata manganese deposit (Katanga, D.R. Congo) belongs to a group of Paleoproterozoic sedimentary manganese ore occurrences that have been recognized for various parts of Africa. It is a relatively small deposit with total estimated reserves of ∼12 Mt ore, but it is of major importance for understanding key aspects of the geology of the Kasai Block of the Congo Craton. The available constraints indicate that the deposit formed in the 2.0-1.9 Ga time interval, in the Orosirian period. The manganese-rich facies of the Kisenge-Kamata deposits range in composition from manganese carbonate rocks to non-calcareous deposits dominated by manganese-rich garnet. Associated rocks are mainly grey graphitic shales. The Kisenge-Kamata sediments are significantly enriched in Cu, Co, Ni, Zn and other accessory metals (V, Mo, Ga). Field constraints and geochemical data suggest that the Kisenge-Kamata sediments were probably deposited in a relatively small intracratonic basin (graben) within the Kasai Block, with an inlet allowing periodical access for ocean water influx carrying Mn, REE and other metals. Primary carbonate ore has relatively low δ 13 C (−3 to −6‰) and high δ 18 O (+13 to +20‰) values, confirming their derivation from marine fluids.
• Tracers used for monitoring groundwater evolution at a high spatial resolution. • Groundwater a... more • Tracers used for monitoring groundwater evolution at a high spatial resolution. • Groundwater arsenic associated with tracers of modern groundwater • Surface-derived organic matter transported in aquifer to depths of N30 m. • Groundwater arsenic accumulation rates are depth dependent. • Dual in-aquifer and near surfaces processes drive arsenic mobilization.
A pumping test at rates of up to 50 L s-1 was carried out in the 256 m-deep Florence Shaft of the... more A pumping test at rates of up to 50 L s-1 was carried out in the 256 m-deep Florence Shaft of the Beckermet-Winscales-Florence haematite ore mine in Cumbria, UK, between 8th January and 25th March 2015. Drawdowns in mine water level did not exceed 4 m and the entire interconnected mine complex behaved as a single reservoir. Pumping did, however, induce drawdowns of around 1 m in the St. Bees Sandstone aquifer overlying the Carboniferous Limestone host rock. During a second phase of the pumping test, a proportion of the 11.3-12°C mine water was directed through a heat pump, which extracted up to 103 kW heat from the water and recirculated it back to the top of the shaft. Provided that an issue with elevated arsenic concentrations (20-30 lg L-1) can be resolved, the Florence mine could provide not only a valuable resource of high-quality water for industrial or even potable uses, it could also provide several hundred to several thousand kW of ground sourced heating and/or cooling, if a suitable demand can be identified. The ultimate constraint would be potential hydraulic impacts on the overlying St Bees Sandstone aquifer. The practice of recirculating thermally spent water in the Florence Shaft produced only a rather modest additional thermal benefit.
The Snowdon caldera of North Wales is host to base metal sulfide-bearing veins and stockworks, mi... more The Snowdon caldera of North Wales is host to base metal sulfide-bearing veins and stockworks, mineralized breccias, disseminated sulfides, and localized zones of semi-massive to massive sulfide, with subordinate magnetite-rich veins. The late Ordovician host volcanic sequence accumulated in a shallow marine, back-arc environment in the Welsh Basin, which forms part of the Avalon Zone of the British and Irish Caledonides. New field evidence, sulfur isotopes, and U-Pb dating indicate that the Snowdon mineralization is genetically and temporally related to Late Ordovician magmatism and caldera formation. It is interpreted to represent volcanogenic pipe-style sulfide mineralization, resulting from focused hydrothermal fluids moving along caldera-related faults and simultaneous dispersal of fluids through the volcaniclastic pile. Sulfur isotope data suggest that, whilst a limited contribution of magmatic S cannot be ruled out, thermochemical reduction of contemporaneous Ordovician seawater sulfate was the dominant mechanism for sulfide production in the Snowdon system, resulting in a mean value of about 12‰ in both the host volcanic strata and the mineralized veins. Despite the tectonic setting being prospective for VMS deposits, strata-bound sulfide accumulations are absent in the caldera. This is attributed to the shallow water depths, which promoted boiling and the formation of sub-seafloor vein-type mineralization. Furthermore, the tectonic instability of the caldera and the high energy, shallow marine environment would have limited preservation of any seafloor deposits. The new U-Pb dates for the base (454.26 ± 0.35 Ma) and top (454.42 ± 0.45 Ma) of the host volcanic rocks, indicate that the Snowdon magmatic activity was short lived, which is likely to have limited the duration and areal extent of the ore-forming system. Here we present new field observations, sulfur isotope data, and U-Pb zircon dates, which suggest that the Snowdon mineralization is genetically and temporally related to Caradoc-age magmatism and caldera formation. We compare the Snowdon mineralization and its geological setting with those of volcanogenic massive sulfide (VMS) deposits, and consider why the Snowdon caldera failed to produce strata-bound sulfide accumulations. 2. Geological setting 2.1. Regional geological setting The Lower Palaeozoic rocks of Wales consist of a thick sequence of principally marine sedimentary and intercalated volcanic rocks that accumulated in the Welsh Basin (Howells et al. 1991). The basin was located on late Precambrian continental crust composed of accreted island arcs (Thorpe et al. 1984), and formed part of Eastern Avalonia (Kokelaar et al. 1984). During the Ordovician, the Welsh Basin was located close to an ensialic convergent plate margin, probably active from late Tremadoc to Caradoc times (Kokelaar et al. 1988; Howells et al. 1991). It is thought that convergence of Eastern Avalonia with Laurentia caused southeast subduction of Iapetus oceanic crust beneath Wales, resulting in arc volcanism in the Tremadoc (c. 485-478 Ma) (Kokelaar et al. 1984; Kokelaar, 1992). Subsequent back-arc extension in Wales during the Arenig (c. 478-467 Ma) to Caradoc (c. 458-448 Ma) resulted in tholeiitic volcanism, with the main arc located further north in the 'Lake District-Leinster Zone' of the Caledonides (Kokelaar et al. 1984). The tectonic regime in Wales during the Ordovician time is thought to have been sinistrally transtensional, principally resulting from oblique subduction (Kokelaar, 1992). The location of Ordovician eruptive centres in Wales was influenced by major deep-seated fractures within the ensialic basement (Campbell et al. 1988, Kokelaar, 1988). Arenig to Caradoc age volcanics developed in a complex extensional marine basin (>150 km wide), close to the site of the earlier Tremadoc arc (Kokelaar, 1992). Major volcanism and subsidence within the basin was focussed along relatively narrow, repeatedly subsiding graben systems, controlled by faults splaying off deep-seated basement fractures (Howells et al. 1991; Kokelaar, 1992). In North Wales, an approximately 40 km-wide structural belt, termed the 'Snowdon Graben' (Kokelaar, 1988) or 'failed rift' (Campbell et al. 1988) developed from east-west extension along a north-northeast basement fracture (Kokelaar, 1992) (Fig. 1). 2.2. The Snowdon Volcanic Group The geology of Snowdonia is described in detail by Howells et al. (1991), Howells and Smith (1997) and references therein. The Caradoc rocks of North Wales can be divided into two eruptive cycles: the lower (1st Eruptive Cycle) Llewelyn Volcanic Group and the upper (2nd Eruptive Cycle)
There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of w... more There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.
Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation ca... more Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation can occur in more intermediate and alkaline magmas. Sron Garbh is an appinite-diorite intrusion emplaced into Dalradian metasediments in the Tyndrum area of Scotland that hosts magmatic Cu-Ni-PGE-Au sulfide mineralisation in the appinitic portion. It is thus an example of magmatic sulfide mineralisation hosted by alkaline rocks, and is the most significantly mineralised appinitic intrusion known in the British Isles. The intrusion is irregularly shaped, with an appinite rim, comprising amphibole cumulates classed as vogesites. The central portion of the intrusion is comprised of unmineralised, but pyrite-bearing, diorites. Both appinites and diorites have similar trace element geochemistry that suggests the diorite is a more fractionated differentiate of the appinite from a common source that can be classed with the high Ba-Sr intrusions of the Scottish Caledonides. Mineralisation is present as a disseminated, primary chalcopyrite-pyrite-PGM assemblage and a blebby, pyrite-chalcopyrite assemblage with significant Co-As-rich pyrite. Both assemblages contain minor millerite and Ni-Co-As-sulfides. The mineralisation is Cu-, PPGE-, and Au-rich and IPGE-poor and the platinum group mineral assemblage is overwhelmingly dominated by Pd minerals; however, the bulk rock Pt/Pd ratio is around 0.8. Laser ablation analysis of the sulfides reveals that pyrite and the Ni-Co-sulfides are the primary host for Pt, which is present in solid solution in concentrations of up to 22 ppm in pyrite. Good correlations between all base and precious metals indicate very little hydrothermal remobilisation of metals despite some evidence of secondary pyrite and PGM. Sulfur isotope data indicate some crustal S in the magmatic sulfide assemblages. The source of this is unlikely to have been the local quartzites, but S-rich Dalradian sediments present at depth. The generation of magmatic Cu-Ni-PGE-Au mineralisation at Sron Garbh can be attributed to post-collisional slab drop off that allowed hydrous, low-degree partial melting to take place that produced a Cu-PPGE-Au-enriched melt, which ascended through the crust, assimilating crustal S from the Dalradian sediments. The presence of a number of PGE-enriched sulfide occurrences in appinitic intrusions across the Scottish Caledonides indicates that the region contains certain features that make it more prospective than other alkaline provinces worldwide, which may be linked the post-Caledonian slab drop off event. We propose that the incongruent melting of pre-existing magmatic sulfides or 'refertilised' mantle in low-degree partial melts can produce characteristically fractionated, Cu-PPGE-Au-semi metal bearing, hydrous, alkali melts, which, if they undergo sulfide saturation, have the potential to produce alkaline-hosted magmatic sulfide deposits.
Ultramafic portions of ophiolitic fragments in the Arabian-Nubian Shield (ANS) show pervasive car... more Ultramafic portions of ophiolitic fragments in the Arabian-Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Despite the extent of the alteration, little is known about the processes that caused it, the source of the CO 2 , or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites, and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between-8.1‰ to-6.8‰ for δ 13 C, +6.4‰ to +10.5‰ for δ 18 O, and 87 Sr/ 86 Sr of 0.7028 to 0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO 2 ±CH 4 ±N 2) and aqueous-carbonic (H 2 O-NaCl-CO 2 ±CH 4 ±N 2) low salinity fluid, with trapping conditions of 270°C to 300°C and 0.7 to 1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO 2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Audeposits. This is the first time that Au has been observed to be remobilized from rocks during the lizardite-antigorite transition.
<p>Volcanoes in island arcs can undergo edifice evolution that includes submarine a... more <p>Volcanoes in island arcs can undergo edifice evolution that includes submarine and subaerial volcanism, providing a dynamic environment of magmatic heat and volatiles that drives hydrothermal fluid flow with potential inputs from sea and/or meteoric water. This, in turn, can generate significant hydrothermal alteration that can result in economic deposits of industrial minerals. One example includes bentonite, a smectitic rock composed dominantly of montmorillonite.</p><p>Economically viable bentonite deposits are typically only 0.5 – 5 meters thick and<strong> </strong>although Wyoming-type bentonites comprise 70% of the world’s known deposits, they are commonly no thicker than 8 m. The island of Milos is Europe’s largest and actively mined calcium bentonite resource from volcanic piles exceeding 80 m thickness. Here, we use the Milos island example to understand how magmatism, volcanic edifice evolution and hydrothermal activity interact. We integrate field relationships of volcanic stratigraphy and alteration zones, with clay mineralogy (XRD), stable (S, O and H) isotope analysis and high precision geochronology (CA-ID-TIMS zircon U-Pb, and alunite Ar-Ar) to elucidate the timescales, thermal drivers and fluid components that lead to the development of a globally important bentonite resource.</p><p>A vertical transect through bentonite-altered volcanic stratigraphy indicates multiple magmatic pulses ca. 2.8 Ma with a submarine andesitic cryptodome and accompanying pepperitic hyaloclastite. Cumulative volcanic and sub-volcanic processes occurred over ca. 170 kyrs, resulting in a vertically and laterally extensive volcanic pile overlain by an episode of magmatic quiescence and brackish-water diatomaceous sediments. It is overlain by a silicic pyroclastic flow host to pervasive silica-alunite-kaolinite alteration. Stable isotopic analyses of bentonite indicate a hydrothermal origin at around 70°C with the fluid being sourced from sea and meteoric waters. The timing of formation is defined by a maximum duration of ca. 170 kyrs, with clear geological evidence that a significant period of alteration occurred within < 20 kyrs at ~ 2.64 Ma. Alunite sulfur isotope compositions reflect steaming ground activity that could be interpreted as the oxidised, shallower level counterpart to a boiling geothermal system linked to development of extensive bentonite. However, the timing of alunite can be clearly resolved to > 1.5 myrs after bentonite formation to ~ 1.0 Ma, supporting a later overprint origin due to relatively recent steam heating of groundwater after emergence of the submarine system.</p><p>This study identifies key parameters that have resulted in the formation of an economic-scale bentonite resource on the emergent island of Milos. We conclude that the hydrology needed to form a bentonite deposit is not constrained to the marine environment and can be connected to emergent parts of the volcanic edifice. High precision geochronology indicates bentonite development happens on volcanic timescales (10 to 100 kyrs). A cumulative volcanic and sub-volcanic pile coeval with the formation of bentonite suggests multiple magmatic episodes over narrow timeframes provide and sustain the thermal driver for significant bentonite development. After emergence and development of a groundwater system, the subsequent steam heating is deleterious to grade and results in the development of alunite-kaolinite overburden.</p>
The livelihood of inhabitants from rural agricultural valleys in the arid Arica and Parinacota Re... more The livelihood of inhabitants from rural agricultural valleys in the arid Arica and Parinacota Region, northernmost Chile, strongly depends on water from high altitude rainfall and runoff to lower elevation areas. However, elevated arsenic, boron, and other potentially harmful elements compromise water quality, especially in rural areas. Samples (n = 90) of surface, underground, cold, geothermal springs, and treated and raw tap water were studied to assess water quality and to determine the main geochemical controls on water composition, origin, and geochemical evolution along dominant flowpaths. Water from major river basins across the region (Lluta, San Jose, Codpa-Chaca, Camarones and Altiplanicas) were collected for hydrogeochemical analysis of a suite of major and trace elements, δD and δ 18 O. Our new dataset was supplemented by hydrochemical data (n > 1500 data points) from secondary sources. Results show that 72% of the collected samples had As >10 μg/L (WHO drinking water provisional guideline) and affected 44% of the studied waters used for drinking (n = 32). Based on Chilean irrigation guidelines, elevated salinity (EC > 0.75 mS/cm) affected 80% of sampled waters, which were also impacted by high B (89% > 0.75 mg/L), and As (31% > 50 μg/L). Water composition was strongly controlled by geothermal water and freshwater mixing in high altitude areas. Magnitude and fate of As and B concentration was determined by the geothermal input type. Highest As (~21 mg/L) was associated with circum-neutral Na-Cl waters in Camarones basin, while lower As (~5 mg/L) with acid SO 4 waters in Lluta basin. Additionally, evaporative concentration and sediment-water interactions were shown to control the
In porphyry ore deposit models, the propylitic alteration facies is widely interpreted to be caus... more In porphyry ore deposit models, the propylitic alteration facies is widely interpreted to be caused by convective circulation of meteoric waters. However, recent field-based and geochemical data suggest that magmatic-derived fluids are likely to contribute to development of the propylitic assemblage. In order to test this hypothesis, we determined the oxygen and hydrogen isotope compositions of propylitic mineral separates (epidote, chlorite, and quartz), selected potassic mineral separates (quartz and magnetite), and quartz-hosted fluid inclusions from around the E48 and E26 deposits in the Northparkes porphyry Cu-Au district, New South Wales, Australia. In addition, the strontium isotope composition of epidote was determined to test for the potential contribution of seawater in the Northparkes system given the postulated island-arc setting and submarine character of some country rocks.Oxygen isotope geothermometry calculations indicate potassic alteration occurred between ~600° an...
The fluid sources of granite-related Sn-quartz vein deposits are commonly obscured by fluid mixin... more The fluid sources of granite-related Sn-quartz vein deposits are commonly obscured by fluid mixing or fluid-rock interactions. As a result, fluid inclusions, minerals and isotopes in these veins indicate an intermediate composition between magmatic and metamorphic, but the degree of mixing between Highlights Peribatholitic Sn-quartz veins often show secondary overprinting signatures. Fluid mixing was quantified by muscovite and fluid inclusion geochemistry Fluid mixing plays an important role in the genesis of Sn-quartz veins.
Highlights Putative stromatolites of the Mesoproterozoic Stoer Group show no evidence for bioge... more Highlights Putative stromatolites of the Mesoproterozoic Stoer Group show no evidence for biogenicity Some are laminated clastic sediment draped over an ancient land surface Clastic carbonate is calcite, probably derived from weathering of older rocks Other possible stromatolites result from soft-sediment deformation Crystalline calcite in these is a result of syn-depositional to burial diagenesis Detecting ancient life: investigating the nature and origin of possible stromatolites and associated calcite from a one billion year old lake
The Klaipeda Geothermal Demonstration Plant (KGDP), Lithuania, exploits a hypersaline sodium-chlo... more The Klaipeda Geothermal Demonstration Plant (KGDP), Lithuania, exploits a hypersaline sodium-chloride (salinity c. 90 g/L) groundwater from a 1100 m deep Devonian sandstone/siltstone reservoir. The hydrogen and oxygen stable isotope composition is relatively undepleted (δ18O=c. -4.5‰), while the δ34S is relatively “heavy” at +18.9‰. Hydrochemical and isotopic data support the existing hypothesis that the groundwater is dominated by a hypersaline brine derived from evapoconcentrated seawater, modified by water-rock interaction and admixed with smaller quantities of more recent glacial meltwater and/or interglacial recharge. The injectivity of the two injection boreholes has declined dramatically during the operational lifetime of the KGDP. Initially, precipitation of crystalline gypsum led to a program of rehabilitation and the introduction of sodium polyphosphonate dosing of the abstracted brine, which has prevented visible gypsum precipitation but has failed to halt the injectivity...
The Kisenge-Kamata manganese deposit (Katanga, D.R. Congo) belongs to a group of Paleoproterozoic... more The Kisenge-Kamata manganese deposit (Katanga, D.R. Congo) belongs to a group of Paleoproterozoic sedimentary manganese ore occurrences that have been recognized for various parts of Africa. It is a relatively small deposit with total estimated reserves of ∼12 Mt ore, but it is of major importance for understanding key aspects of the geology of the Kasai Block of the Congo Craton. The available constraints indicate that the deposit formed in the 2.0-1.9 Ga time interval, in the Orosirian period. The manganese-rich facies of the Kisenge-Kamata deposits range in composition from manganese carbonate rocks to non-calcareous deposits dominated by manganese-rich garnet. Associated rocks are mainly grey graphitic shales. The Kisenge-Kamata sediments are significantly enriched in Cu, Co, Ni, Zn and other accessory metals (V, Mo, Ga). Field constraints and geochemical data suggest that the Kisenge-Kamata sediments were probably deposited in a relatively small intracratonic basin (graben) within the Kasai Block, with an inlet allowing periodical access for ocean water influx carrying Mn, REE and other metals. Primary carbonate ore has relatively low δ 13 C (−3 to −6‰) and high δ 18 O (+13 to +20‰) values, confirming their derivation from marine fluids.
• Tracers used for monitoring groundwater evolution at a high spatial resolution. • Groundwater a... more • Tracers used for monitoring groundwater evolution at a high spatial resolution. • Groundwater arsenic associated with tracers of modern groundwater • Surface-derived organic matter transported in aquifer to depths of N30 m. • Groundwater arsenic accumulation rates are depth dependent. • Dual in-aquifer and near surfaces processes drive arsenic mobilization.
A pumping test at rates of up to 50 L s-1 was carried out in the 256 m-deep Florence Shaft of the... more A pumping test at rates of up to 50 L s-1 was carried out in the 256 m-deep Florence Shaft of the Beckermet-Winscales-Florence haematite ore mine in Cumbria, UK, between 8th January and 25th March 2015. Drawdowns in mine water level did not exceed 4 m and the entire interconnected mine complex behaved as a single reservoir. Pumping did, however, induce drawdowns of around 1 m in the St. Bees Sandstone aquifer overlying the Carboniferous Limestone host rock. During a second phase of the pumping test, a proportion of the 11.3-12°C mine water was directed through a heat pump, which extracted up to 103 kW heat from the water and recirculated it back to the top of the shaft. Provided that an issue with elevated arsenic concentrations (20-30 lg L-1) can be resolved, the Florence mine could provide not only a valuable resource of high-quality water for industrial or even potable uses, it could also provide several hundred to several thousand kW of ground sourced heating and/or cooling, if a suitable demand can be identified. The ultimate constraint would be potential hydraulic impacts on the overlying St Bees Sandstone aquifer. The practice of recirculating thermally spent water in the Florence Shaft produced only a rather modest additional thermal benefit.
The Snowdon caldera of North Wales is host to base metal sulfide-bearing veins and stockworks, mi... more The Snowdon caldera of North Wales is host to base metal sulfide-bearing veins and stockworks, mineralized breccias, disseminated sulfides, and localized zones of semi-massive to massive sulfide, with subordinate magnetite-rich veins. The late Ordovician host volcanic sequence accumulated in a shallow marine, back-arc environment in the Welsh Basin, which forms part of the Avalon Zone of the British and Irish Caledonides. New field evidence, sulfur isotopes, and U-Pb dating indicate that the Snowdon mineralization is genetically and temporally related to Late Ordovician magmatism and caldera formation. It is interpreted to represent volcanogenic pipe-style sulfide mineralization, resulting from focused hydrothermal fluids moving along caldera-related faults and simultaneous dispersal of fluids through the volcaniclastic pile. Sulfur isotope data suggest that, whilst a limited contribution of magmatic S cannot be ruled out, thermochemical reduction of contemporaneous Ordovician seawater sulfate was the dominant mechanism for sulfide production in the Snowdon system, resulting in a mean value of about 12‰ in both the host volcanic strata and the mineralized veins. Despite the tectonic setting being prospective for VMS deposits, strata-bound sulfide accumulations are absent in the caldera. This is attributed to the shallow water depths, which promoted boiling and the formation of sub-seafloor vein-type mineralization. Furthermore, the tectonic instability of the caldera and the high energy, shallow marine environment would have limited preservation of any seafloor deposits. The new U-Pb dates for the base (454.26 ± 0.35 Ma) and top (454.42 ± 0.45 Ma) of the host volcanic rocks, indicate that the Snowdon magmatic activity was short lived, which is likely to have limited the duration and areal extent of the ore-forming system. Here we present new field observations, sulfur isotope data, and U-Pb zircon dates, which suggest that the Snowdon mineralization is genetically and temporally related to Caradoc-age magmatism and caldera formation. We compare the Snowdon mineralization and its geological setting with those of volcanogenic massive sulfide (VMS) deposits, and consider why the Snowdon caldera failed to produce strata-bound sulfide accumulations. 2. Geological setting 2.1. Regional geological setting The Lower Palaeozoic rocks of Wales consist of a thick sequence of principally marine sedimentary and intercalated volcanic rocks that accumulated in the Welsh Basin (Howells et al. 1991). The basin was located on late Precambrian continental crust composed of accreted island arcs (Thorpe et al. 1984), and formed part of Eastern Avalonia (Kokelaar et al. 1984). During the Ordovician, the Welsh Basin was located close to an ensialic convergent plate margin, probably active from late Tremadoc to Caradoc times (Kokelaar et al. 1988; Howells et al. 1991). It is thought that convergence of Eastern Avalonia with Laurentia caused southeast subduction of Iapetus oceanic crust beneath Wales, resulting in arc volcanism in the Tremadoc (c. 485-478 Ma) (Kokelaar et al. 1984; Kokelaar, 1992). Subsequent back-arc extension in Wales during the Arenig (c. 478-467 Ma) to Caradoc (c. 458-448 Ma) resulted in tholeiitic volcanism, with the main arc located further north in the 'Lake District-Leinster Zone' of the Caledonides (Kokelaar et al. 1984). The tectonic regime in Wales during the Ordovician time is thought to have been sinistrally transtensional, principally resulting from oblique subduction (Kokelaar, 1992). The location of Ordovician eruptive centres in Wales was influenced by major deep-seated fractures within the ensialic basement (Campbell et al. 1988, Kokelaar, 1988). Arenig to Caradoc age volcanics developed in a complex extensional marine basin (>150 km wide), close to the site of the earlier Tremadoc arc (Kokelaar, 1992). Major volcanism and subsidence within the basin was focussed along relatively narrow, repeatedly subsiding graben systems, controlled by faults splaying off deep-seated basement fractures (Howells et al. 1991; Kokelaar, 1992). In North Wales, an approximately 40 km-wide structural belt, termed the 'Snowdon Graben' (Kokelaar, 1988) or 'failed rift' (Campbell et al. 1988) developed from east-west extension along a north-northeast basement fracture (Kokelaar, 1992) (Fig. 1). 2.2. The Snowdon Volcanic Group The geology of Snowdonia is described in detail by Howells et al. (1991), Howells and Smith (1997) and references therein. The Caradoc rocks of North Wales can be divided into two eruptive cycles: the lower (1st Eruptive Cycle) Llewelyn Volcanic Group and the upper (2nd Eruptive Cycle)
There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of w... more There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.
Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation ca... more Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation can occur in more intermediate and alkaline magmas. Sron Garbh is an appinite-diorite intrusion emplaced into Dalradian metasediments in the Tyndrum area of Scotland that hosts magmatic Cu-Ni-PGE-Au sulfide mineralisation in the appinitic portion. It is thus an example of magmatic sulfide mineralisation hosted by alkaline rocks, and is the most significantly mineralised appinitic intrusion known in the British Isles. The intrusion is irregularly shaped, with an appinite rim, comprising amphibole cumulates classed as vogesites. The central portion of the intrusion is comprised of unmineralised, but pyrite-bearing, diorites. Both appinites and diorites have similar trace element geochemistry that suggests the diorite is a more fractionated differentiate of the appinite from a common source that can be classed with the high Ba-Sr intrusions of the Scottish Caledonides. Mineralisation is present as a disseminated, primary chalcopyrite-pyrite-PGM assemblage and a blebby, pyrite-chalcopyrite assemblage with significant Co-As-rich pyrite. Both assemblages contain minor millerite and Ni-Co-As-sulfides. The mineralisation is Cu-, PPGE-, and Au-rich and IPGE-poor and the platinum group mineral assemblage is overwhelmingly dominated by Pd minerals; however, the bulk rock Pt/Pd ratio is around 0.8. Laser ablation analysis of the sulfides reveals that pyrite and the Ni-Co-sulfides are the primary host for Pt, which is present in solid solution in concentrations of up to 22 ppm in pyrite. Good correlations between all base and precious metals indicate very little hydrothermal remobilisation of metals despite some evidence of secondary pyrite and PGM. Sulfur isotope data indicate some crustal S in the magmatic sulfide assemblages. The source of this is unlikely to have been the local quartzites, but S-rich Dalradian sediments present at depth. The generation of magmatic Cu-Ni-PGE-Au mineralisation at Sron Garbh can be attributed to post-collisional slab drop off that allowed hydrous, low-degree partial melting to take place that produced a Cu-PPGE-Au-enriched melt, which ascended through the crust, assimilating crustal S from the Dalradian sediments. The presence of a number of PGE-enriched sulfide occurrences in appinitic intrusions across the Scottish Caledonides indicates that the region contains certain features that make it more prospective than other alkaline provinces worldwide, which may be linked the post-Caledonian slab drop off event. We propose that the incongruent melting of pre-existing magmatic sulfides or 'refertilised' mantle in low-degree partial melts can produce characteristically fractionated, Cu-PPGE-Au-semi metal bearing, hydrous, alkali melts, which, if they undergo sulfide saturation, have the potential to produce alkaline-hosted magmatic sulfide deposits.
Ultramafic portions of ophiolitic fragments in the Arabian-Nubian Shield (ANS) show pervasive car... more Ultramafic portions of ophiolitic fragments in the Arabian-Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Despite the extent of the alteration, little is known about the processes that caused it, the source of the CO 2 , or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites, and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between-8.1‰ to-6.8‰ for δ 13 C, +6.4‰ to +10.5‰ for δ 18 O, and 87 Sr/ 86 Sr of 0.7028 to 0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO 2 ±CH 4 ±N 2) and aqueous-carbonic (H 2 O-NaCl-CO 2 ±CH 4 ±N 2) low salinity fluid, with trapping conditions of 270°C to 300°C and 0.7 to 1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO 2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Audeposits. This is the first time that Au has been observed to be remobilized from rocks during the lizardite-antigorite transition.
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Papers by Adrian J Boyce