Mineralogical and experimental study of serpentine minerals and ultramafic rocks with application to carbon capture and storage by mineralisation

The type of feedstock and host rock utilised in ex situ and in situ Carbon Capture and Storage by Mineralisation (CCSM) is an important aspect of these technologies, and detailed appraisal of candidate mineral/rock performance in the presence of CO2 may greatly improve CCSM efficiency. Here, a detailed mineralogical and geochemical investigation of serpentine minerals and ultramafic rocks under laboratory-controlled experiments simulating ex situ and in situ process conditions is presented. Feedstock serpentine minerals were analysed comprehensively, prior to experimental processing. The identification of antigorite was unequivocal using a combination of X-ray diffraction, Fourier Transform Infrared Spectroscopy and/or Thermo-gravimetric Analysis. However, the analysis of chrysotile and lizardite proved to be more challenging, especially when the two polymorphs were finely inter-grown. This study highlighted the structural, textural and chemical complexity of serpentines and showed that great care must be taken when analysing this mineral group for CCSM. Investigation of the acid leaching of serpentine minerals under conditions of 70°C and 1.4M NH4HSO4 provided fundamental insights into the rate and extraction efficiency (EE) of Mg2+ and associated controlling factors, under conditions relevant for ex situ CCSM. It is demonstrated that EE is a function of mineral reactivity and depend on a complex interplay between crystal structure and chemistry. Generally, poorly crystalline and highly disordered phases with low levels of Al3+ were found to be suitable feedstock materials for acid digestion pre-treatment. Chrysotile, lizardite 2H1 and poorly crystalline serpentines showed up to 85% Mg2+ EE after 3 h of acid leaching, and hence are recommended as best feedstock materials for CCSM, whilst antigorite and Al3+-rich serpentines proved to be largely unsuitable, showing low levels of EE of ~ 20%. Examination of dunite, harzburgite and serpentinite under conditions relevant for in situ CCSM, i.e. 70°C and 100 bar CO2 pressure, provided insights into rock reactivity as a function of composition and texture, and the progression of in situ mineral carbonation. The rate of net Mg release and thus the extent of subsequent carbonation were greatest for serpentinite, providing ca. 3% carbonation after six months. However, mineral reactivity within serpentinite was preferential, i.e. significantly enhanced within secondary vein serpentine, being thus, the main source of Mg2+ for magnesite precipitation. Reaction-induced transformation and hence mineral carbonation of dunite and harzburgite were less pronounced over the same time-scale. The reaction of serpentinite with wet supercritical CO2, as opposed to CO2-saturated brine, significantly affected rock integrity, with the exposure of more surface area and promotion of fluid-rock interaction. In particular, it is shown that ferric iron oxidation and the precipitation of goethite impacts upon surface mineral dissolution at exposed surfaces, thereby hindering subsequent carbonation. Overall, this study highlighted the importance of host rock choice for in situ CCSM and the need for detailed petrographical and geochemical investigation of any proposed CCSM repository prior to technological process modelling

The structure and petrology of the Cnoc nan Cuilean Intrusion, Loch Loyal Syenite Complex, NW Scotland

In NW Scotland, several alkaline intrusive complexes of Silurian age intrude the Caledonian orogenic front. The most northerly is the Loch Loyal Syenite Complex, which is divided into three separate intrusions (Ben Loyal, Beinn Stumanadh and Cnoc nan Cuilean). Mapping of the Cnoc nan Cuilean intrusion shows two main zones: a Mixed Syenite Zone (MZ) and a Massive Leucosyenite Zone (LZ), with a gradational contact. The MZ forms a lopolith, with multiple syenitic lithologies, including early basic melasyenites and later felsic leucosyenites. Leucosyenite melts mixed and mingled with melasyenites, resulting in extreme heterogeneity within the MZ. Continued felsic magmatism resulted in formation of the relatively homogeneous LZ, invading western parts of the MZ and now forming the topographically highest terrane. The identification of pegmatites, microgranitic veins and unusual biotite-magnetite veins demonstrates the intrusion's complex petrogenesis. Cross-sections have been used to create a novel 3D GoCad™ model contributing to our understanding of the intrusion. The Loch Loyal Syenite Complex is known to have relatively high concentrations of rare earth elements (REEs), and thus the area has potential economic and strategic value. At Cnoc nan Cuilean, abundant REE-bearing allanite is present within melasyenites of the MZ. Extensive hydrothermal alteration of melasyenites here formed steeply dipping biotite-magnetite veins, most enriched in allanite and other REE-bearing accessories. This study has thus identified the area of greatest importance for further study of REE enrichment processes in the Cnoc nan Cuilean intrusion

The Effect of X-ray Energy Overlaps on the Microanalysis of Chevkinite (Ce, La, Ca, Th)4(Fe2+, Mg)2(Ti, Fe3+)3Si4O22 Using SEM EDS-WDS

A light REE (LREE)-bearing mineral called chevkinite (Ce, La, Ca, Th)4(Fe2+, Mg)2(Ti, Fe3+)3Si4O22, originating from a heavy metal placer deposit Aksu Diamas in Turkey, previously assessed for potential REE extraction as a by-product of magnetite production, was studied using scanning electron microscopy with energy and wavelength-dispersive spectrometers (SEM EDS-WDS). This mineral exhibits analytical challenges associated with severe X-ray energy overlaps between the REE, titanium, and barium. Here, we present an iterative process, showing that SEM EDS-WDS is a viable technique for obtaining good quality quantitative data. SEM EDS-WDS is an in situ, non-destructive, and relatively non-expensive technique, but operator’s experience is essential to obtain good quality data. In cases where the peak fitting remains challenging, in particular, and where the constituents have large differences in abundance, an assessment of the X-ray spectrum to qualitatively assign all peaks is essential prior to quantitative analysi

Acid-dissolution of antigorite, chrysotile and lizardite for ex situ carbon capture and storage by mineralisation

Serpentine minerals serve as a Mg donor in carbon capture and storage by mineralisation (CCSM). The acid-treatment of nine comprehensively-examined serpentine polymorphs and polytypes, and the subsequent microanalysis of their post-test residues highlighted several aspects of great importance to the choice of the optimal feed material for CCSM. Compelling evidence for the non-uniformity of serpentine mineral performance was revealed, and the following order of increasing Mg extraction efficiency after three hours of acid-leaching was established: Al-bearing polygonal serpentine (<5%) ≤ Al-bearing lizardite 1T (≈5%) < antigorite (24-29%) < well-ordered lizardite 2H1 (≈65%) ≤ Al-poor lizardite 1T (≈68%) < chrysotile (≈70%) < poorly-ordered lizardite 2H1 (≈80%) < nanotubular chrysotile (≈85%). It was recognised that the Mg extraction efficiency of the minerals depended greatly on the intrinsic properties of crystal structure, chemistry and rock microtexture. On this basis, antigorite and Al-bearing well-ordered lizardite were rejected as potential feedstock material whereas any chrysotile, non-aluminous, widely spaced lizardite and/or disordered serpentine were recommended. The formation of peripheral siliceous layers, tens of microns thick, was not universal and depended greatly upon the intrinsic microtexture of the leached particles. This study provides the first comprehensive investigation of nine, carefully-selected serpentine minerals, covering most varieties and polytypes, under the same experimental conditions. We focused on material characterisation and the identification of the intrinsic properties of the minerals that affect particle’s reactivity. It can therefore serve as a generic basis for any acid-based CCSM pre-treatment

Volcanic-derived placers as a potential resource of Rare Earth Elements: the Aksu Diamas Case Study, Turkey

Rare earth elements (REE) are essential raw materials used in modern technology. Current production of REE is dominated by hard-rock mining, particularly in China, which typically requires high energy input. In order to expand the resource base of the REE, it is important to determine what alternative sources exist. REE placers have been known for many years, and require less energy than mining of hard rock, but the REE ore minerals are typically derived from eroded granitic rocks and are commonly radioactive. Other types of REE placers, such as those derived from volcanic activity, are rare. The Aksu Diamas heavy mineral placer in Turkey has been assessed for potential REE extraction as a by-product of magnetite production, but its genesis was not previously well understood. REE at Aksu Diamas are hosted in an array of mineral phases, including apatite, chevkinite group minerals (CGM), monazite, allanite and britholite, which are concentrated in lenses and channels in unconsolidated Quaternary sands. Fingerprinting of pyroxene, CGM, magnetite and zircon have identified the source of the placer as the nearby Gölcük alkaline volcanic complex, which has a history of eruption throughout the Plio-Quaternary. Heavy minerals were eroded from tephra and reworked into basinal sediments. This type of deposit may represent a potential resource of REE in other areas of alkaline volcanis

Evidence for fracture-hosted fluid-rock reactions within geothermal reservoirs of the eastern trans-Mexico volcanic belt

Fractures within hydrothermal systems represent major pathways for fluid flow, and it is therefore vital that we understand processes occurring along them as these may have an impact on productivity of hot fluids during geothermal exploitation. This is especially important where hydrothermal activity crosses contrasting rock types, as fluid movement can result in a range of fluid�rock reactions, mineral dissolution and precipitation, and possible changes in fracture permeability. Here we report evidence of fluid-rock reactions within basement carbonates and overlying volcanic rocks within hydrothermally altered rocks of the eastern trans-Mexico volcanic belt, as part of the Europe-Mexico collaborative ‘GEMex’ project (EU-H2020, GA No. 727550). Identified reactions within basement carbonates include initial high temperature Si-metasomatism linked to igneous intrusions to form minerals such as olivine, wollastonite, garnet and diopside, followed by subsequent lower temperature hydration (back reaction) at lower temperatures, where olivine and diopside hydrate to form serpentine and talc. Reactions of overlying andesitic units include Ca-metasomatism and bleaching through interaction of rising acidic, carbonate-equilibrated fluids. Secondary minerals produced during these reactions appear to seal fractures, implying tectonic reactivation of fractures to maintain long-term fluid flow through fracture zones

Volcanological and environmental controls on the Snowdon mineralization, North Wales, UK: a failed volcanogenic massive sulfide system in the Avalon Zone of the British Caledonides

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. The absence of massive sulfide mineralization is consistent with the general paucity of economic VMS deposits in the Avalon Zone. Despite the highly prospective geological setting this study further illustrates the importance of volcanic facies mapping and associated paleo-environmental interpretations in VMS exploration

CO2 sequestration potential in Indian basalts

India currently produces 7 – 8% of annual global CO2 emissions, with energy production accounting for 69% of India’s emissions. As such there is growing interest in the potential for emissions reductions through Carbon Capture and Storage (CCS). Vishal et al., (2021) estimated the theoretical storage capacity of India. However, many large emission sources are located distant from sedimentary basin settings where CO2 storage may conventionally be considered. Continental flood basalts of the Deccan Traps cover an estimated 500,000 km3 in west-central India, and have been identified for potential CO2 storage. The concept of CO2 storage in basalt relies on CCS by mineralisation (CCSM) as basalt can act as a ready source of divalent cations such as Mg2+ and Ca2+. These can combine with carbonate, CO32-, from dissolved CO2, to produce carbonate minerals such as Magnesite (MgCO3). The CarbFix project in Iceland is a successful example of CCSM in basalt. CO2 is captured from the industrial emission site and piped to the CarbFix operation, where CO2 gas is dissolved in water to produce carbonated water and injected into the basalt (Matter and Kelemen, 2009). A key point to note is that a successful basalt storage scheme requires a target formation with sufficient porosity and permeability to sustain the required injection rates. We present an experimental study focused on characterising basalt of the Ambenali and Poladpur formations sampled from the Killari-1 borehole to assess the storage potential of Deccan Trap basalts. A density log for the Killari-1 borehole is shown in Figure 1, illustrating significant physical property variations throughout the site. Rubbly flow tops and vesicular layers which may be considered as potential injection intervals can be identified from the density log. Higher density layers may act as low permeability top seals. X-ray computed tomography (CT) was performed on six selected core samples (c. 120 mm in length and 54 mm in diameter) using a Geotek rotating X-ray computed tomography (RXCT) core scanner at the British Geological Survey’s (BGS) Core Scanning Facility (CSF). Sample depths are shown on Figure 1. Total and connected porosity was calculated using digital rock analysis (PerGeos by ThermoFisher). Figure 2 illustrates the porosity network for sample KIL-110 taken from a low-density zone at a depth of 231 m. The left image (blue) shows total porosity, while the right image (purple) shows total connected porosity which is 15.57%. For comparison, KIL-123, at a depth of 168 m has no porosity and is therefore unlikely to contribute to fluid flow unless open fractures or joint networks are present. Previous geochemical experiments carried out on Deccan Trap samples have indicated that major dissolution of primary carbonates and precipitation of secondary minerals such as siderite occurs during CO2 exposure. This confirms the potential for mineral trapping of CO2. Using samples from the Killari-1 borehole, a new series of experiments has been initiated to explore these geochemical processes more fully, including the reaction rates and implications for CO2 storage. An ongoing series of batch experiments using powdered basalt samples at a range of temperatures is currently underway, with regular fluid sampling and monitoring to provide valuable information on reaction rates. The experiments are conducted under 100 bar CO2 headspace and at temperatures of 50°C, 100°C and 150°C. As well as providing some relatively high temperature data points where reactions will progress more rapidly, the selected temperature range reflects the significant variation in geothermal gradient across the Deccan Volcanic Province. SEM and XRD reacted solids will provide information on changes from pre to post experiment. The experiments will be complemented by additional batch experiments conducted at similar conditions using cut, rather than powdered sample material. As well as generating more realistic data on reaction rates, this approach will enable detailed characterisation of the reacted material surfaces through a before and after comparison of specific surface sites via SEM. A subsequent suite of flow-through experiments will be conducted on core samples to investigate the impact of flowing water with a high dissolved CO2 content through the basalt. These experiments will enable the assessment of the dissolution and precipitation potential of porous basalt systems, and, therefore, the potential impacts on flow within a representative flow zone. Effluent chemistry will be monitored to assess directions and rates of reaction within the system, allowing an assessment of the storage potential of Deccan Trap basalts. The experiments will be conducted under pressure and temperature appropriate to potential storage depths. Acknowledgements The research was part of the BGS International NC programme ‘Geoscience to tackle Global Environmental Challenges’, NERC reference NE/X006255/1. Nimisha Vedanti acknowledges ECCSEL-ERIC for supporting transnational access to ECCSEL research infrastructure at the British Geological Survey. The Director, NGRI is acknowledged for permission to conduct research on NGRI Basalt samples. The extended abstract is published by permission of the Director of the British geological Survey. References Gupta, H.K., Srinivasan, R., Rao, R.U.M., Rao, G.V., Reddy, G.K. and others. 2003. Borehole investigations in the surface rupture zone of the 1993 Latur SCR earthquake, Maharashtra, India: Overview of results. Memoir of the Geological Society of India, 54, 1–22. Matter, J., Kelemen, P. 2009. Permanent storage of carbon dioxide in geological reservoirs by mineral carbonation. Nature Geosci 2, 837–841. Vishal, V., Verma, Y., Chandra, D. and Ashok, D. 2021. A systematic capacity assessment and classification of geologic CO2 storage systems in India. International Journal of Greenhouse Gas Control, 111, 103458

The impact of hydrothermal alteration on the physiochemical characteristics of reservoir rocks: the case of the Los Humeros geothermal field (Mexico)

Hydrothermal alteration is a common process in active geothermal systems and can significantly change the physiochemical properties of rocks. To improve reservoir assessment and modeling of high-temperature geothermal resources linked to active volcanic settings, a detailed understanding of the reservoir is needed. The Los Humeros Volcanic Complex, hosting the third largest exploited geothermal field in Mexico, represents a natural laboratory to investigate the impact of hydrothermal processes on the rock properties through andesitic reservoir cores and outcropping analogs. Complementary petrographic and chemical analyses were used to characterize the intensities and facies of hydrothermal alteration. The alteration varies from argillic and propylitic facies characterized by no significant changes of the REE budget indicating an inert behavior to silicic facies and skarn instead showing highly variable REE contents. Unaltered outcrop samples predominantly feature low matrix permeabilities ( 1.67 W m−1 K−1; > 0.91 10–6 m2 s−1), but a significant loss of magnetic susceptibility (10–3–10–6 SI). In particular, this latter characteristic appears to be a suitable indicator during geophysical survey for the identification of hydrothermalized domains and possible pathways for fluids. The lack of clear trends between alteration facies, alteration intensity, and chemical indices in the studied samples is interpreted as the response to multiple and/or repeated hydrothermal events. Finally, the proposed integrated field-based approach shows the capability to unravel the complexity of geothermal reservoir rocks in active volcanic settings