Jonathan Naden

Active geothermal systems with entrained seawater as modern analogs for transitional volcanic-hosted massive sulfide and continental magmato-hydrothermal mineralization: The example of Milos Island, Greece

The paradigm for low-sulphidation (LS) volcanic-arc associated mineralization is the active geothermal systems located along the Taupo Volcanic Zone (e.g. Broadlands). However, this analogue is inapt where fluid salinities are consistently in excess of 3.5 wt % NaCl. LS mineralization on Milos (Aegean arc) records high paleofluid-salinities. The δD and δ18O data do not exemplify 18O-shifted meteoric waters—typical of terrestrial geothermal systems. Nor is a submarine origin indicated—stable isotope data show mixing between meteoric, seawater and volcanic-arc gases. Strontium isotope data are comparable to a nearby active seawater-entrained geothermal system. These are features seen in hydrothermal systems associated with emergent volcanoes. For the Milos LS mineralization, high-salinity fluids show it cannot be explained by a Broadlands-type model. The absence of saliferous sequences and significant intrusive rocks preclude these as salinity sources. The similarities between paleo and active systems in terms of salinity, δD–δ18O and strontium isotope systematics strongly suggest that seawater is the main source for Na and Cl. We suggest geothermal systems, containing seawater, associated with emergent volcanoes are an alternative analogue for LS epithermal mineralization. Furthermore, they bridge the gap between submarine, and large-scale terrestrial geothermal systems—the modern analogues for VHMS and epithermal mineralisation in the scheme of intrusion-centered hydrothermal mineralization.

From continent to intra-oceanic arc: Zircon xenocrysts record the crustal evolution of the Solomon island arc

The first U-Pb ages from a ca. 26–24 Ma pluton on Guadalcanal, in the intra-oceanic Solomon island arc (southwest Pacific Ocean), reveal Eocene- to Archean-aged zircon xenocrysts. Xenocryst populations at ca. 39–33 Ma and ca. 71–63 Ma correlate with previously obtained ages of supra-subduction magmatism within the arc. A ca. 96 Ma zircon population may be derived from Cretaceous ophiolite basement crust or region-wide continental rift-related magmatism. Xenocryst age populations alternate with periods of oceanic basin formation that fragmented the East Gondwana margin. Early Cretaceous to Archean zircon xenocryst ages imply continental origins and a cryptic source within the arc crust; they may have been introduced by Eocene interaction of a continental fragment with the arc, and concealed by ophiolite obduction. The data demonstrate that continentally derived zircons may be transported thousands of kilometers from their source and added to intra-oceanic arc magmas, a process likely facilitated by cyclical subduction zone advance and retreat. The findings highlight the continuum of arcs that occurs between continental and oceanic end members, and the caution with which zircons should be used to determine the provenance and setting of ancient arc terranes accreted to the continental crust.

Fluid–rock interactions and the role of late Hercynian aplite intrusion in the genesis of the Castromil gold deposit, northern Portugal

Abstract Castromil (northern Portugal) is one of several important orogenic gold deposits located within the “Central Iberian” geotectonic zone of northwest Iberia. The deposit occurs at the margin of a Variscan, syn- to late-D3 biotite granite, and is spatially associated with a small tourmaline aplite body that intrudes the granite at its contact with a secondary anticline of Palaeozoic arenaceous and argillaceous metasediments of the Valongo Belt. Identification of the ore fluids and their pathways, and the reconstruction of the P–T–X conditions during mineralisation were obtained by combining the geometric characteristics of veins and microstructures together with a detailed study of the inclusion fluids. Several stages of fluid percolation following contact metamorphism can be recognised. At each stage, the contact zone, characterised by intrusive aplites, related faults and fractures, appears to have focused the hydrothermal flow and acted as a structural conduit for deeper-sourced hydrothermal fluids. The earliest fluid stage (Stage I) is characterised by aqueous-carbonic fluids dominated by CO2 and CH4 that were probably generated by high-temperature fluid–rock interaction (400–500 °C) with graphitic schists interbedded with the metasediments. These fluids were responsible for significant alteration (greisenisation) of the aplite and its host granite, and the formation of silicified, flat lying structures that can be traced along the strike length of the deposit. At temperatures between 400 and 500 °C, fluid pressure ranges from 230 to 300 MPa, which is equivalent to a depth of 10±1.5 km. The second stage of mineralisation (Stage II: As-ore stage) is also characterised by aqueous-carbonic fluids and represents the main phase of quartz–arsenopyrite–pyrite deposition. The third stage of mineralisation (Stage III: Au-ore stage) was accompanied by intense microfracturing of the preexisting quartz veins and the preferential deposition of gold along microfractures in the sulphides. The introduction of gold corresponds to the percolation and mixing of two distinctive aqueous fluids of contrasting salinity at relatively low temperatures (150–275 °C). Based on compositional and temperature data, it is suggested that during the main phase of uplift, shallow waters penetrated deep into the basement, allowing gold to be leached from potential source rocks (most probably the Palaeozoic metasediments) and deposited in structural and geochemical traps formed during earlier stages of the hydrothermal system. The decrease in pressure during the As-ore stage corresponds to a significant tectonic uplift (around 5–6 km), and probably marks the transition from lithostatic to hydrostatic pressure conditions. Furthermore, if uplift had already been initiated during aplite emplacement, the prevailing sub-isothermal high-temperature conditions provide an explanation for the presence of decrepitated aqueous-carbonic inclusions in metamorphic quartz lenses and veins in the surrounding metasediments. To conclude, localised heat flows linked to late Hercynian magmatism at deeper structural levels appears to be the main cause of fluid circulation at Castromil. Evidence suggests that contact zones related to faulting along a secondary anticline of the Valongo Belt controlled both aplite intrusion and subsequent long-lived hydrothermal fluid circulation. The proposed genetic model differs from orogenic gold deposit models in emphasising the role of late stage aqueous fluids in the development of economic grade (10–15 g/t) gold ores.

Do fluid inclusions preserve δ18O values of hydrothermal fluids in epithermal systems over geological time? Evidence from paleo- and modern geothermal systems, Milos island, Aegean Sea

Stable isotope compositions of quartz (d 18 Oquartz) and fluid inclusion waters (d 18 OFI and dDFI) were analysed from Profitis Ilias, a low-sulphidation epithermal gold mineralisation deposit on Milos island, Greece, to establish if d 18 OFI preserve a record of paleogeothermal processes. Previous studies show that mineralisation at Profitis Ilias resulted from extreme boiling and vaporisation with a zone located at approximately 430 m above sea level (asl) representing the transition between liquid- and vapor-dominated systems [Miner. Depos. 36 (2001) 32]. The deposit is also closely associated with an active geothermal system, whose waters have a well-characterised stable isotope geochemistry [Pflumio, C., Boulegue, J., Liakopoulos, A., Briqueu, L., 1991. Source, Transport and Deposition of Metals. Balkema, Rotterdam]. The samples were collected over an elevation interval of 440 m (210–650 m asl) to give information on the liquid and vapor dominated sections of the paleosystem. The data show systematic variations with sample elevation. Samples from the highest elevations (ca. 650 m asl) have the lightest d 18 OFI ( � 7.3x) and dDFI ( � 68.0x) whilst the deepest (ca. 210 m asl) are isotopically heavier (d 18 OFI, � 0.3x; dDFI, � 19.0x). Relative changes in d 18 OFI closely parallel those in dDFI. d 18 Oquartz shows an opposite trend, from the lightest values (+13.9x) at the lowest elevations to the heaviest (+15.1x) at the highest elevations. d 18 OFI shows correlations with other parameters. For example, variable fluid inclusion homogenisation temperatures in the vapor-dominated part of the system correlate with a rapid shift in dDFI ( � 33.3xto � 50.5x) and d 18 OFI ( � 4.1xto � 6.2x). Gold contents also increase in the same zone (up to 50 ppm Au). Comparable correlations in d 18 Oquartz or d 18 Ocalculated (estimated geothermal fluid from fluid inclusion homogenisation data) are absent. d 18 Ocalculated are always 5–10xheavier than d 18 OFI. Comparison with the present day geothermal field shows that dDFI and d 18 OFI are similar. Isotope data for the modern system and fluid inclusion waters fall on linear trends subparalleling the meteoric water line and project towards seawater values. Numerical modelling favours kinetically controlled fractionation to explain differences in d 18 Ocalculated and d 18 Ofluid rather than diffusive posttrapping reequilibration. The evidence suggests that in low-temperature epithermal systems, d 18 OFI may represent

How the Neoproterozoic S-isotope record illuminates the genesis of vein gold systems: an example from the Dalradian Supergroup in Scotland

Abstract The genesis of quartz vein-hosted gold mineralization in the Neoproterozoic–early Palaeozoic Dalradian Supergroup of Scotland remains controversial. An extensive new dataset of S-isotope analyses from the Tyndrum area, together with correlation of the global Neoproterozoic sedimentary S-isotope dataset to the Dalradian stratigraphy, demonstrates a mixed sedimentary and magmatic sulphur source for the mineralization. δ34S values for early molybdenite- and later gold-bearing mineralization range from −2 to +12‰, but show distinct populations related to mineralization type. Modelling of the relative input of magmatic and sedimentary sulphur into gold-bearing quartz veins with δ34S values of +12‰ indicates a maximum of 68% magmatic sulphur, and that S-rich, SEDEX-bearing, Easdale Subgroup metasedimentary rocks lying stratigraphically above the host rocks represent the only viable source of sedimentary sulphur in the Dalradian Supergroup. Consequently, the immediate host rocks were not a major source of sulphur to the mineralization, consistent with their low bulk sulphur and lack of metal enrichment. Recent structural models of the Tyndrum area suggest that Easdale Subgroup metasedimentary rocks, enriched in 34S, sulphur and metals, are repeated at depth owing to folding, and it is suggested that these are the most likely source of sedimentary sulphur, and possibly metals, for the ore fluids.

Application of airborne LiDAR data and airborne multispectral imagery to structural mapping of the upper section of the Troodos ophiolite, Cyprus

Structural maps are traditionally produced by mapping features such as faults, folds, fabrics, fractures and joints in the field. However, large map areas and the spatially limited ground perspective of the field geologist can potentially increase the likelihood that not all structural features will be identified within a given area. The ability to recognise and map both local and regional structural features using high-resolution remote sensing data provides an opportunity to complement field-based mapping to help generate more comprehensive structural maps. Nonetheless, vegetation cover can adversely affect the extraction of structural information from remotely sensed data as it can mask the appearance of subtle spectral and geomorphological features that correspond to geological structures. This study investigates the utility of airborne Light Detection And Ranging (LiDAR) data and airborne multispectral imagery for detailed structural mapping in vegetated ophiolitic rocks and sedimentary cover of a section of the northern Troodos ophiolite, Cyprus. Visual enhancement techniques were applied to a 4-m airborne LiDAR digital terrain model and 4-m airborne multispectral imagery to assist the generation of structural lineament maps. Despite widespread vegetation cover, dykes and faults were recognisable as lineaments in both data sets, and the predominant strike trends of lineaments in all resulting maps were found to be in agreement with field-based structural data. Interestingly, prior to fieldwork, most lineaments were assumed to be faults, but were ground-verified as dykes instead, emphasising the importance of ground-truthing. Dyke and fault trends documented in this study define a pervasive structural fabric in the upper Troodos ophiolite that reflects the original sea-floor spreading history in the Larnaca graben. This structural fabric has not previously been observed in such detail and is likely to be continuous in adjacent regions under sedimentary cover. This information may be useful to future exploration efforts in the region focused on identification of structurally controlled mineral and groundwater resources. Overall, our case study highlights the efficacy of airborne LiDAR data and airborne multispectral imagery for extracting detailed and accurate structural information in hard-rock terrain to help complement field-based mapping.

The Impact of Vegetation on Lithological Mapping Using Airborne Multispectral Data: A Case Study for the North Troodos Region, Cyprus

Vegetation cover can affect the lithological mapping capability of space- and airborne instruments because it obscures the spectral signatures of the underlying geological substrate. Despite being widely accepted as a hindrance, few studies have explicitly demonstrated the impact vegetation can have on remote lithological mapping. Accordingly, this study comprehensively elucidates the impact of vegetation on the lithological mapping capability of airborne multispectral data in the Troodos region, Cyprus. Synthetic spectral mixtures were first used to quantify the potential impact vegetation cover might have on spectral recognition and remote mapping of different rock types. The modeled effects of green grass were apparent in the spectra of low albedo lithologies for 30%–40% fractional cover, compared to just 20% for dry grass cover. Lichen was found to obscure the spectra for 30%–50% cover, depending on the spectral contrast between bare rock and lichen cover. The subsequent impact of vegetation on the remote mapping capability is elucidated by considering the outcomes of three airborne multispectral lithological classifications alongside the spectral mixing analysis and field observations. Vegetation abundance was found to be the primary control on the inability to classify large proportions of pixels in the imagery. Matched Filtering outperformed direct spectral matching algorithms owing to its ability to partially unmix pixel spectra with vegetation abundance above the modeled limits. This study highlights that despite the limited spectral sampling and resolution of the sensor and dense, ubiquitous vegetation cover, useful lithological information can be extracted using an appropriate algorithm. Furthermore, the findings of this case study provide a useful insight to the potential capabilities and challenges faced when utilizing comparable sensors (e.g., Landsat 8, Sentinel-2, WorldView-3) to map similar types of terrain.

Unusual mixed silica–carbonate deposits from magmatic–hydrothermal hot springs, Savo, Solomon Islands

Abstract: The volcanic island of Savo, Solomon Islands, hosts an active hydrothermal system discharging unusual alkaline (pH 7–8) sulphate-rich, chloride-poor fluid, with variable admixtures of Ca–Mg–HCO3−-rich fluid. Hot springs and their outflow streams precipitate a variety of deposits, including travertine, silica sinter and unusual mixed silica–carbonate rocks. Travertine fabrics are dominated by ray-crystal calcite, associated with rapid abiotic precipitation from a supersaturated solution. Sinter is produced by evaporation of thermal waters, and downstream samples contain preserved traces of micro-organisms, which potentially acted as templates for precipitation. Trace element chemistry of sinters and travertines includes anomalously high levels of Te, indicating a magmatic origin for a component fluid in the hydrothermal system. Springs are close to or at saturation with both calcite and amorphous silica. Increased contributions from the Ca–Mg–HCO3− end-member favours calcite formation; this fluid is of low-temperature origin, and as such is favoured by high rainfall. Mixed samples show cyclical changes between silica and carbonate precipitation, potentially as a result of seasonal variation in rainfall. Supplementary material: A spreadsheet of water and deposit chemical analyses is available at www.geolsoc.org.uk/SUP18495.

The origin and distribution of critical metals (Pd, Pt, Te and Se) within the Skouries Cu–Au porphyry deposit, Greece

Component Analysis was used to reduce noise from spurious pixels associated with high water content in peat. These spurious pixels were subsequently removed using a supervised classification (Minimum-Distance). The data were standardised to zero mean and equal variance. An unsupervised classification (K-means) was used to automatically and objectively classify the remaining data in 9-dimensional space (based on the input variables). Six classes were found to be optimum for delineating geochemical variations within the granites. Here, we present this new classification of the granites using continuous data covering the whole batholith. We also consider how to incorporate ground-based geochemical and satellite multispectral (Landsat) data into the classification.

An assessment of global resources of rocks as suitable raw materials for carbon capture and storage by mineralisation

Abstract Carbon capture and storage by mineralisation (CCSM) is a method proposed for capturing CO2 by reacting it with magnesium in ultramafic rocks to form carbonate minerals and silica. Large quantities of magnesium silicate rocks are required for this process and to demonstrate the feasibility, and adequately plan for the development and supply of mineral resources, their locations and quantities must be known. This study attempts to globally define the spatial extent and quantity of resources that could be used for the CCSM processes and to assess, if based on resources, this could be a viable, widely applicable CO2 sequestration process. It has been estimated that around 90 teratonnes of material is available. This is sufficient to capture global CO2 emissions for over 700 years at current levels of output and highlights the enormous resource. Even if only a small part is utilised, it could make a significant impact on CO2 reduction. The majority of the resource is contained within ophiolitic rocks. The study further attempts to split CCSM resources into altered (serpentine-rich rocks) and unaltered (olivine-rich rocks) due to the different processing requirements for these rock types. Carbon capture and storage by mineralisation is likely to be of most use in areas with no access to underground geological CO2 storage or for small operations where underground storage is not practical. This study demonstrates that substantial resources are available and their supply is unlikely to be a constraint.