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

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

Magmatic Cu-Ni-PGE-Au sulfide mineralisation in alkaline igneous systems: An example from the Sron Garbh intrusion, Tyndrum, Scotland

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

Alkaline fluids in a volcanic-hydrothermal system : Savo, Solomon Islands

Savo volcano, Solomon Islands, is host to an active hydrothermal system that is potentially analogous to high sulfidation epithermal Au deposits. Chemical and stable isotope data from fluids discharged at the surface indicate a relatively shallow condensate layer fed by magmatic volatiles including SO2. Acidic condensates are buffered to high pH by wall rock reaction, leading to the precipitation of unusual carbonate-silica sinters at the surface, in an environment where low pH fluids and associated products would be expected

Anomalous alkaline sulphate fluids produced in a magmatic hydrothermal system : Savo, Solomon Islands

In magmatic-hydrothermal and associated geothermal systems, acidic magmatic-derived fluids (pH5) geothermal fluids are typically limited to lateral outflows some distance from the main vent. Here we describe an unusual hydrothermal system associated with Savo volcano, a recently active (1830–40) trachyte-dominated island arc stratovolcano in the Solomon Islands. Hot springs (~100°C) near to the volcanic crater discharge alkaline waters instead of the more commonly recognised acidic fluids. The hydrothermal system of Savo dominantly discharges sinter and travertine-forming alkaline sulphate (pH 7–8) waters at hot springs on its upper flanks, in addition to a small number of lower discharge acid sulphate springs (pH 2–7). Alkaline sulphate springs discharge dilute, chloride-poor (600 mg/l) and silica-rich (>250 mg/l) fluids. They have restricted δ34SSO4 (5.4 ± 1.5‰) and δ18OH2O values (−4‰; local non-thermal groundwater is −8‰). Acid sulphate springs discharge low chloride (<20 mg/l), high sulphate (300–800 mg/l) waters, with variable silica (100–300 mg/l) and distinctly lower δ34SSO4 values (−0.6 ± 2.5‰) compared to the alkaline sulphate fluids. They also display high δ18OH2O and δDH2O relative to non-thermal groundwater. Geochemical modelling shows that water–rock reaction and dilution in the presence of secondary anhydrite, pyrite and quartz leads to chloride being diluted to low concentrations, whilst maintaining high sulphate and silica concentrations in the fluid. Strontium, oxygen and hydrogen isotopes confirm water–rock reaction and mixing with groundwater as primary controls on the composition of the alkaline sulphate springs. The highly unusual dilute chemistry of all discharges at Savo is a consequence of high regional rainfall, i.e. climatic control, and results from open system mixing at depth between hydrothermal and meteoric waters

An investigation of closure temperature of the biotite Rb-Sr system: The importance of cation exchange

Factors controlling closure in the biotite Rb-Sr system were investigated in a detailed study of an amphibolite-facies metacarbonate from the central Swiss Alps. Oxygen isotope data suggest that the rock cooled as a closed system. Calcite- dolomite thermometry temperatures of similar to 450 degreesC to 500 degreesC and feldspar thermometry temperatures of similar to 300 degreesC to 400 degreesC provide evidence of extensive Ca-Mg and Na-K exchange during cooling. Biotite in the sample is 90 mol.% phlogopite and has high Rb (similar to 900 ppm) compared to Sr (similar to0.3 ppm), giving precise Rb-Sr ages. Carefully separated and sized phlogopite shows a range of Rb-Sr ages that do not simply decrease with grain size as predicted by current models of closure temperature. Rb-Sr ages decrease from 18.1 Ma to 16.6 Ma with a decrease in mean grain diameter from 1.16 mm to 0.74 mm, but grains with mean diameter of 0.54 mm show an increase again to 17.6 Ma. This contrasts with Ar-Ar data for single phlogopites, which do show a decrease in age with decreasing grain size. The Rb-Sr age pattern is due to Rb- loss during cooling, which is most pronounced in the finest fraction. The phlogopites are restricted to a a-cm-thick layer in calcite marble; Sr-87/Sr-86 of the calcite decreases away from the phlogopite band over 4 cm, indicating that the calcite was moving towards Sr-isotope equilibration with the phlogopites over this distance and that the phlogopite was not equilibrating with an "infinite reservoir." Ion microprobe traverses across grains of different minerals reveal systematic core-rim variations in major and trace element concentrations. In particular, Sr decreases from calcite core to rim, but increases from core to rim in K-feldspar, whereas Rb decreases from core to rim in phlogopite but also increases from core to rim in K-feldspar. These gradients are interpreted as indicating the direction of transport of elements during cooling as a result of cation exchange reactions; calcite and phlogopite were sources for Sr and Rb, respectively, whereas K- feldspar acted as a sink for both elements. This chemical equilibration was taking place at the same time as isotopic equilibration during cooling, and was equally important in controlling the apparent ages recorded by the mica grains. In contrast, closure temperature calculations for geochronological systems based on classic Dodsontype models assume parent and daughter element concentrations are homogeneous across grains and do not change with time, only isotopic exchange is modeled. Closure in mica Rb-Sr systems will depend both on the factors that control isotopic exchange (grain size, mode, Sr-87 diffusion coefficients) and those that control chemical exchange (grain size, mode, Rb and Sr diffusion coefficients, Rb and Sr contents of phases and their partition coefficients)

A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H2O-CO2-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V1) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 &#176;C and 0.7–1.8 kbar. Homogeneous quartz &#948;18O across the batholith (9.5 ± 0.4&#8240;n = 12) suggests V1 precipitation at high temperatures (perhaps 600 &#176;C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V1 indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H2O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 &#176;C), high &#948;D (−18 ± 2&#8240;), low &#948;18O (0.5–2.0&#8240;) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V2) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H2O-NaCl-CaCl2-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 &#176;C), &#948;D (−17 to −45&#8240;), &#948;18O (−3 to + 1.2&#8240;), &#948;13CCO2 (−19 to 0&#8240;) and &#948;34Ssulphate (13–23&#8240;) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V3). Correlations of salinity, temperature, &#948;D and &#948;18O are interpreted as the result of mixing of two fluid end-members, one a high-&#948;D (−17 to −8&#8240;), moderate-&#948;18O (1.2–2.5&#8240;), high-&#948;13CCO2 (&#62; −4&#8240;), low-&#948;34Ssulphate (13&#8240;), high-temperature (205–230 &#176;C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-&#948;D (−61 to −45&#8240;), low-&#948;18O (−5.4 to −3&#8240;), low-&#948;13C (&#60;−10&#8240;), high-&#948;34Ssulphate (20–23&#8240;) low-temperature (80–125 &#176;C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V3 veins are late Triassic; the high-&#948;D end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-&#948;D end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated

Tracing carbon: natural mineral carbonation and the incorporation of atmospheric vs. recycled CO2

Mineral carbonation is a process whereby CO2 reacts with ultramafic rocks to form carbonate minerals such as calcite (CaCO3) and magnesite (MgCO3). This process can be induced artificially at high pressures and temperatures and therefore has potential to be adapted as a carbon capture and storage (CCS) technology. Large-scale surface and subsurface carbonate deposits of probable Quaternary age are associated with major faulting across the Oman-UAE ophiolite. Here, fractured rock forms a natural fluid pathway and increases the surface area available for carbonation. Modern springs along these faults typically discharge hyperalkaline (pH ~11), Ca(OH)2-rich waters that precipitate carbonates on reaction with atmospheric CO2. Carbonates formed by absorption of atmospheric CO2 into Ca(OH)2 13 13 produ 13C end member with other carbon sources such as limestones or organic-derived soil bicarbonate. Strontium isotope ratios of samples indicate fluids that formed calcite and magnesite veins may have interacted with limestones around and beneath the ophiolite. These are a carbon source which can easily be reworked and incorporated into carbonate deposits elsewhere. Carbonate deposits may not be created solely from atmospheric CO2, but instead represent a mixture of carbon sources. Failure to account for multiple carbon sources or recycled carbon may result in poor estimates of the rates and volumes of carbon that natural systems sequester. Further investigation is therefore necessary to determine how much of the carbon held within carbonate deposits has been incorporated from reworked sources