Ascent Of Fluids Research Articles

Structural architecture and active deformation pattern in the northern sector of the Aeolian-Tindari-Letojanni fault system (SE Tyrrhenian Sea-NE Sicily) from integrated analysis of field, marine geophysical, seismological and geodetic data

Framed in the current geodynamics of the central Mediterranean, the Aeolian-Tindari-Letojanni fault system is part of a wider NW-SE oriented right-lateral wrench zone which accommodates diverging motion between regional-scale blocks located at the southern edge of the Calabrian Arc. In order to investigate the structural architecture and the active deformation pattern of the northern sector of this tectonic feature, structural observations on-land, high and very-high resolution seismic reflection profiles, swath bathymetry and seismological and geodetic data were merged from the Lipari-Vulcano volcanic complex (central sector of the Aeolian Islands) to the Peloritani Mountains across the Gulf of Patti. Our interpretation shows that the active deformation pattern of the study area is currently expressed by NW-SE trending, right-transtensional en-echelon fault segments whose overlapping gives rise to releasing stepover and pull-apart structures. This structural architecture has favored magma and fluid ascent and the shaping of the Lipari-Vulcano volcanic complex. Similarly, the Gulf of Patti is interpreted as an extensional relay zone between two overlapping, right-lateral NW-SE trending master faults. The structural configuration we reconstruct is also supported by seismological and geodetic data which are consistent with kinematics of the mapped faults. Notably, most of the low-magnitude instrumental seismicity occurs within the relay zones, whilst the largest historical earthquakes (1786, Mw=6.2; 1978, Mw=6.1) are located along the major fault segments.

Genesis of ultra-high-Ni olivine in high-Mg andesite lava triggered by seamount subduction

The Kamchatka Peninsula is a prominent and wide volcanic arc located near the northern edge of the Pacific Plate. It has highly active volcanic chains and groups, and characteristic lavas that include adakitic rocks. In the north of the peninsula adjacent to the triple junction, some additional processes such as hot asthenospheric injection around the slab edge and seamount subduction operate, which might enhance local magmatism. In the forearc area of the northeastern part of the peninsula, monogenetic volcanic cones dated at <1 Ma were found. Despite their limited spatiotemporal occurrence, remarkable variations were observed, including primitive basalt and high-Mg andesite containing high-Ni (up to 6300 ppm) olivine. The melting and crystallization conditions of these lavas indicate a locally warm slab, facilitating dehydration beneath the forearc region, and a relatively cold overlying mantle wedge fluxed heterogeneously by slab-derived fluids. It is suggested that the collapse of a subducted seamount triggered the ascent of Si-rich fluids to vein the wedge peridotite and formed a peridotite–pyroxenite source, causing the temporal evolution of local magmatism with wide compositional range.

New perspectives on volcano monitoring in a tropical environment: Continuous measurements of soil CO2 flux at Piton de la Fournaise (La Réunion Island, France)

AbstractDetecting renewal of volcanic activity is a challenging task and even more difficult in tropical settings. Continuous measurements of soil CO2 flux were carried out at the Piton de la Fournaise volcano during 2013–2016. Since this site is in the tropics, periods of heavy rainfall are in the norm. Measurements covered volcanic unrest after a hiatus of 3.5 years. We find that while temperature has the strongest effect, extreme rainfall causes short‐term noise. When corrected and filtered from the environmental influence, soil CO2 time series permit to detect a major deep magmatic event during March–April 2014, 3 months before the first eruption of the new activity phase. Correlation with geophysical data sets allows timing of further stages of upward fluid ascent. Our study validates soil CO2 flux monitoring in tropical environments as a valuable tool to monitor magma transfer and to enhance understanding of volcano unrest down to the lithospheric mantle.

Multielement statistical evidence for uraniferous hydrothermal activity in sandstones overlying the Phoenix uranium deposit, Athabasca Basin, Canada

The Phoenix U deposit, with indicated resources of 70.2 M lb U3O8, occurs along the unconformity between the Proterozoic Athabasca Group sandstones and the crystalline basement rocks. Principal component analysis (PCA) is applied to the compositions of sandstones overlying the deposit. Among PCs, PC1 accounts for the largest variability of U and shows a positive association of U with rare earth elements (REEs) + Y + Cu + B + Na + Mg + Ni + Be. The evidence suggests that U was dispersed into sandstones together with these elements during the uraniferous hydrothermal activity. Uranium shows an inverse association with Zr, Hf, Th, Fe, and Ti. Since they are common in detrital heavy minerals, such heavy minerals are not the major host of U. The elements positively associated with U are high in concentrations above the deposit, forming a “chimney-like” or “hump-like” distribution in a vertical section. Their enrichment patterns are explained by the ascent of basement fluids through faults to sandstones and the circulation of basinal fluids around the deposit. The Pb isotope compositions of whole rocks are similar to expected values calculated from the concentrations of U, Th, and Pb except for sandstones close to the deposit. The data suggest that in situ decay of U and Th is responsible for the Pb isotope compositions of most sandstones and that highly radiogenic Pb dispersed from the deposit to the proximal sandstones long after the mineralization. This secondary dispersion is captured in PC8, which has low eigenvalue. The data suggests that the secondary dispersion has minor effect on the overall lithogeochemistry of sandstones.

Hydrochemical and Isotopic Characteristics of CO2-rich Groundwater in the Gyeongsang Sedimentary Basin, South Korea: A Natural Analogue Study on the Potential Leakage of Geologically-stored CO2

The potential of geologic carbon storage has been considered in the Gyeongsang sedimentary basin (GSB) at the southeastern part of South Korea. In order to understand the long-term behaviors of geologically-stored CO2, hydrochemical and multi-isotopic data (δ18O-δD of water and δ13C of dissolved inorganic carbon) of groundwater samples were collected from naturally seeping CO2-rich springs and ambient shallow groundwater wells in the GSB. The collected samples were classified into three hydrochemical groups: 1) dilute and acidic, CO2-rich springs, 2) high TDS, CO2-rich springs, and 3) CO2-poor groundwater. CO2-rich springs (i.e., Groups 1 and 2) were differentiated from Group 3 by their high carbon isotope values and partial pressure of CO2, suggesting that the CO2 in Groups 1 and 2 originates from a deep-seated (magmatic) source. Compared to the CO2-rich springs of Group 1, major cations and HCO3-, together with Fe, Mn and Sr, are enriched in CO2-rich springs of Group 2 and these constituents are highly correlated with TDS. This indicates that, during the ascent of CO2-rich fluids via fractures, CO2-rich springs of Group 2 experienced substantial CO2-water-rock interactions, possibly through longer or restricted pathways, whereas CO2-rich springs of Group 1 acquired the chemistry through lower degrees of water-rock interaction via the rapid ascent through more permeable pathways. The results of principal component analysis (PCA) with compositional data demonstrated that pH can be used as a prior monitoring parameter to detect the direct leakage of CO2 as represented by Group 1. In addition, the changes of ionic composition can be used as a supplementary tool to monitor the indirect leakage of CO2 accompanying a substantial CO2-water-rock interaction.

Fluid-driven uplift at Long Valley Caldera, California: Geologic perspectives

Since persistent seismicity began in the Sierra Nevada adjacent to Long Valley caldera in 1978–1980, intracaldera unrest has been marked by (1) episodes of uplift totaling ~83cm, centered on the middle Pleistocene resurgent dome, and (2) recurrent earthquake swarms along a 12-km-long segment of the caldera's ring-fault zone that is contiguous with both the dome and the Sierran seismogenic domain. Others have attributed the recent unrest to magmatic intrusion(s), but it is argued here that evidence for new magma is lacking and that ongoing uplift and ring-fault-zone seismicity are both promoted by ascent of aqueous fluid released by second boiling of the residue of the enormous Pleistocene rhyolitic reservoir terminally crystallizing at depths ≥10km. For 2Myr, eruptive vent clusters migrated southwestward from Glass Mountain to Mammoth Mountain. There has been no eruption on the resurgent dome since 500ka, and since 230ka volcanism has been restricted to the caldera's west moat and contiguous Sierran terrain, both outside the structural caldera. High-temperature hydrothermal activity in the central caldera waned after ~300ka, cooling the Pleistocene rhyolitic focus to the extent that drilling on the resurgent dome found mid-caldera temperature to be only 100°C and isothermal at depths of 2–3km. Beneath most of the resurgent dome, there is little seismicity at any depth, no emission of magmatic CO2 or other magmatic gases, no elevated 3He/4He ratios, and only normal to below-normal heat flow. Most of the 75-km-long ring-fault zone is likewise aseismic, excepting only the 12-km segment contiguous with the extracaldera seismogenic domain in the Sierra. Since 1980, the Sierran seismicity has released 3.6 times more cumulative seismic energy than have intracaldera earthquakes. The caldera seismicity is not driven by stresses associated with the adjacent uplift but, instead, by the extracaldera tectonic stressfield. Sierran seismicity activated the directly contiguous south-moat segment of the ring-fault zone, which had originated in the caldera-forming eruption at 767ka and everywhere else remains sealed. Hypocenter relocation studies of 1000s of earthquakes along the seismic segment have resolved recurrent upward-migrating swarms within networks of cryptic faults, apparently triggered by rapidly ascending pulses of high-pressure low-viscosity aqueous fluid. Entering the brittle crust at depths of 8–10km, such fluid is just what should be expected from second boiling of the late-stage CO2-poor rhyolitic residue. The fluid provides the pressure source above the apex of the crystallizing caldera-wide pluton and then escapes laterally to the newly reactivated southern segment of the ring-fault zone, its only available permeable pathway, where it mediates the ongoing south-moat seismicity.

Structural controls on fluid circulation at the Caviahue-Copahue Volcanic Complex (CCVC) geothermal area (Chile-Argentina), revealed by soil CO2 and temperature, self-potential, and helium isotopes

Natural geothermal systems are limited areas characterized by anomalously high heat flow caused by recent tectonic or magmatic activity. The heat source at depth is the result of the emplacement of magma bodies, controlled by the regional volcano-tectonic setting. In contrast, at a local scale a well-developed fault-fracture network favors the development of hydrothermal cells, and promotes the vertical advection of fluids and heat. The Southern Volcanic Zone (SVZ), straddling Chile and Argentina, has an important, yet unexplored and undeveloped geothermal potential. Studies on the lithological and tectonic controls of the hydrothermal circulation are therefore important for a correct assessment of the geothermal potential of the region. Here, new and dense self-potential (SP), soil CO2 and temperature (T) measurements, and helium isotope data measured in fumaroles and thermal springs from the geothermal area located in the north-eastern flank of the Copahue volcanic edifice, within the Caviahue Caldera (the Caviahue-Copahue Volcanic Complex - CCVC) are presented. Our results allowed to the constraint of the structural origin of the active thermal areas and the understanding of the evolution of the geothermal system. NE-striking faults in the area, characterized by a combination of SP, CO2, and T maxima and high 3He/4He ratios (up to 8.16±0.21Ra, whereas atmospheric Ra is 1.382×10−6), promote the formation of vertical permeability preferential pathways for fluid circulation. WNW-striking faults represent low-permeability pathways for hydrothermal fluid ascent, but promote infiltration of meteoric water at shallow depths, which dilute the hydrothermal input. The region is scattered with SP, CO2, and T minima, representing self-sealed zones characterized by impermeable altered rocks at depth, which create local barriers for fluid ascent. The NE-striking faults seem to be associated with the upflowing zones of the geothermal system, where the boiling process produces a high vapor-dominated zone close to the surface, whereas the WNW-striking faults could act as a boundary of the Copahue geothermal area to the south.

Architecture and temporal variations of a terrestrial CO2 degassing site using electric resistivity tomography and self-potential

The Hartousov mofette field in NW Bohemia, Czech Republic, is characterized by strong CO2 degassing from the Lithospheric Mantle. In a test survey using electrical resistivity tomography, we observed changes over time in the subsurface structure beneath heavily CO2 degassing spots to depths of about 40 m and compared them to CO2 gas flux and soil gas mappings from previous studies. Changes in the electrical resistivity were measured between September 2012 and October 2013 at irregular intervals and highlight large variations in the resistivity distribution where the CO2 degassing was strongest (56 × 103 g m−2 day−1), indicating a fluid-induced change of properties of the underlying clayey sediments. Positive and negative anomalies in the electric self-potential parallel to the ERTs can be found where CO2 degassing occurs which indicates varying ascent or descent of fluids within these spots. Some degassing spots seem not to be actively degassing continuously over time, which can also be observed by other studies in the same area. We suggest that future gas mappings are accompanied by methods that observe the state of fluid systems at subsurface over time, e.g. electric resistivity tomography and self potential.

Monitoring crustal changes at volcanoes by seismic noise interferometry: Mt. Etna case of study

In this work, we analysed the seismic noise recorded at Mt. Etna by 18 stations during the interval 2007–2015 in the frequency band 0.1–0.3Hz, chosen to avoid contamination from volcanic tremor. Variations in time of medium seismic velocity in the range −0.8 to 0.8% were found, mostly affecting the stations located on the volcano summit and flanks. Based on the investigated frequency content, the Δv/v changes took place from the surface to a depth of ~4.5–6.5km. To identify the source mechanism of the observed medium changes, the variations were quantitatively compared by wavelet transform coherence with volcano-tectonic and meteorological parameters. A significant relationship with meteorological parameters with seasonal periodicity (especially air temperature and snow loading) was found, probably caused by thermo-elastic strain and increasing-decreasing surface loading cycles. Moreover, a sharp medium velocity decrease, taking place in mid-December 2009 and clearly time-related to the largest volcano-tectonic strain release phenomenon of the investigated period, was also found. Such a velocity decrease was interpreted as resulting from ascent of fluids and gas exsolution taking place at the same time as the volcano-tectonic swarm.

Fracture network, fluid pathways and paleostress at the Tolhuaca geothermal field

In this study, we examine the fracture network of the Tolhuaca geothermal system located in the Southern Andean volcanic zone that may have acted as a pathway for migration and ascent of deep-seated fluids under the far/local stress field conditions of the area. We collected the orientation, slip-data and mineralogical content of faults and veins recovered on a ca. 1000 m deep borehole (Tol-1) located in the NW-flank of the Tolhuaca volcano. Tol-1 is a non-oriented, vertical borehole that recovered relatively young (<1 Ma) basaltic/andesitic volcanic rocks with subordinate pyroclastic/volcanoclastic interbedded units of Pleistocene age. Here, we examined and measured the inclination, geometry, texture, mineralogy, and relative sense of displacement of veins and faults. To determine the actual azimuthal orientation of fault and veins we reoriented 66 segments (89 standard mini-cores) of Tol-1 using stable Characteristic remanent magnetization component (ChRM) obtained by thermal demagnetization methodology. Paleo-declination of ChRM vectors was used to re-orient the borehole pieces, as well as fault and veins, to a common anchor orientation value consistent with the Geocentric Axial Dipole approximation (GAD). Inversion of RM-corrected fault-slip data reveals a local tensional stress field with a vertically oriented σ1 axis (083/74) and a subhorizontal, NS-trending σ3 axis (184/03). Within the topmost 400 m of the borehole, faults and veins are randomly oriented, whereas below 400 m depth, faults and veins show preferential NE-to EW-strikes and steep (>50°) dips. The EW-striking veins are compatible with the calculated local stress field whereas NE-striking veins are compatible with the regional stress field, the morphological elongation of volcanic centers, alignments of flank vents and dikes orientation. Our results demonstrate that the paleomagnetic methodology proved to be reliable and it is useful to re-orient vertical boreholes such as Tol-1. Furthermore, our data show that the bulk transpressional regional stress field has local variations to a tensional stress field within the NE-striking fault zone belonging to the Liquiñe-Ofqui Fault System, favoring the activation of both NW- and NE-striking pre-existent discontinuities, especially the latter which are favorably oriented to open under the prevailing stress field. The vertical σ1 and NS-trending subhorizontal σ3 calculated in the TGS promote the activation of EW-striking extensional veins and both NE and NW-striking hybrid faults, constituting a complex fluid pathway geometry of at least one kilometer depth.

Structural Control of Auriferous Mineralization in the Birimian: Case of the Agbahou Deposit in the Region of Divo, C&amp;amp;ocirc;te d’Ivoire

Located in the southwestern of the Oume-Fettekro greenstone belt, the Agbahou gold deposit is controlled by structural factors. Geophysics, teledetection and core data suggest the presence of NE and NW faults. However, the NE-faults define two major shear zones (ATZ: Agbahou Tectonic Zone and WTZ: West Tectonic Zone) that control the Agbahou gold mineralization. These first order structures are subparallel to the regional tectonic grain mostly north-east oriented. They seem to respectively develop on the both arms (eastern and western) of an anticline moderately plunging ~25° towards northeast. Each shear-zone contains several second-order shear-zones or lenses of variable direction and of 50° - 80° dip. NW-faults however correspond to strike-slip faults and their development should be related to transcurrent tectonics. They acted as control channels on the distribution of gold mineralizations. The ductile-brittle character of shear-zones favored the ascent of hydrothermal fluids and the formation of multiple auriferous quartz veins: veins Type IIa and veins Type IIb relating respectively to the shear-veins and extensional veins. However, Agbahou also shows the existence of a disseminated sulphides-bearing mineralization within host-rocks. At Agbahou, the precipitation of gold probably occurred in a post to late ductile-brittle deformation period.

87Sr/86Sr Ratios in Shallow and Deep Aquifers and Thermal Water from the Eastern Boundary Fault of the Northern Upper Rhine Graben at the Heidelberg Basin, Germany

Strontium isotopic ratios are important tracers of hydrological processes such as water-rock interaction and the detection of the ascent of deep fluids at hydraulically active faults. The 87Sr/86Sr ratios have been measured in shallow and deep aquifers (n=19) and thermal water at the Eastern Boundary Fault (EBF) of the northern Rhine Graben Rift in Germany. 87Sr/86Sr ratios and Sr contents from Quaternary groundwater conducting sediments of the shallow aquifers range from 0.70896 to 0.70965 and 0.23 to 0.67 mg/l, respectively. Strontium contents of the shallow groundwater are low and highly variable in their 87Sr/86Sr ratios. No unequivocally hints of ascending deep fluids at the EBF fault have been found in the shallow aquifers of the Heidelberg Basin. However, groundwater from production wells of up to 205 m deep aquifers and the 1100 m deep thermal well show radiogenic 87Sr/86Sr ratios and high Sr contents in the range from 0.70984 to 0.71074 and 0.39 to 319 mg/l, respectively. The radiogenic Sr ratios are clear indications for ascending deep fluids into deep aquifers at the EBF fault of the northern upper Rhine Graben.

Ascending Deep Fluids into Shallow Aquifer at Hydraulically Active Segments of the Western Boundary Fault of the Rhine Graben, Germany: Constraints from 87Sr/86Sr ratios

87Sr/86Sr ratios have been used as a tracer of fault-controlled fluid ascent into shallow aquifers at the Western Boundary Fault (WBF) of the northern upper Rhine Graben in Germany, a major rift system of the European continent. The 87Sr/86Sr ratios and Sr contents from the Quarternary groundwater conducting sediments of the uppermost aquifer increase with decreasing distance to the WBF. The 87Sr/86Sr ratios and Sr contents of the shallow groundwater range from 0.70877 to 0.71022 and 0.41 to 2.30 mg/l, respectively. The Strontium isotope ratios are important tracers of hydrological processes such as water-rock interaction and groundwater mixing at active faults and thus a powerful geochemical tool for the detection of the ascent of deep fluids at hydraulically active segments of faults and its mixing with shallow recharged groundwater.

The chemistry and isotopic composition of waters in the low-enthalpy geothermal system of Cimino-Vico Volcanic District, Italy

Geothermal energy exploration is based in part on interpretation of the chemistry, temperature, and discharge rate of thermal springs. Here we present the major element chemistry and the δD, δ18O, 87Sr/86Sr and δ11B isotopic ratio of groundwater from the low-enthalpy geothermal system near the city of Viterbo in the Cimino-Vico volcanic district of west-Central Italy. The geothermal system hosts many thermal springs and gas vents, but the resource is still unexploited. Water chemistry is controlled by mixing between low salinity,HCO3-rich fresh waters (<24.2°C) flowing in shallow volcanic rocks and SO4-rich thermal waters (25.3°C to 62.2°C) ascending from deep, high permeability Mesozoic limestones. The (equivalent) SO4/Cl (0.01–0.02), Na/Cl (2.82–5.83) and B/Cl ratios (0.02–0.38) of thermal waters differs from the ratios in other geothermal systems from Central Italy, probably implying a lack of hydraulic continuity across the region. The δ18O (−6.6‰ to −5.9‰) and δD (−40.60‰ to −36.30‰) isotopic composition of spring water suggest that the recharge area for the geothermal system is the summit region of Mount Cimino. The strontium isotope ratios (87Sr/86Sr) of thermal waters (0.70797–0.70805) are consistent with dissolution of the Mesozoic evaporite-carbonate units that constitute the reservoir, and the ratios of cold fresh waters mainly reflect shallow circulation through the volcanic cover and some minor admixture (<10%) of thermal water as well. The boron isotopic composition (δ11B) of fresh waters (−5.00 and 6.12‰) is similar to that of the volcanic cover, but the δ11B of thermal waters (−8.37‰ to −4.12‰) is a mismatch for the Mesozoic reservoir rocks and instead reflects dissolution of secondary boron minerals during fluid ascent through flysch units that overlie the reservoir. A slow and tortuous ascent enhances extraction of boron but also promotes conductive cooling, partially masking the heat present in the reservoir. Overall data from this study is consistent with previous studies that concluded that the geothermal system has a large energy potential.

Heterogeneous melt and hypersaline liquid inclusions in shallow porphyry type mineralization as markers of the magmatic-hydrothermal transition (Cerro de Pasco district, Peru)

Recently identified occurrences of porphyry-style mineralization evidence the link of the world's second largest known epithermal base metal Cerro de Pasco deposit (Peru) to a porphyry system emplaced at depth. They consist of (i) quartz-monzonite dykes and (ii) the south-western part of the large diatreme-dome complex adjacent to the main ore bodies of Cerro de Pasco, and (iii) stockwork of banded quartz-magnetite-chalcopyrite-(pyrite) porphyry-type veinlets crosscutting trachyte porphyritic intrusion cropping out at surface in the central part of the diatreme-dome complex. The latter porphyry-type mineralization observed at the same erosion level as the main epithermal base metal carbonate-replacement ore bodies is the subject of this work. Geological constraints indicate a shallow emplacement level (depth<1km, P<270bar), implying rather unusual low-pressure and high-temperature environment for the formation of porphyry-style mineralization.The banded porphyry-type veinlets record a multiphase history of formation with two successive high-temperature (>600°C) stages, followed by a lower-temperature (<350°C) stage. More than 90% of the quartz in veinlets precipitates during the first two high-temperature stages. Stage 1 is characterized by the entrapment in hydrothermal quartz of inclusions containing variable proportions of both silicate melt and metal-rich hypersaline (>90wt% NaCl eq.) liquid, hereafter referred to as heterogeneous silicate melt inclusions (HSMIs). The latter are rarely described in porphyry-type mineralization. We suggest that during stage 1, the inclusions result from heterogeneous entrapment of an evolved hydrous rhyolitic melt mixed with a hypersaline fluid phase at low pressure (270bar) and high temperature (>600°C). The stage 2 is marked by the entrapment of metal- and sulfur-rich hypersaline liquid inclusions, with salinity around 70wt% NaCl eq., originated from the adiabatic ascent of magmatic hypersaline fluids transferred from deeper parts of the system. The lower-temperature stage 3 is characterized by an important temperature drop from >600°C to <350°C as revealed by microthermometry of aqueous two-phase liquid-vapor (L-V) inclusions. Quartz textures revealed by SEM-CL imaging allow ascribing the sulfide precipitation to the low-temperature mineralization stage 3. In-situ SIMS 18O/16O isotope analyses of quartz across the veinlets are indicative of a magmatic signature of the fluids during the first two stages; while quartz from stage 3 has oxygen isotopic compositions suggestive of minor contribution of meteoric waters to a predominantly magmatic aqueous fluid (~10vol.% of meteoric input), which probably triggered Cu-Fe sulfide precipitation in the stockwork. High metal and sulfur contents of HSMIs and hypersaline liquid inclusions determined by LA-ICP-MS are interpreted to represent the fluid composition prior to the main sulfide precipitation event. The similar Pb-Zn ratio of the bulk ore extracted from the epithermal ore bodies at Cerro de Pasco and the HSMIs and hypersaline liquid inclusions suggest a common source of the fluids associated with the different mineralization styles at Cerro de Pasco.

New insight from noble gas and stable isotopes of geothermal/hydrothermal fluids at Caviahue-Copahue Volcanic Complex: Boiling steam separation and water-rock interaction at shallow depth

We measured noble gas and stable isotopes of the geothermal and hydrothermal fluids of the Caviahue-Copahue Volcanic Complex (CCVC), one of the most important geothermal systems in Argentina/Chile, in order to provide new insights into fluid circulation and origin. With the exception of Anfiteatro and Chancho-co geothermal systems, mantle-derived helium dominates in the CCVC fluids, with measured 3He/4He ratios up to 7.86Ra in 2015. Their positive δ15N is an evidence for subducted sediment-derived nitrogen, which is commonly observed in subduction settings. Both He-N2-Ar composition and positive correlation between δD-H2O and δ18O-H2O suggest that the fluids from Anfiteatro and Chancho-co (and partly from Pucon-Mahuida as well, on the southern flank of Copahue volcano) represent a meteoric water composition with a minor magmatic contribution. The Ne, Kr and Xe isotopic compositions are entirely of atmospheric origin, but processes of boiling and steam separation have led to fractionation of their elemental abundances. We modeled the CCVC fluid evolution using Rayleigh distillation curves, considering an initial air saturated geothermal water (ASGW) end-member at 250 and 300°C, followed by boiling and steam separation at lower temperatures (from 200°C to 150°C). Between 2014 and 2015, the CCVC hydrogen and oxygen isotopes shifted from local meteoric water-dominated to andesitic water-dominated signature. This shift is associated with an increase of δ13C values and Stotal, HCl and He contents. These characteristics are consistent with a change in the gas ascent pathway between 2014 and 2015, which in turn induced higher magmatic-hydrothermal contribution in the fluid signature. The composition of the magmatic source of the CCVC fluids is: 3He/4He=7.7Ra, δ15N=+6‰, and δ13C=−6.5‰. Mixing models between air-corrected He and N suggest the involvement of 0.5% to 5% of subducted sediments in the magmatic source. The magmatic sulfur isotopic composition is estimated at −2.38‰ (from COP-2), but most samples show elemental fractionation due to boiling and steam separation followed by various degrees of atmospheric contamination. All these geochemical and isotopic characteristics are the direct consequence of tectonic particularities of the CCVC: NE faults promote the ascent of hydrothermal fluids in the geothermal area whereas WNW faults serve as preferential channels for meteoric water infiltration.

The thermal and chemical evolution of hydrothermal vent fluids in shale hosted massive sulphide (SHMS) systems from the MacMillan Pass district (Yukon, Canada)

At Macmillan Pass (YT, Canada), the hydrothermal vent complexes beneath two shale-hosted massive sulphide (SHMS) deposits (Tom, Jason) are well preserved within Late Devonian strata. These deposits provide a unique opportunity to constrain key geochemical parameters (temperature, salinity, pH, fO2, ΣS) that are critical for metal transport and deposition in SHMS systems, and to evaluate the interaction between hydrothermal fluids and the mudstone host rock. This has been achieved using a combination of detailed petrography, isotopic techniques (δ34S, δ13C and δ18O values), carbonate rare earth element analysis (LA-ICP-MS), fluid inclusion analysis (microthermometry, gas analysis via incremental crush fast scan mass spectrometry), and thermodynamic modelling.Two main paragenetic stages are preserved in both vent complexes: Stage 1 comprises pervasive ankerite alteration of the organic-rich mudstone host rock and crosscutting stockwork ankerite veining (±pyrobitumen, pyrite and quartz) and; Stage 2 consists of main stage massive sulphide (galena-pyrrhotite-pyrite±chalcopyrite-sphalerite) and siderite (±quartz and barytocalcite) mineralisation. Co-variation of δ18O and δ13C values in ankerite can be described by temperature dependent fractionation and fluid rock interaction. Together with fluid inclusion microthermometry, this provides evidence of a steep thermal gradient (from 300 to ∼100°C) over approximately 15m stratigraphic depth, temporally and spatially constrained within the paragenesis of both vent complexes and developed under shallow lithostatic (<1km; 250 bars) to hydrostatic (<400m; 40 bars) conditions. There is evidence of mixing between diagenetic and hydrothermal fluids recorded in chondrite-normalised rare earth element (REE) profiles of ankerite and siderite. Middle REE enrichments and superchondritic Y/Ho ratios (>28), characteristic of diagenetic fluids, are coupled with positive europium anomalies and variable light REE depletion, which are more consistent with chloride complexation in hot (>250°C) hydrothermal fluids. In this shallow sub-seafloor setting, thermal alteration of organic carbon in the immature, chemically reactive mudstones also had an important role in the evolution of fluid chemistry. Reduced sulphur generation via thermochemical reduction of Late Devonian seawater sulphate produced positive δ34S values in sulphide minerals (+7.5‰ to +19.5‰), coupled with a suite of volatile components (CO2, CH4, C1–C4 hydrocarbons, N2) trapped in Stage 2 quartz.Many of these geochemical features developed during the final stages of fluid ascent, in a system where the fluid cooled close to the site of mineralisation. Using this information, we have modelled the metal transporting capacity of the deep hydrothermal fluid, which even at modest salinities (6wt.% NaCl) was high (≫100ppm Pb, Zn), owing to the combined effects of high temperature and low pH (⩽4.5). Therefore in SHMS systems, enhanced geothermal gradients and rapid fluid ascent (with minimal fluid cooling) are considered to be the most important factors for transporting high concentrations of base metals to the site of mineralisation.

Mineralization mechanisms in the Shangfanggou giant porphyry-skarn Mo–Fe deposit of the east Qinling, China: Constraints from H–O–C–S–Pb isotopes

The giant Shangfanggou skarn-porphyry Mo–Fe deposit, one of the largest Mo deposits in the East Qinling Orogen, is hosted by the Neoproterozoic Luanchuan Group on the southern margin of the North China Craton. The mineralization occurs mainly as veinlets and in veinlet-disseminated form within altered schists, skarns, hornfelses, and/or marbles. The hydrothermal alteration and mineralization at Shangfanggou took place over four stages. Stage 1 involved potassic alteration and silicification in the porphyry, skarnization and magnetization in carbonate strata, and hornfelsization in schists. Quartz–molybdenite veins associated with phyllic alteration were produced in the porphyry during stage 2, along with sericitization, chloritization, serpentinization, and silicification in the skarns, hornfelses and/or marbles. Quartz–polymetallic sulfide veins related to phyllic alteration were produced during stage 3, and ore-barren quartz±calcite±pyrite veins associated with weak carbonatation were produced during stage 4. C, O, and D isotope compositions of hydrothermal calcite, dolomite, magnetite, phlogopite, and quartz from different stages of mineralization during the formation of the Shangfanggou deposit range from −12.9‰ to +8.8‰, −3.2‰ to +22.7‰, and −100‰ to −80‰, respectively. Sulfur isotope compositions of sulfides from the ores of the Shanfanggou deposit range from 3.6‰ to 4.4‰. 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 17.648 to 18.742 (apart from one magnetite with a value of 20.78), 15.526 to 15.690, and 38.197 to 39.823 in the Shanfanggou deposit. The evolution of the ore-forming fluids in the Shangfanggou deposit, as revealed by δ18O, δD, and δ13C isotope studies, was one of progressive modification of the magmatic fluids through fluid–wall rock interactions along conduits during fluid ascent, and this signifies the involvement of formation/strata water during the earlier stages of mineralization, and the predominance of meteoric waters over magmatic fluids in the later stages. The carbon, sulfur, and lead isotope compositions suggest mixing took place between the magmas and the host rocks within the region. Integrating the data obtained from studies of the regional geology, ore geology, fluid inclusions, and C–H–O–S–Pb isotope geochemistry, we conclude that the Shangfanggou Mo–Fe deposit is a porphyry-skarn type system that formed during a transition from compressional to extensional tectonics during the Jurassic–Early Cretaceous continental collision between the North China Craton and Yangtze Craton. The ore materials were sourced from magmatic ore-forming fluids that mixed with components derived from carbonaceous wall rocks during their ascent, and which were gradually mixed with meteoric waters, especially in the later stages.

Breccia formation by particle fluidization in fault zones: Implications for transitory, rupture-controlled fluid flow regimes in hydrothermal systems

Breccias in the Rusey Fault (Cornwall, UK) and the Roamane Fault (Porgera gold deposit, Papua New Guinea) provide insights about the dynamics of fluid flow and flow velocities when fault ruptures breach overpressured reservoirs of hydrothermal fluid. These faults contain cockade-like breccias in which rock fragments are mantled by spheroidal overgrowths of hydrothermal minerals. At Rusey, the overgrowths are quartz-dominant, whereas in the Roamane Fault overgrowths are composed of calcite, quartz, or gold-rich pyrite. Although none of the rock fragment cores of accretionary spheroids are in contact with their neighbours, the spheroidal overgrowths do contact each other and are at least partially cemented together. The hydrothermal overgrowths mostly comprise either outwards coarsening crystals that radiate from the surface of the core rock fragment, or finer-grained, inequigranular to mesh-like intergrowths. Concentric textural banding and oscillatory growth zones are present in some hydrothermal mantles. The breccias occur as fault-parallel layers and lenses, each up to several tens of centimeters thick. Adjacent layers are characterized by texturally-distinct ranges of clast sizes and different proportions of clasts to hydrothermal overgrowths. In the Rusey Fault, many texturally-distinct breccia layers are present within a 3m wide fault core. Some layers truncate others and many breccia layers exhibit grainsize grading or banding. Clasts in the breccias include fragments of wall-rock, veins and various fault damage products, including fragments of earlier generations of cemented breccia. Brecciation was episodic and separated by periods of cementation. The distinctive textures of the breccias are interpreted to have formed by fluidization of fault damage products in a high fluid flux regime. Hydrothermal coatings developed while clasts were in a suspended state during fluid ascent through dilatant fault segments. Layered breccias record multiple episodes of particle fluidization and indicate that the faults provided conduits for repeated transitory fluid upflow. Particle size distributions indicate that fluid velocities during fluidization were in the range 0.1 ms−1 to 1 ms−1. The maximum flow rates correspond to fluid fluxes of 10 to 100 L.s−1 per meter strike length of fault through dilatant fault apertures up to several tens of centimeters wide. Such high flow rates characteristically induce intense swarm seismicity rather than mainshock-aftershock seismicity, and have implications for the dynamics and rates of formation of fault-related hydrothermal ore deposits.

Eruptive activity of Planchón-Peteroa volcano for period 2010-2011, Southern Andean Volcanic Zone, Chile

Planchón-Peteroa volcano started a renewed eruptive period between January 2010 and July 2011. This eruptive period was characterized by the occurrence of 4 explosive eruptive phases, dominated by low-intensity phreatic activity, which produced almost permanent gas/steam columns (200-800 m height over the active crater). Those columns presented frequently scarce ash, and were interrupted by phreatic explosions that produced ash columns 1,000-3,000 m height in the more intense periods. Eruptive plumes were transported in several directions (NW, N, NE, E and SE), but more than half of the time the plume axis was 130-150° E, and reached a distance up to 638 km from the active crater. Tephra fall deposits identified in the NW, N, NE, E and SE flanks covered an area of 1,265 km2, thickness variable from 4 m (SE border of active crater) to ~0.5 cm 36.8 km SE and ~8 km NW from active crater, respectively, corresponding to a minimum volume of 0.0088 km3. Tephra fall deposit is exclusively constituted of no juvenile fragments including: lithics fragments as main component, quartz and plagioclase crystals, some oxidized lithics, and occasional presence of Fe oxide, and less frequently Cu minerals, as single fragments. We present new field-based measurements data of the geochemistry of gas/water from fumaroles and acid crater lakes, and fall deposit analysis, that integrated with the eruptive record and GOES satellite data, suggests that the eruptive period 2010-2011 has been related to an increasing of heat and mass transfer from hydrothermal-magmatic reservoirs, which would have been favoured by the formation and/or reactivation of cracks after 8.8 Mw Maule earthquake in February 2010. This process also allowed the ascent of fluids from a shallow hydrothermal source, dominated by reduced species as H2S and CH4, during the entire eruptive period, and the release of more oxidizing fluids from a deep magmatic reservoir, dominated by acid species as SO2, HCl and HF, increasing strongly after the end of the eruptive period, probably since October 2011. The eruptive period was scored with a magnitude of 3.36, corresponding to a VEI 1-2.