Input Of Magmatic Fluids Research Articles

Noble-metal mineralization of the Adycha-Taryn metallogenic zone: Geochemistry of stable isotopes, fluid regime, and ore formation conditions

The regional geologic setting of the Adycha-Taryn metallogenic zone, one of the areas most productive for noble-metal mineralization in northeastern Russia, is discussed. The intricate metallogenic history of the zone and the prolonged geodynamic activity of its ore-hosting structures are documented. Different types of mineralization, such as hydrothermal-metamorphogenic, gold-bismuth, gold-quartz, gold-antimony, and silver-antimony, are described. New data on the isotopic compositions of oxygen in quartz, sulfur in sulfides, and oxygen and carbon in carbonates from different mineralization types are presented. The early metamorphogenic quartz beyond the ore zones has δ18O = +20.1 ± 2.0‰. At the gold-bismuth deposits, the δ18O values of quartz are within the narrow range of +12.5 ± 0.4‰. Quartz from the gold-quartz mineralization shows much wider variation in δ18O values, from +14.2 to +19.5‰. A similar range (δ18O = +16.1 to +19.2‰) is observed for the gold-antimony mineralization. Cryptograined quartz from the silver-antimony mineralization is enriched in light oxygen isotopes (δ18O = -3.2 to +4.7‰). The following δ34S values (‰) have been established in sulfides of mineralization of different types: gold-bismuth -3.7 to -2.2 (Apy) and -6.7 to -6.8 (Py); gold-quartz -2.1 to +2.4 (Apy), -6.6 to +5.4 (Py), and -6.1 to +4.2 (St); gold-antimony -2.0 to +1.6 (Apy), -3.5 to +2.1 (Py), and -5.3 to +0.2 (St); and silver-antimony -2.0 to -1.9 (Apy), -2.2 ± 0.1 (Py), and -5.7 to -5.6 (St). The δ13C and δ18O values are contrasting in the studied types of mineralization, varying respectively from -6.9 to -5.9‰ and from +2.1 to +5.7‰ (gold-bismuth), from -9.1 to -6.1‰ and from +12.4 to 18.7‰ (gold-quartz), from -12.1 to -9.5‰ and from +15.0 to +16.3‰ (gold-antimony), from -11.6 to -11.1‰ and from +1.5 to +4.7‰ (silver-antimony). Metamorphogenic calcites are rich in both heavy C (-1.1 to -1.7‰) and heavy O (+20.3 to +20.5‰) isotopes. Microthermometric study and crush-leach analysis of fluid inclusions have revealed differences in the composition of ore-forming fluids and formation conditions for different types of mineralization. The isotopic compositions of O, C, and S of mineral-forming fluids suggest a significant input of magmatic fluids to the formation of gold-bismuth and gold-antimony deposits, the contribution of metamorphic fluids increases at gold-quartz deposits, and meteoric water is involved in the formation of silver-antimony deposits.

The 2012–2016 eruptive cycle at Copahue volcano (Argentina) versus the peripheral gas manifestations: hints from the chemical and isotopic features of fumarolic fluids

This study presents the chemical and isotopic compositions of hydrothermal gases from fumaroles discharging around Copahue volcano (Argentina). Gas samples, including those from two fumaroles at the active summit crater, were collected during 13 surveys carried out by different research teams from 1976 to February 2016. The time-series of H2, CO and light hydrocarbons showed episodic increases related to the main events of the last eruptive cycle that started on 19 July 2012. Concentration peaks were likely caused by enhanced input of hot magmatic fluids affecting the hydrothermal reservoir. These data contrast with the temporal variations shown by Rc/Ra and δ13C-CO2 values in 2012–2014, which indicated an increasing input from a crustal fluid source. In 2015–2016, however, these isotopic parameters showed opposite trends; their composition became closer to that of the two summit fumaroles, which possibly corresponds to that of the deep magmatic-related end-member. The delayed and reduced compositional changes in the peripheral hydrothermal fluid discharge in response to the 2012–2016 eruptive events suggest that geochemical surveys of these emissions are unlikely to provide premonitory signals of volcanic unrest if the volcanic activity remains centered in the main crater. Instead, an instrument which is able to provide measurements of volcanic gases in the air (e.g. MultiGAS) may be used to detect changes at the summit crater. Otherwise, monitoring of seismic activity and ground deformation, as well as the periodic measurement of the chemistry of the water in the Rio Agrio, which is fed by thermal discharge from the summit crater, seem to represent the most reliable means of monitoring at Copahue. However, the relative compositional stability of the hydrothermal reservoir is a great advantage in terms of geothermal resource exploitation and could encourage new investments in the Copahue geothermal project which was abandoned in the 1990s.

Sulfur isotope characteristics of the Permian VHMS deposits in Tasik Chini district, Central Belt of Peninsular Malaysia

Sulfur isotope data from sulfide and sulfate minerals have been measured from the two typical examples of the Permian volcanic-hosted massive sulfide (VHMS) deposits at the Tasik Chini district in the Central Belt of Peninsular Malaysia. In this study, we present the sulfur isotope data for 33 sulfide minerals and 23 barite samples from two VHMS deposits in the Tasik Chini district. Sulfides show a narrow range of sulfur values from -2.9%o to +8.30%o, which can be interpreted to be derived from a mixed sulfur source of reduced seawater sulfate with the possible addition of magmatic sulfur. Sulfate sulfur in barites yields a δ34S range between 11%o and 22%o, which is comparable to that of Permian seawater sulfate. Sulfur isotope results obtained for the VHMS deposits in the Tasik Chini district suggest that the source of ore fluids during the formation of the Tasik Chini VHMS deposit is a seawater-dominated fluid with probably minor magmatic fluid input. This is similar to VHMS associated with ancient and modern submarine hydrothermal systems.

Hydrogeochemical processes occurring in the hydrothermal systems of the Gonghe–Guide basin, northwestern China: critical insights from a principal components analysis (PCA)

The Gonghe–Guide basin is a geologically complex area located in the northeastern Qinghai–Tibet Plateau. Although it is far away from the Himalayan geothermal belt, hot springs with high contents of SiO2, Li, B and As are widely distributed there. Principal components analysis (PCA) was used to identify the geochemical genesis of the major constituents in geothermal waters and cold groundwaters from the study areas as well as the related hydrogeochemical processes. The results indicate that these constituents can be represented by six factors: factor 1 (F−, Li+, K+ and SiO2), factor 2 (EC, Cl− and Na+), factor 3 (pH \({\text{HCO}}_{3}^{ - }\), Mg2+ and Ca2+), factor 4 (B, As, Fe and Al), factor 5 (\({\text{SO}}_{4}^{2 - }\) and Sr) and factor 6 (\({\text{NO}}_{3}^{ - }\) ). Generally, the dissolution of silicate and carbonate minerals is the predominant geochemical processes responsible for the occurrence of the constituents that constitute factors 1 and 3, respectively. In contrast, although halite is not a primary mineral in the aquifer matrix, its leaching results in the enrichment of chloride and sodium in the groundwaters, which make significant contributions to the high EC values of some water samples. Although arsenic and boron are associated with factor 4, elevated levels of these elements in groundwater may be derived from the leaching of crystalline rocks (like granite and gneiss) rather than the input of potential magmatic fluids. Moreover, anthropogenic activity has an impact on the hydrochemistry of the groundwaters as well, as indicated by the elevated concentrations of nitrate (the sole component of factor 6) in some samples. It is also worth noting that the PCA results may not match well with the hydrogeochemical analyses. Factor 5 is composed of sulfate and strontium, whereas celestite is unlikely to be the primary source of sulfate in groundwater.

Advances in Trace Element “Fingerprinting” of Gem Corundum, Ruby and Sapphire, Mogok Area, Myanmar

Mogok gem corundum samples from twelve localities were analyzed for trace element signatures (LA-ICP-MS method) and oxygen isotope values (δ18O, by laser fluorination). The study augmented earlier findings on Mogok gem suites that suggested the Mogok tract forms a high vanadium gem corundum area and also identified rare alluvial ruby and sapphire grains characterised by unusually high silicon, calcium and gallium, presence of noticeable boron, tin and niobium and very low iron, titanium and magnesium contents. Oxygen isotope values (δ18O) for the ruby and high Si-Ca-Ga corundum (20‰–25‰) and for sapphire (10‰–20‰) indicate typical crustal values, with values >20‰ being typical of carbonate genesis. The high Si-Ca-Ga ruby has high chromium (up to 3.2 wt % Cr) and gallium (up to 0. 08 wt % Ga) compared to most Mogok ruby (<2 wt % Cr; <0.02 wt % Ga). In trace element ratio plots the Si-Ca-Ga-rich corundum falls into separate fields from the typical Mogok metamorphic fields. The high Ga/Mg ratios (46–521) lie well within the magmatic range (>6), and with other features suggest a potential skarn-like, carbonate-related genesis with a high degree of magmatic fluid input The overall trace element results widen the range of different signatures identified within Mogok gem corundum suites and indicate complex genesis. The expanded geochemical platform, related to a variety of metamorphic, metasomatic and magmatic sources, now provides a wider base for geographic typing of Mogok gem corundum suites. It allows more detailed comparisons with suites from other deposits and will assist identification of Mogok gem corundum sources used in jewelry.

Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): Insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids

More than three decades of geochemical monitoring of hot springs and fumaroles of La Soufrière of Guadeloupe allows the construction of a working model of the shallow hydrothermal system. This system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption (1530AD) and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions. Because it is confined into the low volume, highly compartmented and partially sealed upper edifice structure, the hydrothermal system is highly reactive to perturbations in the volcanic activity (input of deep magmatic fluids), the edifice structure (sealing and fracturing) and meteorology (wet tropical regime).The current unrest, which began with a mild reactivation of fumarolic activity in 1990, increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome. In 1997 seismic activity increased further and was accompanied by a sudden high-flux HCl-rich gas from summit fumaroles. We focus on the interpretation of the time series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens (F, Cl, Br and I). This extensive geochemical time series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater (scrubbing). It is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series: T, CO2 and total S content in fumaroles, T and Cl− in hot springs and the relative fractionations between F, Cl, Br and I in both fluids.Coupling 35years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976–1977 crisis. The contrasting behaviours of S- and Cl-bearing species in fumarolic gas and in thermal springs suggest that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since ~1992. Although it might still be evolving, the characteristics of this new intrusion indicate that it has already reached a magnitude similar to the intrusion that was emplaced during the 1976–1977 eruptive crisis. The assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurisation of the hydrothermal system on the likelihood of renewed phreatic explosive activity. Moreover, the role of hydrothermal pressurisation on the basal friction along low-strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse. At this stage enhanced monitoring, research, and data analysis are required to quantify the uncertainties related to future scenarios of renewed eruptive activity and magmatic evolution.

CH4-N2 in the Maldon gold deposit, central Victoria, Australia

The Maldon gold deposit in central Victoria has geological, geochronological and fluid chemistry characteristics that distinguish it from typical vein-hosted, ‘orogenic’ gold deposits in this region. The deposit lies within the thermal aureole of the Late Devonian Harcourt Granite and associated granitic dykes that postdate regional metamorphism (~445Ma) and large gold deposits such as Bendigo. The fluid inclusions are characterised by the presence of non-aqueous (i.e. carbonic) fluids, which exhibit complex freezing and heating behaviour, as well as mixed CO2–low-salinity aqueous fluids (mostly ≤10wt.% NaCl eq.). Raman analysis indicates that carbonic inclusions can vary from CO2-rich to CH4+N2-rich. Furthermore, higher-salinity fluid inclusions, containing 20–22wt.% NaCl eq., occur locally. Overall, fluid inclusions in the K-feldspar zone are much less abundant by volume than those in the cordierite zone probably due to recrystallisation, suggesting limited magmatic fluid input. The Harcourt Granite is a moderately reduced, I-type granite and it is suggested that the ‘retrograde’, reduced fluids (e.g. CH4+N2-rich), formed within the thermal aureole of the granite and associated dykes during contact metamorphism, are not part of the regional mineralising fluid system, which was dominated by deeply derived CO2–low-salinity aqueous fluids of metamorphic origin. Thus, the Maldon deposit is an ‘orogenic’ gold deposit that was metamorphosed and/or remobilised during the emplacement of post-orogenic intrusions.

Fluid-Flow Patterns at Brothers Volcano, Southern Kermadec Arc: Insights from Geologically Constrained Numerical Simulations

Numerical, multiphase pure-water simulations were performed to study the first-order geologic and physical parameters controlling the style and distribution of hydrothermal venting at Brothers volcano, southern Kermadec arc, New Zealand. By comparing the results for different permeability scenarios, we can show that the location of venting on the inner slopes of the caldera (e.g., at the NW caldera site) requires the presence of higher permeability faults. Venting at the top of the Upper cone develops naturally by hydrothermal flow in porous rocks above an underlying magma body. However, this magmatic reservoir cannot alone account for present-day hydrothermal venting at the NW caldera site, which implies that a larger magma chamber, which was responsible for caldera collapse, is still active. Modeled venting temperatures for scenarios with homogeneous host-rock permeability correspond well with formation temperatures determined by sulfate-sulfide mineral pairs from different vent sites at Brothers volcano. Direct measurements of vent fluids at the NW caldera site today, however, show higher temperatures than modeled. This may be due to rapid ascent of hot fluids in individual fractures that are not resolved in the simulations. At the cone sites, measured temperatures are lower than modeled, likely the result of mixing with ambient seawater in near-surface permeable rocks. The inferred presence of a constant magmatic fluid source underneath the volcanic edifice leads to a more rapid development of the hydrothermal circulation and stabilizes the system at higher temperatures. We suggest that the hydrothermal evolution and fluid-flow patterns at Brothers volcano are controlled by episodes of varying magmatic fluid input into a seawater-dominated convection system.

Tilt measurements at Vulcano Island

A network of tiltmeters has been operational on Vulcano Island for numerous years. At present, the network comprises five functioning borehole stations, four of which are installed at 8-10 m and allow recording very stable, high precision signals with very low noise. We report observations over the last 12 years that illustrate impulsive variations linked to seismicity and long-term (several years) trends in the signals. We suggest a relationship between tilt changes correlated to the strongest regional seismic events and site acceleration; long-term tilt variations analyzed in combination with other ground deformation data seem to represent evidence of a contraction of the La Fossa cone. We also analyzed how the tilt device has the capability to detect possible magma migrations; we considered previous studies that have imaged spatially well-defined levels of magma accumulation beneath La Fossa, and Vulcanello; we concluded that the Vulcano tilt network should be capable of detecting the upward migration of small magma volumes. Finally, we show that no evidence of changes are visible on tilt signals during anomalous degassing episodes (linked to a building up input of magmatic fluids) at the La Fossa thereby evidencing that no magma migration occurred during such events.

Magmatic fluid input to the Kuju-Iwoyama hydrothermal system prior to the 1995 eruption of the Kuju volcano (Kyushu, Japan)

A schematic model showing the sources of hot waters being discharged at the surface in the Kuju-Iwoyama of the Kuju volcano has been developed. Based on the isotopic characteristics of these fluids it is inferred that deep magmatic fluid mixes with thermal waters derived from rainwater in a shallow geothermal reservoir, and with local groundwaters in a deeper reservoir. These thermal waters feed hot springs that discharge waters with Cl/SO 4 ratios that differ from that of the fumaroles on Kuju-Iwoyama, due to the addition of SO 4 2− ions produced by the decomposition of native sulfur and mixing with magmatic fluid of high Cl content.

Water and gas chemistry at Lake Kivu (DRC): Geochemical evidence of vertical and horizontal heterogeneities in a multibasin structure

Waters and dissolved gases collected along vertical profiles in the five basins (Main, Kabuno Bay, Kalehe, Ishungu, and Bukavu) forming the 485 m deep Lake Kivu (Democratic Republic of the Congo) were analyzed to provide a geochemical conceptual model of the several processes controlling lake chemistry. The measured horizontal and vertical variations of water and gas compositions suggest that each basin has distinct chemical features produced by (1) different contribution from long circulating fluid system containing magmatic CO2, responsible of the huge CO2(CH4)‐rich reservoir hosted within the deep lake water; (2) spatial variations of the biomass distribution and/or speciation; and (3) solutes from water‐rock interactions. The Kabuno Bay basin is characterized by the highest rate of magmatic fluid input. Accordingly, this basin must be considered the most hazardous site for possible gas outburst that could be triggered by the activity of the Nyiragongo and Nyamulagira volcanoes, located a few kilometers north of the lake.

Temporal Variability of Major and Trace Element Concentrations in the Groundwaters of Mt. Etna (Italy): Effects of Transient Input of Magmatic Fluids Highlighted by Means of Cluster Analysis

Data for major, minor and trace elements in groundwaters from Mt. Etna volcano collected in 1994, 1995 and 1997 were analyzed using Cluster Analysis (CA). Two groups of sampling sites were identified (named clusters A and B), mainly on the basis of their different salinity and content of dissolved CO2. The highest levels of both of these parameters were observed in the sites of cluster A, located in the lower southwestern and central eastern flanks of the volcano. For both of the statistical groups CA was repeated, taking into account the mean values of each parameter in time, and the results allowed us to recognize four distinct groups of parameters for each group of sites on the basis of their temporal patterns. Four different types of temporal patterns were recognized: concave, convex, increasing, decreasing. The observed changes were basically interpreted as a result of the different response of dissolved chemical elements to changes in the aqueous environment and/or in their solubility/mobility in water due to different rates of input of magmatic gases to Etna’s aquifers. The main changes occurred in 1995, when Etna’s volcanic activity resumed after a two-year period of rest. The temporal changes of the majority of the studied parameters (water temperature, water conductivity, Eh, pH, Al, Mg, B, Ca, Cl−, Hg, Mn, Mo, Na, Ni, Se, Si, Sr, Cr Zn and pCO2) were not cluster-dependent, therefore they were not apparently affected by differences in water salinity between the two groups of sampling sites. A limited number of parameters (Ti, K, Li, HCO 3 − , As, Fe, SO 4 2− , Cu and V), however, manifested different behaviors, depending on the cluster of sites to which they belonged, thus suggesting their apparent dependency on water salinity.

Submarine gas burst at Panarea Island (southern Italy) on 3 November 2002: A magmatic versus hydrothermal episode

On 3 November 2002, fishermen observed the sudden appearance at the sea surface of three large whitish plumes 3 km offshore of Panarea Island (Aeolian Islands, southern Italy) caused by the uprising from the seafloor of huge columns of gas bubbles, mixed with sediments and colloidal sulfur. The degassing event partly superimposed an already existing gentle‐degassing fumarolic field. Since 12 November 2002, a discontinuous geochemical monitoring program of these new discharging fluids have been carried out. The submarine emissions collected on November 2002 were an emulsion made by a CO2‐dominated continuous gas phase with suspended sediments, colloidal sulfur, and water condensate microdroplets acidified by dissolution of compounds such as SO2, HCl, and HF. In the gas phase, light‐unsaturated hydrocarbons also occurred, possibly related to relatively high temperature and oxidizing conditions due to local input of magmatic fluids at depth, whose occurrence was also supported by the relative high values of 3He/4He isotopic ratio (R/Rair = 4.62) with respect to previously measured values (R/Rair < 4.2). The flux of the submarine emissions significantly decreased in a couple of months together with the almost complete disappearance of the magmatic chemical markers and the decrease of the helium isotopic ratio. Thus the most striking feature of the temporal and spatial evolution in the chemical and isotopic compositions of the submarine fumaroles was the relatively rapid restoration, since January 2003, of the precrisis conditions, i.e., typical of a stationary hydrothermal system.

Renewed ground uplift at Campi Flegrei caldera (Italy): New insight on magmatic processes and forecast

Campi Flegrei caldera, including the extremely urbanised city of Naples, is the most risky volcanic area in the World. The last eruption in the area (1538) occurred at the end of some decades of ground uplift, superimposed to secular subsidence. During the last four decades, it experienced a huge uplift phase, reaching about 3.5 m in 1985, when a subsidence phase started. Recent geodetic data demonstrate that such a subsidence phase has terminated, and a new uplift episode started in November 2004, with a low but increasing rate leading to about 0.04 m of uplift till the end of October 2006. A new indicator, based on the monitoring of maximum horizontal to vertical displacement ratio with continuous GPS, indicates that this uplift is likely to be associated with input of magmatic fluids from a shallow magma chamber. The method is promising to monitor magma intrusion processes, at this and other volcanoes.

Correlated increase in CO2 fumarolic content and diffuse emission from La Fossa crater (Vulcano, Italy): Evidence of volcanic unrest or increasing gas release from a stationary deep magma body?

The peculiarity of the quiescent La Fossa volcano is the occurrence of “crises” characterized by strong increases of fumarole T and output and by chemical changes indicative of an increasing input of magmatic fluids. Several surveys carried out during a new “crisis” began in November 2004 indicate that the total diffuse CO2 emission for the crater area increases by one order of magnitude during crises (up to 1600 ton·d−1 in December 2005). Concern exists on the possibility that these crises be related to an unrest process leading to eruption. The repetition along decades of the same gas compositional variations during crises, their temporal coincidence with increases of the local shallow seismicity, and the lack of any significant ground motion, rather suggest that they correspond to moments of increasing volatile release from a stationary magma system.

Geochemistry of the Submarine Gaseous Emissions of Panarea (Aeolian Islands, Southern Italy): Magmatic vs. Hydrothermal Origin and Implications for Volcanic Surveillance

The marine sector surrounding Panarea Island (Aeolian Islands, South Italy) is affected by widespread submarine emissions of CO2 -rich gases and thermal water discharges which have been known since the Roman Age. On November 3rd, 2002 an anomalous degassing event affected the area, probably in response to a submarine explosion. The concentrations of minor reactive gases (CO, CH4 and H2) of samples collected in November and December, 2002 show drastic compositional changes when compared to previous samples collected from the same area in the 1980s. In particular the samples collected after the November 3rd phenomenon display relative increases in H2 and CO and a strong decrease in the CH4 contents, while other gas species show no significant change. The interaction of the original gas with seawater explains the variable contents of CO2, H2S, N2, Ar and He which characterize the different samples, but cannot explain the large variations of CO, CH4 and H2 which are instead compatible with changes in the redox, temperature and pressure conditions of the system. Two models, both implying an increasing input of magmatic fluids are compatible with the observed variations of minor reactive species. In the first one, the input of magmatic fluids drives the hydrothermal system towards atypical (more oxidizing) redox conditions, slowly pressurizing the system up to a critical state. In the second one, the hydrothermal system is flashed by the rising high-T volcanic fluid, suddenly released by a magmatic body at depth. The two models have different implications for volcanic surveillance and risk assessment: In the first case, the November 3rd event may represent both the culmination of a relatively slow process which caused the overpressurization of the hydrothermal system and the beginning of a new phase of quiescence. The possible evolution of the second model is unforeseeable because it is mainly related to the thermal, baric and compositional state of the deep magmatic system that is poorly known.

Carbon dioxide emissions from vegetation-kill zones around the resurgent dome of Long Valley caldera, eastern California, USA

A survey of diffuse CO 2 efflux, soil temperature and soil-gas chemistry over areas of localized vegetation-kill on and around the resurgent dome of Long Valley caldera California was performed to evaluate the premise that gaseous and thermal anomalies are related to renewed intrusion of magma. Some kill sites are long-lived features and others have developed in the past few years. Total anomalous CO 2 emissions from the thirteen areas average around 8.7 t per day; but the majority of the emissions come from four sites west of the Casa Diablo geothermal power plant. Geochemical analyses of the soil-gases from locations west and east of the plant revealed the presence of isobutane related to plant operations. The δ 13C values of diffuse CO 2 range from − 5.7‰ to − 3.4‰, similar to values previously reported for CO 2 from hot springs and thermal wells around Long Valley. At many of the vegetation-kill sites soil temperatures reach boiling at depths ≤ 20 cm. Soil temperature/depth profiles at two of the high-emissions areas indicate that the conductive thermal gradient in the center of the areas is around 320 °C m − 1 . We estimate total heat loss from the two areas to be about 6.1 and 2.3 MW. Given current thinking on the rate of hydrothermal fluid flow across the caldera and using the CO 2 concentration in the thermal fluids, the heat and CO 2 loss from the kill areas is easily provided by the shallow hydrothermal system, which is sourced to the west of the resurgent dome. We find no evidence that the development of new areas of vegetation kill across the resurgent dome are related to new input of magma or magmatic fluids from beneath the resurgent dome. Our findings indicate that the areas have developed as a response to changes in the shallow hydrologic system. Some of the changes are likely related to fluid production at the power plant, but at distal sites the changes are more likely related to seismicity and uplift of the dome.

Changes in fluid geochemistry and physico-chemical conditions of geothermal systems caused by magmatic input: The recent abrupt outgassing off the island of Panarea (Aeolian Islands, Italy)

Hydrothermal systems and related vents can exhibit dramatic changes in their physico-chemical conditions over time as a response to varying activity in the feeding magmatic systems. Massive steam condensation and gas scrubbing processes of thermal fluids during their ascent and cooling cause further compositional changes that mask information regarding the conditions evolving at depth in the hydrothermal system. Here we propose a new stability diagram based on the CO 2-CH 4-CO-H 2 concentrations in vapor, which aims at calculating the temperatures and pressures in hydrothermal reservoirs. To filter gas scrubbing effects, we have also developed a model for selective dissolution of CO 2-H 2S-N 2-CH 4-He-Ne mixtures in fresh and/or air-saturated seawater. This methodology has been applied to the recent (November 2002) crisis that affected the geothermal field off the island of Panarea (Italy), where the fluid composition and fluxes have been monitored for the past two decades. The chemical and isotopic compositions of the gases suggest that the volatile elements originate from an active magma, which feeds a boiling saline solution having temperatures of up to 350°C and containing ≈12 mol% CO 2 in vapor. The thermal fluids undergo cooling and re-equilibration processes on account of gas-water-rock interactions during their ascent along fracture networks. Furthermore, steam condensation and removal of acidic species, partial dissolution in cold air-saturated seawater and stripping of atmospheric components, affect the composition of the geothermal gases at shallow levels. The observed geochemical variations are consistent with a new input of magmatic fluids that perturbed the geothermal system and caused the unrest event. The present-state evolution shows that this dramatic input of fluids is probably over, and that the system is now tending towards steady-state conditions on a time scale of months.

Stable isotope evidence for magmatic fluid input during large‐scale Na–Ca alteration in the Cloncurry Fe oxide Cu–Au district, NW Queensland, Australia

AbstractSodic–calcic alteration is common in mineralized hydrothermal systems, yet the relative importance of igneous vs. basinal fluid sources remains controversial. One of the most extensive volumes of sodic–calcic rocks occurs near Cloncurry, NW Queensland, and was formed by overlapping hydrothermal systems that were active synchronously with emplacement of mid‐crustal batholithic granitoids (c. 1.55–1.50 Ga). Altered rocks contain albite–oligoclase, actinolite, diopside, titanite and magnetite. Alteration was localized by: (A) composite veins and breccias containing crystallized magma intimately intergrown with hydrothermal precipitates; (B) intrusions that host setting A veins and breccias; and (C) extensive breccia and vein systems linked to regional fault systems. Isotope analyses of actinolites in settings A and B indicate calculated δ18OH2O (+8.2 to +10.6‰) and variably depleted δDH2O (−130 to −54‰) compared with typical magmatic fluids, whereas those from setting C typically indicate calculated δ18OH2O (+8.0 to +12.8‰) and δDH2O (−29 to −99‰). The lowest δDH2O values are interpreted as representing residual fluids after significant (> 90%) open‐system magmatic degassing. Overall the stable isotope, field, geochronological and geobarometric data suggest that these sodic–calcic alteration systems were formed by the episodic incursion of magmatic fluids that underwent minor isotopic modification as a result of varying degrees of interaction with country rocks.

The Spectrum of Ore Deposit Types, Volcanic Environments, Alteration Halos, and Related Exploration Vectors in Submarine Volcanic Successions: Some Examples from Australia

Variations in shape, metal content, alteration mineralogy, and volcanic host rocks of the ore deposits in the two major volcanic-hosted massive sulfide (VHMS) districts of eastern Australia, the Cambrian Mount Read Volcanics and the Cambro-Ordovician Mount Windsor subprovince, strongly reflect their volcanic environment, conditions of ore formation, and hydrothermal alteration processes. Lens and sheet-style polymetallic zinc-rich deposits such as Rosebery, Hellyer, Que River, and Thalanga are considered to have formed in moderate to relatively deep water environments (500-1,000+m). These deposits probably formed either on the sea floor (e.g., Hellyer, Que River) or by replacement of porous volcaniclastic units directly below the sea floor (e.g., Rosebery). The footwall alteration associated with these polymetallic VHMS deposits was controlled by host-rock permeability and porosity, which are in turn related to volcanic facies type, degree of fracturing, and synvolcanic structural architecture. Focusing of hydrothermal fluids along synvolcanic structures has resulted in well-zoned chlorite-sericite footwall alteration pipes within footwall lavas at Hellyer. On the other hand, diffuse fluid flow through very thick pumice breccia at Rosebery and Hercules has resulted in strata-bound, sericite-dominated footwall alteration zones parallel to the paleosea floor and the ore lenses. Massive and disseminated, pyritic Cu-Au deposits, such as those in the Mount Lyell field and at Highway-Reward, formed by subsea-floor replacement and are associated with only minor zinc-lead massive sulfide ore. These deposits formed from higher temperature fluids (>300 degrees C), in which copper transport is enhanced, and are commonly located in felsic volcanic centers dominated by shallow porphyritic intrusions (e.g., Highway-Reward). The Cu-Au ore lenses may be strata-bound (e.g., Mount Lyell) or crosscutting pipes (e.g., Highway-Reward) depending on the structure and permeability characteristics of the felsic volcanic host rocks. The presence of high-sulfidation alteration minerals (e.g., pyrophyllite, zunyite) in some of the Cu-Au deposits (e.g., Mount Lyell field) indicates that fluids were relatively acidic and suggests the possibility of magmatic fluid input into the hydrothermal system. Alteration zonation associated with the Cu-Au VHMS deposits is more symmetrical than that of the Zn-rich deposits, with sericite-rich alteration extending into the hanging wall, in keeping with the subsurface replacement origin of these deposits. Synvolcanic gold-rich deposits, with high gold/base metal ratios are less common than the Cu-Au and Zn-rich VHMS ore types. The gold-rich ores (e.g., Henty, South Hercules) are strata bound in nature, have low sulfide contents, and are associated with central zones of intense silicification, surrounded by envelopes of sericite-pyrite and carbonate alteration. Volcanological and geochemical studies at Henty indicate the gold-rich ore formed by the replacement of particular volcanic units deposited in a relatively shallow water environment dominated by volcaniclastic facies, lavas, and limestones. This spectrum of Cu-Au, Zn-rich, and Au-only deposits in the Mount Read Volcanics and the Mount Windsor subprovince is interpreted to represent a continuum from classic sea-floor VHMS ores toward those with features more akin to porphyry Cu-Au and epithermal Au-Ag deposits. This spectrum relates to the interplay between factors in the submarine volcanic environment and the character of the hydrothermal fluid as follows: (1) proportions of volcaniclastic, lava, and subvolcanic intrusive facies; (2) depth of seawater; (3) permeability and porosity of volcanic host rocks; (4) balance between magmatic components and seawater components in the ore fluid; and (5) temperature and acidity of the ore fluid. Mineralogical, lithogeochemical, and isotopic studies have revealed a range of alteration vectors useful in exploration for both the Zn-rich and Cu-Au VHMS deposits. Carbonate and white mica compositional variations are highlighted as important mineralogical vectors; thallium and antimony halos may be useful trace element vectors; and oxygen and sulfur provide important isotope vectors toward the center of the hydrothermal system.