Fossil Geothermal Systems Research Articles

Integrated approach for geothermal exploration: Case study from Salar area (Afyonkarahisar, Turkey)

The intense investment demand in the geothermal sector in Afyonkarahisar province in recent years has enabled the utilization of geothermal waters such as district heating and greenhouse heating, electricity generation, and spa facilities and accelerated the exploration of new geothermal areas in the region. In this study, the Salar (Afyonkarahisar) region's geothermal potential was investigated using the mineralogy and geochemistry of hydrothermal alteration, hydrogeochemistry, and resistivity models obtained from magnetotelluric data. The Salar region is located within the Afyon-Akşehir Graben (AAG) and 10 km south of Afyonkarahisar province. The most important manifestations of the geothermal system are the geothermal water at temperatures of 25 °C and 31 °C obtained from the boreholes and hydrothermal alteration in Salar. Afyon volcanoclastics are reservoir rocks. Smectite and illite are the most important clay minerals in the hydrothermal alteration zones. The transformation from volcanic glass and alkali feldspar to smectite and illite reflects neutral-alkaline alteration conditions in felsic rocks. The clay minerals' stable isotopes (δD and δ18O) indicate hypogene conditions. Discharge temperature, electrical conductivity and pH of Salar region geothermal waters vary from 25 to 31 °C, 320–357 µs/cm, and 6.8, respectively. The Salar geothermal waters are Ca-(Na)-HCO3 type chemically. The electric resistivity models reveal shallow low resistivity (10 < ρ < 80 Ωm) layer related to the alluvium, Gebeceler formation, and Afyon volcanoclastics and deeper high resistivity (80 <ρ < 200 Ωm and ρ > 200 Ωm) layer based on Deresinek and Değirmendere formation respectively. The difference in electrical resistivity arises from the geothermal waters and hydrothermal alteration zones, influenced by the AAG tectonics.The stable isotopes (δD and δ18O) and alpha cristobalite geothermometer calculations indicate that the condition of the temperature in the active and fossil geothermal systems in the Salar does not change, and the condition of the temperature is between 44 °C and 112 °C.

Characteristics and evolution of the Etili silica sinter epithermal deposits, Çanakkale – Turkey: Relation to alkali chloride vs acid-sulfate fluids

The Oligo-Miocene Etili epithermal deposits are a well-preserved fossil geothermal system in the Çanakkale Region and is one of the largest fossil siliceous hot spring deposits of Turkey. Main E-W and NE-SW trending faults systems occur with minor NW-SE fracture systems perpendicular to the main faults. Silica-rich deposits are commonly observed in or on the calc-alkaline tuffs, ashes and pyroclastic rocks that were the products of the magmatism controlled by the extensional tectonic regime.The Etili epithermal system was examined at two representative locations including the Hamamtepe and Muratlar sites. Morphologies of the proximal apron were defined by lithofacies that included silica infiltrate, spring conduits, nodular and finely laminated geyserite, sinter clast breccia, silicified volcanic rocks, and epithermal veins. Microfossils were detected in mat structures developed in the proximal apron. The silica deposits have low abundances of altered mineral assemblages (e.g., kaolinite, halloysite, and alunite), which are otherwise commonly observed in the region. The origin of the Etili Fossil Silica Sinter Region (EFSSR) was constrained by using geochemical and isotopic data. δ18O and δD isotopic values of kaolinites ranged +9.4/+9.6 ‰; to 84/−74 ‰ respectively. δ18O isotopic values from siliceous and silicified samples ranged from 8.3 to 18.4 ‰, which shows two different formation types for the Etili Fossil Silica Sinter Region. We hypothesized hypogene origins for the kaolins and a mixed hypo/supergene origin for siliceous and silicified samples. The calculated model formation temperature from the δ18O values of the silica and siliceous samples is in the range of 63° to 140 °C. δ34S isotopic values of alunite ranged from −19.6 to +16.6 ‰, which also indicated two different sources that include magmatic hydrothermal and meteoric waters affected by bacterial activity. The timing of acid-sulphate alteration can be grouped into three periods by 40Ar/39Ar dating of alunites. These ages are: (A) 32.4 ± 1.2 to 22.6 ± 0.22 Ma in the Muratlar silica site, (B) 12.3 ± 0.3 to 15.2 ± 0.3 Ma in the northern part of the Hamamtepe site and (C) 5 ± 0.18 to 7 ± 0.3 Ma in the southern part of the Hamamtepe silica site. These consistent findings show that the emplacement periods of the Evciler pluton and the ages of alteration deposits are similar to each other. Silica sinters and geyser mounds represent very late stage near-neutral pH alkali chloride geothermal activities. The epithermal deposits in the EFSSR comprises a complex history of three different hydrothermal processes, each of which have different compositional and temporal emplacement periods related to the Evciler pluton.

Factors controlling the geometry of travertine mounds: Insights from Heinitang (China)

AbstractTravertine mounds are common spring‐constructed landforms with highly variable geometries, but factors controlling their development have not yet been fully understood. To investigate processes linked with their lateral and vertical growth, this study investigated travertine mounds at Heinitang (China) and compared them with typical mounds around the world. Calcium concentration variation modelling and height evaluation of travertine mounds were also performed. The lateral extent of travertine mounds was found to be mainly controlled by Ca2+ and concentrations and water discharge: relatively small metre‐scale mounds, like those from Heinitang, are largely the products of Ca2+‐deficiency (&lt;150 mg L−1, commonly &lt;100 mg L−1) or ‐deficiency (&lt;500 mg L−1) springs with low discharges (&lt;0.1 L s−1), whereas springs with higher Ca2+ and concentrations and discharges give rise to the formation of wider mounds. Additional factors affecting mound lateral development include infiltration, evaporation, micro‐organisms, water cooling, CO2 degassing and flow paths. Height of travertine mounds cannot be simply considered to represent the piezometric surface, because the vertical growth of travertine mounds is also influenced by the water table change triggered by tectonic pulses and climate fluctuations, and the closing of conduits induced by self‐sealing. The extinction of travertine mounds at Heinitang is, for example, probably caused by the water table lowering triggered by earthquakes or climate drying after 4 kyr bp. These findings offer new inputs for the better understanding of factors controlling travertine mound development and might assist in the reconstruction of fluid properties, hydrodynamics, tectonics and climate fluctuations in fossil geothermal systems with travertine mounds.

Challenges of Tunnelling in Volcanic Rock Masses

Volcanic rock masses exhibit temporal and spatial variability, even at the scale and duration of engineering projects. Volcanic processes are dynamic, resulting in rock masses ranging from high-porosity, clay-rich, fractured, and soil-like to low-porosity, high-strength, brittle, and massive. Based on a number of studies in a variety of geological settings, such as active and fossil geothermal systems, on the surface of active volcanoes and up to 3000 m below the surface, the work presented in this article shows the relationship between geological characteristics and mechanical parameters of volcanic rocks. These are then linked to the resultant challenges to tunnelling associated with the mechanical behaviour of volcanic rocks and rock masses, ranging from ductile failure such as squeezing and swelling to dynamic failure such as spalling and rockburst.This article highlights some of the key parameters that should be incorporated in site and laboratory investigations to build representative ground models in volcanic rocks and rock masses. Rock mass characterisation needs to address the highly variable and anisotropic nature of volcanic rocks, ranging from millimetre to decametre scale. Ground models must include not only the mechanical properties, such as strength and stiffness, of typical lab investigations, but also petrophysical properties, such as porosity, and geological conditions, such as alteration. Geomechanical characterisation of these rock masses requires an understanding of geological processes to select appropriate field, lab and design tools. In volcanic rocks, perhaps more than any other rock types, the geology is critical to characterising and understanding the behaviour in response to tunnelling.

40Ar/39Ar dating of high temperature geothermal systems: First attempt on hydrothermally altered pyroxenes from the Saintes archipelago (Lesser Antilles arc, Guadeloupe)

We investigate the potential of 40Ar-39Ar systematics in secondary K-bearing minerals developed at the expense of hydrothermally altered volcanic pyroxenes from a fossil geothermal system located in the Saintes archipelago (Lesser Antilles magmatic arc).Our work reports the first example of dating hydrothermal muscovite crystallization developed during pseudomorphic transformation of pyroxenes within altered rhyodacites at temperatures above 300 °C. The white micas display a well-defined Ar/Ar plateau age of 2.59 ± 0.12 Ma, interpreted as the best age estimate for high temperature fluid circulation in the Saintes geothermal paleo-reservoir. A model of muscovite substitution by illite during cooling supports the interpretation that illitization perturbs the K-Ar and the Ar-Ar ages only when muscovite is highly illitized and/or if illitization is significantly younger than muscovite crystallization. The whole temporal dataset implies a fast cooling rate (>150 °C/200 ka) and a maximal lifetime of 650 ka for the Saintes hydrothermal system.

A cold-water trap as an essential process for the generation of low-sulfidation epithermal deposits: Geological and numerical studies of the Hosokura deposit, northern Japan

Although low-sulfidation epithermal deposits, including the Hosokura deposit (Ag–Pb–Zn) in the Miyagi Prefecture, northern Japan, are globally significant metal sources, their ore deposition mechanisms are not thoroughly understood . This study aimed to construct a genetic ore deposition model for this deposit by focusing on the richest vein, the Fuji, and test the model using a numerical simulation of the formation and transitions of this fossil geothermal system. It was proposed that the mixing of neutral hydrothermal fluids (~260 °C) ascending through the vein with oxygenated cold groundwater caused early stage mineralization and the transport of hydrothermal fluids into the surrounding host rocks, forming a hydrothermal alteration halo around the vein that results in silicification, adularia alteration, and argillization. The progress of this alteration decreased the permeability of the halo and generated a mushroom-shaped low-permeability alteration halo (LPAH) in permeable zones surrounding the vein. The low-permeability and insulating properties of the LPAH increased the fluid temperature, the vein fluid boiled, and consequently the middle stage mineralization occurred. After a further decrease in LPAH permeability, hydrothermal brecciation resulting from increasing fluid pressure occurred at the top of the vein, resulting in late-stage mineralization. A TOUGH2 numerical simulation tested these proposed processes: early stage cooling of hydrothermal fluids by mixing with groundwater, the development of vapor in the vein by the formation of an LPAH during the middle stage, and an increase in fluid pressure leading to hydrothermal brecciation during the late-stage. The results of the simulation confirmed that a cold-water trap is an essential process for the generation of low-sulfidation epithermal deposits. Mushroom-shaped LPAHs, silicification zones, and brecciated veins caused by cold-water traps have been found in representative low-sulfidation epithermal deposits in Japan, such as the richest Au deposit, the Hishikari. Therefore, identification of cold-water-trap footprints can contribute to the exploration of this type of deposit.

Evolution of fluorite-mica-feldspar veins: Evidences of a fossil geothermal system in the são Gabriel terrane and consequences for Pb-Zn-Cu metallogeny

Fluorite-mica-feldspar hydrothermal veins and stockworks host Pb-Zn-Cu sulphides in the São Gabriel Terrane, Southern Brazil. The veins fill second- and third-order structures in Neoproterozoic calcitic marbles on the Cambaizinho Complex. Veins are mostly symmetrically zoned showing, from borders to the centre, fluorite and sericite, muscovite, adularia and albite (minor quartz, calcite and iron oxides). Galena, sphalerite, chalcopyrite, pyrite and minor cassiterite are disseminated in the veins and concentrated in ore pockets. Vein petrography, mineral chemistry and microthermometric measurements in fluorite fluid inclusions indicate that the hydrothermal veins were formed in a shallow crust environment from epithermal fluids near 140 °C. Their origin is attributed to a fossil geothermal system related to the post-collisional alkaline volcanism and caldera system of the Acampamento Velho Formation (574-549 Ma). This hydrothermal system is interpreted as genetically associated to fluorite and Pb-Zn-Cu occurrences on rhyolites and miarolitic cavities of related alkaline granitoids, Pb-Zn-Cu anomalies on stream sediment and soil analyses and Au quartz veins hosted by alkaline granitoids accompanied by pyrite, sericite and fluorite alteration halos in the same region. Although there is no mineral deposit discovered so far, the studied veins linked to the other base metal occurrences would indicate that the northwestern São Gabriel Terrane may host a poorly preserved mineralized epithermal system.

Cerro Prieto Geothermal Field (Baja California, Mexico) – A fossil system? Insights from a noble gas study

Results of a survey of noble gases, water stable isotopes (δD and δ18O), and chlorine (Cl) and bromine (Br) contents measured in geothermal waters from the high-enthalpy, liquid-dominated Cerro Prieto Geothermal Field (CPGF) are reported. Helium isotopic ratios (3He/4He) help to identify two distinct fluids: a magmatic fluid, with 3He/4He (R) ratios of up to 7.3 times that of the atmospheric ratio (Ra = 1.384 × 10−6), and a 4He-rich fluid, with 3He/4He ≤ 3.5 Ra. The helium isotopic systematics point to limited modern freshwater recharge in the CPGF, and thus to the presence of a nearly fossil geothermal system. U-Th/4He-estimated minimum residence times for the 4He-rich fluid component vary from 1.5 to 1.8 Ma, corresponding to the age of the Colorado River paleo-delta sediments, suggesting the presence of connate waters. Cl and Br concentrations indicate that this connate water component is seawater with minor halite dissolution. Together with Cl and Br, the isotopic signature of the water (δD, δ18O) indicates the presence of a third old fluid of meteoric origin, from Late Quaternary to Holocene, corresponding to recharge during colder and wetter periods than today. This is identified as being old groundwater from the Colorado Delta unconsolidated sediments, which cap the Cerro Prieto reservoir. Our results suggest little direct recharge to the CPGF. The spatial distribution of helium isotopes in the reservoir shows that connate waters dominate the central portion of the field and mix with the magmatic fluid containing mantle He, with both fluids rising at the center of the basin where the heat source is located. Deep magmatic fluids containing mantle helium enter laterally into the reservoir, through the main lateral strike-slip faults (i.e., the Cerro Prieto and Imperial faults).

Fracture analysis, hydrothermal mineralization and fluid pathways in the Neogene Geitafell central volcano: insights for the Krafla active geothermal system, Iceland

Studies on hydrothermal fluids circulation and tectonic structures improve the geothermal exploration and exploitation in Iceland. In this paper we present the results of an integrated study (structural, kinematic, fluid inclusions analyses) carried out on a fossil and exhumed geothermal system at Geitafell (SE Iceland), considered an analogue of the active Krafla geothermal system (NE Iceland). Our work is addressed to the relationships between the main structures and the paleo-geothermal fluids circulation at the boundary between the magma chamber (with magma of gabbroic composition) and host rocks, with the aim to get information on those factors having controlled the hydrothermal fluids flow, its storage and the rocks-fluid interaction, at 1–2 km crustal depth. The structural study is mostly based on defining the architecture of mineralized brittle shear zones, where, by measuring length and width of mineralized fractures, permeability was computed in 1.3 × 10−14 m2. As regards the hydrothermal paleo-fluids, fluid inclusions in hydrothermal quartz and calcite, usually associated with epidote and chlorite (i.e., the typical alteration minerals found in Icelandic geothermal fields), recorded a fluid circulation with temperature ranging from about 255 °C to 320 °C, and boiling at the highest temperature. Transient pressure drop, likely related to fracture opening and propagation during tectonic activity, triggered local boiling process testified by fluid inclusions in a calcite sample. During a late evolution the system progressively cooled down to 60–90 °C. Paleo-geothermal fluids were characterized by low-salinity meteoric water, although higher salinity fluids (up to 10.6 wt% NaCl equiv.) entered in the system and mixed with meteoric fluid. In the end, based on results from fluid inclusions, derived fluid density, permeability values and modeling of viscosity, hydraulic conductivity between 2.8 × 10−8 to 1.8 × 10−7 m/s, was also computed within the fluids-channelling brittle shear zones. We finally encourage the key-study of fossil geothermal systems as a tool for getting parameters commonly obtained after drilling in active systems.

Rock mass properties and edifice strength data from Pinnacle Ridge, Mt. Ruapehu, New Zealand

Volcanic edifices exhibit spatially variable physical and mechanical properties. Magmatic intrusions are common at shallow depths within the volcanic edifice and are a poorly-understood contributor to this spatial variability. Intrusion-related alteration has been found to weaken rock mass strength through the development of joints and fractures; however, there is a paucity of research investigating how intrusions affect rock mass strength specific to the geotechnical units that define the rock masses. In this study, we employ a range of field techniques—field permeametry, rock hardness assessment, rock mass classification, and discontinuity mapping—to characterise an exposed fossil geothermal system produced by a shallow intrusion at Pinnacle Ridge, Mt. Ruapehu (New Zealand). We find that intrusions detrimentally affect the rock mass characteristics of altered brecciated lava margins. The resulting change in rock mass strength may be offset by an increase in intact rock strength as a product of alteration mineral precipitation in microfractures. Consequently, the final strength of the rock mass of the altered brecciated lava margins has the potential to be lowest of any of the geotechnical units in the volcanic edifice. We also conclude that these discontinuities increase permeability of the host rock at distances from the intrusion roughly proportional to 1–2 times the thickness of the intrusion itself under near-surface conditions. The data and conclusions presented in this study help to bridge the gap between the lab- and the field-scale and have immediate relevance to engineering geology and geothermal applications worldwide, and to rock mass classification assessments in volcanic environments.

Modelling of hydrothermal fluid flow and structural architecture in an extensional basin, Ngakuru Graben, Taupo Rift, New Zealand

Present-day geothermal activity on the margins of the Ngakuru graben and evidence of fossil hydrothermal activity in the central graben suggest that a graben-wide system of permeable intersecting faults acts as the principal conduit for fluid flow to the surface. We have developed numerical models of fluid and heat flow in a regional-scale 2-D cross-section of the Ngakuru Graben. The models incorporate simplified representations of two ‘end-member’ fault architectures (one symmetric at depth, the other highly asymmetric) which are consistent with the surface locations and dips of the Ngakuru graben faults. The models are used to explore controls on buoyancy-driven convective fluid flow which could explain the differences between the past and present hydrothermal systems associated with these faults.The models show that the surface flows from the faults are strongly controlled by the fault permeability, the fault system architecture and the location of the heat source with respect to the faults in the graben. In particular, fault intersections at depth allow exchange of fluid between faults, and the location of the heat source on the footwall of normal faults can facilitate upflow along those faults. These controls give rise to two distinct fluid flow regimes in the fault network. The first, a regular flow regime, is characterised by a nearly unchanging pattern of fluid flow vectors within the fault network as the fault permeability evolves. In the second, complex flow regime, the surface flows depend strongly on fault permeability, and can fluctuate in an erratic manner. The direction of flow within faults can reverse in both regimes as fault permeability changes.Both flow regimes provide insights into the differences between the present-day and fossil geothermal systems in the Ngakuru graben. Hydrothermal upflow along the Paeroa fault seems to have occurred, possibly continuously, for tens of thousands of years, while upflow in other faults in the graben has switched on and off during the same period. An asymmetric graben architecture with the Paeroa being the major boundary fault will facilitate the predominant upflow along this fault. Upflow on the axial faults is more difficult to explain with this modelling. It occurs most easily with an asymmetric graben architecture and heat sources close to the graben axis (which could be associated with remnant heat from recent eruptions from Okataina Volcanic Centre). Temporal changes in upflow can also be associated with acceleration and deceleration of fault activity if this is considered a proxy for fault permeability. Other explanations for temporal variations in hydrothermal activity not explored here are different permeability on different faults, and different permeability along fault strike.

Upper Jurassic travertine at El Macanudo, Argentine Patagonia: a fossil geothermal field modified by hydrothermal silicification and acid overprinting

AbstractThe Deseado Massif hosts numerous Late Jurassic (150 Ma) fossil geothermal systems related to an extensive volcanic event developed in a diffuse extensional back-arc setting. Detailed mapping, petrography and mineralogical observations of El Macanudo outcrops verify that it represents a hot-spring-related travertine partially replaced by silica and delineated by six sedimentary facies. These are large concentric cones (F1), laminated vertical columnar structures (F2), porous layers (F3), shrubby and irregular lamination (F4), low-amplitude wavy bedding (F5) and mounds and breccias (F6). The Macanudo Norte Outcrop rocks constitute a silica-replaced travertine sequence, with development of large conical stromatolites in a deep pool or geothermally influenced shallow lacustrine environment, surrounded by a subaerial travertine apron terrace; whereas, the Macanudo Sur Outcrop is a subaerial travertine mound sequence. Structurally controlled vent areas occur in both northern (F1) and southern (F6) outcrops, mainly located along regional NNE- and ENE-trending faults. The other sedimentary units display a concentric distribution of travertine facies with respect to the interpreted vent areas. The El Macanudo palaeo-hot spring deposit is situated in an eroded Jurassic volcanic centre, and records a complex evolutionary-fluid history. The sediments archived three different Jurassic events, when large and long-lasting hydrothermal systems were active across the region. This relative temporal sequence was formed by: (1) travertine precipitation; (2) development of a silica cap, where early silicification was responsible for exceptional preservation of some stromatolitic fabrics; and (3) acid alteration, recorded by dissolution textures and clay formation, and caused by a palaeo-phreatic water-level drop.

Gold enrichment in active geothermal systems by accumulating colloidal suspensions

The origins of high-grade hydrothermal ore deposits are debated, but active geothermal systems provide important clues to their formation. The highest concentrations of gold are found in geothermal systems with direct links to island arc magmatism. Yet, similar concentrations have also been found in the absence of any input from arc magmas, for example, in the Reykjanes geothermal field, Iceland. Here we analyse brine samples taken from deep wells at Reykjanes and find that gold concentrations in the reservoir zone have increased over the past seven years from an average of 3 ppb to 14 ppb. The metal concentrations greatly exceed the maximum solubility of gold in the reservoir under saturated conditions and are now nearly two orders of magnitude higher than in mid-ocean ridge black smoker fluids—the direct analogues of Reykjanes deep liquids. We suggest that ongoing extraction of brine, the resulting pressure drop, and increased boiling have caused gold to drop out of solution and become trapped in the reservoir as a colloidal suspension. This process may explain how the stock of metal in the reservoirs of fossil geothermal systems could have increased over time and thus become available for the formation of gold-rich ore deposits.

New isotopic evidence bearing on bonanza (Au-Ag) epithermal ore-forming processes

New Cu, S, and Pb isotope data provide evidence for a magmatic source of metal(loid)s and sulfur in epithermal Au-Ag deposits even though their ore-forming solutions are composed primarily of heated meteoric (ground) waters. The apparent isotopic discrepancy between ore metals and ore-forming solutions, and even between the ore and associated gangue minerals, indicates two different sources of epithermal ore-forming constituents: (1) a shallow geothermal system that not only provides the bulk of water for the ore-forming solutions but also major chemical constituents leached from host rocks (silica, aluminum, potassium, sodium, calcium) to make gangue minerals and (2) metals and metalloids (As, Te, Sb, etc.) and sulfur (±Se) derived from deeper magma bodies. Isotopic data are consistent with either vapor-phase transport of metal(loids) and sulfur and their subsequent absorption by shallow geothermal waters or formation of metallic (Au, Ag, Cu phases) nanoparticles at depth from magmatic fluids prior to encountering the geothermal system. The latter is most consistent with ore textures that indicate physical transport and aggregation of nanoparticles were significant ore-forming processes. The recognition that epithermal Au-Ag ores form in tectonic settings that produce magmas capable of releasing metal-rich fluids necessary to form these deposits can refine exploration strategies that previously often have focused on locating fossil geothermal systems.

Seismic velocity structure of a fossilized Icelandic geothermal system: A combined laboratory and field study

Magmatic geothermal systems, as they exist in Iceland, are complex geological structures. Key features, such as hydrothermal upwelling zones and intrusive heat sources, are embedded in a highly heterogeneous host rock. The latter comprise quasi-horizontally layered basaltic lava flows of variable texture and morphology, repeatedly intruded by magma chambers and numerous intersecting sub-vertical dykes and sub-horizontal sheets. In order to estimate whether seismic techniques can detect the important geothermal features, this study examined the seismic velocity structure of the fossil geothermal system of Geitafell in southeast Iceland.We combined seismic tomography field experiments with ultrasonic measurements in the laboratory to obtain a comprehensive picture of the velocity systematics and to investigate the scale-dependence of experimental velocity determination. We recorded six shallow seismic profiles over outcrops of different parts of the exposed magmatic system and we investigated 10 specimens of basalt, dolerite, and gabbro in the laboratory. Our results demonstrate that in the fossilized geothermal system of Geitafell, seismic velocities can vary over a wide range of around 1500m/s. Considering this large spread of wavespeeds, velocity anomalies caused by geothermal activity are likely to be masked by the heterogeneity of the host rock. This places stringent demands on acquiring high quality data and an optimal survey design for successful seismic exploration. Moreover, we discovered that ultrasonic velocities measured in the laboratory under comparable pressure (depth) conditions are up to 15% higher than seismic velocities inverted from the field data. This is of great importance and must be taken into account when interpreting exploration-scale field tomograms with the help of laboratory test data.

Diagenesis versus hydrothermalism and fluid–rock interaction within the Tuscan Nappe of the Monte Amiata CO2‐rich geothermal area (Italy)

AbstractIn southern Tuscany (central Italy), deep to shallow geothermal systems were active since the Pleistocene and comprise fluids carrying variable amounts of CO2. The Monte Amiata geothermal fields include two main reservoirs: a deep one located in the Palaeozoic metamorphic succession (1300–3000 m deep) belonging to the Tuscan metamorphic ‘basement’ and a shallow one hosted in the Mesozoic sedimentary succession (500–1000 m deep) belonging to the Tuscan Nappe. Multiple sets of calcite veins were investigated in some outcrops of the Tuscan Nappe succession exposed in the Monte Amiata area and surroundings. Two main fluid systems were characterized: the former related to combined diagenetic syn‐tectonic processes from highly saline fluids after interaction with the Late Triassic evaporites (Burano Fm) and the latter related to a fossil geothermal system, Pleistocene in age, fed by low‐salinity meteoric fluids, carrying CO2, radiogenic Sr and heavy O isotopes after interaction with the subsurface rocks. Comparison with the present‐day hydrothermal fluids allowed to sketch the thermal evolution of the system from past to present. Geochemical data proved that portions of the vein–host rocks were reset after interaction with CO2‐rich fluids circulating in the past. Consequently, textural and mineralogical changes are expected to have occurred with respect to the undisturbed rock masses. Understanding these modifications, their location and extension, is a prerequisite to construct (i) geochemical models addressed to the comprehension of the diagenesis induced in sediments after long‐term CO2 storage; and (ii) geological models applied to the prediction of rock heterogeneity distribution within the upper reservoirs, characterizing the shallow (500–1500 m) geothermal systems of southern Tuscany.

Diverse subaerial and sublacustrine hot spring settings of the Cerro Negro epithermal system (Jurassic, Deseado Massif), Patagonia, Argentina

The Late Jurassic (~150Ma) Cerro Negro volcanic–epithermal–geothermal system (~15km2 area), Deseado Massif, Patagonia, Argentina, includes two inferred volcanic emission centers characterized by rhyolitic domes linked along NW–SE regional faults that are associated with deeper level Au/Ag mineralization to the NW, and with shallow epithermal quartz veins and mainly travertine surface hot spring manifestations to the SE. Some travertines are silica-replaced, and siliceous and mixed silica–carbonate geothermal deposits also are found. Five hot spring-related facies associations were mapped in detail, which show morphological and textural similarities to Pleistocene–Recent geothermal deposits at Yellowstone National Park (U.S.A.), the Kenya Rift Valley, and elsewhere. They are interpreted to represent subaerial travertine fissure ridge/mound deposits (low-flow spring discharge) and apron terraces (high-flow spring discharge), as well as mixed silica–carbonate lake margin and shallow lake terrace vent-conduit tubes, stromatolitic mounds, and volcano-shaped cones. The nearly 200 mapped fossil vent-associated deposits at Cerro Negro are on a geographical and numerical scale comparable with subaerial and sublacustrine hydrothermal vents at Mammoth Hot Springs, and affiliated with Yellowstone Lake, respectively. Overall, the Cerro Negro geothermal system yields paleoenvironmentally significant textural details of variable quality, owing to both the differential preservation potential of particular subaerial versus subaqueous facies, as well as to the timing and extent of carbonate diagenesis and silica replacement of some deposits. For example, the western fault associated with the Eureka epithermal quartz vein facilitated early silicification of the travertine deposits in the SE volcanic emission center, thereby preserving high-quality, microbial macro- and micro-textures of this silica-replaced “pseudosinter.” Cerro Negro provides an opportunity to reconstruct paleogeographic, paleohydrologic and paleoenvironmental associations in a well-exposed, extensive and diverse fossil geothermal system. This Late Jurassic hydrothermal deposit will likely contribute to a better understanding of the impact of depositional and post-depositional history on the development and long-term preservation potential of Lagerstätte in epithermal settings and, more generally, in extreme environments of the geological record.

Patrones químicos y origen del depósito de oro de La Libertad, Chontales, Nicaragua

The La Libertad region is situated in the central part of Nicaragua, within a broad discontinuous belt of Tertiary volcanic rocks that contains several epithermal gold and silver deposits and extends from Guatemala (and Mexico) to Costa Rica. A chemical comparision between altered and unaltered basic lavas of this area shows that chemical changes associated with the geothermal field type of alteration centered at the mining district reach more than 5 km away from it. Water content, CO2, K and S have beeen added, titanium seem to have been inmobile and Cl partily lost from the fossil geothermal system. Gold, originally concentrated in the glass of basic lavas, was leached during zeolite facies conditions and precipitated with silica in fractures, forming veins on the center of the geothermal field. An estimate shows that the amount of Au released during the alteration was sufficient to give origin to La Libertad deposit.

Hydrologic models of modern and fossil geothermal systems in the Great Basin: Genetic implications for epithermal Au-Ag and Carlin-type gold deposits

The Great Basin region in the western United States contains active geothermal systems, large epithermal Au-Ag deposits, and world-class Carlin-type gold deposits. Temperature profiles, fluid inclusion studies, and isotopic evidence suggest that modern and fossil hydrothermal systems associated with gold mineralization share many common features, including the absence of a clear magmatic fluid source, discharge areas restricted to fault zones, and remarkably high temperatures (>200 °C) at shallow depths (200–1500 m). While the plumbing of these systems varies, geochemical and isotopic data collected at the Dixie Valley and Beowawe geothermal systems suggest that fluid circulation along fault zones was relatively deep (>5 km) and comprised of relatively unexchanged Pleistocene meteoric water with small (<2.5‰) shifts from the meteoric water line (MWL). Many fossil ore-forming systems were also dominated by meteoric water, but usually exhibit δ18O fluid-rock interactions with larger shifts of 5‰–20‰ from the MWL. Here we present a suite of two-dimensional regional (100 km) and local (40–50 km) scale hydrologic models that we have used to study the plumbing of modern and Tertiary hydrothermal systems of the Great Basin. Geologically and geophysically consistent cross sections were used to generate somewhat idealized hydrogeologic models for these systems that include the most important faults, aquifers, and confining units in their approximate configurations. Multiple constraints were used, including enthalpy, δ18O, silica compositions of fluids and/or rocks, groundwater residence times, fluid inclusion homogenization temperatures, and apatite fission track anomalies. Our results suggest that these hydrothermal systems were driven by natural thermal convection along anisotropic, subvertical faults connected in many cases at depth by permeable aquifers within favorable lithostratigraphic horizons. Those with minimal fluid δ18O shifts are restricted to high-permeability fault zones and relatively small-scale (~5 km), single-pass flow systems (e.g., Beowawe). Those with intermediate to large isotopic shifts (e.g., epithermal and Carlin-type Au) had larger-scale (~15 km) loop convection cells with a greater component of flow through marine sedimentary rocks at lower water/rock ratios and greater endowments of gold. Enthalpy calculations constrain the duration of Carlin-type gold systems to probably <200 k.y. Shallow heat flow gradients and fluid silica concentrations suggest that the duration of the modern Beowawe system is <5 k.y. However, fluid flow at Beowawe during the Quaternary must have been episodic with a net duration of ~200 k.y. to account for the amount of silica in the sinter deposits. In the Carlin trend, fluid circulation extended down into Paleozoic siliciclastic rocks, which afforded more mixing with isotopically enriched higher enthalpy fluids. Computed fission track ages along the Carlin trend included the convective effects, and ranged between 91.6 and 35.3 Ma. Older fission track ages occurred in zones of groundwater recharge, and the younger ages occurred in discharge areas. This is largely consistent with fission track ages reported in recent studies. We found that either an amagmatic system with more permeable faults (10−11 m2) or a magmatic system with less permeable faults (10−13 m2) could account for the published isotopic and thermal data along the Carlin trend systems. Localized high heat flow beneath the Muleshoe fault was needed to match fluid inclusion temperatures at Mule Canyon. However, both magmatic and amagmatic scenarios require the existence of deep, permeable faults to bring hot fluids to the near surface.

Fluid Inclusions in Epidote Minerals and Fluid Development in Epidote-Bearing Rocks

The widespread occurrence of epidote minerals in metamorphic and igneous rocks as well as in many ore deposit types makes it a promising candidate for fluid inclusion studies. Apart from high- to very high-temperature and low- to intermediate-pressure conditions, epidote minerals are stable over a wide range of pressure and temperature in the continental and oceanic crust (e.g., Poli and Schmidt 1998). Yet fluid inclusion studies on epidote minerals are surprisingly scarce, even in fluid-saturated environments like certain vein-type deposits or hydrothermal-volcanic vugs and druses. For example, epidote minerals are not mentioned in the subject index of Roedder’s (1984) outstanding summary and review of fluid inclusion studies and occurrences, which lists more than sixty different host minerals for fluid inclusions. Nonetheless, more recent studies showed fluid inclusions in epidote minerals to be the only direct witness of the physiochemical and compositional fluid evolution during certain geodynamic processes mainly found in fossil geothermal systems, ore deposits and high-pressure to ultra-high pressure rocks. The aim of this review is to outline and summarize some aspects and interpretations of geodynamic processes, which are based on temperature ( T ), pressure ( P ), molar volume ( V ) and composition ( X ) data from fluid inclusions in epidote minerals as well as associated host minerals from various geological environments. The review starts with a chapter on some typical mixed volatile solid-fluid equilibria involving epidote minerals, which are relevant for the here discussed environments. This is followed by a short introduction into the basic concepts of fluid inclusion research and the role of epidote minerals. The next section covers fluid inclusion studies on epidote minerals from active and fossil geothermal systems as well as low-grade metamorphic rocks and constraints on the P - T - X properties of the fluids present in these systems. This is followed by a short introduction …