Hydrothermal Fluid Flow Research Articles

Dynamics of coupled olivine dissolution and serpentine precipitation revealed by hydrothermal flow-through experiments at 260 °C–300 °C

Serpentinization is an important process that influences rock rheology, global fluid circulation, and microbiological activity in the deep sea, and is induced by hydrothermal fluid flow. Elemental olivine dissolution and serpentine precipitation govern the overall dynamics of serpentinization, and its prediction and modeling require an understanding of the reaction mechanism including the rate-limiting process. In this study, we examined the initial stage of coupled olivine dissolution and serpentine precipitation at 260 °C–300 °C and 50 MPa using a hydrothermal flow-through experimental apparatus. Saline water (0.5 m NaCl) was continuously injected into olivine powder at a constant flow rate (3.3 × 10−5 L s−1). Petrologic observation of solid products after experimental runs revealed that serpentine is produced from olivine. The hydration flux of serpentinization in this experiment is similar to estimates made using closed-system experiments, indicating that fluid flow does not affect the hydration rate at the flow rates adopted. Moreover, mass-balance calculations based on outlet fluid compositions during experiments suggest that both the overall serpentine precipitation and olivine dissolution rates decrease over time, and the serpentine precipitation rate is lower than the olivine dissolution rate during serpentinization. Although a comparison of speciated solutions observed in this experiment with results obtained by kinetic reaction modeling did not constrain the relationship between serpentine precipitation and olivine dissolution, a comparison with other experimental data and kinetic modeling results indicates that serpentine precipitation occurs more slowly than olivine dissolution at 170 °C. These results suggest that the relationship between serpentine precipitation and olivine dissolution kinetics varies with temperature and time, with such dynamic behavior governing serpentinization processes in the oceanic crust.

New constraints on the timing and character of the Laramide Orogeny and associated gold mineralization in SE California, USA

Abstract The timing of deformation and associated gold mineralization in SE California, USA, is contentious, partly due to the challenges involved with dating ductile deformation. We therefore combine modern geo- and thermochronology with field and microscopic observations to show that the Cargo Muchacho Mountains preserve evidence of northward thrusting in a kilometer-scale ductile shear zone during the Late Cretaceous Laramide Orogeny, accompanied by hydrothermal fluid flow, gold mineralization, and pegmatite emplacement. Penetrative strain was largely accommodated within the Jurassic metavolcaniclastic Tumco Formation, whereas intrusive Jurassic granitoids behaved as competent bodies. Quartz microstructures suggest deformation at ~500 °C, which is consistent with fabrics defined by amphibolite facies minerals. The timing of thrusting is constrained by dynamically recrystallized titanite with a U-Pb age of 68 ± 1 Ma and late syn-kinematic pegmatites that yield U-Pb zircon ages of 65.0 ± 4.2–63.2 ± 4.8 Ma. Syn-kinematic fluid flow was focused into a lateral thrust ramp where the shear zone foliation was deflected around a relatively rigid pluton, creating zones rich in magnetite-quartz veins and epidote, and precipitating gold associated with pyrite and chalcopyrite. Dating of these sulfides via Re-Os yields an age of 64.7 ± 0.8 Ma, which confirms a Laramide age for the gold mineralization. Together, apatite from the pegmatites and a nearby Jurassic granite yields a U-Pb age of 60.4 ± 3.5 Ma, reflecting cooling to below 530–450 °C. Comparison with published studies suggests that thick-skinned deformation in the Cargo Muchacho Mountains was driven by flat-slab subduction of the conjugate Hess Plateau, which occurred several million years after and to the south of flat-slab subduction of the conjugate Shatsky Rise. This suggests that the conjugate Hess Plateau may have been subducted up to several hundred kilometers farther north than previously thought. Metamorphic devolatilization of underplated Orocopia Schist likely generated the gold-bearing hydrothermal fluids, and anatexis of the schist formed the peraluminous pegmatites, which highlights the importance of schist underplating and devolatilization along much of the Californian and Mexican cordillera.

Effect of Fault Extension Relevant to Unconformity on Hydrothermal Fluid Flow, Mass Transport, and Uranium Deposition

In this study, a conceptual model is developed based on common features of typical unconformity-related uranium deposits in the Athabasca Basin, Canada. Three reactive flow modeling scenarios are designed to address the effect of fault extension on the formation of uranium deposits. Our results indicate that the location of the fault zone relevant to the unconformity is crucial to the fluid circulation in both the sandstone layer and the basement unit, the temperature distribution, the transport of aqueous components, and the uranium deposition. In particular, this research reveals that the circulating pattern of the basement brine is critical for the ore genesis. The reducing basal brine is capable of carrying aqueous uranium from depth to react with the shallow oxidizing fluid, being percolated to the basement from the overlain sandstone layer, for uranium precipitation. Scenarios 1 and 2, in which the fault zone is mainly in the basement, are in favor of focusing ore-forming hydrothermal fluids into the footwall area in the basement, leading to the formation of uranium deposits therein. Scenario 3, in which the fault zone is mainly in the sandstone layer with a limited extension below the unconformity, is unfavorable for the focusing of fluids, and hence no significant deposits can be formed, except for some minor uranium mineralization occurring in the footwall and other areas in the basement that are spatially associated with the upwelling flow zones in the sandstone layer.

Iron oxide chemistry supports a multistage hydrothermal genesis of BIF-hosted hematite ore in the Mt. Tom Price and Mt. Whaleback deposits

This study demonstrates the great value of iron oxide chemistry in low-temperature systems. Hematite records elemental signatures of structural control, partial inheritance of pre-existing alteration, and allows chemical fingerprinting of multistage BIF-hosted ore.Mt. Whaleback and Mt. Tom Price are Western Australia’s largest banded iron formation (BIF) hosted hematite ore deposits. The mechanisms and timing of iron enrichment from BIF (with ∼20–40 wt% Fe) to high-grade iron ore (with up to 69 wt% Fe) are still controversially debated. In-situ iron oxide chemistry support post-metamorphic, structurally controlled, hydrothermal hematite ore formation via multiple stages of hydrothermally altered BIF.Iron oxides in two hydrothermal alteration zones in BIF at Mt. Tom Price (distal alteration assemblages: magnetite-siderite-stilpnomelane ± pyrite and intermediate alteration assemblages: hematite-magnetite-ankerite-chlorite) show similar trace element patterns that are distinct from metamorphic magnetite in least-altered magnetite-quartz ± hematite ± Fe-carbonate ± Fe-silicates BIF. At Mt. Whaleback, the hydrothermal alteration is completely eradicated by subsequent microplaty-martite hematite ore formation, and solely recorded in hematite composition by the inheritance of various conservative element abundances.Petrographically indistinct zones within a given orebody are chemically distinct and demonstrate formation via contrasting structurally controlled hydrothermal fluid flow: At Mt. Whaleback, specifically in the vicinity of Central Fault and Whaleback Fault, systematic trace element group abundance trends are recorded: major controls are fO2, pH, and acquired country rock signatures, i.e., evolved (Al, Mn, Mo, Pb, U) at Central Fault, and primitive (Al, Cr, Ti, V, Cu, Ni, Co) at Whaleback fault. Complex zoned growth textures and element distribution in microplaty hematite support precipitation during protracted hydrothermal fluid/rock interaction. In combination, our results falsify the importance of alternative hematite crystallisation models, including goethite dehydration and coalescent nanoparticles.

Recognition of a widespread Paleoproterozoic hydrothermal system in the southern McArthur Basin, northern Australia, by in-situ analysis of fine-grained pyrite and spatially-associated solid bitumen in the Lamont Pass palaeohigh

The Paleoproterozoic hydrothermal system in the southern McArthur Basin, northern Australia, played a key role in the formation of the world-class clastic-dominant (CD)-type McArthur River Zn-Pb deposit, as well as several small-sized deposits nearby in this basin. However, the spatial scale of this hydrothermal system and its role in controlling mineralisation are uncertain. Such understanding is crucial for improving mineral exploration efficiency. In this study, we characterised the geochemistry of the fine-grained (<10 μm) pyrite and the co-existing solid bitumen in the non-mineralised Paleoproterozoic Barney Creek Formation (BCF) intersected by the drill-hole Lamont Pass 3, with a novel combination of Raman spectroscopy and nanoscale secondary ion mass spectrometry (Nano-SIMS), to underpin any potential hydrothermal activities distal (∼50 km) to the McArthur River deposit. In the studied samples, fine-grained pyrites display sub-rounded, elongated and ultrafine-grained morphologies and occur as grain clusters spatially closely-associated with solid bitumen. Elemental maps obtained from Nano-SIMS reveal that some pyrites are enriched in trace elements, such as Pb, Tl, Ni, Co, and Cu, occurring as individual grains or overgrowth over the trace element-depleted pyrites. Fine-grained pyrites in the studied samples show similar trace element enrichment patterns to those often observed in the McArthur River deposit. The trace element-rich and -poor pyrites are interpreted to form as a result of hydrothermal and diagenetic processes, respectively. Formation temperatures of solid bitumen were estimated as ∼ 170 ℃ based on Raman spectral and reflectance data, approximating temperatures of hydrothermal fluids responsible for the McArthur River deposit. It is interpreted that solid bitumen may be either formed syngenetically with diagenetic pyrites and thermally reworked by hydrothermal fluids or formed contemporaneously with hydrothermal pyrite. The in-situ analysis of fine-grained pyrite and spatially-associated solid bitumen indicates that hydrothermal activity in the study area is consistent with the fluids responsible for the McArthur River deposit with respect to the trace element composition and fluid temperature. Hydrothermal fluid flow in the study area was likely a far-field effect of the McArthur River mineralisation extending along the Emu Fault. The widespread hydrothermal activity in the Barney Creek Formation strongly suggests that other factors, such as the fluid migration conduits (e.g. large-scale faults), are important controls for mineralization in this region.

Hydrothermal fluid flow triggered by an earthquake in Iceland

Microearthquake hypocenters were analyzed in the Krýsuvík geothermal area in SW-Iceland with data taken from two consecutive passive seismic surveys, 2005 and 2009. Five years prior to the 2005 survey, this area was struck by an earthquake initiating a major top-to-bottom fluid migration in the upper crust. We observe from our surveys a complex bottom-to-top migration of seismicity with time following this fluid penetration, suggesting the migration of a pore pressure front controlled by the upper-crust fracture system. We interpret these data as the time and space development of high-temperature hydrothermal cells from a deep upper crustal fluid reservoir in the supercritical field. These results provide an insight into the coupling mechanisms between active tectonics and fluid flow in upper-crustal extensional systems with high thermal flux.

Regional and local controls of hydrothermal fluid flow and gold mineralization in the Sheba and Fairview mines, Barberton Greenstone Belt, South Africa

High-grade gold-quartz-sulphide lodes of the Sheba-Fairview Complex (SFC) of gold mines in the Barberton Greenstone Belt of South Africa describe a wide range of geometries, orientations, controls, and a number of different and often contradicting models have been postulated to account for their formation. This paper aims to present a holistic model that describes the focussing of a regional-scale fluid flow into narrower, mineralized structures during the late-stage refolding and tightening of the first-order folds that host the mineralization.The auriferous reefs of the SFC are hosted by well-bedded, low-grade metamorphic, clastic sedimentary rocks of the Paleoarchaean Fig Tree and Moodies groups that are preserved in two regional-scale, strongly curved and refolded synclinal structures along the northern margin of the belt. The majority (85 %) of presently and historically mined orebodies occur in the broad hinge zone of the refolded synclines. This suggests the regional-scale fluid redistribution from fold limbs into the first-order fold hinge following regional hydraulic gradients during refolding. Ubiquitous bedding-parallel quartz (-carbonate-sulphide) veins and associated alteration underline the significance of bedding anisotropies and bedding-parallel flexural slip for regional-scale fluid flow during refolding. The focussing of these fluids and economic-grade gold mineralization occurs preferentially along contacts between lithologies with pronounced rheological contrasts. Lithological contacts combine the effects of strain localization in weaker units and the formation of fracture permeabilities in competent units across these contacts. Examples include kilometer-scale hinge-parallel ore shoots that are controlled by undulations (jogs) along low-displacement slip horizons between retrogressed ultramafic rocks and competent metasediments. In contrast, low-angle thrusts or steep reverse faults form accommodation structures related to the late-stage tightening of the first-order folds. These structures transect bedding at high angles and are, thus, ideally orientated to collect fluid from the regional system of bedding-parallel fluid flow.Mineralization in the SFC may be explained by the spatial and temporal coincidence of (1) late-stage NW-SE shortening and refolding, of (2) a well-bedded, anisotropic wall-rock sequence, associated with (3) regional-scale fluid redistribution along bedding planes during flexural slip, leading to (4) fluid focussing into either jog geometries or bedding discordant structures. The combination of these factors is unique to the SFC and accounts for the location of the main Barberton gold mines along the northwestern margin of the greenstone belt.

A Mississippi Valley–type Zn-Pb mineralizing system in South China constrained by in situ U-Pb dating of carbonates and barite and in situ S-Sr-Pb isotopes

AbstractThe ages of Zn-Pb deposits are exceptionally challenging to determine owing to the lack of suitable mineral chronometers and techniques. Here we present the first result for in situ LA-ICP-MS U-Pb dating of carbonates and barite from a Mississippi Valley– type (MVT) Zn-Pb deposit in South China. Hydrothermal dolomite in close textural and paragenetic association with Zn-Pb sulfides, and calcite and barite cement from the breccia ores, yield ages of 473.4 ± 2.7 Ma and 368.7 ± 3.1 Ma, respectively. Together with new in situ S-Pb-Sr isotope values, these data reveal an epigenetic Zn-Pb mineralization history, agreeing well with a model involving basinal brine accumulation and MVT Zn-Pb sulfide precipitation. Because carbonate is a common mineral in Zn-Pb deposits worldwide, and other minerals in such deposits suitable for isotope dating are generally absent, in situ U-Pb dating of gangue carbonates opens a new window for better defining the ore genesis of this globally important Zn-Pb deposit type and for tracking hydrothermal fluid flow in sedimentary basins.

Late Jurassic-Early Cretaceous orogenic gold mineralization in the Klamath Mountains, California: Constraints from 40Ar/39Ar dating of hydrothermal muscovite

The Klamath Mountains gold province is the second most important historical producer in California, having produced more than 7 Moz of gold from both lode and placer sources. Hydrothermal muscovite grains from gold-bearing veins provide the first 40Ar/39Ar age constraints indicative of a protracted period of mineralization in the Klamath Mountains. The data indicate that the window for orogenic gold mineralization in the Klamath Mountains was from ~ 160–140 Ma, coinciding in age with the oldest orogenic gold deposits in the Sierra Nevada foothills province to the south, although mineralization continued in the latter along the Mother Lode belt for another 20–30 million years. Despite the broad age overlap between hydrothermal activity and magmatism, the former relates to a regional thermal event, and there is no genetic link between the two. Instead, a correlation exists between the timing of lode gold formation and discrete tectonic events, changes in stress regime, and fault movement. The maximum age corresponds to a major plate reorganization in the Pacific basin and initial gold mineralization continuing into the Sierra Nevada foothills gold province to the south. The minimum age corresponds to the westerly lateral offset of the Klamath Mountains from the Sierra Nevada and the then-active arc, marking the termination of both magmatism and gold-producing hydrothermal activity in the Klamath Mountains. Lode gold mineralization in the Klamath Mountains is compatible with a crustal source of metals and fluids that were released during metamorphic devolatilization and focused via hydrothermal fluid flow along regional faults.

In situ U-Pb and geochemical evidence for ancient Pb-loss during hydrothermal alteration producing apparent young concordant zircon dates in older tuffs

Zircon U-Pb geochronology is vital for providing age constraints for the calibration of the geologic timescale. The presence of zircon grains that yield concordant ages younger than the host sedimentary rock is often ignored, attributed to young outliers and possible contaminants. Using in situ U-Pb dating techniques, which analyze zircon crystals drilled directly from polished thin sections, two tuff beds from the Brockman Iron Formation, Pilbara Craton, Australia, yielded weighted mean 207Pb/206Pb ages of 2466 ± 6 Ma and 2469 ± 5 Ma (95% confidence level), consistent with independent age constraints. The tuffs also contain zircons that gave younger, yet concordant, U-Pb dates (2450–2270 Ma) as well as some with highly discordant dates. The “young” zircons have low or negligible common Pb, and display magmatic-type euhedral outlines, oscillatory zonation and Th/U ratios, indistinguishable from syn-depositional zircons, and cannot be readily dismissed on the grounds of high discordance or common-Pb contents. The “young” zircon dates are statistical outliers that overlap with the age of hydrothermal xenotime in deformed, bedding-parallel veins in shale that underlies the iron formation, which yields an age of 2383 ± 4 Ma (95% confidence level). The xenotime in overpressure veins provides independent evidence for hydrothermal fluid flow broadly coeval with isotopic resetting of “young” zircon crystals. Although SEM-CL imaging found no evidence for alteration, element mapping shows that most zircon grains have been altered by fluid infiltration, which proceeded along fractures or radiation-damaged growth zones leading to elevated Fe, Ca and Y contents in the altered domains. Total and partial Pb loss caused by hydrothermal alteration can explain the young concordant and discordant ages, respectively. These results have implications for the use of U-Pb zircon geochronology, particularly in detrital zircon studies, which routinely use the age of the youngest zircon grains to constrain the age of deposition. The erosion and re-deposition or in situ growth of young zircons in low-grade metasedimentary rocks may lead to erroneously young depositional ages.

Evolution and temporal constraints of a multiphase postglacial rock slope failure

Large rock slope failures are temporal processes which act to modify the landscape after glacial retreat. The slope failure process often shows a lag time of thousands of years after deglaciation, with multiple failure events possible. While global datasets constrain this lag time from extensive mapping and dating of paraglacial rock avalanches, the timeline is poorly refined in northern Norway. We present a case study of multiphase failure at Skredkallen on Vanna, one of a group of coastal islands in Troms, northern Norway. The site contains an actively deforming rock slope above a large rock avalanche deposit. The rock slope deformation (RSD) is a system of fractured and dislocated blocks up to 3 Mm3, and is moving slowly ~5 mm/yr downslope to the north-east. The metasedimentary rock mass contains four pervasive joint sets and a foliation, contributing to a compound structure failure mechanism. The rock mass is further weakened by foliation-parallel sheared mylonite, and the presence of a brittle fault in the immediate area, with evidence of hydrothermal fluid flow though the RSD.The rock avalanche deposit below the slope deformation is calculated to be 3 Mm3, and extends >1 km from the source area, displaying typical mobility for north Norwegian rock avalanches onto undrained sediments. The deposits showcase exceptional lobate morphology with elongated ridge-and-furrow features. Raised shorelines predating and postdating the deposit provide temporal constraints on the deposit and an opportunity to reconstruct a relative timeline for the slope evolution. The postglacial marine limit (>14 cal. ka BP) is obscured by the deposit, while shorelines corresponding to the early Younger Dryas (12.2 cal. ka BP) and the subsequent Tapes transgression maximum (7.6–7.2 cal. ka BP) are prominent across the deposits, implying that the avalanche was emplaced between 15 and 12.2 cal. ka BP. Failure occurred during a time of immense climate instability at the boundary to the early Holocene, consistent with global reports of mountain slope failure following glacial retreat. The avalanche was emplaced into what would have been the marine environment. The anomaly between the rock avalanche source area volume (35 Mm3), and the rock avalanche deposit implies previous failure events, the deposits of which were either removed due to failure of the underlying marine sediments into the fjord, by retreating glacial ice or scour. The initiation of movement at the RSD may be attributed to periods of local climate changes, such as the Holocene Thermal Maximum. Cosmogenic nuclide dating is suggested as the next step to fill gaps in the slope evolution story through the mid to late Holocene.

Insights from modern diffuse-flow hydrothermal systems into the origin of post-GOE deep-water Fe-Si precipitates

Post-GOE deep-water Fe-Si precipitates associated with volcanogenic massive sulfide deposits are an important feature of the Proterozoic rock record. Although it is clear that these enigmatic deposits formed in oxygen-deficient and hydrothermally influenced deep-water settings, the oxidation mechanism(s) resulting in their precipitation remain(s) unclear. Whilst existing genetic models typically couple direct and/or bacterially-mediated iron oxidation with abiotic silica precipitation, low temperature diffuse hydrothermal fluids offer a potential mechanistic alternative to explain the observed layering. Herein, via combination of petrographic observations with elemental and isotopic data (Fe, Si, and O), we explore the genesis of the primary mineral phases present within recent Fe-Si precipitates obtained from the Southwest Indian Ridge. Formation of ferrihydrite and opal-A in these precipitates provides insight into the genesis of widely invoked precursor minerals to post-GOE deep-water Fe-Si precipitates. Specifically, we find that the mineralogical layers that typify these Fe-Si precipitates may have originated via biologically mediated ferrihydrite precipitation and abiotic precipitation of opal-A from oversaturated fluids during diffuse flow. By analogy, we propose that diffuse hydrothermal fluid flow played an important role in the formation of post-GOE deep-water Fe-Si precipitates.

Preface to Advances in the Jurassic of Argentina

Mocho-Choshuenco Volcanic Complex (MCVC) is one of the most hazardous volcanoes in Chile's Southern Volcanic Zone (SVZ). It is located at 40°S 72°W in Los Ríos Region, Chile, where the SVZ coexists with the margin-parallel Liquiñe-Ofqui Fault System (LOFS) and with NW-striking Andean Transverse Faults (ATF). The LOFS is 1200 km-long intra-arc strike-slip fault system located between 38° and 46°S, and partially controls the occurrence, spatial distribution and geometry of major stratovolcanoes. The fault-fracture networks induced by crustal deformation plays an essential role in the generation of pathways, facilitating the transport and emplacement of magmas. Thus, the Chilean Andes provides one of the best natural laboratories in the world to assess the link between tectonics and volcanism. In this study, we provide new structural, kinematic and dynamic insights, which would condition the formation and evolution of the MCVC and laying foundations for future works.Structural field data was collected and analyzed from regional and local scales. The strain analysis shows heterogeneous deformation. The stress field analysis at a regional scale indicates a strike-slip dominated transpressional regime with N61°E-trending σ1 and N26°W-trending σ3, which is consistent with the actual oblique convergence. Deformation driven by this oblique convergence is partially partitioned, where the margin-parallel component originates the dextral strike-slip in LOFS. Results are consistent with reactivation of preexisting NW-striking faults under the regional stress field, facilitating hydrothermal fluid flow and magma migration, associated with volcanoes, dikes and minor eruptive centers present in the study area. As a final result, we propose a simplified, three-dimensional model for the MCVC's basement, contributing to the understanding of the system and the way in which the different factors interact.

Mobilization of ore-forming metals during post-magmatic hydrothermal overprinting of the Huangshandong Ni–Cu sulfide deposit, Eastern Tianshan, NW China

The Huangshandong Ni–Cu sulfide deposit is the largest magmatic sulfide deposit in the Huangshan–Jing’erquan mafic–ultramafic belt in the Eastern Tianshan; it was overprinted by post-magmatic hydrothermal fluids, and metamorphosed and deformed as a result of regional ductile shearing. Ore-forming elements were remobilized as a result of post-magmatic hydrothermal fluid flow, but the extent of this remobilization is still unclear. In this contribution, detailed mineralogical–textural characteristics of silicates and base-metal sulfides in different types of ores in the Huangshandong Ni–Cu sulfide deposit were integrated with spatially resolved trace-element compositions of olivine, serpentine, pyrrhotite, pentlandite, and chalcopyrite to identify the metals that were mobilized, the scale and quantity of metal remobilization, and the phases into which the mobilized metals were incorporated. Based on varying degrees of serpentine and talc alteration, three different types of ores are identified – weakly serpentinized magmatic ores, which are most representative of the primary magmatic mineralization, strongly serpentinized and deformed ores, and talc-altered ores. The cracks consisting of serpentine and magnetite in weakly serpentinized magmatic ores caused by the expansion of olivine during serpentinization; this, serpentine is enriched in ore-forming metals (e.g., Ni, Co, Cu) relative to olivine. The expansion cracks extend from olivine to sulfides also enriched in ore-forming metals, suggesting that some metals were mobilized from olivine during serpentinization. Based on the higher Ni content of secondary pyrrhotite compared to primary pyrrhotite, it is suggested that base-metal sulfides in the primary, weakly serpentinized ores were dissolved and reprecipitated in the strongly serpentinized and deformed ores from metal-rich fluids that interacted with olivine.

Dynamic and kinematic characterization of the basement structures of the Mocho-Choshuenco Volcanic Complex, Southern Andes, Chile

Mocho-Choshuenco Volcanic Complex (MCVC) is one of the most hazardous volcanoes in Chile's Southern Volcanic Zone (SVZ). It is located at 40°S 72°W in Los Ríos Region, Chile, where the SVZ coexists with the margin-parallel Liquiñe-Ofqui Fault System (LOFS) and with NW-striking Andean Transverse Faults (ATF). The LOFS is 1200 km-long intra-arc strike-slip fault system located between 38° and 46°S, and partially controls the occurrence, spatial distribution and geometry of major stratovolcanoes. The fault-fracture networks induced by crustal deformation plays an essential role in the generation of pathways, facilitating the transport and emplacement of magmas. Thus, the Chilean Andes provides one of the best natural laboratories in the world to assess the link between tectonics and volcanism. In this study, we provide new structural, kinematic and dynamic insights, which would condition the formation and evolution of the MCVC and laying foundations for future works.Structural field data was collected and analyzed from regional and local scales. The strain analysis shows heterogeneous deformation. The stress field analysis at a regional scale indicates a strike-slip dominated transpressional regime with N61°E-trending σ1 and N26°W-trending σ3, which is consistent with the actual oblique convergence. Deformation driven by this oblique convergence is partially partitioned, where the margin-parallel component originates the dextral strike-slip in LOFS. Results are consistent with reactivation of preexisting NW-striking faults under the regional stress field, facilitating hydrothermal fluid flow and magma migration, associated with volcanoes, dikes and minor eruptive centers present in the study area. As a final result, we propose a simplified, three-dimensional model for the MCVC's basement, contributing to the understanding of the system and the way in which the different factors interact.

Evolution of Seismicity During a Stalled Episode of Reawakening at Cayambe Volcano, Ecuador

Cayambe Volcano is an ice-capped, 5,790 m high, andesitic-dacitic volcanic complex, located on the equator in the Eastern Cordillera of the Ecuadorian Andes. An eruption at Cayambe would pose considerable hazards to surrounding communities and a nationally significant agricultural industry. Although the only historically documented eruption was in 1785, it remains persistently restless and long-period (LP) seismicity has been consistently observed at the volcano for over 10 years. However, the sparse monitoring network, and complex interactions between the magmatic, hydrothermal, glacial, and tectonic systems, make unrest at Cayambe challenging to interpret. In June 2016 a seismic “crisis” began at Cayambe, as rates of high frequency volcano-tectonic (VT) earthquakes increased to hundreds of events per day, leading to speculation about the possibility of a forthcoming eruption. The crisis began 2 months after the Mw7.8 Pedernales earthquake, which occurred on the coast, 200 km from Cayambe. Here we show that the 2016 seismicity at Cayambe resulted from four distinct source processes. Cross correlation, template matching, and spectral analysis isolate two source regions for VT earthquakes–tectonic events from a regional fault system and more varied VTs from beneath the volcanic cone. The temporal evolution of the LP seismicity, and mean Q value of 9.9, indicate that these events are most likely generated by flow of hydrothermal fluids. These observations are consistent with a model where a new pulse of magma ascent initially stresses regional tectonic faults, and subsequently drives elevated VT seismicity in the edifice. We draw comparisons from models of volcano-tectonic interactions, and speculate that static stress changes from the Pedernales earthquake put Cayambe volcano in an area of dilation, providing a mechanism for magma ascent. Our findings provide a better understanding of “background” seismicity at Cayambe allowing faster characterization of future crises, and a benchmark to measure changes driven by rapid glacial retreat.

Antimony isotope fractionation in hydrothermal systems

In order to characterize antimony (Sb) isotope fractionation in hydrothermal systems, we present Sb isotope compositions of primary stibnite ores from a large Sb deposit in south China. A total number of 39 analyses reveals a large δ123Sb range of −0.27 to +0.86‰, representing an up to 1.13‰ variation in this hydrothermal system. A gradual increase of Sb isotope ratios from the proximal to the distal parts was observed in stibnite ores. Rayleigh distillation models the systematic variation trend of the Sb isotope values, demonstrating that ore fluids are preferentially enriched in heavier Sb isotopes during the precipitation of isotopically light stibnite. In this way, we consider that separation of stibnite from a Sb-bearing fluid related to reaction kinetics as a cause for Sb isotope fractionation. The modeled data constrain a Sb isotope fractionation factor (αfluid-stibnite) between hydrothermal fluid and stibnite at approximately 0.9994. The model also constrains an initial Sb isotope value of ~0.45‰, which indicates metal sources in the basement rocks of the study area. Given that distal stibnites possess higher Sb isotopic values, Sb isotopes could be used to fingerprint hydrothermal fluid flow and unravel different processes in natural systems.

3-D Geologic Controls of Hydrothermal Fluid Flow at Brady geothermal field, Nevada, USA

In many hydrothermal systems, fracture permeability along faults provides pathways for groundwater to transport heat from depth. Faulting generates a range of deformation styles that cross-cut heterogeneous geology, resulting in complex patterns of permeability, porosity, and hydraulic conductivity. Vertical connectivity (a throughgoing network of permeable areas that allows advection of heat from depth to the shallow subsurface) is rare and is confined to relatively small volumes that have highly variable spatial distribution. This local compartmentalization of connectivity represents a significant challenge to understanding hydrothermal circulation and for exploring, developing, and managing hydrothermal resources. Here, we present an evaluation of the geologic characteristics that control this compartmentalization in hydrothermal systems through 3-D analysis of the Brady geothermal field in western Nevada. A published 3-D geologic map of the Brady area is used as a basis to develop structural and geological variables that are hypothesized to control or effect permeability or connectivity. The 3-D distribution of these variables is compared to the distribution of productive and non-productive fluid flow intervals along production wells and non-productive wells via principal component analysis (PCA). This comparison elucidates which geologic and structural variables are most closely associated with productive fluid flow intervals. Results indicate that production intervals at Brady are located: (1) within or near to known and stress-loaded macro-scale faults, and (2) in areas of high fault and fracture density.

High-permeability zones in folded and faulted silicified carbonate rocks – Implications for karstified carbonate reservoirs

Ascending hydrothermal fluid flow and subsequent hypogene karstification can control the formation of high-permeability zones in folded or faulted carbonate rocks. This study presents a multidisciplinary and multiscale approach to investigate the relationships between regional deformation and subseismic karst conduits along fracture corridors. We investigate the Cristal Cave Karst System, which is 6.7 km long and occurs in Mesoproterozoic carbonate-siliciclastic units affected by hydrothermal silicification in the São Francisco Craton, Brazil. Based on integrated techniques of ALOS PALSAR imagery, structural field surveys, stratigraphic and laboratory analyses in the caves and the surrounding surface outcrops, we highlight the following points: (1) the cave system occurs in a regional folded sector; (2) the internal architecture of the large regional folds is composed of subseismic folds; (3) fracture corridors occur along the subseismic fold hinges, forming high-permeability zones; and (4) the principal cave passages, which are oriented NNE-SSW, are concentrated along these fold hinges. We discuss the results in terms of a spatiotemporal conceptual model and unravel the deformation and karstification stages, which are characterized by the development of (i) precontractional and (ii) contractional structural networks followed by (iii) hydrothermal silicification and karstification processes. This cave system can be used as an analog for deformed, silicified, and karstified carbonate reservoirs, allowing for the in situ characterization of subseismic structural features. This study provides the key for first-order prediction and characterization of high-permeability zone internal structures, where both deformation and karstification processes occur.

Unravelling the hydro-mechanical evolution of a porphyry-type deposit by using vein structures

The Permian San Pedro Cu-(Mo) deposit consists of a porphyry system with associated polymetallic veins in the San Rafael Massif, Argentina. This deposit resulted from the emplacement of a dioritic - tonalitic subvolcanic stock into the Early Permian Choiyoi volcanic rocks. Veins and veinlets linked to the hydrothermal alteration stages (potassic, phyllic and carbonitization) show different attitudes and are predominantly steeply dipping; sequentially, they fill four fracture systems: stockwork (potassic), dilatant WNW- & ENE-trending shear-compressional fractures (phyllic), NNE- & ~NS-striking tensional and shear-extensional fractures (phyllic), and NW/NNW- & WNW-trending tensional and shear-extensional fractures (carbonitization). Because of mechanical anisotropies of the host-rock, alteration telescoping, presence of hydrothermal fluids, thermodynamic constraints, and waning orogenic conditions, vein/veinlet overprinting is conspicuous. Focused hydrothermal fluid flow promoted episodic hydraulic reactivation and hydraulic fracturing, and reveals that differences in vein fabrics do not necessarily represents veins emplaced under different stress regimes. Based on the study of the structure of the area (including available geophysical maps) and its kinematics, the control of structures during vein emplacement, the vein/veinlet overprint sequence, and the thermo and physical conditions at the time of vein emplacement (using fluid inclusion data), it was possible to identify, characterize and track the changing hydro-mechanical evolution of this deposit as well as the progressive exhumation of the area during the time of vein emplacement.