Migration Of Hydrothermal Fluids Research Articles

Orange opals from Buriti dos Montes, Piauí: solid inclusions as genetic guides

Orange opals from Buriti dos Montes (Piauí, northeastern Brazil) have gemological properties that favor their use as jewelry; these characteristics include their colors, transparency, relatively high stability and hardness. The exotic content of solid inclusions provides greater beauty to the opals of this region. These opals originated from hydrothermal processes and are found mainly as veinlets and veins in the sandstones of the Serra Grande Group, sectioned by diabase dikes and sills of the Sardinha Formation. Solid inclusions, such as bubbles, botryoidal aggregates, dendrites, and nodules, among others, consist mainly of kaolinite, hematite/goethite and quartz and influence the chemical composition of opals. Intense zoning of quartz crystals and high values of Ba and Fe suggest that opal deposits were formed in a hydrothermal environment. Diabase dykes could have been responsible for heating the hydrothermal fluids. Sandstones, rich in aqueous solutions, also contributed to the available silica for the saturation of these solutions, and fractures enabled the migration and entrapment of hydrothermal fluids, resulting in the mineralized veins.

Phenyl derivatives of polycyclic aromatic compounds as indicators of hydrothermal activity in the Silurian black siliceous shales of the Bardzkie Mountains, Poland

High relative concentrations of phenyl (Ph) derivatives of polycyclic aromatic compounds (PACs) were found in samples from the Silurian Żdanów section (Bardzkie Mountains, Poland). These sedimentary rocks were characterized by the occurrence of all previously described Ph-PAC groups, including: phenylnaphthalenes, phenylphenanthrenes, terphenyls, phenyldibenzofurans, phenyldibenzothiophenes, and quaterphenyls, among which the phenylnaphthalenes and phenylphenanthrenes were most abundant. In all samples the thermally more stable isomers clearly dominated, while those with ortho- or α-substitution were present only as traces or absent. This suggests a high maturity of the investigated section, which was also confirmed by other molecular and petrographic thermal maturity parameters. Considering the highest abundances of Ph-PACs occur in the older part of the section, we suggest that the major factor controlling OM maturation was ascending hydrothermal fluids. Moreover, the significant peri-condensed aromatic compounds detected in the samples may have formed by dehydrocyclization of phenyl- and naphthyl-PAC precursors. If the formation of the PhPACs is strictly connected with alteration by ascending solutions, the interpretation of the inorganic geochemical results should be expanded to consider the impact of hydrothermal fluid migration.

Formation and age of sphalerite mineralization in carbonate rocks of Bajocian age in the Swiss Jura Mountains: evidence of Mesozoic hydrothermal activity

A combination of petrographic and geochemical techniques was applied to better constrain the origin and evolution of the fluid systems responsible for the formation of disseminated, Cd-rich (up to 0.6 wt%), sphalerite (ZnS) mineralization in the northeastern part of the Jura Mountains, Switzerland. The Rb–Sr ages of sphalerite samples indicate that a main phase of sphalerite formation occurred near the boundary between the late Middle and early Late Jurassic, at around 162 Ma. The negative δ34S values (−22.3 to −5.3 ‰) suggest that biogenic sulfide sulfur was involved in ZnS precipitation. The strontium isotope composition is more radiogenic than that of contemporaneous seawater, reflecting the interaction of mineralizing fluids with silicate rocks. Lead isotope signatures are very uniform (206Pb/204Pb = 18.63–18.67, 207Pb/204Pb = 15.63–15.64, 208Pb/204Pb = 38.51–38.63), indicating an isotopically well-homogenized fluid system. The basement rocks underlying the Jurassic strata are considered to be the main source of metals for the sphalerite mineralization. The migration of deep-sourced hydrothermal saline metal-bearing fluids into the Bajocian host carbonates containing sedimentary reduced sulfur resulted in the precipitation of sulfides. The period of sphalerite formation near the Middle–Late Jurassic boundary is characterized by enhanced tectonic and hydrothermal activity in Europe, related to the opening of the Central Atlantic and tectonic/thermal subsidence during spreading of the Alpine Tethys. Our study provides evidence that the Bajocian carbonate rocks in the Jura Mountains area were affected by the circulation of deep-sourced metal-bearing hydrothermal fluids in response to these continent-wide tectonothermal events. The presence of sphalerite mineralization and associated geochemical anomalies in Zn and Cd contents in carbonate rocks may also be used to trace basement features.

Coalified bitumens from the Kłodawa Salt Structure (central Poland) as evidence of migration of hydrothermal fluids in Zechstein (Upper Permian) deposits

The paper presents the results of petrographic and geochemical studies of coaly matter occurring in Stinking Shale and Main Dolomite (PZ2, Upper Permian) deposits in the Klodawa Salt Structure in central Poland. The coaly matter is both dispersed in the PZ2 shales and carbonates and concentrated in veins which crosscut these deposits and, depending on the location, depicts contrasting maceral composition and the rank of coal. The veins are built of colotellinite and micrinite, showing a mean random reflectance of coal in the range of 1.38–1.40%, whereas the dispersed organic matter is composed mostly of semifusinite and fusinite with minor vitrinite, showing a mean random reflectance of 0.82%. It is interpreted that the heterogeneous composition and rank of coal result from migration of hot fluids within a fracture system developed in the PZ2 shale and carbonate beds. The influx of hot fluids into the organic matter-rich beds led to the generation of bitumen which catagenetically altered along the hot fluid conduits. The coalification of bitumens took place at an early stage of tectonic deformation of the Zechstein salt series, initiated by regional extension and normal displacement on a basement fault, most probably in the Late Permian–Early Triassic.

Hydrothermal alteration of dolostones in the Lower Ordovician, Tarim Basin, NW China: Multiple constraints from petrology, isotope geochemistry and fluid inclusion microthermometry

Massive dolostones pervasively occur in the Lower Paleozoic strata in Tarim Basin, NW China. This study focuses on the hydrothermally-altered dolostones in the Lower Ordovician at Sancha, Bachu County where the hydrothermally-altered dolostones, marbles and diabase intrusions co-occur within a large-scale, NW trending strike-slip fault zone. Within this zone, the intrusive diabases mostly occur along the master border faults to the east and west, respectively. The limestones that got in contact with the intrusions commonly were marbleized and the antecedent dolostones were subject to intense hydrothermal alteration, particularly along fractured portions. Based on field investigations and petrographic examinations on the hydrothermally-altered dolostones, five distinctive fabrics of matrix and cement dolomites are distinguished. Two episodes of vein-filling calcites postdate the hydrothermally-precipitated saddle dolomites. Integrated isotopic geochemistry (C, O and Sr) and fluid inclusion microthermometry suggest that dolomite recrystallization and precipitation were associated with the hydrothermal activity induced by fracturing/faulting and intrusive magmatism. CAMECA SIMS U–Pb isotopic dating of zircon grains from the diabases yields an age of 290.5 ± 2.9 Ma in the Early Permian, which reconciles the relative timing revealed by the paragenetic history. The magmatic emplacement was thus linked to the large igneous province (LIP) of Early Permian in Tarim Basin. In this case, the widespread magmatic activities could have caused the thermal anomaly and fracturing in the strata that were penetrated by the intrusions, enhancing the thermal diffusion and hydrothermal fluid migration (although minor volumes of thermal fluids from the basic magmas) through the fracture/fault systems, thereby resulting in contact metamorphism (or marbleization), recrystallization of matrix dolomites and dolomite precipitation commonly in primary dolostone successions. This study provides a useful analogue to understand the dolomitization and relevant issues for the deeply-buried dolostones associated with tectono-magmatism within the Tarim Basin and elsewhere.

Repeating earthquakes in the Yellowstone volcanic field: Implications for rupture dynamics, ground deformation, and migration in earthquake swarms

We evaluated properties of Yellowstone earthquake swarms employing waveform multiplet analysis. Thirty-seven percent of the earthquakes in the Yellowstone caldera occur in multiplets and generally intensify in areas undergoing crustal subsidence. Outside the caldera, in the Hedgen Lake tectonic area, the clustering rate is higher, up to 75%. The Yellowstone seismicity follows a succession of two phases of earthquake sequence. The first phase is defined between swarms. It is characterized by a decay of clustering rate and by foreshock–aftershock sequences. The second phase is confined to swarms and is characterized by an increase in clustering rate, and dominant aftershock sequences. This phase reflects tectonic swarms that occur on short segments of optimally oriented faults. For example, the largest recorded swarm in Yellowstone occurred in autumn 1985 on the northwest side of the Yellowstone Plateau which was initiated as a tectonic source sequence. Fitting experimental dependence of fluid injection with intrusion migration suggests that the 1985 swarm involved, after 10days, hydrothermal fluids flowing outward from the caldera. The 2008–2009 Yellowstone Lake swarm exhibited a high migration rate of 1km/day, a decrease in clustering rate without a main-shock, and appears to be associated with magma injection of 1 to 5m3/s in a succession of migrating magma fronts that incrementally solidify and fracture at its brittle edges. The 2010 Madison Plateau earthquake swarm on the west side of the caldera initiated as a tectonic sequence but the expansion of the swarm front was associated with hydrothermal fluid migration.

Fracture mineralization and fluid flow evolution: an example from Ordovician–Devonian carbonates, southwestern Ontario, Canada

AbstractPetrography, geochemistry (stable and radiogenic isotopes), and fluid inclusion microthermometry of matrix dolomite, fracture‐filling calcite, and saddle dolomite in Ordovician to Devonian carbonates from southwestern Ontario, Canada, provide useful insights into fluid flow evolution during diagenesis. The calculated δ18Ofluid, ΣREE, and REESN patterns of matrix and saddle dolomite suggest diverse fluids were involved in dolomitization and/or recrystallization of dolomite. The 87Sr/86Sr ratios of dolomite of each succession vary from values in the range of coeval seawater to values more radiogenic than corresponding seawater, which indicate diagenetic fluids were influenced by significant water/rock interaction. High salinities (22.4–26.3 wt. % NaCl + CaCl2) of Silurian and Ordovician dolomite–hosted fluid inclusions indicate involvement of saline waters from dissolution of Silurian evaporites. High fluid inclusion homogenization temperatures (>100°C) in all samples from Devonian to Ordovician show temperatures higher than maximum burial (60–90°C) of their host strata and suggest involvement of hydrothermal fluids in precipitation and/or recrystallization of dolomite. A thermal anomaly over the mid‐continent rift during Devonian to Mississippian time likely was the source of excess heat in the basin. Thermal buoyancy resulting from this anomaly was the driving force for migration of hydrothermal fluids through regional aquifers from the center of the Michigan Basin toward its margin. The decreasing trend of homogenization temperatures from the basin center toward its margin further supports the interpreted migration of hydrothermal fluids from the basin center toward its margin. Hydrocarbon‐bearing fluid inclusions in late‐stage Devonian to Ordovician calcite cements with high homogenization temperatures (>80°C) and their 13C‐depleted values (approaching −32‰ PDB) indicate the close relationship between hydrothermal fluids and hydrocarbon migration.

Effects of host rock mineralogical composition and sedimentary facies on development of geochemical halos in Shahmirzad Pb-Zn deposits, central Alborz, Iran

Strata-bound carbonate hosted PbZn deposits or Mississippi Valley-type (MVT) deposits represent an economically important class of ore that received much attention in terms of defining effects of host rock petrography and sedimentary facies on development of geochemical halos. However there are few works on relation between sedimentary facies and MVT mineralizations in Iran. The Cretaceous carbonate rocks in the central Alborz (CA), northern Iran, host numerous small PbZn deposits and occurrences. The Shahmirzad PbZn deposits, located in the south of CA, geographically can be split into two groups, the north east (NEDs) and north west (NWDs) deposits. The Upper Cretaceous sequence in the NWDs area is composed of massive light gray limestone without any signs of later changes such as dolomitization and silicification. Additionally, three principal micro-facies are present: skeletal mudstone/wackestone (F1), biolithoclastic packstone (F2) and allochthonous bioclastic floutstone (F3). In contrast, the Cretaceous sequence hosting the NEDs is composed of massive brown limestone with six different types of dolomite (Rd1, Rd2, Rd3, Rd4, Cd1 and Cd2, the two last types are related to ore mineralization and open space filling) distinguished based on crystal size, distribution, and crystal boundary shape. The sedimentary succession in the NEDs indicates that the sedimentation took place in inner, mid and outer ramp environments, resulting in three different carbonate facies. In this area 15 carbonate facies were identified. The permeability and mineral compositions of these areas have obviously influenced the quantity of major and minor elements such as Fe (in average 0.24% in the NWDs and 0.12% in the NEDs), Si (in average 1.86% in the NWDs and 0.44% in the NEDs), Al (in average 0.50% in the NWDs and 0.13% in the NEDs), K (in average 0.24% in the NWDs and 0.06% in the NEDs), Sr (in average 685ppm in the NWDs and 204ppm in the NEDs), Na (in average 0.03% in the NWDs and 0.01% in the NEDs), Zn (in average 46ppm in the NWDs and 188ppm in the NEDs) and Pb (in average 3.3ppm in the NWDs and 83ppm in the NEDs). Not only are minor and major elements affected by changes in host rock mineral composition, but REEs decreased in concentration in dolomitic host rocks of NEDs. Some of the noticeable differences between the NWDs and the NEDs REEs data are i.e. La (in average 3.52ppm in the NWDs and 1.47ppm in the NEDs), Ce (in average 5.20ppm in the NWDs and 2.21ppm in the NEDs) and Eu (in average 0.13ppm in the NWDs and 0.03ppm in the NEDs). The combination of these data and host rock characteristics indicate that host rock compositions and textures effectively controlled these variations. We believe that perhaps the lack of dolomitization and low permeable facies in the NWDs precluded migration of diagenetic fluids and hydrothermal solutions in the carbonate host rocks. On the other hand, the presence of broad dolomitization and permeable facies in the NEDs carbonate host rocks may create appropriate conditions for diagenetic and hydrothermal fluid migration and interaction with the host rocks.

Tectonic–geochemical exploration modeling for characterizing geo-anomalies in southeastern Yunnan district, China

In this paper, a tectonic–geochemical exploration model is constructed to support mineral exploration in the southeastern Yunnan mineral district, China. Fault systems are significant to mineralization in this district because faulting activities have confined magmatic activities into certain spatial scales and temporal stages, which provide both hydrothermal fluids and heat for mineralization within fracture zones. Analysis of fault density using the singularity theory to characterize development of fault systems suggests that spaces produced by faulting activities favored the migration of hydrothermal fluids and mineralization. In addition, the singularity theory is applied to examine singular distributed ore-forming elements. Principal component analysis was used to model the spatial distribution of enrichment of the association of ore-forming elements, which suggests that geochemical haloes are linked with characteristic hydrothermal mineralization in the study area. For modeling mineral prospectivity, student's t-values were estimated by weights of evidence method to depict patterns of fault density and geochemical anomalies that are highly correlated with known mineral deposit occurrences in the study area. By overlaying these patterns, distributions of enrichments of associated ore-forming elements and spaces due to faulting that favored mineralization can be modeled and areas prospective for mineral exploration can be delineated. The consistency of the results with information from published documents demonstrates that the exploration model discussed in this paper is useful and effective for investigating geological issues related with faulting activities.

REE geochemistry of altered tectonites in the Huize base-metal district, Yunnan, China

Hydrothermally altered tectonites (e.g. mylonites, cataclasites) occur in ore-controlling fault zones in the Huize base-metal district. Non-altered wallrocks and altered but non-mineralized tectonites have similar REE characteristics indicating seawater origin of the host carbonate rocks but with significant terrigenous input. Calcite gangues in mineralized (argillaceous/barite-bearing) tectonites and tectonite ores contain the highest REE values and exhibit convex-upward and roughly roof-shaped REE patterns with positive Eu anomalies, which are typical of hydrothermal crustal fluids. Negative Ce and negative La anomalies in calcite gangues imply hydrothermal fluids that previously interacted with marine and terrigenous rocks underlying the carbonate host rocks. Similarities in the REE characteristics of mineralized tectonites with those of the non-altered wallrocks and non-mineralized tectonites support earlier propositions that hydrothermal fluids were derived from the Kunyang Group basement rocks. REE characteristics of mineralized (argillaceous/barite-bearing) tectonites indicate fluid-wallrock interactions during migration of hydrothermal fluids along faults. Convex-upward REE patterns of tectonite ores were likely due to remobilization of early-formed base-metal sulphides whereas convex-upward REE patterns of strongly mineralized tectonites (i.e. with disseminated–massive base-metal sulphides) were likely due to recrystallization of early formed calcite gangues. REE characteristics of altered tectonites due to fluid-rock interactions during fluid migration and metal/mineral remobilization/recrystallization are depicted in Y/Ho–La/Ho, Tb/Ca–Tb/La, and Yb/Ca–Yb/La plots of the data. Migration of hydrothermal fluids along ore-controlling faults was likely induced by seismic pumping associated with tectonic deformation, whereas the remobilization/recrystallization process likely occurred in ‘pressure shadows’ along ore-controlling faults.

Earthquake swarm and b-value characterization of the Yellowstone volcano-tectonic system

The Yellowstone volcanic field, Yellowstone National Park, is one of the most seismically active areas of the western U.S., experiencing the deadly 1959 M7.5 Hebgen Lake, MT, earthquake adjacent to the 0.64-Ma caldera, as well as more than 30,000 earthquakes from 1973 to 2007. This well-recorded seismic activity offers the opportunity to study the temporal and spatial occurrence of earthquakes and extensive earthquake swarms and how they relate to active volcanic and tectonic processes. We characterize the distribution of earthquakes by analyzing the rate of occurrence characterized by the b-value. To accurately determine b-values, the earthquake catalog was filtered to identify statistically time- and spatially-dependent related events, defined as swarms, from independent single main and aftershocks. An algorithm was employed that identified 69 swarms for 1984–2006 based on inter-event times and spatial clustering. The swarms varied in duration from 1 to 46 days with the number of events varying from 30 to 722 with magnitudes of − 1.2 to 4.8. All of the swarm events as well as the 597 events triggered by the 2002 Denali fault, AK, earthquake were removed from the catalog for analysis. The catalog data were then filtered for a magnitude of completeness ( M COMP) of 1.5 and the b-value distribution for the Yellowstone region was determined with the de-swarmed data. b-values ranged from 0.6 ± 0.1 to 1.5 ± 0.05 with the highest values associated with the youthful 150,000-year old Mallard Lake resurgent dome. These variations are interpreted to be related to variations in stresses accompanying the migration of magmatic and hydrothermal fluids. An area of high b-values (up to 1.3 ± 0.1) associated with the Hebgen Lake fault zone west of the Yellowstone caldera could be related to the transport of magmatic fluids out of the Yellowstone volcanic system or could be indicative of a relative low stress regime resulting from the stress release by the Hebgen Lake earthquake. An area of low b-values (0.6 ± 0.1) south of the Yellowstone caldera is interpreted as evidence of a relatively higher stress regime associated with an area of dominantly extensional stress. This seismicity was associated with a nearly 90° change in the principal stress axes direction to northeast-southwest, compared to east-west within the Yellowstone caldera, and may be influenced by buoyancy loading by the Yellowstone hotspot.

Recent magmatotectonic activity in the Eastern Snake River Plain–Island Park region revealed by SAR interferometry

Synthetic Aperture Radar Interferometry (InSAR) has been applied in this study to address crustal deformation in a 10,000-km 2 region located immediately west of the Yellowstone hotspot. InSAR results show that surface movements in the study area were non-linear and episodic during the period of observation (1993–2006). The Island Park region and its adjacent Eastern Snake River Plain (ESRP) were characterized by northeast-trending zones of uplift (+ 1 cm yr − 1 ) and subsidence (− 1 cm yr − 1 ) with various extents through time. The western edge of Yellowstone caldera experienced episodes of subsidence (− 1 cm yr − 1 ) during 1997–2000 and uplift (+ 3 cm yr − 1 ) during 2004–2006. Differential surface movements of varying rates were also detected between 1993 and 2006 in the vicinity of Basin and Range normal faults to the north of Henrys Fork caldera. Throughout the study area, surface displacements across the Island Park region, the ESRP, and the adjacent Basin and Range province generally reversed the movement direction in 2004, in concert with displacement reversal to uplift in the Yellowstone caldera. Crustal deformation in the general vicinity of major Basin and Range faults is interpreted to reflect diffuse extensional strain adjacent to the deeper segments of faults, rather than near-surface slip. The northeast-trending displacement zones in the ESRP and the Island Park region may indicate folding in response to converging zones of extension in the surrounding Basin and Range province. Surface displacements of the Yellowstone caldera are interpreted to reflect migration of magma or hydrothermal fluids. The inverse relation between vertical displacements in Yellowstone and its surrounding regions may reflect an upper crustal flexural response or a large-scale movement of hydrothermal fluids, though neither hypothesis is completely supported by the processed data.

Formation of saddle dolomites in Upper Cambrian carbonates, western Tarim Basin (northwest China): Implications for fault-related fluid flow

The saddle dolomites occur more intensely in cores closely to fault than that in cores far away from the fault in Upper Cambrian carbonate of western Tarim basin, suggesting that formation of the saddle dolomites is likely related to fault-controlled fluid flow. They partially fill in fractures and vugs of replacement dolomite. The saddle dolomites exhibit complex internal textures, commonly consisting of core and cortex. In comparison with the matrix dolomites, the saddle dolomites show lower Sr-content and 87Sr/ 86Sr ratios, higher Fe- and Mn-content, and more negative δ 18O values. Combined with high Th (100–130 °C) of primary fluid inclusions, it is suggested that the saddle dolomites precipitated from hydrothermal fluid derived from the deep evaporite-bearing Middle Cambrian strata, and the magnesium source may be due to dissolution of host dolomite during hydrothermal fluid migration. Fault activity resulted in petrographic and geochemical difference of the core and cortex of the saddle dolomites. The cores precipitated from the formation water mixed by deep brines at the early stage of fault activity, and the cortexes precipitated from the deep fluid with higher temperatures through the Middle Cambrian later. In summary, the formation of the saddle dolomites implies a hydrothermal fluid event related to fault activity, which also resulted in high porosity in Upper Cambrian carbonate in western Tarim Basin.

On the role of buoyancy force in the ore genesis of SEDEX deposits: Example from Northern Australia

Finite element modeling on a highly conceptualized 2-D model of fluid flow and heat transport is undertaken to simulate the paleo-hydrological system as if the Mount Isa deposits were being formed in the Mount Isa basin, Northern Australia, and to evaluate the potential of buoyancy force in driving basin-scale fluid flow for the formation of sedimentary-exhalative (SEDEX) deposits. Our numerical case studies indicate that buoyancy-driven fluid flow is controlled mainly by the fault penetration depth and its spatial relation with the aquifer. Marine water recharges the basin via one fault and flows through the aquifer where it is heated from below. The heated metalliferous fluid discharges to the basin floor via the other fault. The venting fluid temperatures are computed to be in the range of 115 to 160°C, with fluid velocities of 2.6 to 4.1 m/year over a period of 1 Ma. These conditions are suitable for the formation of a Mount Isa-sized zinc deposit, provided a suitable chemical trap environment is present. Buoyancy force is therefore a viable driving mechanism for basin-scale ore-forming hydrothermal fluid migration, and it is strong enough to lead to the genesis of supergiant SEDEX deposits like the Mount Isa deposit, Northern Australia.

Organic geochemistry and palynofacies of the Early–Middle Frasnian transition (Late Devonian) of the Holy Cross Mountains, Southern Poland

Geochemical and palynological investigation of the Lower to Middle Frasnian reef-to-basin succession at the well-known Kowala Quarry (Holy Cross Mountains) demonstrates the variable character of organic matter (OM) deposited in different oxidative–reductive conditions. During sedimentation of the basinal deposits, anoxic conditions in the water column prevailed. In the organic-rich sediments deposited during the Palmatolepis punctata Zone (set D), anoxia occurred in the photic zone, as inferred from the presence of isorenieratane, aryl isoprenoids and gammacerane. In the same kind of deposits, but occurring during the Palmatolepis hassi Zone (set F), anoxia apparently occurred mainly in the deeper parts of the basin and only sporadically reached the photic zone. This interpretation is based on the lack of isorenieratane, traces of aryl isoprenoids and the presence of gammacerane in the investigated samples. The basinal clay-enriched sediments of sets D and F are divided by an organic-poor set E, typical for the carbonate platform (fore-reef) environment characterized by oxic conditions. A palynostratigraphical study of the Frasnian section from the Kowala Quarry allowed recognition of two miospore zones: Geminospora semilucensa– Perotrilites donensis (SD) and Archaeoperisaccus ovalis–Verrucosisporites grumosus (OG). Relatively small quantitative and qualitative changes of the organic particles (miospores, tracheids, prasinophytes and acritarchs) are interpreted primarily in terms of sea-level fluctuations and enhanced productivity. In particular, the appearance of abundant, abnormally large mature leiospheres in set D can be the result of progressive eutrophication, related to environmental changes recorded in the recently detected major positive carbon isotope anomaly. As during the Frasnian–Famennian mass extinction interval, this eutrophication could have been stimulated by a rapid increase in primary production and subsequently expanding anoxic conditions, but it is evident only in the early phase of the Middle Frasnian δ 13C excursion. The processes of the secondary hydrothermal fluid migration, taking place after the lithification of the sediments, caused partial OM oxidation. The products of these processes are such phenyl derivatives of aromatic hydrocarbons as phenylphenanthrenes, terphenyls, phenyldibenzothiophenes and phenyldibenzofurans. Moreover, an oxygen compound rarely described from sedimentary OM, dibenzo- p-dioxin and its methyl- and dimethyl-derivatives, seems to be the additional product of hydrothermal OM oxidation.

Source rock characterization and petroleum systems of Eocene Tyee basin, southern Oregon Coast Range, USA

The hydrocarbon source rock potential of the Eocene units in the southern Oregon Coast Range was evaluated using Rock-Eval pyrolysis. Most Eocene units in southern Oregon Coast Range are thermally immature and contain lean, gas prone Type III kerogen. However, some beds (coals) are sufficiently organic rich to be sources of biogenic and thermogenic methane discovered in numerous seeps. The overall hydrocarbon source rock potential of the southern Oregon Coast Range is moderately low; however, requirements for commercial accumulations of hydrocarbon probably exist locally within and adjacent areas. Two speculative petroleum systems are proposed. The first includes the southern part of the Oregon Coast Range near the border with the Mesozoic Klamath Mountains and is related to a proposed subduction zone maturation mechanism along thrust faults. The second occurs near the eastern border of the range where maturation is related to heating by sills and migration of hydrothermal fluids associated with mid-Tertiary volcanism in the western cascade arc.

Crustal deformation and source models of the Yellowstone volcanic field from geodetic data

Geodetic observations, comprising Interferometric Synthetic Aperture Radar (InSAR), Global Positioning System (GPS), and precision leveling measurements, are used to infer volume change in the subsurface associated with the dynamics of the Yellowstone volcanic system. We focus primarily on the Yellowstone Caldera and its related magmatic, hydrothermal, and fault systems. It appears that known faults play a significant role in controlling crustal volume increases and decreases due to the migration of volcanic and hydrothermal fluids. For example, over 5 cm of subsidence from 1992 to 1995 is associated with source volume changes 6–10 km beneath the NW‐trending Elephant Back fault zone and a north‐trending fault cutting across the caldera. Furthermore, we are able to image an episode of fluid intrusion near the northern edge of the caldera. The intrusion is elongated in the north‐south direction and is parallel to the north‐trending volume decrease. The primary intrusion and related hydrothermal activity occurred between 1996 and 2000, though the volume changes appear to have continued, shallowed, and changed shape between 2000 and 2002. There is evidence that the intrusive activity influenced extensional faults to the north of the caldera.

Tectonic-hydrothermal brecciation associated with calcite precipitation and permeability destruction in Mississippian carbonate reservoirs, Montana and Wyoming

The Mississippian Madison Formation contains abundant fracture zones and breccias that are hydrothermal in origin based on their morphology, distribution, and geochemical signature. The hydrothermal activity is related to crustal shortening during the Laramide orogeny. Brecciation is accompanied by dedolomitization, late-stage calcite precipitation, and porosity occlusion, especially in outcrop dolomites. The tectonic-hydrothermal late-stage calcite reduces permeability in outcrops and, potentially, high-quality subsurface reservoir rocks of the subsurface Madison Formation, Bighorn Basin. The reduction of permeability and porosity is increased along the margins of the Bighorn Basin but not predictable at outcrop scale. The destruction of porosity and permeability by hydrothermal activity in the Madison Formation is unique in comparison to studies that document enhanced porosity and permeability and invoke hydrothermal dolomitization models. Hydrothermal breccias from the Owl Creek thrust sheet are classified into four categories based on fracture density, calcite volume, and clast orientation. Shattered breccias dominate the leading edge of the tip of the Owl Creek thrust sheet in the eastern Owl Creek Mountains, where tectonic deformation is greatest, whereas fracture, mosaic, and chaotic breccias occur throughout the Bighorn Basin. The breccias are healed by calcite cements with 18O values ranging between 26.5 and 15.1 Peedee belemnite (PDB), indicating that the cements were derived from isotopically depleted fluids with elevated temperatures. In the chaotic and mosaic breccia types, large rotated and angular clasts of the host rock float in the matrix of coarse and nonzoned late-stage calcite. This appearance, combined with similar 18O values across even large calcite veins, indicates that the calcite precipitated rapidly after brecciation. Values for 13C (5–12 PDB) from the frontal part of the Owl Creek thrust sheet indicate equilibrium between methane and CO2-bearing fluids at about 180C. Fluid inclusions from the eastern basin margin show that these cements are in equilibrium with fluids having minimum temperatures between 120 and 140C and formed from relatively low-salinity fluids, less than 5 wt.% NaCl. Strontium isotope ratios of these hydrothermal fluids are more radiogenic than proposed values for Mississippian seawater, suggesting that the fluids mixed with felsic-rich basement before migrating vertically into the Madison Formation. We envisage that the tectonic-hydrothermal late-stage calcite-cemented breccias and fractures originated from undersaturated meteoric groundwaters that migrated into the burial environment while dissolving and incorporating Ca2+ and and radiogenic Sr from the dissolution of the surrounding carbonates and the felsic basement, respectively. In the burial environment, these fluids were heated and mixed with hypersaline brines from deeply buried parts of the basement. Expulsion of these fluids along basement-rooted thrust faults into the overlying strata, including the Madison Formation, occurred most likely during shortening episodes of the Laramide orogeny by earthquake-induced rupturing of the host rock. The fluids were injected forcefully and in an explosive manner into the Madison Formation, causing brecciation and fracturing of the host rock, whereas the subsequent and sudden decrease in the partial pressure of CO2 caused the rapid precipitation of calcite cements. The explosive nature of hydrothermal fluid migration ultimately produces heterogeneities in reservoir-quality carbonates. In general, flow units in the Madison Formation are related to sequence boundaries, which create vertical subdivisions in the porous dolomite. The late-stage calcite cement surrounds hydrothermal breccia clasts and invades the dolomite, reducing porosity and permeability of the reservoir-quality rock. As a consequence, horizontal flow barriers and compartments are established that are locally unpredictable in their location and extent and regionally predictable along the margins of the Bighorn Basin.

Numerical investigation of the influence of volcanic facies architecture on hydrothermal fluid migration: Example of the Mount Read Volcanics, western Tasmania, Australia

A 2-D numerical conceptual model is developed to represent the volcanic facies architecture of part of the submarine Mount Read Volcanics successions in western Tasmania. Numerical results reveal that a synvolcanic fault strongly controls fluid migration paths. The pumice and volcanic breccias are also important, focusing hydrothermal fluid circulation and implying that alteration and mineralization may occur preferentially in such facies. As expected, the synvolcanic intrusion significantly alters the pre-existing regime of fluid flow and temperature distribution. Vigorous fluid circulation is initiated near the intrusion and then shifts to the more permeable host facies. Temperature contours diffuse outwards from the heat source. The intrusion becomes less important with time due to the energy loss by heat conduction and convection. Fluid migration along the synvolcanic fault reveals an important reversing phenomenon: hydrothermal fluid initially travels upwards then downwards and finally upwards again as the intrusion cools.

Geology, petrography, and mineral chemistry of the Vazante non-sulfide and Ambrósia and Fagundes sulfide-rich carbonate-hosted Zn–(Pb) deposits, Minas Gerais, Brazil

The Vazante–Paracatu region represents the most important Zn district known in Brazil and includes the Vazante hypogene non-sulfide Zn deposit composed mainly of willemite (Zn 2SiO 4) and sphalerite-rich carbonate-hosted Zn–(Pb) deposits. Fagundes is a stratabound deposit, characterized by strong silicification, dolomitization and a Fe-rich carbonate alteration halo. The main ore is represented by rhythmically banded, colloform, and zoned pyrite, sphalerite and galena, related to wall rock dissolution and sulfide infilling, which probably occurs late during the burial diagenesis. Later veins and breccia ore types reflect epigenetic mobilization, related to brittle–ductile structures. The Ambrósia mineralization is mainly fault-controlled and related to brecciated dolomite, which is tectonically imbricated with black shales and slates. Typical features include host rock recrystallization, minor silicification, baroque dolomite and ankerite formation. Pyrite, marcasite, sphalerite and minor galena occur in brecciated comb-veins and veinlets, which overprint stylolites and tectonic fractures, suggesting an epigenetic origin for the ore. The Vazante deposit differs from all other deposits of the district due to the presence of willemitic ore, which is composed of willemite, dolomite, siderite, quartz, hematite, Zn-rich chlorite, barite, franklinite and zincite. The willemitic ore occurs tectonically imbricate to small sulfide ore bodies, which comprises sphalerite and galena, metabasites and hydrothermalized dolomites within the Vazante Shear Zone. The relationships between willemite formation and the development of mylonitic structures suggest that willemitic mineralization and deformation are synchronous episodes closely related to the Vazante Shear Zone. Low Zn/Cd ratios in sphalerite from Vazante (64 to 98) and Fagundes colloform (96 to 244) and zoned sphalerite (89 to 305) could reflect the regional role of mineralizing fluids with similar low Zn/Cd ratios and low contents of reduced sulfur (∑S red). High Ge (up to 2200 ppm) and the low Fe, Cu, Mn and Ag contents in late-diagenetic Fagundes sphalerite might suggest that this metal-bearing fluid could be derived from the underlying basin fill, which comprise clastic sediments and organic matter-rich pelitic sequences. Systematic relationships among sphalerite composition and paragenetic evolution of the Fagundes and Ambrósia deposits suggest that progressive fluid mixing processes were important for the genesis of the sulfide-rich deposits in the district. These mixing processes possibly involved the hot metal-bearing fluid with low contents of reduced sulfur and moderate-temperature, highly saline fluids, which could represent an important sulfur supply. The predominance of the highly saline brines in later epigenetic mineralization episodes might be related to episodic migration of hydrothermal fluids mainly derived from reduced shale sequences during the Brasiliano compressive events. The small variation in chemical and sulfur isotopic composition of the Vazante sphalerite could imply that the high-temperature metal-bearing fluid with low Zn/Cd ratios could represent a minor reservoir of reduced sulfur, which permitted only subordinate sphalerite precipitation in the Vazante deposit. The lack of reduced shale sequences above the Vazante deposit could represent a limiting factor for H 2S supply. Additionally, overall mixture between this hot sulfur-deficient metal-bearing fluid and meteoric fluids channeled to the Vazante Shear Zone favor the high fO 2/S 2 conditions responsible for the stability of the Vazante willemitic assemblage.