Calcite Veins Research Articles

Inherited Heterogeneities Can Control Viscous Subduction Zone Deformation of Carbonates at Seismogenic Depths.

This work links mineral-scale deformation mechanisms with structural evolution during subduction, providing examples showing how grain-scale heterogeneities facilitated viscous creep in calcite at nominally seismogenic temperatures. Carbonates commonly enter subduction zones, either highly concentrated in irregularly distributed sediments or as more distributed precipitates in seafloor volcanics. We present shear zones, localized in calcite veins formed during shallow subduction of calcareous sediment and seafloor volcanics, with viscous shear strains of ≥5. Shear strain localized because secondary phases and chemical variations maintained fine grain sizes in calcite aggregates, activating relatively rapid grain size-sensitive and frictional-viscous creep at temperatures (260±10°C), cooler than predicted from extrapolation of experimental data. Creep at increased strain rates may limit elastic strain accumulation during interseismic periods, reducing the likelihood of large megathrust earthquakes. As shown here for calcite, common inherited natural heterogeneities may induce weakening of viscous mechanisms in other rocks, or at larger scales in the lithosphere.

Determining the gas accumulation period using fluid inclusion observations: Xiang Zhong Basin, China

Abstract There are a large number of natural gas shows in the Upper Paleozoic of the central Hunan Basin. However, the hydrocarbon accumulation process was complicated due to the strong tectonic reformation in this area. A study of the natural gas accumulation period is of significance for determining gas accumulation models. In this research, the fluid inclusion samples were tested, the fluid inclusion occurrence characteristics, uniform temperature data, and reservoir evolution were analyzed, and four diagenetic periods and four diagenetic periods and three hydrocarbon accumulation periods were identified. (i) For the syntaxial stage, the filling is dark and nonluminous, and the fluid captured by the overgrowing cement around the particles is mainly seawater; (ii) for the (Early) Mesogenetic burial stage, the calcite cement grows around the particles or micritic calcite matrix, and the fluid captured is mainly brine; (iii) for (Late) Mesogenetic burial stage, two sets of decomposed fissures developed in massive calcite colloids with non-luminous gas-rich hydrocarbon inclusions within the crystals; and (iv) for Telogenetic burial stage, weakly fluorescent pure gaseous hydrocarbon inclusions are developed in luminescent calcite veins. The result indicates that the organic inclusions in the Lower Carboniferous of the central Hunan Basin are dominated by weakly fluorescent and nonfluorescent rich gas inclusions and pure gas-phase inclusions, with three stages of hydrocarbon filling. By integrating the average homogenization temperatures on burial curves, the three charging orders correspond to 260–250 Ma during the Late Permian, 249–245 Ma during the Early Triassic, and 208–170 Ma during the Late Triassic. Dense reservoir characteristics reveal deep basin gas reservoir in deep basin.

Laramide contractional folding in the Devils River Uplift, west Texas, U.S.A.

Structures in Cretaceous strata in the Devils River Uplift of west Texas include folds, thrust faults, and bedding-parallel veins indicative of contractional deformation in a thrust faulting stress regime. In this study we analyze a series of small-wavelength (meters to tens of meters) folds in roadcut exposures northwest of Del Rio, Texas. The folds have WNW-ESE dominant trends, gently plunging axes, and are gentle to tight with upright to inclined axial surfaces of variable vergence. These structures are interpreted as detachment folds, with detachments within volcanic ash or mudrock beds. Observed thickening in anticline cores was by ductile flow of volcanic ash or mudrock, small-scale folding, and thrust or reverse faulting. Bed-perpendicular calcite veins are common and in some cases are offset by flexural slip on fold limbs, indicating that folding occurred after brittle failure and extension vein formation in lithified rock. Although rare, bed-parallel calcite veins are also present in the exposure and indicative of a thrust faulting stress regime. Occasional normal and reverse or thrust faults are also present in the exposures. Fluid inclusion analysis of vein cements from extension fractures and faults yielded homogenization temperatures of 71–101 °C, and estimated trapping depths of 2.2–3.5 km prior to and coeval with folding. We interpret these folds as having formed during NNE-SSW directed Laramide shortening (∼80–40 Ma) in a thrust-faulting stress regime with >2 km of overburden, rather than as either penecontemporaneous with deposition or after exhumation by caliche formation, as previously interpreted.

Timing and ore genesis of the Ciweigou low-sulfidation epithermal gold deposit in the Yanbian Cu-Au district, NE China

The Ciweigou low-17 sulfidation epithermal Cu-Au mining district, NE China, is hosted by Early Cretaceous andesite of the Jingouling Formation. From bottom to top (not explored below the 100 m level), the ore bodies consist of gold-bearing quartz veins, gold-bearing quartz-calcite veins, and calcite veins and the sides are stockwork veins and hydrothermal breccia ore bodies. Five mineralization stages are distinguished: (I) pyrite-sericite, (II) quartz-calcite, (III) grey quartz, (IV) milky quartz-adularia and (V) late carbonate. Au-bearing minerals deposited in stages II and III. Hydrothermal alteration is composed of an inner silicic-pyritic zone, intermediate zone of quartz-sericite-adularia, outer zone of quartz-chlorite-epidote and upper zone of calcite. Systematic fluid inclusion petrography shows that two-phase vapour-rich and liquid-rich inclusions are the dominant fluid inclusions types in the quartz and calcite veins, suggesting that the ore-forming fluids belong to a moderate- to low-temperature (140 °C−320 °C) and low-salinity (0 wt%−14 wt% NaCl-eqv.) H2O-NaCl-CO2 system. HO isotopes (δ18OH2O = -14.0 ‰ to −4.9 ‰ and δD = −123.4 ‰ to −88.1 ‰) indicate that the ore-forming fluids likely originated from mixed fluids driven by residual andesitic magmatic fluid. The precipitation mechanisms are mixing and major fluid boiling. The comprehensive geological and isotopic characteristics show that the Ciweigou deposit is a low-sulfidation epithermal deposit. Six pyrite samples from stages I and II yielded a Rb-Sr isochron age of 102.6 ± 2.2 Ma (MSWD = 1.19), consistent with other low-sulfidation epithermal gold mineralization ages in the Yanbian district and Lesser Xing'an Range. The timing of the Ciweigou gold mineralization at ∼103 Ma postdates the eruption of andesitic magma (106.4 ± 2.1 Ma, zircon U–Pb) and granodiorite porphyry (106.6 ± 1.2 Ma, zircon U–Pb) and diorite porphyry (107.8 ± 1.8 Ma, zircon U–Pb) emplacement, suiting in an extensional environment attributed to the rollback of the subducting Paleo-Pacific Plate.

Effects of different types of fractures on shale gas preservation in Lower Cambrian shale of northern Sichuan Basin: Evidence from macro-fracture characteristics and microchemical analysis

The Lower Cambrian Qiongzhusi shale is the main producing layer of shale gas in the northern Sichuan Basin. Natural fractures are considered an important factor that influences the exploration and development of shale gas. In this study, outcrops, drill cores, thin sections, fluid inclusions and rare earth element (REE) were analyzed to determine the characteristics, changes in fluid chemistry, and fluid source of different types of fractures and then reveal their effects on the accumulation and preservation of shale gas. Natural fractures of Qiongzhusi shale are mainly composed of non-strata-bounded fractures (NSBFs), strata-bounded fractures (SBFs), and bed-parallel fractures (BPFs). NSBFs span multiple layers, which can significantly enhance the vertical connectivity of shale reservoirs. There are three dominant orientations of NSBFs, as N-S, NNE-SSW, and E-W trends. The E-W trending fractures, which intersect with the maximum principal stress (NWW-SEE) at a small angle, have larger permeability and control the vertical leakage of shale gas. SBFs are confined to individual layers. Homogenization temperatures of aqueous inclusions in SBF veins vary from 149.7 to 172.3 °C. Calcite veins exhibit positive Eu anomalies (δEu = 11.71, 27.9). Analysis of fluid inclusions and rare earth elements indicated that SBF vein-precipitating fluids were derived from the surrounding wall rock. The development of SBFs contributes to shale gas enrichment. BPFs are parallel to bedding planes or intersect at a low angle (<20°). Microstructural analysis suggests that cemented BPFs form from the multiple crack-seal processes. The temperature of fluid during mineral precipitation exceeds 200 °C. REE concentrations vary significantly across the BPF veins (δEu = 2.31–119.37), donating that fluid characteristics episodically changed during vein growth. The methane inclusions trapped in cements and mixing externally sourced fluids indicate that BPFs are the preferential fluid-flow pathway of shale gas and controls the lateral migration of shale gas.

Fault zone fluid pathways in the carbonate Irecê basin, NE Brazil

AbstractWe applied field structural data and isotope geochemical (δ13C, δ18O and 87Sr/86Sr) analyses to understand the relationship among calcite veins, fault damage zones and carbonate host rocks in a thrust fault damage zone in the Achado quarry, Irecê Basin in the São Francisco Craton, NE Brazil. Our results reveal three hydrological packages with different rheological behaviours in a stratified carbonate succession. The upper package includes the Achado fault damage zone that is characterised by interlayered dolomitised grainstones and mudstones. These rocks display high positive δ13C values (10‰–13‰), negative δ18O values (mean −6.34‰) and radiogenic (87Sr/86Sr) isotope values (0.70885–0.71519). A second package is marked by a cataclastic brittle shear zone lateral parallel to dolograinstones bedding. These rocks show low to positive δ13C values (−3.41‰ to +8.85‰), more positive δ18O values (mean −3.73‰) and radiogenic (87Sr/86Sr) isotope values (0.71039–0.71373). The lower package is characterised by well‐preserved pristine limestone succession that shows δ13C values ranging between −0.46‰ and +3.17‰, mean δ18O = −5.41‰ and less radiogenic 87Sr/86Sr values (0.70762–0.70818). In contrast to the upper and intermediate packages, rocks from the lower one exhibit very low permeability and behaved as a seal for fluid migration. Fluid flow occurred several times during basin evolution, for example along syn‐rift fault damage zones, bedding‐parallel carbonate breccia, thrust faults, cataclastic shear zones, synorogenic conjugate shear fractures or joints and opening mode I fracture‐fill calcite veins. These fractures allowed pervasive fluid flow in the porous intermediate cataclastic shear zone where fluids flowed and formed veins, as diffuse fluid flow in randomly oriented fracture swarms, or channelised fluid flow in aligned fracture corridors. They record significant centimetre‐scale to km‐scale hydrological behaviour within carbonate layers. Most carbonates that are associated with veins, fault damage zones and hydraulic breccia were formed by fluids of the same origin with low δ13C (−6.0 to −2.0‰) and δ18O (−6.0 to −8.5‰) values, and more radiogenic 87Sr/86Sr values compared to the carbonate host rocks.

Heterogeneous Subgreenschist Deformation in an Exhumed Sediment-Poor Mélange.

Many described subduction complexes (or mélanges) exhumed from seismogenic depths comprise thick, turbidite‐dominated sequences with deformed zones containing clasts or boudins of more competent sandstone and/or basalt. In contrast, many active subduction zones have a relatively small thickness of sedimentary inputs (<2 km), turbidite sequences are commonly accreted rather than subducted, and the role of pelagic sediments and basalt (lavas and hyaloclastites) in the deforming zone near the plate interface at <20 km depth is poorly understood. Field investigation of Neoproterozoic oceanic sequences accreted in the Gwna Complex, Anglesey, UK, reveals repeated lenticular slices of variably sampled ocean plate stratigraphy (OPS) bounded by thin mélange‐bearing shear zones. Mélange matrix material is derived from adjacent OPS lithologies and is either dominantly illitic, likely derived from altered siliciclastic sediment, or chloritic, likely derived from altered volcanics. In the illitic mélange, mutually cross‐cutting phyllosilicate foliation and variably deformed chlorite‐quartz‐calcite veins suggest ductile creep was cyclically punctuated by transient, localized fluid pulses. Chlorite thermometry indicates the veins formed at 260 ± 10°C. In the chloritic mélange, recrystallized through‐going calcite veins are deformed to shear strains of 4–5 within a foliated chlorite matrix, suggesting calcite veins in subducting volcanics may localize deformation in the seismogenic zone. Shear stress‐strain rate curves constructed using existing empirical relationships in a simplified shear zone geometry predict that slip velocities varied depending on pore fluid pressure; models predict slow slip velocities preferentially by frictional sliding in chlorite, at pore fluid pressures greater than hydrostatic but less than lithostatic.

Meta-bauxite deposit in the Tavşanlı Zone, NW Turkey: A new locality for gem-quality diaspore formation

The tectonic units of Sakarya Zone (Pontides) and Tavşanlı Zone (Anatolides) are exposed in the Mihalıççık area of central Anatolia. A Cretaceous accretionary complex that forms the İzmir-Ankara-Erzincan suture zone separates the units. Within this region, the Tavşanlı Zone is made up of a coherent series and tectonically overlying late Cretaceous mélange, which has undergone blueschist facies metamorphism. Coherent series consist of strongly retrogressed schists and conformably overlying platform-type thick marbles, known as the İnönü marbles.The İnönü marbles contain meta-bauxite lenses in the lower levels, which can be classified as karstic bauxites deposited in the karst holes within the original limestone. Considering the general stratigraphy of the Tavşanlı Zone, a middle Jurassic age can be envisaged for the bauxite formation. Geochemical data suggest that the bauxites were largely derived from redeposited argillic sediments, with minor and variable inputs from the weathering of intermediate to acidic rock. The mineral assemblage of the meta-bauxite consists of diaspore, chloritoid, muscovite, paragonite, Al-spinel (hercynite), magnetite, hematite and ilmenite. In addition, apatite, rutile, monazite, xenotime and zircon occur as accessory minerals. Limonite, goethite, akaganeite and kaolinite also formed during the latest stage of alteration. P-T conditions for the meta-bauxites are estimated as 20–27 kbar and 330–470 °C that are indicative of formation under high-P / low-T blueschist facies conditions.The meta-bauxites are cut by veins of diaspore, muscovite-paragonite, goethite, calcite and ankerite, and include gem-quality diaspore crystals up to 6 cm long. Textural evidence indicates that the diaspore formation is related to the extraction of aluminium from the meta-bauxite body and migration of Al-rich fluids into the shear zones during the last stage of the high-P metamorphism. During exhumation of the Tavşanlı Zone, these veins were brecciated by brittle deformation and the fractured minerals were cemented by granoblastic calcite and ankerite during the latest stage of the vein formation.

Ca2+] and [SO[formula omitted]] in Phanerozoic and terminal Proterozoic seawater from fluid inclusions in halite: The significance of Ca-SO4 crossover points

Chemical analyses of 2,618 (1,640 new and 978 published) fluid inclusions in marine halite were used to define paleoseawater [Ca2+] and [SO2−4] over the past 550 million years (Myr). Three types of fluid inclusion brine chemistries were recognized based on measured [Ca2+] and [SO2−4]: (1) SO4-rich with [SO2−4] ≫ [Ca2+]; (2) Ca-rich with [Ca2+] ≫ [SO2−4]; and (3) Ca-SO4 crossover points with [Ca2+] ≈ [SO2−4]. The SO4-rich and Ca-rich fluid inclusion chemistries oscillated twice in the terminal Proterozoic and Phanerozoic. Transitions between SO4-rich and Ca-rich seas, here called “Ca2+−SO42−crossover points” occurred four times: terminal Proterozoic–Early Cambrian (544–515 Ma), Late Pennsylvanian (309–305 Ma), Triassic–Jurassic boundary (∼200 Ma), and Eocene–Oligocene (36–34 Ma). New fluid inclusion analyses using laser ablation-inductively coupled plasma-mass spectrometry better defined the [Ca2+] and [SO2−4] in seawater at the Late Pennsylvanian and Eocene–Oligocene crossover points and the timing of the Triassic–Jurassic crossover point. Crossover points coincide with shifts in seawater Mg2+/Ca2+ ratios, the mineralogies of marine non-skeletal carbonates and shell building organisms (aragonite vs. calcite) and potash evaporites (MgSO4 vs. KCl types). Phanerozoic and terminal Proterozoic trends in seawater [Ca2+] and [SO2−4] also coincide with supercontinent breakup, dispersal, and assembly cycles, greenhouse–icehouse climates, and modeled atmospheric pCO2. Paleoseawater [Ca2+] and [SO2−4] were calculated from the fluid inclusion data using the assumption that the [Ca2+] × [SO2−4] ranged from 150 to 450 mmolal2, which is 0.5–1.5 times the [Ca2+] = 11 × [SO2−4] = 29 product in modern seawater (319 mmolal2). Two additional end-member scenarios, independent of the [Ca2+] × [SO2−4] = 150–450 mmolal2 assumption, were tested using constraints from fluid inclusion [Ca] and [SO4]: (1) constant [SO2−4] = 29 mmolal as in modern seawater, and variable [Ca2+], and (2) constant [Ca2+] = 11 mmolal as in modern seawater and variable [SO2−4]. Mg2+/Ca2+ ratios calculated from the three scenarios were compared to independent data on the Mg2+/Ca2+ ratios from skeletal carbonates (echinoderms and corals) and mid-ocean ridge flank calcite veins. Constant [Ca2+] of 11 mmolal is unlikely because this relatively low concentration generated unreasonably low seawater [SO2−4] during most of the past 550 Myr and high Mg2+/Ca2+ ratios compared to independent data. Constant [SO2−4] of 29 mmolal produced unreasonably high seawater [Ca2+] and lower Mg2+/Ca2+ ratios than those derived from fluid inclusions, echinoderms, corals, and calcite veins. Variable [Ca2+] and [SO2−4] showed the best agreement with the Mg2+/Ca2+ ratios derived from fluid inclusions, echinoderms, corals, and calcite veins.

Multistage fracturing history in the Paleocene lacustrine shale oil reservoirs of the Subei Basin, Eastern China

This study examines sealed fractures in core samples from the Paleocene organic-rich lacustrine shales of the Gaoyou Depression, Subei Basin, China, using a multi-pronged approach to elucidate the interplay between fracture propagation and fluid circulation in shaly successions. Based on petrography, geochemistry, and geochronology of fracture-filling minerals, four generations of vein emplacement (denoted Types A-D) were identified. These were attributed to successive episodes of burial diagenesis, tectonism, and uplift occurring at different phases of basin evolution. These vein generations were then inserted into the temporal-spatial framework derived from basin modeling, leading to a reconstruction of the fracture diagenesis history in the Funing shales. Two of the vein generations contain evidence for two phases of oil migration marked by two types of oil inclusions with different fluorescence.It has been shown that the studied lacustrine shales were deposited at ca. 60.2–58.0 Ma. The earliest vein generation, represented by antitaxial beef calcite veins sub-perpendicular to shale bedding, propagated in incompletely consolidated shales within a regional thrust regime related to the Wubao Movement. The vein nucleation during a short-lived burial following initial deposition was also evidenced by the presence of monoliquid inclusions in the veins and calcite LA-ICP-MS U–Pb age (57.7 ± 3.7 Ma). Tectonic compressive stress together with overpressures arising from horizontal and vertical disequilibrium compaction were interpreted as being responsible for this fracturing episode. The second vein generation is bedding-parallel, oil inclusion-rich stretching calcite veins with a crack-seal texture, formed concurrently with kerogen maturation and early oil generation at moderate burial depths. The vein crack-seal fills containing yellow fluorescing oil inclusions provide evidence for episodic oil expulsion in response to cyclic rises and drops in pore pressure during the oil window. Bedding-parallel beef calcite veins characterized by antitaxial growth represent the next vein generation, emplaced during the maximum burial period (ca. 41.9 ± 2.7 Ma). The vein fibers have tracked the horizontal displacement of the wall rocks, likely related to lateral shortening before or at the initiation of Late Eocene basin inversion. The first three vein generations were generated in a relative closed fluid system, with vein fill sourced locally. Host-rock carbonate components and/or earlier diagenetic carbonates were dissolved and then mobilized into these veins. By contrast, the last vein generation with blocky calcite and barite cementation is associated with input of externally derived hydrothermal fluids. The evidence comes from abnormally high fluid-inclusion homogenization temperature (Th) and salinity data, as well as the fact that Ba and sulfate were sourced from the underlying evaporite layers rather than local shales. We thus infer that cross-stratal faulting during the uplift related to the Sanduo Movement resulted in the opening of initial hydraulically closed fluid system. This episode of tectonically-driven hydrothermal circulation from deeper locations in the basin into the Funing shales led to the mixing of oil-bearing pore fluids with exotic basinal hot brines. This also accounts for the presence of abundant blue-green fluorescing oil inclusions and significantly elevated fluid salinity. Overall, this study emphasizes that during basin evolution, differences in tectonic settings and the diagenetic boundary conditions under which veins formed results in different fracturing mechanisms and associated fluid-flow regimes in shale sequences, with consequent effects on vein attributes and mineralization.

Selective reactivation of inherited fault zones driven by stress field changes: Insights from structural and paleostress analysis of the Pocuro Fault Zone, Southern Central Andes (32.8°S)

This study aims to explain the selective reactivation of normal faults during the Andean orogeny at the Southern Central Andes western flank. We conducted a structural mapping and paleostress field reconstruction in the regional-scale Pocuro Fault Zone (PFZ) at 32.8°S. Results reveal that the architecture of the PFZ results from at least two deformation phases, each revealing an individual progressive and gradual evolution. The earliest deformation phase is recorded by two ∼ NS-striking normal faults involving a 5 km wide damage zone characterized by quartz-laumontite and calcite veins that were developed under an extensional regime with a WNW-ESE-trending σ3-axis. The latest deformation phase is recorded by one NS-striking reverse-dextral fault with goethite-hematite syn-tectonic precipitation and two NW-striking reverse-sinistral faults. Reverse faults were developed under a compressional/transpressional regime characterised by an ENE-WSW-trending σ1-axis with a σ2-/σ3-axis permutation. From a geophysical data reassessment, we inferred that observed faults in the surface within the PFZ are regional-scale deep-seated structures. Considering previous geochronological data, we correlated the earliest and latest phases with the Abanico Basin extension (middle Eocene-early Miocene) and its subsequent inversion (Miocene). Given the neotectonic evidence (geomorphic markers and deformation of unconsolidated deposits), the latter phase likely remains active. Quartz-laumontite cementation of the fault core's cataclastic material promotes mechanical strengthening leading to negative feedback for the reactivation of inherited normal faults as reverse ones. Conversely, the concentration of fractures in the damage zone between the normal faults promotes mechanical weakening resulting in a preferential area for the propagation of reverse fault during the compressive/transpressional phase.

Low‐grade prehnite‐pumpellyite facies metamorphism and metasomatism in basement rocks adjacent to the Permian Oslo rift: The importance of displacive reactions

AbstractThe Kongsberg and Bamble lithotectonic domains of SE‐Norway are known as classical Precambrian high‐grade metamorphic terrains. The area has undergone extensive metasomatism with formation of albitites and scapolite‐rich rocks and numbers of previously economically important deposits including the Kongsberg Silver and the Modum Cobalt mines. We demonstrate here that the central part of the Bamble lithotectonic domain (Kragerø area) has locally developed low‐grade metamorphic minerals (prehnite, pumpellyite, analcime, stilpnomelane and thomsonite) belonging to the prehnite‐pumpellyite and zeolite facies. Structurally, the low‐grade minerals occur as fracture fills, in the alteration selvages around fractures where the rock is albitized, and along shear zones and cataclastic zones. The fracture fill and the alteration selvages vary from millimetres scale to 1 m in thickness. The fractures with low‐grade minerals are part of larger fracture systems. The low‐grade minerals typically formed by both displacive (swelling) and replacive reactions and in a combination of these. Prehnite together with albite, K‐feldspar, quartz, epidote and hydrogarnet form lenses along (001) faces in biotite and chlorite leading to bending of the sheet silicates through a displacive reaction mechanism. Numerous replacement reactions including the earlier minerals as well as the low‐grade minerals occur. As albite, K‐feldspar, talc, quartz, actinolite, titanite, calcite and hydrogrossular form in the same veins and in the same biotite grain as the classical low‐grade minerals, they probably belong to the low‐grade assemblage and some of the albitization in the region presumably occurred at low‐grade conditions. Alteration of olivine (Fo69) at low‐grade conditions results in the formation of clay minerals including ferroan saponite. Reconnaissance studies at the east (Idefjord lithotectonic domain) and the northwest (Kongsberg lithotectonic domain) sides of the Oslo rift together with reports of low‐grade assemblages in south‐western Sweden along the continuation of the rift into Skagerrak suggest that the low grade assembles occur in rocks adjacent to the Oslo rift along its full extent. Ar‐Ar dating of K‐feldspar from the low‐grade assemblages gave an age of 265.2 ± 0.4 Ma (MSWD = 0.514 and P = 0.766), suggesting that the low‐grade metamorphism and some of the metasomatism is induced by fluids and heat from the magmatic activity of the Permian Oslo rift, which requires transport of fluid over distances of several kilometres. The metamorphic conditions are constrained by stability fields of prehnite, pumpellyite and analcime to be less than 250°C and at a pressure less than 5 kbars. The displacive reactions created micro‐fractures and porosity in the adjacent minerals that enhance fluid flow and low‐grade mineral formation on a local scale. On a thin section scale, the displacive growth of albite in biotite results in a local volume increase of several 100%. Whether the opening of the larger, horizontally oriented fracture systems needed to transport the fluid over a distance of several kilometres was also the results of displacive reactions remains unknown. The low‐grade metamorphism and metasomatism formed in the shoulder of the Oslo rift and may have contributed to its uplift.

Chemical interaction driven by deep fluids in the damage zone of a seismogenic carbonate fault

We analyzed from meso-to microscale the chemical interaction (e.g., bleaching) between deep fluids and carbonate rocks in pelagic limestones exposed along the seismically active Gubbio normal fault (Northern Apennines), exhumed from ca. 2 km depth. Bleaching is enhanced by the exploitation of inherited stylolitic seams by fluids and is favored by the progressive leaching and mobilization of primary components (i.e., Ca, Fe, Pb, and Cu) and the precipitation of authigenic baryte due to temperature and/or redox potential contrast.Analysis of fluid inclusions trapped in calcite veins in the damage zone precipitated from a H2O–NaCl-bearing fluid(s), with low salinity (1.22–2.57 wt%NaCleq) and a range of trapping temperature between 107 °C and 185 °C, higher than the host rock peak T conditions. Analysis of fluid inclusions and noble gases from syn-kinematic calcite and mineralogy of altered rocks suggests a mixed contribution between crustal-derived fluids with diagenetic fluids containing sulphates and CO2. We speculate that bleaching is a by-product of the circulation of such aggressive fluids with mixed contributions, likely mobilized during seismic event(s) and injected into the damage zone at shallow depths. Such processes, affecting the chemical-physical properties of limestones, are generally underestimated and should be considered when modelling hydraulic connectivity in fractured carbonate reservoirs.

REE and cathodoluminescence features of scheelites from the Tongshankou Cu–Mo deposit in Eastern China

The Tongshankou porphyry-/skarn-type Cu–Mo deposit is located in the Edong ore district, the largest district of the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in Eastern China. From the ore-causative granodiorite porphyry and skarn at Tongshankou, three types of scheelite are recognized (Sch1 to Sch3): Sch1-subhedral–anhedral, occurring in the chalcopyrite ores within the porphyry; Sch2-granular, occurring inside garnet in the endoskarn and Sch3-subhedral–anhedral, occurring in calcite veins in the exoskarn. Sch2 and Sch3 have different internal texture according to CL images: Sch2-Dark core and bright rim, with the heterogeneous dark core; Sch3-Dark core and narrow dark rim, showing patches of different brightness in core. In addition, three REE types have been differentiated: 1) REE-Type1: LREE-enriched, HREE-depleted, and negative Eu anomalies patterns (Sch1), which is related to the porphyry system; 2) REE-Type2: No apparent fractionation between LREE and HREE, with positive Eu anomalies (Sch2), which is related to the endoskarn; 3) REE-Type3: Higher LREE and lower HREE with extremely positive Eu anomalies (Sch3), which is related to the exoskarn. The negative MoO3 vs. WO3 correlation for the Tongshankou scheelites suggests Mo-W substitution in the scheelite crystal lattice. No humped pattern (MREEs-enriched) of REE, lower Na content and the lack of correlation between Na and REE at Tongshankou excluded the Na substitute into the Ca site. The positive correlation with Nb and REE for Sch 1 and 3, suggests equation [2] and [3] should be more plausible mechanism to generate the Tongshankou scheelite REE patterns.

Insights into the Ore Genesis of the Harla Gold Deposit in Eastern Tianshan, NW China: Evidence from Geology, Fluid Inclusions, and H-O-C-S-Pb Isotopes

The Harla gold deposit is located on the eastern segment of the Kanggur-Huangshan ductile shear belt in Eastern Tianshan, on the southern margin of the Central Asian Orogenic Belt. The orebodies show close spatial association with NEE-trending faults and are hosted in volcanic rocks of the Lower Carboniferous Xiaorequanzi Formation. Three mineralization stages were recognized: an early stage characterized by specularite–pyrite–quartz mineralization, an intermediate stage with pyrite–chalcopyrite–gold–quartz genesis, and a late stage defined by sulfide-poor calcite and quartz veins. Three types of fluid inclusions (FIs) were recognized: daughter mineral-bearing triphase FIs (S-type), liquid-dominated biphase FIs (LV-type), and liquid-only monophase FIs (L-type). S- and LV-type FIs entrapped in the early stage show similar homogenization temperatures (Th) at 216–229 °C and 199–222 °C. However, they display contrasting salinities of 32.2–33.4 and 6.7–8.9 wt.% NaCl eqv., respectively. LV-type FIs entrapped in the intermediate stage display Th of 163–191 °C, with salinities of 5.6–7.4 wt.% NaCl eqv. LV-type FIs occur in the late stage display Th of 135–258 °C and salinities of 3.7–5.6 wt.% NaCl eqv. Insights from hydrogen, oxygen, and carbon isotope analyses (δ18OH2O = −10.7 to 3.3‰, δDH2O = −88.1 to −61.4‰, δ13CH2O = −8.9 to −6.2‰), combined with FI characteristics, indicate that the ore-forming fluids were derived from a hybrid source of magmatic and meteoric water, with the latter becoming dominant in the later stage. Sulfide and lead isotopic compositions (δ34Spyrite = 0.2 to 2.5‰, 206Pb/204Pb = 17.709–18.273, 207Pb/204Pb = 15.483–15.569, 208Pb/204Pb = 37.533–37.754) suggest that ore-forming materials were mainly derived from wall rocks (Xiaorequanzi Formation rocks and plagiogranite). By integrating geological background work and fluid inclusion analyses, as well as isotope data, we conclude that the Harla gold deposit is an epithermal-type deposit.

Formation of the Rock Canyon Creek carbonate-hosted REE–F–Ba deposit, British Columbia, Canada: Constraints from Mg-Sr isotopes of dolomite, calcite, and fluorite

The Rock Canyon Creek REE–F–Ba deposit is a carbonate-hosted REE deposit located in the Alkaline Province of British Columbia, Canada, which hosts most REE mineralization. It shares tectonic, stratigraphic and structural similarities with nearby Mississippi Valley-type deposits. To constrain the dolomitization and mineralization processes, the δ26Mg, 87Sr/86Sr, and elemental compositions of dolomite, calcite, and fluorite were determined. The δ26Mg values of dolomite from the unmineralized zone (−1.83 ‰ and −1.67 ‰) are consistent with marine evaporative dolomite, but those from the mineralized zone (−0.97 ‰ to −0.17 ‰) are higher and more variable. The linear 87Sr/86Sr–Sr/Mg relationships, both in the dolomite and calcite from the unmineralized zone, suggest dolomitization in an evaporative marine environment. The high δ26Mg values with large variation and the non-stoichiometric Ca/Mg ratios in the dolomite from the mineralized zone suggest that this dolomite interacted with a relatively low-temperature hydrothermal fluid (<200 °C). Considering the Na, Ca, Sr, Ba, and LREE-enrichment and low 87Sr/86Sr signature in the dolomite from the mineralized zone as well as mineral paragenesis in the deposit, the most plausible scenario for the formation of the deposit is that marine-evaporitic dolomite re-equilibrated with basinal fluids along permeable zones, and at a later stage, carbohydrothermal REE-bearing fluids infiltrated into the Rock Canyon Creek brecciated zone and deposited the REE mineralization. Late-stage, post-mineralization calcite veins and iron oxides formed in the upper part of the deposit under supergene conditions, as suggested by combination of textural observations, REE patterns, and differences in isotopic compositions between dolomite and calcite from the mineralized zone. This study demonstrates that δ26Mg, 87Sr/86Sr, and element/Mg ratios in carbonate minerals are useful for characterizing mineralizing fluids in carbonate-hosted deposits, and discriminating mineralization-related dolomite from marine evaporative dolomite and unmineralized hydrothermal dolomite.

Deformation of the European Plate (58-0 Ma): Evidence from Calcite Twinning Strains

We present a data set of calcite twinning strain results (n = 209 samples; 9919 measured calcite twins) from the internal Alpine nappes northwestward across the Alps and Alpine foreland to the older extensional margin along the Atlantic coast in Ireland. Along the coast of Northern Ireland, Cretaceous chalks and Tertiary basalts are cross-cut by calcite veins and offset by calcite-filled normal and strike-slip faults. Both Irish sample suites (n = 16 with four U-Pb vein calcite ages between 70–42 Ma) record a sub-horizontal SW-NE shortening strain with vertical extension and no strain overprint. This sub-horizontal shortening is parallel to the margin of the opening of the Atlantic Ocean (~58 Ma), and this penetrative fabric is only observed ~100 km inboard of the margin to the southeast. The younger, collisional Alpine orogen (~40 Ma) imparted a stress–strain regime dominated by SE-NW sub-horizontal shortening ~1200 km northwest from the Alps preserved in Mesozoic limestones and calcite veins (n = 32) in France, Germany and Britain. This layer-parallel shortening strain (−3.4%, 5% negative expected values) is preserved across the foreland in the plane of Alpine thrust shortening (SE-NW) along with numerous outcrop-scale contractional structures (i.e., folds, thrust faults). Calcite veins were observed in the Alpine foreland in numerous orientations and include both a SE-NW layer-parallel shortening fabric (n = 11) and a sub-vertical NE-SW vein-parallel shortening fabric (n = 4). Alpine foreland strains are compared with twinning strains from the frontal Jura Mountains (n = 9; layer-parallel shortening), the Molasse basin (n = 26; layer-parallel and layer-normal shortening), Pre-Alp nappes (n = 39; layer-parallel and layer-normal shortening), Helvetic and Penninic nappes (Penninic klippe; n = 46; layer-parallel and layer-normal shortening plus four striated U-Pb calcite vein ages ~24 Ma) and calcsilicates from the internal Tauern window (n = 4; layer-normal shortening). We provide a chronology of the stress–strain history of the European plate from 58 Ma through the Alpine orogen.

Spatio-temporal variation of fluid flow behavior along a fold: The Bóixols-Sant Corneli anticline (Southern Pyrenees) from U–Pb dating and structural, petrographic and geochemical constraints

This study integrates field structural data, petrographic and geochemical (δ18O, δ13C, Δ47, 87Sr/86Sr, and elemental composition) analyses and U–Pb dating of calcite veins cutting the Bóixols-Sant Corneli anticline (Southern Pyrenees) in order to date and to investigate the spatio-temporal relationships between fluid flow and fold evolution. This E-W trending anticline grew from Late Cretaceous to Paleocene at the front of the Bóixols thrust sheet deforming pre-growth and growth sedimentary sequences. U–Pb dating reveals Late Cretaceous to late Miocene deformation ages, which agree with the age of growth strata deposition and the sequence of deformation interpreted from field and microstructural data. Dates coeval (71.2 ± 6.4 to 56.9 ± 1.4 Ma) and postdating (55.5 ± 1.2 to 27.4 ± 0.9 Ma) Upper Cretaceous to Paleocene growth strata are interpreted to record: (i) the growth of the Bóixols-Sant Corneli anticline during the Bóixols thrust emplacement, and (ii) the tightening of the anticline during the southern tectonic transport of the South-Central Pyrenean Unit. Other ages (20.8 ± 1.2 to 9.0 ± 4.6 Ma) postdate the folding event and have been associated with the collapse of the Bóixols-Sant Corneli anticline. The geochemistry of calcite veins indicates that the fluid flow behavior varied across the Bóixols-Sant Corneli anticline through its growth, showing a compartmentalized fluid system. In the hinge of the anticline and in the upper Santonian to middle Campanian syn-orogenic sequence along the footwall of the Bóixols thrust, the similar petrographic and geochemical features between all calcite cements and host rocks point towards a locally-derived or well-equilibrated fluid system. Contrarily, along large faults such as the Bóixols thrust, and in the anticline limbs, the geochemistry of vein cements indicates a different scenario. Cements in large faults yielded the lightest δ18O values, from −8 to −14 ‰VPDB, and variable enrichment in δ13C, 87Sr/86Sr, elemental composition and δ18Ofluid. This is interpreted as the migration of fluids, through fault zones, that evolved from distinct fluid origins. Cements in the fold limbs exhibit δ18O and δ13C between −8 and −6 ‰VPDB and between −10 and + 2 ‰VPDB, respectively, the lowest Sr contents and the lowest precipitation temperatures, suggesting that the anticline limbs recorded the infiltration and evolution of meteoric waters. The paleohydrological system in the Bóixols-Sant Corneli anticline was restricted to the Bóixols thrust sheet. The Upper Triassic evaporitic basal detachment likely acted as a lower fluid barrier, preventing the input of fluids from deeper parts of the Pyrenean crustal thrust system. This study provides a well-constrained absolute timing of fracturing and fluid flow during basin inversion and folding evolution and highlights the suitability of U–Pb geochronology to refine the age of fractures and veins and their sequential evolution in orogenic belts.

The Effect of Tectonic Stress and Thermal Evolution on Shale Pores of Devonian and Carboniferous Shales in Southern China

Many sets of Paleozoic marine organic-rich shale strata are developed in southern China. However, the exploration and development degree of these strata are different. Cambrian shale and Ordovician-Silurian shale are two horizons with high degree of exploration, while Devonian shale and Carboniferous shale exploration is deficient. Based on XRD, FE-SEM, and gas adsorption experiment, pore development characteristics and controlling factors of Devonian and Carboniferous shale are investigated. There are mainly four lithofacies in Devonian and Carboniferous shale: mixed shale (M), carbonate/siliceous mixed shale (M-1), argillaceous/siliceous mixed shale (M-2), and argillaceous-rich siliceous shale (S-3). Reservoir characteristics of both two sets of shale strata are quite different. The averages of porosity, pore volume, and specific surface area of Devonian shale are 3.81 % , 9.7 × 10 − 3 cm3/g and 11.8 m2/g, while those of Carboniferous shale are 3.57%, 17.3 × 10 − 3 cm3/g and 19.8 m2/g. Thermal evolution (Ro) and tectonic preservation conditions are the main factors affecting the pore development. Carboniferous shale ( Ro ≈ 2 % ) is in the stage of producing a large number of organic pores. Devonian shale ( Ro ≈ 3.5 % ) is having difficulty preserving organic pores due to high thermal evolution. Meanwhile, Devonian shale (well GTD1) is strongly affected by tectonic movement; tectonic fractures and calcite veins are developed. Carboniferous shale (well GRY1) is in relatively stable area; tectonic fractures are not developed. Under the influence of compaction, the pore volume and specific surface area of Carboniferous shale are 78.3% and 67.8% higher than those of Devonian shale, respectively. This research can provide reference for clarifying shale pore development and evolution mechanism and similar shale gas exploration both in study area and around the world.

Continuous versus punctuated vein widening in the Marcellus Formation, USA: the fine line between pressure fringes and hydraulic fractures

AbstractCalcite veins are common in organic-rich mudrocks, but their genesis and ability to transmit fluids are debated. A combined microstructural and isotopic investigation of an array of calcite veins recovered in core from the Marcellus Formation reveals that the veins grew via a combination of continuous fibrous growth and punctuated fracture-opening increments. Continuous opening is the result of pressure-solution creep and involves no mechanical fracturing, but rather the growth of a pressure fringe around a pre-existing, sealed fracture. In contrast, incremental opening is accomplished by overpressured, mineral-saturated fluid, which repeatedly ruptures the rock at the cement / host-rock interface. Punctuated growth increments occurred repeatedly throughout an otherwise protracted, continuous growth history, indicating that the present structures preserve hybrid deformation conditions between brittle, fluid-assisted cracking and plastic strain. Stable isotopic signatures match those of a regional opening-mode fracture set that formed in response to catagenetic fluid overpressures amid a tectonically imposed (Alleghanian) stress field. It is concluded that calcite veins form as opening-mode hydraulic fractures and are susceptible to increments of brittle reactivation, even while inelastic growth processes widen and fill the veins with fibrous cement.