Effects Of Hydrothermal Alteration Research Articles

Anatomy of a fumarole field: drone remote-sensing and petrological approaches reveal the degassing and alteration structure at La Fossa cone, Vulcano, Italy

Abstract. Hydrothermal alteration and mineralization processes can affect the physical and chemical properties of volcanic rocks. Aggressive acidic degassing and fluid flow often also lead to changes in the appearance of a rock, such as changes in surface coloration or intense bleaching. Although hydrothermal alteration can have far-reaching consequences for rock stability and permeability, limited knowledge exists on the detailed structures, extent, and dynamic changes that take place near the surface of hydrothermal venting systems. By integrating drone-based photogrammetry with mineralogical and chemical analyses of rock samples and surface gas flux, we investigate the structure of the evolving volcanic degassing and alteration system at the La Fossa cone on the island of Vulcano, Italy. Our image analysis combines principal component analysis (PCA) with image classification and thermal analysis through which we identify an area of approximately 70 000 m2 that outlines the maximum extent of hydrothermal alteration effects at the surface, represented by a shift in rock color from reddish to gray. Within this area, we identify distinct gradients of surface coloration and temperature that indicate a local variability in the degassing and alteration intensity and define several structural units within the fumarole field. At least seven such larger units of increased activity could be constrained. Through mineralogical and geochemical analysis of samples from the different alteration units, we define a relationship between surface appearance in drone imagery and the mineralogical and chemical composition. Gradients in surface color from reddish to gray correlate with a reduction in Fe2O3 from up to 3.2 % in the unaltered regime to 0.3 % in the altered regime, and the latter coincides with the area of increased diffuse acid gas flux. As the pixel brightness increases towards higher alteration gradients, we note a loss of the initial (igneous) mineral fraction and a change in the bulk chemical composition with a concomitant increase in sulfur content from close to 0 % in the unaltered samples to up to 60 % in samples from the altered domains. Using this approach of combined remote-sensing and in situ analyses, we define and spatially constrain several alteration units and compare them to the present-day thermally active surface and degassing pattern over the main crater area. The combined results permit us to present a detailed anatomy of the La Fossa fumarole field, including high-temperature fumaroles and seven larger units of increased alteration intensity, surface temperature, and variably intense surface degassing. Importantly, we also identify apparently sealed surface domains that prevent degassing, likely as a consequence of mineral precipitation from degassing and alteration processes. By assessing the thermal energy release of the identified spatial units quantitatively, we show that thermal radiation of high-temperature fumaroles accounts for < 50 % of the total thermal energy release only and that the larger part is emitted by diffuse degassing units. The integrated use of methods presented here has proven to be a useful combination for a detailed characterization of alteration and activity patterns of volcanic degassing sites and has the potential for application in alteration research and for the monitoring of volcanic degassing systems.

On the origin and evolution of deuterium enrichment in type 1 and 2 chondritic organic solids

Rotationally resonant Deuterium Nuclear Magnetic Resonance spectroscopy (D MAS NMR) was applied to IOM isolated from a CR1 chondrite Grosvenor Mountains (GRO) 95577 and a CM2 chondrite (Murchison). It is shown that in IOM D strongly prefers the aliphatic hydrogen reservoir over the aromatic hydrogen reservoir. For GRO 95577, that has a bulk δD of 3303 ‰ (Alexander et al., 2010), the average δD value of the aromatic reservoir is 1740 ± 128 ‰ and the aliphatic reservoir is 4477 ± 105 ‰, i.e., D/H enrichments of 1.27 and 0.64, respectively, relative to the bulk. For Murchison IOM, that has a bulk δD of 811 ‰ (Alexander et al., 2010), the average δD of the aromatic reservoir is 512 ± 88 ‰ and the aliphatic reservoir is 1033 ± 64 ‰ i.e., D/H enrichments of 1.12 and 0.82, respectively, relative to the bulk. D-H exchange between D-enriched water and a type III kerogen reveals nearly equivalent D up take by both aromatics and aliphatics. Laboratory synthesis of IOM-like material in the presence of D2O reveals a high degree of deuteration with a strong preferential deuteration of the aliphatic hydrogen reservoir indicating that the δD of the water during IOM synthesis is the primary determinant of syn-IOM’s δD. The IOM in GRO 95577 and Murchison (FA and H/C × 100) lie on the molecular evolution line as defined by the IOM of the Tagish Lake clasts and Murchison IOM has experienced more molecular evolution relative to that exhibited by GRO 95577 IOM. A forward prediction derived from the D/H ratios for the aliphatic and aromatic hydrogen reservoirs in Murchison and GRO 95577, relative to their bulk D/H ratios, derived from D MAS NMR, is applied to explain the origin of the Tagish Lake trend of δD vs molecular evolution (H/C × 100). The results of this forward prediction suggest that the Tagish Lake isotopic trend results from a combination of molecular evolution (loss of predominantly aliphatic H and D) and partial D-H exchange with D depleted chondritic water during a short-term hydrothermal alteration event. Such events may be faithfully identified in chondritic organic solids and be a common occurrence, but not necessarily revealed in the mineralogy of type 1 and 2 carbonaceous chondrites.

Temperature dependent loss of noble gases from insoluble organic matter and presolar grains during hydrothermal alteration

The effects of hydrothermal alteration on primordial noble gases were investigated by analysing noble gases in aliquots of insoluble organic matter (IOM) from the Murchison meteorite (CM2) that were hydrothermally altered for different durations (27–163 days) and at a range of temperatures (250–450 °C). The samples contained Q gases, HL from presolar nanodiamonds, and Ne-E from presolar SiC and graphite. We observed changes in the noble gases that correlated with changes in the experimental reaction temperature. Losses of Ne-E occurred in samples reacted at 250–300 °C (23 % of the Ne-E present in unaltered samples) and in samples reacted at 350–450 °C (72 %). This indicates that temperature drives degassing of Ne-E from presolar SiC and graphite and that Ne-E can be degassed at temperatures below 400 °C under hydrothermal conditions. Elemental ratios of 20Ne/36Ar correlate with reaction temperature. This relationship can be used to estimate the peak hydrothermal alteration temperature experienced, as Ne is predominantly carried in presolar diamonds and Ar in Phase Q, which respond differently to hydrothermal alteration. The calculated temperatures using the correlation between 20Ne/36Ar and temperature with 20Ne/36Ar ratios from previously published data agree well with temperatures in the literature determined by other techniques when ratios are between 0.15 and 0.30. Therefore 20Ne/36Ar ratios have the potential to be used as a parent body hydrothermal alteration thermometer.

Compositional heterogeneity of insoluble organic matter extracted from asteroid Ryugu samples

AbstractWe report a Fourier transform infrared analysis of functional groups in insoluble organic matter (IOM) extracted from a series of 100–500 μm Ryugu grains collected during the two touchdowns of February 22 and July 11, 2019. IOM extracted from most of the samples is very similar to IOM in primitive CI, CM, and CR chondrites, and shows that the extent of thermal metamorphism in Ryugu regolith was, at best, very limited. One sample displays chemical signatures consistent with a very mild heating, likely due to asteroidal collision impacts. We also report a lower carbonyl abundance in Ryugu IOM samples compared to primitive chondrites, which could reflect the accretion of a less oxygenated precursor by Ryugu. The possible effects of hydrothermal alteration and terrestrial weathering are also discussed. Last, no firm conclusions could be drawn on the origin of the soluble outlier phases, observed along with IOM in this study and in the preliminary analysis of Ryugu samples. However, it is clear that the HF/HCl residues presented in this publication are a mix between IOM and the nitrogen‐rich outlier phase.

Effects of hydrothermal alteration on shear localization and weakening in the mantle lithosphere

Hydrothermal alteration occurs in upper-mantle fault zones and significantly modifies the rheological properties of ultramafic rocks. To understand how hydrothermal alteration affects shear localization and the strength of the fault zones, we conducted simple-shear deformation experiments on water-saturated harzburgite (70 wt% olivine +30 wt% orthopyroxene) gouges sandwiched between three types of shear pistons (tungsten, yttria-stabilized zirconia, and corundum) using two deformation-DIA apparatuses at confining pressures of 1–4 GPa, temperatures of 500–580 °C, average shear strain rates of 6.1 × 10−6 s−1 to 1.0 × 10−4 s−1, and water contents of 4–30 wt%. The harzburgitic gouges with water contents of 10 and 30 wt% exhibited steady-state sliding or strain-hardening behavior, and the shear strength of the latter sample was less than one-third of that of the former sample, possibly reflecting the difference in pore fluid pressure. Olivine/orthopyroxene grains exhibit distributed microfracturing and grain rotation towards P foliation, indicating the operation of cataclastic flow. Shear displacement is also accommodated by the development of serpentine- or talc-bearing multiple shear zones including B, R1, and/or Y shears, where intense cataclasis-related grain size reduction and dissolution/precipitation occur. For experimental runs using tungsten and zirconia shear pistons, the serpentine or talc blades that wrap finer-grained and dissolved olivine/orthopyroxene grains form an interconnected network, indicative of frictional sliding or dislocation glide on their basal planes as a dominant deformation mechanism. Raman spectra of the serpentine minerals indicate that they represent antigorite with low crystallinity. These findings suggest that when deep lithospheric fault zones undergo hydrothermal alteration, deformation is controlled by competition between cataclastic flow in olivine/orthopyroxene matrix and slip or glide of antigorite or talc in semi-brittle fault zones, the predominance of which depends on the amount of the hydrous phyllosilicates and the magnitude of pore fluid pressure.

Effect of hydrothermal alteration of Ca-bentonite in K-rich solutions with different pHs on the physicochemical, swelling, and Cs adsorption properties

Understanding the stability of the bentonite buffer when exposed to cement leachates at temperatures exceeding 100°C is crucial for optimizing the design of deep geological repositories (DGRs) for high-level radioactive wastes (HLRWs) and minimizing the required site area. Experiments were conducted under hydrothermal conditions at 150°C for 30–150 days using deionized (DI) water (pH 6) and K-rich solutions (1 mol/L KCl, pH 6 and 1 mol/L KOH, pH 13) on Ca-bentonite. The 1 mol K concentration is comparable to 15 times the cation exchange capacity (CEC) of raw bentonite. Using various analytical techniques, the mineralogical, physicochemical, swelling, and Cs adsorption characteristics of raw and reacted bentonite samples were determined. Almost no changes in the properties of bentonite reacted with DI water were found for a given reaction time. For bentonite reacted with a 1 mol/L KCl solution, the cation exchange of Ca by K was a primary alteration process, resulting in a slight decrease in swelling capacity. However, almost no mineralogical changes were observed and consequently, there was minimal change in the Cs adsorption capacity. In contrast, for the bentonite reacted with a 1 mol/L KOH solution, the dominant alteration process was the transformation of minerals in bentonite into zeolite minerals, which resulted in significant changes in physicochemical properties, in particular, a decrease in the swelling capacity. On the other hand, the Cs adsorption capacity increased up to 2 times compared with the raw bentonite.

New constraints on fluid chemistry and elemental mobility during hydrothermal alteration from thallium isotope systematics of the battle mountain district, North-central Nevada

The Battle Mountain area is a major porphyry style Au (–Cu) district in northern Nevada, forming one end of the Battle Mountain-Eureka mineral trend. This trend, along with several others in the north-central Nevada region, hosts some of the world's most prolific gold mineralization, which is oftentimes associated with a suite of trace elements including arsenic, antimony, tungsten, and thallium and can be related to or overprinted by hydrothermal alteration. However, determining the extent of control hydrothermal fluids exert on some of the deposits in the region has been difficult to ascertain. One of the associated trace metals in the region, thallium (Tl), is a highly incompatible element and, as such, dominantly resides in the continental crust, where it can be easily remobilized during hydrothermal alteration. While previous work has demonstrated the effects of hydrothermal alteration on Tl distribution and fractionation, the controls responsible for the observed Tl fractionation during hydrothermal alteration are still poorly characterized but may provide insight into fluid behavior. Here, we present new Tl isotope composition and concentration data for a suite of 54 mineral separates obtained from 43 samples from the Battle Mountain area, which range from unaltered intrusive igneous rocks through varying types and degrees of hydrothermally altered rocks.Measured Tl concentrations vary by more than an order of magnitude, from below detection limit (0.2 ppm in this study) to 2.0 ppm, while ε205Tl ranges between −5.0 and +2.2 (ε205Tl is the deviation of the 205Tl/203Tl isotope ratio of a sample from a standard in parts per 104). Thallium concentrations correlate positively with whole-rock potassium (K), thus show strong increases during K alteration, and demonstrate significantly lower ε205Tl values within K-altered samples. Conversely, during later, overprinting Na–Ca alteration, both K and Tl are removed, resulting in a noted decrease in Tl concentrations coupled with a shift to significantly higher ε205Tl values. It appears that during hydrothermal alteration, 203Tl is more easily (re)mobilized and (re)distributed, which reflects: 1) a first-order hydrothermal alteration control that relates to the transport of Tl during the formation of a new, metasomatic mineral assemblage (particularly the breakdown and/or formation of K-bearing minerals) and 2) a second-order mineralogical control, relating to inter-mineral equilibrium, which also results in a small fractionation effect.

Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars

AbstractSecondary minerals in martian nakhlites provide a powerful tool for investigating the nature, composition, and duration of aqueous activity in the martian crust. Northwest Africa (NWA) 998 crystallized early from the nakhlite magmatic source and has evidence of minimal signatures of the late hydrothermal alteration event that altered the nakhlites. Using FIB‐TEM techniques to study a cumulus apatite grain in NWA 998, we report the first evidence of a submicron‐scale vein consisting of fluorapatite and an SiO2‐rich phase. Fluorapatite grew epitaxially on the walls of an opened cleavage plane of host F‐bearing chlorapatite and the SiO2‐rich phase filled the center of the vein. The presence of nanoporosity and nanometer‐scale amorphous material and the sharp interface between the vein and the host apatite indicate the vein represents a coupled dissolution–reprecipitation process that generated apatite of a different composition that was more stable with the fluid. Using experimental data and diffusion coefficients of Cl in apatite from the literature, we conclude that the vein was caused by a low temperature (~300°C), slightly acidic, F‐, Si‐rich, aqueous fluid that acted as a closed system. Based on the characteristics of the vein (formation by rapid injection of fluid) and the fluid (composition, temperature, pH), and the lack of terrestrial weathering products in our SEM and TEM images, we infer that the vein is pre‐terrestrial in origin. Our observations support the hypothesis that the heat source triggering a hydrothermal system was a low‐shock velocity impact and rule out a magmatic origin. Finally, the vein could have formed from a late‐stage fluid different from that reported in other nakhlites, but formation during the same magmatic event by, for example, a less evolved fluid might also be plausible.

Spectral characterisation of hydrothermal alteration associated with sediment-hosted Cu–Ag mineralisation in the central European Kupferschiefer

Abstract. The analysis of hydrothermal alteration in exploration drill cores allows for fluid–rock interaction processes to be traced, for fluid flow paths to be identified, and thus for vectors in mineral systems to be determined. Hyperspectral imaging techniques are increasingly being employed to fill the scale gap between lab-based petrographic or geochemical analyses and the typical size of exploration targets. Hyperspectral imaging permits the rapid, cost-efficient, and continuous characterisation of alteration mineralogy and texture along entire drill cores, with a spatial sampling of a few millimetres. In this contribution, we present the results of an exploratory study on three mineralised drill cores from the Spremberg–Graustein Kupferschiefer-type Cu–Ag deposit in the Lusatia region of Germany. We demonstrate that hyperspectral imaging is well-suited to recognising and tracking the effects of hydrothermal alteration associated with strata-bound hydrothermal mineralisation. Micro X-ray fluorescence spectrometry was used to corroborate the alteration mineral assemblages identified in hyperspectral data acquired in the visible, near- (400 to 970 nm), shortwave (970 to 2500 nm), mid-wave (2700 to 5300 nm), and longwave infrared (7700 to 12 300 nm). We identified two main shortcomings of the technique, namely the overlapping of some mineral features (e.g. carbonate and illite absorption in the shortwave infrared) and the darkness of the organic-matter-rich dolostones and shales that results in low reflectance. Nevertheless, spectral features associated with iron oxide, kaolinite, sulfate, and carbonates were successfully identified and mapped. We identified different markers of hydrothermal alteration spatially associated with or stratigraphically adjacent to Cu–Ag mineralisation. Importantly, we can clearly distinguish two mineralogically distinct styles of alteration (hematite and ferroan carbonate) that bracket high-grade Cu–Ag mineralisation. Intensive hydrothermal alteration is characterised by the occurrence of well-crystallised kaolinite in the sandstone units immediately below the Kupferschiefer horizon sensu stricto. Proximal Fe-carbonate and kaolinite alteration have not previously been documented for the high-grade Cu–Ag deposits of the central European Kupferschiefer, whereas hematite alteration is well-known in Kupferschiefer-type ore deposits. The latter marks the flow path of oxidising, metal-bearing hydrothermal fluids towards the site of hydrothermal sulfide mineralisation. In contrast, ferroan carbonate alteration in carbonate rocks located above the main mineralised zone is interpreted as a mark of hydrothermal fluid discharge from the mineralising system. Although this study is limited to a small number of drill cores, our results suggest that hyperspectral imaging techniques may be used to identify vectors towards high-grade Cu–Ag mineralisation in Kupferschiefer-type mineral systems.

Effects of Hydrothermal Alteration and Mineralization on the Paleomagnetic Properties of Rocks from IODP Expedition 376 at Brothers Volcano

AbstractThe 3-D subseafloor architecture of submarine hydrothermal systems is largely unknown, particularly at arc volcanoes. The alteration of volcanic rocks in these systems produces dramatic changes in their magnetic properties. Here, we present the first comprehensive study of paleomagnetic measurements from oriented samples of hydrothermally altered dacites from Brothers volcano (Kermadec arc), drilled during International Ocean Discovery Program (IODP) Expedition 376. These data have enabled insight into the progressive evolution of magnetic minerals in subseafloor volcanic rocks affected by variable types and degrees of hydrothermal alteration in response to varying fluid temperatures, chemistry, and associated mineralization; from initial chloritization typical of relatively low-temperature interaction with seawater to extremely altered rocks affected by higher-temperature, very acidic magmatic fluids.Hydrothermally altered samples show a significant reduction in natural remanent magnetization intensity (10–4 to 10–2 A/m) compared with unaltered samples (1–10 A/m), suggesting that primary titanomagnetite grains are destroyed during the hydrothermal alteration process. Except for a small region in proximity to the mineralized stockwork zone, no chemical remanent magnetization is observed in association with hydrothermal alteration, consistent with the widespread formation of diamagnetic and/or paramagnetic minerals such as pyrite, rutile, and leucoxene, which do not carry any natural remanent magnetization.Demagnetization experiments show that most of the oriented samples possess a stable characteristic remanent magnetization induced by the residual primary magnetic minerals formed at the time the rocks cooled on the sea floor. Partially chloritized dacites, however, are characterized by large magnetic susceptibilities, low Koenigsberger ratios, and very low magnetic coercivities, consistent with initial dissolution of smaller, singledomain magnetic grains, indicating that intensely hydrothermally altered rocks are better paleomagnetic indicators than initially chloritized samples at the periphery of the hydrothermal systems.The significant magnetic contrast between fresh and hydrothermally altered rocks, in addition to a thick layer (>300 m) of demagnetized rocks observed at Brothers volcano, confirms the empirical results that magnetic anomalies are important geophysical tools to determine the geometry of hydrothermal systems at submarine arc volcanoes.

Unravelling magmatic-hydrothermal processes at Salobo and GT-46 IOCG deposits, Carajás mineral province, Brazil: Constraints from whole-rock geochemistry and trace elements in apatite

The iron oxide-copper-gold (IOCG) deposits of the northern portion of the Carajás Province, Amazonian Craton, Brazil, were primarily formed at the 2.5 Ga IOCG event. These deposits commonly exhibit wide Fe–K hydrothermal alteration halos (i.e., almandine-grunerite-biotite-magnetite) and display a temporal and spatial association with the 2.5 Ga granitic bodies, particularly in the GT-46 and in the world-class Salobo deposit. However, this association still lacks investigation concerning the metallogenic influence of the 2.5 Ga granitic bodies in the IOCG formation. Within these deposits, ca. 2.5 hydrothermally altered granites are recognized and include the Old Salobo, at the Salobo deposit, and G1 and G2, at the GT-46 deposit. Whole-rock geochemistry combined with petrography has pointed out that the G1 and Old Salobo are represented by granites to monzogranites, ferroan, metaluminous to weakly peraluminous with negative Eu anomaly. On the contrary, the G2 granite seems to represent a distinct body with granite composition, ferroan, metaluminous to weakly peraluminous with positive Eu anomaly. This granite exhibit pegmatitic textures, high Sr/Y ratios and listric-shaped normalized REE patterns, which suggest crystallization under high water contents. These granites primarily plots in the field of the volcanic arc granites, though some shifts towards to syn-collisional granites are observed. Effects of hydrothermal alteration are commonly seen in these granites both in textural aspects and geochemistry. Apatite mineral chemistry studies shows that magmatic signature in apatite is preserved in the G2 granite (G2core apatite), Old Salobo granite (OSA) and in the GT-46 ore zone (OZGT-A apatite), represented by enrichment in LREE and negative Eu anomaly. Hydrothermally altered apatite is recognized in the Old Salobo (OSB) and G2 granite (G2rim) and in the ore zone of GT-46 deposit and (OZGT-B) with depletion in LREE, which suggest that the same fluid might have affected both deposits. Truly hydrothermal apatite crystals from the Salobo and GT-46 ore zones shows high (Eu/Eu*)N values, indicative of high oxygen fugacity and alkaline fluids. Strong HREE depletion of the apatite grains from the central zone of the deposits suggest coeval almandine crystallization in the Fe–K alteration halos and evolved hydrothermal fluids. These results indicate that magmatic and hydrothermal process coexist in the IOCG deposits from the northern sector of Carajás Province. This might suggest that the 2.5 Ga IOCG deposits from the Carajás Province can have formed during the development of wide magmatic-hydrothermal systems.

Multiple generation magmatic and hydrothermal processes in a Martian subvolcanic environment based on the analysis of Yamato‐000593 nakhlite meteorite

AbstractWe studied the occurrence of secondary minerals and inferred their formation in the Yamato‐000593 Martian meteorite using multiple technological approaches such as electron probe micro analysis, optical microscope, Raman spectroscopy, scanning electron microscopy, as well as Fourier transform‐infrared microscopy and spectroscopy. Two separate hydrothermal alteration events and their sequence of formation (based on superpositional relationship) can be identified: an elevated temperature phase producing high‐temperature sulfidic hydrothermal alteration and a lower temperature hydrothermal alteration phase by iron‐rich fluids. This meteorite shows signatures more compatible with magmatic effects, rather than impact‐induced hydrothermal alteration, as has been proposed earlier. The sulfidic alteration probably formed by magmatic hydrothermal fluids, whereas iron‐rich hydrothermal fluid circulation after a possible early impact event has also been proposed, when the fluids cooled down to 50 °C. Most of the secondary minerals formed at alkaline‐neutral conditions, and the few observed signatures (clay–silica‐bearing veins, siderite‐iron‐oxide veins) of briny conditions are probably from local spatial effects in larger cavities. The ferrous minerals (hematite and siderite) along the fractures could be crystallized from Fe‐HCO3‐bearing fluids. Alternatively, the primary magmatic minerals could have been oxidized easily (Fe‐rich olivines, magnetite) during the cooling to iron oxides (hematite, goethite). The results suggest the possible existence of at least ephemerally habitable environments on Mars, mainly at volcanically heated locations. Following published geochemical models, the carbonates formed within acidic‐circumneutral condition, which was followed by formation of phyllosilicates in alkaline condition.

Multi-scale impact of climate change and cascade reservoirs on hydrothermal regime alteration in regulated rivers

Study regionThe upper reach of the Yangtze River (URYR) basin, China. Study focusThere are few studies on the factors driving the change in hydrothermal regime and its ecological response. This study proposes a framework to quantify the impact of climate change and reservoirs on hydrothermal and fish reproduction. The framework identifies mutation years of water temperature by the TFPW-MK method and characterizes hydrothermal variability on multiple time scales using hydrologically meaningful indicators (Sample Entropy and Range of Variability Approach). The differences in hydrothermal driving forces are then clarified utilizing the Long Short-Term Memory model and contribution calculation method. In addition, we established water temperature scenarios to assess the ecological effects of hydrothermal alteration. New hydrological insights for the regionAfter mutation, URYR hydrothermal tends to be uniform. With the increase in hydrothermal complexity, the relative contribution of reservoirs is 98%. On an annual scale, climate change is the dominant factor in river warming. On the seasonal and monthly scales, reservoirs are used as heat sources in autumn and winter, and as cold sources in other seasons. Moreover, the reservoirs have a significant impact on hydrothermal in the non-flood season, especially in March and December, which are − 0.72 °C (48%) and 1.17 °C (87%), respectively. The reservoirs delayed the spawning time for Coreius guichenoti and Myxocyprinus asiaticus but increased the duration of Myxocyprinus asiaticus (19 days).

Emplacement ages of diamondiferous kimberlites in the Wafangdian District, North China Craton: New evidence from LA-ICP-MS U-Pb geochronology of andradite-rich garnet

This contribution presents new U-Pb geochronological data and attempts to elucidate the complex evolution history of various garnet types identified from two kimberlite pipes in the Wafangdian diamond mining district, southern Liaoning Province. These calcic garnets are dominated by andradite with relatively low proportions of schorlomite, grossular and pyrope. Abundant euhedral to subhedral, highly brecciated andradite phenocrysts hosted by LN30 “carbonatite-like” kimberlite samples yield a lower-intercept age of 459.3 ± 3.4 Ma, which is in excellent agreement with the previously reported phlogopite Ar-Ar (463.9 ± 0.9 Ma) and Rb-Sr ages (461.7 ± 4.8 Ma). Based on their trace element and C-O isotopic compositions of associated groundmass carbonate, we infer that these primary magmatic andradites probably originated from kimberlitic magmas. By comparison, three compositionally and texturally distinct groups of Ti-bearing andradites from LN42 hypabyssal kimberlites separately define three well-fitted regression lines with lower intercept ages at 581 ± 12 Ma, 414.9 ± 9.3 Ma and 292.0 ± 5.7 Ma, respectively. Relict andradite xenocrysts implies that ancient lower crust of the North China Craton (NCC) might have been affected by a significant but less-known tectonothermal event to varying degrees at ∼ 0.6 Ga. By contrast, fresh grains of magmatic Ti-andradites with chemical zoning produce a relatively young age of ∼ 415 Ma, which can still provide minimum age estimates for the most recent pulses of Paleozoic kimberlite magmatism in this study area. Noteworthily, a yet unrecognized local-scale hydrothermal alteration event at ∼ 292 Ma has been recorded in the texturally distinct population of secondary hydroandradites, whose age reported here for the first time is geologically meaningful. To sum up, this study further highlights andradite U-Pb dating as a potential robust geochronometer for constraining the late-stage evolution of kimberlite magmas as well as post emplacement hydrothermal alteration.

Geochemistry of hydrothermal zircon as a proxy to fingerprint ore fluids in late Mesozoic decratonic gold deposits

Decratonic gold deposits constitute the most important type of gold endowment in the North China Craton (NCC), although the nature and evolution of ore-related fluids in these deposits remain controversial. Here we investigate geochemistry of hydrothermal zircon grains from the Dahu gold deposit in the Xiaoqinling region in the southern margin of the NCC. We also compile the published information on trace elements in hydrothermal zircon from deposits in the other major gold districts of the NCC. The zircon U-Pb data show ages in the range of 101–148 Ma reflecting prolonged hydrothermal alteration that occurred within an extensional tectonic setting. Based on the trace elements, three distinct groups of hydrothermal zircons are recognized. Group 1 shows low La contents (<1 ppm), high Ce/Ce* (>10), and steep positive LREE patterns ((Sm/La)N > 20); Group 2 possesses high La contents (<10 ppm), low Ce/Ce* (<10), and weak positive LREE patterns ((Sm/La)N < 20); Group 3 displays ultra-high La contents (>10 ppm), low Ce/Ce* (<10), and flat LREE patterns ((Sm/La)N < 20). This classification is also applicable to orogenic and porphyry gold deposits. Group 1 grains initially crystallized from magma and underwent subsequent modification by post-magmatic fluids, whereas Group 2 and Group 3 directly crystallized and regrew from zirconium-saturated aqueous fluids. The La-enriched signature in hydrothermal zircons may mainly originate from undetected REE-bearing phases within zircon that formed during the hydrothermal alteration events. The fluid flow has a significant effect on the redox state of gold metallogenic system. The hydrothermal zircons from decratonic gold deposits display higher Ce/Ce* ratios (1–258) and Ce/Nd ratios (0.7–32.2) than those from orogenic and porphyry gold deposits, indicating that the zircon grains formed from a relatively oxidized fluid.

Effect of alteration on the geochemistry and mechanical properties of granite from Pingjiang, Hunan Province, China

The effect of alteration on the geochemistry and mechanical properties of granite from Pingjiang, Hunan Province, China was investigated. Six weathered and 14 hydrothermally altered samples in 3 adits were collected for mechanical strength tests, mineralogical and geochemical analysis. The types of alteration observed within the samples were chloritization and argillization. Samples taken from a silicified fracture zone were enriched in quartz. Weathering was observed to significantly weaken the granite whereas the effects of hydrothermal alteration on strength were more complex. The porosity increased with the enrichment of the altered minerals, indicating that the formation of altered minerals degrades the strength of physical bonds between minerals within the granite. The granite Na2O, CaO, K2O, MgO and SiO2 contents decreased while Fe2O3T increased due to weathering. Variations of major elements within the hydrothermally altered granite were distinguished from those observed in weathered samples—notably Mg was removed whilst Si and Fe were generally stable during the hydrothermal alteration. Whereas the quartz-enriched samples gained Si with slight depletions in Mg and Fe. Trace and rare earth elements were both removed in hydrothermal alteration. The variable behavior of major element was quantified by the mobility index which indicated that the different geochemical changes were attributed tochloritization and argillization. Furthermore, the mobility index of Mg was used to identify the dominated alteration in granite and evaluate the effects of chloritization and argillization. Generally, chloritization was found to be more dominant than argillization in weakening the granite.

Increasing sulfur and chlorine contents in ore-forming magmas: The key to Pulang porphyry Cu-Au formation, SW China

The giant Pulang porphyry Cu-Au district (446.8 million tonnes at 0.52% Cu and 0.18 g/t Au) is located in the southern Yidun terrane of the Sanjiang (Three-river) region, southwest China. It has six phases of porphyry intrusions: premineralized fine-grained quartz diorite (FQD) and coarse-grained quartz diorite porphyry (CQD), the main synmineralized quartz monzonite porphyry (QMP), and late synmineralized granodiorite porphyry (GP), diorite porphyry (DP), and aplite. New zircon U-Pb ages for the premineralized FQD and late synmineralized GP, DP, and aplite are 226.3 ± 2.6 Ma (MSWD = 1.9), 216.4 ± 1.0 Ma (MSWD = 2.0), 216.0 ± 1.1 Ma (MSWD = 1.7), and 216.0 ± 1.5 Ma (MSWD = 1.7), respectively. All Pulang porphyry rocks have high whole-rock Sr/Y ratios (37.8–184) with minor or no negative Eu anomalies (0.73–1.04), and are characterized by high large-ion-lithophile-element (LILE: Th, U, Ba, Rb) abundances and low high-field-strength-element (HFSE: Nb, Ta, Ti, Zr, Hf).Zircons from premineralized FQD and synmineralized GP, DP, and aplite rocks have consistently high EuN/EuN* ratios (mostly > 0.4). The magmatic oxygen fugacity calculated from zircon compositions, show that the FQD (ΔFMQ = 1.59 ± 0.43), GP (ΔFMQ = 1.52 ± 0.18), DP (ΔFMQ = 1.52 ± 0.30), and aplite (ΔFMQ = 1.75 ± 0.60) were all derived from moderately oxidized magmas. Magmatic water contents estimated from amphibole compositions and the extensive presence of amphibole phenocrysts suggest that H2O contents are >4 wt%. Together with the published results for premineralized CQD and main synmineralized QMP, it is concluded the causative magmas for the six porphyry phases at Pulang were all relatively oxidized and hydrous.The difference between premineralized FQD and CQD and synmineralized QMP, GP, and DP are reflected in magmatic chlorine and sulfur contents. To minimize the effect of hydrothermal alteration, the apatite microphenocrysts enclosed by least altered phenocrysts were selected for analyses. The Cl contents of magmatic apatite microphenocrysts from the synmineralized QMP (avg = 0.340 ± 0.192, n = 26), GP (avg = 0.317 ± 0.151, n = 21), and DP (avg = 0.179 ± 0.112, n = 18) are significantly higher than those from the premineralized FQD (avg = 0.046 ± 0.032, n = 9) and CQD (avg = 0.043 ± 0.034, n = 13). Similarly, the SO3 contents of igneous apatite microphenocrysts from the synmineralized QMP (avg = 0.259 ± 0.156, n = 25), GP (avg = 0.203 ± 0.163, n = 20), and DP (avg = 0.306 ± 0.173, n = 18) are higher than those from the premineralized FQD (avg = 0.181 ± 0.136, n = 10) and CQD (avg = 0.085 ± 0.068, n = 6). The S-Cl contents of synmineralized melts estimated from apatite S-Cl compositions are higher than those of premineralized magmas, suggesting that the elevated magmatic S-Cl contents in the synmineralized porphyries might have been important for Cu-Au mineralization. This was likely due to S-Cl rich mafic magma recharge, supported by the presence of abundant mafic magmatic enclaves in the synmineralized QMP.

Geochronology of mafic magmatism and hydrothermal alteration during early stages of South Atlantic opening

40Ar/39Ar laser incremental-heating geochronology of whole-rock fragments and minerals from primitive subalkaline mafic dykes from the Cabo Frio Tectonic Domain, southeastern Brazil, reveals a complex combination of intrusive ages, apparent ages affected by excess argon, and ages that record late-stage hydrothermal alteration. Incremental-heating analysis of encapsulated magmatic amphibole-biotite clusters reveals a minimum intrusive age of 132.83 ± 0.30 Ma. Sericitized plagioclase crystals show the percolation of K-rich fluids at ~106 Ma. Albitized plagioclase phenocrysts suggest the percolation of Na-rich fluids in the 60–30 Ma interval. Hydrothermal alteration events overlap in time with pulses of alkaline magmatism in the extending Brazilian crust, suggesting a link between magmatic activity and generation of fluid circulation cells along the margin and adjacent sedimentary basins. Evidence for pervasive hydrothermal re-setting of the K-Ar system in the Cabo Frio Tectonic Domain dykes suggests that young 40Ar/39Ar geochronology results obtained for mafic dykes here and elsewhere along the continental margins may not record multiple intrusive events, reflecting instead partial or total resetting of the K-Ar system by water-rock interaction during fluid circulation.

Chicxulub impact structure, IODP‐ICDP Expedition 364 drill core: Geochemistry of the granite basement

AbstractThe IODP‐ICDP Expedition 364 drilling recovered a 829 m core from Hole M0077A, sampling ˜600 m of near continuous crystalline basement within the peak ring of the Chicxulub impact structure. The bulk of the basement consists of pervasively deformed, fractured, and shocked granite. Detailed geochemical investigations of 41 granitoid samples, that is, major and trace element contents, and Sr–Nd isotopic ratios are presented here, providing a broad overview of the composition of the granitic crystalline basement. Mainly granite but also granite clasts (in impact melt rock), granite breccias, and aplite were analyzed, yielding relatively homogeneous compositions between all samples. The granite is part of the high‐K, calc‐alkaline metaluminous series. Additionally, they are characterized by high Sr/Y and (La/Yb)N ratios, and low Y and Yb contents, which are typical for adakitic rocks. However, other criteria (such as Al2O3 and MgO contents, Mg#, K2O/Na2O ratio, Ni concentrations, etc.) do not match the adakite definition. Rubidium–Sr errorchron and initial 87Sr/86Srt=326Ma suggest that a hydrothermal fluid metasomatic event occurred shortly after the granite formation, in addition to the postimpact alteration, which mainly affected samples crosscut by shear fractures or in contact with aplite, where the fluid circulation was enhanced, and would have preferentially affected fluid‐mobile element concentrations. The initial (ɛNd)t=326Ma values range from −4.0 to 3.2 and indicate that a minor Grenville basement component may have been involved in the granite genesis. Our results are consistent with previous studies, further supporting that the cored granite unit intruded the Maya block during the Carboniferous, in an arc setting with crustal melting related to the closure of the Rheic Ocean associated with the assembly of Pangea. The granite was likely affected by two distinct hydrothermal alteration events, both influencing the granite chemistry: (1) a hydrothermal metasomatic event, possibly related to the first stages of Pangea breakup, which occurred approximately 50 Myr after the granite crystallization, and (2) the postimpact hydrothermal alteration linked to a long‐lived hydrothermal system within the Chicxulub structure. Importantly, the granites sampled in Hole M0077A are unique in composition when compared to granite or gneiss clasts from other drill cores recovered from the Chicxulub impact structure. This marks them as valuable lithologies that provide new insights into the Yucatán basement.

Using multispectral and radar remote sensing data for geological investigation, Qena-Safaga Shear Zone, Eastern Desert, Egypt

In the present study, Qena-Safaga Shear Zone (QSSZ) has been the subject matter of detailed remote sensing investigation using Landsat-8, ASTER, and SRTM DEM data. Gathering information from these sensors allowed revealing the different lithological, geological, and structural features. Field observations were carried out to verify the remotely sensed data analysis and interpretation. A fused Landsat-8 OLI and ALOS/PALSAR data clearly improved the view of structural elements that are difficult to be revealed by optical images. The results showed that the area is controlled by the dextral ENE-WSW shearing trend which extends across the Nile at Qena Bend. Lineament analysis and lineament density map demonstrated that the area was subjected to different phases of stress; accordingly, conjugate faults were formed; most of these trends were rejuvenated. The ENE- WSW, NE-SW, and NW-SE are the major tectonic trends affecting the area but the N-S direction is less dominant. WNW, NW, NE, E, and N directions are indicated by sinistral strike-slip movements. The ENE, NE, and WNW are governed by right-lateral strike-slip movements and most of these trends controlled the emplacement of granitoids. The image transformation technique also allowed revealing hydrothermal alteration effects related to the NW-SE and NE-SW fault systems. Overall, combining of multispectral and radar data and field observations is significant for characterizing the geological features in arid regions.