Gravitational instabilities triggered by fluid overpressure and downslope incision – Insights from analytical and analogue modelling

Fluid overpressure at the base of low-permeability strata reduces effective stress, allowing for gravitational sliding of the overlying cover. The force driving sliding is the slope-parallel component of the weight of the cover, whereas the resisting forces are the friction at the base of the cover and the buttressing resistance to shortening, which can be critically reduced by incision at the base of the slope. We developed an analytical model and undertook a series of analogue experiments to better understand the evolution of a sedimentary cover sliding above a low-permeability layer subjected to fluid overpressure. Where a downslope buttress was present, the sliding sheet length decreased with increasing pore-fluid pressure. In the absence of such buttress, the slide's length increased exponentially with increasing pore-fluid pressure. Another important difference dealt with geometry and kinematics. Buttressed slides consisted of one large slope-parallel mass rigidly translated and bounded by downslope thrusts and upslope normal faults. With increasing pore-fluid pressure, the contractional structures propagated upslope. By contrast, non-buttressed slides showed intense strain: deformation started with normal faults forming near the incision, then propagating upslope throughout the slide's evolution.     

The significance of cherts as markers of Ocean Plate Stratigraphy and paleoenvironmental conditions: New insights from the Neoproterozoic–Cambrian Blovice accretionary wedge,Bohemian Massif

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.     

Genesis of some tertiary Indian coals from the chemical composition of ash – a statistical approach: Part 1

In the present investigation, 37 numbers of high sulphur tertiary coal samples from Meghalaya, India have been studied on the basis of proximate and ash analysis. Various statistical tools like Bivariant Analysis, Principal Component Analysis (PCA) and Hierarchical Clustering Analysis (HCA), and also the geochemical indicators were applied to determine the dominant detrital or authigenic affinity of the ash forming elements in these coals. The genetic interpretation of coal as well as the coal ash has been carried out based on chemical compositions of high temperature ash (HTA) by using Detrital/Authigenic Index. X-Ray Diffraction (XRD) analysis was also carried out to study the mineralogy of the studied coal ashes. Both statistical tools and geochemical indicators have confirmed the detrital nature of these coals as well as the ash forming elements.     

Evolution of fault zones in carbonates with mechanical stratigraphy – Insights from scale models using layered cohesive powder

We present analogue models of the formation of dilatant normal faults and fractures in carbonate fault zones, using cohesive hemihydrate powder (CaSO₄·½H₂O). The evolution of these dilatant fault zones involves a range of processes such as fragmentation, gravity-driven breccia transport and the formation of dilatant jogs. To allow scaling to natural prototypes, extensive material characterisation was done. This showed that tensile strength and cohesion depend on the state of compaction, whereas the friction angle remains approximately constant. In our models, tensile strength of the hemihydrate increases with depth from 9 to 50 Pa, while cohesion increases from 40 to 250 Pa. We studied homogeneous and layered material sequences, using sand as a relatively weak layer and hemihydrate/graphite mixtures as a slightly stronger layer. Deformation was analyzed by time-lapse photography and Particle Image Velocimetry (PIV) to calculate the evolution of the displacement field. With PIV the initial, predominantly elastic deformation and progressive localization of deformation are observed in detail. We observed near-vertical opening-mode fractures near the surface. With increasing depth, dilational shear faults were dominant, with releasing jogs forming at fault-dip variations. A transition to non-dilatant shear faults was observed near the bottom of the model. In models with mechanical stratigraphy, fault zones are more complex. The inferred stress states and strengths in different parts of the model agree with the observed transitions in the mode of deformation.     

Deformation inside a paleosubduction channel – Insights from microstructures and crystallographic preferred orientations of eclogites and metasediments from the Tauern Window,Austria

The Eclogite Zone, of the Tauern Window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high-pressure metasediments. The rocks were exposed to 600 °C and 20–25 kbars, and then retrogressed during their exhumation, first under blueschist facies and later under amphibolite facies metamorphism. To gain insights into the deformation within the subduction channel during subduction and exhumation, both fresh and retrogressed eclogites, as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations (CPOs). Pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite. A misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep. The omphacite CPOs of the eclogites correspond to dominant SL-fabrics characteristic of plane strain deformation, though there are local variations towards flattening or constriction within the paleosubduction channel. The glaucophane CPOs in retrogressed eclogites match those of omphacite, suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions. Plastic deformation of the host high-pressure metasediments outlasted that of the eclogites, as indicated by white mica fabrics and quartz CPO. The latter is consistently asymmetric, pointing to the operation of non-coaxial deformation. The microstructures and CPO data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units.     

Dissolution–reprecipitation process of magnetite from the Chengchao iron deposit: Insights into ore genesis and implication for in-situ chemical analysis of magnetite

Magnetite formed in different environments commonly has distinct assemblages and concentrations of trace elements that can potentially be used as a genetic indicator of this mineral and associated ore deposits. In this paper, we present textural and compositional data of magnetite from the Chengchao iron deposit, Daye district, China to provide a better understanding in the formation mechanism and genesis of the deposit and shed light on analytical protocols for in-situ chemical analysis of hydrothermal magnetite. Magnetite grains from the ore-related granitoid pluton, mineralized endoskarn, magnetite-dominated exoskarn, and vein-type iron ores hosted in marine carbonate intruded by the pluton were examined using scanning electron microscopy and analyzed for major and trace elements using electron microprobe. Back-scattered electron images reveal that primary magnetite from the mineralized skarns and vein-type ores were all partly reequilibrated with late-stage hydrothermal fluids, forming secondary magnetite domains that are featured by abundant porosity and have sharp contact with the primary magnetite. These textures are interpreted as resulting from a dissolution–reprecipitation process of magnetite, which, however, are mostly obscure under optically.Primary magnetite grains from the mineralized endoskarn and vein-type ores contain high SiO₂ (0.92–3.21 wt.%), Al₂O₃ (0.51–2.83 wt.%), and low MgO (0.15–0.67 wt.%), whereas varieties from the exoskarn ores have high MgO (2.76–3.07 wt.%) and low SiO₂ (0.03–0.23 wt.%) and Al₂O₃ (0.54–1.05 wt.%). This compositional contrast indicates that trace-element geochemical composition of magnetite is largely controlled by the compositions of magmatic fluids and host rocks of the ores that have reacted with the fluids. Compared to its precursor mineral, secondary magnetite is significantly depleted in most trace elements, with SiO₂ deceasing from 1.87 to 0.47 wt.% (on average) and Al₂O₃ from 0.89 to 0.08 wt.% in mineralized endoskarn and vein type ores, and MgO from 2.87 to 0.60 wt.% in exoskarn ores. On the contrary, average content of iron is notably increased from 69.2 wt.% to 71.9 wt.% in the secondary magnetite grains. The results suggest that the dissolution–reprecipitation process has been important in significantly removing trace elements from early-stage magnetite to form high-grade, high-quality iron ores in hydrothermal environments. The textural and compositional data confirm that the Chengchao iron deposit is of hydrothermal origin, rather than being crystallized from immiscible iron oxide melts as previously suggested. This study also highlights the importance of textural characterization using various imaging techniques before in-situ chemical analysis of magnetite, as is the case for texturally complicated UTh-bearing accessory minerals that have been widely used for UPb geochronology study.     

Chemistry of magnetite-apatite from albitite and carbonate-hosted Bhukia Gold Deposit,Rajasthan, western India – An IOCG-IOA analogue from Paleoproterozoic Aravalli Supergroup: Evidence from petrographic,LA-ICP-MS and EPMA studies

The Bhukia gold (+copper) deposit hosted by albitite and carbonates that occur within the Paleoproterozoic Aravalli-Delhi Fold Belt (ADFB) in western India consists of magnetite, graphite, apatite and tourmaline along with sulfide mineralization. Ubiquitous presence of magnetite and apatite in gold-sulfide association, alteration patterns and shear controlled mineralization suggest it to be IOCG (Iron-oxide copper gold) type deposits. The detailed mineral chemistry of magnetite and apatite are generated and interpreted in terms of their genetic significance, hydrothermal and magmatic origin vis-à-vis their affiliation with IOCG deposition. The data suggest that the magnetite has hydrothermal affiliation. The Ni/Cr ratio is greater than 1, which is explained by differences in solubility and mobility of Ni and Cr in hydrothermal fluids and is corroborated with other key evidences including that of wide ranging Mg concentration further supports a strong hydrothermal input that is envisaged for the deposition of magnetite. Concentration of vanadium in magnetite is generally <1000 ppm in case of barren hydrothermal occurrences while in the study area, it is relatively higher as it is attributed to the gold-sulfide-Cu mineralization. Ti vs Ni/Cr, Ni/(Cr+Mn) vs Ti+V, Ca+Al+Mn vs Ti+V and Al+Mn vs Ti+V variations are interpreted in terms of magnetite genesis. EPMA data suggests that apatite present in Bhukia is of fluorapatite variety with F content >1 wt% and F/Cl >1. Higher concentration of F and moderate Mn along with lower concentration of Cl attests their magmatic hydrothermal character and its derivation from meta-volcano sedimentary source. REE patterns obtained from LA-ICP-MS analysis suggest enrichment of LREE relative to MREE and HREE with negative Eu anomaly. Y/∑REE, La/Sm, Ce/Th and Eu/Eu¹ vs Ce/Ce¹ values of apatite is indicative of their origin in a highly oxidized environment. Presence of magnetite along with apatite is a common feature in IOCG-IOA (Iron-Oxide Apatite) association. Bhukia Gold Deposit has many similarities with Kiruna type Iron-Oxide Apatite (IOA) deposits particularly with respect to their similar tectonic setting, alteration patterns, mineral assemblages such as abundance of magnetite, apatite and presence of late stage sulfides based on EPMA and Laser ablation ICP-MS (LA-ICP-MS) studies. Lithological, petro-mineralogical and geochemical signatures of magnetite and apatite infer that the Bhukia is a possible IOCG-IOA type gold deposit typically associated with sulfides and graphite which may be used as petrogenetic indicators and pathfinders for exploration.     

Detailed compositional analysis of hydrocarbons in soil gases above multi-horizon petroleum deposits – A case study from western Poland

A surface geochemical survey was carried on in an area with a cluster of petroleum deposits in western Poland. The largest of these accumulations is the Papro?-Cicha Góra dry gas deposit reservoired in the Rotliegend sediments. Other, similar gas deposits are: the Papro? gas field discovered in the top part of the Carboniferous formation and the Papro? W, Papro? N, and Nowy Tomy?l gas fields localized in the Zechstein Limestone reef structures. Moreover, the Papro? gas-condensate and the Jastrz?bsko oil deposits were found at shallower depths, in the Main Dolomite formation. This paper aims to analyze the variability of composition and distribution of near-surface gaseous hydrocarbons in relation to hydrocarbon accumulations reservoired in various stratigraphic units at depths.