Geological Constraints Research Articles

Explorations on footprints of salt-rich fluid and salt-depleted fluid immiscibility in hydrothermal systems: Insights from divergent partitioning of sulfate and perchlorate in the ZnSO4–Zn(ClO4)2–H2O system

A variety of salt fluids (e.g., aqueous Li2SO4, ZnSO4 and UO2SO4 fluid) were observed to separate into a heavy salt-rich fluid (F1) and a light salt-depleted fluid (F2) when heated to appropriate temperatures. The F1–F2 immiscibility (FFI) likely provides an efficient pathway for hydrothermal transport and accumulation of certain ore elements and influences the compositional evolution of hydrothermal fluids, as it considerably concentrates these elements into the F1 phase. However, geological studies to date have paid little attention to the occurrence and influence of this kind of fluid immiscibility. The discrepancy may be explained from two perspectives. First, the complex pressure–temperature–composition (P–T–x) conditions of geological fluids may destabilize the FFI or suppress its occurrence. Alternatively, the FFI in geological fluids may be neglected during geologic investigations due to a lack of knowledge. To bridge the gap between experimental research and geological studies, this study aims to reveal more properties of the FFI and establish some geological constraints for discerning the FFI in geological systems. The partitioning behaviors of sulfate (FFI-relevant component) and perchlorate (FFI-irrelevant component) during the FFI and boiling process in the ZnSO4–Zn(ClO4)2–H2O system were quantified using Raman spectroscopic analyses integrated with the mass conservation law. Results show inconsistent partitioning behaviors of SO42− and ClO4− during the FFI; SO42− is highly concentrated into the F1 phase relative to the light F2 phase, while ClO4− partitions almost equally between the two phases. On the contrary, SO42− and ClO4− are concentrated proportionally in the residual liquid with boiling. Accordingly, some possible geological constraints, in addition to microthermometry, are postulated for the identification of the FFI in hydrothermal fluids, including (a) the P–T–x trajectory of the fluid intersecting with the FFI's stability region; (b) fluid inclusions showing divergent contents of FFI-relevant components but consistent concentrations of FFI-irrelevant components. This study sheds new light on the research of the salt-rich fluid and salt-depleted fluid immiscibility in geological systems.

Vector-based three-dimensional (3D) well-path optimization assisted by geological modelling and borehole-log extraction

This paper describes a novel strategy to optimize the drilling time of three-dimensional (3D) directional wellbore trajectories using a vector-based approach subject to drilling and geological constraints. Many existing well-path models require manual entry for certain geological constraints such as formation dip or kick-off limit. In contrast, this vector-based approach ensures that geological constraints are automatically satisfied by building a geological model, and extracting a borehole log of key points along the well-path. The presented approach applies and compares a deterministic optimization technique known as Constrained Optimization by Linear Approximation (COBYLA) with a Genetic-Algorithm (GA) global optimization to determine the optimum 3D well path to drill the target. While optimizing the path, the model determines the optimum kick-off point based on the subsurface-formation strength and depth subject to predetermined doglog severity, inclination and azimuth angles. The methodology is applied to well paths with different number of build-up and drop sections in unconstrained and constrained geological settings. Results show that COBYLA and GA are comparable when not using geological modelling while GA is superior for complex well-path geology-assisted optimization problems. The technique is applicable for a single well path planning, and can be expanded to a set of wells being optimized during Field Development Planning (FDP).

Lithology identification from well-log curves via neural networks with additional geologic constraint

Lithology identification is of great importance in reservoir characterization. Recently, many researchers have applied machine-learning techniques to solve lithology identification problems from well-log curves, and their works indicate three main characteristics. First, most works predict lithofacies using features measured during logging, whereas very few consider adding stratigraphic sequence information that is available prior to drilling to solve this problem. Second, most studies predict lithofacies using measured properties of one depth point, whereas few take the influence of the neighboring formation into account. Third, due to a lack of publicly available interpreted well-log data, previous research has concentrated on applying different algorithms on their private data set, making it impossible to perform a comparison. We have developed a machine-learning framework to solve the lithology classification problem from well-log curves by incorporating an additional geologic constraint. The constraint is a stratigraphic unit, and we use it as an additional feature. We evaluate three types of recurrent neural networks (RNNs), bidirectional long short-term memory, bidirectional gated recurrent unit (Bi-GRU), and GRU-based encoder-decoder architecture with attention, as well as two types of 1D convolutional neural networks (1D CNNs), temporal convolutional network and multiscale residual network, on a publicly available data set from the North Sea. The RNN-based networks and 1D CNN-based networks can process sequential data, enabling the model to have access to information from neighboring formations when performing lithofacies prediction at a particular depth. Our experiments indicate that geologic constraint improves the performance of the models significantly, and that the overall performance of RNN-based networks is better and more consistent.

Crustal structure of the East African Limpopo margin, a strike-slip rifted corridor along the continental Mozambique Coastal Plain and North Natal Valley

Abstract. Coincident wide-angle and multi-channel seismic data acquired within the scope of the PAMELA Moz3-5 project allow us to reconsider the formation mechanism of East African margins offshore of southern Mozambique. This study specifically focuses on the sedimentary and deep-crustal architecture of the Limpopo margin (LM) that fringes the eastern edge of the Mozambique’s Coastal Plain (MCP) and its offshore southern prolongation the North Natal Valley (NNV). It relies primarily on the MZ3 profile that runs obliquely from the northeastern NNV towards the Mozambique basin (MB) with additional inputs from a tectonostratigraphy analysis of industrial onshore–offshore seismic lines and nearby or crossing velocity models from companion studies. Over its entire N–S extension the LM appears segmented into (1) a western domain that shows the progressive eastward crustal thinning and termination of the MCP/NNV continental crust and its overlying pre-Neocomian volcano-sedimentary basement and (2) a central corridor of anomalous crust bounded to the east by the Mozambique fracture zone (MFZ) and the oceanic crust of the MB. A prominent basement high marks the boundary between these two domains. Its development was most probably controlled by a steep and deeply rooted fault, i.e., the Limpopo fault. We infer that strike-slip or slightly transtensional rifting occurred along the LM and was accommodated along this Limpopo fault. At depth we propose that ductile shearing was responsible for the thinning of the continental crust and an oceanward flow of lower crustal material. This process was accompanied by intense magmatism that extruded to form the volcanic basement and gave the corridor its peculiar structure and mixed nature. The whole region remained at a relative high level during the rifting period and a shallow marine environment dominated the pre-Neocomian period during the early phase of continent–ocean interaction. It is only some time after break-up in the MB and the initiation of the MFZ that decoupling occurred between the MCP/NNV and the corridor, allowing for the latter to subside and become covered by deep marine sediments. A scenario for the early evolution and formation of the LM is proposed taking into account both recent kinematic and geological constraints. It implies that no or little change in extensional direction occurred between the intra-continental rifting and subsequent phase of continent–ocean interaction.

The influence of paleoclimate and a marine transgression event on organic matter accumulation in lacustrine black shales from the Late Cretaceous, southern Songliao Basin, Northeast China

Lacustrine black shales are widely developed within the Late Cretaceous Nenjiang Formation (k2n) of the Songliao Basin. These black shales provide an ideal natural laboratory for reconstructing the paleoclimate and paleoenvironment during the Late Cretaceous in Northeast China and have great significance in the exploration for potential oil and gas resources. According to the systematic analysis of organic carbon isotopes, elemental geochemistry and clay mineral composition, the organic matter (OM) source, paleoenvironment, paleoclimate, paleoproductivity and marine transgression event of the Nen 1 member (k2n1) were investigated, and the influence of the paleoclimate and marine transgression event on OM accumulation in lacustrine black shales was revealed. Black shales in the lower unit are characterized by a high OM abundance (max. 4.19 wt%, avg. 3.02 wt%), I-II1 kerogen (with abundant lamalginite) and low maturity. The element ratios (Fe/Mn, Mg/Ca and Sr/Cu), paleoweathering indexes (CIA and PIA), δ13Corg values and clay mineral composition together indicate a warm paleoclimate in the Late Cretaceous. The humid paleoclimate promoted the flourishing of lacustrine microalgae and surrounding terrestrial vascular plants and enhanced the lake level, conducive to favorable sources and preservation conditions for OM accumulation. Based on paleontological and radioisotope dating, the global paleotemperature rise in the Late Santonian stage probably led to the marine transgression event. It can be explained by the high lake level and sea-level conditions under the global humid climate background that resulted in the Songliao paleolake connecting with the paleo-Pacific Ocean. Alkaline and saline seawater incursion into the paleolake increased the salinity of the lacustrine water column and further aggravated the anoxic conditions of the lake, and abundant marine nutrient inputs promoted the primary bioproductivity and contributed to the accumulation of OM. The discovery of the OM accumulation mechanism under the influence of the warm-humid paleoclimate and intermittent marine transgression will add critical geological constraints on further studies of geological events in the deposition of late Cretaceous black shale.

Thermal Structure of Eastern Australia's Upper Mantle and Its Relationship to Cenozoic Volcanic Activity and Dynamic Topography

AbstractSpatio‐temporal changes of upper mantle structure play a significant role in generating and maintaining surface topography. Although geophysical models of upper mantle structure have become increasingly refined, there is a paucity of geologic constraints with respect to its present‐day state and temporal evolution. Cenozoic intraplate volcanic rocks that crop out across eastern Australia provide a significant opportunity to quantify mantle conditions at the time of emplacement and to independently validate geophysical estimates. This volcanic activity is divided into two categories: age‐progressive provinces that are generated by the sub‐plate passage of mantle plumes and age‐independent provinces that could be generated by convective upwelling at lithospheric steps. In this study, we acquired and analyzed 78 samples from both types of provinces across Queensland. These samples were incorporated into a comprehensive database of Australian Cenozoic volcanism assembled from legacy analyses. We use geochemical modeling techniques to estimate mantle temperature and lithospheric thickness beneath each province. Our results suggest that melting occurred at depths ≤80 km across eastern Australia. Prior to, or coincident with, onset of volcanism, lithospheric thinning as well as dynamic support from shallow convective processes could have triggered uplift of the Eastern Highlands. Mantle temperatures are inferred to be ∼50–100°C hotter beneath age‐progressive provinces that demarcate passage of the Cosgrove mantle plume than beneath age‐independent provinces. Even though this plume initiated as one of the hottest recorded during Cenozoic times, it appears to have thermally waned with time. These results are consistent with xenolith thermobarometric and geophysical studies.

Mélanges in flysch-type formations: Reviewing geological constraints for a better understanding of complex formations with block-in-matrix fabric

The geological term “mélange” is used to indicate a vast group of “chaotic rock” units with a block-in-matrix internal arrangement exhumed in orogenic belts. Geotechnically such units are often defined as “complex formations”, and are commonly associated with flysch-type formations. Such units range from coherent, bedding-concordant broken-formations to fully disrupted, discordant block-in-matrix rock assemblages. Mélanges derive from the progressive disruption and mixing of flysch-type successions from their earliest phases of development to their complete lithification and exhumation. They are the product of sedimentary (submarine mass transport), tectonic (thrust-related shearing) and mud-diapiric (fluid/gas driven remobilization of mud/shales) processes. Throughout a review of geological data, we show that the different intrinsic anatomical features and relationships with the host rocks shown by these various units allow differential internal anisotropy of the lithological properties, and, depending on their origin, size and distribution, they can variably influence the overall mechanical behavior of the host flysch-type formation. The review of geological data thus provides useful observations to distinguish different types of “complex formations” and block-in-matrix rocks with different geotechnical and mechanical characteristics, and then potentially suggesting the application of different rock or soil mechanics procedures.

India‐Tarim Lithospheric Mantle Collision Beneath Western Tibet Controls the Cenozoic Building of Tian Shan

AbstractThe ongoing India‐Asia collision principally regulates the Cenozoic tectonic deformation of the Asian interior, and builds a far‐away but active spectacular intraplate orogen—Tian Shan. However, the deep processes and dynamics of far‐field deformation propagation and the resultant Tian Shan building remain ambiguous. Here, we construct systematic numerical models with variable thermo‐rheological properties of the orogen‐featured blocks and convergence rates, which reveal that the far‐field effect of India‐Asia collision on the Tian Shan building is strongly controlled by the direct collision of Indian lithospheric mantle with the rigid Tarim block beneath western Tibet. The model results, together with the well‐established geological and geophysical constraints, not only reconcile the first‐order crustal and lithospheric structures of the western Tibetan plateau and Tian Shan, but also confirm a >30 Myr time lag between the initial India‐Asia collision and the far‐field Tian Shan building.

Creating the funerary landscape of Eastern Sudan.

Funerary landscapes are eminent results of the relationship between environments and superstructural human behavior, spanning over wide territories and growing over centuries. The comprehension of such cultural palimpsests needs substantial research efforts in the field of human ecology. The funerary landscape of the semi-arid region of Kassala (Eastern Sudan) represents a solid example. Therein, geoarchaeological surveys and the creation of a desk-based dataset of thousands of diachronic funerary monuments (from early tumuli up to modern Beja people islamic tombs) were achieved by means of fieldwork and remote sensing over an area of ∼4100 km2. The wealth of generated information was employed to decipher the spatial arrangement of sites and monuments using Point Pattern Analysis. The enormous number of monuments and their spatial distribution are here successfully explained using, for the first time in archaeology, the Neyman-Scott Cluster Process, hitherto designed for cosmology. Our study highlights the existence of a built funerary landscape with galaxy-like aggregations of monuments driven by multiple layers of societal behavior. We suggest that the distribution of monuments was controlled by a synthesis of opportunistic geological constraints and cultural superstructure, conditioned by the social memory of the Beja people who have inhabited the region for two thousand years and still cherish the ancient tombs as their own kin's.

Structure of an accreted intra-oceanic arc: potential-field model of a crustal cross-section through the Macquarie Arc, Lachlan Orogen, southeastern Australia

Construction of a 3D geological model for the eastern Lachlan Orogen provided a new reference model for a combined magnetic and gravity model of the Macquarie Arc, an intra-oceanic arc accreted to Gondwana during the Late Ordovician to early Silurian Benambran Orogeny. Geologically constrained, iterative 2.5 D forward modelling represented the entire thickness of the crust over a west-to-east profile extending for 230 km, approximately coinciding with seismic reflection profiles 99AGSL1 and 99AGSL2. The output model, referred to here as the Forbes model, confirms west-dipping thrust stacks of the Junee-Narromine and Molong volcanic belts of the Macquarie Arc, in packages that also comprise underlying arc basement, overlying Ordovician turbidites, and overlying Silurian–Devonian volcanic and sedimentary units. Ordovician rocks in the Parkes Fault Zone are distributed in wedges within a flower-fault structure. These structural elements are broadly similar to an earlier interpretation of the seismic profiles and corresponding potential-field models but differ in the nature of the substrate to the Macquarie Arc. The arc substrate is best fitted with physical properties broadly that of an intermediate calc-alkaline rock, rather than mid-ocean ridge basalt (MORB) as previously inferred. Highly magnetic metamafic rocks of the Jindalee Group appear to occupy relatively narrow fault-bounded slivers and may correspond to mafic volcanic rocks in the forearc rather than fragments of MORB basement. The thick intermediate-composition crustal profile required by the Forbes model cannot distinguish between an Izu-Bonin-like thickened arc or a compound of an Ordovician arc developed on a pre-existing Cambrian arc, but the simple MORB substrate previously assumed for the Macquarie Arc can be excluded, as can obduction of the Macquarie Arc as a nappe over the Wagga Belt. KEY POINTS A 3D model for the Lachlan Orogen provides geological constraints on joint magnetic and gravity model across the accreted intra-oceanic Macquarie Arc. The substrate of the arc is best fitted with magnetic susceptibility and density typical of an intermediate composition, in contrast with previous interpretations that the substrate preserves an oceanic crust basement. Western and central belts of the Macquarie Arc are stacked by west-dipping thrusts, inconsistent with emplacement as an allochthonous nappe.

CO2 mobility control improvement using N2-foam at high pressure and high temperature conditions

The high mobility of CO2 in porous media is an issue in most subsurface CO2 projects worldwide, demonstrated by uncontrolled and rapid migration through the reservoir, early well breakthroughs and poor sweep efficiency. Improved mobility control of CO2 is needed to accelerate and improve the value-creation from subsurface CO2 projects, both in terms of energy produced and volumes of CO2 stored. Field-proven conformance and mobility control techniques, such as foam, provide a cost-effective and sustainable alternative to overcome geological and process constraints observed with CO2 flow in reservoirs.In this laboratory study, we have investigated CO2 mobility control by foam in sandstone core samples at typical North Sea reservoir conditions, 220 barg and 100°C, respectively. Two important aspects related to any field application of foam have been evaluated and discussed: I) Can strong CO2-foams be generated at reservoir conditions using AOS surfactant? If not, II) can relatively simple modifications to the foam system be made to improve foam properties and CO2 mobility control?Our results show that only high-mobility CO2-foams with low degree of CO2 mobility reduction are obtained at 220 barg and 100°C. An easy way to improve the foam properties at reservoir conditions is suggested by changing the gas-phase in foam to nitrogen (N2). The N2-foams display improved foam generation performance, larger mobility control in terms of mobility reduction factors (MRF) and ability to block subsequent CO2 injection.An alternative strategy for applying foam for CO2 conformance and mobility control in subsurface CO2 projects, which emerges from this study, is to initiate the foam treatment with nitrogen followed by subsequent CO2 injection to change the main direction of CO2 flow to enhance the displacement area or reduce uncontrolled CO2 production between the injecting and producing wells (as illustrated in the graphical abstract). The possible mechanisms explaining the observed differences in foam properties using CO2 versus N2, including implications that could be helpful for future studies and CO2 field applications are discussed further in more detail.The efficiency and limitations of miscible CO2 flooding to recover oil and simultaneously store CO2 after extensive water flooding are also demonstrated in this article, which are relevant to enhanced oil recovery (EOR) and subsequent CO2 storage applications, known as the Carbon Capture Utilization and Storage (CCUS).

Elastic FWI for orthorhombic media with lithologic constraints applied via machine learning

Full-waveform inversion (FWI) of 3D wide-azimuth data for elastic orthorhombic media suffers from parameter trade-offs which cannot be overcome without constraining the model-updating procedure. We present an FWI methodology that incorporates geologic constraints to reduce the inversion nonlinearity and increase the resolution of parameter estimation for orthorhombic models. These constraints are obtained from well logs, which can provide rock-physics relationships for different geologic facies. Because the locations of the available well logs are usually sparse, a supervised machine-learning (ML) algorithm (Support Vector Machine) is employed to account for lateral heterogeneity in building the lithologic constraints. The advantages of the facies-based FWI are demonstrated on the modified SEG-EAGE 3D overthrust model, which is made orthorhombic with the symmetry planes that coincide with the Cartesian coordinate planes. We employ a velocity-based parameterization, whose suitability for FWI was studied using the radiation-pattern analysis. Application of the facies-based constraints substantially increases the resolution of the P- and S-wave vertical velocities ([Formula: see text], [Formula: see text], and [Formula: see text]) and, therefore, of the depth scale of the model. Improvements are also observed for the P-wave horizontal and normal-moveout velocities ([Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]) and the S-wave horizontal velocity [Formula: see text]. However, the velocity [Formula: see text] that depends on Tsvankin’s parameter [Formula: see text] defined in the horizontal plane is not well recovered from the surface data. On the whole, the developed algorithm achieves a much higher spatial resolution compared to unconstrained FWI, even in the absence of recorded frequencies below 2 Hz.

Three-Dimensional Structural Geological Modeling Using Graph Neural Networks

Three-dimensional structural geomodels are increasingly being used for a wide variety of scientific and societal purposes. Most advanced methods for generating these models are implicit approaches, but they suffer limitations in the types of interpolation constraints permitted, which can lead to poor modeling in structurally complex settings. A geometric deep learning approach, using graph neural networks, is presented in this paper as an alternative to classical implicit interpolation that is driven by a learning through training paradigm. The graph neural network approach consists of a developed architecture utilizing unstructured meshes as graphs on which coupled implicit and discrete geological unit modeling is performed, with the latter treated as a classification problem. The architecture generates three-dimensional structural models constrained by scattered point data, sampling geological units and interfaces as well as planar and linear orientations. The modeling capacity of the architecture for representing geological structures is demonstrated from its application on two diverse case studies. The benefits of the approach are (1) its ability to provide an expressive framework for incorporating interpolation constraints using loss functions and (2) its capacity to deal with both continuous and discrete properties simultaneously. Furthermore, a framework is established for future research for which additional geological constraints can be integrated into the modeling process.

Geophysical imaging of multiple volcanic structures at the Huichapan Caldera Complex, Mexico

Detailed processing and analyses of aeromagnetic data supported by geological constraints shed new light on the structural framework and plumbing system of Huichapan Caldera and Nopala volcano in the northern sector of the Mexican Volcanic Belt. The interpretation of magnetic anomalies is complex due to the bipolar nature of the magnetic field so we applied reduction to the pole using a double fast Fourier transform to convert the magnetic anomalies into simplified patterns. Short optimal continuous analytical field filters enhanced subsurface and surface structures. The reduction to pole analysis filtered for the upward/downward continuation of potential field data improved both regional and detailed imaging since it was applied to different depths. Shallow subsurface structures as well as at depths of 1.25 km and 0.65 km, were correlated with the surface and regional features.The Huichapan Caldera has at least two contrasting ignimbrites. Results show that there are two large magnetic anomalies associated with different volcanic structures. The first one, a circular positive anomaly corresponds to the caldera rim and the other to the structure of the younger Nopala volcano to the west. Strong magnetic sources indicate the feeder dikes. This study provides geophysical evidence for the location of the head wall of the Huichapan caldera as well as a second rim on the south but E-W and NE-SW regional faulting has destroyed this older caldera on the north. Results show that the structures are tectonically controlled and that the caldera is open on the east and southwest. The trap door structure was associated with this regional fault.

Rise of the Colorado Plateau: A Synthesis of Paleoelevation Constraints From the Region and a Path Forward Using Temperature-Based Elevation Proxies

The Colorado Plateau’s complex landscape has motivated over a century of debate, key to which is understanding the timing and processes of surface uplift of the greater Colorado Plateau region, and its interactions with erosion, drainage reorganization, and landscape evolution. Here, we evaluate what is known about the surface uplift history from prior paleoelevation estimates from the region by synthesizing and evaluating estimates 1) in context inferred from geologic, geomorphic, and thermochronologic constraints, and 2) in light of recent isotopic and paleobotanical proxy method advancements. Altogether, existing data and estimates suggest that half-modern surface elevations were attained by the end of the Laramide orogeny (∼40 Ma), and near-modern surface elevations by the mid-Miocene (∼16 Ma). However, our analysis of paleoelevation proxy methods highlights the need to improve proxy estimates from carbonate and floral archives including the ∼6–16 Ma Bidahochi and ∼34 Ma Florissant Formations and explore understudied (with respect to paleoelevation) Laramide basin deposits to fill knowledge gaps. We argue that there are opportunities to leverage recent advancements in temperature-based paleoaltimetry to refine the surface uplift history; for instance, via systematic comparison of clumped isotope and paleobotanical thermometry methods applied to lacustrine carbonates that span the region in both space and time, and by use of paleoclimate model mediated lapse rates in paleoelevation reconstruction.

Crustal melting vs. fractionation of basaltic magmas: Part 1, granites and paradigms

Granitoids are a major component of the continental crust. They play a pivotal role in its evolution, either by adding new material (continental growth), or by reworking older continental crust. These two roles correspond to two main ways of forming granitic magmas, either by partial melting of pre-existing crustal rocks yielding granitic melts directly, or by fractionation of mantle-derived mafic to intermediate magmas. Both models represent endmembers, or paradigms that have shaped the way the geological community envisions granitoids, their occurrence, features, formation and meaning for crustal evolution and differentiation of the whole planet.In this paper, we expose the two competing paradigms and their implications. We explore the evidence on which each model is based, and how each school of thought articulates a comprehensive view of granitic magmatism based on field geological, petrological, geochemical (including isotopes) and physical constraints; and how, in turn, each view shapes the thinking on crustal growth and evolution, and the interpretation of proxies such as age and Hf isotopic patterns in detrital zircon databases. We emphasize that both schools of thought build a different, but internally consistent view based on a large body of evidence, and we propose that each of them is, or has been, relevant to some portions of the Earth. Thus, the key question is not so much “which” model applies, but “where, when and to which extent”.

Hydrodynamic Constraints on Ore Formation by Basin‐Scale Fluid Flow at Continental Margins: Modelling Zn Metallogenesis in the Devonian Selwyn Basin

AbstractThe clastic‐dominant (CD‐type) deposits that are contained within sedimentary basins are major resources of Zn, Pb and Ag, but their formation by basin‐scale hydrothermal mass and energy transport processes is still poorly understood. Using geological constraints from the Late Devonian Selwyn Basin (Canada), we apply quantitative numerical fluid flow modeling to explore the effect of strata permeability, timing of fault opening and increased heat flow in controlling fluid migration, metal leaching and ore formation during an extensional tectonic event. The results indicate that tapping hot fluids from a confined and permeable aquifer at several km depths by means of permeable normal faults is a key factor for the formation of large Zn‐Pb deposits. The hot (282°C) ore‐forming fluids are transported to the shallow subsurface shortly after the initiation of a rifting event (within 100 kyr), before the development of extensive basin‐scale convection patterns that lead to stronger cooling and a reduction in the capacity of the hydrothermal system to make an economic deposit. Such a hydrothermal event can result in metal endowments comparable to the deposits of the Selwyn Basin.

Seismic Velocity Variations in a 3D Martian Mantle: Implications for the InSight Measurements

AbstractWe use a large data set of 3D thermal evolution models to predict the distribution of present‐day seismic velocities in the Martian interior. Our models show a difference between maximum and minimum S wave velocity of up to 10% either below the crust, where thermal variations are largest, or at the depth of the olivine to wadsleyite phase transition, located at around 1,000–1,200 km depth. Models with thick lithospheres on average have weak low‐velocity zones that extend deeper than 400 km and seismic velocity variations in the uppermost 400–600 km that closely follow the crustal thickness pattern. For these cases, the crust contains more than half of the total amount of heat‐producing elements. Models with limited crustal heat production have thinner lithospheres and shallower but prominent low‐velocity zones that are incompatible with Interior exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) observations. Seismic events suggested to originate in Cerberus Fossae indicate the absence of S wave shadow zones in 25°–30° epicentral distance. This result is compatible with previous best fit models that require a large average lithospheric thickness and a crust containing more than half of the bulk amount of heat‐producing elements to be compatible with geological and geophysical constraints. Ongoing and future InSight measurements that will determine the existence of a weak low‐velocity zone will directly bear on the crustal heat production.

Shearing and fluid evolution of the Porto Nacional orogenic gold district, western Brazil: microstructural, fluid inclusion, and C-O isotopic evidence

The Porto Nacional Gold District, in the northern Tocantins Province, Brazil, is one of the few crustal-scale shear systems that contain both hypozonal and mesozonal orogenic gold deposits. It is situated within a Paleoproterozoic terrane, hosted by the crustal-scale Neoproterozoic Transbrasiliano-Kandi Shear System. The Cachimbo Shear Zone (CSZ) hypozonal deposits are hosted by phyllonites characterized by staurolite, garnet, and median to high-crystallinity graphite, consistent with amphibolite facies conditions. The mesozonal deposits, hosted in the Mutum Shear Zone (MSZ) and the Conceição Shear Zone (CoSZ), show a paragenesis that reflects greenschist-facies conditions. All zones host quartz fault-fill veins and adjacent extensional quartz-carbonate veinlet swarms. Two mineralizing stages are recognized. Stage 1 occurs in old quartz porphyroclasts in the fault-fill veins of the CSZ and MSZ deposits, and comprises pyrite, Type-2 graphite, gold, and other sulphides. In the CSZ stage 1, pressure-temperature data derived from mineral thermometry, quartz microstructure, and fluid inclusion isochores are compatible with a ductile deformation regime (433° to 580 °C and 3.5 to 6.3 kb), equivalent to 16 to 24 km depths under lithostatic conditions. Conditions for MSZ stage 1 are 350° to 415 °C and 2.0 to 3.1 kb, where the calculated depth (9 to 12 km) suggests the MSZ developed at shallower levels in the crust than the CSZ. In both shear zones the ore fluids are represented by H2O-CO2 fluid inclusions that are CH4-bearing and low salinity (0.3 to 6 wt% NaCl), and have low ƒO2. Reactivation along the shear zones caused folding and boudinaging of the extensional veinlets and recrystallization by subgrain rotation in the vein quartz, suggesting temperatures of ~ 400 °C. Ore stage 2 (gold-galena) occurs in vug-fill quartz in the veins and in extensional veinlets of the MSZ and CoSZ. It is estimated to have formed at lower temperatures (340° and 390℃) and pressures (1.8 to 4.5 kb) than ore stage 1, at depths of 6 to 18 km. The fluids were H2O-CO2 rich, with salinities of 3 to 8 wt% NaCl. The high and fluctuating pressures, above lithostatic conditions, suggest supralithostatic conditions. The wide fluid pressure variation is consistent with the fault-valve model for both ore stages. Later, higher salinity aqueous fluids (up to 15 wt% NaCl) with no CO2 or CH4 circulated at 300℃ and at 1.0 to 1.7 kb, which is more compatible with hydrostatic conditions at depths of 4 to 7 km. The stable isotopic composition of the fluids (δ18OH2O = 2 to 12‰, δ13CCO2 = −4 to −8‰) and the local geological constraints indicate that the mineralizing fluids were metamorphic. The P-T characteristics of the system, which indicate a decrease in T and P from the CSZ to the MSZ and CoSZ suggests that the PNSZ developed concurrently in the greenschist-amphibolite window. The further development of footprints for the hypozonal-mesozonal orogenic gold deposits of the PNGD could help identify new target areas for exploration in Brazil and elsewhere in this Neoproterozoic crustal-scale shear system.

An integrated study on the geochemical, geophysical and geomechanical characteristics of the organic deposits (Coal and CBM) of eastern Sohagpur coalfield, India

Coal mining, especially the underground mines are often encountered with several types of geological constraints, where massive-hard roof and trapped gas are the most serious threats leading to catastrophic (fatal) effects in form of roof falls, mine explosion and gas contamination. Thus, for enhancing the safety of underground coal mines, identification of aforesaid constraints during detailed exploration is very essential. The minable seams of Sohagpur Coalfield are often associated with rich Coal Bed Methane (CBM) content, which enhances their economical importance, but the gas content along with the presence of stable sandstone roof parallely induces challenges during their extraction. CBM also acts as an environmental hazard having twenty-one times more greenhouse effect than carbon dioxide (CO2). Therefore, the present paper attempts to investigate the depositional environment of the high-quality methane-rich sub-bituminous reserves of eastern Sohagpur Coalfield through integration of geochemical, geophysical and geomechanical analysis of the drilled boreholes along with their respective gas content. The results showed that the six major Barakar coal seams possessed nearly consistent distribution in terms of reserve distribution and chemical properties. These seams consisted of good quality reserves (Grade: G5-G8) with high heat values and organic content, and had less impurity content. These coal seams are overlain by stable roof having lower cavablity in form of thick hard medium-grained sandstone beds with moderate elasticity and strength. The seam structures showed a stable profile without any abrupt discontinuity except Bahmni-Chilpa fault in southern part of block. Some of the higher rank coal seams also contained potential CBM content. Statistical analysis was also carried out for understanding the distribution of reserve properties and monitored their interdependence using suitable fitting tools. Thus, the present study helped in summarizing different aspects related to reserve quality, roof properties and gas content for understanding the geology and working environment of coal reserves of less explored but economically important Central Indian coalfields.