Sedimentary Host Rocks Research Articles

Uranium (VI) sorption on illite under varying carbonate concentrations: Batch experiments, modeling, and cryogenic time-resolved laser fluorescence spectroscopy study

Dissolved inorganic carbon (DIC) present in groundwaters can affect both the aqueous and surface species of hexavalent uranium (U(VI)) owing to its strong complexation ability with U(VI). However, the effect of DIC on U(VI) sorption on illite, which is a critical component of argillaceous rocks, is still unclear. In this study, we investigated the sorption of U(VI) on conditioned illite du Puy with varying DIC concentrations (up to 250 mM DIC) as a function of pH through batch sorption experiments, surface complexation modeling, and cryogenic time-resolved laser fluorescence spectroscopy (TRLFS). The distribution coefficients of U(VI) decreased with an increase in the DIC concentration. At relatively high DIC concentrations with respect to experimentally investigated range (≥100 mM DIC), no U(VI) sorption was observed. The U(VI) sorption on illite was modeled using the two-site protolysis non-electrostatic surface complexation and cation exchange model. Two ternary surface complexation reactions with carbonate were needed to depict the experimental sorption data in addition to binary and ternary hydroxo surface complexation reactions used to describe the U(VI) sorption to illite without carbonate. Cryogenic TRLFS data revealed that U(VI) did sorb to illite at high DIC concentrations (up to 10 mM DIC). The spectra were unchanged with varying DIC concentrations (2.0, 5.0, and 10 mM DIC) at pH ∼8.5, indicating that the surface speciation of U(VI) remained the same. The decay curves were biexponential, which further indicates that at least two surface species were responsible for the sorption. These results could improve the performance assessment of a deep geological disposal system in a sedimentary host rock.

In situ U-Pb and geochemical evidence for ancient Pb-loss during hydrothermal alteration producing apparent young concordant zircon dates in older tuffs

Zircon U-Pb geochronology is vital for providing age constraints for the calibration of the geologic timescale. The presence of zircon grains that yield concordant ages younger than the host sedimentary rock is often ignored, attributed to young outliers and possible contaminants. Using in situ U-Pb dating techniques, which analyze zircon crystals drilled directly from polished thin sections, two tuff beds from the Brockman Iron Formation, Pilbara Craton, Australia, yielded weighted mean 207Pb/206Pb ages of 2466 ± 6 Ma and 2469 ± 5 Ma (95% confidence level), consistent with independent age constraints. The tuffs also contain zircons that gave younger, yet concordant, U-Pb dates (2450–2270 Ma) as well as some with highly discordant dates. The “young” zircons have low or negligible common Pb, and display magmatic-type euhedral outlines, oscillatory zonation and Th/U ratios, indistinguishable from syn-depositional zircons, and cannot be readily dismissed on the grounds of high discordance or common-Pb contents. The “young” zircon dates are statistical outliers that overlap with the age of hydrothermal xenotime in deformed, bedding-parallel veins in shale that underlies the iron formation, which yields an age of 2383 ± 4 Ma (95% confidence level). The xenotime in overpressure veins provides independent evidence for hydrothermal fluid flow broadly coeval with isotopic resetting of “young” zircon crystals. Although SEM-CL imaging found no evidence for alteration, element mapping shows that most zircon grains have been altered by fluid infiltration, which proceeded along fractures or radiation-damaged growth zones leading to elevated Fe, Ca and Y contents in the altered domains. Total and partial Pb loss caused by hydrothermal alteration can explain the young concordant and discordant ages, respectively. These results have implications for the use of U-Pb zircon geochronology, particularly in detrital zircon studies, which routinely use the age of the youngest zircon grains to constrain the age of deposition. The erosion and re-deposition or in situ growth of young zircons in low-grade metasedimentary rocks may lead to erroneously young depositional ages.

Petrogenesis and U[sbnd]Pb (titanite) age of Cu[sbnd]Ag skarn mineralization in the McKenzie Gulch area, northern New Brunswick, Canada

Copper–silver skarn occurrences in the McKenzie Gulch (MG) area are spatially associated with Middle Devonian (386.3 ± 2.3 Ma; UPb zircon) intermediate to felsic dyke swarms. Mineralization occurs as veins and stockwork veinlets, disseminations, patches, and locally as replacement of calc-silicate skarns in argillaceous limestone. Fluid inclusion data from the MG deposit suggest that hydrothermal systems associated with skarn formation evolved from an early magmatic-dominated stage to a late formation water and/or meteoric water-dominated stage. An early prograde endoskarn and exoskarn formed from high temperature (444–865 °C) and high salinity (36–40 wt% CaCl2 + NaCl) fluids, whereas retrograde skarn formed from low temperature (97–257 °C) and low salinity (1–15 wt% NaCl equiv.) fluids. The salinity gap between prograde and retrograde fluids (~ 4-fold dilution) may be due to throttling in the magmatic-hydrothermal system resulting in a pressure regime that alternated between dominantly lithostatic and hydrostatic. The δ34S values of sulfides from the skarn deposit range from +4.6 to +9.1‰. These values are lower than the corresponding δ34S values from the adjacent gold-bearing quartz-calcite-sulfide veins whose values range from +7.9 to +10.0‰. These high positive values suggest that magmatic hydrothermal fluids interacted with and incorporated significant amount of sulfides from sedimentary rocks, which typically have higher δ34S values in the region.Hydrothermal titanites from retrograde skarn ores were successfully dated by laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) in an effort to constrain the timing of mineralization. These analyses yield weighted mean 206Pb/238U age of 387.2 ± 3.6 Ma, in agreement with 206Pb/238U age of 386.3 ± 2.3 Ma of zircon from associated porphyry dykes. The felsic porphyry dykes spatially associated with CuAg skarn at MG, coupled with fluid inclusion data, provide evidence for magmatic activity coincident with the mineralizing events. In addition, Pb isotope compositions of galena and other sulfides (chalcopyrite, pyrite and pyrrhotite) are compatible with a model linking the skarns with Early to Middle Devonian magmatic activity at depth that eventually interacted with and incorporated significant volumes of sedimentary host rocks. Consequently, the MG CuAg skarn mineralization occurred contemporaneously with the intrusion of porphyry dyke swarms.

USING HIGH-RESOLUTION AMMONITE BIOCHRONOSTRATIGRAPHY TO DATE VOLCANOGENIC DEPOSITS PRESERVED IN MIDDLE JURASSIC CARBONATE PLATFORMS SUCCESSIONS OF THE WESTERNMOST TETHYS (SOUTHEASTERN IBERIAN RANGE, SPAIN)

The Middle Jurassic successions that are currently exposed in eastern Spain (southeastern Iberian Range) were deposited in a system of shallow carbonate platforms, constituting the western margin of the Alpine Ocean, and display a series of successively interbedded volcaniclastic rocks and minor lava flows. The large-scale mapping reveals that Middle Jurassic volcanic rocks crop out along NW‒SE pathways, highlighting the importance of the role played by tectonics in controlling the spatial distribution of volcanic outcrops. The progressive dismemberment of Pangea impelled a geographic provinciality increase and, thus, might have contributed to a growing speciation rate of marine invertebrate organisms, notably ammonites. The present study provides new data constraining in detail the age of the Middle Jurassic volcanic deposits. The biochronostratigraphic calibrations of the sedimentary host rocks encasing the volcanics have been obtained from the study of the fossil content (mainly ammonites) from16 well-exposed stratigraphic sections. The study of the fossils from these sections has enabled the identification of three volcanic episodes during the Murchisonae Zone (Aalenian), the Concavum‒Discites zonal boundary (Aalenian-Bajocian), and the late Laeviuscula Zone or Laeviuscula‒Propinquans zonal boundary (Bajocian). Precise age constraints for volcanic accumulations can be a significant contribution in reconstructing the geodynamic history of the Iberian Basin.

Halogen Enrichment of Siberian Traps Magmas During Interaction With Evaporites

Volatile emissions to the atmosphere associated with the Siberian Traps eruptions at the Permian-Triassic boundary were sourced from the outgassing of primary magmas and the sedimentary host rocks into which they were intruded. Halogens in volcanic gases may have played an important role in environmental degradation and in stratospheric ozone destruction. Here we investigate how halogens behave during the interaction between salts and basalt magma emplaced as sills and erupted as lava. We present whole-rock, trace, and halogen concentrations for a suite of samples from three locations in the Siberian Traps Large Igneous Province, including basalt lavas erupted, and dolerites intruded into both organic-bearing shales and evaporites. Dolerites are enriched in Cl, Br, and I; their enrichment in Cl is similar to MORB and OIB that have been inferred to have assimilated seawater. The dolerites exhibit halogen compositional systematics, which extend towards both evaporites and crustal brines. Furthermore, all analyzed samples show enrichment in Rb/Nb; with the dolerites also showing enrichment in Cl/K similar to MORB and OIB that have been inferred to have assimilated seawater. We infer that samples from all three locations have assimilated fluids derived from evaporites, which are components of crustal sedimentary rocks. We show that up to 89% of the chlorine in the dolerites may have been assimilated as a consequence of the contact metamorphism of evaporites. We show, by thermal modeling, that halogen transfer may occur via assimilation of a brine phase derived from heating evaporites. Halogen assimilation from subcropping evaporites may be pervasive in the Siberian Traps Large Igneous Province and is expected to have enhanced emissions of Cl and Br into the atmosphere from both intrusive and extrusive magmatism.

Early and Late Mineralization of the Intrusive Rocks in Lokpaukwu, Southern Benue Trough, Nigeria.

The intrusive rocks (dolerites) in Lokpaukwu were studies to evaluate the kind of mineralization in them. The dolerites erupted at three regions in the Lokpaukwu area (Eziama, Eluama, Umuchieze). The results of the petrographic studies show that the dolerites in Eziama, Eluama, are characterized by olivine, pyroxene, plagioclase, biotite, hornblende, hematite. The first pattern of mineralization of the dolerites at LokpaUkwu appears to start from the Eziama or Eluama region, the dolerites of these two regions conforms to the mineralization of dolerites that exist in some other parts of the southern Benue trough. Umuchieze dolerites have some peculiarities that differentiate it from other dolerites of the Southern Benue trough. The second pattern of mineralization exist in dolerites of umuchieze which are characterized by quartz, pyroxene, plagioclase, biotite, and hematite. These phenomena indicate that two types of dolerites exist in LokpaUkwu area, the Olivine dolerites and the quartz porphyry dolerites. The interaction of the magma with sedimentary host rocks or hydrothermal influx or late stage of crystallization of magma probably played vital roles in the formation of the quartz porphyry dolerites, whereas the olivine dolerites may have originated from early stage of crystallization of basaltic magma. The basaltic magma that gave rise to these rocks(dolerites) probably has a single source region but erupted at different rifts which gave opportunity for some magmatic evolutionary processes (fractionation, contamination, assimilation) to play major roles. &nbsp

Observations of reservoir quality alteration in proximity to igneous intrusions for two distinct sandstones in Scotland

Igneous intrusions are common features within rifted sedimentary basins. With advances in technology, exploration has progressed into more complicated petroleum systems where sediments are segmented and altered by networks of dykes and sills. Magmatism increases temperatures and fluid pressures within adjacent sedimentary host rocks. This can lead to mobilisation of mineral-rich hot fluids through pores and fractures, which alters the host rock microstructure via both mineralisation and dissolution. However, few studies have quantified the impact of intrusions on reservoir quality, specifically on host rock pore networks and their ability to conduct fluid. This study uses a combination of plug porosity and permeability, optical, SEM and cathodoluminescence microscopy and XRD to investigate the mineralogical and reservoir quality changes to sandstones within thermal aureoles. In contrast to published models, associating igneous bodies with reduction of reservoir quality in the host rock, our analyses of two intruded sandstones show that alteration and mobilisation of pore filling minerals and clays can also result in enhancement of porosity and permeability at the intrusion-host rock contact. This study identifies the microstructural and chemical changes which produce these reservoir characteristics and discusses the implications of these processes on reservoir quality and fluid migration in intrusion-rich sedimentary basins.

U-Pb Dating on Hydrothermal Rutile and Monazite from the Badu Gold Deposit Supports an Early Cretaceous Age for Carlin-Type Gold Mineralization in the Youjiang Basin, Southwestern China

Abstract The Youjiang basin on the southwestern margin of the Yangtze block in southwestern China is the world’s second largest Carlin-type gold province after Nevada, USA. The lack of precise age determinations on gold deposits in this province has hindered understanding of their genesis and relation to the geodynamic setting. Although most Carlin-type gold deposits in the basin are hosted in calcareous sedimentary rocks, ~70% of the ore in the Badu Carlin-type gold deposit is hosted by altered and sulfidized dolerite. Although in most respects Badu is similar to other Carlin-type gold deposits in the province, alteration of the unusual dolerite host produced hydrothermal rutile and monazite that can be dated. Field observations show that gold mineralization is spatially associated with, but temporally later than, dolerite. In situ secondary ion mass spectrometry (SIMS) U-Pb dating on magmatic zircon from the least altered dolerite yielded a robust emplacement age of 212.2 ± 1.9 Ma (2σ, mean square of weighted deviates [MSWD] = 0.55), providing a maximum age constraint on gold mineralization. The U-Th/He ages of detrital zircons from hydrothermally mineralized sedimentary host rocks at Badu and four other Carlin-type gold deposits yielded consistent weighted mean ages of 146 to 130 Ma that record cooling from a temperature over 180° to 200°C and place a lower limit on the age of gold mineralization in the basin. Hydrothermal rutile and monazite that are coeval with gold mineralization have been identified in the mineralized dolerite. Rutile is closely associated with hydrothermal ankerite, sericite, and gold-bearing pyrite. It has high concentrations of W, Fe, V, Cr, and Nb, as well as growth zones that are variably enriched in W, Fe, Nb, and U. Monazite contains primary two-phase fluid inclusions and is intergrown with gold-bearing pyrite and hydrothermal minerals. In situ SIMS U-Pb dating of rutile yielded a Tera-Wasserburg lower intercept age of 141.7 ± 5.8 Ma (2σ, MSWD = 1.04) that is within error of the in situ SIMS Th-Pb age of 143.5 ± 1.4 Ma (2σ, MSWD = 1.5) on monazite. These ages are ~70 m.y. younger than magmatic zircons in the host dolerite and are similar to the aforementioned U-Th/He cooling ages on detrital zircons from hydrothermally mineralized sedimentary host rocks. We, therefore, conclude that the Badu Carlin-type gold deposit formed at ca. 144 Ma. The agreement of the rutile and monazite ages with the U-Th-He cooling ages of Badu and four other Carlin-type gold deposits in the Youjiang basin suggests that ca. 144 Ma is representative of a regional Early Cretaceous Carlin-type hydrothermal event formed during back-arc extension.

Linking the Siberian Flood Basalts and Giant Ni‐Cu‐PGE Sulfide Deposits at Norilsk

AbstractThe world‐class magmatic sulfide deposits in the Norilsk region exhibit a remarkable spatial and temporal association with the Siberian large igneous province (LIP). However, the details of the causal connection between the Siberian LIP and the ore deposits have remained contentious. Here we address the problem by modeling of assimilation, crystallization and flow behavior of magmas based on aggregated data from the Norilsk camp. Crustal assimilation at depth by mantle‐derived picritic magma can account for the compositional and isotopic variations of the early tholeiitic basalts and the sequestering of sulfide liquid in a mid‐crustal conduit system. When the rate and volume of asthenospheric magma production dramatically increased, the metal tenors of the mid‐crustal sulfide reservoir were first upgraded by consequent reaction with the fresh, undepleted magmas, then entrained and flushed to higher crustal levels. The resulting sulfide‐rich emulsions were too dense to reach the surface to form lavas but were instead emplaced in multiple pulses within blind sill‐like bodies at shallow depth. Sulfide ores transported from the mid‐crust were deposited at the bases of the upper‐crustal sills, where sedimentary host rocks were assimilated wholesale by the already sulfide‐charged replenishing magmas; this shallow assimilation was itself not instrumental in forming the ores. Although the evolution and dynamics of flood basalts are thus inextricably linked with coeval subvolcanic economic intrusions, the mineralization itself is argued not to occupy feeders with direct links to the overlying lavas, suggesting major implications for exploration in other LIPs.

Iron isotope fractionation in reduced hydrothermal gold deposits: A case study of the Wulong gold deposit, Liaodong Peninsula, East China

Abstract Iron isotope fractionation in hydrothermal systems is a useful diagnostic tool for tracing ore-forming processes. Here, we report on the Fe isotopic compositions of a suite of hydrothermal minerals from ores (pyrite, pyrrhotite, and quartz) from the Wulong gold deposit, Liaodong Peninsula, East China. Pyrites from quartz sulfide ores show a δ56Fe (56Fe/54Fe in the sample relative to IRMM-14) range from +0.11 ± 0.03‰ to +0.78 ± 0.03‰ (2SD), and pyrrhotites from the same vein are isotopically lighter than the pyrites, varying between −0.85 ± 0.01‰ and −0.07 ± 0.00‰. This result is consistent with theoretical predictions of equilibrium fractionation and published mineral compositions. For the first time, to our knowledge, we report the Fe isotopes of hydrothermal quartz that records the isotopic compositions of the ore-forming fluids. Two quartz separates in the quartz-sulfide vein yield δ56Fe values of −0.02 ± 0.02‰ and +0.07 ± 0.07‰. Our Fe isotope fractionation calculations show that pyrrhotite with light Fe isotopes crystallized first from the ore-forming fluids, which indicates a relatively reduced condition for the initial ore-forming fluids. Then the remaining fluids with heavy Fe isotopes precipitated pyrites with positive δ56Fe values, and their mineral crystallization sequence records an increase of oxygen fugacity during mineralization. Gold deposits in Wulong and Jiaodong share many similar geological characteristics. The pyrites from the Wulong deposit have higher δ34S values (+0.5 to +4.1‰) than the pyrrhotites (–1.2 to +1.2‰). Pyrites from the Jiaodong gold deposits show a wide range of both positive and negative δ56Fe values as well as high δ34S values, whereas those from the Wulong deposit have a relatively narrow range of positive δ56Fe values and near-zero δ34S values. The differences in Fe isotopes may be due to early precipitation of pyrrhotite with light Fe isotopes under a relatively low oxygen fugacity environment in the Wulong deposit, resulting in pyrite precipitated from the remaining fluids with heavy isotopes. The sulfur isotope variations between Wulong and Jiaodong gold deposits reflect differences in their source regions rather than oxygen fugacities. In addition, we have compiled Fe isotopic compositions of pyrites and pyrrhotites from different types of ore deposits to investigate their Fe isotopic behavior in magmatic-hydrothermal systems. Pyrite grains show a wide range of δ56Fe values. Positive pyrite δ 56Fe values reflect an equilibrium isotope effect, whereas negative pyrite δ56Fe values may be due to kinetic isotope effects or to a mixture of sedimentary host rocks. Pyrrhotite grains show similar negative δ56Fe values, and they have a strong influence on the Fe isotope systematics in magmatic and hydrothermal systems. Our data show that Fe isotopes can be used to trace precipitation orders of pyrite and pyrrhotite and oxygen fugacity evolution in relatively reduced hydrothermal deposits.

Shale gas leakage in lower Ecca shales during contact metamorphism by dolerite sill intrusions in the Karoo Basin, South Africa

Abstract Contact metamorphism along widespread dolerite sills and dykes, emplaced at 182 to 183 Ma through the sedimentary host rocks of the Karoo Basin, triggered devolatilization of carbon-rich shales of the Lower Ecca Group. Hornfel samples collected from drill cores that intersect dolerite sills were analyzed for mineral phase equilibria, chemistry and porosity to characterize thermal aureoles at various distances from sill intrusions. Andalusite-chiastolite and cordierite porphyroblasts with biotite and muscovite occur within 10 to 20 m of many intrusive contacts. These metamorphic minerals crystallized when host shales attained maximum temperatures ranging between 450 and 600°C. Scanning electron microscopy imaging confirms that the hornfels are compact and that their metamorphic minerals limit porosity along grain boundaries. In few cases intra-mineral porosity occurs within individual crystals such as calcite, andalusite and cordierite. Disequilibrium metamorphic textures such as irregular grain boundaries, and inclusions in andalusite and cordierite reveal that the elevated temperatures were too short-lived to accomplish complete (re)crystallization. Thermal modeling results are consistent with the observed metamorphic mineral assemblages. Gas leakage calculations along a 7 m and a 47 m thick dolerite sill that intrude toward the top of the Whitehill Formation suggest that methane volumes ranging between 8 to 15 Tcf were generated during the sill emplacement. Methane was likely released into the atmosphere through hydrothermal vent complexes that are well preserved in the western Karoo Basin. If such loss was widespread across the entire basin, the implications for paleo-climate change and preserved shale gas reserves in the Karoo Basin of South Africa would be significant.

The Abra sedimentary-hosted Pb-Ag-Cu-Au deposit, Western Australia: A geophysical case study

Abra is a high-grade sedimentary-hosted Pb deposit located in the Paleoproterozoic Edmund Basin in Western Australia. Mineralization is blind, with the top of the deposit occurring 250 m beneath the land surface. The deposit consists of a stratiform apron of Pb-Ag-Ba mineralization in laminated iron-oxide- and barite-altered dolomite and siltstone, which overlies a feeder zone of chlorite-altered, brecciated, and veined carbonatic siltstone that contains Pb-Ag mineralization in the core that transitions to Pb-Cu and Cu-Au at depth. Abra is characterized by discrete geophysical anomaly responses in magnetic, gravity, time-domain electromagnetic (TDEM), and induced polarization survey data. A +450 nT magnetic anomaly is caused by magnetite in the lower stratiform zone. Dense galena, barite, dolomite, and iron-oxide mineralization in the apron and galena in the feeder zone is surrounded by lower-density sedimentary host rocks, which results in a +1 mGal gravity anomaly. Airborne, ground, and downhole TDEM surveying resolved known mineralization as weak electromagnetic conductor responses, and petrophysical testing on core samples shows that this is caused by galena. Pole-dipole-induced polarization surveying resolved a +20 ms chargeability anomaly on the southern flank of the deposit. This chargeable anomaly response is related to disseminated galena, pyrite, chalcopyrite, and alteration. Joint audiomagnetotelluric-magnetotelluric 2D inverted data sections resolved Abra as a broad weakly conductive anomaly. Weak conductor responses associated with Abra were also resolved in 2D and 3D inversion modeling of airborne Z-axis tipper electromagnetic data. 2D seismic reflection surveying resolved Abra as strong flat-lying seismic reflectors, which are bounded and offset by faults and surrounded by a seismically bland zone. The seismic reflections are related to significant density contrasts between high-density stratiform mineralization that is in contact with low-density sedimentary host rocks, as the mineralization and host rocks have similar seismic velocities. Passive seismic horizontal to vertical spectral ratio surveying resolved the top of the deposit as a subtle layer sitting below a flat impedance contrast horizon that is interpreted as weathered siltstone on top of diagenetically cemented siltstone.

Hydrocarbon-bearing sulphate-polymetallic deposits at the Colipilli area, Neuquén Basin, Argentina: Implications in the petroleum system modeling

This work deals with the hydrocarbon-bearing barite-polymetallic mineralizations of the Colipilli area, located in the western sector of the Agrio Fold and Thrust Belt (Neuquén Basin, Argentina). The mineralizations consist of bed- and vein-type deposits mainly composed of barite (barite96.99%–celestine2.93%) with minor amounts of Fe-oxyhydroxides and sulfides. The bed-type deposits have zebra texture and are emplaced along the contact between Late Cretaceous–Paleocene igneous rocks (Naunauco Group) and their Early Cretaceous sedimentary host rocks (e.g., Huitrín Formation). In contrast, the vein-type deposits have breccia texture and are crosscutting the Mulichinco, Agrio and Huitrín formations or the andesitic/dioritic stocks and sills of the Naunauco Group. Different types and families of primary fluid inclusions (FI) were identified in the barite crystals. Fluorescence techniques with UV incident light and Raman spectroscopy allowed FI from completely aqueous to completely organic, including all the intermediate terms, to be identified. The organic FI have blue fluorescence and contain liquid hydrocarbons. The blue fluorescence is correlated with medium to high API gravity values (ca. 40°), indicating the presence of light hydrocarbons of advanced maturity related with the window for the generation of liquid/gaseous hydrocarbons. Microthermometry studies carried out on aqueous FI revealed that vein-type deposits formed at higher temperatures and salinities (249.7 °C and 0.5–9.3 wt % NaCl equivalent) than bed-type deposits (162.2 °C and 0.2–7.2 wt % NaCl equivalent). The heat influx provided by the Late Cretaceous–Paleocene magmatism promoted the circulation of inorganic and organic fluids of connate origin and the leaching of metallic and non-metallic elements from the sedimentary pile. During its crystallization, barite trapped fluids with variable hydrocarbon contents. The thermal anomaly associated with the magmatic activity could also have contributed with the maturation of the nearby source rocks and to the development of an atypical petroleum system.

Ground Magnetic Surveying and Susceptibility Mapping Across Weathered Basalt Dikes Reveal Soil Creep and Pedoturbation

Ground magnetic survey profiles across a soil-covered and weathered mafic dike in sedimentary host rock not only permit to delineate the strike, width and burial depth of the intrusive basalt sheet, but also reflect the subsurface deformation of its clayey weathering products. We illustrate this finding and its practical geomorphological applicability by an example from the mid-German Heldburg Dike Swarm, where blue- and olive-gray basalt-derived clays inherited not just the dike space previously occupied by the basalt, but also large parts of its magnetic iron minerals and their strong induced and remanent magnetization. Such ductile basaltic “marker soils” deform and move with the surrounding low-magnetic host soils, but remain distinguishable by their contrasting colors and high magnetic susceptibility. Ground magnetic surveys can therefore delineate soil creep distance at meter- and basalt weathering depth at decimeter-precision. Magnetic mapping of a weathered dike’s cross-section from an exploration trench by in-situ susceptometry permits to analyze past soil deformation in great detail. Weathering and solifluction transforms the simple “vertical sheet” anomalies of dikes into complex, but still interpretable composite patterns, providing a new and promising exploratory approach for field studies concerned with soil creep and pedoturbation.

A 3D Geological Model Constrained by Gravity and Magnetic Inversion and its Exploration Implications for the World‐class Zhuxi Tungsten Deposit, South China

AbstractThe Zhuxi tungsten deposit in Jiangxi Province, South China, contains a total W reserve of about 2.86 Mt at an average grade of 0.54 wt% WO3, representing the largest W deposit in the world. Numerous studies on the metallogeny of the deposit have included its timing, the ore‐controlling structures and sedimentary host rocks and their implications for mineral exploration. However, the deep nappe structural style of Taqian–Fuchun metallogenic belt that hosts the W deposit, and the spatial shape and scale of deeply concealed intrusions and their sedimentary host rocks are still poorly defined, which seriously restricts the discovery of new deposits at depth and in surrounding areas of the W deposit. Modern 3D geological modeling is an important tool for the exploration of concealed orebodies, especially in brownfield environments. There are obvious density contrast and weak magnetic contrast in the ore‐controlling strata and granite at the periphery of the deposit, which lays a physical foundation for solving the 3D spatial problems of the ore‐controlling geological body in the deep part of the study area through gravity and magnetic modeling. Gravity data (1:50000) and aeromagnetic data (1:50000) from the latest geophysical surveys of 2016–2018 have been used, firstly, to carry out a potential field separation to obtain residual anomalies for gravity and magnetic interactive inversion. Then, on the basis of the analysis of the relationship between physical properties and lithology, under the constraints of surface geology and borehole data, human–computer interactive gravity and magnetic inversion for 18 cross‐sections were completed. Finally, the 3D geological model of the Zhuxi tungsten deposit and its periphery have been established through these 18 sections, and the spatial shape of the intrusions and strata with a depth of 5 km underground were obtained, initially realizing “transparency” for ore‐controlling bodies. According the analysis of the geophysical, geochemical, and geological characteristics of the Zhuxi tungsten deposit, we discern three principles for prospecting and prediction in the research area, and propose five new exploration targets in its periphery.

Mineral Deposit Model of Cu–Fe–Au Skarn System in the Edongnan Region, Eastern China

AbstractCu and Fe skarns are the world's most abundant and largest skarn type deposits, especially in China, and Au‐rich skarn deposits have received much attention in the past two decades and yet there are few papers focused on schematic mineral deposit models of Cu–Fe–Au skarn systems. Three types of Au‐rich deposits are recognized in the Edongnan region, Middle–Lower Yangtze River metallogenic belt: ∼140 Ma Cu–Au and Au–Cu skarn deposits and distal Au–Tl deposits. 137–148 Ma Cu–Fe and 130–133 Ma Fe skarn deposits are recognized in the Edongnan region. The Cu–Fe skarn deposits have a greater contribution of mantle components than the Fe skarn deposits, and the hydrothermal fluids responsible for formation of the Fe skarn deposits involved a greater contribution from evaporitic sedimentary rocks compared to Cu–Fe skarn deposits. The carbonate‐hosted Au–Tl deposits in the Edongnan region are interpreted as distal products of Cu–Au skarn mineralization. A new schematic mineral deposit model of the Cu–Fe–Au skarn system is proposed to illustrate the relationship between the Cu–Fe–Au skarn mineralization, the evaporitic sedimentary rocks, and distal Au–Tl deposits. This model has important implications for the exploration for carbonate–hosted Au–Tl deposits in the more distal parts of Cu–Au skarn systems, and Fe skarn deposits with the occurrence of gypsum‐bearing host sedimentary rocks in the MLYRB, and possibly elsewhere.

The Hatillo Limestone, Pueblo Viejo district, Dominican Republic: Marginal reef or impermeable cap?

The Hatillo Limestone and the underlying Los Ranchos Formation are exposed over an east-west distance of 100 km in the eastern Dominican Republic. The lowermost portion of the Hatillo Limestone in the Pueblo Viejo district contains a Late Lower Albian fossil assemblage including corals and rudist bivalves indicative of a near-shore reef environment. Diamond drilling in the Pueblo Viejo district and exposures in the open pits show that the Hatillo Limestone conformably overlies the Early Cretaceous Los Ranchos Formation. Volcanogenic massive sulfide beds, exposed in the Moore pit, provide evidence for an Early Cretaceous, syn-mineralization paleosurface. Altered and mineralized clasts in the epiclastic, sedimentary host-rock section at the Pueblo Viejo mine indicate that the ore deposits were open to erosion during hydrothermal alteration and mineralization. The Hatillo Limestone did not overlie the ore deposits during the mineralizing event and, consequently, could not have acted as an impermeable cap to ascending hydrothermal fluids. Intra-oceanic island arc volcanism (Los Ranchos Formation) overlapped at the Aptian–Albian boundary (112 Ma) with a marginal fringing reef (basal Hatillo Limestone). The marginal reef gradually gave way to deeper-water facies as Hatillo Limestone deposition progressed through the middle Albian. Low-angle reverse faulting, penetrative deformation, and metamorphic recrystallization affected the Hatillo Limestone as well as the Los Ranchos and Maimón formations during the Late Cretaceous. Deformation intensity and metamorphic grade progressed from incipient metamorphism in the Pueblo Viejo district to schists in the Maimón Formation to amphibolite near a faulted contact with the Loma Caribe peridotite.

Middle Ordovician (Darriwilian) conodonts from southern Tibet, the Indian passive margin: implications for the age and correlation of the roof of the world

AbstractNew occurrences of middle–late Darriwilian (Middle Ordovician) conodonts are reported from the Nyalam region, southern Tibet. The conodont-yielding strata, referred to the Chiatsun Group, accumulated on the north Indian continental margin of northern Gondwana. These Middle Ordovician conodonts include the informal speciesHistiodellasp. A in the middle part of the Lower Formation of the Chiatsun Group succeeded by a fauna of thePygodus serraZone in the upper part of that formation.Pygodus anserinusis recorded from the base of the Upper Formation of the Chiatsun Group. The Nyalam succession and its conodont taxa allow for precise correlation of the strata preserved on top of Mount Qomolangma (Mount Everest), eastern Tibet and the Peri-Gondwana Lhasa (north central Tibet), South China, North China, Tarim Basin and Thailand-Malaysia (Sibumasu Terrane) terranes and/or microcontinents. The middle Darriwilian positive increase in δ13Ccarbvalues (carbon isotope excursion, or MDICE) is recorded from most terranes, and can be related to a late middle Darriwilian global short-term cooling and sea-level drop. The cooling event prompted temperate- to warm-water taxa to migrate towards the palaeoequator and constrained the Australasian Province to locations near and at the palaeoequator. The intensified oceanic circulation and upwelling on continental margins probably caused some characteristic taxa to become extinct. The incoming fauna was mainly of cool-water taxa. The conodont specimens from southern Tibet are black to pale grey, corresponding to conodont colour index (CAI) values of 5 to 6, which demonstrates that the host sedimentary rocks were once heated to more than 360°C.

Genesis of the Shangxu orogenic gold deposit, Bangong-Nujiang suture belt, central Tibet, China: Constraints from H, O, C, Si, He and Ar isotopes

Shangxu is an orogenic gold deposit in the Bangong-Nujiang suture zone, central Tibet, China, formed in the Early Cretaceous orogene, related to convergence and collision between the Qiangtang and Lhasa terranes. The mineralization at Shangxu is hosted by Jurassic turbidite sedimentary rocks of the Mugagangri Group, and is associated with a regional fault system. Hydrothermal minerals develop muscovite, carbonate, sulfides and chlorite. Hydrothermal fluids record three main hydrothermal stages based on mineral paragenesis. The earliest is barren quartz stage (H1), which is pre-ore. During the early mineralization quartz-pyrite stage (H2a), defined by massive quartz veins with minor euhedral pyrite and gold, hydrothermal fluids had a δ18Ofluid of 6.1–6.4‰, δD of −74 to −116‰, δ13CCO2 of −5.4 to −7.6‰, and δ30Si of −0.1‰. In the quartz-pyrite-sulfides stage (H2b), characterized by abundant quartz, granular pyrite, muscovite with minor chalcopyrite, galena, sphalerite and gold, fluids had a δ18Ofluid of 7–8.2‰, δD of −109 to −120‰, δ13CCO2 of −9.6‰ and δ30Si of −0.1 to −0.2‰. During the ankerite-sulfide stage (H3a), distinguished by abundant ankerite, muscovite, sulfides with minor quartz and chlorite, hydrothermal system had a fluid with δ18Ofluid of 4.9–5.3‰, δD of −125.3‰, δ30Si of −0.1‰, δ13CCO2 of −12.4‰ in quartz inclusion fluid and δ13CCO2 of −2.7‰ in ankerite. The calcite-sulfide stage (H3b) is characterized by calcite, sulfides, with minor quartz and chlorite. Quartz formed earlier than calcite from a fluid having a δ18Ofluid of 6.4‰, δD of −112.6‰, δ30Si of −0.1‰, and δ13CCO2 of −7.4‰, after which calcite precipitated from a hydrothermal fluid with δ18Ofluid of 5.5–9‰, δ13CCO2 of −0.5 to −2.8‰. Hydrothermal fluids in H2b pyrite have 3He/4He ratios of 0.27–0.42Ra and 40Ar/36Ar ratios of 313–372.The stable isotope composition of hydrothermal fluids from the Shangxu gold deposit is similar to that of typical orogenic gold deposits. The early stage (H1), methane-bearing fluids were probably sourced from the basin sediments, leading to precipitation of early barren quartz veins and siderite alteration. From the quartz-sulfide stage (H2) to carbonate-sulfide stage (H3), hydrothermal fluids were likely derived from dissolution of the marine carbonate cement from sedimentary host rocks during metamorphism at depth. The decreasing δD and δ13C of the ore-forming fluids from early to late suggests mixing with δ13C-depleted oxidized graphite in sedimentary rocks, meteoric waters or reation between δD-depleted organic matters. Generally, these fluids were likely generated at depth through prograde metamorphic devolatilization of hydrous minerals in the deeper equivalents of the sedimentary rocks of the Mugagangri Group. Sulfur and gold, by inference, likely originated from sedimentary/diagenetic pyrite or the metasedimentary rocks between the greenschist and amphibolite facies, and migrated with the metamorphic fluids. During transportation to the site of deposition, gold-bearing fluids variably reacted with country rocks.

U–Pb Age and Geochemistry of Detrital Zircons from Quartzites of the Daldyn Group, Anabar Shield, Northern Central Siberia, Russia

The paper presents U–Pb zircon dates (SHRIMP-II) and isotopic-geochemical data (SIMS and Sm–Nd method) on detrital zircon and its sedimentary host rocks of the Daldyn Group at the Anabar Shield. The rocks were metamorphosed to the granulite facies. The oldest Eoarchean (3.7 Ga) source of terrigenous material (determined based on single zircon grains) is confirmed by the Nd model age of the protolith: TNd(DM)2 = 3.69–3.71 Ga. The rocks are dominated by zircon dated at 3.5 Ga, which reflects that the area from which the terrigenous material was brought consisted mostly of Paleoarchean magmatic rocks.