Sulfide Deposits Research Articles

A review on theory, modeling, inversion, and application of self-potential in marine mineral exploration

The self-potential (SP) method is one of the passive geophysical exploration techniques, which employs measurements of naturally occurring electric potential differences on the ground surface and in boreholes or (sea-)water due to causative sources from electrochemical, electrokinetic, and thermoelectric mechanisms. SP signals are directly related to groundwater flow and (electro-)chemical gradient. There has been increasing interest in the application of SP measurements in mineral exploration, environmental surveys, and hydrogeophysical problems. This review focuses primarily on the theory concerning the causative source mechanism, numerical modeling, and inversion and interpretation of SP signals related to ore bodies and applications in mineral exploration. Three field examples focusing on seafloor sulfide deposits exploration are summarized to show the application effect of SP measurements in marine mineral exploration. This study would be helpful to investigating the use of SP surveys in ore prospecting, especially in marine environments.

Gahnite, garnet and magnetite compositions of metamorphosed sediment-hosted Zn-Pb-(Cu[sbnd]Ag) deposits of the Mesoproterozoic Nova Brasilândia Group: Vectors for SEDEX deposits with Broken Hill-type affinities in the western Amazonian Craton, Brazil

Stratiform and stratabound Zn-Pb-Ag sulfide deposits hosted in siliciclastic sedimentary rocks constitute some of the world's largest accumulations of base metals. This research paper is the first to describe the DM-1 Zn-(PbAg) and PQ Zn-(CuAu) sulfide deposits, located about 20 km apart in the Mesoproterozoic Nova Brasilândia belt at the southwest margin of the Amazonian Craton. These deposits occur in a NW-SE trending sequence of metamorphosed siliciclastic rocks. The Zn-rich mineralization in both deposits occurs as massive, semi-massive and banded sulfide orebodies concordant with the layering of the country rocks. The DM-1 deposit (1.2 Mt at 12 wt% Zn) has up to 30 m thick zones of sulfide ore consisting predominantly of sphalerite, galena, pyrrhotite and pyrite while the PQ deposit (5 Mt at 5.6 wt% Zn) contains up to 20 m thick zones of sulfide ore consisting mainly of pyrite, pyrrhotite, and sphalerite along with minor chalcopyrite. The host rocks of both deposits consist of schist, gneiss and quartzite with variable modal proportions of garnet, sillimanite, gahnite, magnetite and biotite. These host rocks are up to 50 m thick and commonly show gradational contacts with country rocks. Country rocks, Zn-rich orebodies and their host rocks were overprinted by a post-ore upper amphibolite facies regional metamorphic event.Gahnite compositions of the PQ and DM-1 deposits are characterized by high contents of Zn (68 to 81 % of the gahnite end member) and moderate contents of Fe+2 and Mg. Their major elements compositions plot within the field for metamorphosed massive sulfide deposits hosted by hydrothermally altered Fe-Al-rich metasedimentary and metavolcanic rocks. Moreover, they display compositional trends like those reported for Pb-Zn-Ag deposits in the Broken Hill Domain, Australia. Gahnite from the DM-1 and PQ deposits shows unusually low Cd concentrations (<0.04 ppm), which are 1 to 2 orders of magnitude lower than values reported for gahnite in other Zn-rich sulfide deposits. Very high (from 400 to 900 ppm) Cr concentrations were obtained in gahnite from the PQ deposit. Garnet from the DM-1 and PQ deposits varies from Mn-rich (30 to 62 % of the spessartine end member) in samples from the ore zone or closely associated sulfide-rich host rocks to Mn-poor in sulfide-poor host rocks (<5.5 % of the spessartine end member). Magnetite crystals from ore samples from both deposits have distinctively higher MnO contents than those from host rocks, which follows the Mn-rich and Mn-poor garnets distribution. Our results support the concept that the DM-1 and PQ deposits share common features with BHT deposits as well as some SEDEX deposits affected by high-grade metamorphism, suggesting that they may be classified as clastic-dominated SEDEX deposits with BHT affinities. In addition, gahnite in heavy mineral concentrates is the best pathfinder for regional exploration, and the identification of Mn-rich garnet and/or Mn-rich magnetite in heavy mineral concentrates or preliminary drilling may be used as a proximal pathfinder for Zn-rich sulfide ore.

Halogen characteristics and their implications for geochemical exploration in the Qujia gold deposit, Laizhou, Jiaodong

The ore-forming fluids in the Jiaodong gold deposits are characterized by low to medium salinities and Au-HS complexes are the predominant Au species in the fluid. The role of halogen elements in geochemical exploration for the gold deposits is still unclear. This study takes the Qujia gold deposit as an example to explore the issue. Scatter plots and 3D modeling were conducted based on the geochemical data of 1130 borehole samples. The halogen characteristics are governed by fluid-rock interactions, and rock types and heterogeneity. Nevertheless, the halogen characteristics in the alteration zone along the Jiaojia fault zone are still interesting and appealing. In the mineralized zone along the Jiaojia fault, the F and Cl concentrations generally reduce with a decrease in depth; F anomalies evolved from strong positive anomalies at deep depths to negative anomalies at shallow depths; positive Cl anomalies appeared in the upper part or above the mineralized zone while negative Cl anomalies appear in the middle and lower parts of the mineralized zone. The zonation pattern may be attributed to the preferential replacement of OH− by F− due to the acidic pH and the intense beresitization. The consumption of halogen elements may cause the destabilization of metal-Cl complexes and increase the pH of ore-forming fluid, and the deposition of metal sulfides further results in the destabilization of Au-HS complexes and Au deposition. Therefore, F, Cl, and F/Cl ratio can be jointly used as exploration indicators for the geochemical exploration only if rock types, and characteristics of hydrothermal fluid and alteration are examined in advance.

Contrasting Features and Volcanostratigraphy of the Mafic-Hosted Mandoos and Shinas Volcanogenic Massive Sulfide Deposits, Samail Ophiolite, Oman

AbstractDespite the substantial amount of research on the Cretaceous Samail ophiolite in Oman, the factors controlling the size and metal endowment of the mafic-hosted, Cu-Au(-Zn-Ag) volcanogenic massive sulfide (VMS) deposits remain elusive. This work shows that the volcanostratigraphic position, hydrothermal venting style, and oxidation processes are critical factors controlling the distinct features of the Shinas and Mandoos deposits.Mandoos is a large (8 Mt, 1.8 wt % Cu, 0.18 g/ton Au) orebody preserving abundant primary vent-related features formed via mound growth and collapse within a wide hydrothermal field, overlying a poorly developed stockwork. The smaller Shinas deposit (0.8 Mt, 2.6 wt % Cu, 0.63 g/t Au) represents a higher-temperature system evolving from low fS2/fO2 conditions, locally sealed by jaspers, to a mound growth stage with widespread subseafloor brecciation/replacement with associated zone refining. Mandoos formed at the onset of the postaxial stage (Geotimes-Tholeiitic Alley transition), and Shinas is hosted within the Alley units. Volcanism in Samail was seemingly continuous, and the low ɛNd and Nb/Ta of the Shinas hanging-wall lavas record the onset of significant modifications of the mantle source during the postaxial stage.Mandoos is enriched in Te + As + Se ± Zn ± Ga ± Sb relative to Shinas, where higher Cu + Au + Tl ± Mo grades possibly reflect leaching of protoarc-like lavas. Rare earth element patterns in the ores mimicking the deposit footwall can be employed to constrain volcanostratigraphic positioning and indicate that the footwall lavas may also represent a source of metals. Formation of metal-rich ochres at the sea floor likely led to Cu + Au upgrading in the ores during seawater-induced oxidation, which was enhanced during subaerial gossan formation.

Genesis of the Questa Mo Porphyry Deposit and Nearby Polymetallic Mineralization, New Mexico, USA

Abstract The Oligocene Latir magmatic center in northern New Mexico is an exceptionally well exposed volcano-plutonic complex that hosts a variety of magmatic-hydrothermal deposits, ranging from relatively deep, F-rich porphyry Mo mineralization to shallower epithermal deposits. We present new whole-rock chemical and isotopic data for plutonic rocks from the Latir magmatic center, including extensive sampling of drill core samples of intrusive rocks from the Questa porphyry Mo deposit. These data document temporal chemical trends of porphyry-related mineralization that occurred after caldera-forming magmatism and during postcaldera batholith assembly. Silicic magmas were generated multiple times throughout the history of the Latir magmatic center, but few are associated with the formation of a mineral deposit. Whole-rock trace element ratios and Sr, Nd, and Pb isotope compositions vary throughout the protracted history of silicic magmatism. The caldera-forming ignimbrite and early phase of postcaldera intrusions are unmineralized, more enriched in high field strength elements, and generally contain less radiogenic Sr and Pb and more radiogenic Nd than later intrusions. The Questa porphyry Mo deposit formed immediately after the most isotopically primitive phase of the batholith was assembled, ruling out simple reworking of juvenile mantle-derived crust as the source for mineralizing magmas. Rhyolite dikes associated with polymetallic sulfide deposits intruded ~800 k.y. after Mo mineralization, and Nd isotope data indicate that these dikes are associated with different batches of magma and are unrelated to the Mo-mineralizing intrusions at the Questa mine. Together, these data indicate that the source of magmas changed significantly throughout the 10-m.y. history of the magmatic center. We assess multiple genetic models for porphyry-related magmatism against this data set, favoring models with discrete periods of magma genesis from a deep hybridized zone in the lower crust giving rise to the punctuated periods of mineralization. These observations suggest that the formation of mineral deposits within a central magmatic locus is likely the result of the piecemeal assembly of individual hydrothermal-magmatic systems, and that distal and younger polymetallic mineralization commonly observed near known porphyry deposits represents decoupled processes.

PGE geochemical and Os-S-Sr-Nd isotopic constraints on the genesis of the Shitoukengde magmatic sulfide deposit in the East Kunlun Orogenic Belt, NW China

Numerous early Paleozoic mafic–ultramafic intrusions that host important Ni-Cu sulfide mineralization occur in the northern part of the Qinghai-Tibet Plateau, NW China. The sulfide-bearing Shitoukengde Ni-Cu deposit, which is hosted in a ∼424 Ma mafic–ultramafic intrusion, is one of the most represent magmatic sulfide deposits in the East Kunlun Orogenic Belt. The intrusion is composed of a large gabbroic body, cut by lherzolite, olivine websterite, websterite, and minor dunite. Disseminated and net-textured sulfide mineralization occurs as lenses in the center and towards the bottom of the ultramafic bodies. The sulfide mineralization has variable sulfur contents of 0.3 to 13.6 wt% with Ni grade of 0.41 wt% on average, and low platinum-group element (PGE) concentrations (<8.2 ppb Ru and <11.9 ppb Pd). Pd and Ru tenors of bulk sulfides in the deposit are characterized by a positive correlation, indicating that PGE tenor variations are mainly controlled by varying R-factors (magma/sulfide mass ratio). The estimated PGE concentrations in the Shitoukengde magma are about three orders of magnitude lower than those in continental tholeiites. The significant PGE depletions in the Shitoukengde deposit may be due to sulfide removal in the magma conduit at depth. Our new sulfur isotope data reported range in δ34S values from −5.0 to 4.9‰ for the sulfide minerals of the Shitoukengde deposit, clearly distinct from those of mantle-derived sulfur (0 ± 2‰) and immediate country rocks (1.1 to 2.0‰). This, together with the modeling for S isotope variation as a function of R-factor, suggest that the current δ34S range is attributed to the addition of external sulfides with anomalous δ34S into the magma at depth, followed by progressive modification to crustal signature by uncontaminated pulses of magma. The ultramafic rocks exhibit low εNd(t) values (−4.71 to 2.12) and high (87Sr/86Sr)i ratios (0.7073 to 0.7130), which are more enriched relative to the isotopic compositions of depleted mantle. Mixing calculations indicate that about 15 to 20 wt% of lower crustal contamination plus variable upper crustal contamination is required to explain the variations of Sr-Nd isotopes in the Shitoukengde intrusion. Significant crustal contamination is also supported by the mineralization at Shitoukengde having high (187Os/188Os)i (0.24 to 4.51) and γOs(t) values (0.24 to 2534) that are comparable to or higher than those of the important Ni-Cu deposits worldwide. Current results provide evidence that assimilation of crustal sulfur at deep crust and siliceous material with variable crustal contamination during magma ascent and emplacement played critical roles in the attainment of sulfide saturation in the parental magma and the formation of the Shitoukengde deposit.

Diffraction denoising using self‐supervised learning

AbstractDiffraction wavefield contains valuable information on subsurface composition through velocity extraction and sometimes anisotropy estimation. It can also be used for the delineation of geological features, such as faults, fractures and mineral deposits. Diffraction recognition is, therefore, crucial for improved interpretation of seismic data. To date, many workflows for diffraction denoising, including deep‐learning applications, have been provided, however, with a major focus on sedimentary settings or for ground‐penetrating radar data. In this study, we have developed a workflow for a self‐supervised learning technique, an autoencoder, for diffraction denoising on synthetic seismic, ground‐penetrating radar and hardrock seismic datasets. The autoencoder provides promising results especially for the ground‐penetrating radar data. Depending on the target of the studies, diffraction signals can be tackled using the autoencoder both as the signal and/or noise when, for example, a reflection is a target. The real hardrock seismic data required additional pre‐ and post‐autoencoder image processing steps to improve automatic delineation of the diffraction. Here, we also coupled the autoencoder with Hough transform and pixel edge detection filters. Along inlines and crosslines, diffraction signals have sometimes a similar character as the reflection and may spatially be correlated making the denoising workflow unsuccessful. Coupled with additional image processing steps, we successfully isolated diffraction that is generated from a known volcanogenic massive sulphide deposit. These encouraging results suggest that the self‐supervised learning techniques such as the autoencoder can be used also for seismic mineral exploration purposes and are worthy to be implemented as additional tools for data processing and target detections.

Textures, trace element compositions, and sulfur isotopes of pyrite from the Honghai volcanogenic massive sulfide deposit: Implications for ore genesis and mineral exploration

Textures, trace element compositions, and sulfur isotopes of pyrite from the Honghai volcanogenic massive sulfide deposit: Implications for ore genesis and mineral exploration

Sulfide anatexis during high-grade metamorphism: a case study from the Bodenmais SEDEX deposit, Germany

Many sedimentary-exhalative (SEDEX) sulfide deposits have been subject to regional metamorphism and, if the metamorphic grade was high enough, this could have resulted in sulfide anatexis. Although experiments and textures indeed showed that some deposits were partially molten, there is an ongoing debate as to the extent to which metamorphosed ore deposits were molten. Since some SEDEX deposits underwent amphibolite to granulite facies metamorphism, not only sulfides but also the host silicate rocks should have reached anatectic conditions. Due to the two immiscible silicate and sulfide melts, the formation of typical mingling and emulsion textures, as already known from magmatic sulfide deposits, should form. To test this hypothesis, we investigate sulfide-silicate textures from the granulite-facies Bodenmais SEDEX deposit (Germany). Textures from Bodenmais are similar to magmatic sulfide deposits including sulfide-matrix breccia, emulsion textures, pegmatitic leucosomes, and massive sulfides overlain by net-textured intergrowths of refractory quartz, which is interpreted to be a relic of silicate anatexis. Minerals crystallized during the interaction of both immiscible melts differ in their chemistry compared to the same minerals found in the adjacent migmatitic host rocks: for example, garnet in sulfides is Mn-rich (spessartine), but Fe-rich (almandine) in the migmatites and sulfide-enclosed cordierite is more enriched in Mg (Mg/(Mg + Fe): 0.84) than migmatitic cordierite (Mg/(Mg + Fe): 0.54). The textures themselves, their spatial arrangement within the deposit, the differences in mineral chemistry, and the observed crystallization sequence provide unequivocal evidence that the sulfides at Bodenmais were molten to a large extent under granulite facies conditions.

Ultramafic-Hosted Ni-Cu-Co-(As) Mineralization from an Ancient Oceanic Transform Fault Zone in the Troodos Ophiolite, Cyprus: An Analogue for Ultramafic Sea-Floor Massive Sulfide Mineralization?

Abstract Accumulations of sulfide minerals that are enriched in Ni-Cu-Co-(As) occur as sea-floor massive sulfide (SMS) deposits associated with ultramafic rock types on the sea floor and in ophiolite terranes as Outokumpu-type mineralization. In this study we focus on similar mineralization at Lakxia tou Mavrou in the Limassol Forest Complex of Cyprus, which represents the on-land exposure of an oceanic transform fault zone preserved within the Troodos ophiolite. Mineralization here consists of massive lenses of pyrrhotite associated with veins of isocubanite, chalcopyrite, Co pentlandite, and chrome spinel hosted in serpentinized mantle peridotite. We reexamine the field context of mineral occurrences and use in situ mineral chemistry, element mapping, and sulfur isotope ratios (δ34S) to constrain metal sources and provide an updated paragenetic model for Lakxia tou Mavrou. Highly variable S/Se ratios (304–108,571), a depletion in platinum group elements relative to mantle values, and an average δ34S value of –3.7 ± 2.4‰ (1σ, n = 17) in sulfide minerals support a hybrid hydrothermal and magmatic origin for the mineralization. Metals at Lakxia tou Mavrou were sourced from both the serpentinization of peridotites and from crosscutting intrusions, with later intrusions into the already serpentinized mantle lithosphere host providing a heat source to drive prolonged hydrothermal circulation. The reexamination of the field context of mineralization shows that the Ni-Cu-Co-(As) mineralization at Lakxia tou Mavrou originally formed because of the fault-guided intrusion of hot primitive magma bodies into serpentinized shallow mantle lithosphere in the active domain of an ocean-floor transform fault zone. The mineralization was subsequently partially disrupted by structures related to emplacement of the Troodos ophiolite. We show that the relationship between serpentinization, magmatism, and hydrothermal circulation at Lakxia tou Mavrou can be used to understand the formation of ultramafic-hosted SMS deposits in transform fault and other ultramaficdominated slow- and ultraslow-spreading mid-ocean ridge settings.

Deciphering the diagenetic evolution of the lower Carnian succession in the Gorno and Pian Resinelli sulphide mining districts (Southalpine Domain, N Italy)

The stratabound Mississippi Valley-type (MVT) sulphide deposits of the Gorno and Piani Resinelli mining districts are hosted in the lower Carnian carbonate succession, mainly peritidal limestones, of the Lombardian Basin (Southalpine Domain, Northern Italy). In addition to the formation of the well-known sulphide deposits, these carbonate sediments experienced a complex diagenetic evolution, including different stages of dolomitization, silicification, dissolution, brecciation, calcite cement precipitation. Stratigraphic and petrographic features suggest that, apart from an early calcite cementation and a first penecontemporaneous dolomitization affecting the supratidal beds, diagenetic processes and sulphide precipitation, are intimately and genetically associated. They document the occurrence of a Carnian hydrothermal system developed shortly after deposition at shallow burial depth, confirmed by fluid inclusion microthermometry and U-Pb radiometric datings.

Interaction Between Mineralization and Rock Magnetization: New Constraints From a Silurian‐Lower Devonian Volcanogenic Massive Sulfide (VMS) Deposit

AbstractThe unclear relationship between mineralization, hydrothermal alteration, and rock magnetization in volcanogenic massive sulfide (VMS) deposits limits us from fully understanding the magnetic anomalies and remagnetization process in this type of deposit. We address the issue by conducting systemic paleomagnetic, rock magnetic, petrographic, and X‐Ray diffraction studies in the Dapingzhang VMS deposit on the northwestern Indochina Block. Magnetite is the dominant magnetic carrier of hydrothermally altered surrounding rocks and orebody. Magnetite consumption and secondary magnetite formation occurred at different stages and types of hydrothermal alteration. The overall decrease in magnetite concentration from chloritization via silicification to mineralization implies that the magnetite was mostly consumed during hydrothermal alterations. Secondary single domain magnetite, which can carry a remagnetization direction, is proposed to be formed during illitization via the smectite‐to‐illite transformation. Secondary superparamagnetic magnetite, which was most likely formed during the late stage of chloritization, is unable to carry the stable characteristic remanent magnetization. The site‐mean direction of high‐temperature and high‐coercivity components is Dg/Ig = 324.5°/43.1°, and kg = 35.1, with α95 = 6.8° before tilt correction, and Ds/Is = 316.2°/37.6°, and ks = 16.4, with α95 = 10.1° after tilt correction, with a negative fold test. However, plate reconstruction is limited by the uncertainty of the tilting process following mineralization and the possibility of remagnetization during burial alteration. Therefore, this study provides a mechanism for rock magnetic variation and remagnetization during VMS mineralization.

Lithium isotopic and fluid mobile trace element systematics of the Bay of Islands altered forearc upper to lower ophiolitic crust

Bulk rock Li isotope and select fluid mobile element traverses through the crustal section of the Late Cambrian Bay of Islands Ophiolite Complex (BOIC) in the Taconic Humber Arm Allochthon in the Newfoundland Appalachians are presented to examine the extent and conditions of alteration at different pseudo-stratigraphic levels in the ancient forearc oceanic crust that was obducted during the Middle Ordovician.N-MORB normalized enrichment factors (EFs) calculated for Li and fluid mobile elements (FMEs) show variable extents of enrichments or depletions from shallow to deep oceanic crustal sections in the BOIC that are consistent with a broad range of alteration-related δ7Li values from −2.97 to +20.69 ‰ (vs. unaltered MORB range of +1.5 to +6.1 ‰ with a mean of +3.4 ± 1.4 ‰). EFs and fluid-rock reaction modeling for Li contents and δ7Li values indicate alteration dominantly controlled enrichments and depletions of Li isotopic and FMEs systematics through all crustal levels of the BOIC ophiolite. Alteration fluids are dominated by modern seawater-like δ7Li composition rather than the recently proposed Cambrian seawater that is based on ancient carbonate shell compositions.BOIC basaltic samples proximal to the York Harbor Mine (YHM), similar to the classic mafic Cu-Zn Cyprus-type volcanic massive sulfide (VMS) deposits, show light δ7Li values of −2.97 to +1.59 ‰, but highly enriched Li contents (14.0–40.7 μg/g). Modeling results indicate that these YHM basalts were altered by metal-rich magmatic fluids, in part, derived from deeper plagiogranite intrusions near the base of the sheeted dikes that are possibly admixed with seawater-derived hydrothermal fluids below the VMS deposit. Magmatic volatile exsolution may be associated with the crystallization of directly underlying subvolcanic silicic intrusions, resulting in over-pressuring and fracturing of the carapace that may assist in localizing the high temperature (400–200 °C) upflow zones of the seafloor hydrothermal system.The BOIC layered gabbroic rocks also show evidence of variable extent of alteration as reflected by both Li content (EF both >1 and < 1) and δ7Li composition (+0.79 to +9.47 ‰), likely indicating high-temperature amphibolite (800–600 °C within the ductile deformation field) to lower temperature greenschist facies paragenesis, with down-temperature complex retrograde petrogenetic lineages. Mylonites and ductile shear zones commonly show heavier δ7Li values between +5.47 and + 18.83 ‰. We suggest that mylonite zones and coarser-grained high-strain zones allowed near-axis fluid penetration at high to lower temperatures that extended to the basal gabbroic sections in the crust during ongoing igneous activity. Shear zones were associated with rheologic weakening and initial permeable pathways for hydrothermal fluids. Shallow on-axis and deep off-axis hydrothermal circulation likely occurred at a relatively fast-spreading rate, which may account for the significant shallow to deep crustal heat extraction as predicted by the multiple sill model of lithospheric cooling.

A fast 3-D inversion for airborne EM data using pre-conditioned stochastic gradient descent

SUMMARY Airborne electromagnetic (AEM) exploration produces large amounts of data due to its high sampling rate, so that the 3-D inversions take extremely big computation and time consumption. We present a fast 3-D inversion framework for large-scale AEM explorations using a pre-conditioned stochastic gradient descent combined with Gauss–Newton (PSG-GN) method. We adopt a compressed sensing (CS) in the 3-D forward modelling, in which a random undersampling is used to reduce the calculation, while the responses for all survey stations are obtained via a reconstruction technique. For our 3-D AEM inversions, a method of combining the stochastic gradient descent with Gauss–Newton (SG-GN) that requires only a small data set in each iteration instead of the conventional full-batch data (complete original data) inversion have been investigated. To further speed up the 3-D inversion, we develop a pre-conditioner considering the random sampling rate and gradient noise to achieve a fast convergence. We use two synthetic models to test the accuracy, convergence and efficiency of our algorithm. The results show that the conventional inversion with full-batch data and the PSG-GN method can both converge quickly, but our method can enhance the inversion efficiency up to 78 per cent. Finally, we invert a field data set acquired from a massive sulfide deposit in Ireland and obtain the results that agree well with the known geologies.

Gold endowment and unloading along pathway for giant gold mineralization: Insights from spatiotemporal variations of in-situ pyrite geochemistry and gold fineness from the Jiaodong gold deposits, north China Craton

Over 5000 tonnes of gold were accumulated in the Jiaodong Peninsula at the early Cretaceous, the sources and critical ore-forming processes of which are still controversial. Here, we conducted comprehensive in-situ textural, elemental and isotopic analyses on pyrite and gold grains from the different mineralization styles, hydrothermal stages and depths for three representative gold deposits (Linglong, Xiadian and Jiangjiayao) from the northwestern Jiaodong Peninsula, with the aim to exactly constrain the sources and ore-forming processes. Three groups of δ34Spyrite were identified in the studied deposits, of which the Group 1 shows stable and low δ34S (4.8–7.6‰) throughout the hydrothermal evolution while the Group 2 has elevated δ34S (6.0–12.8‰) coupled with increasing As and Au concentrations. The Group 3 shows moderate δ34S (5.2–9.8‰) accompanied by high Al and Si concentrations. The Group 1 suggests a low-δ34S primary auriferous fluid derived from a mantle-related source, whereas the Group 2 indicates the primary auriferous fluid leaching additional S, As and Au from the Precambrian metasedimentary rocks along pathway. The Group 3 is petrographically coupled with numerous silicate inclusions, indicating enhanced fluid-rock interaction between the auriferous fluids and the granitic wallrcoks. Gold fineness decreases from the early to the late hydrothermal stages, combined with increasing deposition of polymetallic sulfides, suggesting an increase of pH resulting in efficient Au deposition. Gold fineness and Bi concentrations in pyrite decrease with shallowing, suggesting that cooling affected Au deposition as well. Bismuth-Te-S minerals were identified in the deep depths, combined with the well correlated Bi and Ag, implying that Bi-Te complexes might play a role in transporting Au and Ag at high-temperature conditions. The above results corroborate that multiple sources and ore-forming processes were responsible for the giant gold mineralization, of which the gold endowment from Precambrian wallrocks needs to be further concerned.

Comparative Study of Lithium Halide-Based Electrolytes for Application in Lithium-Sulfur Batteries

Among the next-generation energy storage technologies, lithium-sulfur batteries are considered one of the most appealing solutions owing to their remarkable theoretical capacity. However, to become commercially competitive, there is a strong need to address some issues still characterizing this technology. One of the explored strategies is the optimization of the electrolyte formulation. To this aim, we compared 1,3-dioxolane/1,2-dimethoxyethane-based electrolytes containing two lithium halides, i.e., lithium bromide (LiBr) and lithium iodide (LiI), with lithium bis (trifluoromethane)sulfonylimide (LiTFSI) as a reference electrolyte. The obtained results show how the donicity of the lithium-salt anions might affect the solid electrolyte interphase stability and the lithium sulfide deposition morphology, therefore influencing the electrochemical performance of the cells. Among the tested electrolytes, the sulfur cell containing LiBr salt exhibited the best electrochemical performance maintaining a specific capacity of 900 mAh g−1 at C/4 and a stable trend along cycling at 1C with a specific capacity of about 770 mAh g−1 for 200 cycles.

Evidence of hydrothermal fluid migration pathways in pores and microfractures based on nuclear magnetic resonance and electron microscopic analyses from the Southwest Indian Ridge

The formation of massive sulfide deposits on the seafloor is closely related to sub-seafloor hydrothermal circulation. Hydrothermal circulation depends on the connected pores and fractures. Most previous studies have focused on hydrothermal circulation at the macroscopic scale; however, the transport pathways of hydrothermal fluids have rarely been studied at the microscopic scale. This study showed that multiple measurement techniques could characterize the pores of rocks and the mineral fraction surrounding these pores. Nuclear magnetic resonance (NMR) was used to measure the porosity of fresh basalt, altered basalt, massive sulfide, and sulfide chimneys obtained from the Southwest Indian Ridge. Meanwhile, the distribution of pore radius and pore throats in these samples was obtained using the NMR transverse relaxation time distribution. The scanning electron microscopy results and polarizing light microscope images showed that the pores of altered basalts are more developed than those of fresh basalts, which is consistent with the NMR measurement results that altered basalts have a large pore size distribution. In addition, metallic minerals such as pyrite were found to be precipitated in these pores, and microfractures were observed in altered basalts. This demonstrates that the pores and microfractures in these altered basalts provide pathways for fluid transport and are enriched with metal minerals. Altered basalt is closely related to the formation of massive sulfide deposits and is an important ore-prospecting indicator. Therefore, the study of rock pore structures and components can provide a better understanding of hydrothermal circulation and contribute to the exploration of mineral resources.

Metallogenic model of the Hongqiling Cu–Ni sulfide intrusions, Central Asian Orogenic Belt: Insight from long-period magnetotellurics

Abstract The Hongqiling magmatic Cu–Ni sulfide deposit is one of the most important deposits in the easternmost segment of the Central Asian Metallogenic Belt, northeast China. However, the existence or non-existence of magmatic chambers is still not been determined, which is the key to decipher the formation of Hongqiling Cu–Ni deposit. Therefore, this study proposes to adopt long-period magnetotellurics method to image the deep-seated metallogenic system in Hongqiling Cu–Ni deposit. Two-dimensional (2D) nonlinear conjugate gradient inversion technology allows us to describe entire magma plumbing system, including the mantle-derived magma channels (banded low-resistivity anomalies), deep separated chamber (low-resistivity anomaly C2), and shallow magma conduits (low-resistivity anomaly C1). These results indicated that the mantle-derived primitive magma gave rise to the parental magma for the mafic–ultramafic intrusions in Hongqiling Cu–Ni deposit and triggered the segregation of Cu–Ni sulfides in the shallow chamber. By combining the experimental simulation, geochemistry, geochronology, and geotectonics data in the study area, we suggest that the partial melting processes which produced the large magma plumbing system probably have been triggered by lithospheric delamination.

Discrimination of Pb-Zn deposit types using sphalerite geochemistry: New insights from machine learning algorithm

Due to the combined influences such as ore-forming temperature, fluid and metal sources, sphalerite tends to incorporate diverse contents of trace elements during the formation of different types of Lead-zinc (Pb-Zn) deposits. Therefore, trace elements in sphalerite have long been utilized to distinguish Pb-Zn deposit types. However, previous discriminant diagrams usually contain two or three dimensions, which are limited to revealing the complicated interrelations between trace elements of sphalerite and the types of Pb-Zn deposits. In this study, we aim to prove that the sphalerite trace elements can be used to classify the Pb-Zn deposit types and extract key factors from sphalerite trace elements that can discriminate Pb-Zn deposit types using machine learning algorithms. A dataset of nearly 3600 sphalerite spot analyses from 95 Pb-Zn deposits worldwide determined by LA-ICP-MS was compiled from peer-reviewed publications, containing 12 elements (Mn, Fe, Co, Cu, Ga, Ge, Ag, Cd, In, Sn, Sb, and Pb) from 5 types, including Sedimentary Exhalative (SEDEX), Mississippi Valley Type (MVT), Volcanic Massive Sulfide (VMS), skarn, and epithermal deposits. Random Forests (RF) is applied to the data processing and the results show that trace elements of sphalerite can successfully discriminate different types of Pb-Zn deposits except for VMS deposits, most of which are falsely distinguished as skarn and epithermal types. To further discriminate VMS deposits, future studies could focus on enlarging the capacity of VMS deposits in datasets and applying other geological factors along with sphalerite trace elements when constructing the classification model. RF’s feature importance and permutation feature importance were adopted to evaluate the element significance for classification. Besides, a visualized tool, t-distributed stochastic neighbor embedding (t-SNE), was used to verify the results of both classification and evaluation. The results presented here show that Mn, Co, and Ge display significant impacts on classification of Pb-Zn deposits and In, Ga, Sn, Cd, and Fe also have relatively important effects compared to the rest elements, confirming that Pb-Zn deposits discrimination is mainly controlled by multi-elements in sphalerite. Our study hence shows that machine learning algorithm can provide new insights into conventional geochemical analyses, inspiring future research on constructing classification models of mineral deposits using mineral geochemistry data.

Water Wall Tubes’ High Temperature Corrosion Root Cause Investigation: A 300 MW Level Boiler Case

High temperature corrosion poses a great threat to boiler water wall safe operation. To investigate the corrosion root cause, a 300 MW level boiler water wall high temperature H2S corrosion case was reported. The typical hydrogen sulfide H2S corrosion feature was large amounts of sulfur which could be found in the cut down sample tube corrosion layer, with a thickness of 482 μm. In addition, huge amounts of lead (Pb) could be found in the corrosion layer, which resulted from the lead sulfide (PbS) deposition when the high temperature flue gas condensed at the water wall tubes. Meanwhile, the sulfur in the corrosion layer was closely related to the H2S concentration in the water wall ambience. The related ambience test showed that the H2S could achieve up to 1000 ppm when the boiler was in operation, far larger than the suggested reference value of 100 ppm. Hence, the overlarge H2S concentration was a vital factor for the tube corrosion. To further investigate the reason why the H2S was kept in such high concentration in the boiler long term operation, and the reasons for the over-high sulfur content in the coal and the over-large diameter of the imaginary circle of primary air (DICPA), two factors were obtained. The peak sulfur content reached 2.5% and the suggested sulfur content was below 1%. The DICPA was so large (1580 mm) that the pulverized coal easily scoured the water wall tubes, which would boost the thinning process of the tubes. To relieve the high temperature corrosion, improve the coal qualities, decrease the DICPA, adjust the operation diameter and adopt a coating technology four measures were suggested.