Gold Deposits Research Articles

Machine learning on white mica short-wave infrared (SWIR) spectral data in the Tengjia Au deposit, Jiaodong peninsula (Eastern China): A prospecting indicator for lode gold deposits

Lode gold deposits are the primary source of global gold resources and possess significant mineralization potential at depth, necessitating new strategies to locate deep concealed orebodies. The Tengjia Au deposit, a newly-discovered concealed altered rock-type lode gold deposit (50 t @ 3.89 g/t), is located within the Zhaoping metallogenic belt of the illustrious gold-rich Jiaodong peninsula in Eastern China. It is distinguished by pervasive phyllic alteration associated with gold mineralization, making it an ideal target for mineral geochemical exploration in lode gold deposits. The mineralization and alteration at Tengjia unfold across three distinct stages, delineated by mineral assemblages and textural relationships: K-feldspar-quartz (I), quartz-sericite-native gold-sulfide (Ⅱ), quartz-calcite-galena-sphalerite (Ⅲ) stages.Systematic analysis of short wavelength infrared (SWIR) spectra, coupled with petrographic observation, has unveiled an abundance of white micas (montmorillonite, muscovite, illite, paragonite, and phengite) within Stage Ⅱ at Tengjia. The Al-OH absorption feature wavelengths (Pos2200), as well as illite crystallinity (IC) values, exhibit a discernible shift towards longer wavelengths (>2204 nm) and higher values (>1.4) in the vicinity of ore deposition, which likely resulted from intense water–rock interaction between ore-forming fluid and wall rocks. Discriminant analysis of the orthogonal partial least squares method (OPLS-DA) shows that the absorption wavelengths corresponding to Water, –OH, and Al-OH effectively differentiate between ore and wall-rock samples. Additionally, analysis using the random forest algorithm (RF) demonstrates that spectral data from Tengjia white micas can reliably classify orebodies, achieving an accuracy of 83.2 %. Hence, the findings suggest that the unique SWIR spectral features of white micas offer a valuable tool for detecting the concealed Tengjia gold mineralization. This study proposes a novel approach that integrates machine learning technology with SWIR analysis for the identification of concealed lode gold deposits.

Using Airborne Magnetic and Radiometric Data to Unravel Auriferous Prospective Areas in the Pitangui Greenstone Belt, NW of Quadrilátero Ferrífero – MG

The Pitangui Greenstone Belt (PGB), NW of the Quadrilátero Ferrífero, Brazil, is a meta-volcano-sedimentary sequence that hosts some important gold deposits recently discovered. Research has been made in the PGB to understand its gold mineral system, but the information regarding the prospective vectors associated with gold mineralization is still unclear. Here, we show the application of airborne magnetic and radiometric data to find the spatial relation between hydrothermal alteration zones and the structural framework of the PGB region. The results showed that the occurrence of gold deposits is associated with NW-SE structures and hydrothermal alteration zones. This association does not exist with E-W structures, which has direct implications on the relevance of these structures in the prospective modeling. Additionally, the application of radiometric data to map hydrothermal alteration zones showed that the limitations of this technique can generate anomalies not strictly associated with hydrothermal alteration processes. In this case, some anomalies could be linked with secondary processes like weathering, leaching, transportation and accumulation of mobile elements, such as K and U.

3D mineral prospectivity modeling using deep adaptation network transfer learning: A case study of the Xiadian gold deposit, Eastern China

Mineralization distribution is commonly heterogeneous in space due to the various geology and structure conditions or fluid flow and evoluation, causing the possible difference in mineralization distribution regularity between shallow and deep at a deposit. This presents important challenges for three-dimensional mineral prospectivity modeling (3D MPM) for deep zones with the scarcity data conditions. Transfer learning has shown promising generalization performance in tasks involving shifts in data distribution, reducing reliance on labeled samples and enhancing learning capability with limited data. In this study, we propose an approach of 3D MPM, namely DAN-CBAM, based on the deep adaptation network (DAN) augmented with the convolutional block attention module (CBAM). It theoretically can harmonize the distribution of ore-controlling features between shallow and deep zones of deposits while effectively extract critical high-dimensional features and spatial patterns. The Xiadian orogenic gold deposit was selected as a case study to validate the approach. The DAN integrates multi-layer, multi-kernel adaptation at the top layer of the CNN, resulting in improved alignment of marginal distributions across domains. Metrics for distribution similarity such as Wasserstein distance, were reduced by 0.250 and KL divergence decreased by 0.032. Additionally, the inclusion of the CBAM module led to a lower MK-MMD loss and a faster convergence rate. And the DAN-CBAM model achieves superior prediction accuracy (0.85) compared to traditional deep neural network (DNN) models (0.81). These highlight CBAM's effectiveness in enhancing the model's ability to capture spatial similarities in mineralization. Furthermore, the area under the curve (AUC) value of the DAN-CBAM model (0.869) significantly outperforms traditional machine learning methods, including CNN (0.786) and Random Forest (0.703) models, underscoring its superior predictive efficiency in 3D MPM for deep mineralization. Therefore, the proposed DAN-CBAM model is promising to be applied in the 3D MPM, in particular for the deposits with different mineralization distribution regularities in space.

Evaluation of gold mineralisation potential using AHP systems and weighted overlay analysis

The demand for sustainable development goals and the absence of systematic development and organised exploration for gold has prompted this study to integrate magnetic and radiometric datasets with lithology to evaluate the gold mineralisation potential in the Ilesha schist belt. This study considers 3168.72 km2 of the Ilesha schist belt in southwestern Nigeria, a frontier belt for gold deposits. The high-resolution airborne magnetic and radiometric datasets were processed using enhancement techniques, including the analytical signal, lineament density, and K/Th ratio. CET grid analysis, Euler deconvolution, and analytical signal depth estimation methods were used to aid the interpretation. The spatial integration and interpolation were performed using the Analytical Hierarchy Process (AHP) and weighted overlay analytical tools within the ArcGIS environment. The dominant structural controls for potential mineralisation are ENE–WSW and ESE–WNW trends. The depth of the magnetic sources revealed by the analytical signal ranged from 63.17 to 629.47 m, while depths ranging from 47.32 to 457.22 m were obtained from Euler deconvolution. The delineated highly magnetic edge sources, dense lineaments, radiometrically highlighted alteration zones, and lithological hosts for gold mineralisation were integrated to establish the gold mineralisation potential map. The AHP deductions reveal that 10.52% of the study site is within the high mineralisation potential class, a remarkable 60.39% falls within the moderate class, a significant portion (28.86%) falls within the poor class, and 0.23% is considered unfavourable. The result was optimised by validation using known mines, with 94% (i.e., 15 out of 16 mining sites) plotting within the high mineralisation potential class. This assessment provides invaluable insight for stakeholders and policymakers to embark on gold exploration and exploitation and promote sustainable mineral development.

Controlling the growth mode of Au depositing on Au nanobipyramids via ligand coverage for SERS enhancement

Gold deposition on Au nanoparticles is a common method to control the shape and further modify the properties of nanoparticles as their properties have a strong correlation with their nanostructures. For Au nanobipyramid (Au NBP), it has advantages such as the enhancement of electric field and a higher tunability in plasmon wavelength than the Au nanorod and thus owns a greater potential in shape control. In this paper, we demonstrate a scheme of depositing Au on the surface of Au NBP with the presence of a type of ligand 2-mercaptobenzoimidazole-5-carboxylic acid (MBIA) to synthesize Au NBP@Au dimers. The growth mode of Au depositing on Au NBP can be controlled by the coverage of MBIA. As the coverage is low, with a concentration of MBIA below 0.4 mM, the rough core–shell nanostructure is synthesized; However, as the coverage is high, with a concentration of MBIA over 0.8 mM, gold deposition may form islands on the surface of Au NBP. The SERS performance of Au depositing on Au NBP can also be enhanced by growth mode. For the rough-surface core–shell growth mode, the enhancement is more significant as the EF is improved from 3.5 × 105 to 1.06 × 106 than the islands-growing growth mode due to the coupling between core and shell. And our results show that with multiple types of nanosturctures easy to obtained by changing modified ligand coverage, the controlled growth has a great potential in the dimer design and SERS enhancement using Au NBP.

Metallogenic model of the Lykling ophiolite-hosted lode Au deposit, Scandinavian Caledonides: Insight from fluid inclusions, mineral chemistry and stable isotope geochemistry

The Lykling lode Au deposit represents a unique example of gold mineralization in the Upper Allochthone of the Scandinavian Caledonides. The mineralization is hosted by the Early Ordovician Lykling Ophiolite Complex and the intruding trondhjemite unit and spatially associated with two generations of mafic dykes that crosscut both the ophiolitic complex and trondhjemite. Field observations indicate that the mafic dykes did not play an active role in the emplacement of Au at Lykling, however their contacts with the immediate host rocks (i.e., gabbro and trondhjemite) may have focused ore-forming fluids.Three main stages in the evolution of the hydrothermal system in the Lykling area are documented by two generations of auriferous quartz veins: 1) older quartz-carbonate veins hosted by moderately angled brittle-ductile shear zones and 2) younger quartz-sulfide veins that fill steeply dipping brittle faults. Stage 1 resulted in deposition of barren quartz and gold-free pyrite from moderately saline NaCl-CaCl2-H2O ± CO2 fluids at temperatures between ∼310–330 °C and pressures in the range from 2.7 to 3.5 kbars. Under the given physicochemical conditions Au was mobile in the form of its chloride complexes. Stage 2 is associated with multiple episodes of ductile deformation punctuated by concomitant brecciation reflecting brittle-ductile transition processes. Fluid inclusion data reveals a decrease in temperature and pressure that may result in the formation of stable Au-bisulfide complexes and making the hydrothermal mobility of Au sensitive to changes in pressure. Consequently, the fluctuation in pressure controlled by the development of brittle-ductile structures likely resulted in the precipitation of native gold in quartz veins. Stage 3 represents a pure brittle event which is accompanied by a significant decrease in the fCO2/fS2 ratio. Mixing of higher-temperature and moderate-salinity fluids with cooler fluids has been recognized as the principal trigger for deposition of sulfides and a paragenetically late phase of native gold in the Lykling hydrothermal system.The Lykling ophiolite-hosted lode Au deposit shows numerous similarities with orogenic gold deposits elsewhere, including structural controls during mineralizing events, deposition from moderately to low salinity CO2-bearing aqueous solutions, deposition from a focused fluid flow along trans-crustal fault zones with a mixed brittle-ductile character and the spatial association with regionally metamorphosed terranes. In contrast to the great majority of the known orogenic gold deposits, the Lykling ophiolite-hosted lode Au deposit records a magmatic origin of volatiles. Therefore, taking into consideration field relationships and the regional setting of the Lykling deposit, we argue that its formation is concomitant with emplacement of the Sunnhordland Batholith during post-collisional thinning of the crust and associated localized uplift within the Lykling Ophiolite Complex.

Growth Mediated Disassembly of Cucurbit[8]uril Cross‐linked Static Cubic Self‐assembly of Silver Nanoparticles

AbstractStatic self‐assembly resides in thermodynamically stable global minima of the energy landscape, whereas dynamic self‐assembly occupies local minima of the energy profile and remains in the ordered state for a limited time via dissipation of energy to surroundings. This makes the spatiotemporal control over the assembly and disassembly mechanism easily controllable in the case of dynamic self‐assembly. However, due to the higher thermal stability of static self‐assembly, it is very challenging to perform reverse engineering on these types of systems. Herein we report growth reaction‐based reversal of static silver cubes obtained via cucurbit[8]uril (CB[8]) crosslinked self‐assembly of silver nanoparticles (AgNP). The AgNP building units with variable CB[8] surface coverage have been used as seeds onto which deposition of gold via growth reaction has been performed. The disassembly of supracube structure has been controlled by the formation of [AuCl4]−–CB[8] complex and successive reduction of [AuCl4]− to Au0 on the surface of the seed. The resulting monodispersed isotropic nanoparticles, formed from the CB[8] based cubic self‐assembly after growth, exhibit Au−Ag bimetallic nature. Quenching of the fluorogenic response from the hydrophobic dye coumarin‐7, added after growth, suggests direct interaction with the metallic nanoparticle surface after disassembly and thereby confirms successful growth reaction mediated reversal of self‐assembly.

Advanced soil-gas geochemical exploration methods for orogenic gold deposits: A case study of Chalapu deposit, Xizang

Orogenic gold deposits play a significant role in the world’s gold reserves, and their distinctive characteristics pose challenges for exploration. The low sulfide content and narrow, fault-controlled sulfide-quartz veins hinder the use of traditional mineral exploration methods to locate deeply buried ore bodies, particularly in regolith-covered areas. Enhanced methods are required to detect deposits at greater depths. Soil gas composition shows promising potential for mineral exploration by conveying information about sulfur-rich and carbon-rich gases related to deep-seated mineralization into surface soils. However, the prospects of this method for gold deposits remain uncertain. In this study, a novel method was introduced to perform an integrated H2S, SO2, CH4, and CO2 soil-gas geochemical survey on gold ore bodies of different scales at the Chalapu deposit in Tibet. The results unveiled notable gas geochemical anomalies of H2S, SO2, CH4, and CO2 above the multi-layered or thick gold ore bodies. For instance, on exploration line 16 at point A1607, H2S reached the regional maximum value of 1.064 ppm, coinciding with the surface exposure of the thickest gold ore body, measuring 19.81 m. While the presence of CH4 and CO2 anomalies alone may not signify potential mineralized zones, the combined presence of anomalous H2S and SO2 values along with CH4 and CO2 concentrations appears to be effective in identifying mineralization. These combinations of geochemical anomalies not only uncover concealed ore bodies, but also delineate the trend and extension direction in strike and dip of ore bodies. Gas intensity may provide insights into the scale of the ore bodies, with stronger gas anomalies observed in areas with thicker ore bodies at similar depths. Thus, the study reveals that soil-gas exploration shows great promise as an exploration technique for orogenic gold deposits, especially in areas with regolith cover hindering traditional methods from detecting mineralized zones for gold exploration.

The Carboniferous Wuzunbulake orogenic gold deposit in South Tianshan Orogen (NW China): Sericite Rb[sbnd]Sr geochronology, pyrite geochemistry, and metallogeny

The Kumishi area is located in the eastern part of the South Tianshan Orogen, which hosts several gold deposits and has substantial gold discovery potential. The timing of gold mineralization at Kumishi, however, has been poorly constrained owing to the absence of suitable dating minerals. Hydrothermal activity at Wuzunbulake is divided into the pre-ore stage 1 pyrite-quartz, syn-ore stage 2 quartz(-sulfide) and post-ore stage 3 quartz-calcite alteration/mineralization. Three types of pyrite have been recognized, i.e., Py1 (stage 1), Py2 (stage 2), and PyWR (from wallrock). Our in-situ RbSr dating on stage 2 sericite yielded an isochron age of 351.0 ± 17.4 Ma, indicating Early Carboniferous gold mineralization. Py1 and Py2 have δ34SΣS = 8.28–15.97 ‰ (avg. 12.88 ‰) and 6.92–8.70 ‰ (avg. 7.67 ‰), respectively, indicating that the sulfur in Py1 was metamorphic fluid sourced, while that of Py2 may have a mixed metamorphic fluid and wallrock source (0.84–3.27 ‰; avg. 2.31 ‰). For Py1, its contents of Au, As, Ag, Bi, Co, Cu, Mn, Ni, Pb, Sb, Tl are the lowest. Py2 has significantly higher Au-As-Ag, slightly higher Co-Cu-Ni-Sb-Tl, but lower Bi-Mn-Pb contents than those in PyWR. Considering also the sulfur isotope features, we considered that Py1 was primarily originated from the initial ore-forming fluid, and Py2 was derived from both the ore fluid and PyWR, with the former being more important and represents the source of gold. Based on that Py2 was formed by metasomatism on the PyWR margin and the element spatial coupling characteristics shown in EPMA geochemical maps, we inferred that the Au enrichment and precipitation are associated with fluid-rock reactions. The initial ore fluid is likely featured by the enrichments in Au, As, Ag, Co, Cu, Ni, Sb, Tl, and depletions in Bi, Mn, and Pb. The Wuzunbulake is best classified as an orogenic gold deposit based on its tectonic background, wallrock alteration style, and the ore-fluid source and characteristics.

Orogenic Gold Deposits of Northern Transbaikalia, Russia: Geology, Age, Sources, and Genesis

The paper presents newly obtained and summarizes preexisting data of long-term geochronological and isotope studies of orogenic mesothermal gold deposits in the world’s largest Lena metallogenic province and reviews interpretations of their genesis. Geochronologic data indicate that the gold mineralization was formed during three ore-forming Paleozoic events. The early Late Ordovician–Early Silurian (450–430 Ma) event produced the abundant veinlet—disseminated gold–sulfide mineralization in Neoproterozoic carbonaceous terrigenous–carbonate rocks of the Baikal–Patom foldbelt (BPB). The mineralization was formed simultaneously with regional metamorphic processes. The rejuvenation of hydrothermal activity in the BPB resulted in gold-bearing quartz veins, which was produced in the Middle Carboniferous (340–330 Ma) in relation to postcollisional granitoid magmatism. The latest ore-forming event occurred in the Early Permian (290–280 Ma) and affected exclusively in Precambrian structures of the Baikal-Muya foldbelt (BMB). It was synchronous with the development of intraplate alkaline and subalkaline magmatism in the region. Newly obtained and preexisting isotopic-geochemical (87Sr/86Sr, 143Nd/144Nd, Pb–Pb, and δ34S) data indicate that the mesothermal ore-forming systems of northern Transbaikalia were heterogeneous in their isotopic characteristics, which distinguishes them from the ore–magmatic (intrusion-related type) systems. Comparison of the Sr, Nd, and Pb isotopic composition of the ores and rocks, including magmatic rocks coeval with the gold mineralization, indicates that the Precambrian continental crust was the dominant source of mineral-forming components for the hydrothermal systems of the gold deposits. The contribution of the magmatic source to the genesis of the orogenic gold mineralization was limited and is identifiable only for the Early Permian ore-forming systems of the BMB, for which the input of metals from alkaline mafic melts was suggested.

3D mineral prospectivity modeling at the Axi epithermal gold deposit, NW China by using a feature adaptive fusion strategy

Mineralization distribution is always intricately affected by multiple ore-controlling geological units that play different roles in a mineral system (e.g., driver, trap, and throttle). How to effectively balance and integrate ore-controlling features from various 3D geological models during 3D mineral prospectivity modeling (MPM) is still a challenging task. In this paper, we introduce a novel approach, the feature adaptive fusion convolutional neural networks (CNN), which is designed to learn multiple 3D geological models with ore-controlling functions. The method is validated in the Axi epithermal gold deposit, northwestern China that mineralization distribution is jointly controlled by fault, volcanic phase, and phyllic alteration. The geology units are firstly constructed by explicit-implicit modeling and their ore-controlling features are subsequently described by high-frequency Laplace-Beltrami eigenfunctions and reassembled into multi-channel images as input to CNN. To learn the differences in ore-controlling effects among various geological units, we designed a fully connected layer to achieve adaptive quantification and weighted integration of the ore-controlling features by automatically optimizing weight allocation parameters and bias vectors using the neural network intelligence. Comparison results between the proposed method and other prospectivity methods suggest that the feature adaptive fusion CNN produces more reliable predictions, characterized by: (1) high consistency with known mineralization, (2) the highest AUC value and success rate, and (3) accurate prediction of deep voxels explored by drilling. Therefore, the proposed method effectively integrates the ore-controlling effects of multiple geological units and is suitable for complex scenarios of 3D MPM. Utilizing the prospectivity results generated by our method, we identified five potential mineralization in the Axi gold deposit, laying a robust foundation for future gold exploration.

Fractal dimension and its implication to mineral exploration: a case study from the Jonnagiri and Gadag gold deposits in India

Fractal dimensions characterize the spatial distribution pattern of mineral deposits, allowing the mineralization processes beneath and at the surface to be understood. This study considered a mineralized vein along the fracture/shear zone near the Dona Temple in the Jonnagiri gold deposit and borehole data from the Sangli Block in the Gadag gold deposit. The above-mentioned zones were considered using fractal dimensional analysis to constrain the fluid-flow pathways. The box-counting technique was used to measure the fractal dimension of the mineralized vein along the surface from a detailed to a regional scale. The fractal correlation integral method was used to provide the fractal dimension values from the gold assay (in g t −1 ). The box-counting fractal dimension values obtained varied from 1.07 to 1.36, whilst the fractal dimensions obtained from the fractal correlation integral method ranged from 0.03 to 1.97. Higher fractal values signify a fully matured interlinked fracture/fault network with steep fluid-flow pathways, while lower values indicate the converse condition. Further, the study showed that the fractal analysis of the mineralized vein along the fracture/shear zone appeared to correlate with the fault system rather than with the litho-contact. Economic gold deposition occurs mainly along a structure with a steep fractal gradient, which is the target area for economic gold exploration.

Discreteness and mosaic characteristics of mineralization and ore-free blocks of the Jeruy stockwork gold deposit

Discreteness and mosaic characteristics of mineralization and ore-free blocks of the Jeruy stockwork gold deposit

ESTIMATING THE GOLD MINERALIZATION ZONE BY INDUCED POLARIZATION AND RESISTIVITY METHOD IN X VILLAGE, SUMATERA, INDONESIA

This paper identifies the gold mineralization zone in X Village using the Induced Polarization and resistivity geoelectric method. The measurements were performed using the Wenner arrangement on six tracks, each separated by a distance of 235 meters and spaced 5 meters apart. The distribution of chargeability values was determined using the Induced Polarization method, while the distribution of resistivity values was determined using the resistivity geoelectric method at the research site. The identification results indicate that the suspected gold mineralization zone is on tracks 2 and 6. On track 2, the zone of suspected gold mineralization is suspected to be located 70-75 meters away from the starting point of measurement (point A). On track 6, it is located 65-75 meters away. The gold deposits are found in phyllite rocks that have undergone alteration, specifically philic, argillic, and silicified rock.

Fuzzy c-mean (FCM) integration of geophysical data from an iron-oxide copper gold (IOCG) deposit under thick cover

Geophysical methods depend on a range of physical and chemical mechanisms, and each method has different sensitivities and resolution of Earth parameters, and over different scale lengths. Additionally, such geophysical methods will also have their own statistical characteristics. Thus, it is a significant challenge to combine different methods through a single inversion framework. Rather than attempting to find an optimal and single model for different geophysical responses, an alternative approach is to use FCM clustering to identify clusters of parameters from two or more different geophysical data sets or models that have similar statistical properties. In this paper, we apply the FCM approach to integrate data and model sets for an array of 100 broadband magnetotelluric (MT) and 100 passive seismic receivers spaced 1 km apart on a 10 by 10 grid above the Vulcan IOCG prospect in the Olympic Cu–Au Province, southern Australia. The challenge for exploration of the Vulcan prospect is that it lies beneath 750 m of regolith sedimentary cover, no single geophysical method provides a unique characterization of the deposit geometry, and drilling is very expensive. Fuzzy c-mean cluster analyses are undertaken in 2D for gravity data and shear-wave velocity model data for basement depths beneath cover and in 3D for shear-wave velocity and electrical resistivity model data. Fuzzy c-mean clustering is shown to provide a simple and efficient method of integrating different geophysical measurements to produce a geological framework that can be verified with drilling information.

The Ore-Forming Process of Washan Porphyrite Iron Deposits in the Ningwu District Associated with Iron Oxide Apatite (IOA) Deposits and Iron Oxide Copper Gold (IOCG) Deposits

The Washan iron deposits in Ningwu district contain different magma-related genetic natures, including magmatic, magmatic–hydrothermal and hydrothermal types, and their ore-forming processes remain a subject of debate. To elucidate the ore-forming processes of iron ores from Washan, we present textural, major element analytical, and thermal data of magnetites from various ore bodies in Washan, probing the crystallization conditions and subsequent formation sequence of magnetites. SEM analysis with back-scattered electron (BSE) imaging reveals diverse magnetite textures, including mineral inclusions, exsolution lamellae, and recrystallization features, reflecting the transitional environment from magmatic to hydrothermal. Based on Ti, V, and Cr compositions of magnetite from different ore bodies, two distinct evolution trends of genetic processes are identified, including evolution paths from porphyry-type to IOA- and IOCG-type magnetite. High-resolution WDS mapping highlights the intensifying alterations during this process. Calculated magnetite crystallization temperatures among different types of magnetite range from 597 °C to 378 °C, suggesting a cooling trend from porphyry-type magnetite (~558 °C) to IOA-type magnetite (~515–439 °C) and IOCG-type magnetite (~378 °C). These results underscore the significant role of magma-derived hydrosaline liquids and vapors in the formation of iron ores from Washan, where variations in the salinity of ore-forming fluids lead to different evolutionary paths for subsequent generations of magnetite. The metallogenic model of the Washan iron deposit suggests that highly saline, iron-rich fluids connect the varying geneses of magnetite, transitioning from deeply formed porphyry-type magnetite to IOA- or IOCG-type magnetite generated in the subaerial zone.

Geology and S-Pb isotope geochemistry of the Hatu gold deposit in West Junggar, NW China: Insights into ore genesis and metal source

Geology and S-Pb isotope geochemistry of the Hatu gold deposit in West Junggar, NW China: Insights into ore genesis and metal source

Inferring fault structures and overburden depth in 3D from geophysical data using machine learning algorithms – A case study on the Fenelon gold deposit, Quebec, Canada

AbstractWe apply a machine learning approach to automatically infer two key attributes – the location of fault or shear zone structures and the thickness of the overburden – in an 18 km2 study area within and surrounding the Archean Fenelon gold deposit in Quebec, Canada. Our approach involves the inversion of carefully curated borehole lithological and structural observations truncated at 480 m below the surface, combined with magnetic and Light Detection and Ranging survey data. We take a computationally low‐cost approach in which no underlying model for geological consistency is imposed. We investigated three contrasting approaches: (1) an inferred fault model, in which the borehole observations represent a direct evaluation of the presence of fault or shear zones; (2) an inferred overburden model, using borehole observations on the overburden‐bedrock contact; (3) a model with three classes – overburden, faulted bedrock and unfaulted bedrock, which combines aspects of (1) and (2). In every case, we applied all 32 standard machine learning algorithms. We found that Bagged Trees, fine K‐nearest neighbours and weighted K‐nearest neighbour were the most successful, producing similar accuracy, sensitivity and specificity metrics. The Bagged Trees algorithm predicted fault locations with approximately 80% accuracy, 70% sensitivity and 73% specificity. Overburden thickness was predicted with 99% accuracy, 77% sensitivity and 93% specificity. Qualitatively, fault location predictions compared well to independently construct geological interpretations. Similar methods might be applicable in other areas with good borehole coverage, providing that criteria used in borehole logging are closely followed in devising classifications for the machine learning training set and might be usefully supplemented with a variety of geophysical survey data types.

Refold structures of the Archaean Nyanzaga gold Project, Sukumaland Greenstone Belt, Tanzania: Precursors to the gold-bearing fault system

This study presents a complete investigation into the structural-hydrothermal evolution of the Nyanzaga and Kilimani gold deposits within the Neoarchean Sukumaland Greenstone Belt, Tanzania Craton. The first compressional stage, D1a, creates upright E-W folds in response to N-S-oriented shortening. The second stage, D1b, refolds the previously formed structures during ongoing compression/transpression under N-S-oriented shortening. Due to the brittle nature of late deformation, during D1c, a set of faults emerges, with the NW-SE ones exhibit dextral shearing and control the formation of normal N-S vertical faults.A protracted hydrothermal history is recorded, beginning with a disseminated silicate minerals- and pyrite-rich, gold-barren stage associated with the D1a and D1b deformations, followed by the development of a gold-endowed vein system during progressive D1b and D1c stages. Gold was mainly concentrated within vertical NW-SE (Kilimani) and N-S (Nyanzaga) faults. Our findings challenge traditional polyphase deformation models presented in the other gold deposits of the area, by proposing a progressive and continuous deformation-hydrothermal history. In addition, we highlight the influence of refold structures on fault geometry, which is crucial in concentrating gold mineralization under continuous N-S compression and transpression.

The gold content of mafic to felsic potassic magmas

Many epithermal gold and gold-rich porphyry-type ore deposits are associated with potassic magmas. Hence, potassic magmas are commonly assumed to have been unusually Au-rich or to have contained high Au/Cu ratios. However, these hypotheses remain poorly tested. Here, we report Au concentrations and Au/Cu ratios in silicate melt inclusions analyzed in potassic rocks worldwide. The results suggest that mafic potassic magmas generally contain only 2‒7 ng/g Au, despite common sulfide exhaustion during partial mantle melting. Both the absolute Au concentrations and Au/Cu ratios are comparable to those of mafic calc-alkaline magmas, and they vary little during subsequent magma differentiation because magmatic sulfide precipitation is strongly dominated by monosulfide solid solution that is relatively poor in Au and Cu. We thus suggest that the close association of Au-rich deposits with potassic magmas is not due to Au enrichment in the magma, but rather due to selective Au precipitation at the hydrothermal stage.