查看更多>>摘要：? 2022 Elsevier B.V.In regional exploration for Au mineralization using stream sediment geochemistry, multielement analysis following cyanide leaching of bulk samples or aqua regia digestion of the <180 μm fraction are the two most common sampling and analytical methods. Using data from extensive regional stream sediment and rock chip surveys in Western Turkey a comparison is made between the efficiency of fractal modelling of various spatial and frequency-based statistical methods to isolate patterns or populations related to known mineral deposits. This includes different combinations of pre-processing raw data, such as logistic-transformations and principal component analysis, together with fractal, U-spatial statistics and singularity index modelling. Due to variations in regional geology and the effects of a substantial number of base and precious metal in the region, all approaches indicate the presence of multiple geochemical populations with clearly defined thresholds. The efficiency in detecting known deposits is higher for spatially based methods than for simpler frequency distribution-based techniques, especially if weak geochemical anomalies are excluded to limit the risk of generating false positives. Maximum efficiency (between 70 and 80%) in linking the spatial distribution of anomalous geochemical populations with known Au ± Ag mineral deposits, in both in BLEG and the <180 μm fraction, was obtained using classification based on singularity indices, or logistical-transformed data with fractal analysis or U-spatial statistics. The results demonstrate the advantage of spatially based anomaly detection methods applied to multivariate data over simpler frequency distribution methods applied to univariate data.
查看更多>>摘要：? 2022 Elsevier B.V.Magnetite is a reliable indicator mineral for exploration because it records petrogenetic processes and discriminate deposit types. Binary discriminant diagrams have limits to accurately predict deposit types (prediction accuracy ~40%). This study aims to determine the best predictive supervised multivariate classification method using the geochemical composition of magnetite in order to provide a model usable by industry and government for mineral exploration. After screening a comprehensive database of ~30 k magnetite analyses, ~17 k observations are selected for our study. These data are from 303 different deposits that belong to 9 major deposit types (BIF, Fe-Ti, IOCG, IOA, Ni-Cu-PGE, Porphyry, VMS, Skarn, V) and a varied class of non-mineralized rocks (Country rocks). We tested the three most promising supervised machine learning algorithms on our dataset (Naive Bayes, K-Nearest Neighbor, Random Forest) with 2 open source statistical platforms: Orange and R. The Random Forest (RF) algorithm yield the best predictive outcome on untransformed data with prediction accuracies of 0.80 with Orange and 0.81 with R. We also tested our RF model on three case studies: 1) IOCG-like deposits, 2) porphyry Cu-Au and Cu skarn, and 3) Scandinavian Ni-Cu-PGE deposits. Our model was able to effectively predict the deposit type for the first two case studies to the large family of Porphyry/IOCG/Skarn. For the 3rd case study, almost 61% of the observations were correctly identified as belonging to Ni-Cu-PGE deposits. Our RF model is therefore accurate enough to be used with confidence for mineral exploration.
查看更多>>摘要：? 2022 Elsevier B.V.Uranium reserve estimates in ore deposits can be significantly impacted by 226Ra/238U disequilibria arising from the differential mobility of uranium and radium during groundwater transport. 1D reactive transport models were developed to investigate the long-term effects of retention processes (UO2(am) precipitation, U(VI) and Ra sorption on smectite, Ra co-precipitation with barite) on the repartitioning of 238U and 226Ra during formation of roll-front type deposits. Analytical solutions to radioactive decay chains were used in complement to examine the influence of geochemical parameters, including fluid 234U/238U activity ratios and α-recoil loss, on 226Ra/238U disequilibria in uranium ores. Model results demonstrate that smectite and barite can produce 226Ra/238U ratios >1 at low uranium contents and may explain 226Ra/238U disequilibria occurring in altered rock up- and downstream of roll-front deposits. The capacity of these phases to take up Ra and generate 226Ra/238U disequilibria depends on both mineral contents and groundwater compositions, and is thus expected to be site-specific. Simulations of ore deposits that advance downstream with time demonstrate the formation of stronger 226Ra/238U disequilibria, as expected, in the downgradient side or nose of the ore, reflecting both younger mineralization ages and the presence of active uranium precipitation. Whether disequilibria are positive or negative with respect to secular equilibrium, however, depends on the 234U/238U activity ratio in the fluid from which uranium minerals precipitate. Smaller hydraulic conductivities are shown to generate a narrower range in 226Ra/238U activity ratios with distance, and may explain the occurrence of disequilibria in the limb ore that are less pronounced than those in the nose. Furthermore, the ability of α-recoil loss to decrease 226Ra/238U activity ratios at secular equilibrium may account for negative disequilibria in high grade ores. The South Tortkuduk uranium deposits (Kazakhstan) are subsequently used as a case study to identify the processes and parameters that may contribute to 226Ra/238U disequilibria at this site. Variations in multiple parameters, including clay contents, barite contents, and mineralization ages, are found to reproduce measured 226Ra/238U activity ratios in the roll-front ore. Prioritization of these parameters will necessitate field measurements targeting both groundwater fluids and the host rock. Results from this study will ultimately aid geologists in building appropriate hydrogeochemical data sets to more efficiently locate and exploit uranium ore deposits.
查看更多>>摘要：? 2022 Elsevier B.V.Known as the Guadalupian-Lopingian boundary (GLB), the unconformity between the Guadalupian (Middle Permian) and Lopingian (Late Permian) is widespread in the South China Block. This boundary presents an important period with actives of the Emeishan Large Igneous Provenance (ELIP), the regression and the mass extinction. In Zunyi and Liupanshui areas of Guizhou Province, southwestern China, marine deposits of the Middle Permian Maokou Formation were covered by the terrigenous deposits of the Upper Permian Longtan Formation, the bauxitic claystone at the bottom of the Longtan Formation indicates the exposure during the Middle - Late Permian transition. The provenance of this bauxitic claystone in the South China is complicated, previous studies argue that the flood basalt and submarine volcanic rocks in the ELIP and the aeolian tuff in the Indochina Block are potential sources. In this study, new mineralogical, geochronological and geochemical data are got from the NW Guizhou, our new data constrain the age of bauxitic claystone in Zunyi and Liupanshui are 262.1 ± 1.5 Ma and 267.0 ± 2.7 Ma, respectively. Combined with trace element geochemical analysis, the source materials of bauxitic claystone in Zunyi is mainly derived from the intermediate-acid extrusive rocks in the ELIP, while the provenance of bauxitic claystone in Liupanshui is derived from the submarine volcanic activities during the initial stage of the ELIP. From the central area of ELIP to the outer area, the provenance of early Late Permian sediments transforms from the volcanic rocks of ELIP to Paleo-Tethys magmatic arc. Mass balance calculation and geochemical multivariate statistical methods show that the bauxitic claystone in Zunyi experienced two groundwater level variations in the land surface leaching stage, and Fe2O3、P2O5、HREE show similar geochemical behaviors. HREE is adsorbed by iron phase minerals and phosphate minerals, resulting in simultaneous migration. Our study also finds that paleogeographic location is an important factor for Late Permian bauxitization in South China. The different paleogeographic location determines the supply mode of provenance, provenance lithology, exposure time, leaching degree and other factors.
查看更多>>摘要：? 2022 Elsevier B.V.Shitouban Au deposit is a vein-type deposit, which located in the Dehua-Youxi area (Fujian Province, southeast China) and hosted by late Jurassic volcanic-sedimentary rocks. In this study, a detailed investigation on fluid inclusions, multiple isotopic compositions (H-O-S-Pb-Sr) and Rb[sbnd]Sr isotope dating of quartz have been carried out in an attempt to constrain the ore-forming processes and ore genesis. The deposit shares a characteristic of low-sulfidation epithermal Au deposit, such as low-temperature (peak Th 160–180 °C, average 168 °C) and low-salinity (2-3 wt% NaCl equiv., average 2.47). The hydrogen and calculated oxygen isotope values for fluids are ?64 to ?54‰ (average ? 60‰) and - 0.95 to 0.76‰ (average 0.15‰) respectively, indicating that the ore-forming fluid was a mixture of dominant meteroric water and subsidiary magmatic fluid. Pyrite from the deposit shows a δ34S range from - 4.5 to - 1.2‰ (average ?2.4‰), indicating a magmatic sulfur. The lead isotope ratios of pyrite, 206Pb/204Pb (18.399–18.494), 207Pb/204Pb (15.695–15.841), 208Pb/204Pb (38.842–39.297), are more radiogenic than the Jurassic magmatic rocks in the area, indicating a mixing metal source of magmatic and basement rocks. Based on the Rb[sbnd]Sr isotopic isochron age of Au-bearing quartz, the metallogenic epoch of Shitouban Au deposit is 157 ± 1 Ma, and consistent with the Jurassic volcanism in the Dehua-Youxi area. It is proposed that the primary hydrothermal fluids may have exsolved from the late Jurassic magma chamber and subsequently ascent into the shallower crust level near paleosurface to mix with meteoric water, which cause precipitation of Au and sulfides in the epithermal environment due to a decrease in metal solubility triggered by fluid boiling, cooling, and mixing.
查看更多>>摘要：? 2022 The AuthorsAs part of the European-Peruvian ION4RAW project (Horizon 2020 framework programme of the European Commission), which aims at developing mineral-processing technology to recover selected by-products (e.g., Te, Bi, Co, Re, Mo, Pt, Sb, Ge, Se, In) from primary Cu-Ag-Au deposits, we assessed a geographical inventory of selected elements. However, not all elements of economic interest today have been systematically assayed and/or studied in the past, and the existing European databases commonly are incomplete from a 2022 viewpoint. The DataBase Querying (DBQ) geostatistical mineral prospectivity method helps address this gap between potential mineral occurrences and ‘piecemeal’ historical inventories. In addition to a ‘classical’ application of the DBQ method, we developed a new approach. This is based on the assessment of more global predictive metallogenic-signature aspects (e.g., VMS, orogenic, epithermal), by clustering studied elements known to occur in various metallogenic families, using ArcGIS software. Development of this method at a continental scale allowed identifying several areas of great interest in Europe for exploration of the targeted by-products. It also helps in assessing the favourability for the occurrence of commodities that are ‘by-products’ in their parageneses and that were, until recently, rarely reported in geochemical studies.
查看更多>>摘要：? 2022 The AuthorsResource estimates are crucial to assess the economic potential of tailings storage facilities (TSF) for re-mining and the extraction of critical raw materials. However, a lack of consensus exists on best practices in sampling for this purpose. This study aims to address this gap by assessing different sampling schemes for the resource classification of TSFs. To do so, one layer of a TSF was sampled with regular and nested grids of varying sizes and additional random holes. Systematic spatial trends in the data were fitted with low-order polynomial functions of the coordinates and the grayscale values of a historical aerial photograph of the same layer of the TSF. Variography was performed on the trend residuals, and exponential or Gaussian variogram models were fitted. Universal Sequential Gaussian Simulation was then used to produce 1000 realisations of the potential ground truth. The optimum sampling strategy was investigated by re-sampling these realisations with varying sample densities and configurations and using geostatistical modelling of the re-sampled data to assess uncertainties on the estimated metal tonnages. Robust estimates of metal tonnages can be achieved at relatively low sampling densities, particularly with regular grids. When historical image information is used, spatial variability is better reproduced, and a lower number of samples is required to reach a certain confidence level. Furthermore, an approach to approximate expected errors on grades/tonnages estimated with a given sampling scheme was developed to assess whether further sampling is required. Overall, the findings of this study have clear and transferrable implications for the best-practice sampling and modelling of TSFs and their critical raw material resources.
查看更多>>摘要：? 2022 Elsevier B.V.The basis for understanding continental tectonics to a large degree relies on terrane recognition. However, discrepant identifications and interpretations of the same terrane as well as its boundary are still commonly seen in South China. For years, the central debate has been revolving around the Yangtze-Cathaysia boundary and West-East Cathaysia boundary. Here we characterized the geochemical patterns (or differences) of the proposed terranes and located their boundaries by using a big geochemical dataset of South China, which consists of compositions of 75 elements in 5231 sediment samples. To extract valuable information from this big geochemical dataset, we applied multiple data mining techniques, including principal component analysis, sequential binary partition, Sobel operator, and support vector machine. Our results show that the geochemical patterns vary distinctly at regional scale across both the subblocks (i.e., Yangtze and Cathaysia) and terranes (i.e., Jiuling, Huaiyu, and Youjiang). Accordingly, we conclude that 1) the Northeast Jiangxi fault marks not only the Jiuling-Huaiyu boundary but also the north-eastern Yangtze-Cathaysia boundary; 2) the Yangtze-Cathaysia boundary extends westward along the Jiangshan-Pingxiang-Chaling fault; and 3) a fault zone comprising the Northwest Fujian fault and Zhenghe-Dapu fault possibly demarcates the West and East Cathaysia. The boundaries defined by the contrasting geochemical patterns in this study are also supported by geophysical, Nd isotopic, and geological evidence. Our observations will allow better explanation of where the proposed terranes originated and how South China was assembled. Our work demonstrates that data mining techniques are effective in deriving and integrating information for solving geoscience problems.
查看更多>>摘要：? 2022 Elsevier B.V.Lithium (Li) was recently added to the list of critical raw materials by the European Union due to its significance for the green energy transition. Thus, the development of new toolchains to make Li exploration more economic and more effective is needed. Stream sediment analysis can play an important part in these new tool chains. In this work, two historical stream sediment datasets covering parts of the Fregeneda-Almendra pegmatite field in the Douro region (Portugal) were reprocessed considering two distinct approaches: spatial interpolation through inverse distance weighting (IDW) and the catchment basin approach using the concentration area (C-A) fractal analysis. The following objectives were delineated: (i) determine pathfinder elements for Li, considering relevant associations in the mineralization sources; (ii) compare the performance of both approaches; (iii) identify new target areas for Li. In the case of spatial interpolation, the highest Li values were associated to granites although the use of key elements allowed lithological discrimination and the delineation of target areas. In the catchment basin approach, fractal analysis proved to be effective in decreasing the number of areas of interest with high accuracy (>75%) when considering the previously mapped Li-pegmatites. One of the limitations identified was the number of anomalous basins related to the granites, despite the use of pathfinder elements allowing discriminating granite- from pegmatite-related Li anomalies. Comparing the two approaches, the spatial interpolation method is more adequate for the early stages of exploration (reconnaissance), while the catchment basin approach is more suited for prospect-scale exploration. Field validation of the results identified one pegmatite containing Li mineralization and three others with favorable signs for Li mineralization in the Douro region.
查看更多>>摘要：? 2022 Elsevier B.V.The world's lithium demand is currently increasing due to its use in batteries and in green energy technologies. As a consequence, Li has recently been added to the list of critical materials in Europe. Finding new deposits is necessary in order to guarantee a mid- to long-term stable supply. Granites and associated pegmatites are the primary ‘hard-rock’ source for Li, where it is present in a variety of aluminosilicate minerals. Li is a light element that typically occurs at low concentrations in rocks and minerals, which presents challenges for some analytical techniques (e.g., portable X-ray fluorescence) for exploration. Laser induced breakdown spectroscopy (LIBS) is a powerful analytical technique for the acquisition of simultaneous almost all elements of the periodic table. LIBS has particular sensitivity to the light elements (Z < 13) and, therefore, handheld LIBS has significant potential for the exploration of pegmatite- or granite-related Li mineralization. For this study, a suite of 16 pegmatite-hosted Li ore minerals comprising the three main aluminosilicate species (spodumene, lepidolite and petalite) that contained between 1.36 and 3.54 wt% Li was analyzed using a handheld LIBS instrument. A univariate calibration curve was developed for LIBS quantification of Li in these minerals using the strong Li emission lines at 610.36 nm and 670.79 nm (R2 > 0.9, RMSD = 0.2 wt% Li). Simultaneous quantification of Li contents and measurement of major element ratios (e.g., Si/Al) using portable LIBS permits rapid discrimination between different Li-bearing silicates, such as spodumene and petalite, that can be difficult to identify in the field on the basis of macroscopic criteria. Moreover, the detection of minor/trace elements such as Be, Rb and Cs is possible by handheld LIBS, which can provide useful information for rare metal pegmatite and granite exploration. Thus, qualitative and quantitative analysis by handheld LIBS represents a very promising approach for Li exploration.
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