Genesis Of Deposits Research Articles

Sedimentary exhalative Pb-Zn deposit model of the Ban Lin – Phia Dam ore range, Vietnam

Purpose. The aim of this study is to construct a typical ore model for Pb-Zn ore range in Ban Lin – Phia Dam and to compare it with the existing ones. The model is constructed based on three main elements of the mineral generation process, such as formation environment, generated-ore fluid source, and deposited-ore mechanism. The obtained results are aimed at determining and predicting the resource of sedimentary exhalative (SEDEX) Pb-Zn ore deposit in Vietnam. Methods. To comprehend the characteristics of the SEDEX Pb-Zn ore deposits in the Ban Lin – Phia Dam area, we employ a combination of approaches that include mineralography to determine the composition and tectonic structure of rocks and ore, inclusion analysis to determine the temperature and inclusion composition, Pb isotopic analysis and stable S isotopic analysis to explore the environment and sources of ore-forming materials. Findings. The sedimentary exhalative (SEDEX) Pb-Zn ore deposit arises from the dissolution of metal-rich salt deposits due to heating at shallow depths (several kilometers). Subsequently, this fluid migrates through faults and fractures to preferred locations where galena and sphalerite precipitate alongside sedimentary basin deposition. Originality. SEDEX ore deposits have been extensively studied in various regions of the world. Although there are multiple theories regarding their origin, including explosion, biological debris accumulation, and surface replacement, each model has its own distinct advantages and limitations in explaining the genesis and development of SEDEX ore deposits. Practical implications. The acquired findings are intended to identify and predict the resources of sedimentary exhalative Pb-Zn ore deposits in Vietnam.

Exploration geochemical evidence for the genesis of the Jiaodong gold deposits in eastern China: A case study of the Haiyu gold deposit

Jiaodong is the largest gold province in China and has great potential for further prospecting. It has been confirmed that the enrichment of Au is always accompanied by pyritization in Jiaodong gold deposits, so the genesis of pyritization in gold deposits and the mechanism of Au enrichment associated with pyritization has always been an issue of discussion to the researchers, the relationship between pyritization and Au enrichment is still unclear. In order to solve this problem, this paper studied the occurrence state and transformation characteristics of S in the ore-bearing geological bodies and discussed the mechanism of Au enrichment. 84 samples were collected from the drill hole ZK3006 of the Haiyu gold deposit, the average sampling interval was 15m, and 7 specific elements/analytical indexes were measured (Total sulfur, Reduced sulfur, Na2O,CaO, Sr, Ba, and Au), Burning-iodine quantity method was used to measure the content of Total sulfur, Combustion neutralization method was used to measure the content of Reduced sulfur Inductively coupled plasma-atomic emission spectrometry was used to measure the content of Na2O and CaO, Wavelength dispersive X-Ray fluorescence spectrometry was used to measure the content of Sr and Ba, and Foamed plastic enrichment-inductively coupled plasma-mass spectrometry was used to measure the content of Au.

Genesis studies of Li–Rb deposits in pegmatites from Bailongshan, western China: Evidence from chronology, fluid inclusions, and H–O isotope analysis

The Bailongshan Pegmatite deposit, located in the West Kunlun Orogenic Belt, Northwest China, is a newly discovered, super-large Li–Rb (Be–Ta–Nb) rare-metal deposit. Since complex magmatic-hydrothermal processes are responsible for the mineralization of such rare-element pegmatites, it is desirable to study the evolution and sources of ore-forming fluids to analyze the genesis of ore deposits. In this study, the 40Ar/39Ar plateau ages of muscovite and biotite were determined to be 171.36 ± 1.87 and 172.39 ± 1.66 Ma, respectively, indicating that the duration of hydrothermal mineralization was approximately 170 Ma. Based on the zonal nature of the mineral assemblage, the Bailongshan area was divided into four zones and stages (I–IV), namely the albite–quartz–lithium tourmaline (AQT, stage I), albite–quartz-bearing mica (AQM, stage II), albite–quartz–spodumene (AQS, stage Ⅲ), and spodumene–quartz (SQ, stage IV) zones. Among these, AQS and SQ were the main ore-bearing areas. In terms of the fluid inclusions found in quartz and spodumene, the different types include a gas-rich phase (V-type), a liquid-rich phase (L-type), a daughter mineral-bearing three-phase (S-type), and a carbon dioxide-bearing phase (C-type). In stage I, the homogeneous temperatures of the V- and S-type fluid inclusions varied from 365 to 415 °C, while their corresponding salinities were 8.5–12.9 and 44.8–47.2 wt% NaCl equiv., respectively. In stage II, the homogeneous temperature and salinity of the L-type inclusions were 315–365 °C and 9.9–13.3 wt% NaCl equiv., respectively, while in stages Ⅲ and IV, the homogeneous temperatures of the L- and S-type fluid inclusions were between 235 and 335 °C, while their salinities were 7.2–12.3 and 32.1–37.0 wt% NaCl equiv., respectively. Furthermore, for the C-type inclusions, the homogeneous temperature and salinity were 235–320 °C and 4.9–10.6 wt% NaCl equiv., respectively. The laser Raman results showed that the fluid in the metallogenic stage was an H2O–NaCl–CO2–CH4 system. Based on the homogeneous temperature and salinity results, the fluid capture pressure from stage III to stage IV was calculated to be 280–150 MPa, and the depth of the capture was >6 km. Moreover, the H–O isotope results suggested that the early ore-forming fluids are mainly magmatic hydrothermal fluids, whereas the later (stage IV) mineralizing fluids may be mixed with a small amount of meteoric water. The subsequent immiscibility of the fluid may be one of the factors responsible for the discharge and precipitation of minerals.

The genesis of intersection-type uranium deposits in south China: Insights from zircon and pitchblende U-Pb geochronology and pyrite sulfur isotopes of the Egongtang uranium deposit, southern Jiangxi

Intersection-type uranium deposits refer to the uranium deposits that are located at the intersection of mafic dykes and silicified faults and are an important type of granite-related uranium deposits in South China. However, the genesis of these deposits still remain poorly understood. The Egongtang uranium deposit is a typical intersection-type uranium deposit in the Huangsha uranium ore district (southern Jiangxi) that bears a number of NWW-trending mafic dykes, thus providing an excellent opportunity to study the genesis of intersection-type uranium deposits. In this study, the mineralization age and genesis of this deposit were constrained using in situ zircon and pitchblende U-Pb geochronology, whole-rock geochemistry, and pyrite sulfur isotope data. The zircon U-Pb dating constrains the crystallization ages of 240.4 ± 1.3 Ma and 160.7 ± 1.6 Ma for granites in the Huangsha ore district. The samples (including altered and unaltered) of the granites are characterized by variable whole-rock U contents of 5.8–14.1 ppm and Th/U ratios of 2.4–7.7, which favor the crystallization of uraninite in the granites and suggest U leaching from the granites during alteration. In situ U-Pb dating on pitchblende from the Egongtang uranium deposit yielded a lower intercept age of 89.8 ± 1.1 Ma, indicating that the uranium mineralization took place at the Late Cretaceous. Pyrite associated with pitchblende has δ34S values ranging from 1.6‰ to 3.8‰, suggesting a magmatic source. This study indicates that granites in this area might have represented the source of uranium for the Egongtang deposit, and that the uranium mineralization was probably related to the Late Cretaceous crustal extension in South China.

Mercury isotope evidence for the importance of recycled fluids in collisional ore systems.

The sources of fluids and metals in porphyry systems of continental-collision settings are poorly constrained. Mercury isotopes display unique mass-independent fractionation (expressed as Δ199Hg) and may provide important constraints on metal and volatile sources given that Hg is a highly volatile metal. Here, we report Hg isotope data on ore-forming porphyries, barren magmatic rocks, and mantle-derived mafic magmas from southern Tibet. The fertile porphyries and coeval mafic magmas display mainly positive Δ199Hg values (up to +0.25 per mil), while Δ199Hg values in barren magmatic rocks and mafic magmas are largely negative (-0.54 to 0.00 per mil). The positive Δ199Hg values observed here are consistent with seawater and marine sediments, suggesting that the ultimate source of fluids involved in the genesis of post-subduction porphyry copper deposits was the mantle lithosphere metasomatized by previous oceanic plate subduction. Our Hg isotope data provide an alternative view to current metallogenetic models on collisional porphyry systems that focus on melting of the lower continental crust.

International Association on the Genesis of Ore Deposits

International Association on the Genesis of Ore Deposits

Osmium and oxygen isotope constraints on magma-crust interactions and the transport of copper at the roots of arcs

The formation of copper-rich cumulates at the base of arc crusts has been proposed as a key process modulating the geochemical evolution of the continental crust and the genesis of giant ore deposits of copper. Despite the importance of these phenomena, the degree to which the lower crustal evolution of magmatic systems is influenced by open-system interaction and the assimilation of pre-existing crustal materials remains unclear. To tackle this issue, we provide direct isotopic constraints on the evolution of deep magmatic systems in arcs by measuring the osmium and oxygen isotope composition of hydrous copper-rich (∼730 μg·g−1 Cu) ultramafic cumulates formed at the base of the Acadian orogen (∼40 km deep) in the New England Appalachians (northeastern USA). The radiogenic 187Os/188Os initial ratios (ranging from 0.31 to 0.67) and the elevated δ18O values (8.97 ± 0.42 ‰ for orthopyroxene; 9.25 ± 0.26 ‰ for phlogopite) suggest a significant role of open-system magmatic differentiation, involving crustal assimilation, in the formation of these cumulates. Modeling of the 187Os/188Os and δ18O composition of the cumulates suggests that the observed isotopic compositions result from the initial evolution of parental magmas under sulfide-undersaturated conditions, followed by saturation after approximately 15% to 20% of assimilation and fractional crystallization progression. These results suggest that the assimilation of crustal material led to a drop in the magmatic system's fO2 (∼ΔFMQ < −1), triggering sulfide segregation and the formation of copper-rich cumulates. Our findings align with the hypothesis that magma-crust interactions can lead to the formation of lower crustal domains enriched with Cu, which may constitute a pre-stage in the formation of some porphyry copper deposits, particularly in collisional orogens.

Enhanced classification of pyrite generations based on mineral chemistry using uniform manifold approximation and projection (UMAP)

Trace element signatures are tracked to unravel the genetic history of ore deposits in several mineral systems. This is particularly true for pyrite, a ubiquitous component of many ore-forming systems, including orogenic gold deposits. Here, we critically compare the efficacy of utilizing Uniform Manifold Approximation and Projection (UMAP) versus Principal Component Analysis (PCA) as dimensionality reduction tools applied to a 31 element dataset collected using Laser Ablation Inductively Coupled Mass Spectrometry of pyrite grains from the Kibali gold district (Democratic Republic of Congo). Because of the non-linearity inherent to mineral chemistry and because of its superior preservation of local (distances within clusters) and global (separation between clusters) data relationships, the UMAP approach outperforms dimensionality reduction by PCA. We further present a workflow in which UMAP dimensionality reduction is followed by k-means clustering to guide the classification of pyrite generations in the Kibali case study. Validating this approach with trace elements and UMAP + k-means on a seed-by-seed petrography basis shows that the workflow significantly enhances the original pyrite classification, previously based on texture. This study thus emphasizes the utility of employing advanced statistical analysis methods to capture the intricate nature of pyrite formation. These findings will shape best practices for handling large multi-element datasets in pyrite mineral chemistry studies and are extrapolatable to other mineral systems in which trace element signatures are used to infer the conditions of ore deposit genesis.

Discovery and Exploration of the Luming Porphyry Mo Deposit, Northeastern China: Implications for Regional Prospecting

Over the past two decades, significant deposit discoveries were made in Northeastern China, including the super-large Chalukou, Daheishan, and Luming porphyry Mo deposits. The discovery of the Luming deposit was accomplished through verification of stream sediment anomalies, with mineralization closely associated with early Jurassic monzogranite and granite porphyry. Previous studies primarily focused on the mineralization mechanisms of these deposits without adequately addressing the exploration methods and prospecting criteria. This study involved a comprehensive re-evaluation of geological observations, analysis of rock primary halo, gravity and magnetic surveys, and induced polarization surveys conducted during exploration campaigns at the Luming porphyry Mo deposit. The results suggest that hydrothermal breccias play a critical role in controlling the mineralization by forming a central low-grade core within the deposit while the Mo mineralization and hydrothermal alteration exhibit a donut-shaped distribution around it. The primary halo shows a distinct metal zonation moving from a central W-Bi-Mo-(Sb) to a peripheral Cu-Co-Ni and a distal Pb-Zn-Ag-In. The mineralization zone exhibits a low Bouguer gravity anomaly, negative magnetic anomaly, medium to low resistivity, and moderate to high chargeability, indicating the effectiveness of geophysical methods in defining the extent of the ore body. The Luming porphyry Mo deposit and distal skarn-epithermal Pb-Zn mineralization are parts of a porphyry-related magmatic-hydrothermal system. The results of this study offer valuable insights into the genesis of porphyry Mo deposits and their implications for prospecting in the forested region of Northeastern China.

Iron isotope constraints on the genesis of giant Beiya Au-base metal deposits, Yunnan, Southwest China

The Beiya Au-base metal deposit in southwest China is characterised by a huge amount of iron associated with gold mineralization. The formation of the Beiya deposit is generally thought to be related to Tertiary potassic intrusion, as porphyry- and skarn-type mineralization are the two most important types of mineralization in the region. However, the lack of direct evidence to constrain the source of iron and gold as well as other metals makes it difficult to accurately conceptualise the ore genesis model. In this study, we report high-precision (0.03 ‰; 2sd) Fe isotope data on Fe-bearing minerals including magnetite, pyrite and chalcopyrite, and major igneous rocks exposed in the area, including monzogranite porphyry (MGP), mafic microgranular enclave (MME), lamprophyre and basalt. Magnetites have a wide range of δ Fe from 0.15 to 0.64 ‰, reflecting substantial isotopic fractionation at different mineralization stages. MMEs have heaviest Fe isotopic composition (0.35 ‰ to 0.88 ‰) compared to MGPs (0.19 ‰ to 0.48 ‰), lamprophyre (∼0.2 ‰) and basalt (∼0.0 ‰). A negative correlation between δ56Fe and TFe2O3 is found for both MPGs and MMEs, which indicates the leaching of iron by hydrothermal fluids mobilizes light Fe and leaves behind isotopically heavy Fe in the remaining source rock. Fe isotopic compositions of MPG and MME overlap with those of magnetite, both of which are proposed to be the source of Fe in magnetite. The basalt has δ56Fe value of 0 ± 0.03 ‰, similar to those of global basalts, suggesting a negligible contribution to the formation of iron ores. Therefore, we propose that magmas parental to the MGP and MME may be the main source of iron to form the Beiya deposit; post-magmatism hydrothermal activities play a key role in leaching the metals out and transporting them to the mineralization area.

MECHANISED COAL EXPLOITATION IN THE "BREZJE" PIT OF THE "DJURDJEVIK" BROWN COAL MINE

In addition to the coal and electricity market and changing geopolitical circumstances, there are other sufficiently rational reasons for opening new underground mining facilities and modernizing existing mining methods.Significant limiting factors and influences, in complex mining-geological conditions, most significantly affect the possibilities of the mechanized way of mining brown coal. The "Brezje" pit is a part of the deposit where the mining-geological factors are complex and determined by the natural conditions of the deposit's genesis. Excavation methods and technologies in complex deposit conditions must be based on great flexibility and adaptability.The complexity of the tectonic structure in the "Đurđevik" pit justified the application of chamber excavation methods with borehole blasting, in various technical modifications and solutions. In all previous solutions, the degree of mechanization of the method was not significantly improved and all modifications of the chamber method were non-mechanized or semi- mechanized with a large participation of manual work, technology based on drilling and blasting, high risk of injuries, low utilization of deposits, unjustified economic effects of work. The aim of this paper is to give an adequate answer to the hypothesis "in the block structure of the deposit with pronounced tectonics, it is possible to reduce the unfavorable influence of the complex mining-geological factors of the deposit on the excavation method by choosing modern mechanization for underground exploitation". The mechanized chamber method of excavation is a conceptual solution for the possible application in complex conditions of the proposed flexible mechanization for work in the "Brezje" pit. The advantage of modern multifunctional mining machines is a high degree of adaptability to the elements of the coal layer with the use of different working mechanisms (tools) that can be changed in a very short time depending on the working phase of the process. Key words: Pit, complexity, factors, chamber method, flexible, multifunctional machine

Sedimentary dolomite in Western Australia and the dolomite problem: Genesis of channel and playa uranium deposits

ABSTRACTQuaternary to Recent sedimentary dolomite in groundwater calcrete and saltmarsh sediments of Lake Way and Lake Maitland, Western Australia, provide new information about the formation of low‐temperature dolomite. Dolomite can form through numerous pathways depending on the depositional and diagenetic environment. Many pathways involve microbial processes and/or the presence of a nucleation substrate which help overcome kinetic barriers preventing precipitation in the laboratory. Petrographic, mineralogical, hydrogeochemical and stable isotopic data in this study reveal the importance of Mg‐clays as nucleation sites for dolomite precipitation in a range of aquifer environments. There is also a close association between authigenic Mg‐clays, dolomite and the potassium–uranyl–vanadate ore mineral carnotite in channel and playa uranium deposits. It is interpreted that evaporation‐driven precipitation of dolomite establishes a positive feedback loop promoting the dissociation of aqueous uranyl–carbonate complexes and concomitant increase in carnotite saturation. Critical to this model is the presence of authigenic Mg‐clays because they facilitate dolomite precipitation and promote carnotite nucleation by concentrating potassium ions on clay surfaces via adsorption. This Mg‐clay–dolomite–carnotite relationship is widespread throughout Western Australian channel and playa uranium deposits and has been observed in similar deposits in Namibia and Botswana. In addition to this economic implication, Mg‐clay mediated nucleation of dolomite potentially has global relevance as a precipitation mechanism for low temperature dolomite in sedimentary deposits where detrital and/or authigenic Mg‐clays are present. Maturation of sedimentary dolomite from disordered high‐calcian dolomite to ordered low‐calcian dolomite occurs very early in the meteoric realm making it resistant to alteration during burial diagenesis. Diagenetic resistance may be further increased by early meteoric silicification related to the degradation of associated Mg‐clays. These findings indicate that Mg‐clay associated sedimentary dolomite has potential to retain a primary isotopic signature indicative of its origin, making it a useful recorder of palaeoenvironmental conditions.

Clarification of the genesis of deposits of Qala Suite of the Hovsan-Zykh area by methods of stratigraphic and lithofacies analysis based on 3D seismic and Well Logging data (Absheron oil and gas bearing region, Azerbaijan)

Clarification of the genesis of deposits of Qala Suite of the Hovsan-Zykh area by methods of stratigraphic and lithofacies analysis based on 3D seismic and Well Logging data (Absheron oil and gas bearing region, Azerbaijan)

Archean atmospheric sulfur stocked in the Cretaceous gold deposits

The giant Cretaceous Jiaodong gold province is the world's third largest gold mining district, while the source of the ore-forming fluids and AuS remains controversial. The elemental sulfur is a key carrier for gold and the isotopic composition is an ideal tracer of the sulfur source, which makes it crucial for understanding gold deposit genesis and guiding ore exploration. Here, we show the Au-bearing pyrite in the gold deposits from Jiaodong bears a non-zero Δ33S signature from −0.07 ‰ to 0.31 ‰ showing the mass-independent fractionation (S-MIF). This clearly indicates involvement of Archean atmospheric sulfur during the Cretaceous gold mineralization. The MIF-bearing sulfur was likely remobilized from the Archean supracrustal rocks in the basement of the Jiaodong region via fluid-rock interaction while the hydrothermal ore-forming fluids passed through the crustal-scale fault zones. The Phanerozoic gold deposits from Jiaodong have relatively minor S-MIF signals compared with the Archean orogenic gold deposits due to more material (S) cycling and dilution. This new discovery of S-MIF signal in the Phanerozoic ore deposits implies that the recycling of crustal sulfur may be a more common process and play an important role in forming hydrothermal gold deposits.

Two stages of gold mineralization in the Hebaoshan deposit: Evidence from the U-Pb dating and trace element geochemistry of rutile, monazite and apatite

The Hebaoshan gold deposit (41.5 t Au @ 3.5 g/t) is located in the southeastern region of the South China Block, central part of the Wuyishan metallogenic belt. The ore-hosting rocks in this area are predominantly Precambrian metasedimentary rocks and Caledonian granitic rocks. Two hydrothermal mineralization stages can be distinguished: a quartz-sericite-pyrite-native gold (stage I) and a chlorite-quartz-sericite-chalcopyrite-electrum (stage II), with hydrothermal monazite and rutile are firstly identified in separate stages. The complex geological history of the region has resulted in ongoing debates regarding the age of gold mineralization and the genesis of the major gold deposits in this area. In order to precisely constrain the mineralization age of the deposit and further establish a genetic model for the ore deposit, LA-ICP-MS U-Pb dating and trace element analysis on accessory minerals were conducted. Based on the textures mineral assemblages, and geochemical features of the accessory minerals, magmatic apatite, hydrothermal rutile, and both magmatic and hydrothermal monazite were identified. The U-Pb ages of magmatic apatite and monazite are determined to be 445.3 ± 15.80, 441.3 ± 15.10 Ma and 446.57 ± 1.03 Ma, respectively, suggesting that these ages represent the emplacement ages of the Caledonian intrusive rocks. The ages of hydrothermal monazite and hydrothermal rutile are determined to be 238.46 ± 2.01 Ma (single-mineral analysis), 238.46 ± 2.01 Ma (in-suit analysis) and 179.54 ± 7.28 Ma, respectively, suggesting that represent two mineralization events during the Late Triassic to early Jurassic in the Hebaoshan area. These data provide new constraints on the mineralization process in the Hebaoshan deposit and excludes the link between gold mineralization and the intrusion of the Caledonian granites. Regionally, It is speculated that the two mineralization events at Hebaoshan are respectively associated with intracontinental orogenic movements between the Yangtze Block and Cathaysia Block, the flat-slab subduction of the Paleo-Pacific Plate (stage I), and the subsequent extensional tectonics related to the collision between the Yangtze Block and Cathaysia Block (stage II).Our study indicates that the timing of multiple episodic mineralization can be constrained by analysis of accessory minerals, which provides a geological basis for better genetic model for the deposit and provide geological evidence for unraveling the relationships between magmatic activities and mineralization events in the region.

THE SIGNIFICANCE OF THE PLEISTOCENE AQUATIC FAUNA OF MOLLUSKS FOR THE REPRODUCTION OF SEDIMENTATION CONDITIONS IN WESTERN UKRAINE

The territory of Western Ukraine is characterized by the development of a wide range of genetic types of continental formations. The Carpathian Mountain Country (properly the Carpathians, East Carpathian Foreland, and Transcarpathia) and the highly fragmented parts of the Volhynia and Podolia Uplands are characterized by a special diversity. The time range of their formation is vast – from the Early Pleistocene to the present. The complexity of the structure of the Quaternary sequence gives rise to discussions about the origin of some components of anthropogenic accumulations. Despite the fact that most of the challenging issues have already been resolved, we find debatable information in the works of some researchers of the 20th and even the 21st centuries about the aqueous origin of the thick sequences of anthropogenic sediments covering the terraces of the Dniester and Dnieper basins. The researchers use many methods to solve the issue of the deposit genesis. Some of them are based on the study of paleontological (including paleomalacological) remains, which are often found in both aqueous (alluvial, lake, swamp) and aerial (primarily loess) formations. Malacofauna has been used since the second half of the 19th century to determine the age, conditions of formation, and origin of the deposits. To establish the origin of sediments we have used the mollusk fauna in this work, which can be often found in the formations of the studied territory. The interpretation of the fauna is debatable in the sections where several genetic types of formations are exposed, primarily alluvial and eolian–deluvial loess. This type of combination of deposits is found in outcrops on the numerous terraces of the Dniester and the terraces of its East Carpathian Foreland’s and Podolia tributaries. The results of the research obtained by us using paleomalacological studies correlate well with data obtained using other methods. Such methods include actual field observations, as well as lithological, palaeopedological, paleontological (study of fossil mammals, fish, spores, and pollen), archaeological, and other methods. The results presented in this work made it possible to establish the genesis of some components of polygenetic complex strata, as well as testify to the effectiveness of the applied research method. Keywords: anthropogen; malacofauna; alluvium; loess; paleogeography; Podolia; East Carpathian Foreland.

Unraveling marine phosphogenesis along the Miocene coast of Peru: Origin and sedimentological significance of the Pisco Formation phosphorites

Phosphorite deposits form through phosphogenesis, a mechanism of diffusion and concentration of interstitial phosphate in sediments leading to Ca-phosphate mineral precipitation in a favorable sedimentary regime at the seafloor. Stretching along the Peruvian coast, the East Pisco Basin is known to host phosphorite beds of Miocene age. In this study, we investigate the petrographic, mineralogical and chemical composition of the Miocene phosphorite layers of the East Pisco Basin, and discuss the phosphogenetic mechanism(s) and sedimentary dynamics that led to the genesis and accumulation of P-rich deposits. Phosphorite samples were collected from layers overlaying stratigraphic unconformities within the Pisco Formation and subsequently analyzed through multiple methods: optical microscopy, XRD, XRF, SEM-EDS, EPMA, ICP-MS and LA-ICP-MS. Our results indicate that these layers consist of phosphatic clasts and nodules along with dolomite clasts, basement boulders, shark teeth, mollusk molds and vertebrate bone fragments. The P-rich clasts are mainly composed of phosphatic intraclasts and small phosphatic nodules. They occur associated with minor amounts of dolomite clasts coated by Fe-oxyhydroxides and laminites. Clasts and nodules are cemented by both early and late diagenetic cements, such as dolomite, calcite, silica, gypsum/anhydrite and baryte. Our observations suggest that the Pisco phosphorite deposits are transgressive lags laying on ravinement surfaces. They formed just below the sediment-water boundary during the early transgression phases, when the porewaters were enriched in phosphorus due to the abundance of organic-rich sediments and the microbial activity, as typical of upwelling settings. These deposits formed in shallow marine conditions, characterized by high biological productivity and low net sedimentation rates, with suboxic conditions at and beneath the sediment-water interface. The concentration of phosphate-coated clasts was then favored by a sedimentary condensation mechanism of dynamic bypassing. Overall, this study provides new insights into the genesis of phosphorite layers in the East Pisco Basin and highlights the importance of understanding the complex interplay between authigenic and sedimentological processes in the formation of these economically and geologically relevant sedimentary rocks.

The role of CO2 in the genesis of Dabie-type porphyry molybdenum deposits

Porphyry-type molybdenum deposits, many of which are in China, supply most of the World’s molybdenum. Of particular importance are the molybdenum deposits located in the Qinling-Dabie region that are responsible for more than half of China’s molybdenum production. A feature that distinguishes this suite of deposits from the better-known Climax and Endako sub-types of porphyry molybdenum deposits is their formation from CO2-rich magmatic-hydrothermal fluids. The role of CO2, if any, in the transport of molybdenum by these fluids, however, is poorly understood. We conducted experiments on the partitioning of molybdenum between H2O-CO2, H2O-NaCl, and H2O-NaCl-CO2 fluids and a felsic melt at 850 °C and 100 and 200 MPa. Here we show that the exsolution of separate (immiscible) brine and vapor leads to the very high brine DMo values needed for efficient extraction of Mo from the magmas forming Dabie-type porphyry molybdenum deposits.

Discrimination of deposit types using magnetite geochemistry based on machine learning

The application of trace elements in magnetite for deposit genesis research is significant, highlighting its potential as a valuable mineral exploration tool. However, traditional low-dimensional analysis methods are not effective in revealing the genetic types of deposits using magnetite trace elements, as they fail to fully utilize the rich high-dimensional information provided by magnetite trace element analysis. To address this limitation, we implemented a supervised machine learning method, eXtreme Gradient Boosting (XGBoost), to correlate the multi-element composition of magnetite with deposit types. Our study encompassed 3,865 magnetite trace element datasets from six distinct deposit types (BIF, IOA, IOCG, magmatic, porphyry, and skarn deposits). It demonstrates the XGBoost classifier’s efficiency and accuracy in classifying high-dimensional magnetite trace element data based on deposit types, achieving an impressive overall accuracy of 96% with an F1 score of 95%. Interpretation of the model using the SHAPley Additive exPlanations (SHAP) tool shows that Ni, Ga, Sc, and V are the most indicative elements for classifying deposit types using magnetite trace element chemistry. Additionally, a visualization method based on XGBoost and t-SNE was proposed. Finally, Xgboost and SHAP were applied in Jinchuan magmatic sulfide Ni-Cu-PGE deposit. Based on optical characteristics and machine learning, Jinchuan magnetite can be classified into three types. Compared with type I magmatic magnetite and type III hydrothermal magnetite, type II magnetite’s paragenetic with sulfides, calcite and apatite and high Ni content may indicate that it’s origin of interaction between sulfide melts–volatile fluids. These results indicate that deep volatile fluid plays a key role for the formation of Jinchuan Cu-Ni sulfide deposit.

International Association on the Genesis of Ore Deposits

International Association on the Genesis of Ore Deposits