Ore Section Research Articles

Genesis of Pb–Zn Mineralization in the Pulang Cu Polymetallic Deposit in Yunnan Province, China: Insights from Analyses of Geology, Fluid Inclusions and C–H–O–S Isotopes

The Pulang super-large porphyry Cu polymetallic deposit, located in the Sanjiang area of Yunnan Province, is one of the largest Cu deposits in China. This deposit hosts Cu resources of ~5 × 106 t and other ore-forming elements, such as Mo, Au, Ag, Pb, Zn, Pt and Pd. Recently, obvious hydrothermal vein-type Pb–Zn mineralization, with a Pb + Zn resource of ~0.4 × 106 t, has been detected in the North Ore Section of the deposit. However, the genesis of these Pb–Zn ore bodies, especially their relationship to the major Cu ore bodies in the South Ore Section, remains controversial. We conducted geologic description, fluid inclusion petrography and microthermometry, and C, H, O and S isotope studies to uncover the genesis of Pb–Zn vein-type mineralization in North Pulang. As a result, three types of Pb–Zn veins were identified: a quartz–pyrrhotite–chalcopyrite–sphalerite–galena vein, a quartz–pyrrhotite–sphalerite–galena vein, and a calcite–quartz–pyrrhotite–galena vein. All fluid inclusions in the quartz from different veins are liquid-rich inclusions, with homogenization temperatures in the range of 184 °C–235 °C and salinities between 10.4 wt.% and 17.8 wt.% NaCl eq., indicating that the Pb–Zn ore-forming fluid was a single-phase fluid with a low temperature and low-to-medium salinity. Hydrothermal quartz in different stages displays δDwater values ranging from −46.9‰ to −120.0‰ (V-SMOW), and the calculated δ18Owater values range from 2.4‰ to 4.3‰ (V-SMOW), implying that the mineralization fluids likely originated from magma, with a minor involvement of meteoric water. The δ13CCal values (−2.3‰ to −7.9‰ V-PDB) of calcite indicate that C likely originated from a deep-seated source. The δ34S values of chalcopyrite, pyrite, pyrrhotite and sphalerite reveal that S was possibly derived from magmatic rocks. Based on the above data, it is suggested that the Pb–Zn mineralization in North Pulang was the result of the northward migration of ore-forming fluids that originated from South Pulang along the NE-trending structural fractures. A strong water–rock interaction occurred during the migration process. However, the involvement of meteoric water and accompanied cooling of fluids were most likely responsible for the precipitation of galena and sphalerite.

Textures and trace element geochemistry of pyrite in Gaoloushan ore section of the Yangshan gold belt, West Qinling: Implications for ore genesis

The Devonian Sanhekou Group phyllite in the Yangshan gold belt includes multiple stages of pyrite development, and Au is mainly distributed within the pyrite. Bodies within the Gaoloushan ore section of the gold belt are located in the broken and altered carbonaceous phyllite and granitic rock dykes, where quartz vein-type gold ores are more common as compared to other ore sections (Anba, Getiaowan). Studying the trace element distribution characteristics of gold-bearing and non-gold-bearing pyrite can constrain the evolution of ore-forming fluids and the types of ore deposit genesis. Based on detailed fieldwork and microscopic identification, 48 pyrite samples from the 69th exploration line of the Gaoloushan ore section were analyzed in situ by LA-ICP-MS. The following conclusions were drawn: (1) The gold-bearing pyrite formed during the ore-forming stage while the non-gold-bearing pyrite formed during the sedimentary and metamorphic period. The morphology of the pyrites differs, with gold-bearing pyrite mainly characterized by medium-fine grains, impregnation, fine veins, and reticulate structure, with crystal morphology mainly pentagonal dodecahedra or pentagonal octahedra, whereas non-gold-bearing pyrite is mainly characterized by a medium or medium-coarse grains, with stellate, strawberry, or cluster structures, and crystal morphology mainly in the form of cubes or pentagonal octahedra; (2) The As and Te content of the sedimentary pyrite is low, locally below the detection limit, and the Co and Ni contents and ratios vary greatly, with a distribution of 117–945 ppm and ratio distribution of 0.11–1.92. Pyrite from the ore-forming stage is primarily characterized by high As, Pb, Co, Ni, Cu, Sb, Zn, and Se concentrations, with Bi, Au, and trace amounts of Tl, Te, and Sn observed in later formations; (3) Au and As are positively correlated. The average Au content of 60.53 ppm in pyrite from granite dykes is higher than the average Au 15.97 ppm in carbonaceous phyllite. Pyrite particles with Au contents below the detection limit are sourced in carbonaceous phyllite. In addition, the higher As enrichment is observed in pyrite from granite dykes, averaging 35 770.60 ppm with a maximum value of 72 930.19 ppm. In comparison, the average As content in carbonaceous phyllite is 8974.66 ppm, with a maximum of 69 800.50 ppm, and the Au-As diagram indicates that gold in the Gaoloushan ore section is mainly in the form of solid solution gold within the pyrite lattice; (4) Statistical analysis of the Co/Ni ratio variation in the different lithologies and pyrite stages reveals an initial increase followed by a decrease. The median Co/Ni ratio variations during the mineralization stages of Py1s (0.51–0.7), Py1m (1.35–2.25), Py2 (1.43–1.87), Py1-2 m (1.53), Py2a (1.59), Py2b (1.4), Py2s (1.2), Py2m (1.63), and Py3 (0.19–1.40) suggest that magmatic-hydrothermal fluids dominated during the early stage of mineralization in the gold belt and metamorphic fluids during the main mineralization stage, with deep-seated metamorphic fluids playing a significant role in the late mineralization stage. The varying proportions of ore-forming fluids are likely to have been crucial in controlling gold mineralization in the Yangshan gold belt.

Characteristics of Ore-Forming Fluids and Genesis of the First Mining Area and Eastern Ore Section of the Pulang Porphyry Copper Deposit, Southeastern China: A Comparative Study

The Pulang copper deposit, formed in the Late Triassic, is the largest porphyry Cu-Mo-Au deposit in the eastern Tethys, and its genetic type and mineralization potential have received widespread attention. Identifying the characteristics of ore-forming fluids and the sources of ore-forming materials in the deep and peripheral ore bodies of Pulang is particularly important for constructing a complete porphyry copper mineralization system. Based on detailed core logging and geological observations, this article provides extensive petrographic, fluid inclusion micro-thermometry, laser Raman spectroscopy, and H-O-S isotope data on the veins of the main mineralization stage (B veins) in the first mining area and eastern ore section of the Pulang porphyry copper deposit. The genetic correlation between the eastern ore section and the first mining area is clarified, and their mineralization potential is inferred. The results indicate that the deep vein bodies in the first mining area exhibit multi-stage characteristics, and the fluid in B veins exhibits both high-temperature and salinity characteristics. The magma-derived early ore-forming fluids underwent processes such as boiling and experienced immiscibility during meteoric water mixing, which could be the primary mechanism of the precipitation of Cu, Mo, Au, and other metals. The outer eastern ore section is located in a medium-to-low-temperature hydrothermal mineralization zone far from the mineralization center. This outer eastern ore section is a distant part of the magmatic–hydrothermal system of the first mining area.

The REY geochemistry of phosphorites during metamorphism of the Haizhou Group, NW Yangtze Block, China

Rare earth elements (REE including Y—REY) in phosphorites have been used widely as a proxy for seawater chemistry at the time of their formation. However, it is unclear whether metamorphism can lead to REE remobilization and alteration of primary seawater signatures recorded in phosphorites. In this study, we examine similarly aged metamorphosed phosphorite ore deposits from the northeastern margin of the Yangtze Block (Jinping and Xinpu ore sections), with contemporaneously deposited unmetamorphosed equivalents in the southwestern margin of the Yangtze Block (Weng'an ore section). New insights have been provided into the migration of rare earth elements and the redistribution of Zn and O isotopes in metamorphosed phosphorite, which suggests that the varying degrees of metamorphic fluid–rock reaction during the metamorphism process significantly reduced the δ18O value of phosphate (13.7‰ ± 2.2‰ of Jinping phosphate and 7.3‰ ± 1.8‰ of Xinpu phosphate) and added light Zn isotope. These metamorphic fluids were likely generated by the metamorphic dehydration of upper slab uplift during the plate subduction in the Triassic. The negative correlation between the O (and Zn) isotope compositions and REY contents in P-rich rocks suggests that REY were likely re-enriched during the recrystallization process of apatite. Preferential migration of LREE led to significant LREE and MREE enrichment in Haizhou metamorphosed P-rich rocks compared to Weng'an unmetamorphosed phosphorites.

Soil Environmental Study Analysis of Mining Areas in the Yangshan Gold Belt,Gansu

The Yangshan gold deposit in Gansu province is a super large gold deposit in the West Qinling metallogenic belt, with a cumulative gold accumulation of 400 tons. The soil environment conditions in Anba- Getiaowan ore section were investigated and analyzed to provide a reference basis for mine construction, development and treatment. A total of 120 samples (95 horizontal profile samples and 25 vertical profile samples) were collected, taking into account the topography, cover type, soil use type and soil type in the study area, and the distribution characteristics and change patterns of soil heavy metals As, Cd, Hg and Pb in the study area were evaluated sterically from both horizontal and vertical dimensions. Meanwhile, the distribution characteristics of F and Se elements in soil were compared and analyzed. The results of the study showed that: (1) the value of heavy metal elements As exceeded the standard, and the rest of the heavy metal elements were above the second grade of soil quality, among which the exceedance rate of heavy metal elements As in Anba area was 15%, and that in Getiaowan area was 84%; (2) the average value of As, F and Se content was higher than the background value of Longnan City, which was speculated to be significantly related to the presence of high arsenic and high fluorine water in the evaluation of poor quality water in the study area; (3) the excessive content of As element in the soil in the mining area is related to the occurrence of arsenic-containing minerals in gold, and As element pollution problem should be emphasized in the later construction and development of the mine.

The genetic association between vein and skarn type tungsten mineralization in the Yaogangxian tungsten deposit, South China: Constraints from LA-ICP-MS analysis of individual fluid inclusion

The large-scale Yaogangxian W deposit, located in central of the Nanling metallogenic belt, South China, comprises independent quartz vein-type and skarn-type W mineralization. Wolframite-bearing quartz veins occur invariably in metasandstone and slate to the northwest of the Yaogangxian granitic intrusion, whereas scheelite-skarn are mainly hosted by the limestone to the east. The vertical extension of single wolframite-bearing quartz vein can reach up to 1000 m and we focused on the vein samples collected from the deepest level (26 level) which likely recorded early magmatic fluids. For skarn mineralization, two scheelite precipitation stages were recognized, namely scheelite-1 in retrograde stage and scheelite-2 from sulfide-stage. To reveal the fluid sources and characteristics of two mineralizing systems, detailed petrography and microthermometry analyses were carried out on fluid inclusion assemblages (FIA) in scheelite, wolframite and their coexisting quartz. Fluid compositions were obtained from individual fluid inclusion (FI) in ore and gangue minerals via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. These data aid elucidation on the source and nature of fluids in the two distinct W mineralization styles. The chemical compositions of FIs hosted in wolframite and scheelite points to a common magmatic source. The fluid in the scheelite ore section exhibits lower As and B than the wolframite ore section, indicating that boiling may have occurred at early skarn stage. Fluid evolution in the two mineralizing systems suggest that formation of diverse W mineralization styles is mainly controlled by different host lithology and fluid processes. In the wolframite ore section, ore-forming fluid recorded in the deepest level of vein show elevated As, B, K and Rb contents than previously reported ore-forming fluid obtained from the tip of the vein. This likely indicate absence of intensive fluid immiscibility in the deep during wolframite formation, which may rather be accompanied by fluid-rock interaction. In the scheelite ore section, constant injection of meteoric water is revealed. FI hosted in scheelite-1 and 2 exhibit similar compositions, and the lower K content in scheelite-2 fluid is thought to be caused by sericitization in the retrograde stage skarn. The Mo content of scheelite decreased from the retrograde stage to sulfide-stage, indicating a decrease in oxygen fugacity. Based on these conclusion and previous geological and geochronological studies, we propose a metallogenic model of the coupled wolframite-quartz vein and scheelite-skarn mineralization.

Preliminary research on developing the methodology for predicting and estimation of productive sulfide mineralization of the Norilsk type using polarizing-optical and petrophysical methods (on the example of the Talnakh Deposit)

The quantity of the mineral composition of ore and parent rocks was carried out. Ore sections from the core samples of two wells of the Talnakh field were used for these studying. Also a number of their physical, mechanical, magnetic and electrical properties were determined. The content of the main ore minerals and useful components in the studied samples were calculated due to comparison of these data of their petrophysical properties. Additionally petrophysical anomalies in the external contact of ore horizons were identified. Using of this methodology may contribute to improving the efficiency of survey and evaluation work on Norilsk type sulfide mineralization.

Forming conditions of tellurides and their constraints on gold enrichment in Linglong gold district, Jiaodong gold province

胶东地区探明金储量超过5000t，是我国最重要的金矿集区。玲珑金矿区位于胶东西北部招-平断裂北段，具有典型含金石英脉矿床的特征。矿区部分矿段产出富碲铋化物的明金矿石，是研究富碲铋化物金矿床成因和金超级富集机制的理想对象。通过显微岩相学观察、扫描电镜及电子探针分析，首次对胶东玲珑金矿区富碲铋化物明金矿石的矿物组合和形成机制进行了较为详细的研究。玲珑金矿区含明金矿石中的金矿物主要为含银自然金，成色整体较高，平均为894。共发现了五种与自然金共生的碲化物，分别是辉碲铋矿、碲铋矿、碲银矿、碲铋银矿和碲镍矿，其中辉碲铋矿含量最多。通过矿物共生组合研究及物理化学条件分析，确定成矿流体的碲逸度范围为-12.6 < logf_Te₂ <-9.1，硫逸度范围为-13.6 < logf_S₂ <-9.6。碲、铋元素的富集指示了深成幔源的信息，碲、铋等物质可能来源于古太平洋板块俯冲。玲珑金矿区大颗粒自然金的超级富集与碲、铋等元素密切相关，且银碲化物的产出提高了自然金的成色。;Jiaodong gold province has proved gold reserves of more than 5000t, which is the most important gold concentration area in China. Linglong gold district is located in the northern section of Zhao-Ping fault in the northwest of Jiaodong Peninsula, and it has the typical characteristics of a gold-bearing quartz vein deposit. Some of the ore sections in this mining area produce tellurium-bismuth enriched gold ore, therefore, it is an ideal object to study the relationship of the genesis of ore deposits and the extreme enrichment mechanism of gold with the enriched tellurium-bismuth. Through microscopic observation, scanning electron microscope analysis (SEM) and electron probe micro-analysis (EPMA), the mineral assemblages and formation mechanism of tellurium-bismuth enriched gold ore in Linglong gold district were studied in detail for the first time. The gold minerals in Linglong mining area are mainly silver-bearing native gold, and the gold fineness is high with an average of 894. Five kinds of tellurides co-occurring with native gold have been found, namely, tetradymite, tellurobismuthite, hessite, volynskite and melonite, among which the tetradymite is the most abundant. According to the study of mineral assemblages and analysis of physical and chemical conditions, the range of tellurium fugacity in the ore-forming fluid is determined to be -12.6 < logf_Te₂ < -9.1, while the sulfur fugacity range of -13.6 < logf_S₂ < -9.6. The enrichment of tellurium and bismuth indicates they are formed by a deep mantle source, and to be more specifically, the tellurium and bismuth may have been derived from the subduction of the paleo-Pacific plate. The rich concentration of large grains of native gold in Linglong gold district is closely related to tellurium and bismuth, and the precipitation of silver telluride improves the fineness of native gold.

Enrichment of rare earth elements in the early Cambrian Zhijin phosphorite deposit, SW China: Evidence from francolite micro-petrography and geochemistry

The Zhijin phosphorite deposit is a giant early Cambrian rare earth element (REE) deposit, with particular enrichment in heavy REEs, and is of high economic significance. The REEs of the deposit occur almost entirely in apatite, which, on the basis of petrographic characteristics, can be divided into francolite grains (90%), amorphous apatite, and phosphate fossils. In this paper, we report an investigation into francolite grains in different types of phosphorite from ore sections of the Zhijin deposit. Detailed petrographic and transmission electron microscopy observations reveal that the francolite grains are composed of nano-apatite crystals measuring 120–160 nm long. Trace-element contents of pyrite and whole-rock geochemical indicators (such as Cr/Zr and V/Ni ratios and Th/U–Co/Ni plots) suggest that the Zhijin phosphorite formed in an anoxic hydrothermal sedimentary environment. However, the negative Ce/Ce* and Eu/Eu* anomalies indicate that the diagenetic environment of the francolite grains was oxidic. Different diagenetic redox environments and the presence of transportation fractures in francolite grains identified under back-scattered electron imaging reveal that the francolite grains underwent prolonged abrasion and transportation in seawater before sedimentation. Petrographic observations and laser ablation–mass spectrometry element surface scanning analyses indicate that the REEs are uniformly distributed in nano-apatite in isomorphic form, and no nano-inclusions or REE minerals were found, implying that late mineralization was not the cause of REE enrichment. The higher (La/Yb)N ratios of francolite grains relative to seawater indicate that the francolite grains captured abundant REEs during the early diagenetic stage. We infer that prolonged exposure to seawater and the high specific surface area of francolite grains are the key factors that led to the enrichment in REEs. Combining the timing of the metallogenic epoch (543 Ma), Sr–Nd isotopic data, and the Y/Ho ratio, which is close to those of modern seawater, indicates that the REEs were sourced from paleo-seawater. Paleogeographic information suggests that during the early Cambrian, extensive continental collisions caused great amounts of continental weathering products to enter the paleo-ocean, leading to the production of abundant amounts of hydrothermal–sedimentary phosphorus and REEs in shallow seas, thus forming an ideal sedimentary environment for generating the REE resources of the Zhijin phosphorite deposit.

Современное состояние проблемы прогнозирования самовозгорания и взрывчатости сульфидных руд и вмещающих пород на глубине более 1500 м.

The mining factors of ore fire hazard during mining of the lower horizons of the Oktyabrskiy and Talnakhskiy northern deposits are considered. It is noted that the probability of self-heating of sulfide ores and the sulfide dust’s tendency to spontaneous combustion and explosiveness in certain sections of rich sulfide copper-nickel ores are quite high. The oxidation of sulfide ores occurs continuously due to the absorption of oxygen from the mine atmosphere and is accompanied by the release of heat. The oxidation can be accompanied by intense heating of the ore in mining conditions, with the accumulation of large volumes of broken rock mass for a long time in treatment and preparation workings and with free access of air to the bulk of the ore mass. The processes of ore and rock oxidation are especially intense when their moisture content is 1–4%. When the ore is heated above 35 °C, sulfurous gas (SO2) may be released. The main signs of the above-mentioned oxidative processes’ development and signs of the initial phase of a possible underground endogenous fire are indicated along with a constant increase in the temperature of the air coming from the bottom of the face. It is noted that in case of detecting at least one of the signs of a possible underground endogenous fire’s initial phase, urgent measures are taken to improve the ventilation of this working face, to ensure maximum intensity of shipped ore from the fresh stream and the content of sulfurous gas and hydrogen sulfide and mine air temperature are determined every 4 hours. If after two days on the outgoing stream there is no decrease in the content of sulfur dioxide and air temperature, then it should be considered that an endogenous fire has occurred. Measures for the prevention, localization and elimination of foci of spontaneous combustion are given. As an additional safety measure, it is recommended to moisten the dust, since sulfide dust becomes non-explosive at a moisture content of 9–9,5%, and at a humidity of 10% the dust does not transmit an explosive impulse.

Study on the PTX Parameters and Fractal Characteristics of Ore-Forming Fluids in the East Ore Section of the Pulang Copper Deposit, Southwest China

In this paper, the east ore section of the Pulang porphyry copper deposit is selected as the research object. The micro-thermometer and laser Raman spectroscopic technique are utilized to study the parameters of ore-forming fluids such as pressure, temperature, and compositions. In the meantime, the fractal models, including the perimeter-area (P-A) model and number-size (N-S) model, are introduced to quantify the shape of fluid inclusions, and distinguish the stages of ore-forming fluids, respectively. The results show that the types of fluid inclusions are diversified, namely two-phase liquid-rich type, two-phase vapor-rich type, three-phase CO2-rich type, three-phase halite-bearing type and pure liquid type. The fluids of main mineralization stage are characterized by medium-high temperature (170.2–421.4 °C), medium-high salinity (9.3 wt.%–33.3 wt.%), and low density (0.73–1.06 g/cm3). With the migration and evolution, the temperature, salinity, and pressure of ore-forming fluids gradually decrease, while the density of fluids increases. The liquid-phase compositions mainly include H2O, and the vapor-phase compositions consist of H2O, CH4, N2, and CO2, indicating the characteristics of reducing fluids and the mixing of atmospheric precipitation. In general, the characteristics of ore-forming fluids in the east ore section are similar to those of the first mining area, suggesting that the ore-forming fluids in the east ore section may not migrate from the first mining area. And the east ore section may be a relatively independent metallogenic system. Moreover, the fractal analysis results demonstrate that the shape of fluid inclusions formed in the same hydrothermal activity features self-similarity. The DAP values of fluid inclusions in B veins, ED veins, and D veins are 1.04, 1.06 and 1.10, respectively, showing a gradually increasing trend from the main stage to the late stage of mineralization. Meanwhile, the shape of fluid inclusions ranging from B veins to D veins becomes increasingly irregular. It also reveals that the homogenization temperature satisfies fractal distribution with four scale-invariant intervals, suggesting that all B veins, ED veins, and D veins have experienced at least four hydrothermal activities. Compared with histogram, the N-S fractal model is able to describe the distribution characteristics of the ore-forming fluids’ homogenization temperature more precisely. Therefore, it presents a potential tool for the stage division of ore-forming fluids. This research provides information about the characteristics of ore-forming fluids in the east ore section of the Pulang porphyry copper deposit, which is beneficial for further exploration in this region, and the extension of the application of fractal models in the study of fluid inclusions. However, further testing of fractal models on the fluid inclusion study is warranted to fully determine the universality.

Data Balancing Method for Training Segmentation Neural Networks

Data imbalance is a common problem in machine learning and image processing. The lack of training data for the rarest classes can lead to worse learning ability and negatively affect the quality of segmentation. In this paper, we focus on the problem of data balancing for the task of image segmentation. We review major trends in handling unbalanced data and propose a new method for data balancing, based on Distance Transform. This method is designed for using in segmentation convolutional neural networks (CNNs), but it is universal and can be used with any patch-based segmentation machine learning model. The evaluation of the proposed data balancing method is performed on two datasets. The first is medical dataset LiTS, containing CT images of liver with tumor abnormalities. The second one is a geological dataset, containing of photographs of polished sections of different ores. The proposed algorithm enhances the data balance between classes and improves the overall performance of CNN model.

The early stages of ferromanganese ore genesis on the guyots of the Magellan Seamounts (The Pacific Ocean)

Fe-Mn crusts play an important role in marine mineral deposit research because of their widespread occurrence and high concentrations of valuable and rare metals. Most Fe-Mn crust deposits occur on the tens of thousands of seamounts found in the ocean. Data on the structure, texture, composition, age, and deposit characteristics will help define which factors are key for the creation of mineral accumulation and which combination of factors leads to the formation of potentially economic concentrations of metals. In this paper, we address the structure and characteristics of the oldest Fe-Mn crust stratigraphic sections (Late Cretaceous and Paleocene) collected from the Magellan seamounts. A complete section of the crusts on the Magellan Seamounts includes four layers, each 2—4 cm thickness: the Late Paleocene (?) Early Eocene layer I 1, the Mid Late Eocene layer I—2, the Miocene layer II and the Quaternary layer III. In some cases, the main CMC section is underlain by relict layers. The chemical and mineral composition of the layers was determined both by X-ray diffraction and precision methods; concentrations of the main ore components and phosphorus were determined by the methods of classical chemistry. The age of 12 samples was determined, the mineral composition of four, the chemical composition of 22 samples. The results of the relict layers analysis allow to distinguish two groups of samples among them. Among the relict layers, two age ranges are established — the second half of Late Cretaceous (R1) and the first half of Paleocene (R2). High concentrations of barium, lithium, gallium, and zinc suggest that hydrothermal sources could be the source of the material. But not through direct delivery, but via the phase of transfer of sea bottom water. Thus, the analysis of lithological and geochemical parameters and fossil fauna of foraminifera in the relict layers of the Magellan Seamounts ore section indicates two stages of their formation: Late Campan Maastricht and Early Middle Paleocene. The discreteness of the formation of relict layers in time once again proves that the sharply changing environmental conditions controlled the growth of the CMC ore section.

Automated ore microscopy based on multispectral measurements of specular reflectance. I – A comparative study of some supervised classification techniques

Automated mineralogy, including quantitative compositional and textural information, is a requirement for an efficient ore processing, and is comprised as an important input for geometallurgical planning. Classical ore microscopy is seen by many potential users as an outdated, time consuming, tool. Thus, SEM-based systems are often the choice for those who can afford them, in spite of their evident limitations for some minerals (e.g. for iron oxide ores). However, automated and quantitative mineral characterisation of metallic ores is also possible with optical (reflected light) microscopes, attaining similar performance at a much lower price than SEM systems, as shown by the AMCO (Automated Microscopic Characterisation of Ores) prototype. This system relies on the measurement of multispectral specular reflectance, R, on polished ore sections, to achieve automated identification of the ore species (reflectance is computed from grey levels of digital images acquired by automated scanning of the sample). The performance of this approach, supported by a multispectral reflectance database covering the VNIR range (370–1000 nm) built with the AMCO System, is analysed in this paper, comparing the reliability of different classification methods to achieve ore identification.The work outlined in this article focuses on checking the actual behaviour of four classification techniques, based respectively on spectral angle mapper, euclidean distance, Mahalanobis distance and linear discriminant analysis. The tests carried out reveal that the last two techniques are powerful tools to determine to which mineral corresponds a pixel based on its reflectance spectrum.The obtained results prove that automated multispectral optical microscopy is a reliable tool for mineral characterisation of common/industrial ores, with few exceptions (distinction of cassiterite and chromite, or sphalerite and wolframite). For optimisation of its performance, the multispectral information may be complemented with some additional criteria, such as paragenesis or type of deposit (e.g. to differentiate cassiterite from chromite). The reflectance database can also be enriched with measures of local ores, to account for existing reflectance variations due to local conditions or to learn new ores. The performance is particularly good for iron oxide ores (hematite, magnetite, goethite), a very important commodity. In the tests carried out, they are successfully identified in more than 99% of the cases.

Breccias of Svoboda ore area of Malmyzhskoe gold-copper porphyry deposit (Khabarovsk territory)

The texture and variety of types of breccia bodies of the ore section of the Svoboda at the Malmyzhskoye deposit have been studied and described: a large one — the complex structure of eruptive (hydrothermal-magmatic) breccias and a relatively small — the columnar body of phreatic breccias. Eruptive breccias are intra-ore with respect to gold-copper mineralization. The detrital part in them is represented mainly by metasomatically altered intrusive rocks of the 1st phase of introduction and sedimentary formations of the cretaceous Largasinsky suite. Breccia cementing material is potassium feldspar-quartz-chlorite-sericite mass, which is an intensively metasomatically altered rock of the 2nd intrusive phase of intrusion. Ore mineralization in breccias has a veindisseminated texture and is part of the clastic part of breccias and is also superimposed on the already formed breccia bodies in the process of their metasomatic alternation. Phreatic breccias formed at the final stages of the development of the porphyry system. They are distinguished by low copper and gold contents and sharp secant contacts with the rocks surrounding them. The composition of the debris is generally similar to eruptive breccia, cement is quartz-sericite-epidote-chlorite. The position of ore mineralization is similar to that in eruptive breccias, but it is manifested to a much lesser extent. According to the proposed genetic model, the formation of the body of eruptive breccias occurred as a result of fluidization of rocks located in the arches of the intrusive body, followed by the introduction of significant volumes of magmatic melt. Subsequently, when rising, the fluids interacted with the cold near-surface waters, which caused the formation of phreatic breccias. The studied features of breccia formations are in a good agreement with the classical model of copper-porphyry deposits of the world.

REE geochemistry of gangue minerals and their geological significance in the Muli antimony ore deposit in Yunnan, China

The Muli antimony deposit is located in the Au–Sb polymetallic metallogenic belt in south-eastern Yunnan, China. In this paper, we investigated the concentrations of trace elements in gangue minerals, mainly calcite, quartz, and pyrite, which were formed at different metallogenic stages. Meanwhile, the host rocks, predominantly composed of limestone, are also analysed for comparison. The calcite from the Nadan ore section is enriched with medium-heavy rare earth elements (M-HREEs), likely due to the presence of a high concentration of Fe and Mn impurities, which results in the preferential enrichment of M-HREEs in the calcite. Alternatively, the calcite may be precipitated from the M-HREE-rich granitic leaching fluid. In the Muli ore section, both quartz and pyrite in the metallogenic period show enrichment with light rare earth elements (LREEs), and the wall rock is also enriched with LREEs, which indicates that the wall rock material was involved in the metallogenic process. The W-shaped tetrad effect of quartz in the late metallogenic stage was interpreted to determine extensive fluid–rock interactions in highly fractionated Si-rich systems. Fe and Mn impurities cause M-HREE to be preferentially enriched with calcite to some extent. Whether mineralization is related to granite deserves further study. Eu and Ce anomalies of different types of gangue minerals indicate that the temperature and the fO2 were constantly changing during mineralization, and the temperature of the main ore-stage was higher than 200 °C in an oxidized state. The various REE patterns, LREE/HREE and (La/Yb)N values, reveal that there may be multi-sources and multi-stage hydrothermal activities in the Muli antimony deposit. The REE distribution patterns of minerals are likely interfered with by many internal and external factors. Studies on REE characteristics of calcite, quartz, pyrite and limestone in the Muli antimony deposit have greatly improved the understanding of ore-forming fluids. When we traced the origin and evolution of ore-forming fluids by means of mineral REE distribution patterns, in addition to the determination of inclusions of ore minerals related to mineralization and the in situ analysis methods performed by LA-ICP-MS, we should also combine the REE characteristics of various minerals or trace the ore-forming fluids with multiple methods.

The metallogenic environment of the Dounan manganese deposit, Southeast Yunnan, China: evidence from geochemistry and Mössbauer spectroscopic

The Dounan manganese deposit is a typical large-scale marine sedimentary manganese deposit of the Middle Triassic in China. The metallogenic environment and change process directly dictate the migration, enrichment, and precipitation of Mn. To better understand its metallogenetic environment, a detailed study was undertaken involving field observation, mineralogical and geochemical and Mossbauer spectroscopic analyses. The major findings are as follows: (1) Lithofacies paleogeography, sedimentary structural characteristics, and geochemical indexes indicate that the deposits were formed in an epicontinental marine sedimentary basin environment of normal salinity; (2) there were three ore phases including Mn oxides, Mn carbonates, and mixed Mn ores. The ore minerals found were braunite, manganite, Ca-rhodochrosite, manganocalcite, and kutnahorite. Petrographic and mineralogical information indicates that the metallogenic environment was a weakly alkaline and weakly oxidized to weakly reduced environment, and the mineralization occurred near the redox interface; (3) the V/(V + Ni) ratios, δCe and Fe2+/Fe3+ found in profiles of Baigu and Gake ore sections show that the redox conditions of the ore-forming environment were continuously changing; and (4) three Fe species, α-Fe2O3, para-Fe3+, and para-Fe2+, were found in hematite and clay mineral samples using Mossbauer spectrum analysis. The presence and distribution of these Fe species indicate that the deposit was formed in a typical sedimentary environment during the mineralization process. In summary, our study showed that redox was a key factor controlling the mineralization of the Dounan manganese deposit. Our results have led us to the conclusion that transgression and regression caused fluctuations in sea level, which in turn caused the change of the redox environment. Mossbauer spectroscopy is an effective tool for studying the redox conditions of the paleoenvironment in which sedimentary manganese deposits were formed.

Geology, fluid inclusion, and stable isotope systematics of the Dongyang epithermal gold deposit, Fujian Province, southeast China: Implications for ore genesis and mineral exploration

The Dongyang gold deposit is a recently discovered large gold deposit located within the Dehua–Youxi–Yongtai mining camp in central–east Fujian Province, southeast China. The deposit contains two individual ore sections: the northwestern Jitou section and the southeastern Dadongkeng section. Most of the mineralization is hosted by rhyolite porphyry and volcanic rocks of the Nanyuan Formation. The locations of subvolcanic intrusions in this area are controlled by NE–SW and NW–SE trending faults that are also closely related to mineralization. Detailed drillhole logging and adit mapping indicates that the deposit contains massive, polymetallic quartz–sulfide veins, and cryptoexplosive breccia type ores. Ore minerals consist of gold, electrum, silver minerals, pyrite, marcasite, and arsenopyrite with lesser amounts of sphalerite and chalcopyrite. The mineralization is hosted by quartz, sericite, illite, chalcedony, and calcite gangue and is associated with sericite and argillic alteration as well as silicification. Fluid inclusion microthermometry indicates that the inclusions within quartz from auriferous quartz–sulfide veins formed during the main stage of mineralization and homogenize at temperatures (Th) of 172 °C–217 °C (average of 192 °C) and have salinities of 0.4–2.6 wt% NaCl equivalent. These low to moderate temperatures and low salinities suggest that the mineralization occurred in an epithermal environment. The precipitation of gold mineralization at Dongyang was most likely caused by the mixing of a large amount of meteoric fluid with a smaller amount of magmatic fluid. Quartz-hosted fluid inclusions have H and O isotope compositions (δDH2O = −63.1 to −54.2‰; δ18OH2O = −5.7‰ to −3.8‰) that are indicative of derivation from a predominantly meteoric source. The S isotope values of pyrite vary from a narrow range of −3.4‰ to 0.9‰, suggesting that the S within the deposit was derived from a homogeneous magmatic source. The mineralization and alteration, gangue and ore mineral assemblages, fluid inclusion and H–O–S isotope compositions indicate that the Dongyang gold deposit is a low-sulfidation epithermal-type deposit. The estimated ore formation depth of Donyang is nearly identical to its actual depth, indicating minor denudation in the region. These results imply a great prospecting potential for other epithermal mineralization at the Dehua–Youxi–Yongtai mining camp.

Investigating variations in background response in measurements of downhole natural gamma in a banded iron formation in the Pilbara, Western Australia

Measurements of downhole natural gamma radiation (NGR) provide important information about the location of shale or clay bands within stratigraphical sequences (e.g. in Banded Iron Formations; BIF). An ability to link NGR with other kinds of measurements that are acquired at greater spatial and stratigraphic resolution, such as those acquired by hyperspectral sensing, would open up possibilities for improving the resolution of boundary models. To do this, measurements made by NGR and hyperspectral sensing must be highly correlated and any inconsistencies between these data must be understood. Observations made from the literature and from NGR measurements made in a BIF formation of the Hamersley Group, Pilbara, Western Australia, suggest that NGR measurements in some sections of ore or BIF are elevated compared with other sections; laboratory assays of drill chips do not however suggest the presence of shale or clay. These apparent inconsistencies were investigated using hyperspectral measurements and chemical assays of rock cores in the laboratory and NGR measurements made in the field. We show that the patterns of elevated NGR were a consistent feature of the stratigraphy for this region. Comparison of NGR and Al2O3 made by laboratory assay and from hyperspectral sensing show that elevated NGR measurements were caused by Uranium which was not associated with the presence of shale. Neither Thorium nor Potassium contributed to the elevated gamma signal in the ore. Thorium was strongly correlated with Al2O3 and was found to provide the best indicator of the presence of shale in the stratigraphy.

MECHANISMS OF CRYSTAL GROWTH AND ZONING OF HYDROTHERMAL PYRITE FROM Sb-Au DEPOSITS IN NIZKE TATRY, SLOVAKIA

Pyrite is one of the most common ore minerals in hydrothermal ores and a hallmark of many types of hydrothermal deposits, e.g. , mesothermal quartz veins with pyrite, arsenopyrite, and visible or invisible gold. In addition to any other mineralogical investigation, it is possible to examine and interpret the crystal size distribution (CSD) of the mineral which is under investigation. The conditions of mineral-forming processes can be diverse in terms of temperature, pressure, and compositional variables, but the CSD shapes are usually convergent and by far the most common CSD type is the lognormal distribution. In this study, we determined the CSD for pyrite crystals from mesothermal quartz-pyrite-arsenopyrite-stibnite-gold ores from the Nizke Tatry Mountains (Western Carpathians, Slovakia). A large number of crystals were measured manually from ore sections or from stereographic images. All samples show a crystal size spectrum from ≈2 μm up to ≈550 μm. We discovered that all CSDs show a lognormal or pseudo-lognormal shape with low logarithmic size variances (β2). In order to understand the history of nucleation and growth of pyrite formed in hydrothermal milieus, we compared our measurements to simulations with different growth mechanisms using the GALOPER (Growth According to the Law of Proportionate Effect) program (after [Eberl et al. 1998][1]). From the shape of the distributions, the logarithmic mean (α) and β2, we inferred that the crystals grew in an open system with its associated growth mechanisms (surface-controlled growth and supply-controlled growth), since no evidence for ripening processes in the data or in the numerical simulations of our data could be found. Since ripening as a growth mechanism can be excluded, we have to assume that the growth of the pyrite crystals and so the formation of the whole ore body occurred on a relatively short geological time scale. This preliminary result opens more questions related to the emplacement, circulation, and supersaturation of hydrothermal fluids and to the formation of large ore bodies over short time scales. [1]: #ref-10