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21 pages, 12225 KiB  
Article
Mineral Characterization of Gold Ore Occurrences in the Khaptasynnakh Ore Zone, Anabar Shield, Far East Region, Russia
by Boris Gerasimov and Larisa Kondratieva
Minerals 2025, 15(8), 774; https://doi.org/10.3390/min15080774 - 24 Jul 2025
Viewed by 248
Abstract
Mineral characterization of gold-bearing metasomatites in the Khaptasynnakh ore zone of the Anabar Shield is provided in detail. The following ore formation sequence of mineral associations in the Khaptasynnakh zone was found: pyrite and pyrrhotite → gersdorffite and molybdenite → chalcopyrite, sphalerite, and [...] Read more.
Mineral characterization of gold-bearing metasomatites in the Khaptasynnakh ore zone of the Anabar Shield is provided in detail. The following ore formation sequence of mineral associations in the Khaptasynnakh zone was found: pyrite and pyrrhotite → gersdorffite and molybdenite → chalcopyrite, sphalerite, and galena → bornite and chalcocite → tellurides, native gold, stibnite, cinnabar, and native bismuth. Native gold is characterized by varying fineness (550 to 926‰) and Cu impurity (up to 7.87%) values. Most often, it forms symplectite intergrowths with Au telluride–calaverite. Native gold and Au tellurides showed inclusions of chalcocite, bornite, altaite, tellurobismuthite, rickardite, petzite, and clausthalite. A two-stage formation process of the examined gold is suggested: Low-fineness gold was introduced into the system during early potassium metasomatism, while higher-fineness gold related to silica metasomatism resulted from its additional mobilization by fluid during late-stage formation. The low-temperature gold–telluride association observed in the mineral paragenesis of ore-bearing rocks, as well as its inclusions in native gold, suggests epithermal gold–telluride mineralization. Mineral inclusions examined in placer gold validate a genetic relation between the examined ores and gold placers in the Khaptasynnakh ore zone. Full article
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26 pages, 2032 KiB  
Review
A Cross-Disciplinary Review of Rare Earth Elements: Deposit Types, Mineralogy, Machine Learning, Environmental Impact, and Recycling
by Mustafa Rezaei, Gabriela Sanchez-Lecuona and Omid Abdolazimi
Minerals 2025, 15(7), 720; https://doi.org/10.3390/min15070720 - 9 Jul 2025
Viewed by 938
Abstract
Rare-earth elements (REEs), including lanthanides, scandium, and yttrium, are important for advanced technologies such as renewable energy systems, electronics, medical diagnostics, and precision agriculture. Despite their relative crustal abundance, REE extraction is impeded by complex geochemical behavior, dispersed distribution, and environmental challenges. This [...] Read more.
Rare-earth elements (REEs), including lanthanides, scandium, and yttrium, are important for advanced technologies such as renewable energy systems, electronics, medical diagnostics, and precision agriculture. Despite their relative crustal abundance, REE extraction is impeded by complex geochemical behavior, dispersed distribution, and environmental challenges. This review presents a comprehensive overview of REE geochemistry, mineralogy, and major deposit types including carbonatites, alkaline igneous rocks, laterites, placer deposits, coal byproducts, and marine sediments. It also highlights the global distribution and economic potential of key REE projects. The integration of machine learning has further enhanced exploration by enabling deposit classification and geochemical modeling, especially in data-limited regions. Environmental and health challenges associated with REE mining, processing, and electronic waste (e-waste) recycling are studied, along with the expanding use of REEs in agriculture and medicine. Some recycling efforts offer promise for supply diversification, but significant technological and economic barriers remain. Ensuring a secure and sustainable REE supply will require integrated approaches combining advanced analytics, machine learning, responsible extraction, and coordinated policy efforts. The present review offers a general overview that can be useful for informing future studies and resource-related discussions. Full article
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18 pages, 8700 KiB  
Article
The Application of Integrated Geochemical and Geophysical Exploration for Prospecting Potential Prediction of Copper and Gold Polymetallic Deposits in the Fudiyingzi–Bacheli Area, Heilongjiang Province
by Liang Chen, Huiyan Wang, Chengye Sun, Xiaopeng Chang and Weizhong Ding
Minerals 2025, 15(6), 597; https://doi.org/10.3390/min15060597 - 2 Jun 2025
Viewed by 482
Abstract
The Duobaoshan mineralization area in Heilongjiang Province is a key copper–molybdenum–gold polymetallic region in China. Its southeastern Fudiyingzi–Bacheli area, located at the intersection of the NW-trending copper and NE-trending gold belts, exhibits favorable mineralization conditions. Despite over 70 years of placer gold mining [...] Read more.
The Duobaoshan mineralization area in Heilongjiang Province is a key copper–molybdenum–gold polymetallic region in China. Its southeastern Fudiyingzi–Bacheli area, located at the intersection of the NW-trending copper and NE-trending gold belts, exhibits favorable mineralization conditions. Despite over 70 years of placer gold mining and the discovery of one small copper deposit and one gold deposit, the area remains underexplored with significant peripheral exploration potential. This study integrates 1:50,000 geological mapping, high-precision magnetic surveys, phase-induced polarization, and soil geochemistry through multi-source data fusion for comprehensive mineral prediction. Key steps include delineating Cu, Au, and Mo anomalies and analyzing their associations with Zn, Cd, Ag, As, etc.; inferring NE-, NW-, and near-EW-trending linear structures via magnetic boundary enhancement; dividing high/low resistivity zones and identifying nine significant and six weak phase anomalies using phase-induced polarization; establishing a mineralization model based on typical deposits; and delineating four priority exploration targets. These results provide a scientific basis for further exploration in shallow coverage areas. Full article
(This article belongs to the Section Mineral Exploration Methods and Applications)
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31 pages, 14774 KiB  
Article
Morphostructural and Genetic Features of Native Gold in Apocarbonate Tremolite–Diopside Skarns (Ryabinovoye Deposit, Far East, Russia)
by Veronika I. Rozhdestvina and Galina A. Palyanova
Minerals 2025, 15(6), 571; https://doi.org/10.3390/min15060571 - 27 May 2025
Viewed by 467
Abstract
We studied the chemical composition and morphostructural features of micron and submicron-sized particles of native gold in apocarbonate tremolite–diopside skarns of the Ryabinovoye deposit located on the southeastern margin of the Aldan Shield (Far East, Russia). Polished sections of lump ore samples containing [...] Read more.
We studied the chemical composition and morphostructural features of micron and submicron-sized particles of native gold in apocarbonate tremolite–diopside skarns of the Ryabinovoye deposit located on the southeastern margin of the Aldan Shield (Far East, Russia). Polished sections of lump ore samples containing native gold were analyzed by scanning electron microscopy in combination with X-ray microanalysis using different modes of visualization and X-ray diffraction methods. Gold particles, clearly visible after etching the surface of some polished sections with acids and partial or complete dissolution of some host minerals, were also examined. Native gold from the studied deposit is of high fineness (above 970‰) and contains (in wt.%) <1.59 Ag and less commonly <0.37 Cu and <0.15 Zn. Native gold is found intergrown with tremolite, diopside, and other magnesian silicates, as well as calcite, fluorite, magnetite, and sphalerite. Rare microinclusions of pyrrhotite, galena, and clinohumite are present in gold grains. It was found that native gold inherits the morphology of tremolite crystals and aggregates, which is determined by the size and shape of the voids bounded by its crystals. Gold localized in the intercrystalline spaces and in the zones of conjugation with remobilized calcite has irregular, lumpy shapes and partially or completely faceted grains with a dense structure. The nature of the localization and distribution of native gold in ores is due to the crystallization of the tremolite component of skarns. Apparently, the processes of gold accumulation are caused by the thermal activation of solid-phase differentiation of the substance of carbonate rocks, in which the processes of destruction of the original minerals and collective recrystallization play a significant role. It is likely that at some gold skarn deposits, carbonate rocks could be the source of gold. Data on the morphology and sizes of native gold segregations, as well as on the intergrown minerals, can be used to improve gold extraction technologies. A specific group of minerals intergrown with native gold in gold skarn deposits can be used as a diagnostic feature in the primary search for placer gold. The obtained results will help to better understand the formation of native gold in apocarbonate tremolite–diopside skarns. Full article
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17 pages, 6114 KiB  
Article
Spectral Angle Mapper Application Using Sentinel-2 in Coastal Placer Deposits in Vigo Estuary, Northwest Spain
by Wai L. Ng-Cutipa, Ana Lobato, Francisco Javier González, Georgios P. Georgalas, Irene Zananiri, Morgana Carvalho, Joana Cardoso-Fernandes, Luis Somoza, Rubén Piña, Rosario Lunar and Ana Claudia Teodoro
Remote Sens. 2025, 17(11), 1824; https://doi.org/10.3390/rs17111824 - 23 May 2025
Cited by 1 | Viewed by 1130
Abstract
Remote sensing applications for marine placer deposit exploration remain limited due to the mineralogical complexity and dynamic coastal processes. This study presents the first medium- to high-level detailed multi-scale remote sensing analysis of placer deposits in the Rías Baixas, NW Spain, focusing on [...] Read more.
Remote sensing applications for marine placer deposit exploration remain limited due to the mineralogical complexity and dynamic coastal processes. This study presents the first medium- to high-level detailed multi-scale remote sensing analysis of placer deposits in the Rías Baixas, NW Spain, focusing on five beaches within the Vigo Estuary. Ten beach samples were analyzed for their heavy mineral (HM) content and spectral signatures, using bromoform separation and FieldSpec 4 spectroradiometer equipment, respectively. The spectral signatures of beach samples with a high HM content were characterized and resampled for the Sentinel-2 application, employing the Spectral Angle Mapper (SAM) algorithm. Field validation and an unmanned aerial vehicle (UAV) survey confirmed surface placer occurrences and the SAM’s results. Santa Marta Beach exhibited significant placer anomalies (up to 30% HM), correlating with low SAM values (minimum value–0.10), indicating high spectral similarity. The SAM-derived anomaly patches aligned with the field observations, demonstrating Sentinel-2’s potential for placer deposit mapping. This work highlights the application of Sentinel-2 in the exploration of placer deposits and the use of a specific spectral range of these deposits in coastal environments. These tools are non-invasive, more environmentally friendly, and sustainable, and can be extrapolated to other regions of the world with similar characteristics. Full article
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19 pages, 10300 KiB  
Article
Comparative Performance of Sentinel-2 and Landsat-9 Data for Raw Materials’ Exploration Onshore and in Coastal Areas
by Morgana Carvalho, Joana Cardoso-Fernandes, Francisco Javier González and Ana Claudia Teodoro
Remote Sens. 2025, 17(2), 305; https://doi.org/10.3390/rs17020305 - 16 Jan 2025
Cited by 5 | Viewed by 3014
Abstract
The demand for Critical Raw Materials (CRM) is increasing due to the need to decarbonize economies and transition to a sustainable low-carbon future achieving climate goals. To address this, the European Union is investing in the discovery of new mineral deposits within its [...] Read more.
The demand for Critical Raw Materials (CRM) is increasing due to the need to decarbonize economies and transition to a sustainable low-carbon future achieving climate goals. To address this, the European Union is investing in the discovery of new mineral deposits within its territory. The S34I project (Secure and Sustainable Supply of Raw Materials for EU Industry) is developing Earth observation (EO) methods to support this goal. This study compares the performance of two satellites, Sentinel-2 and Landsat-9, for mineral exploration in two geologically distinct areas in northern Spain. The first area, Ria de Vigo, contains marine placer deposits of heavy minerals, while the second, Aramo, hosts Co-Ni epithermal deposits. These sites provide exceptional case studies to improve EO-based methods for CRM exploration onshore and coastal regions, focusing on deposits often overlooked in remote sensing studies. Standard remote sensing methods such as RGB combinations, Principal Component Analysis (PCA), and band ratios were adapted and compared for both satellites. The results showed similar performance in the Ria de Vigo area, but Sentinel-2 performed better in Aramo, identifying a higher number of zones of mineral alterations. The study highlights the advantages of Sentinel-2’s higher spatial resolution, especially for mapping smaller or more scattered mineral deposits. These findings suggest that Sentinel-2 could play a larger role in mineral exploration. This research provides valuable insights into using EO data for diverse CRM deposits. Full article
(This article belongs to the Section Remote Sensing in Geology, Geomorphology and Hydrology)
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10 pages, 8208 KiB  
Article
The Dependence of Ilmenite’s Dissolution Rate in Hydrochloric Acid on the Fe(III)/Fe(II) Ratio, with Fe K-Edge XANES Pre-Edge Peak Analysis
by Thomas Y. Sun, Jeremy L. Wykes, Karla Wolmarans, Peter Kappen and Richard G. Haverkamp
Minerals 2025, 15(1), 20; https://doi.org/10.3390/min15010020 - 27 Dec 2024
Viewed by 809
Abstract
The complete dissolution of the Ti and Fe content of ilmenite is a necessary first step for the production of TiO2 directly from ilmenite. Hydrochloric acid is one of the possible solubilizing agents. However, the ability to dissolve ilmenite in hydrochloric acid [...] Read more.
The complete dissolution of the Ti and Fe content of ilmenite is a necessary first step for the production of TiO2 directly from ilmenite. Hydrochloric acid is one of the possible solubilizing agents. However, the ability to dissolve ilmenite in hydrochloric acid depends on the nature of the source material. Here, we consider the effect that the oxidation state of Fe in the ilmenite has on the dissolution rate. Three placer ilmenite fractions from New Zealand and India were dissolved in concentrated hydrochloric acid in a stirred vessel. The dissolution rate constants for Fe and Ti for each fraction based on a shrinking sphere model were compared with the Fe(III)/Fe(II) ratio. Pre-edge Fe K-edge XANES as a measure of Fe(III)/Fe(II) has been shown to extend to ilmenite, which has a complex pre-edge region due to the involvement of Ti electronic levels. It was found that there is a relationship between the oxidation state of Fe and the dissolution rate, with a higher Fe(II) content resulting in more rapid dissolution. A higher Fe(II) content reflects a younger, less weathered material, closer to the “standard” stoichiometry of ilmenite. These data and the presented correlation may support the design of industrial processes to digest ilmenite in hydrochloric acid from varying feedstocks. Full article
(This article belongs to the Section Mineral Processing and Extractive Metallurgy)
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16 pages, 7943 KiB  
Article
Unraveling a Rare Polygonal Gold–Pyrite Assemblage: Insights from the Lena Province, Bodaibo Mining District (Russia)
by Isabella Pignatelli, Gaston Giuliani, Christophe Morlot and Louis-Dominique Bayle
Crystals 2025, 15(1), 4; https://doi.org/10.3390/cryst15010004 - 24 Dec 2024
Cited by 1 | Viewed by 1266
Abstract
A rare polygonal gold assemblage from the Bodaibo mining district (Russia) was analyzed in this study. It resembles cubic native gold from the same area described as a gold pseudomorph after pyrite. The polygonal assemblage differs from these cubic gold samples by the [...] Read more.
A rare polygonal gold assemblage from the Bodaibo mining district (Russia) was analyzed in this study. It resembles cubic native gold from the same area described as a gold pseudomorph after pyrite. The polygonal assemblage differs from these cubic gold samples by the absence of striations, its stepped morphology, and the presence of euhedral pyrite. It was analyzed with non-destructive techniques (SEM, VSI, and X-ray CT) in order to preserve the integrity of this exceptional sample. The experimental data allowed us to understand how this rare sample could be formed. A formation of secondary deposits, i.e., eluvial placers, is compatible with the mobilization and precipitation of gold by surface effects on primary pyrite, as well as oxidation episodes producing iron oxides/hydroxides. The redox condition in the geological environment caused the pyrite dissolution and release of gold in its structure, leading to the formation of a thin layer of gold on pyrite by epitaxy rather than pseudomorphism. Full article
(This article belongs to the Section Mineralogical Crystallography and Biomineralization)
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15 pages, 4732 KiB  
Article
About the Origin of Carbonado
by Valentin Afanasiev, Vladimir Kovalevsky, Alexander Yelisseyev, Rudolf Mashkovtsev, Sergey Gromilov, Sargylana Ugapeva, Ekaterina Barabash, Oksana Ivanova and Anton Pavlushin
Minerals 2024, 14(9), 927; https://doi.org/10.3390/min14090927 - 11 Sep 2024
Viewed by 2537
Abstract
Carbonado is a specific variety of diamonds, typical representatives of which are distributed in the diamond placers of Central Africa, Brazil, and Venezuela. Carbonado consists of the microcrystalline aggregates of diamonds, with inclusions of mineral matter. These aggregates appear as fragments that are [...] Read more.
Carbonado is a specific variety of diamonds, typical representatives of which are distributed in the diamond placers of Central Africa, Brazil, and Venezuela. Carbonado consists of the microcrystalline aggregates of diamonds, with inclusions of mineral matter. These aggregates appear as fragments that are rounded to varying degrees. Carbonado has been known for a long time, but its primary sources have not been found and its genesis remains unclear. We have substantiated the hypothesis that the most probable precursor of carbonado is shungite. Shungite is a specific form of non-crystalline, non-graphitic, fullerene-like carbon. Shungite rocks, currently known in Karelia (Russia), are natural microdispersed composite materials containing shungite—carbonaceous matter and mineral components of different compositions. The content of carbonaceous matter in shungite rocks is from less than 10% to 98%. The carbon isotopic composition of shungite is light ẟ13C from −25‰ to −40‰. The age of shungite rock is more than 2 billion years old, but earlier shungite was probably much more widespread. Known shungite rocks are more than 2 billion years old, but earlier shungite was probably much more widespread. Shungite rocks could recrystallize into diamond rock upon subduction to high pressure and temperature. The diamond rocks could then be exhumed to the Earth’s surface, where they could undergo disruption and reworking with formation of those very fragments that are known as “carbonado”. Full article
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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14 pages, 11301 KiB  
Article
Application of Multiple Geophysical Exploration Methods in the Exploration of Marine Sand Resources in the Northern Offshore Waters of the South China Sea
by Gang Yu, Xichong Hu, Jie Fang, Ying Yang, Yongcong Zhang, Jinhui Lin, Jingyi Liu and Libing Qian
J. Mar. Sci. Eng. 2024, 12(9), 1561; https://doi.org/10.3390/jmse12091561 - 5 Sep 2024
Cited by 2 | Viewed by 1196
Abstract
Marine sand, in addition to oil and gas resources, is the second-largest marine mineral resource. The rational development and utilization of marine sand resources are conducive to the growth of the marine economy. In the process of marketing marine sand in China, local [...] Read more.
Marine sand, in addition to oil and gas resources, is the second-largest marine mineral resource. The rational development and utilization of marine sand resources are conducive to the growth of the marine economy. In the process of marketing marine sand in China, local authorities are required to delineate auctioned sand mining areas after a general survey, commonly referred to as preliminary exploration. Marine sand can be categorized into surface marine sand and buried marine sand. Buried marine sand deposits are buried beneath the sea floor, making it challenging to locate them due to their thin thickness. Consequently, there exist numerous technical difficulties associated with marine sand exploration. We conducted the preliminary research work in the waters off Guangdong Province of the South China Sea, employing a reduced drilling and identifying a potentially extensive deposit of marine sand ore. In this study, various geophysical methods such as sub-bottom profile survey, single-channel seismic survey, and drilling engineering were employed in the northern offshore waters of the South China Sea. As a result, two distinct marine sand bodies were delineated within the study area. Additionally, five reflective interfaces (R1, R2, R3, R4, and R5) were identified from top to bottom. These interfaces can be divided into five seismic sequences: A1, B1, C1, D1, and E1, respectively. Three sets of strata were recognized: the Holocene Marine facies sediment layer (Q4m), the Pleistocene alluvial and pluvial facies sediment layer (Q3al+pl), as well as the Pleistocene Marine facies sedimentary layer (Q3m). In total, two placers containing marine sand have been discovered during this study. We estimated the volume of marine sand and achieved highly favorable results of the concept that we are proposing a geologic exploration approach that does not involve any previous outcropping analogue study. Full article
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23 pages, 48615 KiB  
Article
Precious and Base Metal Minerals in Black Sands of the Egyptian Mediterranean Coast: Mineralogical and Geochemical Attributes
by Abdel-Aal M. Abdel-Karim and Ahmed Gad
Resources 2024, 13(8), 109; https://doi.org/10.3390/resources13080109 - 9 Aug 2024
Cited by 3 | Viewed by 3417
Abstract
This paper investigates the mineralogical and geochemical characteristics, as well as the possible sources, of gold, silver, platinum group elements (PGE), copper, and lead found in the beach sands along Egypt’s Mediterranean coast. Using scanning electron microscopy and electron probe micro-analysis, this study [...] Read more.
This paper investigates the mineralogical and geochemical characteristics, as well as the possible sources, of gold, silver, platinum group elements (PGE), copper, and lead found in the beach sands along Egypt’s Mediterranean coast. Using scanning electron microscopy and electron probe micro-analysis, this study determines the morphology and micro-chemistry of separated grains to assess their economic potential and how various minerals respond to different transport distances. The analysis reveals that gold grains are of high purity (94.11 to 98.55 wt.%; average 96 wt.% Au) and are alloyed with Ag (1.28–2.32 wt.%) and Cu (0.16–3.15 wt.%). Two types of gold grains were identified, indicating differences in transport distances. Variations in morphology, surface features, inclusion types, rims, and chemistry of the native metals, including gold grains, suggest differences in composition, weathering degree, transport distance, deposit types, and host rocks. The average Ag concentration in gold grains (1.86 wt.%) suggests a link to mesothermal or supergene deposits. Most silver, copper, and lead grains are spherical, with some variations in shape. Silver grains have 71.66–95.34 wt.% Ag (avg. 82.67 wt.%). Copper grains have 92.54–98.42 wt.% Cu (avg. 94.22 wt.%). Lead grains contain 74.22–84.45 wt.% Pb (avg. 79.26 wt.%). The identified platinum group minerals (PGM) belong to the Pt–Fe alloys and sperrylite, both of which are PPGE-bearing minerals. These metals likely originate from the weathering of upstream Nile tributaries surrounded by igneous and metamorphic rocks from Ethiopian and Central African regions, with a minor contribution from the Egyptian Eastern Desert Mountains. Full article
(This article belongs to the Special Issue Mineral Resource Management 2023: Assessment, Mining and Processing)
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21 pages, 4180 KiB  
Article
Mineralogical Method as an Effective Way to Predict Gold Ore Types of Deposits in Platform Areas (East of the Siberian Platform)
by Zinaida Nikiforova
Minerals 2024, 14(6), 631; https://doi.org/10.3390/min14060631 - 20 Jun 2024
Cited by 1 | Viewed by 1596
Abstract
The study of the mineralogical and geochemical features of placer gold and the mechanisms of its distribution in the territory east of the Siberian platform, overlain by a thick cover of Mesozoic–Cenozoic deposits, where traditional methods of searching for gold fields are not [...] Read more.
The study of the mineralogical and geochemical features of placer gold and the mechanisms of its distribution in the territory east of the Siberian platform, overlain by a thick cover of Mesozoic–Cenozoic deposits, where traditional methods of searching for gold fields are not effective, allowed researchers, for the first time, to establish the stages of ore formation and to predict the types of gold deposits and their location. The identified indicators of placer gold (morphology, granulometry, chemical composition, micro-inclusions, and internal structures) indicate that ore occurrences in both the Precambrian and Mesozoic stages of ore formation were primary sources of placer gold. The identification of characteristic indicators in placer gold for certain types of gold deposits allowed researchers to prove the formation of gold ore sources east of the Siberian platform for the first time: low-sulfide quartz gold, gold–ferruginous quartzite, gold–copper–porphyry, and gold–platinoid formations are found in the Precambrian stage of ore formation and gold–silver, gold–sulfide–quartz, and gold–rare metal formations are found in the Mesozoic stages of ore formation. Thus, for the first time, based on a huge amount of factual material, it is proved that the mineralogical and geochemical features of placer gold carry enormous information about both the endogenous origin of gold (stages of ore formation—Precambrian and Mesozoic) and the expected type of formation of the predicted deposits. It is established that the predicted type of ore sources corresponds to a certain geological and structural position; this contributes to a more correct selection of methods for searching for ore and placer gold deposits in closed territories and assessing their prospects. In general, the application of the mineralogical method for the first time makes it possible to develop criteria for predicting resources and types of gold deposits, and to assess the prospects of gold mining potential in platform areas at a new level of knowledge. Full article
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29 pages, 26294 KiB  
Article
Minerals of the Au-Cu-Ag System in Grains from the Placers of the Olkhovaya-1 River (Eastern Kamchatka, Russia)
by Galina A. Palyanova, Tatiana V. Beliaeva, Dmitry P. Savelyev and Yurii V. Seryotkin
Minerals 2024, 14(5), 448; https://doi.org/10.3390/min14050448 - 24 Apr 2024
Cited by 3 | Viewed by 1663
Abstract
Heterogeneous grains in the heavy fractions of the placers of the Olkhovaya-1 river (Kamchatka Cape Peninsula, Eastern Kamchatka, Russia) containing Au-Ag, Au-Cu, and Au-Ag-Cu particles were analyzed using scanning electron microscopy, electron-probe microanalysis, and X-ray powder diffractometry. The analyses showed that auricupride dominates [...] Read more.
Heterogeneous grains in the heavy fractions of the placers of the Olkhovaya-1 river (Kamchatka Cape Peninsula, Eastern Kamchatka, Russia) containing Au-Ag, Au-Cu, and Au-Ag-Cu particles were analyzed using scanning electron microscopy, electron-probe microanalysis, and X-ray powder diffractometry. The analyses showed that auricupride dominates in some grains, whereas in others, the main phases are tetra-auricupride or Ag-rich and (or) Au-rich alloys. It was revealed that in the central parts of some grains of exsolution structures, auricupride Cu3.04–2.74Au0.96–1.26 (Ag < 1 wt.%) occurs in intergrowths with low-fineness Ag0.86–0.67Au0.09–0.33Cu<0.05 solid solutions (160‰–480‰), and tetra-auricupride Cu1.12–0.87Au0.88–1.11Ag<0.02 (Ag < 2 wt.%) occurs with higher-fineness Au0.73–0.62Ag0.2–0.38Cu<0.07 solid solutions (520‰–850‰). We also observed, mainly in the peripheral parts of the grains, insignificant amounts of secondary phases that were compositionally similar to Cu2Au, Cu3Au2, Au3Cu2, Au2Cu, Au3Cu, high-fineness gold with impurities of Cu and occasionally Ag (>850‰), and pure gold. In intergrowths with auricupride and tetra-auricupride, we also observed earlier-formed silicates (garnet, pyroxene, chlorite, epidote, titanite), syngenetic sulfides (pyrrhotite, bornite, galena), and later minerals (chalcocite, covellite, anilite, cuprite, goethite, etc.). The XRD analysis of the peripheral parts of some grains showed the presence of auricupride (Cu3Au), tetra-auricupride CuAu (I) and Cu(Au0.92Ag0.08) (II), and gold. The profiles show the absence of peaks of the Au3Cu phase and other Au-Ag-Cu phases identified by the EMPA, which is likely due to their low concentrations in the samples or their structural similarity to gold. It is assumed that the probable source of Au-Ag-Cu mineralization in the Olkhovaya-1 river placers is located in the upper reaches of watercourses that erode the ultrabasic massif of Mounts Soldatskaya and Golaya (Kamchatka Cape Peninsula). Full article
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15 pages, 1713 KiB  
Article
Stability of CO2 Fluid in Eclogitic Mantle Lithosphere: Thermodynamic Calculations
by Yulia G. Vinogradova and Anton Shatskiy
Minerals 2024, 14(4), 403; https://doi.org/10.3390/min14040403 - 15 Apr 2024
Viewed by 1330
Abstract
Findings of solid and liquefied CO2 in diamonds from kimberlites and placers have indicated its presence in the form of a fluid phase in the Earth’s mantle at depths of 150–250 km. However, this is inconsistent with the results of experiments and [...] Read more.
Findings of solid and liquefied CO2 in diamonds from kimberlites and placers have indicated its presence in the form of a fluid phase in the Earth’s mantle at depths of 150–250 km. However, this is inconsistent with the results of experiments and existing thermodynamic calculations. To clarify this, we carried out thermodynamic modeling of garnet–CO2 and bimineral eclogite–CO2 systems using the Perple_X v. 7.1.3 software package, which establishes the most thermodynamically favorable assemblages for a given bulk composition of the system, unlike previous calculations, for which the phase relationships were simply assumed. The key difference between our results and previously known data is the presence of a region of partial carbonation. In this region, the garnet and clinopyroxene of the new compositions, CO2 fluid, carbonates, kyanite, and coesite are in equilibrium. The calculations revealed that unlike endmember systems (pyrope–CO2 and diopside–CO2) in the eclogite–CO2 system, the carbonation and decarbonation lines do not coincide, and the Grt+Cpx+CO2 and Carb+Ky+Coe+Cpx fields are separated by the Grt+Cpx+CO2+Carb+Ky+Coe region, which extends to pressures exceeding 4.3–6.0 GPa at 1050–1200 °C. This should extend the CO2 stability field in the eclogitic mantle to lower temperatures. Yet, owing to the short CO2 supply in the real mantle, the CO2 fluid should be completely spent on the carbonation of eclogite just below the eclogite + CO2 field. Thus, according to the obtained results, the CO2 fluid is stable in the eclogitic mantle in the diamond stability field at temperatures exceeding 1250 °C and pressures of 5–6 GPa. Full article
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15 pages, 6874 KiB  
Article
Formation of Intergrowths of Platinum-Group Minerals and Gold from Magmatogenic Ores in Relation to Phase Changes in Pt-Pd-Fe-Cu-Au System
by Alexander Okrugin and Boris Gerasimov
Minerals 2024, 14(3), 326; https://doi.org/10.3390/min14030326 - 21 Mar 2024
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Abstract
The article discusses the features of the chemical composition and the formation of intergrowths of platinum-group minerals, gold, gold-bearing phases, and other ore minerals present in placers collected from the Anabar River in the northeast part of the Siberian platform. Based on an [...] Read more.
The article discusses the features of the chemical composition and the formation of intergrowths of platinum-group minerals, gold, gold-bearing phases, and other ore minerals present in placers collected from the Anabar River in the northeast part of the Siberian platform. Based on an analysis of changes in the phase compositions of these intergrowths of noble metals with other ore minerals on (Pt, Pd)-Fe-Au and Pd-Cu-Au phase equilibrium diagrams, potential trends in the crystallization of natural polymineral alloys from multicomponent low-sulfide metallic liquids are discussed. The similarity of the microstructures of natural and metallurgical alloys indicates that the formation of natural multiphase Au-PGE intergrowths occurred in a similar manner to the crystallization of multicomponent synthetic alloys. The authors suggest that magmatic Au-PGE mineralization occurs during the crystallization of a noble-metal-containing, low-sulfide, Cr-rich oxide melt separated from silicate mafic–ultramafic magma. Magmatic gold–platinum deposits are commonly associated with sulfide or oxide disseminated-schlieren ores in layered mafic–ultramafic intrusions. However, due to the high solubility of gold and platinoids in sulfide minerals, PGMs in sulfide ores occur as isomorphic impurities or as microphases and dispersed inclusions that cannot form placers. Therefore, the authors suggest that magmatic Au-PGE mineralization occurs during the crystallization of an immiscible low-sulfide, high-Cr oxide liquid separated from silicate mafic–ultramafic magma. In the northeast part of the Siberian platform, potential sources for these placers are likely alkaline, high-Ti mafic–ultramafic intrusions, as confirmed by the presence of silicate inclusions in ferroan platinum similar in composition to melteigite. Full article
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