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32 pages, 4199 KB

Open AccessArticle

Initial-Model-Guided 3D Long-Offset Transient Electromagnetic Inversion for Concealed Vein-Type Ore Body Identification

by Mingyang Gong, Liangjun Yan, Xingbing Xie, Xinyu Wang and Lei Zhou

Minerals 2026, 16(7), 701; https://doi.org/10.3390/min16070701 (registering DOI) - 4 Jul 2026

Ore districts are important targets for deep mineral exploration. As exploration depths increase, exploration strategies are shifting from ore-body identification based mainly on existing metallogenic models toward refined characterization of subsurface structures and deep mineralization targets. Owing to its high sensitivity to low-resistivity [...] Read more.

Ore districts are important targets for deep mineral exploration. As exploration depths increase, exploration strategies are shifting from ore-body identification based mainly on existing metallogenic models toward refined characterization of subsurface structures and deep mineralization targets. Owing to its high sensitivity to low-resistivity targets, the long-offset transient electromagnetic method (LOTEM) is an important geophysical tool for mineral exploration. However, complex geological settings and electromagnetic interference in ore districts often reduce interpretation accuracy and increase inversion non-uniqueness. To address these challenges, this study proposes an initial-model-guided 3D LOTEM inversion strategy. Background resistivity trends and structural information are incorporated into initial-model construction to provide a better-informed starting model before inversion. Taking concealed vein-type ore bodies in the deep part of the Zhaoxian ore district, northwestern Jiaodong, as the target, an approximate 3D geoelectrical model was built by integrating geological data, borehole information, profile interpretations, and electrical-property constraints. Based on this model, LOTEM forward-response characteristics, inversion comparisons, recovery behavior, and quantitative target-identification metrics were analyzed. The results show that the layered alteration background modifies the response of low-resistivity vein-type targets and reduces the distinguishability between ore-body anomalies and background structural responses, thereby affecting inversion recovery. The structure-constrained initial model concentrates model updates near the target zone and significantly improves anomaly recovery. Sensitivity tests further indicate that one-dimensional electrical information mainly constrains the approximate position and resistivity level of the low-resistivity anomaly, whereas layer-interface information improves the spatial correspondence between the recovered anomaly, the layered background, and the ore-body extension direction. When both types of prior information are integrated, ore-body recovery is significantly enhanced. Under the synthetic-model conditions considered in this study, the initial-model optimization strategy improves the recovery of concealed vein-type low-resistivity targets in a layered alteration background and may inform subsequent LOTEM survey design and inversion interpretation in the Zhaoxian ore district. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

29 pages, 17021 KB

Open AccessArticle

Integrated LIBS-EPMA and Multivariate Statistical Analysis for Ge-Bearing Mineral Characterization: A Tool for High-Tech Critical Metals Exploration

by Nicolas Afanassieff, Emilie Janots, Octave Reignier, Vincent Motto-Ros, Valentina Batanova, Dennis Lahondès, Etienne Le Goff, Jérémie Melleton and Bénédicte Cenki

Minerals 2026, 16(7), 685; https://doi.org/10.3390/min16070685 - 29 Jun 2026

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Abstract

Germanium (Ge) is a high-tech critical metal typically hosted at trace levels in sphalerite, making its detection and characterization challenging in both primary ores and mine residues. This study presents a multi-scale analytical workflow combining laser-induced breakdown spectroscopy (LIBS), electron probe micro-analysis (EPMA), [...] Read more.

Germanium (Ge) is a high-tech critical metal typically hosted at trace levels in sphalerite, making its detection and characterization challenging in both primary ores and mine residues. This study presents a multi-scale analytical workflow combining laser-induced breakdown spectroscopy (LIBS), electron probe micro-analysis (EPMA), and multivariate statistics to detect, map and quantify Ge distribution in a representative Pb-Zn sample from the Les Malines deposit (France). µ-LIBS mapping enables rapid centimeter-scale screening at 15 µm resolution and identifies Ge-bearing domains over large areas, which are subsequently investigated at micrometer scale using EPMA chemical mapping and quantitative analyses. Results reveal a strong µm-scale heterogeneity of Ge distribution within sphalerite, with Ge systematically concentrated in an Fe-rich intermediate zonation associated with prismatic growth textures, while Cu/Cd/Ag are enriched in distinct collomorph domains. Multivariate statistical analyses (correlation matrices and PCA) confirm a strong geochemical structuring opposing an Fe/Ge association against a Cu/Cd/Ag pole. These findings demonstrate that Ge incorporation is controlled by localized growth conditions rather than bulk composition. The proposed workflow provides an efficient and scalable framework for exploration, enabling rapid targeting of critical metal enrichments and supporting their extension to multiple mineralization stages, Pb-Zn deposits, and other high-tech critical metals (HTCMs) such as Ga and In. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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24 pages, 8798 KB

Open AccessArticle

Chemical Changes in Quartz and Micas During Greisenization: Examples from European Variscan Plutons

by Karel Breiter

Minerals 2026, 16(6), 626; https://doi.org/10.3390/min16060626 - 11 Jun 2026

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Abstract

Metals like Li, Sn, W, Nb and Ta accumulate mostly during the magmatic–hydrothermal transition and subsequent hydrothermal alteration of highly fractionated granites, especially greisenization. Evaluation of about 450 bulk-rock analyses, 1500 LA-ICP-MS analyses of quartz and 1600 EPMA and LA-ICP-MS analyses of mica [...] Read more.

Metals like Li, Sn, W, Nb and Ta accumulate mostly during the magmatic–hydrothermal transition and subsequent hydrothermal alteration of highly fractionated granites, especially greisenization. Evaluation of about 450 bulk-rock analyses, 1500 LA-ICP-MS analyses of quartz and 1600 EPMA and LA-ICP-MS analyses of mica from parental granites and related greisens and quartz–mica veins from four typical areas of European Variscan granite plutons with greisen mineralization (Beauvoir, France; Panasqueira, Portugal; and Cínovec and Nejdek, Erzgebirge, Czech Republic) illustrate diversity in initial magma composition (S- vs. A-types), in style of greisenization (pervasive greisenization in granite cupolas vs. vein-like greisen strings along joints), and in chemical evolution of quartz and micas during magmatic–hydrothermal transition. The contents of all monitored elements in quartz and mica from greisen and veins are of very high variability, with principal differences among studied localities. Generally, very low contents of Al (<100 ppm), Ti (<1 ppm) and Li (<10 ppm) or, on the contrary, extremely high contents of Al (>1000 ppm) or Li (>100 ppm) in quartz may indicate its hydrothermal origin. Contents of Sn, W, Nb, and Ta in micas tend to become depleted during greisenization, this trend is more pronounced in Nb and Ta than in Sn and W. Transition from magmatic to hydrothermal crystallization leads to an increase in the Ta/Nb values in mica: from 0.20 to 0.24 in S-type magmatic systems, and from 0.13 to 0.34 at Cínovec as a representative of A-type granites. Whether granite belongs to the S- or A-type is not essential for the development of greisenization. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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25 pages, 25509 KB

Open AccessArticle

Mineralogical and Geochemical Features of Sulphide Mineralization: A Comparative Study of Pb-Zn Deposits in the Laki Ore District, Central Rhodopes, Bulgaria

by Georgi Milenkov, Sylvina Georgieva, Rossitsa D. Vassileva, Yana Georgieva and Elitsa Stefanova

Minerals 2026, 16(6), 616; https://doi.org/10.3390/min16060616 - 8 Jun 2026

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Abstract

The Djurkovo and Govedarnika deposits represent hydrothermal Pb-Zn systems spatially associated with the Eocene–Oligocene tectono-magmatic evolution of the Rhodope Metamorphic Complex. This study presents new mineralogical and geochemical data for galena, sphalerite, pyrite, and chalcopyrite obtained by electron probe microanalysis (EPMA) and LA-ICP-MS [...] Read more.

The Djurkovo and Govedarnika deposits represent hydrothermal Pb-Zn systems spatially associated with the Eocene–Oligocene tectono-magmatic evolution of the Rhodope Metamorphic Complex. This study presents new mineralogical and geochemical data for galena, sphalerite, pyrite, and chalcopyrite obtained by electron probe microanalysis (EPMA) and LA-ICP-MS in order to evaluate the compositional variations of sulphides among the vein and metasomatic mineralization types and between the two deposits. The analysed sulphides exhibit distinct compositional signatures reflecting the different mineralization stages and hydrothermal environments. Sphalerite from the Govedarnika metasomatic ores is enriched in Mn (up to 5200 ppm), Fe (up to 5.13 wt.%) and Co due to interaction with Mn-rich skarn assemblages, whereas Djurkovo sphalerite shows elevated Cd (up to 3000 ppm), In and Hg concentrations. Trace-element systematics indicate coupled Fe-Mn incorporation, competitive Cd-Fe substitution and local re-equilibration processes associated with “chalcopyrite disease” textures. Late pyrite from the quartz-carbonate stage is enriched in As (up to 3.87 wt.%), Au (up to 78 ppm), Ag, Se, Sb and Tl, with positive Au-As and Au-Ag correlations suggesting invisible gold and possible submicroscopic precious-metal inclusions. The obtained data demonstrate prolonged hydrothermal evolution and highlight the potential role of the studied sulphides as concentrators of economically important elements. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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25 pages, 13505 KB

Open AccessArticle

Mineralogical Characterization of Rare Earth Element (REE) Enrichment Within Regolith Overlying the Doradilla Tin-Skarn System, NSW, Australia

by Rory Carter, Ian Graham, David French, Indrani Mukherjee, Mathias Kapo, Karen Privat, Simon Hager, Huixin Wang, Benjamin P. Wade and Oliver Davies

Minerals 2026, 16(6), 612; https://doi.org/10.3390/min16060612 - 8 Jun 2026

Viewed by 329

Abstract

This study presents the first detailed mineralogical characterization of rare earth element (REE) enrichment (up to 1.39 wt% TREO (total rare earth oxide)) within the regolith overlying the Doradilla tin-skarn prospect, northern New South Wales, Australia. The REE mineralogy was investigated using petrography, [...] Read more.

This study presents the first detailed mineralogical characterization of rare earth element (REE) enrichment (up to 1.39 wt% TREO (total rare earth oxide)) within the regolith overlying the Doradilla tin-skarn prospect, northern New South Wales, Australia. The REE mineralogy was investigated using petrography, field-emission scanning electron microscopy (FE-SEM), electron microprobe analysis (EPMA), micro-X-ray fluorescence (µ-XRF) elemental mapping, and laser Raman spectroscopy. Hydrated REE phosphate minerals are identified as the dominant REE hosts within the regolith. Raman spectroscopy confirms the presence of water in both rhabdophane and churchite. Three compositionally distinct rhabdophane populations are identified, including one La-dominant and two Y-rich groups, whereas churchite-(Y) represents the only HREE-enriched phase identified. The predominance of hydrated REE phosphates within the regolith highlights the importance of secondary phosphate minerals as major REE hosts within the Doradilla profile. This study emphasizes the need for detailed mineral characterization when studying REEs in previously undescribed settings. Furthermore, the results demonstrate that regolith developed over tin-skarn protoliths may host compositionally diverse and REE-enriched phosphate assemblages. With growing global demand for REEs, a greater understanding of REE mineralization in underexplored geological settings is increasingly important. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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19 pages, 15723 KB

Open AccessArticle

Petrology and Geochemistry of Scandium in Wailukum Ni Laterites, East Halmahera, Indonesia

by Abdul Bari, Mega Fatimah Rosana, Euis Tintin Yuningsih, Ade Kadarusman, Rubima Aisha Yulman, Muhammad Chandra R. M. and Thaha Rizal Ulhaque

Minerals 2026, 16(2), 222; https://doi.org/10.3390/min16020222 - 22 Feb 2026

Cited by 1 | Viewed by 894

Abstract

The Wailukum area in North Maluku Province, Indonesia, is an ultramafic rock complex with a high degree of serpentinization. The mineral composition of ultramafic and mafic rocks strongly influences the distribution and enrichment of scandium (Sc) during lateritization. In this study, we aim [...] Read more.

The Wailukum area in North Maluku Province, Indonesia, is an ultramafic rock complex with a high degree of serpentinization. The mineral composition of ultramafic and mafic rocks strongly influences the distribution and enrichment of scandium (Sc) during lateritization. In this study, we aim to analyze three types of geological materials in a lateritic profile that contains Sc, specifically bedrock, saprolite, and limonite, in terms of element distribution, mineral composition, and rock identification. We used the analytical methods of petrography, X-ray diffraction (XRD), X-Ray Fluorescence (XRF), and Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP-OES). The results show that Sc in bedrock is mainly associated with clinopyroxene minerals such as augite and diopside. In saprolite, Sc content decreases due to higher mobility but remains partly associated with clinopyroxene, and in limonite zone, Sc reaches maximum enrichment. Among rock types, gabbro contains the highest absolute Sc concentration (23.25 ppm in bedrock and up to 58.5 ppm in limonite), while wehrlite records the greatest enrichment ratio, with a 9.18-fold increase from bedrock to limonite. By contrast, gabbro shows the lowest enrichment ratio (2.52-fold) despite its high initial Sc content. These patterns indicate that Sc enrichment is controlled by clinopyroxene as the primary host in bedrock, affecting its relative stability during weathering. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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21 pages, 4280 KB

Open AccessArticle

Geochemical and Textural Features of Apatites from Propylitic to Advanced Argillic Hydrothermal Alteration Zones in the Sharlo Dere Area, Chelopech Cu-Au Deposit, Bulgaria

by Radoslav Kalchev, Irena Peytcheva, David Chew, Atanas Hikov and Elitsa Stefanova

Minerals 2026, 16(2), 150; https://doi.org/10.3390/min16020150 - 29 Jan 2026

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Abstract

Apatite is a widespread accessory mineral, which can provide information on the geochemical characteristics of magma and the conditions of hydrothermal alteration of the rocks in magmatic–hydrothermal deposits. This study aims to understand the relationships between the geochemical and textural features of apatites [...] Read more.

Apatite is a widespread accessory mineral, which can provide information on the geochemical characteristics of magma and the conditions of hydrothermal alteration of the rocks in magmatic–hydrothermal deposits. This study aims to understand the relationships between the geochemical and textural features of apatites from diorite porphyries that have undergone different degrees of hydrothermal alteration in the Sharlo Dere area, Chelopech epithermal Cu-Au deposit, Bulgaria. The apatites were characterized by laser ablation–inductively coupled plasma mass spectrometry, scanning electron microscopy with energy-dispersive X-ray spectroscopy, electron probe microanalysis with wave-dispersive spectroscopy, optical cathodoluminescence and multi-element mapping. Magmatic apatites from “hematitic”, propylitic and propylitic-sericitic zones of alteration are distinguished by euhedral crystals with oscillatory zoning and brown luminescence in CL images. In quartz-sericitic alteration zones, apatite has a yellow CL response. Hydrothermally altered apatites in the diorite porphyries overprinted by advanced argillic alteration have corroded, irregular forms and pink-green luminescence. Apatite crystals of magmatic origin reveal high contents of chlorine, strontium, light rare earth elements (LREE), negative Eu anomalies and high La_N/Sm_N and Ce_N/Yb_N ratios. Hydrothermally altered or hydrothermal apatites are distinguished by their higher contents of Na₂O, F, SO₃, Y and middle rare earth elements (MREEs) and their low La_N/Sm_N and Ce_N/Yb_N ratios. The intensity of hydrothermal alteration affects the luminescence and major and trace element contents, including the rare earth element patterns in the apatites, implying apatite can be used as a geochemical indicator to study magmatic–hydrothermal ore deposits. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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34 pages, 10510 KB

Open AccessArticle

Evolutionary Model of the Sepid-Sarve Manto-Type Copper Mineralization, Doruneh Fault Volcanic-Plutonic Belt (Central Iran Domain, NE Iran): An Integrated Geological, Geochemical, Fluid-Inclusion and Stable O–S Isotope Study

by Morteza Esform, Hasan Zamanian, Urs Klötzli, Alireza Zarasvandi, Alireza Almasi and Mohammad Goudarzi

Minerals 2025, 15(12), 1246; https://doi.org/10.3390/min15121246 - 25 Nov 2025

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Abstract

The Sepid-Sarve copper deposit is part of an Eocene volcano-sedimentary sequence located in the southern Sabzevar Zone. The copper mineralization occurs at the contact between pyroclastic and lava units with various limestone layers (including marly, Nummulitic, sandy, and clastic limestones). The ore minerals [...] Read more.

The Sepid-Sarve copper deposit is part of an Eocene volcano-sedimentary sequence located in the southern Sabzevar Zone. The copper mineralization occurs at the contact between pyroclastic and lava units with various limestone layers (including marly, Nummulitic, sandy, and clastic limestones). The ore minerals consist of malachite, azurite, chalcocite, digenite, cuprite, tenorite, covellite, and occasionally native copper. The associated hydrothermal fluids show moderate to high salinities, ranging from 3.08 to 13.38 wt.% NaCl equivalent, with homogenization temperatures between 90 and 356 °C, indicating fluid mixing during ore formation. Chalcocite is rarely accompanied by quartz, suggesting low silica content in the ore-forming fluids. The δ³⁴S values of sulfide samples from the Sepid-Sarve deposit range from −23.9 ± 0.3‰ to −2.9 ± 0.2‰, while δ³⁴S values of hydrothermal H₂S range from −24.1 ± 0.3‰ to −21.0 ± 0.3‰. The δ¹⁸O values of hydrothermal fluids associated with mineralization fall within the range of basaltic rocks, meteoric waters, and sedimentary rocks. Geochemical variations in major and trace elements suggest the involvement of continental crustal contamination in the magmatic evolution. The studied volcanic rocks fall within the calc-alkaline to shoshonitic fields, formed in a continental arc setting, and are derived from an enriched mantle source influenced by subduction-related fluids. These rocks are characterized by HREE depletion, moderate LREE enrichment, and a weak negative Eu anomaly. Based on the results, the Sepid-Sarve deposit is classified as a stratabound (Manto-type) copper sulfide deposit, formed in a volcano-sedimentary setting associated with a subduction-related magmatic arc environment. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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29 pages, 9237 KB

Open AccessArticle

Estimating Content of Rare Earth Elements in Marine Sediments Using Hyperspectral Technology: Experiment and Method Series

by Dalong Liu, Shijuan Yan, Gang Yang, Jun Ye, Chunhui Yuan, Mu Huang, Yiping Luo, Yue Hao, Yuxue Zhang, Xiaofeng Liu, Xiangwen Ren, Zhihua Chen and Dewen Du

Minerals 2025, 15(11), 1102; https://doi.org/10.3390/min15111102 - 23 Oct 2025

Viewed by 1071

Abstract

Marine sediments enriched with rare earth elements (REEs) serve as a significant reservoir, particularly for heavy REEs. Conventional lab-based REE exploration restricts rapid and large-scale assessment, whereas hyperspectral imaging provides a promising approach for quantitative evaluation. This study evaluates the capacity of hyperspectral [...] Read more.

Marine sediments enriched with rare earth elements (REEs) serve as a significant reservoir, particularly for heavy REEs. Conventional lab-based REE exploration restricts rapid and large-scale assessment, whereas hyperspectral imaging provides a promising approach for quantitative evaluation. This study evaluates the capacity of hyperspectral data for the quantitative determination of REEs in marine sediments. A total of 53 samples from various locations were analyzed to determine their chemical composition and spectral characteristics within the 380–1000 nm range under natural light. The influence of surface conditions on spectral integrity was evaluated, and multiple preprocessing and spectral feature extraction methods were applied to enhance data reliability. This study proposes a novel approach, termed Feature Importance within Pearson Correlation Coefficient-Based High-Correlation Spectral Range (PCCR-FI), designed for the identification of characteristic spectral bands associated with REEs. Machine learning models were subsequently constructed to estimate REE concentrations, and the following key findings were observed: (a) technical adjustments effectively addressed variations in sediment surface conditions, ensuring data reliability. (b) The PCCR-FI technique identified characteristic REEs spectral bands, enhancing processing efficiency and prediction accuracy. (c) The integration of the reciprocal logarithmic first derivative (TLOG-FD) technique with a multilayer perceptron (MLP) model, termed TLOG-FD-MLP, efficiently captured critical spectral features, resulting in improved prediction accuracy. For light REEs, the model achieved coefficient of determination (R²) values exceeding 0.60 and relative performance deviation (RPD) values exceeding 1.60, with some elements demonstrating R² values as high as 0.81 with RPD values surpassing 2.00. Furthermore, several heavy REEs exhibited moderate prediction performance, with R² values consistently exceeding 0.60. When considering the total REE content, an R² of 0.73 and an RPD of 1.97 were achieved. These findings demonstrate the use of hyperspectral imaging as a viable tool for quantitative evaluation of REE concentrations in marine sediments, providing valuable guidance for resource mapping and the exploration of seafloor polymetallic deposits. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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15 pages, 10364 KB

Open AccessArticle

Reconstruction of Ancient Carboniferous Zhibo Volcanic Edifices in Western China Using Magnetotelluric Observations and Comparisons with Active Volcanoes

by Lanfang He, Ping Shen, Zhongxing Wang, Xi Zhang and Song Huang

Minerals 2025, 15(10), 1089; https://doi.org/10.3390/min15101089 - 19 Oct 2025

Cited by 3 | Viewed by 1077

Abstract

Volcanoes serve as the primary pathways for heat and material transfer from Earth’s interior to its surface, providing valuable insights into subsurface processes. Active and potentially active volcanoes have influenced human history and are closely related to current tectonic activity. Consequently, many active [...] Read more.

Volcanoes serve as the primary pathways for heat and material transfer from Earth’s interior to its surface, providing valuable insights into subsurface processes. Active and potentially active volcanoes have influenced human history and are closely related to current tectonic activity. Consequently, many active volcanoes have been studied using geophysical methods. However, the internal structure of ancient volcano complexes remains poorly understood. We investigated ancient volcano complexes by comparing magnetotelluric (MT) observations from Zhibo (ZB) ancient volcano with active mid-oceanic ridge volcanoes from Iceland and intracontinental volcanoes from north China. The MT responses of magma chambers in these active volcanoes showed similar low-resistivity values ranging from several to tens of Ω·m, indicating a comparable resistivity of the active magma. Assuming that the ancient active volcano chambers had a similar resistivity to that of current active volcanoes, we reconstructed the ancient Carboniferous volcano complex in ZB using the ratio of the lower portion of the MT responses from ZB ancient volcanic edifices and active volcanoes. The results implied the existence of fossil magma chambers at a depth of 5 to 7 km marking the site of a former volcanic center. This finding supports the magmatic origin of the ZB volcanic rock-hosted iron deposits. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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33 pages, 8120 KB

Open AccessArticle

Origin of the World-Class Eagle, Eagle East, and Tamarack Ni-Cu-PGE Deposits

by Robert Nowak, Chad Deering and Espree Essig

Minerals 2025, 15(8), 871; https://doi.org/10.3390/min15080871 - 18 Aug 2025

Viewed by 2542

Abstract

The 1.1 Ga Mesoproterozoic Midcontinent rift hosts the Eagle, Eagle East, and Tamarack Ni-Cu-PGE deposits and Embayment Prospect. These deposits are hosted by ultramafic igneous rocks and have some of the highest Ni-Cu grades on Earth. We use new bulk-rock data and published [...] Read more.

The 1.1 Ga Mesoproterozoic Midcontinent rift hosts the Eagle, Eagle East, and Tamarack Ni-Cu-PGE deposits and Embayment Prospect. These deposits are hosted by ultramafic igneous rocks and have some of the highest Ni-Cu grades on Earth. We use new bulk-rock data and published datasets (bulk-rock, mineral chemistry, and isotopic analyses) to examine major, minor, and trace element trends of both Midcontinent rift-related alkaline and tholeiitic intrusions. In addition, we compare the geochemical data to local kimberlite-hosted lower-crustal xenoliths and local igneous (Archean) and sedimentary (Paleoproterozoic) country rocks. We found the peridotite magma compositions dominantly consist of primitive mantle compositions with varying abundances of subduction-related components, alkaline-transitional melts, and local country rock contaminates (e.g., Baraga and Animikie Basin sediments). The subduction-related components are interpreted to be derived from previous Archean and Paleoproterozoic subduction events and likely hosted within the sub-continental lithospheric mantle. Importantly, these subduction-related components are also interpreted to have acted as oxidizing agents within the melt, stabilizing sulfate (+2 FMQ (fayalite–magnetite–quartz) to FMQ) while inhibiting sulfide crystallization as the magma ascended through ~50 km of the Superior craton. This study largely corroborates the previous findings with respect to the contribution of local country rock contamination to the Eagle–Tamarack peridotite host rocks, which is estimated to be minimal (<5%). However, the incorporation of <5% reductive pelitic siltstone contamination results in strong shifts in the oxygen fugacity of the peridotite melt, from +2 FMQ to slightly below FMQ, as determined from spinel Fe³⁺/∑Fe ratios. This shift in oxygen fugacity resulted in the transition from total sulfate (+2 FMQ) to sulfate + sulfide (<+2 FMQ to FMQ) to total sulfide (<FMQ). This shift in oxygen fugacity is a key contributor to the formation of Ni-Cu-PGE-rich massive sulfides within the Eagle peridotite. This study presents an expanded geochemical interpretation for the exploration of Midcontinent rift-related Ni-Cu-PGE deposits to include peridotites with subduction-like signatures and contaminated via <5% reductive sedimentary country rocks. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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59 pages, 14350 KB

Open AccessReview

REE Mineralogical Evolution in a F-Rich Peralkaline System: A Review on the REE Mineralization Associated with the Madeira Sn-Nb-Ta-Cryolite (REE, U, Th, Zr, Li) Deposit (Amazonas, Brazil)

by Artur C. Bastos Neto, Ingrid W. Hadlich, Harald G. Dill and Vitor P. Pereira

Minerals 2026, 16(4), 417; https://doi.org/10.3390/min16040417 - 17 Apr 2026

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Abstract

This study is centered on REE distribution in several minerals exhibiting exceptionally rare mineralogical and chemical compositions in the 1.8 Ga Madeira albite-enriched granite (AEG). This is a peralkaline A-type granite and corresponds to the Madeira Sn-Nb-Ta-cryolite (REE, Th, U, Zr, Li) world-class [...] Read more.

This study is centered on REE distribution in several minerals exhibiting exceptionally rare mineralogical and chemical compositions in the 1.8 Ga Madeira albite-enriched granite (AEG). This is a peralkaline A-type granite and corresponds to the Madeira Sn-Nb-Ta-cryolite (REE, Th, U, Zr, Li) world-class deposit (195 Mt) (Amazonas, Brazil). The REE mineralization ranks among the major deposits associated with alkaline and peralkaline magmatism in intracontinental and extensional anorogenic environments in terms of tonnage and grades. However, with respect to REE paragenesis and structure, it differs from all other known REE deposits. The REE mineralization (xenotime, gagarinite, fluocerite, thorite, pyrochlore, zircon, fluorite, and cryolite) is disseminated and zoned. In addition, in the central part of the deposit, there is a massive hydrothermal cryolite body, whose feasibility for REE extracting has been demonstrated. The evolution of rare earth minerals followed a precise order, with minimal formation of compound minerals and minerals with compositions distinct from their typical occurrences. Small pegmatites very rich in xenotime and gagarinite occur in the core AEG. These characteristics are due to the very high F activity in the magma, buffered by cryolite crystallization, to progressive, undisturbed crystallization from the margins toward the center, and to minimal CO₂ activity. The alteration of primary REE minerals by F-rich hydrothermal fluids, the origin of these fluids, and the formation of secondary REE minerals are also discussed. Full article

(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)

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