The Unconventional Peridotite-Related Mg-Fe-B Skarn of the El Robledal, SE Spain

Citation: González-Pérez, I.; Fanlo, I.; Ares, G.; Gervilla, F.; González-Jiménez, J.M.; Acosta-Vigil, A.; Arranz, E. The Unconventional Peridotite-Related Mg-Fe-B Skarn of the El Robledal, SE Spain. Minerals 2023,13, 300. https://doi.org/ 10.3390/min13030300 Academic Editor: Evgeny Galuskin Received: 11 January 2023 Revised: 30 January 2023 Accepted: 1 February 2023 Published: 21 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article The Unconventional Peridotite-Related Mg-Fe-B Skarn of the El Robledal, SE Spain Igor González-Pérez 1,* , Isabel Fanlo 2, Gonzalo Ares 3, Fernando Gervilla 1,4 , JoséMaría González-Jiménez 4, Antonio Acosta-Vigil 4and Enrique Arranz 2 1Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18002 Granada, Spain 2Departamento de Ciencias de la Tierra, Cristalografía y Mineralogía, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain 3Instituto de Geociencias (IGEO, CSIC-UCM), c/Severo Ochoa, 7, 28040 Madrid, Spain 4 Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC-UGR, Avda. de las Palmeras 4, 18100 Armilla, Spain *Correspondence: [email protected] Abstract: The El Robledal deposit is a Mg-Fe-B skarn hosted in a dismembered block from the footwall contact of the Ronda orogenic peridotites in the westernmost part of the Betic Cordillera. The skarn is subdivided into two different zones according to the dominant ore mineral assemblage: (1) the ludwigite–magnetite zone, hosted in a completely mineralized body along with metasomatic forsterite, and (2) the magnetite–szaibelyite zone hosted in dolomitic marbles. In the ludwigite–magnetite zone, the massive mineralization comprises ludwigite (Mg 2 Fe 3+ (BO 3 )O 2 ), Mgrich magnetite, and magnetite, with minor amounts of kotoite (Mg 3 (BO 3 ) 2 ), szaibelyite (MgBO 2 (OH)), accessory schoenfliesite (MgSn 4+ (OH) 6 ), and pentlandite. The ratio of ludwigite–magnetite decreases downwards in the stratigraphy of this zone. In contrast, the mineralization in the magnetite– szaibelyite zone is mainly composed of irregular and folded magnetite pods and bands with pull-apart fractures, locally associated with a brucite-, szaibelyite-, and serpentine-rich groundmass. The set of inclusions identified within these ore minerals, using a combination of a focused ion beam (FIB) and high-resolution transmission electron microscope (HRTEM), supports the proposed evolution of the system and reactions of the mineral formation of the skarn. The analysis of the microstructures of the ores by means of electron backscatter diffraction (EBSD) allowed for the determination that the ores experienced ductile deformation followed by variable degrees of recrystallization and annealing. We propose a new classification of the deposit as well as a plausible genetic model in a deposit where the heat source and the ore-fluid source are decoupled. Keywords: boron; Mg-skarn; Ronda Peridotites; EBSD; crustal emplacement 1. Introduction To date, over 250 boron-bearing minerals have been identified, which may be divided into three broad groups according to their origin and geological environments: (1) Mgbearing borates intimately associated with silicates and iron oxides in magnesian skarn (hereafter Mg-skarn) typically related to intermediate calk-alkaline intrusive igneous rocks; (2) Mg-bearing oxides, hosted by marine evaporitic sediments; and (3) Naand Ca-bearing borate hydrates associated with lacustrine (playa lake) sediments and explosive volcanic activity [ 1 ]. Mg-skarn deposits hosting boron mineralizations are spread worldwide [ 2 – 4 ], and they are of particular interest, as they are an economically reliable source not only for B but also for Fe and Sn [ 3 ]. A common feature of all of these types of deposits is their characteristic mineralogical zoning due to the fact of fluid-related metasomatism at varying fluid and/or melt/rock ratios and temperatures [ 5 ]. B-bearing Mg-skarns are usually characterized by the occurrence of mineralogical zoning that from the intrusion outwards are represented by (1) proximal magnesian silicates (e.g., diopside, forsterite, and spinel), Minerals 2023,13, 300. https://doi.org/10.3390/min13030300 https://www.mdpi.com/journal/minerals Minerals 2023,13, 300 2 of 28 (2) magnetite and magnesian borates ores, and (3) periclase or monticellite marble in direct contact with Mg-rich carbonate protolith [ 6 ]. These mineral assemblages are accompanied by their resulting altered minerals (i.e., humite, chondrodite, serpentine, phlogopite, calcite, and brucite). Thus, endogenous borates in Mg-skarn deposits are systematically developed on the outermost part of the contact aureole formed at the contact between igneous rocks and dolomite protolith [2,6–14]. Broadly speaking, B-bearing Mg-skarns are classified into four broad categories based on the prevailing borate mineralogy [ 14 ]: (1) type (Mg-B) is Fe-poor, containing basically kotoite (Mg 3 (BO 3 ) 2 ), suanite Mg 2 ((B 2 O 5 )), and szaibelyite (Mg 2 BO 2 (OH)); (2) type (MgFe-B) is Fe-rich, with ludwigite (Mg 2 Fe 3+ (BO 3 )O 2 ) associated with magnetite; (3) type (Mg-Ca-B-CO 2 ) includes carbonate-bearing Ca-Mg borates; and (4) type (Mg-Ca-B) is rare and contains kurchatovite or clinokurchatovite (Ca(Mg, Mn, Fe 2+ )B 2 O 5 ) and develops at high pressures. In this paper, we report the first detailed mineralogical characterization of the borate mineralization from the El Robledal Mg-Fe-B-skarn deposit in SW Spain. The singularity of this deposit is that it is hosted in dolomitic marbles of the contact aureole produced by the crustal emplacement of the Ronda Peridotites, the largest outcrops of Subcontinental Lithospheric Mantle peridotites exposed on the Earth’s surface ([ 15 ] and references therein). This is a very uncommon geological framework for the formation of a Mg-Fe-B-skarn deposit, where the source of heat (i.e., Ronda Peridotites) and fluid (migmatites) are apparently decoupled, unlike conventional Mg-Fe-B-skarn deposits, where heat and fluids come from intrusive igneous rocks. This offers an opportunity to better constrain the physicochemical conditions of this type of mineral deposit. We sought to unravel the potential role played by country rocks (i.e., Ronda Peridotites, migmatites, and related leucogranites) for the generation of mineralizing fluids responsible for the formation of the skarn. To achieve these targets, we provide and discuss the micro-analytical data obtained using a combination of techniques including (1) high-resolution scanning electron microscopy (HRSEM), (2) an electron probe micro-analyzer (EPMA), (3) electron back-scattered diffraction (EBSD), and (4) a focused ion beam (FIB) combined with high-resolution transmission electron microscopy (HRTEM). These data were integrated into an innovative model that links the genesis of this B-rich skarn deposit to fluids released from the anatexis and dehydration of metasediments during the hot emplacement of the mantle peridotites into the continental crust. 2. Geological Setting The El Robledal deposit is located in the southwestern area of the Sierra de Las Nieves, in the western part of the Betic Cordillera, southern Spain, ~22 km northwestwards from the city of Marbella (Figure 1). The Betic Cordillera along with the Rif in north Morocco constitute an arched alpine orogenic belt formed during the N–S to NW–SE convergence of the Eurasian and African plates along with the westward migration of the Alborán domain [ 16 – 19 ]. Three different domains comprise the geology of the Betic Cordillera: External Domain, Internal Domain, and Flysch (Figure 1inset). The External Domain is in the north of the belt and corresponds to Mesozoic to Cenozoic sedimentary rocks. The Internal Domain exposes three stacked tectonic complexes that from bottom to top are the Nevado-Filábride Complex, the Alpujárride Complex, and the Maláguide Complex. They consist of a variable degree of metamorphized Paleozoic to Mesozoic sediments deposited between the Iberia and Africa plates. The Flysch domain is sandwiched between the aforementioned domains, representing Cretaceous to Miocene turbiditic sediments [ 20 ]. The Alpujárride complex in its westernmost part comprises two main units: Los Reales Unit and Blanca Unit. The Los Reales Unit is the uppermost unit including in its lower portion a ca. 4 km thick slice of a subcontinental lithospheric mantle (SCLM)—the Ronda Peridotites—and an overlying <5 km thick crustal sequence (Balanyáet al. [ 21 ], identified as the Jubrique group). Minerals 2023,13, 300 3 of 28 Minerals 2023, 13, x FOR PEER REVIEW 3 of 29 the Ronda Peridotites—and an overlying <5 km thick crustal sequence (Balanyá et al. [21], identified as the Jubrique group). Figure 1. Geological map of the western Betic Cordillera, southern Spain showing the three main peridotite massifs cropping out in the Betic Cordillera: Ronda (~300 km2), Ojén (~70 km2), and Carratraca (~60 km2). Modified from Gervilla et al. [15]. The red rectangle marks the localization of the El Robledal area. Inset: different domains of the Betic Cordilleras. The Ronda Peridotites are the larger known exposure (~450 km2) of the SCLM peridotites on Earth [22]. Altogether they are portions of a Proterozoic SCLM (1.2–1.8 Ga) now cropping out in three main ultramafic massifs: Ronda (~300 km2), Ojén (~70 km2), and Carratraca (~60 km2) [23–26]. These peridotites are sandwiched between crustal sequences (i.e., Jubrique group at the top and Blanca Unit at the bottom), which show an increasing metamorphic grade towards the mantle rocks, forming mylonitic bands at the contacts [27]. Large blocks from the top of the Blanca Unit are commonly embedded in the mylonitic band at the contact with the Ronda Peridotites [28]. The El Robledal deposit is hosted by one of these blocks, similar to other skarn-related deposits widespread along the Ronda Peridotites–Blanca Unit contact [29,30]. The rocks from the crustal sequence of the Jubrique group range from garnet-bearing granulitic gneisses (i.e., kinzigites; 12–14 kbar and 850 °C) in contact with the Ronda Peridotites to metapelites, marbles and low-grade phyllites (7–8 kbar and 200–350 °C) at the top of the sequence [31–33]. These rocks represent the pre-Mesozoic continental crust that was extremely thinned during Alpine Orogeny ([19] and references therein), although some authors suggested that there was a previous event of orogenic accretion during the Hercynian [34–36]. The Blanca Unit is composed of high-grade metapelites and migmatites underlying a Middle to Upper Triassic carbonate-rich metasedimentary sequence [37,38]. This unit includes several low-pressure and mediumto coarse-grained diatexites and metatexites Figure 1. Geological map of the western Betic Cordillera, southern Spain showing the three main peridotite massifs cropping out in the Betic Cordillera: Ronda (~300 km 2 ), Ojén (~70 km 2 ), and Carratraca (~60 km2). Modified from Gervilla et al. [15]. The red rectangle marks the localization of the El Robledal area. Inset: different domains of the Betic Cordilleras. The Ronda Peridotites are the larger known exposure (~450 km 2 ) of the SCLM peridotites on Earth [ 22 ]. Altogether they are portions of a Proterozoic SCLM (1.2–1.8 Ga) now cropping out in three main ultramafic massifs: Ronda (~300 km 2 ), Ojén (~70 km 2 ), and Carratraca (~60 km 2 ) [ 23 – 26 ]. These peridotites are sandwiched between crustal sequences (i.e., Jubrique group at the top and Blanca Unit at the bottom), which show an increasing metamorphic grade towards the mantle rocks, forming mylonitic bands at the contacts [ 27 ]. Large blocks from the top of the Blanca Unit are commonly embedded in the mylonitic band at the contact with the Ronda Peridotites [ 28 ]. The El Robledal deposit is hosted by one of these blocks, similar to other skarn-related deposits widespread along the Ronda Peridotites–Blanca Unit contact [29,30]. The rocks from the crustal sequence of the Jubrique group range from garnet-bearing granulitic gneisses (i.e., kinzigites; 12–14 kbar and 850 ◦ C) in contact with the Ronda Peridotites to metapelites, marbles and low-grade phyllites (7–8 kbar and 200–350 ◦ C) at the top of the sequence [ 31 – 33 ]. These rocks represent the pre-Mesozoic continental crust that was extremely thinned during Alpine Orogeny ([ 19 ] and references therein), although some authors suggested that there was a previous event of orogenic accretion during the Hercynian [34–36]. The Blanca Unit is composed of high-grade metapelites and migmatites underlying a Middle to Upper Triassic carbonate-rich metasedimentary sequence [ 37 , 38 ]. This unit includes several low-pressure and mediumto coarse-grained diatexites and metatexites (equilibrated at 3–5 kbar and 700–750 ◦ C) [ 31 , 39 , 40 ]. These rocks are interpreted as the product of the anatexis of former metapelitic rocks during the high-temperature thrust emplacement of the Ronda Peridotites over these crustal rocks [27,28,37,41,42]. Minerals 2023,13, 300 4 of 28 Leucogranitic dikes emanating from the migmatites crosscut the peridotite slab and the crustal rocks of the Blanca Unit along steep-dipping open joints that mostly trend NW–SE [ 43 , 44 ]. These dykes show cm to m thicknesses and crystallized at low pressure (<4 kbar) and high temperatures (>750 ◦ C) [ 33 , 37 , 45 , 46 ]. Previous geochronological Rb–Sr ( 22 ±4 Ma ) [ 47 ] and, particularly U–Pb sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS) studies (18.8 ± 4.9 Ma) [ 38 ]; (22.3 ± 0.2 Ma) [ 48 ]; (21.5 ± 3.8 and 22.6 ± 1.8 Ma) [ 49 ], determined that the dikes crystallized during the 19–22 Ma timespan. Moreover, González-Jímenez et al. [ 50 ] dated magmatic zircons in plagioclasite and chromitite from the Ojén Massif that also yielded ~18–21 Ma. These data support the Alpine intracrustal emplacement of peridotites and suggest a genetic link between mineralizations and anatectic melts that originated during such emplacement. Nevertheless, Acosta-Vigil et al. [ 33 ] have found in migmatites from the Blanca unit zircons with magmatic overgrowths of ~280–290 Ma a later thermal and fluid overprint ca. 30 Ma, suggesting that Alpine anatexis is only limited to the very contact with the peridotites—see also [33,34,43]. 3. Materials and Methods A selected set of 30 samples was employed in this study to perform polished thin sections to characterize the mineralogy of the targeted skarn deposit. These thin sections were first inspected by conventional petrographic microscopy under both transmitted and reflected light to identify rock fabrics and textural relationships between transparent and opaque minerals. This task was achieved using an OLYMPUS-BX microscope at the Department of Mineralogy and Petrology at the University of Granada. The preliminary chemical identification and microphotographs of the minerals were acquired by electron microscopy using a Carl Zeiss MERLIN Field Emission Scanning Electron Microscope (FESEM) by applying the back-scattered electron (BSE) mode at the Universidad de Zaragoza. The accelerating voltage was 20 kV, the beam current was 10 nA, and the working distance was 15 mm. The EBSD measurements on the selected areas of the analyzed samples were carried out using a Carl Zeiss MERLIN FE-SEM at the Universidad de Zaragoza. It was equipped with an HKL detector operated at an acceleration voltage of 20 kV (beam current: ~10 nA) and with a tiltable XY stage, including a rotation function around the stage normal z. The samples were tilted 70 ◦ relative to the incident beam at 18.5 mm from the pole piece, and at each position, an electron backscatter pattern (EBSP) was collected and stored. The signal was corrected in a postprocessing step using Phi-rho-Z, which showed a better correlation to stoichiometry than ZAF. The dwell time was 15 ms per point (average of 2 EBSD frames). The magnification was 750 × , with a working distance of 18.5 mm and a step width of 0.5 µm. The quantification of the major element compositions in the minerals was carried out using a JEOL JXA-8230 electron probe microanalyzer at the Serveis Científics i Tècnics of the Universitat de Barcelona, which has a specific crystal (LDE2) for light elements. The analytical conditions were a 15 kV accelerating voltage, 15 to 30 nA focused electron beam current, 5 µ m spot diameter, and counting times of 30 s for boron, 10 s for the rest of the elements on peaks, and 15 and 5 s on the backgrounds, respectively. The ZAF corrections were performed using the program supplied by JEOL. Diopside (Si), corundum (Al), periclase (Mg), hematite (Fe), wollastonite (Ca), vonsenite (B), fluorite (F), rutile (Ti), and cassiterite (Sn) were used as reference materials for the calibration. The atoms per formula (apfu) were calculated assuming the ideal stoichiometry of the borate minerals (4 apfu for the ludwigite group minerals and 2 apfu for szaibelyite). Supplementary Material includes all the EPMA analyses of the skarn minerals. Several regions of the samples were selected for mapping. For each map, several orientation-contrast mapping methods were applied to identify the distinctively different characteristics of the microstructures within phases, such as band contrast (BC), band slope (BS), and pattern quality (PQ) maps. These revealed deformed regions, grain boundaries, Minerals 2023,13, 300 5 of 28 strain, and distinguished between crystallographically similar phases but different in defect content. The IPF (inverse pole figure) maps and pole figures show crystallographic orientations in the X-, Y-, or Z-direction, producing a different color for each orientation according to the crystallographic axis. The overall percentage of indexing in the maps was between 80% and 85%, and most of the no-index points corresponded to small holes and cracks. To estimate the pattern indexing reliability, we used the mean angular deviation (MAD), which describes the average angular difference between the position of Kikuchi bands in both the simulated and the acquired pattern. It is expressed as an angle in degrees. All calculations and simulations used to calculate these two parameters were acquired using the Flamenco software, which is used for pattern acquisition in the Channel 5 system. Indexation is possible with a MAD value smaller than 1◦. Three thin-foil samples (two from the ludwigite–magnetite zone and another one from the magnetite–szaibelyite zone) were prepared and extracted from Mg-rich magnetite grains using a focused ion beam scanning electron microscope (FIB-SEM) in the Laboratorio de Microscopías Avanzadas (LMA) at the Instituto de Nanociencia de Aragón (INA)— University of Zaragoza, Spain. The thin foil preparation was performed using a Dual Beam FEI Thermo-Fisher Scientific, model Helios 650. The selected regions of interest were first covered by a thin strip (~300 nm) of C via focused electron beam-induced deposition (FEBID) and, subsequently with a second strip (~1 µ m) of Pt. The bulk material was first removed on both sides of the lamella by a rough Ga+ ion milling with a 30 kV current at 2.5 nA and the subsequent polishing with a 30 kV current at 0.23 nA. Electron transparency was reached by the final polishing and subsequent milling of the thin foil with a 5 kV current at 68 pA. The electron transparency was monitored by an Everhart-Thornley SE detector and using a 5 kV electron beam. After achieving the electron transparency, the thin foil was rapidly polished using a low energy 5 kV current at 10 pA to reduce the amorphization until a final thin foil thickness of ~90 nm was attained. Subsequently, the thin foil was undercut with a 30 kV at 2.5 nA current, lifted out and transferred from the sample to a TEM grid using an OmniProbe nanomanipulator with a tungsten tip. A ThermoFisher FEI Titan G2 transmission electron microscope (TEM) equipped with a field emission gun XFEG was used to analyze the thin foil at the Centro de Instrumentation Científica of the University of Granada, Spain. The FEI Titan G2 microscope was equipped with 4 energy-dispersive analyses of X-ray (EDX) detectors (FEI microanalysis Super X), a high-angle annular dark-field detector (HAADF), as well as spherical correction for the objective lens (Eindhoven, Netherlands). The selected mineral areas of interest were imaged using the combination of HAADF to obtain Z high contrast images, and HRTEM images to characterize the texture and the ordering of the mineral aggregates. All these images were treated using the Digital Micrograph ®® software in its Version 1.71.38 developed by Gatan, while the maps were processed with the VELOX ®® software package developed by ThermoFisher. The FEI Titan G2 was operated at 300 kV working conditions, while the HRTEM images were acquired using a Gatan CCD camera (Pleasanton, CA, USA). The compositional elemental mappings of the samples were obtained using 200 kV of accelerating voltage and image drift correction. 4. Results 4.1. The El Robledal Deposit The El Robledal deposit is hosted in a dismembered block of the Blanca Unit, including dolomitic marble overlying migmatites. The block is embedded in the mylonitic band of the contact aureole surrounding the Ronda Peridotites and crops out at the southern part of Sierra de Las Nieves, along the contact between the Ronda Peridotites and the partly metamorphosed carbonates of the Las Nieves Unit (Figure 2). The outcrop is now an open-pit mine trench that exposes remnants of the mineralized body over 70–80 m in the E–W direction with a thickness of 25 m at a depth of 10–15 m. The mineralized body is mainly composed of magnetite and borates with two contrasting zones: (1) structurally Minerals 2023,13, 300 6 of 28 upper mineralized body, representing the ludwigite–magnetite zone ca. 9 × 12 × 4 m in contact with highly serpentinized peridotites (i.e., the Ronda Peridotites) to the south, clamped between two strike-slip faults trending N60–80 ◦ , limiting the mineralization; (2) structurally lower mineralized dolomitic marble body representing the magnetite– szaibelyite zone in contact with the underlying cordieriteand garnet-bearing migmatites to the north (Figure 2C). The contact between the dolomitic marbles from the skarn and the underlying migmatites is defined by a reaction zone (i.e., black wall) ca. 30 cm thick of green-colored amphibolite exhibiting sharp contacts with the host rocks (Figure 3I,J). Minerals 2023, 13, x FOR PEER REVIEW 6 of 29 of the contact aureole surrounding the Ronda Peridotites and crops out at the southern part of Sierra de Las Nieves, along the contact between the Ronda Peridotites and the partly metamorphosed carbonates of the Las Nieves Unit (Figure 2). The outcrop is now an open-pit mine trench that exposes remnants of the mineralized body over 70–80 m in the E–W direction with a thickness of 25 m at a depth of 10–15 m. The mineralized body is mainly composed of magnetite and borates with two contrasting zones: (1) structurally upper mineralized body, representing the ludwigite–magnetite zone ca. 9 × 12 × 4 m in contact with highly serpentinized peridotites (i.e., the Ronda Peridotites) to the south, clamped between two strike-slip faults trending N60–80°, limiting the mineralization; (2) structurally lower mineralized dolomitic marble body representing the magnetite–szaibelyite zone in contact with the underlying cordieriteand garnet-bearing migmatites to the north (Figure 2C). The contact between the dolomitic marbles from the skarn and the underlying migmatites is defined by a reaction zone (i.e., black wall) ca. 30 cm thick of greencolored amphibolite exhibiting sharp contacts with the host rocks (Figure 3I,J). Figure 2. Geological sketch map of the El Robledal area: (A) simplified geological map of the northern contact of the Ronda Peridotites with the Las Nieves unit. Red lines mark the cross-section in B (B) geological cross-section of the northern contact of Ronda Peridotites and carbonates of the Las Nieves unit. Modified from Mazzoli and Algarra [51]. The red square highlights the location of the El Robledal deposit; (C) schematic column showing the structural disposition of the different zones of the El Robledal skarn. The column is rotated 90 degrees approximately with respect to the ore deposit distribution in the field, (which follows a lateral disposition) to ease the understanding of the deposit. Figure 2. Geological sketch map of the El Robledal area: ( A ) simplified geological map of the northern contact of the Ronda Peridotites with the Las Nieves unit. Red lines mark the cross-section in B ( B ) geological cross-section of the northern contact of Ronda Peridotites and carbonates of the Las Nieves unit. Modified from Mazzoli and Algarra [ 51 ]. The red square highlights the location of the El Robledal deposit; ( C ) schematic column showing the structural disposition of the different zones of the El Robledal skarn. The column is rotated 90 degrees approximately with respect to the ore deposit distribution in the field, (which follows a lateral disposition) to ease the understanding of the deposit. 4.2. Mineralogy of the Skarn Zones The mineralization of the El Robledal deposit consists of ludwigite, Mg-rich magnetite, magnetite, and szaibelyite, with minor kotoite and schoenfliesite (MgSn 4+ (OH) 6 ). However, their distribution is nonhomogeneous and exhibits marked mineralogical and textural variations depending on the skarn zone. 4.2.1. Ludwigite–Magnetite Zone The mineral assemblage in this zone includes ludwigite, Mg-rich magnetite, magnetite, forsterite, two types of brucite (i.e., brucite-I and brucite-II), chondrodite, and variable proportions of double-layered hydroxides belonging to the hydrotalcite supergroup (hydrotalcite (Mg 6 Al 2 CO 3 (OH) 16· 4H 2 O), pyroaurite (Mg 6 Fe 3+2 (OH) 16 (CO 3 ) · 4H 2 O), and coalingite (Mg 10 Fe 3+2 (OH) 24 (CO 3 ) · 2H 2 O)), hydrous borates (szaibelyite and scarce wightmanite (Mg 5 (BO 3 )O(OH) 5· 2H 2 O)), kotoite, periclase, schoenfliesite, hydrous Mgcarbonates (i.e., hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2· 4H 2 O)), serpentine, and calcite. The latter crosscuts the whole ludwigite–magnetite zone. Forsterite commonly forms trails of Minerals 2023,13, 300 7 of 28 partially serpentinized, euhedral to subhedral grains (<2 mm across) embedded within the ludwigite and magnetite groundmass (Figure 3H). Minerals 2023, 13, x FOR PEER REVIEW 7 of 29 Figure 3. Field images of the El Robledal deposits and host rocks. The ludwigite–magnetite zone (i.e., lud–mag) is marked with a dark-blue marked area, whereas the magnetite–szaibelyite (i.e., mag–sza) zone is marked with a light-brown area. The Ronda Peridotites are marked with a palegreen area. A fault zone separating the lud–mag zone from the serpentinized Ronda Peridotites is marked with a white-colored area. (A) Close-up view of the tectonic contact between Ronda Peridotites and the deposit; (B) panoramic view of the mining area showing the whole sequence of the deposit; (C) upper mineralized body representing the ludwigite–magnetite zone; (D) massive magnetite bands of the magnetite–szaibelyite zone, where the white arrow marks a highly deformed magnetite band; (E) detailed view of the pull-apart fractures of the magnetite bands from the magnetite–szaibelyite zone; (F) small-scale magnetite body within the dolomitic marble associated with a brucite+szaibelyite groundmass; (G) chondrodite-bearing marble with folded magnetite bands. Note that chondrodite is overall associated with magnetite bands; (H) close-up view of the massive texture of the upper body of the deposit (i.e., ludwigite–magnetite zone). Note the occurrence of trails of green-colored forsterite and late calcite veins; (I) field view of the amphibolite reaction zone limiting the skarn from the migmatites of the Blanca Unit. The dashed, red line highlights the sharp contact of the amphibolite with surrounding rocks; (J) amphibolite section obtained from the reaction zone. Lud: ludwigite; Mag: magnetite; Sza: szaibelyite. 4.2. Mineralogy of the Skarn Zones The mineralization of the El Robledal deposit consists of ludwigite, Mg-rich magnetite, magnetite, and szaibelyite, with minor kotoite and schoenfliesite (MgSn4+(OH)6). However, their distribution is nonhomogeneous and exhibits marked mineralogical and textural variations depending on the skarn zone. 4.2.1. Ludwigite–Magnetite Zone The mineral assemblage in this zone includes ludwigite, Mg-rich magnetite, magnetite, forsterite, two types of brucite (i.e., brucite-I and brucite-II), chondrodite, and variable proportions of double-layered hydroxides belonging to the hydrotalcite supergroup (hydrotalcite (Mg6Al2CO3(OH)16·4H2O), pyroaurite (Mg6Fe3+2(OH)16(CO3)·4H2O), and coalingite (Mg10Fe3+2(OH)24(CO3)·2H2O)), hydrous borates (szaibelyite and scarce wightmanite (Mg5(BO3)O(OH)5·2H2O)), kotoite, periclase, schoenfliesite, hydrous Mg-carbonates (i.e., hydromagnesite (Mg5(CO3)4(OH)2·4H2O)), serpentine, and calcite. The latter crosscuts the whole ludwigite–magnetite zone. Forsterite commonly forms trails of partially serpentinized, euhedral to subhedral grains (<2 mm across) embedded within the ludwigite and magnetite groundmass (Figure 3H). This zone is dominated by massive ludwigite, Mg-rich magnetite, and magnetite the proportion of which within the orebody varies according to their distance to the tectonic contact with the Ronda Peridotites. Thus, ludwigite is the most abundant mineral close to the peridotites, whereas Mg-rich magnetite and magnetite abundances increase gradually in the magnetite–szaibelyite zone. Figure 3. Field images of the El Robledal deposits and host rocks. The ludwigite–magnetite zone (i.e., lud–mag) is marked with a dark-blue marked area, whereas the magnetite–szaibelyite (i.e., mag–sza) zone is marked with a light-brown area. The Ronda Peridotites are marked with a pale-green area. A fault zone separating the lud–mag zone from the serpentinized Ronda Peridotites is marked with a white-colored area. ( A ) Close-up view of the tectonic contact between Ronda Peridotites and the deposit; (B) panoramic view of the mining area showing the whole sequence of the deposit; ( C ) upper mineralized body representing the ludwigite–magnetite zone; ( D ) massive magnetite bands of the magnetite–szaibelyite zone, where the white arrow marks a highly deformed magnetite band; ( E ) detailed view of the pull-apart fractures of the magnetite bands from the magnetite–szaibelyite zone; ( F ) small-scale magnetite body within the dolomitic marble associated with a brucite+szaibelyite groundmass; ( G ) chondrodite-bearing marble with folded magnetite bands. Note that chondrodite is overall associated with magnetite bands; ( H ) close-up view of the massive texture of the upper body of the deposit (i.e., ludwigite–magnetite zone). Note the occurrence of trails of green-colored forsterite and late calcite veins; ( I ) field view of the amphibolite reaction zone limiting the skarn from the migmatites of the Blanca Unit. The dashed, red line highlights the sharp contact of the amphibolite with surrounding rocks; ( J ) amphibolite section obtained from the reaction zone. Lud: ludwigite; Mag: magnetite; Sza: szaibelyite. This zone is dominated by massive ludwigite, Mg-rich magnetite, and magnetite the proportion of which within the orebody varies according to their distance to the tectonic contact with the Ronda Peridotites. Thus, ludwigite is the most abundant mineral close to the peridotites, whereas Mg-rich magnetite and magnetite abundances increase gradually in the magnetite–szaibelyite zone. Ludwigite shows a string of grey–brown to grey–pink pleochroism and a marked blueto-pink–white anisotropy when observed under a reflected light microscope ( Figure 4A ). It occurs as massive masses of fibrous or prismatic crystals up to 1.5 mm in size, forming parallel, radiating, or interwoven aggregates including partially serpentinized olivine and/or Mg-rich magnetite crystals (Figures 4B and 5A–D). Ludwigite is commonly deformed, forming curved and elongated crystals, particularly in the upper portion of the zone, close to the contact with the Ronda Peridotites (Figure 4B). However, ludwigite crystals that locally form 120 ◦ triple junctions have also been found (Figure 5B), defining a polygonal structure. Some ludwigite crystals show partially dissolved cores, infilled by an intergrowth of Mg-rich magnetite and fine-grained, fibrous, or flake-like hydrotalcite group minerals (Figure 5D). A mixture of cataclasized ludwigite crystals occasionally fills a network of fractures among euhedral Mg-rich magnetite (Figure 4C). Minerals 2023,13, 300 8 of 28 Minerals 2023, 13, x FOR PEER REVIEW 8 of 29 Ludwigite shows a string of grey–brown to grey–pink pleochroism and a marked blue-to-pink–white anisotropy when observed under a reflected light microscope (Figure 4A). It occurs as massive masses of fibrous or prismatic crystals up to 1.5 mm in size, forming parallel, radiating, or interwoven aggregates including partially serpentinized olivine and/or Mg-rich magnetite crystals (Figure 4B and Figure 5A–D). Ludwigite is commonly deformed, forming curved and elongated crystals, particularly in the upper portion of the zone, close to the contact with the Ronda Peridotites (Figure 4B). However, ludwigite crystals that locally form 120° triple junctions have also been found (Figure 5B), defining a polygonal structure. Some ludwigite crystals show partially dissolved cores, infilled by an intergrowth of Mg-rich magnetite and fine-grained, fibrous, or flake-like hydrotalcite group minerals (Figure 5D). A mixture of cataclasized ludwigite crystals occasionally fills a network of fractures among euhedral Mg-rich magnetite (Figure 4C). Figure 4. Transmitted light and reflected light microscopy photomicrographs of the El Robledal deposit. The microphotographs are arranged from the top (ludwigite–magnetite zone) to the bottom (magnetite–szaibelyite zone) of the mineralized area. (A) Massive ludwigite with interstitial forsterite. (B) Ductile deformed ludwigite crystals. (C) Cataclasized ludwigite filling fractures among granoblastic Mg-rich magnetite hosting partially serpentinized forsterite. (D) Fibrous-like, bruciteFigure 4. Transmitted light and reflected light microscopy photomicrographs of the El Robledal deposit. The microphotographs are arranged from the top (ludwigite–magnetite zone) to the bottom (magnetite–szaibelyite zone) of the mineralized area. ( A ) Massive ludwigite with interstitial forsterite. ( B ) Ductile deformed ludwigite crystals. ( C ) Cataclasized ludwigite filling fractures among granoblastic Mg-rich magnetite hosting partially serpentinized forsterite. ( D ) Fibrous-like, brucite-II and szaibelyite intergrowths from the magnetite–szaibelyite zone crosscut by a late serpentine vein. ( E ) Brucite-II and szaibelyite intergrowths around Mg-rich magnetite crystal. The dusty-brown rims around the Mg-rich magnetite are brucite-II crystals. ( F ) Mg-rich magnetite with brucitized cores. ( G , H ) Partially brucitized periclase marble in a nonmineralized area. ( I ) Irregular brucitized periclase bands crosscut periclase marble. The red-dashed line separates the periclase marble band from the brucite-rich band after periclase hydration. ( J ) Granoblastic aggregate of subhedral tremolite–actinolite grains from the reaction zone. ( K , L ) Porphyroclastic amphibolite with large actinolite grains partially replaced by a second generation of fibrous amphibole. Act: actinolite; Brc: brucite; Cal: calcite; Fo: forsterite; Lud: ludwigite; Mg-mag: Mg-rich magnetite; Mag: magnetite; Per: periclase; Srp: serpentine; Sza: szaibelyite. Minerals 2023,13, 300 9 of 28 Minerals 2023, 13, x FOR PEER REVIEW 9 of 29 II and szaibelyite intergrowths from the magnetite–szaibelyite zone crosscut by a late serpentine vein. (E) Brucite-II and szaibelyite intergrowths around Mg-rich magnetite crystal. The dusty-brown rims around the Mg-rich magnetite are brucite-II crystals. (F) Mg-rich magnetite with brucitized cores. (G,H) Partially brucitized periclase marble in a nonmineralized area. (I) Irregular brucitized periclase bands crosscut periclase marble. The red-dashed line separates the periclase marble band from the brucite-rich band after periclase hydration. (J) Granoblastic aggregate of subhedral tremolite–actinolite grains from the reaction zone. (K,L) Porphyroclastic amphibolite with large actinolite grains partially replaced by a second generation of fibrous amphibole. Act: actinolite; Brc: brucite; Cal: calcite; Fo: forsterite; Lud: ludwigite; Mg-mag: Mg-rich magnetite; Mag: magnetite; Per: periclase; Srp: serpentine; Sza: szaibelyite. Figure 5. Scanned electron microscopy images showing the different textures of the El Robledal deposit ore minerals. The pictures are arranged from the top to the bottom of the deposit. Ludwigite–magnetite zone: (A) Deformed ludwigite from the upper part of the mineralized body including partially serpentinized forsterite and anhedral Mg-rich magnetite interstitial to ludwigite. (B) Granoblastic, massive ludwigite with a 120° triple junction (marked with yellow lines). (C) Euhedral Mg-rich magnetite inclusion in ludwigite. (D) A partially dissolved core of ludwigite infilled by an intergrowth of Mg-rich magnetite and fine-grained, fibrous, or flake-like hydrotalcite group minerals. (E) Mg-rich magnetite grain partially altered to magnetite along fractures. (F) Mg-rich magnetite replaced by magnetite along boundaries and fractures. Portions of magnetite are dismembered within the fracture filled with cataclasized ludwigite. (G) Partially altered Mg-rich magnetite grain embedded in a szaibelyite and brucite-II matrix. (H) Porous textures of magnetite replacing Mg-rich magnetite. (I) Euhedral Mg-rich magnetite with brucitized cores after periclase. (J) Non-fractured Mg-rich magnetite grains with unaltered periclase core. (K) Dismembered magnetite defining the borders of a pseudomorph, now replaced by szaibelyite and brucite-I, including rounded, tiny magnetite particles. (L) Kotoite relicts partially replaced by tabular szaibelyite. Magnetite–szaibelyite Figure 5. Scanned electron microscopy images showing the different textures of the El Robledal deposit ore minerals. The pictures are arranged from the top to the bottom of the deposit. Ludwigite– magnetite zone: ( A ) Deformed ludwigite from the upper part of the mineralized body including partially serpentinized forsterite and anhedral Mg-rich magnetite interstitial to ludwigite. ( B ) Granoblastic, massive ludwigite with a 120 ◦ triple junction (marked with yellow lines). ( C ) Euhedral Mg-rich magnetite inclusion in ludwigite. ( D ) A partially dissolved core of ludwigite infilled by an intergrowth of Mg-rich magnetite and fine-grained, fibrous, or flake-like hydrotalcite group minerals. ( E ) Mg-rich magnetite grain partially altered to magnetite along fractures. ( F ) Mg-rich magnetite replaced by magnetite along boundaries and fractures. Portions of magnetite are dismembered within the fracture filled with cataclasized ludwigite. ( G ) Partially altered Mg-rich magnetite grain embedded in a szaibelyite and brucite-II matrix. ( H ) Porous textures of magnetite replacing Mg-rich magnetite. ( I ) Euhedral Mg-rich magnetite with brucitized cores after periclase. ( J ) Non-fractured Mg-rich magnetite grains with unaltered periclase core. ( K ) Dismembered magnetite defining the borders of a pseudomorph, now replaced by szaibelyite and brucite-I, including rounded, tiny magnetite particles. ( L ) Kotoite relicts partially replaced by tabular szaibelyite. Magnetite–szaibelyite zone: ( M ) Magnetite replacing Mg-rich magnetite along fractures and boundaries including brucitized cores. Note that the grade of replacing is higher than in the ludwigite–magnetite zone. ( N ) Elongated and deformed Mg-rich magnetite grains with pull-apart fractures. ( O ) Edges of Mg-rich magnetite with brucitized cores after periclase included in a dolomite and calcite matrix. ( P ) Dolomite rims around Mg-rich magnetite replaced by szaibelyite and brucite-I, including rounded, tiny magnetite particles. The Mg-rich magnetite exhibits different textures within the ludwigite–magnetite zone: (1) Mg-rich cores partially replaced by magnetite along fractures and boundaries, Minerals 2023,13, 300 16 of 28 boundaries in deformed and elongated porphyroclasts were systematically medium-angle boundaries (rectangles A7 to A8 in Figure 12C). The pole figures reveal a strong CPO of the ludwigite grains (Figure 12C). Open fractures partially filled with hydrotalcite group minerals were common in the ludwigite samples. Minerals 2023, 13, x FOR PEER REVIEW 16 of 29 4.6. Microstructure of the Ores Three EBSD maps were performed on selected areas from the massive Mg-rich magnetite with a polygonal microstructure (ROB-3) and massive ludwigite (ROB-2) from the ludwigite–magnetite zone (Figure 12). The massive Mg-rich magnetite sample consisted of medium-grained crystals (up to 0.4 mm) exhibiting a 120° triple junction with straight boundaries (Figure 12A). The BC and IPF map revealed the coexistence of irregular grains having appreciable internal crystal bending (rectangles A1 to A3 in Figure 12A) with slightly smaller but completely euhedral, free-of-strain Mg-rich magnetite grains. The internal crystal bending is marked by low-angle (<5°), subgrain boundaries within the grains, whereas the misorientation between grain boundaries was higher than 15°. The inverse pole figure acquired in 3D space reference X0, Y0, and Z0, displays near random to weak patterns of bulk CPO of the grains with several orientation maxima crystallographic preferred orientation (CPO) patterns, with numerous orientation maxima at (100), (110), and (111) (Figure 12A). The BC and IPF maps acquired from the massive ludwigite reveal two texturally different areas: 1) massive, granoblastic ludwigite (Figure 12B), and 2) porphyroclastic, deformed, and elongated ludwigite crystals (Figure 12C). Figure 12. Ore microstructures from the El Robledal. The data are presented using band contrast maps (left) combined with a color-coded inverse pole figure (center), representing the crystal orientation relative to the X0, and the pole figures (right) represent the crystallographic preferred orientation relative to (001). The misorientation angles between the crystal orientation of neighboring data points were grouped into (1) low-angle boundaries, with misorientation lower than 5° (dashed, black lines), and (2) medium-angle boundaries, with misorientation of 5–10° (dashed, red lines). (A) Massive Mg-rich magnetite sample with 120° triple junctions. The IPF maps show grains with internal crystal bending (rectangles A1 to A3) that coexist with grains free of deformation. The sample shows near-random to weak patterns of CPO with numerous orientation maxima at (100), (110), and (111). (B) Granoblastic ludwigite sample with 120° curved junctions. Note the higher Figure 12. Ore microstructures from the El Robledal. The data are presented using band contrast maps (left) combined with a color-coded inverse pole figure (center), representing the crystal orientation relative to the X0, and the pole figures (right) represent the crystallographic preferred orientation relative to (001). The misorientation angles between the crystal orientation of neighboring data points were grouped into (1) low-angle boundaries, with misorientation lower than 5 ◦ (dashed, black lines), and (2) medium-angle boundaries, with misorientation of 5–10 ◦ (dashed, red lines). ( A ) Massive Mg-rich magnetite sample with 120 ◦ triple junctions. The IPF maps show grains with internal crystal bending (rectangles A1 to A3) that coexist with grains free of deformation. The sample shows near-random to weak patterns of CPO with numerous orientation maxima at (100), (110), and (111). ( B ) Granoblastic ludwigite sample with 120 ◦ curved junctions. Note the higher misorientation angles in the deformed grains compared to A (rectangles A4 to A6). This sample exhibits a strong CPO pattern, especially at (100). ( C ) Porphyroclastic ludwigite with elongated and deformed grains surrounding nondeformed porphyroclasts. The elongated grains show the highest misorientation degrees (rectangles A7-A8) Note the similar size of the neoblast compared to subgrains developed on deformed ludwigite porphyroclasts (rectangle A7). The pole figure shows a strong CPO pattern, especially at (100). 5. Discussion 5.1. Skarn Topology As stated above, the El Robledal deposit is an ore body with two contrasting zones: the uppermost zone dominated by massive ludwigite, Mg-rich magnetite, and magnetite (i.e., ludwigite–magnetite zone) close to the tectonic contact with the Ronda Peridotites; Minerals 2023,13, 300 17 of 28 lower zone (i.e., magnetite–szaibelyite zone) dominated by Mg-rich magnetite, magnetite, and szaibelyite hosted in dolomitic marble. Accounting for that mineralogy we suggest that the El Robledal deposit is an Mg-Fe-B skarn deposit in the terms defined by Marincea and Dumitras [ 14 ] and not a magnesian exoskarn of magnetite–ludwigite, as suggested previously by Curras and Torres-Ruíz [ 63 ]. Noteworthy is that similar mineral assemblages have been previously noted in other borate-bearing Mg-skarns included in the Mg-Fe-B type, particularly in the Banatitic Magmatic and Metallogenetic Belt [14]. 5.2. Mineral Paragenesis and Skarn Evolution Figure 13 summarizes the proposed paragenetic sequence for the El Robledal deposit. Minerals 2023, 13, x FOR PEER REVIEW 18 of 29 Figure 13. Proposed paragenetic sequence for the El Robledal deposit. 5.2.1. Metamorphic Stage In the El Robledal area, periclase is widespread and homogeneously distributed in the dolomitic marble samples free of mineralization (Figure 4G–I). Moreover, crystals of periclase are systematically included in Mg-rich magnetite in both the ludwigite–magnetite and magnetite–szaibelyite zones (Figures 10 and 11). Interestingly, periclase is associated with calcite in the magnetite–szaibelyite zone. This may indicate that periclase was an early-formed mineral unrelated to the infiltration of mineralized fluids (i.e., metamorphic origin), as suggested in other similar skarns worldwide [3]. This mineral may effectively form in anhydrous conditions (i.e., XCO2 = 1) upon increasing temperature via thermal decomposition of dolomite at temperatures above 610 °C at 1–2 kbar [64–66] according to the following reaction: CaMg(CO3)2 = MgO + CaCO3 + CO2 (1 ) From the observations above, we suggest that periclase was the first mineral to form under dry conditions by metamorphism of the Blanca Unit carbonate rocks during the hot emplacement of the Ronda Peridotites. 5.2.2. Prograde Stage Forsterite is systematically embedded in masses of both Mg-rich magnetite and ludwigite crystals and exhibits well-developed shapes (Figure 4A,C); therefore, it may be considered one of the earliest minerals formed. This is in concordance with the scheme proposed by Aleksandrov [3] for Mg-skarn formation in which forsterite + calcite form the primitive metasomatic zoning by the interaction of dolomite with silica-bearing hydrothermal fluids, according to the following reaction: CaMg(CO3)2 + 0.5H4SiO4 = 0.5Mg2SiO4 + CaCO3 + H2O + CO2 (2 ) The analyzed forsterite had a very high Mg# (>96) unlike forsterite from the country Ronda Peridotites (Mg# < 93), suggesting a metasomatic origin related to fluid infiltration v Mg-rich magnetite Magnetite Kotoite Late Alkaline Dolomite Periclase Prograde Retrograde Early Alkaline Sulfide (Acid) ? Schoenfliesite Hydrous Mg-carbonates Mineral Before skarn Tremolite-Actinolite ? Ludwigite Szaibelyite Brucite-I Brucite-II Serpentine Sulfides Calcite Forsterite Figure 13. Proposed paragenetic sequence for the El Robledal deposit. 5.2.1. Metamorphic Stage In the El Robledal area, periclase is widespread and homogeneously distributed in the dolomitic marble samples free of mineralization (Figure 4G–I). Moreover, crystals of periclase are systematically included in Mg-rich magnetite in both the ludwigite–magnetite and magnetite–szaibelyite zones (Figures 10 and 11). Interestingly, periclase is associated with calcite in the magnetite–szaibelyite zone. This may indicate that periclase was an early-formed mineral unrelated to the infiltration of mineralized fluids (i.e., metamorphic origin), as suggested in other similar skarns worldwide [ 3 ]. This mineral may effectively form in anhydrous conditions (i.e., XCO 2 = 1) upon increasing temperature via thermal decomposition of dolomite at temperatures above 610 ◦ C at 1–2 kbar [ 64 – 66 ] according to the following reaction: CaMg(CO3)2= MgO + CaCO3+ CO2(1) From the observations above, we suggest that periclase was the first mineral to form under dry conditions by metamorphism of the Blanca Unit carbonate rocks during the hot emplacement of the Ronda Peridotites. Minerals 2023,13, 300 18 of 28 5.2.2. Prograde Stage Forsterite is systematically embedded in masses of both Mg-rich magnetite and ludwigite crystals and exhibits well-developed shapes (Figure 4A,C); therefore, it may be considered one of the earliest minerals formed. This is in concordance with the scheme proposed by Aleksandrov [ 3 ] for Mg-skarn formation in which forsterite + calcite form the primitive metasomatic zoning by the interaction of dolomite with silica-bearing hydrothermal fluids, according to the following reaction: CaMg(CO3)2+ 0.5H4SiO4= 0.5Mg2SiO4+ CaCO3+ H2O + CO2(2) The analyzed forsterite had a very high Mg# (>96) unlike forsterite from the country Ronda Peridotites (Mg# < 93), suggesting a metasomatic origin related to fluid infiltration [ 59 ]. Mazzoli et al. [ 46 ] estimated that forsterite in dolomitic marble from the las Nieves Unit originated from fluid-driven replacement reactions at ~510 ◦ , which is fully consistent with temperatures of 550–600 ◦ C, proposed for the formation of forsterite in B-bearing skarns worldwide [ 67 ] and in the nearby Mg-Fe skarn deposit of San Manuel, also located in the Blanca Unit [30]. In most Mg-skarns, forsterite is usually isothermally replaced by diopside giving rise to primitive zoning, thus evidencing an increasing aSiO 2 in the system [ 68 , 69 ]. However, this mineral was not detected in our studied rocks, which may indicate low aSiO 2 , similar to those reported in other Mg-skarns worldwide that are spatially related to both basic and ultrabasic rocks—e.g., Skye Island, Scotland; Bor-Uryakh, Yakutia, Russia [ 3 ]; and San Manuel in southern Spain, [30]. Mg-rich magnetite is locally included in ludwigite (Figure 5C) suggesting its formation before the early alkaline phase of the retrograde stage [ 3 ]. On the other hand, cores of Mg-rich magnetite are made of brucitized periclase at the lower part of the ludwigite– magnetite zone. This, along with periclase inclusions in Mg-rich magnetite, allows us to constrain its formation after periclase and before ludwigite, quite probably starting at the end of the prograde stage and continuing during the early alkaline phase of the retrograde stage. Mg-rich magnetite is systematically altered to porous magnetite showing sharp and irregular contacts (Figure 5E–H). This texture suggests coupled dissolution and reprecipitation processes (CDR) [ 70 ] that promoted Mg removal and Fe enrichment. Moreover, cracks and voids of ludwigite are partially filled with Mg-rich magnetite, suggesting the circulation of Fe-bearing fluids or magnetite remobilization after ludwigite formation, during the retrograde stage. This observation is consistent with reports in the literature where the formation of Mg-rich magnetite in skarns is related to the retrograde stage of skarns, especially with the early alkaline phase [ 3 ]. In fact, Pavlov [ 71 ] stated that magnesioferrite formation is promoted by alkaline conditions at very low pressures. However, Aleksandrov [ 3 ] pointed to the possibility of magnesioferrite formation during the prograde stage. This was also noted by González-Pérez et al. [ 30 ] in the San Manuel skarn. They suggest that both magnesioferrite and their replacement by magnetite took place at increasing temperatures during the prograde stage. Moreover, they approximate a minimum temperature for the prograde stage thermal peak of ~650 ◦ C based on spinel exsolution within magnetite. Kotoite has been locally found as small, rounded crystals almost completely replaced by szaibelyite (Figure 5L). Kotoite formation by dolomite replacement has been proposed by Watanabe [72], according to the reaction: 3CaMg(CO3)2+ 2B(OH)3= Mg3(BO3)2+ 3CaCO3+ 3CO2+ 3H2O (3) Barsukov and Deryugina [ 73 ] experimentally confirmed this reaction at 400 ◦ C for alkaline B-bearing solutions. The presence of kotoite is indicative of hypabyssal conditions (i.e., at depths lower than 2–3 km) [3], which agrees with the occurrence of periclase. Kotoite is scarce whereas ludwigite is widespread in the ludwigite–magnetite zone forming massive ores along with magnetite. This is indicative of a high Fe concentration in Minerals 2023,13, 300 19 of 28 ore fluids percolating through the El Robledal. According to Barsukov and Egorov [ 74 ], the presence of Fe in the hydrothermal fluids would enhance ludwigite formation. In fact, these authors suggest that Fe fixed in magnetite seems to be the main reason for ludwigite abundance in most of the Mg-skarn deposits of Romania. The effect of Fe in mineralizing fluids to enhance ludwigite formation was indirectly demonstrated by the experiments of Bloise et al. [ 55 ]. As noted before, the formation of ludwigite has been traditionally ascribed to the early alkaline phase of the prograde stage [ 3 ]. According to Grigorev and Nekrasov [ 75 ], ludwigite forms at temperatures above 450–500 ◦ C by dolomite replacement in a Fe-bearing system according to: Fe3O4+ 3CaMg(CO3)2+ B2O3= 2(Mg,Fe)2Fe(BO3)O2+ 3CaCO3+ 3CO2(4) Similar temperatures (i.e., 500–650 ◦ C) were stated by Kravchuk et al. [ 76 ] in their synthesis of Fe-ludwigite with a Fe molar fraction of 25% (i.e., f = 25%). Marincea [ 56 ] obtained temperatures around 600–650 ◦ C for ludwigite formation in the Ocna de Fier deposit, Romania. Later, Bloise and Barrese [ 77 ] reported equivalent temperatures experimentally. They noted that Fe-bearing ludwigite formed systematically over 550 ◦ C at both 0.5 and 1 kbar in the MgO-FeO-Fe 2 O 3 -B 2 O 3 system for mixtures rich in MgO, as expected in the El Robledal deposit. However, Bilohušˇcin et al. [ 13 ] in the Vysoká-Zlatno deposit, Slovakia, stated that ludwigite formed during the prograde stage at 700 ◦ C and 0.7 kbar. According to these statements, ludwigite in the El Robledal deposit should have formed at temperatures above 500–550 ◦ C, after kotoite, Mg-rich magnetite, and periclase. This range of temperature marks almost the highest temperatures of the early alkaline stage, falling from, and quite close to, that of the thermal peak of the prograde stage. This is in line with temperatures falling from ~650 ◦ C reached during the thermal peak of the prograde stage of the San Manuel skarn [ 30 ]. This is in accordance with temperatures estimated for the anatexis of crustal rocks of the Blanca Unit during the Ronda Peridotites emplacement (600–750 ◦ C at ~3 kbar) [ 33 , 45 ]. On the other hand, according to Bloise et al. [ 55 ], the presence of small amounts of Al in the system can reasonably increase the temperature of ludwigite crystallization up to 700 ◦ C, diminishing ludwigite formation. Thus, the dominance of ludwigite in the El Robledal deposit suggests a low activity of alumina in the mineralizing fluids. 5.2.3. Retrograde Stage During the evolution of the retrograde stage in borate-bearing Mg-skarns, the anhydrous minerals are usually replaced by hydrated ones. Fe-bearing ludwigite (f = 34–21%) and kotoite are systematically replaced by szaibelyite. Kotoite is replaced by szaibelyite following the reaction: Mg3(BO3)2+ 2H2O = 2MgBO2(OH) + Mg(OH)2(5) However, szaibelyite commonly occurs associated with brucite-I hosting micrometric magnetite crystals (Figure 5K) in the El Robledal deposit. Barsukov and Kuril’chikova, [ 78 ] suggested that ludwigite is transformed to szaibelyite, magnetite, and brucite at temperatures below 300 ◦C down to 160 ◦C according to: 6Mg2Fe(BO3)O2+ 9H2O = 3Mg2(B2O4OH)(OH) + 2Fe3O4+ 6Mg(OH)2+ 0.5CO2(6) Similar szaibelytization of ludwigite with f = 25% was proposed by Pertsev [ 79 ], according to the reaction: 2(Mg0.75Fe2+0.25)Fe3+(BO3)O2+ 2H2O = Mg2(B2O4OH)(OH) + Fe3O4+ Mg(OH)2(7) The decomposition of ludwigite to form fibrous szaibelyite along with fine-grained magnetite was also noted by Peng and Palmer [ 80 ]. The presence of such pseudomorphs was already noted by Pertsev [81] and Aleksandrov [82]. Minerals 2023,13, 300 20 of 28 On the other hand, brucite-I also forms banded textures in periclase marbles (Figure 4I). This textural evidence reveals that brucite-I replaces periclase in nonmineralized rocks following the reaction: MgO + H2O = Mg(OH)2(8) Such a fluid infiltration-driven process is very likely related to zones of higher permeability in the periclase marble rocks as a result of weakness promoted by deformation. Brucite-I replacing periclase is interpreted as an earlier phase compared to brucite-II, which forms groundmasses along with szaibelyite and minor serpentine. Brucite-I and brucite-II have similar B 2 O 3 whereas brucite-II has higher Fe contents compared to brucite-I (Figure 8). According to Aleksandrov [ 58 ], the hydration of high-temperature borates and szaibelyite led to the formation of B-bearing brucites. However, we could not ascribe the formation of B-bearing brucite to high-temperature borate replacement and, thus, their formation may be explained by the infiltration of B-bearing fluids through the El Robledal during the retrograde stage. As Fe contents in brucite-II are higher than those of brucite-I, we suggest that Fe content in fluids should have decreased during evolving retrograde stage. This could be ascribed to serpentinization, which is normally accompanied by an increase of aFe in the fluid. Serpentinized forsterite is widespread in the El Robledal deposit embedded within the ore masses (Figure 4A), as well as included in Mg-rich magnetite (Figure 10). Moreover, late serpentine-rich veins crosscut the mineralization in the magnetite–szaibelyite zone. This manifests the late origin of serpentine as a consequence of forsterite replacement according to: 4Mg2(SiO4) + 6H2O = Mg6(Si4O10)(OH)8+ 2Mg(OH)2(9) The acid stage is poorly represented due to the almost complete lack of sulfides. Some schoenfliesite grains have been locally found in the El Robledal deposit. Its formation has been traditionally ascribed to late retrograde stages and low temperatures <260 ◦ C [ 83 ], which allows us to relate its formation to the late alkaline phase of the retrograde stage [ 84 ]. The former reactions promote a subsequent CO 2 excess of the system, which should be consumed in carbonation reactions. Nevertheless, the preservation of brucite in the El Robledal deposit is indicative of an open system in which CO 2 can break out of it. Moreover, according to Ferry and Rumble [ 65 ] and Simandl et al. [ 66 ], low XCO 2 avoids the destabilization of brucite to form retrograde dolomite during the late retrograde stage. The formation of retrograde dolomite is locally observed in zones of the magnetite–szaibelyite zone according to: CaMg(CO3)2+ H2O = Mg(OH)2+ CaCO3+ CO2(10) In these zones, brucite-I pseudomorphs after periclase in direct contact with calcite develop rings of dolomite (Figure 5P), similar to textures described by Simandl et al. [ 66 ]. Brucite destabilization promoted CO 2 consumption leading to its transformation into hydrous Mg carbonates—e.g., [ 85 ]. This could explain the occurrence of late veins filled with acicular hydrotalcite-group minerals, as well as late calcite veins crosscutting the whole deposit. It is assumed that these late veins represent the escape conduits liberating CO2from the system. 5.3. Effects of Deformation All analyzed samples exhibited grains with variable intracrystalline deformation developing subgrain boundaries (rectangles A1 to A8 in Figure 12), which is indicative of crystal–plastic deformation. The massive Mg-rich magnetite sample showed a well-developed foam-like, polygonal microstructure, where internally deformed grains with low-angle subgrain boundaries cohabited with undeformed grains developing a 120 ◦ triple junction. This microstructure is indicative of a high grade of recrystallization (i.e., annealing) after crystal–plastic deformation [ 70 , 86 – 92 ]. The foam-like, polygonal texture has been related either to (1) high- Minerals 2023,13, 300 21 of 28 temperature annealing in a closed system [ 93 ] or to (2) fluid-assisted replacement in an open system [ 94 , 95 ]. Here, Mg-rich magnetite is systematically replaced by porous magnetite with sharp contacts, along boundaries and fractures, suggesting CDR process, which is indeed indicative of fluid-assisted replacement [ 70 ]. Moreover, the occurrence of folded magnetite bands with pull-apart fractures filled with szaibelyite and brucite (Figure 3D–G) indicates that deformation was active during ore formation. The fluid-assisted replacement of ores in the El Robledal is in concordance with the interpretation of similar microstructures observed in the nearby San Manuel skarn [ 30 ]. Moreover, the absence of newly formed grains and the low-angle subgrain boundaries in the deformed grains from the Mgrich magnetite sample point toward high-temperature grain boundary migration (GBM) followed by strong annealing [ 96 ]. Thus, high temperatures would have been preserved allowing oxides to recrystallize almost completely. In the porphyroclastic ludwigite, elongated and/or internally deformed coarse grains are accompanied by free-of-strain neoblasts of apparently homogeneous size (similar to the subgrains size in porphyroclasts) Moreover, the sample exhibited a strong CPO pattern. These microstructures are also indicative of deformation and subsequent dynamic recrystallization processes by subgrain rotation (SGR) at intermediate temperatures and intermediate to high strain rates [91,96]. 5.4. Source of Boron The measured B 2 O 3 content in borates present in the El Robledal deposit ranged from 13.97 to 19.51 wt.% in ludwigite and 32.62–58.02 wt.% in szaibelyite. The formation of these borates is with the percolation of B-bearing fluids through dolomitic marble and subsequent metasomatism. [ 3 ]. The source of these fluids in other B-bearing skarn deposits has been carried by hydrothermal fluids responsible for skarn formation. The source of these B-bearing fluids in other B-bearing skarn deposits has been traditionally ascribed to volatile-rich fluids released from the surrounding cooling igneous intrusions of intermediate to acid composition—e.g., [ 3 ]. In the vicinity of the El Robledal area, the only igneous-related rocks are migmatites and a thin leucogranite dike approximately 10 cm wide located to the north of the deposit. The origin of leucogranites in the area has been associated with the partial melting of migmatites during the emplacement of Ronda Peridotites [43,45,97]. Acosta-Vigil [ 43 ] detected relatively high B concentration (~30 ppm) in migmatites around Ronda Peridotites compared with other Hercinian migmatites (~20 ppm) [ 98 ]. According to Moran et al. [ 99 ], as the metamorphism of pelitic and/or quartz-feldspathic rocks progress, the B content in the rocks decreases. Thus, the destabilization of phyllosilicates and/or tourmaline and dehydration of the metapelite/migmatite with progressive metamorphism may promote the partitioning of B into the escaping fluid/melt phase [ 100 , 101 ]. Later, Acosta-Vigil et al. [ 102 ] identified relatively high B contents ranging from 3.6 to 5.7 ppm in equivalent migmatites located west of the town of Istán (see Figure 1), whereas these contents increased up to an average of 400 ppm (with 1250 ppm maximum) in anatectic leucogranites derived from the mentioned migmatites. It would be necessary for a B-enriched protolith to generate leucogranites with such high B contents. Tourmaline textures and chemical analyses of B 2 O 3 in leucogranites led these authors to suggest a high initial B concentration in the parental melts of the leucogranites (i.e., migmatite). Moreover, [ 97 ] assessed that B from leucogranites could be lost to the country rock since miarolitic cavities are frequent and serpentinites in the contact between peridotites and such dikes are B enriched. A notable B concentration (up to 619 ppm) was also found by Pereira et al. [ 98 ] in tourmaline-bearing leucogranites crosscutting Ronda Peridotites. They also noted that Ronda Peridotites are notably enriched in B (1 ppm on average) compared to mantle composition [ 103 ]. Moreover, these values increase significantly in serpentinites contiguous to leucogranite outwards from the dike (21 ppm). Thus, we interpreted that migmatite is the most plausible source of B, which was liberated by escaping fluids and/or melt that formed the leucogranites upon increasing Minerals 2023,13, 300 22 of 28 metamorphism and anatexis. Melt derived from anatexis and/or fluids percolating through migmatites should have leached B and transferred it into marbles constituting now the El Robledal deposit. This is consistent with (1) the primary high-temperature contact between mineralized marbles and migmatites with igneous microstructures through the amphibolitic reaction zone and (2) the detection of B in the amphibolite black wall, which may indicate a bimetasomatism process with the B coming from the migmatite. 5.5. Genetic Model Figure 14 shows a sketch illustrating the proposed genetic model for the El Robledal deposit. The hot emplacement of the Ronda Peridotites over the carbonates and metapelites/migmatites of the Blanca Unit allows us to assume that the heat source of the system should have been the Ronda Peridotites itself. Such emplacement promoted the dehydration and melting of the metapelites of the Blanca Unit, as attested by the presence of leucogranite dykes emanating from them [ 45 , 102 ]. According to the melting textures observed in migmatites, with increasing melting degree toward the deposit, and B contents detected in both migmatites and associated tourmaline-bearing leucogranites, as well as evidence of B remobilization from leucogranites [ 43 , 97 , 102 ], we interpreted that the source of B is related to the fluids/melt liberated during dehydration and anatexis of metapelites of the Blanca Unit during the hot emplacement of Ronda Peridotites over these crustal rocks. Such anatectic melts and fluids derived from migmatites should have been the source of the fluids percolating through the carbonates responsible for the formation of the deposit. By assuming that, the ore-fluid source (i.e., migmatites) and heat source (i.e., heat emplacement of the Ronda Peridotites) in the El Robledal deposit are decoupled. The heat provided by Ronda Peridotites during emplacement promoted the metamorphism of dolomite marble and subsequent formation of periclase marble (~610 ◦ C) before mineralizing reached the dolomitic marble (Figure 14A). The prograde stage of the skarn began with the infiltration of fluids through dolomitic marble that promoted the formation of high Mg# > 96 forsterite. The fluid temperature during this stage should have reached over ~550–650 ◦ C [ 30 , 45 , 65 ]. Inclusions of periclase and forsterite within Mg-rich magnetite indicate that the latter postdates periclase and olivine formation, likely during the late prograde stage. The chemical evolution of fluids promoted Mg-rich magnetite replacement by magnetite probably via CDR. According to microstructures detected in oxides, this replacement should have been taken under deformation, suggesting that the emplacement of Ronda Peridotites was still active (Figure 14B). Most of the borate mineralization in Mg-Fe-B skarns takes place during the early alkaline period of the retrograde stage [ 3 ]. Kotoite is one of the first borates normally formed at the very beginning of the retrograde stage, and it has been found as relicts partially replaced by ludwigite. Its formation marks the beginning of the retrograde stage. Kotoite was replaced by ludwigite at temperatures around 500–550 ◦ C at 0.5–1 kbar, suggesting cooling from the temperatures reached during the prograde stage (Figure 14C). During the evolution of the retrograde stage, the system is continuously cooling. The presence of brucite-I and szaibelyite with tiny magnetite inclusions attests to such cooling down to temperatures below 300 ◦ C, promoting the szaibelytization of former borates (Figure 14D). Textural and compositional evidence allows us to define two different generations of brucite (i.e., brucite-I and brucite-II). As brucite-II is slightly richer in Fe, we can assume that mineralizing fluids enriched in Fe during evolving retrograde stage. The acid stage is marked by an increase in fS 2 during the evolution of the retrograde stage and subsequent sulfide mineralization. However, only scarce schoenfliesite crystals have been found in the El Robledal deposit. During schoenfliesite formation, the temperatures of mineralizing fluids should have decreased to below 260 ◦ C [ 84 ], which is consistent with the cooling of the system from temperatures that promoted the szaibelytization of borates. All of the aforementioned mineral transformations led to an increase in CO 2 in the system. During the late alkaline stage, CO 2 was consumed in carbonation reactions, promoting the formation of dolomite. In the zones where the excess of CO 2 was channelized Minerals 2023,13, 300 23 of 28 in fractures, it led to late hydrous Mg-carbonates and calcite veins crosscutting the whole deposit (Figure 14E). Minerals 2023, 13, x FOR PEER REVIEW 24 of 29 Figure 14. A proposed genetic model for the El Robledal deposit formation. Red arrows represent the percolating mineralizing fluids released from migmatite. (A) Metamorphic stage. The heat provided by the ultramafic rocks promoted dolomite decomposition and periclase formation. Fluids/melts emanating from incipient anatexis should not have interacted with the dolomites of the Blanca unit during this stage. (B) Prograde stage. The interaction of mineralizing fluids emanating from migmatites and dolomite of the Blanca unit led to olivine and Mg-rich magnetite formation. (C) Further fluid/rock interaction promoted kotoite and ludwigite formation in the outermost part of the skarn. (D) Retrograde stage. The decreasing temperature of the mineralized fluids led to primary borate alteration into szaibelyite and B-bearing brucite. Such a process is widespread close to the fluid source (i.e., migmatites) where the fluid/rock ratio is expected to be higher than in the zone close to the ultramafic rocks. (E) Late retrograde stage. The former reactions promote an increase of CO2 and thus enhanced hydrous-carbonate formation. In zones of fractures, the excess of CO2 should have been channelized generating hydrous Mg-carbonates and calcite veins. After complete mineralization in the El Robledal deposit, it is expected that relatively high temperatures were maintained for some time, associated with the presence of Ronda Peridotites, which allowed for the recovery and recrystallization of ore minerals. The timing of amphibolite formation is not well constrained. It has been usually ascribed to diopside replacement during the late alkaline retrograde stage [3] promoting Caskarn formation [30]. However, the field relationship suggests that amphibolite formation was related to bimetasomatism rather than infiltration metasomatism. Therefore, B2O3, Figure 14. A proposed genetic model for the El Robledal deposit formation. Red arrows represent the percolating mineralizing fluids released from migmatite. ( A ) Metamorphic stage. The heat provided by the ultramafic rocks promoted dolomite decomposition and periclase formation. Fluids/melts emanating from incipient anatexis should not have interacted with the dolomites of the Blanca unit during this stage. ( B ) Prograde stage. The interaction of mineralizing fluids emanating from migmatites and dolomite of the Blanca unit led to olivine and Mg-rich magnetite formation. ( C ) Further fluid/rock interaction promoted kotoite and ludwigite formation in the outermost part of the skarn. ( D ) Retrograde stage. The decreasing temperature of the mineralized fluids led to primary borate alteration into szaibelyite and B-bearing brucite. Such a process is widespread close to the fluid source (i.e., migmatites) where the fluid/rock ratio is expected to be higher than in the zone close to the ultramafic rocks. ( E ) Late retrograde stage. The former reactions promote an increase of CO 2 and thus enhanced hydrous-carbonate formation. In zones of fractures, the excess of CO 2 should have been channelized generating hydrous Mg-carbonates and calcite veins. After complete mineralization in the El Robledal deposit, it is expected that relatively high temperatures were maintained for some time, associated with the presence of Ronda Peridotites, which allowed for the recovery and recrystallization of ore minerals. Minerals 2023,13, 300 24 of 28 The timing of amphibolite formation is not well constrained. It has been usually ascribed to diopside replacement during the late alkaline retrograde stage [ 3 ] promoting Caskarn formation [ 30 ]. However, the field relationship suggests that amphibolite formation was related to bimetasomatism rather than infiltration metasomatism. Therefore, B 2 O 3 , SiO 2 , Al 2 O 3 , and volatiles should have been provided by migmatites, whereas the dolomitic marble may have fundamentally been the source of MgO and CaO. 6. Conclusions The El Robledal deposit is located in an uncommon peridotite-related geological setting. Based on our results, we conclude that it is a hypabyssal (i.e., formation below a 2–3 km depth) deposit that belongs to the Mg-Fe-B skarn type defined by Marincea and Dumitras [ 14 ]. Our interpretation suggests that ore deposition and replacement took place during high-temperature deformation in the presence of fluids, and such deformation is ascribed to the hot emplacement of the Ronda Peridotites, which also acted as the thermal source of the system. Likewise, the heat emanating from the ultramafic rocks promoted anatexis and dehydration of underlying metapelites of the Blanca Unit liberating hydrothermal fluids, which are the most plausible source of B. Thus, we propose that the heat source (i.e., Ronda Peridotites) and the mineralizing-fluid origin (i.e., anatexis of metapelites) of the skarn system were decoupled. Thus, the thermal gradient decreased outward to the peridotite, which allowed it to reach temperatures high enough to form periclase via dolomitic marble decomposition (i.e., ~610 ◦ C) before the participation of fluids escaping from metapelites of the Blanca Unit. Finally, we conclude that the participation of ultramafic rocks in the system does not markedly modify the mineral assemblage and their chemical composition. Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/min13030300/s1, Table S1: Results of EPMA analyses of oxides, Table S2: Results of EPMA analyses of ludwigite, Table S3: Results of EPMA analyses of forsterite, Table S4: Results of EPMA analyses of carbonates, Table S5: Results of EPMA analyses of brucite, Table S6: Results of EPMA analyses of szaibelyite, Table S7: Results of EPMA analyses of serpentine group minerals, Table S8: Results of EPMA analyses of hydrous Mg-carbonates, Table S9: Results of EPMA analyses of amphibole. Author Contributions: Conceptualization, I.F., F.G., and J.M.G.-J.; methodology, I.G.-P., I.F., and E.A.; software, I.F., and E.A.; formal analysis, I.G.-P., I.F., and G.A.; resources, F.G. and J.M.G.-J.; data curation, I.G.-P., I.F., and G.A.; writing—original draft preparation, I.G.-P., I.F., and G.A.; review and editing, I.F., F.G., J.M.G.-J., A.A.-V., and E.A.; supervision, F.G., and J.M.G.-J.; All authors have read and agreed to the published version of the manuscript. Funding: This study was funded by the grant PRE2019-088262 “Ayudas para contratos predoctorales para la formación de doctores”, defrayed by the “Ministerio de Ciencia, Innovación y Universidades” and the MECRAS Project A-RNM-356-UGR20 “Proyectos de I+D+i en el marco del Programa Operativo FEDER Andalucía 2014-2020” defrayed by the “Junta de Andalucía”. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data used in this research are properly cited and reported in the main text. Acknowledgments: The authors would like to acknowledge the use of “Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza”, particularly the help of C. Gallego and L. Casado Zueras from the “Advanced Microscopy Laboratory” (LMA) for their careful preparation of the EBSD maps and FIB samples, respectively. We would also like to thank X. Llovet from “Serveis Cientifico-Técnics” (CCiTUB) of the Universidad de Barcelona for his invaluable assistance with the electron probe micro-analyzer. Conflicts of Interest: The authors declare no conflict of interest. Minerals 2023,13, 300 25 of 28 References 1. Helvacı, C. Borate deposits: An overview and future forecast with regard to mineral deposits. J. Boron 2017,2, 59–70. 2. Pertsev, N.N. Magnesian skarns. In Skarns. 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