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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 (Mg2Fe3+(BO3)O2), Mg-rich magnetite, and magnetite, with minor amounts of kotoite (Mg3(BO3)2), szaibelyite (MgBO2(OH)), accessory schoenfliesite (MgSn4+(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.

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<p>Chemical signatures of magnetite are commonly used to track the evolution of mineralizing systems in many geological settings. However, the impact of deformation processes on magnetite chemistry remains still underexplored. Here, we report a rare case of composite crystals consisting of magnetite and magnesioferrite recording different degrees and styles of deformation in order to evaluate how deformation promotes chemical modification. The samples employed in this study come from two different Mg-skarn iron deposits (i.e., El Robledal and San Manuel) from the Serranía de Ronda (SW Spain). Chemical data acquired by Electron Probe Microprobe Analyzer (EPMA) and Field Emission Scanning Microscopy (FESEM) are contrasted against microstructural data obtained by using Electron Back-Scattered Diffraction (EBSD). Our results show that magnesioferrite crystals [Fe<sup>2+</sup># (Fe<sup>2+</sup>/Fe<sup>2+</sup>+Mg<sup>2+</sup>) = 0.22-0.46 and Fe<sup>3+</sup># (Fe<sup>3+</sup>/Fe<sup>3+</sup>+Al<sup>3+</sup>) = 0.99-1.00] from El Robledal deposit are characterized by a ductile deformation that led to different crystallographic orientation domains along with the replacement of magnesioferrite by magnetite (Fe<sup>2+</sup># (Fe<sup>2+</sup>/Fe<sup>2+</sup>+Mg<sup>2+</sup>) = 0.51-0.99 and Fe<sup>3+</sup> (Fe<sup>3+</sup>/Fe<sup>3+</sup>+Al<sup>3+</sup>) =0.98-1.00] via coupled dissolution – reprecipitation. A replacement of magnesioferrite [Fe<sup>2+</sup># (Fe<sup>2+</sup>/Fe<sup>2+</sup>+Mg<sup>2+</sup>) = 0.43-0.64 and Fe<sup>3+</sup> (Fe<sup>3+</sup>/Fe<sup>3+</sup>+Al<sup>3+</sup>) = 0.99-1.00] by magnetite Fe<sup>2+</sup># (Fe<sup>2+</sup>/Fe<sup>2+</sup>+Mg<sup>2+</sup>) = 0.78-1.00 and Fe<sup>3+</sup># (Fe<sup>3+</sup>/Fe<sup>3+</sup>+Al<sup>3+</sup>) = 0.98-1.00] via a coupled dissolution – reprecipitation mechanism is also preserved in the composite (i.e., zoned) crystals from the San Manuel deposit, which was additionally overprinted by an additional recrystallization event as a result of grain boundary migration recrystallization. Our results show that deformation in a fluid-assisted deformation regime has induced chemical modification of the original magnesioferrite aggregates as well as strain localization. This close physicochemical link offers new avenues of interpreting the chemical signatures of Mg-Fe oxides, utilizing their microstructurally controlled variation or lack thereof.</p>

The Iberian Pyrite Belt (IPB), in the southwestern Iberian Peninsula, is a large metallogenic province exploited since ancient times. As a result of historical and current mining activity, a vast volume of metallic mineral waste, mainly derived from the processing of pyrite, is still in situ and polluting the environment. A specific mine waste residuum locally known in the area as “morrongos”, which was produced during pyrite roasting mainly in the 19th century, is evaluated here in order to unravel untapped resources of high-tech metals commonly used in high-tech devices. Applying a combination of whole-rock geochemical (ICP-AES, ICPMS, FA-AAS) and single-grain mineralogical techniques (EPMA, LA-ICP-MS, FESEM, and FIB-HRTEM) on the “morrongos”, we unhide the still-present remarkable concentrations of Au, Ag, Pb, Zn, and Cu in them. The mineralogical expressions for these economic metals include oxides (hematite, magnetite, and hercynite), arsenates, sulfates of the jarosite group, native metals, and, to a lesser extent, relictic sulfides. This first-ever estimation of these economic metals in this type of residue allows their revalorization, highlighting them as suitable sources for the exploitation and recovery of metals necessary for the clean energy transition.