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.

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.

Magnetite is a widespread ore mineral in skarn systems and usually hosts a wide variety of inclusions. Micro- to nano-sized solid inclusions in magnetite are unique tools to track the evolutionary processes of its host mineral and, subsequently, to constrain the timing of the mineralization event. In this study, we characterize micro- to nano-sized solid inclusions in magnetite from the La Víbora magnesian skarn (Málaga, Spain) using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM energy-dispersive X-ray spectrometry (EDS) analyses and compositional mapping expose two types of nano-inclusions oriented along the (111) of magnetite: type 1 includes dolomite, spinel, and Mg–Fe–Al silicate, and type 2 is made up of Mg–Fe–Al silicates enveloping the Mg-bearing amorphous silica phase. High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and fast Fourier transform (FFT) patterns reveal that the majority of the solid inclusions display non-oriented matrices compared to the host magnetite, precluding the possibility of sub-solidus processes. Instead, these inclusions are thought to preserve skarn mineral assemblages that were entrapped during the growth of magnetite. However, the local supersaturation of fluids trapped in the boundary layer of crystallizing magnetite is evidenced by coherent lattice orientation of precipitated and host magnetite and by the occurrence of an Mg-bearing amorphous silica phase. Our findings reveal that skarn reactions observed at field and microscopic scales are also recorded in nano-sized inclusions within magnetite. These observations underscore the significance of micro- to nano-scale solid inclusions in magnetite to decipher overprinted skarn reactions as well as constraining the timing of Fe mineralization events in skarns.

The Triassic sedimentary rocks of the external zone of the Betic Cordillera in southern Spain host abundant igneous basic rocks (“ophites”) metamorphosed in the greenschist facies. Investigation based on high-resolution transmission electron microscopy (HRTEM) reveals magmatic sulfides intimately intergrown with primary silicates (pyroxene, plagioclase and olivine), consisting of pyrrhotite (FeS to Fe₇S₈; monoclinic C2/c polytype 4C) and pentlandite ((FeNi)₉S₈; cubic Fm3m), bounded by tetragonal I$\stackrel{\mathrm{‾}}{\mathrm{4}}$ 2d chalcopyrite (CuFeS₂). Whole-rock and clinopyroxene geochemical data suggest that Fe–Ni–Cu sulfides crystallized from melt droplets segregated by immiscibility from transitional-alkaline magmas, after their fractional crystallization and volatile oversaturation within crustal conduits developed during intraplate rifting. In contrast, hydrothermal sulfides are pyrite (FeS₂) and/or chalcopyrite associated with secondary silicates (chlorite, epidote, saussurite, prehnite, sericite and quartz). Porous pyrite (cubic Pa3) hosts nanodomains of pyrrhotite (monoclinic C2/c polytype 4C and orthorhombic Cmca polytype 11C) and orthorhombic Pnnm marcasite (FeS₂), evidencing disulfide formation via coupled Fe loss and S addition in pyrrhotite due to reaction with high-fS₂ and high-fO₂ fluids at < 300 °C. Tetragonal I$\stackrel{\mathrm{‾}}{\mathrm{4}}$ 2d chalcopyrite replacing pyrite also preserves nanodomains of pyrrhotite as well as of pyrite $/$ marcasite, suggesting reaction of pre-existing Fe sulfides with Cu transported by low-temperature (∼ 200–100 °C) acidic-to-neutral fluids. Additionally, inclusions of sphalerite (ZnS), galena (PbS) and vaesite (NiS₂) in pyrite and chalcopyrite spotlight metal remobilization during hydrothermal alteration, which is further confirmed by the crystallization of cobaltite coronas (CoAsS) around pyrite. Our new results contribute to the current debate of the full sequence of processes operating in ore-forming mafic systems.

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 [Fe2+# (Fe2+/Fe2++Mg2+) = 0.22-0.46 and Fe3+# (Fe3+/Fe3++Al3+) = 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 (Fe2+# (Fe2+/Fe2++Mg2+) = 0.51-0.99 and Fe3+ (Fe3+/Fe3++Al3+) =0.98-1.00] via coupled dissolution – reprecipitation. A replacement of magnesioferrite [Fe2+# (Fe2+/Fe2++Mg2+) = 0.43-0.64 and Fe3+ (Fe3+/Fe3++Al3+) = 0.99-1.00] by magnetite Fe2+# (Fe2+/Fe2++Mg2+) = 0.78-1.00 and Fe3+# (Fe3+/Fe3++Al3+) = 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.