¿Esférulas de hierro y vítreas subducidas en el manto superior?

Spherules are documented in ophiolitic mantle rocks such as peridotites and associated chromitites. They consist of: (1) native iron having variable amounts of Ni with/without inclusions of silicate glass or oxides (wüstite), (2) dendritic intergrowth of oxides (magnetite, wüstite and hematite) with/without silicate glass and, (3) silicate glass. Consensually, they are interpreted as indigenous to chromitites and related with high-temperature processes operating in the Earth’s upper mantle. However, their similarity with terrestrial and extraterrestrial spherules found in other settings of the geological record is remarkable. We raise the question on such indigenous origin, relating them to volcanic and cosmic material recycled back to the mantle wedge where chromitites form during subduction.Rocas del manto superior ofiolitico tales como peridotitas y cromititas contienen esferas. Estas consisten de: (1) hierro native con cantidades variables de Ni con y sin inclusiones de vidrio silicatdo y óxidos (wüstite), (2) intercrecimientos dendríticos de óxidos (magnetita, wüstita y hematites) con o sin vidrio silicatado y, (2) vidrio silicatado. Unísonamente, estas esferulas se interpretan como indígenas a las cromititas y relacionadas con procesos de alta temperatura que tienen lugar en el manto superior terrestre. Sin embargo, su parecido con aquellas esferas de origen terrestre y extraterrestre descritas en otros contextos geológicos es reseñable. En este trabajo cuestionamos el origen autóctono de las esferas en las rocas mantélicas, interpretándolas como material de origen volcánico y cósmico que ha sido reciclado atraves de la cuña de manto superior donde ser forman las cromititas durante los procesos de subducción.Junta de Andaluci

Isotopic constraints on the age and source of ore-forming fluids of the Bou Azzer arsenide ores (Morocco)

The authors greatly acknowledge the geological survey of CTT-Bou Azzer mine for facilitating our geological field campaigns and specially to Clemente Recio (University of Salamanca) for his invaluable help to IS during the development of the analytical procedure to mea-sure S isotope compositions from the minor amounts of S extracted from arsenides and sulfarsenides. Authors would like to acknowledge the use of Servicio General de Apoyo a la Investigacion-SAI, Universidad de Zaragoza. This research was financially supported by the Spanish project RTI2018-099157-A-I00 granted by the "Ministerio de Ciencia, Innovacion y Universidades". The Swedish Research Council (infra-structure grant: Dnr. 2017-00671) is thanked for financial support to the Vegacenter national laboratory. This is Vegacenter publication number 124The Bou Azzer district in Morocco has a long mining history since the beginning of the XXst century during which it has become the only world producer of Co from primary, hydrothermal Co arsenide ores. Orebodies are structurally controlled, and mainly distributed along fault contacts between Cryogenian ophiolite-related serpentinite bodies and intrusive quartz diorite or, locally, ophiolitic gabbros or Ediacaran volcanic rocks. Ore formation took place through a multi-stage mineralizing process that included an early stage composed by gold, quartz, chlorite, muscovite and calcite, followed by the main arsenide and sulfarsenide stage (subdivided into three substages, IIa: Ni-rich, Co ores, IIb: Co-Fe ores and IIc: Fe-Co ores), and ending with an epithermal stage characterized by the precipitation of sulfides along with quartz and calcite. Field relations and most previous geochronologic dating pointed to a post Pan-African age of ore formation, mainly coincident with the Hercynian orogeny. The isotopic study presented in this paper includes S, Pb, Rb/Sr and Sm/Nd data of a set of ore mineral samples from three deposits (Aghbar, Tamdrost and Aït Ahmane), as well as of regional samples representative of the different lithologies occurring in the Bou Azzer area. The isotope data set was completed with S isotope analyses of arsenide and sulfarsenide minerals from five ore deposits (Filon 7/5, Aghbar, Tamdrost, Ightem and Aït- Ahmane) and of some whole-rock regional samples. Results show that ores formed during multi-episodic hydrothermal events connected with hercynian reactivation of Devonian-Carboniferous faults, supporting previous geochronologic dating. The obtained Pb, Sr, Nd and S isotopic signatures of ore minerals and regional rocks further show that ophiolite-related lithologies became isotopically modified by interaction with crustal material and afterwards acted as the main source of ore-forming elements. Nevertheless, isotopic data do not fully concur with such a simple scenario but are quite consistent with a rather complex interpretation based on multi-source origin of some elements and isotopes scavenged from a number of isotopically different lithologies both from the inferred basement and the volcanic and sedimentary cover.Spanish Government RTI2018-099157-A-I00Swedish Research Council European Commission Dnr. 2017-0067

Geochemical Constraints on the Origin of the Ni–Cu Sulfide Ores in the Tejadillas Prospect (Cortegana Igneous Complex, SW Spain)

After the discovery of theAguablanca ore deposit (the unique Ni–Cu mine operating in SW Europe), a number of mafic-ultramafic intrusions bearing Ni–Cu magmatic sulfides have been found in the Ossa–Morena Zone of the Iberian Massif (SW Iberian Peninsula). The Tejadillas prospect is one of these intrusions, situated close to the border between the Ossa–Morena Zone and the South Portuguese Zone of the Iberian Massif. This prospect contains an average grade of 0.16 wt%Ni and 0.08 wt%Cu with peaks of 1.2 wt%Ni and 0.2 wt%Cu. It forms part of the Cortegana Igneous Complex, a group of small mafic-ultramafic igneous bodies located 65 km west of the Aguablanca deposit. In spite of good initial results, exploration work has revealed that sulfide mineralization is much less abundant than in Aguablanca. A comparative study using whole-rock geochemical data between Aguablanca aand Tejadillas shows that the Tejadillas igneous rocks present a lower degree of crustal contamination than those of Aguablanca. The low crustal contamination of the Tejadillas magmas inhibited the assimilation of significant amounts of crustal sulfur to the silicate magmas, resulting in the sparse formation of sulfides. In addition, Tejadillas sulfides are strongly depleted in PGE, with total PGE contents ranging from 14 to 81 ppb, the sum of Pd and Pt, since Os, Ir, Ru and Rh are usually below or close to the detection limit (2 ppb). High Cu/Pd ratios (9700–146,000) and depleted mantle-normalized PGE patterns suggest that the Tejadillas sulfides formed from PGE-depleted silicate magmas. Modeling has led us to establish that these sulfides segregated under R-factors between 1000 and 10,000 from a silicate melt that previously experienced 0.015% of sulfide extraction. All these results highlight the importance of contamination processes with S-rich crustal rocks and multiple episodes of sulfide segregations in the genesis of high-tenor Ni–Cu–PGE ore deposits in mafic-ultramafic intrusions of the region

Reflectancia multiespectral (visible e inferarrojo cercano) y composición de las cromitas. Datos preliminares

Se aborda por primera vez la relación entre el espectro VNIR de reflectancia especular y los parámetros composicionales del grupo de las espinelas, particularmente cromit

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

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), Mgrich 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.PRE2019-088262 “Ayudas para contratos predoctorales para la formación de doctores”, defrayed by the “Ministerio de Ciencia, Innovación y Universidades”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

Metal Mobility in Embryonic-to-Proto-Ni-Laterite Profiles from Non-Tropical Climates

We evaluated the mobility of a wide suite of economic metals (Ni, Co, REE, Sc, PGE) in Ni-laterites with different maturities, developed in the unconventional humid/hyper-humid Mediterranean climate. An embryonic Ni-laterite was identified at Los Reales in southern Spain, where a saprolite profile of ~1.5mthick was formed at the expense of peridotites of the subcontinental lithospheric mantle. In contrast, a more mature laterite was reported from Camán in south-central Chile, where the thicker (~7 m) weathering profile contains well-developed lower and upper oxide horizons. This comparative study reveals that both embryonic and mature laterites can form outside the typical (sub)-tropical climate conditions expected for lateritic soils, while demonstrating a similar chemical evolution in terms of major (MgO, Fe2O3, and Al2O3), minor (Ni, Mn, Co, Ti, Cr), and trace (REE, Y, Sc, PGE, Au) element concentrations. We show that, even in the earliest stages of laterization, the metal remobilization from primary minerals can already result in uneconomic concentration values.MECRAS Project A-RNM-356-UGR20 “Proyectos de I+D+i en el marco del Programa Operativo FEDER Andalucía 2014-2020” of the Consejería de Economía, Conocimiento, Empresa

Metallogenic fingerprint of a metasomatized lithospheric mantle feeding gold endowment in the western Mediterranean basin

We thank the Associate Editor W.U. Reimold, and the referees T. Jalowitzki and M.L. Fioren-tini for their constructive reviews of the submitted version of the manuscript. This research was sup-ported by the BES-2017-079949 Ph.D. fellowship to ES. The Spanish projects PID2019-111715GB-I00/AEI/10.13039/501100011033 and RTI2018-099157-A-I00 provided funding for field emission gun-environmental scanning electron microscopy (FEG-ESEM) and electron microprobe microanaly-ses (EMPA) /laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of sulfides, respectively, while the Junta de Andalucia project FUMESA B-RNM-189-UGR18 financed LA-ICP-MS analyses of silicates. Research grants, infrastructures, and human resources leading to this research have benefited from funding by the European Social Fund and the European Regional Development Fund. We thank Jesus Montes Rueda (Universidad de Granada) , Isabel Sanchez Almazo (Centro de Instru-mentacion Cientifica [CIC] , Universidad de Granada) , Xavier Llovet (Centres Cientifics i Tecnolgics, Uni-versitat de Barcelona) , Miguel Angel Hidalgo La-guna (CIC, Universidad de Granada) , and Manuel Jesus Roman Alpiste (Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Cientificas-Universidad de Granada) for their careful technical assistance during sample preparation, FEG-ESEM, electron microprobe analyzes, and LA-ICP-MS analyses, respectively.Spinel peridotite xenoliths (one plagioclase- bearing) hosted in alkaline basalts from Tallante (southeast Spain) record the mineralogical and geochemical fingerprint of the subcontinental lithospheric mantle (SCLM) evolution beneath the southern Iberian margin. Mantle metasomatism in fertile lherzolites caused the crystallization of clinopyroxene + orthopyroxene + spinel clusters through the percolation of Miocene subalkaline melts during the westward migration of the subduction front in the western Mediterranean. In the Pliocene, heat and volatiles provided by alkaline host-magmas triggered very low melting degrees of metasomatic pyroxene-spinel assemblages, producing melt quenched to silicate glass and reactive spongy coronae around clinopyroxene and spinel. Refertilization of the Tallante peridotites induced the precipitation of base-metal sulfides (BMS) included in metasomatic clino- and orthopyroxene. These sulfides consist of pentlandite ± chalcopyrite ± bornite aggregates with homogeneous composition in terms of major elements (Ni, Fe, Cu) and semi-metals (Se, As, Te, Sb, Bi), but with wide variability of platinum-group elements (PGE) fractionation (0.14 < PdN/IrN < 30.74). Heterogeneous PGE signatures, as well as the presence of euhedral Pt-Pd-Sn-rich platinum-group minerals (PGM) and/or Auparticles within BMS, cannot be explained by conventional models of chalcophile partitioning from sulfide melt. Alternatively, we suggest that they reflect the incorporation of distinct populations of BMS, PGM, and metal nanoparticles (especially of Pt, Pd, and Au) during mantle melting and/or melt percolation. Therefore, we conclude that Miocene subalkaline melts released by asthenosphere upwelling upon slab tearing of the Iberian continental margin effectively stored metals in metasomatized domains of this sector of the SCLM. Remarkably high Au concentrations in Tallante BMS (median 1.78 ppm) support that these metasomatized domains provided a fertile source of metals, especially gold, for the ore-productive Miocene magmatism of the westernmost Mediterranean.Junta de Andalucia B-RNM-189-UGR18European Social Fund (ESF)European Commission BES-2017-079949 PID2019-111715GB-I00/AEI/10.13039/501100011033 RTI2018-099157-A-I0

Mantle-to-crust metal transfer by nanomelts

The transfer of chalcophile metals across the continental lithosphere has been traditionally modeled based on their chemical equilibrium partitioning in sulfide liquids and silicate magmas. Here, we report a suite of Ni-Fe-Cu sulfide droplets across a trans-lithospheric magmatic network linking the subcontinental lithospheric mantle to the overlying continental crust. Petrographic characteristics and numerical calculations both support that the sulfide droplets were mechanically scavenged from the mantle source during partial melting and transported upwards by alkalinemagmas rising through the continental lithosphere. Nanoscale investigation by high-resolution transmission electron microscopy (HR-TEM) documents the presence of galena (PbS) nanoinclusions within the sulfide droplets that are involved in the mantle-to-crust magma route. The galena nanoinclusions show a range of microstructural features that are inconsistent with a derivation of PbS by exsolution from the solid products of the Ni-Fe-Cu sulfide liquid. It is argued that galena nanoinclusions crystallized from a precursor Pb(-Cu)-rich nanomelt, which was originally immiscible within the sulfide liquid even at Pb concentrations largely below those required for attaining galena saturation. We suggest that evidence of immiscibility between metal-rich nanomelts and sulfide liquids during magma transport would disrupt the classical way by which metal flux and ore genesis are interpreted, hinting for mechanical transfer of nanophases as a key mechanism for sourcing the amounts of mantlederived metals that can be concentrated in the crust.BES-2017-079949The Spanish projects PID2019-111715GB-I00/AEI/10.13039/501100011033NANOMET PID2022- 138768OB-I00MECOBE ProyExcel_00705(FEG-ESEM), focused-ion beam (FIB)High-resolution transmission electron microscopy (HR-TEM)Australian Research Council through ARC Linkage Project LP190100785European Social FundEuropean Regional Development Fun