¿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

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

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

Petrogénesis del cuerpo de cromitita de la ofiolita del Cerro Colorado, península de Paraguaná, Venezuela

This research was financially supported by FEDER Funds through the projects CGL2015-65824-P and CGL2014-55949-R granted by the Spanish "Ministerio de Economia y Competitividad." Additional funding was provided by the Ramon y Cajal Fellowship RYC-2015-17596 granted by the Spanish MINECO to JMGJ.Ultramafic-mafic rocks of ophiolitic affinity crop out along the Venezuelan Caribbean region. They have been interpreted as remnants of the oceanic lithosphere of the Caribbean volcanic arc (135-70 Ma) as well as relicts of proto-Caribbean oceanic lithosphere (Upper Jurassic-Lower Cretaceous) related to Pangea’s break-up. The Cerro Colorado ophiolite, located in the Paraguaná Peninsula, together with the case of the Cordillera de la Costa in north-central Venezuela, are a unique case of these Venezuelan ophiolites containing chromitite bodies. However, the petrogenesis of such a mafic-ultramafic complex and associated chromite ore remains are unknown to date. To advance our understanding of chromite ores in the Caribbean region, the genesis of the Cerro Colorado chromitite body is challenged. The Cerro Colorado chromitite body is characterized by a low-Cr content [Cr# =Cr/ Cr+Al= 0.44-0.60] and a distribution of trace elements in chromite as is typical of high-Al chromitites found in the shallower portions of the petrological Moho Transition Zone of Mesozoic ophiolites. The calculated melts in equilibrium with chromite forming this high-Al chromitite body are back-arc basin basalts. These melts were extracted after ~20 % partial melting of moderately depleted peridotites, which resulted in the precipitation of high-Al chromitite relatively impoverished in PGE (≤ 100 ppb total PGE). A comparison of the geochemical signatures of minor and trace elements in chromite and bulk-PGE contents of the Cerro Colorado chromitite with those of other known chromitites in the peri-Caribbean ophiolites show certain similitude with those high-Al described in the Moa-Baracoa ophiolite in eastern Cuba. The obtained results allow us to suggest that the ultramafic rocks of the Cerro Colorado and the chromitite body associated with it are closely related to the formation of a back-arc basin developed between ca. 125-120 Ma in the rear of the Great Antilles Arc.El margen caribeño de Venezuela se caracteriza por la presencia de algunos afloramientos de rocas máficas y ultramáficas de afinidad ofiolíca. Dichas rocas se han intepretado como fragmentos de la litosfera oceánica del Arco volcánico Caribeano (~135-70 Ma) así como relictos de la litosfera oceánica del proto-Caribe (Jurásico Superior-Cretácico Inferior) relacionados con la ruptura de Pangea. La ofiolita de Cerro Colorado, localizada en la Península de Paraguaná es, conjuntamente con el caso de la Cordillera de la Costa en la parte centro norte de Venezuela, el único caso de esas ofiolitas Venezolanas que contienen cuerpos de cromititas. Sin embargo, se desconoce aún la petrogénesis de dichos complejos de rocas máficas y ultramáficas. Con el objeto de avanzar en el estado del conocimiento de la génesis de menas de cromita en la región del Caribe, en este trabajo se aborda el estudio de la génesis del cuerpo de cromitita de Cerro Colorado. El cuerpo de cromitita de Cerro Colorado está constituido esencialmente por cromita con bajo contenido en Cr [Cr# =Cr/ Cr+Al= 0.44-0.60] y una distribución de elementos trazas similar a la descrita en otras cromititas con alto contenido en Al documentadas en las zonas más someras de la Zona de Transición de la Moho petrológica de otras ofiolitas de edad Mesozoica. Los fundidos calculados en equilibrio con la cromita que forma este cuerpo de cromititas con alto Al son basaltos de trasera de arco. Estos fundidos fueron extraídos de una mantélica formada por peridotitas empobrecidas como resultado de tasas de fusión parcial de ~20 %, lo que dió lugar a la precipitación de cromititas con alto Al relativamente empobrecidas en EGP (≤ 100 ppb suma total de EGP). Una comparación de las firmas geoquímicas de los elementos menores y trazas en la cromita y contenidos de EGP obtenidos a partir del análisis de muestras de roca total de la cromitita de Cerro Colorado con otras cromititas documentadas en el manto de ofiolitas peri-Caribeñas muestra cierta similitud con aquellas altas en Al descritas en la ofiolita de Moa-Baracoa en Cuba oriental. Los resultados obtenidos nos permiten sugerir que las rocas ultramáficas de Cerro Colorado y el cuerpo de cromitita que éstas albergan están íntimamente relacionados con la formación de una cuenca de retro arco desarrollada en un el intervalo temporal 125-120 Ma en la trasera del Arco de las Antillas Mayores.FEDER Funds - Spanish "Ministerio de Economia y Competitividad CGL2015-65824-P CGL2014-55949-RRamon y Cajal Fellowship - Spanish MINECO RYC-2015-1759

Los depósitos de cromita en complejos ofiolíticos: discusion de un modelo de formación a partir de las particularidades de las cromititas de Cuba Oriental.

En la parte oriental de la isla de Cuba aflora la denominada Faja Ofiolítica MayaríBaracoa (FOMB), la cual alberga abundantes depósitos de cromita. Esta faja incluye tres distritos mineros, con una marcada zonación composicional de oeste a este: El distrito de Mayarí (contiene cromitas ricas en Cr), el distrito de Sagua de Tánamo (contiene cromitas ricas en Cr y cromitas ricas en Al), y el distrito de Moa-Baracoa (contiene cromitas ricas en Al). En el distrito de Mayarí las cromititas encajan en harzburgitas y dunitas representativas de sectores mantélicos relativamente profundos y están cortadas por diques de piroxenitas. La cromita presenta altos contenidos en Cr (Cr# = O. 70-0.80), similares a los de la cromita accesoria en las dunitas (0.70-0.74), pero mayores que los de la cromita accesoria en las harzburgitas (0.56-0.69). La cromitita muestra contenidos relativamente altos de EGP (hasta 227 ppb). En el distrito de Sagua de Tánamo, los depósitos de cromita encajan en dunitas y harzburgitas que contienen anfíboles. Algunas cromititas incluyen lentes concordantes y/o discordantes de gabro-hornblenda. El #Cr de la cromita varía entre 0.46 y O. 72, y el contenido total de EGP entre 19 y 539 ppb, existiendo una perfecta correlación entre ambos parámetros. En las cromititas con #Cr � 0.6, él #Cr aumenta progresivamente desde la cromita accesoria en las harzburgitas encajantes, a la accesoria en las dunitas, hasta la cromita del cuerpo de cromitita. Por el contrario, en las cromititas con #Cr � 0.6, él #Cr de la cromita muestra una tendencia de variación opuesta. En el distrito de Moa-Baracoa los depósitos de cromita se localizan en la zona de transición entre el manto y la corteza, frecuentemente contienen cuerpos tabulares de gabros paralelos al eje mayor de los pods de cromitita y están cortados por diques de gabros y pegmatoides gabroicos. La cromita es rica en Al (#Cr = 0.41 -0.54) y pobre EGP (I.EGP = 20- 1 14 ppb)

Los modelos genéticos de los depósitos de cromita en complejos ofiolíticos, a través de las peculiaridades de las cromititas de Cuba Oriental

El origen de las cromititas podiformes, especialmente el mecanismo de concentración de cromita y el ambiente tectónico de formación, continua siendo un tema sujeto a debate (Arai y Yurimoto, 1 994; Zhou y Robinson, 1 997; Melcher et al., 1 997; Bédard y Hérbert, 1 998; Proenza et al., 1 999)

Coeficientes de reparto de Elementos del Grupo del Platino entre fundidos sulfurados y arseniurados

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Nanoscale Structure of Zoned Laurites from the Ojén Ultramafic Massif, Southern Spain

We report the first results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of zoned laurite (RuS2)-erlichmanite (OS2) in mantle-hosted chromitites. These platinum-group minerals form isolated inclusions (<50 um across) within larger crystals of unaltered chromite form the Ojén ultramafic massif (southern Spain). High-magnification electron microscopy (HMEM), high angle-annular dark field (HAADF) and precession electron diffraction (PED) data revealed that microscale normal zoning in laurite consisting of Os-poor core and Os-rich rims observed by conventional micro-analytical techniques like field emission scanning electron microscope and electron microprobe analysis (FE-SEM and EPMA) exist at the nanoscale approach in single laurite crystals. At the nanoscale, Os poor cores consist of relatively homogenous pure laurite (RuS2) lacking defects in the crystal lattice, whereas the Os-richer rim consists of homogenous laurite matrix hosting fringes (10–20 nm thickness) of almost pure erlichmanite (OsS2). Core-to-rim microscale zoning in laurite reflects a nonequilibrium during laurite crystal growth, which hampered the intra-crystalline diffusion of Os. The origin of zoning in laurite is related to the formation of the chromitites in the Earth’s upper mantle but fast cooling of the chromite-laurite magmatic system associated to fast exhumation of the rocks would prevent the effective dissolution of Os in the laurite even at high temperatures (~1200 ºC), allowing the formation/preservation of nanoscale domains of erlichmanite in laurite. Our observation highlights for the first time the importance of nanoscale studies for a better understanding of the genesis of platinum-group minerals in magmatic ore-forming systems.This research was supported by Spanish projects: RTI2018-099157-A-I00 and CGL2015-65824-P granted by the “Ministerio de Ciencia, Innovación y Universidades” and Ministerio de Economía y Competitividad” (MINECO) respectively. Additional funding was provided by the Ramón y Cajal Fellowship RYC-2015-17596 granted by the Spanish MINECO to JMGJ. A. Jiménez-Franco is supported with a postdoctoral grant (CVU 350809) from the National Council on Science and Technology (CONACYT) of Mexico