Fernando Gervilla

High-pressure aluminous mafic rocks from the Ronda peridotite massif, southern Spain: significance of sapphirine- and corundum-bearing mineral assemblages

We firstly present detailed microtextural relationships in sapphirine- and corundum-bearing aluminous mafic rocks alternating with peridotites from the Ronda massif, southern Spain. Garnet and clinopyroxene are the main phases in the aluminous mafic rocks. Garnet is partially to completely kelyphitized. Clinopyroxene (CPX I) has high Al2O3 content in the core (up to 16 wt.%) and is partially converted to spherical symplectitic aggregates consisting of lower-Al2O3 clinopyroxene (CPX II) (<5 wt.%) and plagioclase. Corundum is associated with spinel and plagioclase. Sapphirine occurs in two different mineralogical associations, i.e., as elongated lamellae within CPX I and as very fine-grained crystals in plagioclase-rich domain. The sapphirine+plagioclase aggregate suggests the former presence of kyanite as one of reactants. Sapphirine is partially converted to symplectitic aggregate of spinel and plagioclase. The earliest metamorphic mineral assemblage recorded in the aluminous mafic rock was garnet+clinopyroxene±kyanite±corundum, i.e., eclogitic mineral assemblages, at P≥1.5 GPa and T≥900°C. Many reaction textures were developed during decompression possibly coupled with cooling. The latest P–T conditions recorded in the aluminous mafic rock were at P=1 GPa and T=800–900°C. This decompression had possibly followed the compression of plagioclase-rich, low-pressure cumulate or residue.

Zoning of laurite (RuS2)–erlichmanite (OsS2): implications for the origin of PGM in ophiolite chromitites

We have investigated several chromite deposits in the Mayari-Baracoa Ophiolite Belt (eastern Cuba) and in the Dobromirtsi metamorphosed ultramafic (ophiolitic) massif (SE Bulgaria) with regard to zoning in platinum-group minerals (PGM) of the laurite (RuS 2 )–erlichmanite (OsS 2 ) solid solution series. We found several zoned laurite–erlichmanite grains all included in unaltered chromite crystals. On the basis of internal ordering and compositional variations, three different patterns of zoning have been distinguished: (i) grains with Os-poor (laurite) core and Os-rich rim (normal zoning), (ii) grains with Os-rich core and Os-poor rim (reverse zoning) and (iii) grains made up of a complex intergrowth of Os-rich, Os-poor laurite and erlichmanite (oscillatory zoning). The origin of zoning is interpreted mainly as a result of changes in f (S 2 ), f (O 2 ) and to a lesser extent in melt temperature, before PGM trapping in chromite. A possible case of heterogeneous physicochemical environment in which such changes can take place is when chromite forms during magma mingling of silicate melts in the upper mantle. The preservation of laurite–erlichmanite zoning is attributed to the low diffusion coefficient of Ru and Os in pyrite-type structures.

Petrochemical characteristics of felsic veins in mantle xenoliths from Tallante (SE Spain): an insight into activity of silicic melt within the mantle wedge

Felsic and related veins within mantle-derived peridotite xenoliths from Tallante, Spain, were examined in order to understand the mantle-wedge processes related to the behaviour of Si-rich melt. The thickest part of the vein has a quartz diorite lithology, and is composed mainly of quartz and plagioclase, with pyroxenes, hydrous mineral, apatite, zircon and rutile present as minor phases. The thinner parts are free of quartz and predominantly composed of plagioclase. Orthopyroxene always intervenes between the internal part (plagioclase ± quartz) and host peridotite, indicating that it is a product of interaction between silica-oversaturated melt and olivine. This indicates that a sufficiently high melt/wall rock ratio enabled the melt to retain its silicaoversaturated character. The quartz diorite part has adakite-like geochemical signatures, except for negative Ba, Rb Eu and Sr anomalies, and positive Th and U anomalies. These negative anomalies indicate that fractionation of plagioclase and hydrous minerals was achieved between the upper most mantle and the slab melting zone. The shape of the rare-earth element (REE) pattern of clinopyroxene in quartz diorite is strikingly similar to that of clinopyroxene phenocrysts from Aleutian adakites. However, the former has one order higher REE contents than the latter, except for Eu which shows a prominent negative spike. This feature was caused by the precipitation of large amounts of plagioclase and small amounts of clinopyroxene from a fractionated adakitic melt before and during solidification. This adakitic melt was produced by partial melting of a detached and sinking slab beneath the Betic area in the Tertiary.

Platinum-, palladium- and gold-rich arsenide ores from the Kylmäkoski Ni-Cu deposit

SummaryThe Kylmäkoski deposit consists of a disseminated primary Ni-Cu mineralization hosted by a differentiated ultramafic body. It also shows sulfide veins (tens of meters long and up to 20 cm thick) that evolve laterally to massive Ni-arsenide ores. In these sulfide/arsenide veins, three different ore assemblages can be distinguished: 1) sulfide ores (S ores) composed of pyrrhotite, pentlandite and chalcopyrite with minor amounts of cubanite, sphalerite and argentopentlandite which locally occurs intergrown with Ag-free pentlandite; 2) sulfide/arsenide ores (S/As ores) made up of the former S ores corroded and replaced by nickeline (locally with graphite), with gersdorffite filling discordant veins, abundant minute grains of sudburyite and accessory molybdenite, ullmanite, stibnite, galena and breithauptite; 3) arsenide ores (As ores) composed of nickeline, maucherite and disseminated, zoned cobaltite, with minor chalcopyrite, cubanite, sperrylite, sudburyite, electrum, galena, altaite and pilsenite. These veined ore assemblages were generated by the remobilization of primary, late magmatic arsenide-rich ores (well represented in the Vammala mine) by the intrusion of pegmatitic fluids derived from the partial melting of the metasedimentary country rocks.The early fractional crystallization of the monosulfide solid solution produced a residual As-rich melt that collected most noble metals (specially Pt, Pd and Au) leaving the primary Ni-Cu sulfide ores impoverished in these elements. In fact, late magmatic arsenide ores from Vammala contain up to 42.5 ppm Pd (in the form of extremely fine inclusions of sudburyite in nickeline and maucherite, and dissolved in trace amounts in the lattice of the latter Ni arsenides) and 9.6 ppm Au (concentrated in abundant minute inclusions of electrum in Ni arsenides). Later, during the remobilization of the primary arsenide ores of Kylmäkoski, Pd concentrated both in S/As and As ores in the form of sudburyite and in a rare PdBi compound. It also occurs in trace amounts in nickeline from S/As ores and in maucherite from As ores. Pt mainly concentrated in As ores as sperrylite and, in minor amounts in pilsenite and in cobaltite coronas around sperrylite. It occurs in trace amounts in the cores of zoned cobaltite. Gold is always present in the form of irregular grains of electrum in As ores.ZusammenfassungDie Lagerstätte Kilmäkoski ist eine disseminierte primäre Ni-Cu-Vererzung, die in einem differenzierten ultramafischen Körper aufsitzt. Hier treten auch Sulfid-Gänge, die bis zu Zehnern von Metern lang und bis zu 20 cm mächtig sein können, auf; aus diesen entwickeln sich lateral massive Nickel-Arsenid Erze. Drei Erzparagenesen können in diesen Sulfid-Arsenid-Gängen unterschieden werden: 1. Sulfidische Erze mit Pyrrhotin, Pentlandit, Kupferkies und geringen Mengen von Cubanit, Zinkblende und Argentopentlandit der örtlich mit Ag-freiem Pentlandit verwachsen ist 2. Sulfid-Arsenid Erze, die aus korrodierten und durch Rotnickelkies verdrängten Sulfid-Erzen bestehen. Diese führen örtlich Graphit, Gersdorffit kommt als Füllung diskordanter Gänge vor. Außerdem gibt es verbreitet kleine Körner von Sudburyit und akzessorischem Molybdänit, Ullmanit, Antimonglanz, Bleiglanz und Breithaup tit. 3. Arsenid-Erze, die aus Rotnickelkies, Maucherit und disseminiertem, zonarem Kobaltit, mit Kupferkies, Cubanit, Sperrylit, Sudburyit, Elektrum, Bleiglanz, Altait und Pilsenit als Nebengemengteile bestehen. Diese gangförmigen Erzparagenesen entstanden durch die Remobilisation von primären, spätmagmatischen Arsenidreichen Erzen, die in der Vammala-Mine sehr gut aufgeschlossen sind, und auf die Intrusion pegmatitischer Fluide zurückgehen, die durch teilweises Aufschmelzen der metasedimentären Nebengesteine entstanden sind.Die frühe fraktionierte Kristallisation der Monosulfid Solid Solution führte zu einer residualen As-reichen Schmelze, die den Großteil der Edelmetalle (besonders Pt, Pd und Au) aufgenommen und die primären Ni-Cu Sulfiderze an diesen Elementen verarmt zurückgelassen hat. Spätmagmatische Arseniderze aus Vammala enthalten bis zu 42,5 ppm Pd (in Form von extrem feinkörnigen Einschlüssen von Sudburyit in Rotnickelkies und Maucherit, und als Spurengehalte im Gitter der späten Nickel-Arsenide), sowie 9,6 ppm Au, das hauptsächlich in den verbreiteten winzigen Einschlüssen von Electrum in Nickelarseniden vorkommt. Während der späteren Remobilisierung der primären Arseniderze von Kylmäkoski wurde Pd sowohl in S/As und As-Erzen in der Form von Sudburyit und in einer seltenen Pd-Bi Verbindung konzentriert. Es kommt auch als Spurenelement im Rotnickelkies aus S/As-Erzen und im Maucherit aus As-Erzen vor. Pt is vorwiegend in As-Erzen konzentriert, und zwar als Sperrylit, sowie in geringen Mengen in Pilsenit und in Colbaltit-Rändern um Sperrylit. Es kommt in Spurenelementen in den Kernen von zonaren Kobaltiten vor. Gold liegt stets in Form unregelmäßiger Elektrum-Körner in As-Erzen vor.

Platinum-group elements-bearing pyrite from the Aguablanca Ni-Cu sulphide deposit (SW Spain): a LA-ICP-MS study

Despite the fact that pyrite is a relatively common phase in Ni-Cu-Platinum-Group Elements (PGE) magmatic sulphide deposits, the trace element content of the pyrite has been neglected in the studies of these deposits with most attention being paid to the PGE concentrations of pyrrhotite, pentlandite and chalcopyrite. Pyrite in these deposits exhibits a range of textures, from euhedral to xenomorphic. The origin of the different pyrites is not always clear; they could have formed by exsolution from monosulphide solid solution (mss), by replacement of the existing minerals during cooling or metamorphism or directly from hydrothermal fluids. In order to provide data on trace element contents of pyrite in a magmatic sulphide deposit and to investigate the origin of the pyrite, we have measured the content of PGE and other chalcophile elements (Au, Ag, Co, Ni, Cu, Se, Sb, As, Bi and Te) by laser ablation ICP-MS in pyrite exhibiting different textures from the Aguablanca Ni-Cu deposit (Spain). The results show that 1) large idiomorphic pyrite is compositionally-zoned with Os-Ir-Ru-Rh-As-rich layers and Se-Co-rich layers; 2) some idiomorphic pyrite grains contain unusually high PGE contents (up to 32 ppm Rh and 9 ppm Pt); 3) ribbon-like and small-grained pyrite hosts IPGE (i.e., Iridium-group PGE, Os, Ir, Ru and Rh) in similar contents (100–200 ppb each) than the host pyrrhotite; and 4) pyrite replacing plagioclase is depleted in most metals (i.e., PGE, Co, Ni and Ag). Overall, the different textural types of pyrite have similar abundances of Pd, Au, Se, Bi, Te, Sb and As. Mineralogical and compositional data suggest that the formation of pyrite is the result of the activity of late magmatic/hydrothermal fluids that triggered the partial replacement of pyrrhotite and plagioclase by pyrite, probably due to an increase in the sulphur fugacity on cooling. During this episode, pyrite inherited the IPGE content of the replaced mineral, whereas other elements such as Pd, Au and semi-metals were likely partially introduced into pyrite via altering fluids. These results highlight that pyrite can host appreciable amounts of PGE and therefore it should not be overlooked as a potential carrier of these metals in Ni-Cu-(PGE) sulphide deposits.

Closed-system geochemical recycling of crustal materials in alpine-type peridotite

We examined aluminous mafic rock (with or without corundum or sapphirine) alternating with peridotite from the Ronda peridotite massif, southern Spain. On the basis of petrographic characteristics, these mafic rocks show a decompression history from high pressure (P > 1.5 GPa), but on the basis of their geochemical characteristics, they are crystal accumulates of plagioclase, clinopyroxene, and olivine formed within the lower crust (P < 1 GPa). A complex evolution history, including higher-pressure recrystallization after initial formation as cumulate gabbros at lower-pressure conditions, is proposed. The aluminous mafic rocks and their peridotite hosts are inferred to be recycled crustal materials now observed as centimeter-scale layered components in alpine-type peridotite. The rocks retained their original cumulate compositions; that is, their compositions were not affected by melting and metasomatic modifications during subduction, intense deformation within the upper mantle, and upwelling to the surface.

Origin and emplacement of the Aguablanca magmatic Ni-Cu-(PGE) sulfide deposit, SW Iberia: A multidisciplinary approach

A model is proposed for the origin and emplacement of the ca. 341 Ma Aguablanca magmatic Ni-Cu-(platinum group element [PGE]) sulfi de deposit (SW Iberia) integrating petrological, geochemical, structural, and geophysical data. The Aguablanca deposit occurs in an unusual geodynamic context for this ore type (an active plate margin) as an exotic , magmatic subvertical breccia located at the northern part of the coeval gabbronorite Aguablanca stock (341 ± 1.5 Ma). Structural and gravity data show that mineralized breccia occurs inside the inferred feeder zone for the stock adjacent to the Cherneca ductile shear zone, a Variscan sinistral transpressional structure. The orientation of the feeder zone corresponds to that expected for tensional fractures formed within the strain fi eld of the adjacent Cherneca ductile shear. Two distinctive stages are established for the origin and emplacement of the deposit: (1) initially, the ore-forming processes are attributed to magma emplacement in the crust, assimilation of crustal S, and segregation and gravitational settling of sulfi de melt (a scenario similar to most plutonic Ni-Cu sulfi de ores), and (2) fi nal emplacement of the Ni-Cu sulfi de-bearing rocks by multiple melt injections controlled by successive opening events of tensional fractures related to the Cherneca ductile shear zone.

The architecture of the European-Mediterranean lithosphere: A synthesis of the Re-Os evidence

Rhenium-depletion model ages ( T RD ) of sulfides in peridotite xenoliths from the subcontinental mantle beneath central Spain (the Calatrava volcanic field) reveal that episodes of mantle magmatism and/or metasomatism in the Iberia microplate were linked to crustal growth events, mainly during supercontinent assembly and/or breakup at ca. 1.8, 1.1, 0.9, 0.6, and 0.3 Ga. A synthesis of available in situ and whole-rock Os-isotope data on mantle-derived peridotites shows that this type of mantle (maximum T RD of ca. 1.8 Ga) is widespread in the subcontinental mantle of Europe and Africa outboard from the Betics-Maghrebides-Appenines front. In contrast, the mantle enclosed within the Alpine domain records T RD as old as 2.6 Ga, revealing a previously unrecognized Archean domain or domains in the central and western Mediterranean. Our observations indicate that ancient fragments of subcontinental lithospheric mantle have played an important role in the development of the present architecture of the Mediterranean lithosphere.

D, O and C isotopes in podiform chromitites as fluid tracers for hydrothermal alteration processes of the Mayari-Baracoa Ophiolitic Belt, eastern Cuba

The Mayari — Baracoa Ophiolitic Belt (MBOB, eastern Cuba) is composed of two large, chromite-rich massifs: Mayari —Cristaland Moa-Baracoa. The chromitites and hosting dunites were firstly affected by a regional serpentinization event, a subsequent episode of hydrothennal alteration (chloritization mainly) and, finally, these already altered bodies were crosscut by thin calcite-dominated veins. Analysed serpentines from serpentinized chromitites and dunites present very similar isotopic compositions (δ18O=+4.7 to +6.3 and δD= −67 to −60 , suggesting that the serpentinization process took place at moderate temperatures, in an oceanic environment. Serpentine formation by interaction with ocean water is also supported by the isotopic composition of chlorite and calcite. These results suggest that the serpentinization, chloritization and fracture filling processes of the Mayari — Baracoa Ophiolite Belt took place in a subocean floor scenario and, thus, that the Mayari — Baracoa serpentines represent a good example of serpentine formed during interaction with seawater. The oceanic origin of the serpentines from serpentinized chromitites and dunites from the MBOB indicate that the serpentinization of the mantle sequence occurred pre-thrusting (pre-emplacement in age).

Compositional effects on the solubility of minor and trace elements in oxide spinel minerals: Insights from crystal-crystal partition coefficients in chromite exsolution

Abstract Chromite from Los Congos and Los Guanacos in the Eastern Pampean Ranges of Córdoba (Argentinian Central Andes) shows homogenous and exsolution textures. The composition of the exsolved phases in chromite approaches the end-members of spinel (MgAl2O4; Spl) and magnetite (Fe2+Fe23+