Ophiolites in the Eastern Cordillera of the central Peruvian Andes

A discoutinuous NNW-SSE trending belt of scattered ultraiuafic (UM) and subordínate mafic (M) rocks ís exposed alona some 250 km in the Eastern Cordillera of the peruvian Andes (Junin and Huanuco Departnients. -°"-12° S). New data questiou tlieír pieviousty assuuned [1.2] intrusive origin. Work, in progress shows tLat the essential geologic and tecronk featiires are comnion to most of them, as will t e shown on the southeniniost occurrences: Tapo and Acobaniba (Tarraa proviuce). The Tapo massif is the most conspkuoiis and the oaty one with chiomite mining history. It is a lens-shaped body, 5 km long [NV-SE direction) and 1 -2 km wide. lying on detritaí sedirnents of the Lower Carboniferous Ambo Group [3], and comprising extreniely tectomsed and serpentinised peridotiles wilh subordínate podiform chroirdtite bodies. nieta-gabbros or amphibolites The Acobamba oecurrences couiprise serpentinites aud subordínate meta-gabbros [A]. in contact with phyllites of the Precarnbnan (?) Huacar Group (Maraáón Complex)

Neoproterozoic ultramafic and mafic magmatism in the Eastern Cordillera of the central peruvian Andes: the Tapo Massif

A highly dismembered assemblage of ultramafic and mafic rocks is exposed in the Eastern Cordillera of the Central Peruvian Andes, extending along a discontinuous NW-SE belt over some 250 km between 12° and 9° S of latitude. One of the most important occurrences is the Tapo Mafic-Ultramafic Complex, which occurs at 3750 to 4200 m above sea level, 2 km to the west of Tapo locality, in the Tarma province, about 200 Km west of Lima. The Tapo complex is a lens-shaped body, 5 km long and 1-2 km wide, that consists mainly of strongly serpentinized peridotites and some gabbros. Several small open pits won chromite from podiform chromitite lenses ( ≥60 chromite) and from disseminated chromite in serpentinite. The main structural trend of the Tapo Complex is NW – SE and the massif is tectonically emplaced upon Lower Carboniferous sedimentary rocks. The rocks of the Tapo massif are overprinted by metamorphism reaching amphibolite facies (see Willner et al, 2010, for more information on the metamorphic conditions). The main purpose of this work is to constrain the age determination of the Tapo Complex, using Sm-Nd technique direct dating of chromites and, also, amphibole, plagioclase and whole-rock samples from the host gabbro. In addition K-Ar age determination on amphibole is presented to date the metamorphic overprint

New data on the ophiolitic VMS deposits of Moeche (Cabo Ortegal Complex, NW Spain)

As a result of the variscan collision, several allochtonous complexes were emplaced on the Iberian margin in Devonian times, among them the Cabo Ortegal Complex comprising the Moeche ophiolitic sequence. Copper has been won from several mines (Piquitos I & II, Barqueira, Maruxa) from disseminated ores and thin massive sulphide layers in the Moeche Unit, a strongly deformed meta-volcanic sequence comprising mainly quartz-chlorite schists and mylonites, which defines the top of the ophiolite. The ores were metamorphosed and strongly deformed under brittle conditions (for pyrite), but their textures are often apparently post-deformational, due to very common solution-transfer processes; they are composed mostly of pyrite and chalcopyrite, with minor sphalerite, pyrrhotite, etc., and with traces of native gold and PGE. The geology, mineralogy, and geochemistry of the orebodies relate closely to VMS of the Cu-Zn (Cyprus) type. Fluid inclusion studies allowed an estimation of metamorphic conditions at pressures of 2/2’5 kb and T 325/350ºC. New determinations using the chlorite geothermometer yield temperatures around 320 ºC, corresponding to pressures near 2 kb according to the isochores deduced from the fluid inclusion study, although in the Barqueira mine higher temperatures, up to 350 ºC, are found, corresponding to presssures up to 2’5 kb. Pb isotopic compositions of pyrite point to a double source of Pb, i.e. a main mantle and a subordinate crustal source. The values for 87SR/86Sr in pyrite support this interpretation, but some results suggest later mobilization in an open system, corresponding to solution-transfer. Age determinations of pyrite deduced from the Pb isotope uranogenic graph, ≈ 480 Ma, do not fit with the metamorphic ages published for the Moeche Unit, and might point to the age of Pb extraction from the mantle

Inclusiones fluídas e isótopos de Pb y Sr en Chipmo y Poracota, región minera de Orcopampa, Perú. Implicaciones para la exploración

Chipmo y Poracota forman parte de los depósitos epitermales miocénicos de oro y plata de los Andes centrales en el sur del Perú, entre los 3900 y 4900 metros sobre nivel del mar. Figura 1. Políticamente pertenecen a las provincias de Castilla y Condesuyos en la región de Arequipa y actualmente representan la mayor producción subterranea de oro del Perú. La producción total, a finales del 2009 fue de 2.1 Moz Au – 2.0 Moz Chipmo y 0.1 Moz Poracota–, cuentan con reservas del orden de 0.7 Moz Au y recursos estimados superan los 1.0 Moz Au. Chipmo es el deposito mas importante de la región de Orcopampa, en los últimos 15 años importantes publicaciones se han venido desarrollando: Swanson, 1998; Noble, 1999; Caddey and Sabastizagal, 1999; Mayta, 1999, Salazar et al., 2009 y Salazar 2008 y en Poracota cabe destacar a Bradford, 1999; Miranda Vidal, 2006 y Sarmiento, 2008

Garnet-biotite diffusion mechanisms in complex high-grade orogenic belts : understanding and constraining petrological cooling rates in granulites from Ribeira Fold Belt (SE Brazil)

Cooling rates based on the retrograde diffusion of Fe2+ and Mg between garnet and biotite inclusions commonly show two contrasting scenarios: a) narrow closure temperature range with apparent absence of retrograde diffusion; or b) high result dispersion due to compositional variations in garnet and biotite. Cooling rates from migmatites, felsic and mafic granulites from Ribeira Fold Belt (SE Brazil) also show these two scenarios. Although the former can be explained by very fast cooling, the latter is often the result of open-system behaviour caused by deformation. Retrogressive cooling during the exhumation of granulite-facies rocks is often processed by thrusting and shearing which may cause plastic deformation, fractures and cracks in the garnet megablasts, allowing chemical diffusion outside the garnet megablast – biotite inclusion system. However, a careful use of garnets and biotites with large Fe/Mg variation and software that reduces result dispersion provides a good correlation between closure temperatures and the size of biotite inclusions which are mostly due to diffusion and compositional readjustment to thermal evolution during retrogression. Results show that felsic and mafic granulites have low cooling rates (1–2 °C/Ma) at higher temperatures and high cooling rates (~100 °C/Ma) at lower temperatures, suggesting a two-step cooling/exhumation process, whereas migmatites show a small decrease in cooling rates during cooling (from 2.0 to 0.5 °C/Ma). These results agree with previously obtained thermochronological data, which indicates that this method is a valid tool to obtain meaningful petrological cooling rates in complex high-grade orogenic belts, such as the Ribeira Fold Belt

C-O-H isotopic evidences for fluid sources of granulites in Ribeira Belt, SE Brazil

Publicado em: Geochemica et Cosmochimica Acta, Vol. 72, issue 12, Suppl. 1, A7

Whole-rock geochemistry and Sr-Nd isotopic evidences for sources and processes of granulite formation in Ribeira Belt, Brazil

Geochemica et Cosmochimica Acta, Vol. 72, issue 12, Suppl. 1, A93