Actividad de fluidos hidrotermales del Neoproterozoico tardío en el cinturón de Tandilia, Argentina

In the Barker - Villa Cacique area, Tandilia belt, alteration mineral assemblages were studied by petrography, XRD and EMPA at three different stratigraphic levels: (1) a phyllic alteration at the unconformity palaeoproterozoic basement-neoproterozoic sedimentary succession (TLPU); (2) an advanced argillic alteration in the Las Águilas Formation (middle level); and (3) a phyllic alteration on pyroclastic rocks of the Olavarría Formation (upper level). Special emphasize was placed on the chemical characterization of K-white micas and chlorites. Secondary K-white micas of altered migmatites, from the Las Aguilas and Olavarría Formations have a low paragonite content (Na* 9.5 km) for the entire sedimentary pile. Hot fluids would rise from deep-seated realms, metamorphic and/or hidden igneous sources. A correlation with a rasiliano thermo-tectonic event is hypothesized.En el área de Barker - Villa Cacique, Tandilia, se estudiaron mediante petrografía, DRX, y microsonda electrónica, las para- génesis de minerales de alteración de tres niveles estratigráficos diferentes: (1) alteración fílica de la discordancia basamento paleoproterozoico-secuencia sedimentaria neoproterozoica (TLPU); (2) alteración argílica avanzada en la Formación Las Águilas (nivel medio); y (3) alteración fílica en rocas piroclásticas de la Formación Olavarría (nivel superior). Las micas potási- cas y cloritas, de dichos niveles de alteración, fueron caracterizadas químicamente. Las micas potásicas de migmatitas alteradas y de las formaciones Las Águilas y Olavarría tienen un bajo contenido en paragonita (Na* 9,5 km) para el total de la pila sedimentaria. Los fluidos hidrotermales habrían ascendido desde ambientes profundos con una fuente metamórfica y/o cuerpos ígneos ocultos. Una correlación con un evento termo-tectónico Brasiliano es hipotetizada.Fil: Martinez, Juan Cruz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina. Universidad Nacional del Sur; ArgentinaFil: Dristas, Jorge A.. Universidad Nacional del Sur. Departamento de Geologia. Catedra de Petrologia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; ArgentinaFil: Van Den Kerkhof, Alfons M. . Geowissenschaftliches Zentrum der Universität Göttingen; AlemaniaFil: Wemmer, Klaus . Geowissenschaftliches Zentrum der Universität Göttingen; AlemaniaFil: Massonne, Hans J.. Universität Stuttgart; AlemaniaFil: Theye, Thomas. Universität Stuttgart; AlemaniaFil: Frisicale, Maria Cristina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina. Universidad Nacional del Sur; Argentin

Phase transitions and molar volumes of CO2-CH4-N2 inclusions

The combined application of Raman microspectroscopy and the cooling stage to the study of fluid inclusions opens the possibility determining the molar volumes (v) of fluid mixtures in rocks. Isochoric XT-diagrams are introduced and it is demonstrated that the equations of state presently available (for example the Heyen equation for CO2-N2 mixtures) are not valid outside the range of experimental data used for their calibration. Measurements on CO2-N2 inclusions of extremely low molar volume (about 40 cm3/mole) from the Furua Granulite Complex (Tanzania) are used as an example. Studies on fluid inclusions may therefore contribute, in a semi-quantitative way, to the improvement of equations of state for multi-component systems.L'application combinée de la microsonde Raman et de la platine réfrigérante permet la détermination des volumes molaires des mélanges fluides dans les inclusions des roches. On introduit la notion de diagrammes X-T isochores et on montre que les équations d'état actuellement utilisées (par exemple l'équation de Heyen pour le mélange CO2-N2) ne sont pas valables en dehors du domaine des données expérimentales qui ont servi à leur établissement. Les inclusions CO2-N2 du Complexe granulitique de Furua (Tanzanie), de très faible volume molaire (environ 40 cm3 /mole), sont prises pour exemples. Ainsi, l'étude des inclusions fluides peut contribuer d'une manière semi-quantitative au perfectionnement des équations d'état pour des systèmes complexes.van den Kerkhof Alfons M. Phase transitions and molar volumes of CO2-CH4-N2 inclusions. In: Bulletin de Minéralogie, volume 111, 3-4, 1988. Inclusions fluides. IXth symposium on fluid inclusions. University of Oporto – May 1987

Development of sigmoidal en-échelon microfractures in beryl and the role of strain localization evidenced by fluid inclusions

A combined fluid inclusion and microstructural study was carried out in beryl crystals from the San Cayetano Nb-Ta-bearing pegmatite (San Luis, Argentina). Primary aqueous-carbonic fluids (T0) were subsequently re-trapped during shearing, resulting in en-échelon microfractures. The more brittle behaviour of beryl compared to quartz makes this mineral more suitable for the preservation of fluid inclusions and microstructures. The primary inclusions T0 are preserved in strain-free domains, whereas the pseudo-secondary T1-to T3-type inclusions occur in domains showing intracrystalline deformation. CO2 was relatively immobile or reacted to form carbonate in early T1-type inclusions, whereas H2O preferentially migrated along microfractures and was trapped as T2-and T3-type inclusions. Short intragranular trails of fluid inclusions, representing en-échelon healed microfractures, resulted from progressive strain localization. Contrary to previous proposals, this new model implies the progressive thickness reduction of intracrystalline micro-shear zones. Besides, hydrolytic weakening linked to dislocation glide is the most likely mechanism to explain the evolution of fluid inclusions, with intracrystalline deformation enhancing anisotropic diffusion. This study highlights the potential of combined fluid inclusion and microstructural studies in order to understand the interaction between fluid activity and deformation. In this way, valuable insights can be provided into the progressive development of overprinted fabrics and low-to medium-temperature deformation mechanisms of minerals.Fil: Sosa, Graciela M.. Universität Göttingen; AlemaniaFil: Oriolo, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: van den Kerkhof, Alfons. Universität Göttingen; Alemani

Internal differentiation of the archean continental crust : fluid-controlled partial melting of granulites and TTG - amphibolite associations in Central Finland

Fault bound blocks of granulite and enderbite occur within upper amphibolite-facies migmatitic tonalitic-trondhjemitic-granodioritic (TTG) gneisses of the Iisalmi block of Central Finland. These units record reworking and partial melting of different levels of the Archean crust during a major tectonothermal event at 2.6-2.7 Ga. Anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 8-11 kbar and 750-900°C. A nominally fluid-absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions. The geochemical signature of light rare earth element (LREE)-depleted mafic granulites can be modelled by 10-30 wt % partial melting of an amphibolite source rock leaving a garnet-bearing residue. The degree of melting in intermediate granulites is inferred to be less than 10 wt % and was restricted by the availability of quartz. Pressure-temperature estimates for the TTG gneisses are significantly lower than for the granulites at 660-770°C and 5-6 kbar. Based on the P-T conditions, melting of the TTG gneisses is inferred to have occurred at the wet solidus in the presence of an H2O-rich fluid. A hydrous mineralogy, abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water-fluxed melting regime. Low REE contents and strong positive Eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions, but represent plagioclase-quartz assemblages that crystallized early from felsic melts. Furthermore, similar plagioclase compositions in leucosomes and adjacent mesosomes are not a 'migmatite paradox', as both record equilibration with the same melt phase percolating along grain boundaries.33 page(s

The hydrothermal Waterberg platinum deposit, Mookgophong (Naboomspruit), South Africa. Part 1: Geochemistry and ore mineralogy

ABSTRACTThe Waterberg platinum deposit is an extraordinary example of a vein-type hydrothermal quartz-hematite-PGE (platinum-group element) mineralization. This study concentrates on the geochemical character of the ores and the platinum-group mineral (PGM) assemblage by application of reflected-light and scanning electron microscopy followed by electron probe microanalysis.The PGM-bearing quartz veins show multiple banding indicating numerous pulses of fluid infiltration. Mineralization was introduced contemporaneously with the earliest generation of vein quartz and hematite. High oxygen and low sulfur fugacities of the mineralizing fluids are indicated by hematite as the predominant opaque mineral and the lack of sulfides.The ‘Waterberg type’ mineralization is characterized by unique metal proportions, namely Pt&gt;Pd&gt;Au, interpreted as a fingerprint to the cradle of the metals, namely rocks and ores of the Bushveld Complex, or reflecting metal fractionation during ascent of an oxidized, evolving fluid. The PGM assemblage signifies three main depositional and alteration events. (1) Deposition of native Pt and Pt–Pd alloys (&gt;90% of the PGM assemblage) and Pd–Sb–As compounds (Pt-rich isomertieite and mertieite II) from hydrothermal fluids. (2) Hydrothermal alteration of Pt by Cu-rich fluids and formation of Pt–Cu alloys and hongshiite [PtCu]. (3) Weathering/oxidation of the ores producing Pd/Pt-oxides/hydroxides.Platinum-group element transport was probably by chloride complexes in moderately acidic and strongly oxidizing fluids of relatively low salinity, and depositional temperatures were in the range 400–200°C. Alternatively, quartz and ore textures may hint to noble metal transport in a colloidal form and deposition as gels.The source of the PGE is probably in platiniferous rocks or ores of the Bushveld Complex which were leached by hydrothermal solutions. If so, further Waterberg-type deposits may be present, and a prime target area would be along the corridor of the Thabazimbi-Murchison-Lineament where geothermal springs are presently still active.</jats:p

Cathodoluminescence and Trace Elements in Quartz : Clues to Metal Precipitation Mechanisms at the Asachinskoe Gold Deposit in Kamchatka

The Asachinskoe epithermal gold deposit in South Kamchatka, Russia, is a low-sulfidation type deposit which consists of Au-Ag bearing quartz-adularia-illite veins. Cathodoluminescence (CL) analysis using optical microscope and scanning electron microscope and trace element analysis of quartz using electron probe micro-analyzer (EPMA) were performed to elucidate the relationships between CL structures, trace element concentrations of different quartz generations, and metal precipitation mechanism of the Asachinskoe deposit. Five sequences of quartz crystallization can be distinguished within the mineralized vein of the bonanza zone. Most distinctive trace elements in quartz are Al (av. 1463 ppm) and K (av. 350 ppm). Colloform and microcrystalline quartz with moderate to dull red-brown CL coexists with electrum (Au-Ag alloy), naumannite-aguilarite (ss) and polybasite-pearceite (ss). The Au-Ag-Se precipitation is related to fluid boiling, and the Se enrichment is attributed to relative oxidizing mineralization conditions. Almost non-luminescent, dark brown luminescent quartz forming the matrix of the hydrothermal breccia coexists with electrum, tetrahedrite-tennantite (ss) and covellite. The Au-Ag-Cu precipitation is associated with rapidly precipitated quartz and adularia, and is due to sudden decompression and fluid boiling related to the hydrothermal brecciation.International Symposium, "The Origin and Evolution of Natural Diversity". 1–5 October 2007. Sapporo, Japan