Person:
Ortega Menor, Lorena

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First Name
Lorena
Last Name
Ortega Menor
URI
https://hdl.handle.net/20.500.14352/75233
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Department
Mineralogía y Petrología
Area
Cristalografía y Mineralogía
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2009 - 200922010 - 20121
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    Contrasting Mineralizing Processes in Volcanic-Hosted Graphite Deposits
    (Smart Science for Exploration and Mining, Proceedings of the Tenth Biennial SGA Meeting, 2009) Luque del Villar, Francisco Javier; Barrenechea, José F.; Ortega Menor, Lorena; Rodas, Magdalena; Millward, David; Williams, Jean-Pierre
    The only two known graphite vein-deposits hosted by volcanic rocks (Borrowdale, United Kingdom, and Huelma, Southern Spain) show remarkable similarities and differences. The lithology, age of the magmatism and geodynamic contexts are distinct, but the mineralized bodies are controlled by fractures. Evidence of assimilation of metasedimentary rocks by the magmas and hydrothermal alteration are also common features to both occurrences. Graphite morphologies at the Borrowdale deposit vary from flakes (predominant) to spherulites and cryptocrystalline aggregates, whereas at Huelma, flaky graphite is the only morphology observed. The structural characterization of graphite indicates a high degree of ordering along both the c axis and the basal plane. Stable carbon isotope ratios of graphite point to a biogenic origin of carbon, most probably related to the assimilation of metasedimentary rocks. Bulk į13C values are quite homogeneous in both occurrences, probably related to precipitation in short time periods. Fluid inclusion data reveal that graphite precipitated from C-O-H fluids at moderate temperature (500 ºC) in Borrowdale and crystallized at high temperature from magma in Huelma, In addition, graphite mineralization occurred under contrasting fO2 conditions. All these features can be used as potential exploration tools for volcanic-hosted graphite deposits.
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    Deposition of highly crystalline graphite from moderate-temperature fluids
    (Geology (Boulder Colo.), 2009) Luque del Villar, Francisco Javier; Ortega Menor, Lorena; Barrenechea, José F.; Millward, David; Beyssac, Olivier; Huizenga, Jan-Marten
    Recognized large occurrences of fl uid-deposited graphite displaying high crystallinity were previously restricted to high-temperature environments (mainly granulite facies terranes). However, in the extensively mined Borrowdale deposit (UK), the mineralogical assemblage, notably the graphite-epidote intergrowths, shows that fully ordered graphite precipitated during the propylitic hydrothermal alteration of the volcanic host rocks. Fluids responsible for graphite deposition had an average XCO2/(XCO2 + XCH4) ratio of 0.69, thus indicating temperatures of ~500 °C at the fayalite-magnetite-quartz buffered conditions. Therefore, this is the fi rst reported evidence indicating that huge concentrations of highly crystalline graphite can precipitate from moderate-temperature fluids.
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    Carbon isotopes of graphite: Implications on fluid history
    (Geoscience frontiers, 2012) Luque del Villar, Francisco Javier; Crespo Feo, María Elena; Barrenechea, José F.; Ortega Menor, Lorena
    Stable carbon isotope geochemistry provides important information for the recognition of fundamental isotope exchange processes related to the movement of carbon in the lithosphere and permits the elaboration of models for the global carbon cycle. Carbon isotope ratios in fluid-Deposited graphite are powerful tools for unravelling the ultimate origin of carbon (organic matter, mantle, or carbonates) and help to constrain the fluid history and the mechanisms involved in graphite deposition.Graphite precipitation in fluid-deposited occurrences results from CO2- and/or CH4-bearing aqueous fluids. Fluid flow can be considered as both a closed (without replenishment of the fluid) or an open system (with renewal of the fluid by successive fluid batches). In closed systems, carbon isotope systematics in graphite is mainly governed by Rayleigh precipitation and/or by changes in temperature affecting the fractionation factor between fluid and graphite. Such processes result in zoned graphite crystals or in successive graphite generations showing, in both cases, isotopic variation towards progressive 13C or 12C enrichment (depending upon the dominant carbon phase in the fluid, CO2 or CH4, respectively). In open systems, in which carbon is episodically introduced along the fracture systems, the carbon systematics is more complex and individual graphite crystals may display oscillatory zoning because of Rayleigh precipitation or heterogeneous variations of d13C values when mixing of fluids or changes in the composition of the fluids are the mechanisms responsible for graphite precipitation.