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MINERALOGIE MAGNETIQUE ET PALEOMAGNETISME DES GRANDS GISEMENTS TYPE PORPHYRE CUPRIFERE DE CHUQUICAMATA ET EL TENIENTE, CHILI

Abstract : The hydrothermal alteration and mineralization registered in an orebody produce changes in the magnetic mineralogy of country rocks and, hence, in their magnetic properties. Considering that the remanent magnetization can be used as a marker of the geological history of rocks, a paleomagnetic study was realized in two Chilean porphyry copper mega deposits:
(1) CHUQUICAMATA (Eocene-Oligocene), where the mineralization is hosted in granodioritic rocks; and (2) EL TENIENTE (Miocene-Pliocene), with andesitic country rocks. The principal objective of this research was to determine the effects of hydrothermal alteration on the magnetic mineralogy and the paleomagnetic records. The Chuquicamata orebody is longitudinally limited by a master fault (West fault) and the economic mineralization is concentrated on the eastern side. Magnetic susceptibility and remanent magnetization strongly decrease, while quartz sericite alteration increases when getting closer to the fault. The intensity of the tectonic deformation superimposed to the mineralization effects does not allow preserving a coherent paleomagnetic signal in the mineralized rocks of Chuquicamata.
In the western block of the mine, the Fiesta granodiorite is without mineralization of economic interest, the situation is different and magnetic susceptibility (k ~0.01-0.05 IF) and remanent natural magnetization intensity (~0.1 Am-1) do not present major spatial variation. The magnetic susceptibility is carried by large grains of magnetite, being this mineral the carrier of a strong anisotropy of magnetic susceptibility (1.1< P <1.4). On the other hand, the characteristic directions of normal polarity are carried by another magnetic phase, with high coercivity, related to the formation of titanohematite lamellas in response to biotitic-chloritic alteration. Magnetic foliation is subvertical, with large variations in the orientation correlated to the counterclockwise deviations of characteristic directions with respect to late Eocene reference directions. The paleomagnetic records are interpreted as the consequence of a counterclockwise rotation of small blocks associated with deformation along the West Fault, in agreement with the previous tectonic interpretations that suggest a 35 km of sinistral strikeslip movement, leading to the juxtaposition of Chuquicamata porphyry copper deposit with the Fiesta granodiorite. In El Teniente orebody, the magnetic contrasts are strong between weakly magnetic dacites and quartz-diorites (k <0.001 SI) and the mineralized andesite complex (<0.01 < k < 0.2 SI). The principal magnetic mineral present in these rocks is magnetite. Considering the minerals association, petrographic observations allow classifying that magnetite in 5 subcategories: [i] TYPE 1A: magnetite in fine grains within plagioclases; [ii] TYPE 1B: magnetite in fine grains+ crypto-crystalline quartz (groundmass and veins); [iii] TYPE 2: large magnetite grains+biotite+(>>anhydrite-quartz); [iv] TYPE 3: large magnetite grains +chlorite+diffuse quartz+rutile; and [v] TYPE 4: large magnetite grains+tourmaline+diffuse quartz. A very stable residual magnetization carried by fine grained magnetite is recognized particularly in the mineralized andesites. A spatial zonation of magnetic polarity is identified within the mine. In the N-NE sector of the mine nearby the Teniente dacite porphyry with reversemagnetic polarity, the mafic complex records also a reverse polarity magnetization. In the SSE sector, magnetic polarity is normal. The remanent magnetization is likely a chemical and or thermoremanent magnetization and is recorded during mineralization. Considering the frequent polarity reversals of the earth magnetic field in the time interval 4.5-5Ma, the lack of superposed magnetizations of both polarities within the same sample suggests that mineralization and subsequent cooling occurred within short time intervals (~0.1-0.2Ma). In the deposit, characteristic directions do not show tectonic rotation or tilt posterior to mineralization.
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Natalia Astudillo Leyton. MINERALOGIE MAGNETIQUE ET PALEOMAGNETISME DES GRANDS GISEMENTS TYPE PORPHYRE CUPRIFERE DE CHUQUICAMATA ET EL TENIENTE, CHILI. Planète et Univers [physics]. Université Paul Sabatier - Toulouse III, 2008. Français. ⟨tel-00332775⟩

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