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Mina Chuquicamata TTHHÈÈSSEE En vue de l'obtention du DOCTORAT DE L’UNIVERSITÉ DE TOULOUSE Délivré par l'Université Toulouse III - Paul Sabatier Discipline ou spécialité : Sciences de la Terre Présentée et soutenue par Natalia Astudillo Leyton Le 16 mai 2008 Titre : MINERALOGIE MAGNETIQUE ET PALEOMAGNETISME DES GRANDS GISEMENTS TYPE PORPHYRE CUPRIFÈRE DE CHUQUICAMATA ET EL TENIENTE, CHILI JURY MARTINOD Joseph, Président du Jury MAKSAEV Victor, Examinateur VIVALLO Waldo, Rapporteur CHAUVIN Annick, Rapporteur Ecole doctorale : SDU2E Unité de recherche : LMTG Directeur(s) de Thèse : ROPERCH Pierrick TOWNLEY Brian Rapporteur : CARLIER Gabriel Université de Toulouse III - Paul Sabatier U.F.R SCIENCES DE LA VIE ET DE LA TERRE THÈSE Pour obtenir le grade de: DOCTEUR DE L’ UNIVERSITÉ TOULOUSE III Discipline: Sciences de la Terre Paléomagnétisme - Minéralogie Présentée et soutenue publiquement par Natalia ASTUDILLO LEYTON le 16 mai 2008 à Santiago du Chili MINERALOGIE MAGNETIQUE ET PALEOMAGNETISME DES GRANDS GISEMENTS TYPE PORPHYRE CUPRIFERE DE CHUQUICAMATA ET EL TENIENTE, CHILI Directeurs de Thèse: Pierrick ROPERCH Brian TOWNLEY JURY Joseph MARTINOD President du Jury Victor MAKSAEV Examinateur Waldo VIVALLO Rapporteur Annick CHAUVIN Rapporteur Gabriel CARLIER Rapporteur Nom du Candidat Natalia ASTUDILLO LEYTON Date, heure et lieu de soutenance 16 Mai 2008 à 16 hr à Santiago du Chili Titre de la Thèse MINERALOGIE MAGNETIQUE ET PALEOMAGNETISME DES GRANDS GISEMENTS TYPE PORPHYRE CUPRIFÈRE DE CHUQUICAMATA ET EL TENIENTE, CHILI. Spécialité: Paléomagnétisme-Minéralogie Directeurs de Recherche M. Pierrick ROPERCH, LMTG, Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400 TOULOUSE M. Brian TOWNLEY, Departamento de Geología, Universidad de Chile, Plaza Ercilla #803, Casilla 13518, Correo 21, Santiago du Chili. Jury M Joseph MARTINOD M Victor MAKSAEV M Waldo VIVALLO Mme Annick CHAUVIN M Gabriel CARLIER Mots-clés: Paléomagnétisme ; Minéralogie Magnétique ; Porphyre Cuprifère, Altération Hydrothermale, Chili Key words: Paleomagnetism ; Magnetic Mineralogy ; Porphyry Copper Deposit, Hydrothermal Alteration, Chile Référence des publications Astudillo, N., Roperch, P., Townley, B., Arriagada, C., Maksaev, V., 2008. Importance of small-block rotations in damage zones along transcurrent faults. Evidence from the Chuquicamata open pit, Northern Chile. Tectonophysics, v.450, p.1–20. Recherches effectuées à: Departamento de Geología, Universidad de Chile, Santiago du Chili LMTG, Observatoire Midi-Pyrénées, Toulouse, France RESUME EN FRANÇAIS Les altérations hydrothermales et minéralisations enregistrées dans un gisement produisent des changements dans la minéralogie magnétique de la roche encaissante et, par conséquent, dans leurs propriétés magnétiques. En considérant que l’aimantation rémanente peut être utilisée comme marqueur de l'histoire géologique d'une roche, on a effectué une étude paléomagnétique dans deux méga-gisements de type porphyre cuprifère au Chili : (1) CHUQUICAMATA (Eocène- Oligocène), dont la minéralisation est hébergée dans des roches granodioritiques; et (2) EL TENIENTE (Miocène supérieur-Pliocène inférieur) avec un complexe de roches andésitiques minéralisées. L'objectif principal de cette recherche a été de déterminer les effets de l’altération hydrothermale sur la minéralogie magnétique et les enregistrements paléomagnétiques. Le gisement de Chuquicamata est limité longitudinalement par une faille majeure (Falla oeste) et les minéralisations d’intérêt économique sont principalement concentrées à l’est de la faille. La susceptibilité magnétique et l’intensité de l’aimantation rémanente diminuent fortement en fonction de l’altération quartz-séricite qui augmente à l’approche de la faille. L’intensité de la déformation tectonique qui se surimpose aux effets de minéralisations ne permet pas de conserver un signal paléomagnétique cohérent dans les roches minéralisées de Chuquicamata. A l’ouest de la Falla Oeste, la situation est différente au niveau de la granodiorite Fiesta sans minéralisation d’intérêt économique. La susceptibilité magnétique (k ~0.01-0.05 SI) et l’intensité de l’aimantation rémanente naturelle (~0.1 Am-1) ne présentent pas de variation spatiale majeure. La susceptibilité magnétique est dominée par de la magnétite en gros grains qui est le porteur de la forte anisotropie de la susceptibilité magnétique (1.1<P<1.4) de la Granodiorite Fiesta. Par contre les directions caractéristiques de l’aimantation rémanente de polarité normale sont portées par une autre phase magnétique de plus haute coercivité liée à la formation de lamelles de titano-hématite lors de l’altération à biotite et chlorite. La foliation magnétique est sub-verticale et présente de larges variations d’orientation corrélées aux déviations antihoraires de la déclinaison magnétique de l’aimantation rémanente (entre 330° à 230°). Les enregistrements paléomagnétiques sont interprétés comme la conséquence de rotations antihoraires de petits blocs associés à la déformation le long de la Falla Oeste en accord avec les interprétations tectoniques antérieures suggérant un mouvement sénestre de ~35km ayant permis la juxtaposition du porphyre cuprifère de Chuquicamata avec la granodiorite Fiesta. Dans le gisement d’EL Teniente, les contrastes magnétiques sont forts entre les dacites et quartz-diorite faiblement magnétique (k <0.001 SI) et le complexe d’andésites (CMET) (<0.01 < k < 0.2 SI). Le principal minéral "ferromagnétique" présent dans ces roches est la magnétite. Sur la base des associations minérales, les observations pétrographiques permettent de classer la magnétite dans 5 sous-types : [i] TYPE 1a: magnétite en grains fins dans les plagioclases; [ii] TYPE 1b: magnétite en grains fins +quartz crypto-cristalin (masse fondamentale et veines); [iii] TYPE 2: magnétite en gros grains+biotite+(>>anhydrite-quartz); [iv] TYPE 3: magnétite en gros grains+chlorite+quartz diffus +rutile; et [v] TYPE 4: magnétite en gros grains+tourmaline+ quartz diffus. Une aimantation rémanente très stable portée par la magnétite en grains fins est mise en évidence particulièrement dans les « andésites » minéralisées. Une zonation spatiale de la polarité magnétique est clairement mise en évidence au niveau de la mine. Dans le secteur N-NE une polarité INVERSE est enregistrée en probable relation avec l'intrusion du Porphyre Dacitique El Teniente aussi de polarité inverse. Dans le secteur S-SE, la polarité est normale. L’aimantation rémanente naturelle est d’origine chimique/ thermorémanente et est acquise au cours de la minéralisation. Compte tenu des fréquentes inversions de polarité du champ magnétique terrestre au cours de la période 4.5-5Ma, l’absence de superposition d’aimantation de polarité opposée au sein du même échantillon suggère une acquisition d’aimantation et de minéralisation très rapide (~0.1-0.2Ma) au cours d’épisodes successifs. Les directions caractéristiques ne montrent ni rotation tectonique ni basculement du gisement postérieur à la minéralisation. RESUMEN EN CASTELLANO “MINERALOGÍA MAGNÉTICA Y PALEOMAGNETISMO EN LOS MEGAYACIMIENTOS TIPO PÓRFIDO CUPRÍFERO CHUQUICAMATA Y EL TENIENTE, CHILE” Los fenómenos de alteración hidrotermal y mineralización registrados en un yacimiento producen cambios en la mineralogía magnética de la roca huésped y, por ende, en sus propiedades magnéticas. Considerando que la magnetización remanente representa un registro de la historia geológica de una roca, se realizó un estudio paleomagnético en dos megayacimientos tipo pórfido cuprífero chilenos: (1) CHUQUICAMATA (Eoceno-Oligoceno), cuya mineralización se hospeda en rocas granodioríticas y (2) EL TENIENTE, (Mioceno-Plioceno) de roca huésped andesítica. El objetivo principal de esta investigación fue determinar los efectos de la alteración hidrotermal en la mineralogía ferromagnética de ambos yacimientos, para así aplicarlos a la interpretación de sus resultados paleomagnéticos. Las rocas del yacimiento CHUQUICAMATA (limitado longitudinalmente por la Falla Oeste) presentan una disminución del magnetismo remanente natural (MRN) y susceptibilidad magnética (k) en función de la alteración cuarzo-sericita penetrativa, de mayor desarrollo en zonas cercanas a la falla. Esta situación no es reproducible al oeste de la Falla Oeste, ya que en la Granodiorita Fiesta ambos parámetros magnéticos no muestran variaciones espaciales mayores (MRN: 0.1 A/m y 0.01-0.05 SI). En esta unidad, la susceptibilidad magnética se correlaciona con magnetita gruesa hidrotermal, controlando también la fuerte anisotropía de susceptibilidad magnética (1.1<P<1.4). Por el contrario, el mineral portador de la magnetización remanente es de alta coercividad, probablemente titanohematita lamellar producto de alteración biotítico-clorítica. La foliación magnética es subvertical y presenta grandes variaciones de orientación correlacionadas con desviaciones antihorarias de la dirección característica (entre 330° a 230°) respecto a la polaridad normal esperada en el sector para el Eoceno. Los registros paleomagnéticos indica la presencia de rotaciones antihorarias (>100°) de pequeños bloques a escala hectométrica, asociadas a deformación a lo largo de la Falla Oeste, en acuerdo con interpretaciones tectónicas anteriores que sugieren un movimiento siniestral de 35 km. La polaridad inversa reconocida en ciertas rocas del Pórfido Este e intrusivos encajantes probablemente fue adquirida en relación a alteración potásico-silícica y/o fílica, reconociéndose sólo rotaciones y/o basculamientos localizados de pequeños bloques independientes
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