HR: 0800h
AN: GP41B-02    [Abstracts]
TI: Magnetic Properties of Mineralized Rocks in El Teniente Porphyry Copper Deposits, Central Andes, Chile
AU: * Astudillo, N
EM: nastudil@ing.uchile.cl
AF: Dpto. de Geologia, Universidad de Chile and IRD-LMTG, Plaza Ercilla 803, Santiago, Chile
AU: Roperch, P
EM: pierrick.roperch@ird.fr
AF: IRD UR154 & Geosciences Rennes, Université de Rennes 1 Campus de Beaulieu, Rennes, 35042, France
AU: Townley, B
EM: btownley@cec.uchile.cl
AF: Dpto. de Geologia, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile
AU: Arriagada, C
EM: cearriag@cec.uchile.cl
AF: Dpto. de Geologia, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile
AB: El Teniente porphyry copper deposit, the world's greatest intrusion related Cu-Mo orebody (central Chile) is hosted within basaltic-andesitic volcanic and gabbroic rocks. This orebody is strongly affected by multiple events of alteration/mineralization with pervasive potassic and chloritic alteration. The mineralization is induced principally by felsic apophyses and dykes (quartz diorite to tonalite) and porphyritic stocks, with U/Pb ages between 7-2.8 Ma, and occurs as a stockwork of mineralized veins and hydrothermal breccias. Alteration biotite and sericite minerals have ⁴⁰Ar/³⁹Ar ages between 6.3-4.4 Ma. We have undertaken paleomagnetic sampling of the deposit using mining drill cores and oriented block samples to test the possibility of a post-mineralization deformation and tilt. Taking into account the large size and the young age of this giant deposit, the aims of the paleomagnetic study are also to characterize the magnetic properties and better constrain the timing and duration of the mineralization events. Magnetic experiments (hysteresis loop, IRM, k-T curves, thermal and AF demagnetization) show the presence of prevailing magnetite. Microscope and SEM observations and chemical analyses suggest several magnetite families. Large multidomain magnetite grains, associated with biotite and chlorite are related to different hydrothermal alteration pulses. However, plagioclases showing an early Na-Ca-Fe alteration contain numerous small (1-10μm) to very small magnetites (<1μm). While the felsic intrusions have low magnetic susceptibility (<0.0005SI) and low NRM, the mineralized mafic rocks have usually high susceptibility (>0.01 and up to 0.2SI) with NRM in the range 0.1- 2A/m. Most samples have univectorial magnetizations with high MDF values (>20mT) and high unblocking temperatures (500-580°C). Within the mine the magnetic polarity is spatially distributed and associated to different stocks. Variations of the magnetic polarity indicate different mineralization events occurred in time and space within the deposit. There is no paleomagnetic evidence for post-mineralization deformation.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1519 Magnetic mineralogy and petrology
DE: 1527 Paleomagnetism applied to geologic processes
DE: 8400 VOLCANOLOGY
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly