Paper No. 4
Presentation Time: 1:30 PM-5:30 PM
EL TENIENTE PORPHYRY COPPER DEPOSIT IN THE CHILEAN ANDES: NEW GEOCHRONOLOGICAL TIMEFRAME AND DURATION OF HYDROTHERMAL ACTIVITY
El Teniente is a Neogene giant Cu-Mo porphyry deposit (75 Mt Cu) of the Chilean Andes. Three distinct mineralized felsic intrusive units occur at this deposit whose ages have been resolved by SHRIMP U-Pb zircon dating, these are: (i) eastern quartz-diorite to tonalite four stocks with bimodal age distributions at 6.46 ± 0.11 to 6.11 ± 0.13 Ma (dominant populations) and at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (subordinate populations). These represent two stages of zircon crystallization, with the younger one probably corresponding to the crystallization age of the stocks and the older to inherited zircons (ii) northern dacite porphyry intrusion at 5.28 ± 0.10 Ma, and (iii) emplacement of central latite ring dykes at 4.82 ± 0.09 Ma. The zircon U-Pb data of the earliest tonalite stocks reveal a complex igneous history, and confirm that they represent practically coeval intrusions from a common magma source. In contrast, unimodal U-Pb zircon age distributions reveal a subsequent simple crystallization history for dacite porphyry and latite dykes. A cataclysmic explosion related to the emplacement of latite magma and ring dykes created a diatreme breccia pipe in the central section of the deposit. Postore, amphibole-rich andesitic dikes that yield a hornblende 40Ar-39Ar date of 3.85 ± 0.18 Ma represent the last igneous intrusion within the deposit. Our data indicates three Re-Os molybdenite ages that reflect hydrothermal Mo mineralization episodes at 5.60 ± 0.02, 4.87 ± 0.03, and 4.42 ± 0.02 Ma. Systematic 40Ar-39Ar step-heating dating of hydrothermal biotite, sericite and whole rock altered samples from different sections of the orebody produced 33 dates within the narrow range from 4.81 ± 0.05 to 4.37 ± 0.05 Ma. The 40Ar-39Ar dating hydrothermal minerals reveal a period of at least 440,000 ± 5,000 years (±1s), which is only the duration of the latest hydrothermal activity within the deposits related to latite injections. This late hydrothermal activity has almost completely obliterated the 40Ar-39Ar record of two preceding hydrothermal/mineralizing events identified by Re-Os and U-Pb data. The gigantic El Teniente deposit resulted from the superposition of three separate episodes of hydrothermal mineralization of magmatic affiliation within a time span of approximately 1.3 Ma (from 5.6 to 4.3 Ma).