THE LONGEVITY OF PORPHYRY COPPER SYSTEMS: A RE-OS APPROACH

The relationship between the amount of ore contained in a particular system and the duration of magmatism - hydrothermalism is one of the most important questions in understanding the evolution and formation of ore deposits. Only recently, with the development of more precise geochronological techniques is it possible to constrain different magmatic and hydrothermal events within a single deposit. Precise Re-Os age determinations of molybdenite, using the Carius tube technique and negative thermal ionization mass spectrometry makes this system useful in descriminating potential or possible multiple molybdenite mineralization events, which may be associated with the size of the deposit.

In the Bagdad deposit, in northern Arizona, (current reserves of about 916 Mt with 0.36% Cu and 0.02% Mo) magmatism lasted for aproximately 6 myr, and it is characterized by two different mineralization events at 76 and 72 Ma. Sierrita, Arizona has current sulfide reserves of 1063 Mt grading 0.27%Cu and oxide reserves of 64 Mt grading 0.18 percent copper. The magmatic activity in Sierrita was episodic and three distinctive events are recognized (68-67, 64-63 and 60 Ma), with Cu-Mo mineralization associated to the last two magmatic events. In the Tameapa prospect, Mexico, magmatism has an aproximate duration of 5 myr. Re-Os molibdenite ages suggest at least two mineralization episodes at ~57 and ~52-53 Ma for this deposit. Copper resources have been estimated at 27 Mt averaging 0.37% Cu. In the megadeposit El Teniente, Chile estimated reserves are over 2855 Mt with 1.31% Cu and 0.022% Mo. Four molibdenite events have been reported for this deposit (at 6.3, 5.6, 4.8 and 4.4 Ma, respectively) within a period of aprox. 2 myr. In Los Pelambres, reported sulfide reserves are at 2400 Mt with 0,67% Cu and 0,016% Mo. Previously reported molybdenite ages for this deposit are 11.33 ± 0.09, 11.45 ± 0.01 Ma, 10.40 ± 0.05, 10.75 ± 0.05 and 10.77 ± 0.05 Ma. These results suggest a very short-lived system in Los Pelambres.

This geochronological information coupled with previous information using other isotopic systems (i.e. Ar-Ar, U-Pb) for other porphyry copper deposits suggest that there is no relation between duration of magmatism or number of mineralization events and the size of the deposit.