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New ELA-ICP-MS U-Pb zircon ages of the Alumbrera magmatic-hydrothermal system in the Farallon Negro district, NW Argentina

Anthony C. Harris , Charlotte M. Allen Rodney J. Holcombe2, Ian H. Campbell,
Scott E. Bryan2 and J. Michael Palin

The Late Miocene volcanic rocks of the Farallon Negro district, NW Argentina host a large porphyry Cu-Au deposit, Bajo de la Alumbrera (644.7Mt @ 0.5% Cu and 0.58g/t Au). ELA-ICP-MS U-Pb zircon ages have shown that the exposed magmatism in the volcanic complex lasted only 2.5. m.y. Cu-Au mineralisation occurred throughout this period, but the bulk of the mineralisation at Alumbrera was emplaced towards the end of magmatism. This differs from other porphyry districts where ore is emplaced at the culmination of a protracted history of tens of millions of years (e.g. El Salvador and Potrerillos districts, Chile).

Intrusions within the Alumbrera porphyry Cu-Au deposit are multiphased occurring at two discrete intervals. The earliest at 8 Ma (7.99±0.12 Ma) is believed to be associated with minor Cu-Au disseminations while the later intrusions at 7 Ma (7.12±0.04 Ma) are emplaced synchronously with the bulk of the mineralisation. Comparison of these intrusion ages with previously published 40Ar/39Ar ages for potassic and phyllic alteration (Sasso and Clark, 1998, Society of Economic Geologists Newsletter, v. 34) shows that the magmatic-hydrothermal system was possibly sustained for as much as 2 to 3 m.y. Thermal collapse of the system may have occurred as much as a million years after the final intrusion. It is commonly reported that porphyry-related hydrothermal systems should cool in less than 100,000 yr. These results redirect attention away from the porphyritic intrusions being the principal heat source and focuses on the possible role of the much larger underlying pluton in driving the magmatic-hydrothermal system.