LONGEVITY OF PORPHYRY COPPER FORMATION AT QUELLAVECO, PERU

Metal introduction at the late Paleocene to early Eocene Quellaveco porphyry copper-molybdenum deposit in southern Peru spans several phases of quartz monzonite porphyry emplacement and is bracketed by a precursor granodiorite pluton and a late-mineral porphyry body that postdates essentially all copper introduction. Together, the U-Pb ages of zircons from these intrusive rocks show that 1.08 +/- 0.58 m.y. elapsed between the precursor pluton and initiation of stock emplacement; the porphyry system was active intermittently for at least 3.25 m.y. (4.07 +/- 0.82 m.y.); and at least three-quarters of the copper inventory was deposited in a maximum of 3.12 m.y. (2.51 +/- 0.61 m.y.). Recent U-Pb zircon dating of several other major central Andean porphyry copper deposits, in combination with other isotopic techniques, suggests that 2.5- to 4-m.y. life spans are commonplace. The longevity of porphyry copper systems implied by these studies appears to reflect the protracted time gaps between the multiple intrusions that intermittently replenished porphyry stocks. Other precise isotopic methods (Re-Os, 40Ar/39Ar) typically document shorter life spans because it is more difficult, if not impossible, to date the full sequence of events involved in porphyry copper formation.