A Long-lived Porphyry Ore Deposit and Associated Upper Crustal Silicic Magma Bidy, Bajo de la Alumbrera, Argentina

Porphyry Cu deposits form within and adjacent to small porphyritic intrusions that are apophyses to larger silicic magma bodies that reside in the upper parts of the Earth's crusts. Centred on these intrusions are hydrothermal systems of exsolved magmatic fluid with a carapace of convectively circulating meteoric water. We have applied several different dating techniques to assess the longevity of the magmatic-hydrothermal system and to define the cooling history of porphyry intrusions at the Bajo de la Alumbrera porphyry Cu-Au deposit, Argentina. The closure temperatures of these techniques range from 800°C (zircon U-Pb) to ~70°C (apatite (U-Th)/He; Fig. 1). The resulting cooling history indicates that the magmatic-hydrothermal system cooled to ca. 200°C by ~1.5 m.y. after the last porphyry intrusion (i.e., 6.96�0.09 Ma; U-Pb zircon age). Based on (U-Th)/He apatite data (closure temperature ~60-70°C), exposure and cessation of the system occurred before 4 Ma. The longevity of the magmatic-hydrothermal system indicated by these results is inconsistent with accepted mechanisms for porphyry Cu deposit formation. Depending on wallrock permeability, depth and cooling method, a 2 km wide by 3 km high intrusion has been predicted to cool between 0.01 to 0.1 m.y. (marked as the grey interval; Cathles et al., 1997 Economic Geology). We have obtained numerous age determinations younger than the U-Pb zircon age of the last known intrusion at Bajo de la Alumbrera. These imply that simple cooling of the small, mineralized porphyries did not happen. For the magmatic-hydrothermal system to have been sustained for longer than 0.1 m.y., either 1) younger small intrusions have been episodically emplaced below the youngest known intrusions, thus prolonging heat flow, or 2) fluids derived from a deeper and larger parental intrusion have been episodically discharged through the ore deposit long after the porphyry intrusion had lost its available heat. In either case, the longevity of the deeper-seated parental magma body from which ore-forming fluid and melt were derived, must have exceeded one million years or more