Geology and Geochronology of the Miocene Rio Blanco Porphyry Cu-Mo Deposit, Northern Peru

Abstract

The Rio Blanco porphyry Cu-Mo deposit occurs at the north end of the Miocene metallogenic belt of northern Peru. It has a thick supergene enrichment blanket; while normal for hyperarid Chile, this is unusual in mountainous, cloud forest terrain. Rio Blanco is hosted by the Portachuela batholith. Zircon U-Pb dating shows that the youngest part of the batholith was emplaced at 12.43 ± 0.13 Ma. The deposit formed during three magmatic-hydrothermal cycles. Cycle 1, by far the most important, occurred at 11.50 ± 0.17 to 10.92 ± 0.14 Ma (zircon U-Pb). Two intermineralization intrusions caused early potassic and propylitic alteration. This was then overprinted by a blanket of quartz-sericite, grading down into sericite-chlorite alteration. Cycle 1 was finally cut by a quartz-sericite–cemented breccia, which contains the highest-grade hypogene Cu-Mo grades. A cycle 1 molybdenite-bearing vein has a molybdenite Re-Os model age of 11.43 ± 0.16 Ma. Molybdenite Re-Os dating of the quartz-sericite–cemented breccias shows brecciation occurred at 11.28 ± 0.24 to 11.11 ± 0.18 Ma. Cycle 2 was restricted to the east side, where narrow porphyritic dacite 1 dikes (dated by zircon U-Pb at 10.62 ± 0.16 Ma) show biotite alteration and economic copper. Cycle 3, at 10.02 ± 0.12 to 9.06 ± 0.09 Ma (zircon U-Pb), was triggered by a swarm of NE-striking quartz-plagioclase porphyry and porphyritic dacite 2 dikes. Alteration was milder, and this cycle did not introduce economic copper. Nonmineralized pebble dikes cut the system, emanating from a major diatreme, about 3 × 1.3 km in size, on the north side of the deposit. The magmatic-hydrothermal history spanned about 2.5 m.y., with economic mineralization over about 1.48 m.y. However, metals were mostly introduced during cycle 1, which lasted approximately 0.58 m.y. Our work shows that while multiple magmatic-hydrothermal cycles produced Rio Blanco, sufficient metals were introduced to form a giant porphyry deposit within a single magmatic-hydrothermal cycle.