Magmatic evolution and geochemistry of the Piedras Verdes deposit, Sonora, Mexico

Abstract

Piedras Verdes is a supergene-enriched porphyry-copper deposit. It contains 290 Mt at a total copper grade of 0.37%. The average thickness of the chalcocite blanket is 110 m. It is 400-500 m wide in a north-south direction and approximately 4 km long. The oxide-sulfide interface ranges from 80 m to 340 m in depth. The country rocks are a Triassic-Jurassic or Paleozoic meta-sedimentary sequence and a Tertiary intrusive porphyry suite. Geochemical studies define six principal intrusive phases of volcanic arc affinity, from quartz monzodiorite to granodiorite in composition. The first magmatic event at Piedras Verdes, was the emplacement of the Sinaloa-Sonora batholith at 67.3 ± 1.4 Ma. This was followed by the emplacement of quartz monzodiorite, "tall" biotite granodiorite, quartz-feldspar and biotite-hornblende granodiorite porphyries, fractionated from a less evolved magma than the preceding batholith. The latest magmatic activity at Piedras Verdes originated andesitic dikes (48.4 ± 1.2 Ma). Mineragraphic studies indicate three mineralizing pulses. A first pulse, related to emplacement of the batholith and the quartz monzodiorite, produced skarns. The second event was associated with the "tall" biotite granodiorite porphyry, introducing sulfides. The third pulse, related to the quartz-feldspar porphyry introduced quartz-molybdenite-chalcopyrite veins. Neodymium isotopic analyses, indicate that the four main intrusive phases at Piedras Verdes were formed by a mixture of primitive and crustal materials. Re-Os isotopic analyses on sulfides yield no considerable amount of Re. It is assumed that the sulfides suffered Re loss during alteration after primary mineralization.