EVOLUTION OF LA CARIDAD PORPHYRY COPPER DEPOSIT, SONORA AND GEOCHRONOLOGY OF PORPHYRY COPPER DEPOSITS IN NORTHWEST MEXICO

by Valencia, Victor A.

In order to improve our understanding of poorly studied Mexican Porphyry Copper Deposits in the SW regional metallogenetic province, a detailed study of the hydrothermal fluid evolution of La Caridad porphyry copper-molybdenum deposit, and its connection to a high sulfidation epithermal deposit, was performed using oxygen, hydrogen and sulfur stable isotopes combined with fluid inclusion studies. In addition, U- Pb and Re-Os geochronology from La Caridad, Milpillas and El Arco porphyry deposit were performed to constrain the timing of mineralization and magmatism in northwest Mexico. Uranium-lead zircon ages from La Caridad suggest a short period of magmatism, between 55.5 and 53.0 Ma. Re-Os molybdenite ages from potassic and phyllic hydrothermal veins yielded identical ages within error, 53.6 ± 0.3 Ma and 53.8 ± 0.3 Ma, respectively. Four stages of hypogene alteration and mineralization are recognized at La Caridad porphyry copper deposit. The isotopic composition of the water in equilibrium with hydrothermal alteration minerals is consistent with highly evaporated lacustrine waters mixed with magmatic waters or vapor separated from magmatic fluids, however, sulfur isotopes and fluid inclusions data support the lacustrine-magmatic water hypothesis. Milpillas porphyry copper deposit in the Cananea Mining District, yielded a crystallization age of 63.9 ± 1.3 Ma. Two Re-Os molybdenite ages yielded an identical age of 63.1 ± 0.4 Ma, Suggesting a restricted period of mineralization. Re-Os data 16 indicate that mineralization in Cananea District, spanned ~4 m.y. in three discrete pulses at ~59 Ma, ~61 Ma and ~63Ma. El Arco porphyry copper deposit, Baja California, Mexico, yielded a Middle Jurassic crystallization age (U-Pb) of 164.7 ± 6.7 Ma and a Re-Os mineralization age of 164.1 ± 0.4 Ma and not ~100 Ma as previously determinated. Porphyry copper deposits in Mexico range in age from 164 Ma to 54 Ma and the mineralization in Sonora state occurred in two different periods, but magmatism overlaps in space and time. 17