MAGMA MIXING IN THE DON MANUEL PORPHYRY COPPER DEPOSIT, CHILE

Porphyry copper deposits (PCDs) share many characteristics with arc volcanoes. The Don Manuel PCD in central Chile is no exception. Part of the Miocene-Pliocene porphyry Cu-Mo belt, Don Manuel is located ~30 km east of the giant El Teniente porphyry Cu-Mo deposit and ~25 km southwest of the Holocene Diamante Caldera–Maipo volcano complex.

The occurrence of multiple hypabyssal intrusions and mineralizing events in PCD systems indicates pulsed magmatism with parallels to eruptions and degassing events observed in arc volcanoes. Textural and mineralogical observations of drill core and thin sections from Don Manuel indicate that magma mixing and hybridization may be important factors in understanding mineralization. Mineral disequilibria textures provide information about the fluid dynamics of the magma and the evolution of the magma mixing. Several features found in the intrusives at Don Manuel such as mafic enclaves, igneous net-veining, heterogeneous hybrid rock textures, and complex zoning and sieve textures in plagioclase phenocrysts are also common in the products of arc volcanoes of the central volcanic zone of the Andes, such as Uturuncu volcano in Bolivia and the nearby Diamante Caldera–Maipo volcano complex.

Within the Don Manuel drill core, some mafic enclaves show chilled margins with the more felsic host while in other areas plagioclase phenocrysts and tonalite inclusions are engulfed by the more mafic enclave. In thin section, mafic enclaves have a diktytaxitic texture, suggesting rapid crystallization. Microprobe analyses of the plagioclase phenocrysts from Don Manuel show oscillatory and reverse zoning with An_23-34 cores and An_48-60 rims, whereas the matrix plagioclase displays normal zoning. Plagioclase also occurs as glomerocrysts, perhaps indicating crystal-rich zones which were disturbed by influxes of magma. The repeated injection of mafic magma into a magma reservoir has been suggested to trigger periodic eruptions in arc volcanoes and as an explanation of excess discharges of SO₂. In the case of the Don Manuel PCD, whether or not there was a coexistent volcano above the deposit, these mafic pulses may have driven the exsolution of fluids and volatiles, as well as having been essential for transfer of metals and sulfur to a shallower reservoir.