The geology, geochemistry and geochronology of the El Abra Mine, Chile, and the adjacent Pajonal-El Abra suite of intrusions

Abstract

The El Abra Porphyry Copper Deposit is a major deposit located 1650 km north of Santiago in the El Loa Province, Northern Chile. It occurs within the Late Eocene- Oligocene aged, north-south trending, Domeyko Cordillera which hosts a number of major copper-porphyry deposits including Chuquicamata. The deposit is spatially associated with the El Abra - Pajonal igneous suite consisting of eleven calc-alkaline intrusions ranging in composition from alkali feldspar granite to quartz monzodiorite. El Abra - Pajonal suite whole-rock and mineral chemistry show that these intrusions were emplaced between 43.4 - 35.5 Ma as a series of short-lived, shallow-crustal chambers located at pressures less than 2 kbar. A long-lived, midcrustal chamber located at pressures between 5 - 11.5 kbar underpinned these chambers. Crystal fractionation and assimilation were the key magmatic processes influencing the suites’ evolution within magma chambers at both crustal levels. Pyroxene fractionation was initially dominant in the mid-crustal chamber, however after ~41 Ma, decreasing whole-rock Fe, Mg. Dy. Y. Ni concentrations showed that amphibole fractionation became progressively dominant reflecting increasing magmatic water concentrations. Magma was tapped periodically from the midcrustal chamber producing a series of upper-crustal chambers in which plagioclase and amphibole were the dominant crystallizing phases. Plagioclase fractionation is recognized by decreasing whole-roek Sr, Ba and A1 concentrations and large negative Eu anomalies. At 40.8 Ma, a mixing event occurred between an injected, mantle-derived, mafic magma with the magma evolving in the mid-crustal chamber. The mafic magma is interpreted to have provided a source for copper and sulfur in addition to providing a heat source that promoted partial melting of the surrounding wall rock, composed of amphibolites and meta-andesites. Magnetite assimilation was one process causing the oxygen fugacity of the evolving mid-crustal magma to increase through time. Due to the magmas' high oxygen fugacity, the sulfur species in the melt on the basis of S concentrations in apatite was S , and therefore at sulfur-saturation, copper remained in the silicate melt. Assimilation of amphibolites and metaandesites at the mid-crustal level also enhanced the water concentration of the evolving magma through the break-down of amphibole. As the magma became water-saturated, a fluid phase formed in the lower chamber and copper dissolved in the silicate melt partitioned into that phase. Copper then precipitated from the mineralizing fluid phase when it reached shallow crustal levels. Differences in the averaged Cu concentrations between shallowly crystallized plagioclase cores and rims show that as intermediate composition intrusions were emplaced into shallow levels, minor mineralization events occurred. Zircon 0 / O values up to l%o greater than mantle-derived zircon, rare Cu-rich xenoliths and the El Abra - Pajonal suite's zircon inheritance record suggest that assimilation may have contributed additional copper to El Abra - Pajonal suite magmas as they continued to evolve. However, the main mineralization event is associated with the emplacement of the El Abra porphyry at 36.9 Ma. because the parental mid-crustal magma to this intrusion was the most oxidized and hydrous of the suite and hence the most efficient in extracting Cu from the silicate melt to the fluid phase. The rapid emplacement of the El Abra porphyry into the upper crust aided precipitation of copper from the exsolving fluid phase and helped to generate the highest yielding hydrothermal fluid that is associated with this intrusion. Show more