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Major and trace element geochemistry of the Los Picos-Fortuna/Pajonal-El Abra batholith: implications for magma evolution associated with porphyry copper mineralization

Julian Ballard, Ian Campbell, Joerg Hermann and Alejandro Faunes

The Los Picos-Fortuna/Pajonal-El Abra batholith is a composite system of Eocene calc-alkaline intrusions that is associated with porphyry copper mineralization at El Abra in northern Chile. Major and trace element data from the intrusions have been used to identify the principal controls on the magmatic evolution of the batholith. Linear major element variations show that differentiation was driven principally by fractional crystallisation of plagioclase and variable amounts of a mafic phase such as pyroxene and/or amphibole. Trace element data support this hypothesis but also provide evidence for two end-member differentiation trends, a high water (HW) trend in which amphibole replaces pyroxene early in the fractionation sequence, and a low water (LW) trend in which the replacement of pyroxene occurs late.

A study of Ce(IV)/Ce(III) in zircon has shown that intrusions belonging to the HW trend crystallised from more oxidised magmas than those belonging to the LW trend. The early appearance of amphibole in the HW intrusions results in the progressive depletion of the HREE with increased fractionation. The LW intrusions crystallised from drier, more reduced magmas, with plagioclase crystallisation being more important and amphibole crystallisation being less important. As a consequence, the evolving LW magma developed a strong negative Eu-anomaly, not seen in the HW trend, but only minor HREE depletion. Porphyry copper mineralization is associated with the El Abra mine porphyry, the youngest, most oxidised and most fractionated member of the HW trend. The HW and LW magmas are thought to have evolved together in a deep, long-lived, zoned magma chamber that underlay the Los Picos-Fortuna/Pajonal-El Abra batholith, as shown schematically in figure 1.

Figure 1: A schematic cross-section of the Los Picos-Fortuna section of the Los Picos-Fortuna/Pajonal-El Abra batholith, showing a zoned magma chamber deep in the crust that is believed to underlie the batholith and to be responsible for the developments of the HW and LW trends. The high water/low density (HW) magma overlies the drier/high density (LW) magma. The LW magmas develop a pronounced negative Eu-anomaly during differentiation and a weak concave HREE pattern, whereas the HW magmas lose their weak negative Eu-anomaly and develop a strong Er-centred HREE concave pattern.