Tracking isotopic sources of immiscible melts at the enigmatic magnetite-(apatite) deposit at El Laco, Chile, using Pb isotopes

Abstract

The long-standing controversy about the origin of magnetite-(apatite) mineral deposits pertains to how they form. The Pleistocene El Laco deposit in northern Chile is a critical location because the host andesite contains immiscible melt inclusions trapped in plagioclase and clinopyroxene phenocrysts that reveal the involvement of immiscible melts in the evolution of the El Laco Volcanic System hosting the magnetite-(apatite) mineralization. We present results from the first-ever whole-rock and in situ Pb isotope investigation at El Laco, which provides a better understanding of the relationships between immiscible melts preserved in the melt inclusions, the magnetite ore, and the host andesite, and helps identify sources of the ore metals by analyzing potential sources of crustal lead. Our study reveals that the phenocrysts and the melt inclusions contain homogenous Pb isotope compositions that overlap with the host andesite, which confirms that they are coeval and cogenetic. The magnetite ore, however, has significantly more primitive 206Pb/204Pb ratios, which points to Pb isotopic disequilibrium between the magnetite ore and host andesite. Model ages of 367−167 Ma for the magnetite ore suggest that the Pb was inherited from a U−Th-depleted reservoir that could be represented by sedimentary rocks found in the basement of the Andean Cordillera under El Laco, for example, the Palaeozoic P-rich ironstones sequence. These results are consistent with the major role of crustal contamination in the formation of magnetite-(apatite) mineralization elsewhere and suggest that the magnetite ore crystallized from immiscible Fe-rich melts contaminated by the underlying sedimentary sequences.