A melt inclusion and isotope investigation of the El Laco, Chile, magnetite-(apatite) deposit

Abstract

This two-part investigation focuses on Fe- and Si-rich melt inclusions in plagioclase phenocrysts, in andesite, that hosts the El Laco magnetite-(apatite) (MtAp) deposit in Chile. The first part, a high-resolution study of the mineralogy, micro-textures, chemical composition, and phase relationships of the immiscible melt inclusions reveals that the Fe-rich immiscible melt hosts Cusulfide melt, phosphate-rich melt, and carbonate-REE-Nb-rich residual melts. The composition of these melts at the micro- and nano-scale is controlled by relatively high fO₂ conditions, and is analogous to the macro-scale assemblage forming the MtAp deposits that includes massive magnetite, REE-rich fluorapatite, diopside, and anhydrite. The second part, a Pb isotope investigation of the melt inclusions, phenocrysts, the bulk sample MtAp ore, host andesite, and selected country rocks, indicates isotopic disequilibrium between the melt inclusions and the magnetite ore, suggesting that the Fe-rich immiscible melt preserved in the melt inclusions is not the direct parental melt from which the El Laco deposit was derived. The magnetite ore also has a different Pb isotopic composition than the host andesite, suggesting a Pb isotopic inheritance from the underlying basement reservoir, e.g., Paleozoic ironstone, that was intruded into and crustally contaminated the andesitic magma leading to separation of Fe-rich immiscible melts.