Immiscible iron- and silica-rich melts and magnetite geochemistry at the El Laco volcano (northern Chile): Evidence for a magmatic origin for the magnetite deposits

Abstract

The Pliocene El Laco volcano (northern Chile) hosts a large iron oxide deposit that occurs as stratiform bodies with underlying feeder zones (dykes) hosted by hydrothermally altered andesite lava flows. Phenocrysts in the andesite hosting the mineralization have abundant melt inclusions displaying strong evidence for melt immiscibility. Most of the pyroxene and the plagioclase phenocrysts contain abundant Fe-rich melt inclusions with low-Ti magnetite, pyroxene, apatite, and minor anhydrite. The glassy groundmass of these melt inclusions is chemically similar to sub-alkaline rhyolitic rocks (e.g., >70% SiO2; Si-Al-K-Na-rich). Large resorbed plagioclase phenocrysts also host another group of melt inclusions with a similar silica-rich groundmass but hosting evenly distributed immiscible and complex Fe-rich spheroidal globules that are enriched in Ti, P, Mg, Ca, and S. These immiscible globules and the Ti-poor magnetite daughter crystals in the melt inclusions in pyroxene phenocrysts are thought to be the parental iron oxide-melt liquid, after which their coalescence leads to the formation of magnetite melts, from which the magnetite that formed the El Laco iron oxide deposits crystallized and were emplaced.The mineralogy and chemistry of these melt inclusions tracks the chemical evolution of the Fe-Ox melts and supports a magmatic origin for the El Laco magnetite. This is mainly recognized by the continuum of compositions between volcanic, separate immiscible Fe-oxide melts, and magnetite ores. The trace element composition of the Fe-rich melt inclusions and of the magnetite in the host andesite and the ore bodies reveals a systematic depletion in most of the elements that are compatible in magnetite. The Ti content of the magnetite also records the separation and crystallization of the iron-rich melt. Typically, the magnetite in the orebodies and the melt inclusions is Ti-poor compared with titanomagnetite found in the andesite groundmass and that hosted by the hydrothermally-altered andesite; that is interpreted as being attributed to its crystallization under oxidizing and sulfur-rich conditions. The cause of this unusual chemical behaviour is probably due to the mechanism(s) governing the crystallization of the iron-rich melt, including the temperature, changes in chemistry prior to eruption, and an increase in the redox conditions of the silicate melt. Magma oxidation facilitates the prevalence of ferric iron and formation of Ti-poor magnetite, preventing the incorporation of Ti in the exsolving iron-rich melt. The likely reason for the formation of these large Ti-poor magnetite deposits is likely the contamination of the parental andesitic magma by oxidized crustal rocks and concomitant crystallization of magnetite under oxidizing conditions, something that had a significant impact on the elemental distribution.