The geochemistry of magnetite and apatite from the El Laco iron oxide-apatite deposit, Chile: Implications for ore genesis

• Published in: Economic Geology
• Main Authors: La Cruz, Nikita L., Ovalle, J. Tomás, Simon, Adam C., Konecke, Brian A., Barra Pantoja, Fernando, Reich Morales, Martín, Leisen, Mathieu, Childress, Tristan M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Society of Economic Geologists, USA 2020
• Subjects: Hydrothermal processes evidence; Trace element compositions; Field evidence bearing; Gold IOCG deposits; Kiruna-type; Crystal chemistry; Magmatic apatite; Oxidation state; Phase relations; Silicate melts; Kiruna
• Online Access: https://doi.org/10.5382/econgeo.4753; https://repositorio.uchile.cl/handle/2250/178555

The textures of outcrop and near-surface exposures of the massive magnetite orebodies (>90 vol % magnetite) at the Plio-Pleistocene El Laco iron oxide-apatite (IOA) deposit in northern Chile are similar to basaltic lava flows and have compositions that overlap high- and low-temperature hydrothermal magnetite. Existing models-liquid immiscibility and complete metasomatic replacement of andesitic lava flows-attempt to explain the genesis of the orebodies by entirely igneous or entirely hydrothermal processes. Importantly, those models were developed by studying only near-surface and outcrop samples. Here, we present the results of a comprehensive study of samples from outcrop and drill core that require a new model for the evolution of the El Laco ore deposit. Backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to investigate the textural and compositional variability of magnetite and apatite from surface and drill core samples in order to obtain a holistic understanding of textures and compositions laterally and vertically through the orebodies. Magnetite was analyzed from 39 surface samples from five orebodies (Cristales Grandes, Rodados Negros, San Vicente Alto, Laco Norte, and Laco Sur) and 47 drill core samples from three orebodies (Laco Norte, Laco Sur, and Extension Laco Sur). The geochemistry of apatite from eight surface samples from three orebodies (Cristales Grandes, Rodados Negros, and Laco Sur) was investigated. Minor and trace element compositions of magnetite in these samples are similar to magnetite from igneous rocks and magmatic-hydrothermal systems. Magnetite grains from deeper zones of the orebodies contain >1 wt % titanium, as well as ilmenite oxyexsolution lamellae and interstitial ilmenite. The ilmenite oxyexsolution lamellae, interstitial ilmenite, and igneous-like trace element concentrations in titanomagnetite from the deeper parts of the orebodies are consistent with ...