TRADITIONAL AND NON-TRADITIONAL STABLE ISOTOPES FROM FIVE EL LACO IRON OXIDE ORE BODIES INDICATE A MAGMATIC/MAGMATIC-HYDROTHERMAL GENESIS WITH VARIABLE ALTERATION BY METEORIC WATER

The Plio-Pleistocene El Laco iron oxide – apatite deposit, hosted within a ~20-km2 andesite stratovolcano complex located in northern Chile, remains one of the most enigmatic mineral deposits on Earth. Proposed genetic models include magnetite crystallization from an erupting immiscible Fe- and P-rich melt, metasomatic replacement of andesitic lava flows by a hypogene hydrothermal fluid, and iron oxide precipitation from a basinal brine. In this study, we fingerprint the source of the ore forming fluids at El Laco by measuring ẟ⁵⁶Fe, ẟ¹⁸O, Δ¹⁷O, and ẟ²H stable isotopes in bulk iron oxide (primarily magnetite with minor, secondary hematite and goethite) from five surficial ore bodies. Values of ẟ⁵⁶Fe for magnetite from all five ore bodies range from 0.04 to 0.70‰ (avg = 0.29, n = 26, SD = 0.15 ‰), with a narrow range of only 0.14 to 0.20 (n = 5) for magnetite from Rodados Negros. Magnetite from Laco Sur, Cristales Grandes, and San Vicente Alto yielded ẟ¹⁸O values that range from 4.4 to 4.5‰ (n = 5), 3.0 to 3.9‰ (n = 5), and -8.5 to -0.6‰ (n = 5) respectively. Magnetite from Rodados Negros yielded ẟ¹⁸O values from 2.6 to 3.8 ‰ (n = 9), and Δ¹⁷O values that range from -0.13 to 0.10 (n = 5). Magnetite from Laco Norte yielded ẟ¹⁸O values that range from -10.2 to 4.4 ‰ (avg = 0.6 ‰, n = 27), and δ²H ranges from -189.4 to -61.1 (n = 10). The Fe isotope data indicate a magmatic source for Fe in all the El Laco ore bodies. Oxygen and H data at Laco Norte and San Vicente Altos reveal a magmatic/magmatic-hydrothermal signature that has been altered to meteoric values consistent with goethite in equilibrium with local O and H meteoric isotopic values.