T151 Geology and Fluid Dynamics of Cr, Ni-Cu-PGE, and PGE Mineralized Magmatic Plumbing Systems

Tuesday, 6 November 2018
8:00 AM - 12:00 PM
Room 233

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

Paper number: T151-14
Program: Technical Sessions
Time: 11:25 AM – 11:40 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-321821

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