Triple oxygen (delta O-18, Delta O-17), hydrogen (delta H-2), and iron (delta Fe-56) stable isotope signatures indicate a silicate magma source and magmatic-hydrothermal genesis for magnetite orebodies at El Laco, Chile

• Main Authors: Childress, Tristán, Simon, Adam C., Reich Morales, Martín, Barra Pantoja, Fernando, Bilenker, Laura D., La Cruz, Nikita L., Bindeman, Ilya N., Ovalle Ortega, José
• Format: Article in Journal/Newspaper
• Language: English
• Published: Society of Economic Geologists, USA 2020
• Subjects: Kiruna type; Liquid immiscibility; Fe isotope; Apatite deposits; Titanium dioxide; Ore; Origin; Melt; Bearing; Mantle; Kiruna
• Online Access: https://doi.org/10.5387/econgeo.4760; https://repositorio.uchile.cl/handle/2250/178657

The Plio-Pleistocene El Laco iron oxide-apatite (IOA) orebodies in northern Chile are some of the most enigmatic mineral deposits on Earth, interpreted to have formed as lava flows or by hydrothermal replacement, two radically different processes. Field observations provide some support for both processes, but ultimately fail to explain all observations. Previously proposed genetic models based on observations and study of outcrop samples include (1) magnetite crystallization from an erupting immiscible Fe- and P-rich (Si-poor) melt and (2) metasomatic replacement of andesitic lava flows by a hypogene hydrothermal fluid. A more recent investigation of outcrop and drill core samples at El Laco generated data that were used to develop a new genetic model that invokes shallow emplacement and surface venting of a magnetite-bearing magmatic-hydrothermal fluid suspension. This fluid, with rheological properties similar to basaltic lava, would have been mobilized by decompression-induced collapse of the volcanic edifice. In this study, we report oxygen, including O-17, hydrogen, and iron stable isotope ratios in magnetite and bulk iron oxide (magnetite with minor secondary hematite and minor goethite) from five of seven orebodies around the El Laco volcano, excluding San Vicente Bajo and the minor Laquito deposits. Calculated values of delta O-18, Delta O-17, delta D, and delta Fe-56 fingerprint the source of the ore-forming fluid(s): Delta O-17(sample) = delta O-17(sample) - delta O-18(sample) center dot 0.5305. Magnetite and bulk iron oxide (magnetite variably altered to goethite and hematite) from Laco Sur, Cristales Grandes, and San Vicente Alto yield delta O-18 values that range from 4.3 to 4.5 parts per thousand (n = 5), 3.0 to 3.9 parts per thousand (n = 5), and -8.5 to -0.5 parts per thousand (n = 5), respectively. Magnetite samples from Rodados Negros are the least altered samples and were also analyzed for O-17 as well as conventional O-16 and O-18, yielding calculated delta O-18 values that range from 2.6 to ...