Magmatic-hydrothermal evolution of the El Laco iron deposit revealed by trace element geochemistry and high-resolution chemical mapping of magnetite assemblages

Ovalle, J. Tomás; Reich, Martín; Barra, Fernando; Simon, Adam C.; Deditius, Artur P.; Vaillant, Margaux Le; Neill, Owen K.; Palma, Gisella; Romero, Rurik; Román, Nelson; Cruz, Nikita L. La; Roberts, Malcolm P.; Morata, Diego

doi:10.1016/j.gca.2022.03.012

Cited by 12 publications

(17 citation statements)

References 103 publications

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“…Particularly, our magnetite data plot mostly in the hydrothermal field of the discrimination diagram proposed by Wen et al (Figure b), which allows to distinguish magnetite crystallized from a silicate melt from magnetite precipitated or reequilibrated from hydrothermal fluids. In particular, Ti concentrations well below the thousands of ppm range (150 ppm average), as well as Zn concentrations below the hundreds of ppm range (31 ppm average) are typically reported for hydrothermal magnetite and consistent with comprehensive geochemical datasets published for magnetite from different hydrothermal ore deposits. ,,− Thus, for the El Laco system, these new magnetite data are in agreement with the chemical composition of the massive magnetite in the outcropping orebodies. , Notably, Ti, V, Al, and Cr concentrations are 1 order of magnitude lower than magnetite from deeper levels of the orebodies and 2 orders of magnitude lower than magnetite from the host andesites (Figure and Table S1 in Supporting Information).…”

Section: Formation Of Magnetite Microspheressupporting

confidence: 83%

“…Finally, we stress that the data presented for Laco Sur, which point to a fluid-dominated system, agree with genetic models invoking vapor-rich magmatic-hydrothermal fluids to explain the origin of the deposit. − ,, Furthermore, the abundant presence of (colloidal) magnetite microspheres in the outcropping orebodies at El Laco might have unforeseen implications for magnetite precipitation in hydrothermal systems. It is increasingly recognized that transport of metals as colloidal particles may play a key role during ore genesis, particularly in geothermal environments and epithermal Au–Ag vein systems. − Most importantly, those studies proposed that high-grade Au veins might involve physical transport of Au as colloidal suspensions rather than as a dissolved species. , In conclusion, and considering recent evidence suggesting that significant fractions of Fe in hydrothermal plumes exist in the colloidal size range ( i.e.…”

Section: Implications For Iron Mineralizationsupporting

confidence: 73%

“…Bivariant trace element diagrams showing EPMA and LA-ICP-MS analyses of magnetite grains forming the vesicular ores at El Laco. These new data are plotted along with chemical data reported for magnetite from the sub-surface orebodies (drill cores samples) and magnetite from the host andesites at El Laco deposit. ,, (A) [Ti + V] vs [Al + Mn] diagram (concentrations in elemental wt %). (B) Discriminant diagram from Wen et al for magnetite of magmatic vs hydrothermal origin.…”

Section: Resultsmentioning

confidence: 99%

“…The trace element composition of magnetite was determined on selected magnetite grains using a Teledyne-Photon Machines Analyte G2 193 nm ArF excimer laser ablation system coupled to a Thermo Fisher Scientific iCAP-Q quadrupole mass spectrometer at the CEGA Mass Spectrometry Laboratory, Department of Geology, Universidad de Chile. The analytical protocols for both methods follow the procedures described in Ovalle et al, 20 and the detailed analytical conditions are provided in Supporting Information.…”

Section: ■ Methodsmentioning

confidence: 99%

Section: ■ Implications For Iron Mineralizationmentioning

confidence: 99%

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Fluid-Assisted Aggregation and Assembly of Magnetite Microparticles in the Giant El Laco Iron Oxide Deposit, Central Andes

Ovalle

Reich

Barra

et al. 2023

ACS Earth Space Chem.

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The El Laco iron oxide mineral deposit in the Central Andes of Chile has attracted significant attention because of its uniquely preserved massive magnetite orebodies, which bear a remarkable similarity to volcanic products. To date, the outcropping highly vesicular and porous massive magnetite orebodies have received little attention from a microtextural point of view, limiting our understanding about the role of volcanogenic processes on iron mineralization. Here, we report the chemical composition of vesicular magnetite at El Laco using EPMA and LA-ICP-MS methods and provide detailed 2D and 3D imaging of the internal structure of these texturally complex magnetite ores by combining SEM observations, synchrotron radiation micro-X-ray fluorescence chemical mapping, and high-resolution X-ray computed microtomography. Our observations reveal the presence of abundant magnetite microspheres with diameters ranging from ∼100 to ∼900 μm, as well as dendritic microstructures forming interconnected networks up to a few millimeters in size. Two-dimensional microtextural and geochemical imaging of the microspheres show that these features are formed by multiple euhedral magnetite crystals growing in all directions and occur immersed within a porous matrix conformed by smaller-sized (∼2–20 μm) and irregularly shaped magnetite microparticles. These types of morphologies have been reported in hydrothermal vents associated with hydrovolcanic processes and commonly described in hydrothermal synthesis experiments of magnetite microspheres, suggesting precipitation from iron-rich fluids. A hydrothermal origin for the magnetite microparticles reported here is further supported by their geochemical signature, which shows a strong depletion in most minor and trace elements typical from magnetite precipitated from hydrothermal fluids in ore-forming environments. We propose that decompression, cooling, and boiling of fluids triggered massive iron supersaturation, resulting in the nucleation of magnetite microparticles or colloids, followed by self-assembly into larger and more complex microstructures. Our data from El Laco deposit agree with models invoking magmatic-hydrothermal fluids to explain the origin of the deposit and provide new insights on the potential role of iron colloids as agents of mineralization in volcanic systems.

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