Geology and Rare Earth Element Geochemistry of Magnetite Deposits at El Laco, Chile
Amy Larson Rhodes, Noami Oreskes, Sossity Sheets, 1999. "Geology and Rare Earth Element Geochemistry of Magnetite Deposits at El Laco, Chile", Geology and Ore Deposits of the Central Andes, Brian J. Skinner
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The magnetite deposits at El Laco, Chile, have been widely cited as the type example of an iron deposit formed from direct consolidation of a magma. This study presents field, petrographic, and rare earth element (REE) evidence that shows that magnetite deposition was largely the result of hydrothermal activity. Hydrothermal activity at El Laco is clearly manifested in alteration assemblages observed in host-rock andesite: magnetite occurs in association with a series of hydrothermal alteration assemblages. The earliest-formed alteration assemblages include weak propylitic (chlorite-sericite-clay), sodic-potassic mineral phases (scapolite-albite-K feldspar-sphene), and silicification by cation leaching. Alteration of andesite to calcium-rich pyroxene (diopside) ± magnetite followed. Bulk iron-oxide mineralization then occurred, producing an assemblage of magnetite ± apatite ± quartz ± calcite. The final stages of hydrothermal alteration of andesite included argillic alteration (kaolinite-sericite-alunite), silicification associated with anhydrite, and retrograde alteration (sericite-clinozoisite-talc-illite/smectite-biotite) of sodic and calcic alteration phases. Oxidation of magnetite to hematite (±jarosite ± goethite) postdated magnetite emplacement.
Textural evidence also supports the interpretation of a metasomatic-hydrothermal origin:
1. Abundant layers of magnetite mimic the morphology of andesite flow layers, suggesting direct replacement of andesite by magnetite.
2. Porous textures in magnetite resemble andesite breccia fragments that were partially or completely replaced by magnetite. These textures are associated with bladed diopside and pyroxene casts, confirming that, prior to iron mineralization, andesite experienced calcic alteration. Porous regions in magnetite that occur on a larger scale than typical breccia fragments (>10 cm) indicate direct replacement of unbrecciated andesite.
3. Microscopy shows direct replacement of diopside by both magnetite and apatite.
4. Abundant hydrothermal magnetite textures include coarse-grained magnetite octahedra encrustations, magnetite veins occurring with drusy quartz, brecciation, geyserlike magnetite terraces, and fumarole-like tube structures.
5. Magnetite veins crosscut diopside-rich altered breccia fragments, indicating that hydraulic fracturing of footwall andesite was followed by hydrothermal precipitation of magnetite.
REE patterns of magnetite and andesite alteration assemblages show negative Eu anomalies during early alteration (diopside and scapolite) and magnetite-apatite phases, and positive Eu anomalies in later alteration. This likely reflects a change in oxidation state from Eu2+ to Eu3+ as the system developed. REE concentrations in magnetite are the same as REE concentrations in unaltered country rocks, suggesting that the total REE content of magnetite was inherited from precursor andesite during replacement processes. These results are most consistent with the interpretation that magnetite-apatite formation represents a hydrothermal-alteration event, in which iron- or phosphorous-rich fluids reacted with and replaced host-rock andesite and formed by hydrothermal open-space filling. Silica that was removed during replacement was later deposited as quartz veins and as siliceous alteration associated with argillic and sulfate deposition.
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Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages:
1. Normal subduction extended into Eocene time.
2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera.
3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle.
4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab.
5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner