“…The pluton is faulted by steeply dipping, northwest to northeast oriented faults that can be traced over several kilometres and represent the southernmost part of the AFS. Figure 1 ( a ) Simplified geological map of northern Chile between 25 °S and 31 °S indicating the Atacama Fault System in the present-day forearc as based on maps in Tornos et al 61 . Traces of the Atacama fault system are based on Cembrano et al 31 .…”
Section: Geological Setting and Drilling Proceduresmentioning
confidence: 99%
“… ( a ) Simplified geological map of northern Chile between 25 °S and 31 °S indicating the Atacama Fault System in the present-day forearc as based on maps in Tornos et al 61 . Traces of the Atacama fault system are based on Cembrano et al 31 .…”
Section: Geological Setting and Drilling Proceduresmentioning
The weathering front is the boundary beneath Earth’s surface where pristine rock is converted into weathered rock. It is the base of the “critical zone”, in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow.
“…The pluton is faulted by steeply dipping, northwest to northeast oriented faults that can be traced over several kilometres and represent the southernmost part of the AFS. Figure 1 ( a ) Simplified geological map of northern Chile between 25 °S and 31 °S indicating the Atacama Fault System in the present-day forearc as based on maps in Tornos et al 61 . Traces of the Atacama fault system are based on Cembrano et al 31 .…”
Section: Geological Setting and Drilling Proceduresmentioning
confidence: 99%
“… ( a ) Simplified geological map of northern Chile between 25 °S and 31 °S indicating the Atacama Fault System in the present-day forearc as based on maps in Tornos et al 61 . Traces of the Atacama fault system are based on Cembrano et al 31 .…”
Section: Geological Setting and Drilling Proceduresmentioning
The weathering front is the boundary beneath Earth’s surface where pristine rock is converted into weathered rock. It is the base of the “critical zone”, in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow.
“…Magnetic anomalies at El Laco have also been interpreted as evidence of a further intrusive magnetite body beneath 56 . While vapour-driven ascent along fracture zones is a viable process in shallow subvolcanic environments such as at El Laco and Cerro del Mercado and other caldera-related MtAp systems, intrusive emplacement by pressurisation of an unmixing magma body by collapse- or regional tectonics-related faulting in transpressional/transtensional environments may still be a relevant process in deeper environments such as in the nearby Coastal Cordillera of Chile 57 . Whereas some degree of Fe-Si liquid immiscibility may be more ubiquitous, the evidence suggests that contamination by Fe-, P-, and/or S-rich basement lithologies and moderate extension (or trans-tension) around a cooling and deflating subvolcanic magma body provided an ideal environment for massive ore extrusion at El Laco.…”
Magnetite-apatite deposits are important sources of iron and other metals. A prominent example are the magnetite lavas at the El Laco volcano, Northern Chile. Their formation processes remain debated. Here, we test the genetic hypothesis that an Fe-rich melt separated from silicate magma and ascended along collapse-related fractures. We complement recent analyses with thermodynamic modelling to corroborate Fe-Si liquid immiscibility evident in melt inclusions at El Laco and present viscometry of Fe- and Si-rich melts to assess the time and length scales of immiscible liquid separation. Using a rock deformation model, we demonstrate that volcano collapse can form failure zones extending towards the edifice flanks along which the ore liquid ascends towards extrusion driven by vapour exsolution despite its high density. Our results support the proposed magmatic genesis for the El Laco deposits. Geochemical and textural similarities indicate magnetite-apatite deposits elsewhere form by similar processes.
“…The genesis of this massive magnetite mineralization that resembles lava flows and related feeder dykes remains unclear despite numerous studies for six decades [1][2][3][4] . Even today there is no consensus on how MtAp deposits form 1,[5][6][7][8][9][10] , whether they are an independent style of mineralization, or genetically related to other systems, like iron oxide-copper-gold deposits (IOCG) 5,11 . Yet, these Paleoproterozoic to Holocene deposits are a significant source of iron in Chile, China, Iran, Peru, and Sweden, and have significant resource for fluorite, REE, cobalt, and phosphorus 5,11 .…”
The origin of magnetite-(apatite) iron deposits (MtAp) is among the most contentious issue in ore geology with competing models involving purely hydrothermal to magmatic processes. We study here melt inclusions trapped in plagioclase phenocrysts in andesite hosting the emblematic MtAp mineralization at El Laco, Chile. Our high-resolution study reveals that individual melt inclusions preserve complex processes of melt immiscibility including the separation of Si- and Fe-rich melts, the latter hosting other Cu-sulfide, phosphate-rich, and C-O-HFSE-rich residual melts. This assemblage is a small-scale analogue of the ore mineralization and establishes the missing link between silicate and Fe-P-rich melts which subsequently produce extrusive magnetite. These results strongly indicate that El Laco mineralization derives from crystallization of Fe-P-rich melts providing insight into the formation of similar deposits elsewhere.
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