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Title: Emplacement and exhumation of Andean granites : implications for porphyry copper mineralisation and enrichment in the Middle Eocene-Early Oligocene metallogenic belt of northern Chile
Author: Dahlstrom, Simon I. R.
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2020
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Abstract:
The Middle Eocene-Early Oligocene metallogenic belt of northern Chile is one of the richest porphyry copper belts in the world, hosting world-class deposits such as Collahuasi, Chuquicamata and Escondida. The belt is situated along the Precordillera, a N-S trending morphotectonic unit on the western margin of the Central Andes, which has been largely hyperarid since at least the Early Miocene. The porphyry copper deposits (PCDs) are temporally and spatially associated with plutonic rocks of the Eocene-Oligocene magmatic arc, and formed during the waning stages of the Eocene-Oligocene Incaic Orogeny. PCDs of the belt are characterised by large sizes (several >1 Gt at >1% Cu), and well-developed supergene enrichment zones. These features result from the favourable interplay between magmatism, tectonics, and climate, which (1) generated and intruded Cu-fertile magmas into the upper crust, and (2) enriched and preserved them at the surface. In this thesis, I use U-Pb zircon geochronology and whole-rock geochemistry to track the spatial and temporal evolution of intrusive magmatism along a 700 km-long segment of the belt. I then use trace element modelling to constrain the P-T-H2 O conditions under which the porphyry Cu-fertile magmas formed. Exhumation histories of individual plutons, from emplacement at depth to exposure at the surface, are determined by combining the U-Pb zircon ages with Al-in-hornblende geobarometry, zircon and apatite (U-Th)/He thermochronology, and thermal-kinematic modelling. The path each pluton has taken from emplacement to exposure reflects spatial and temporal variations in tectonics and climate, which are critical controls on the potential for porphyry Cu formation, enrichment, and preservation. The geochemical data suggest that magmatism transitioned in the Middle-Late Eocene towards increasingly adakite-like porphyry Cu-fertile magmas, characterised by high Sr/Y and La/Yb ratios, and steeper REE curves. This transition has previously been ascribed to an increasing fractionation of amphibole ± garnet, and decreasing fractionation of plagioclase during magma evolution, due to higher magma water contents and higher pressures of fractionation, likely related to contractional deformation and crustal thickening. The highest Sr/Y and La/Yb ratios are found in the PCDs, which represent the youngest intrusive rocks within the belt. The presence of the transition towards adakite-like signatures along the entire 700 km length of the study area suggests that the first-order control on generating porphyry Cu-fertile magmas is an arc-scale process, consistent with crustal thickening. Experimentally constrained trace element modelling shows that the adakite-like signatures of the PCDs must be generated by amphibole-dominated magma fractionation, where only minor garnet and/or plagioclase can be present. Published data from equilibrium crystallisation experiments suggests that this requires magma evolution between ~7 and 14 kbar, and H2 O contents of >6 %. Sampled plutonic rocks along the belt were emplaced between ~4 and ~7 km depth, except for one Carboniferous pluton that was emplaced at ~12 km. Modelled exhumation histories between 40 Ma and the Present suggest that the highest exhumation rates (>0.30 km/My) in most areas occurred between 40 and 30 Ma, and the lowest rates (<0.10 km/My) between 20 and 0 Ma. Areas that have experienced the most exhumation (> ~3.5 km) since PCD formation (El Abra, Queen Elizabeth, and the Los Picos-Fortuna pluton) are the most poorly enriched, whereas areas with less post-mineralisation exhumation (< ~3 km) (Collahuasi, Centinela and Escondida) have preserved thick and Cu-rich enrichment blankets.
Supervisor: Cooper, Frances ; Blundy, Jon Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.801616  DOI: Not available
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