The redox state and metal-volatile budget of primitive arc magmas: implications in the formation of ore-mineralized porphyry systems in the shalow crust
At global scale, prominently mineralized porphyry copper systems (>40 Mt Cu) are located exclusively along thick crustal segments (>~40 km) of active continental margins. These magmatic-hydrothermal systems accumulate sulfide ore involving huge anomalies of Cu, Au and Mo in the shallow crust (...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2020 |
| País: | Chile |
| OAI Identifier: | oai:repositorio.anid.cl:10533/253021 |
| Acceso en línea: | https://hdl.handle.net/10533/253021 |
| Access Level: | acceso abierto |
| Palabra clave: | Ciencias Naturales Otras Ciencias Naturales |
| Sumario: | At global scale, prominently mineralized porphyry copper systems (>40 Mt Cu) are located exclusively along thick crustal segments (>~40 km) of active continental margins. These magmatic-hydrothermal systems accumulate sulfide ore involving huge anomalies of Cu, Au and Mo in the shallow crust (<10 km depth). The specific processes that allow the large-scale transport of sulfur (S) and metals (Cu-Au) to the upper crust remain unclear, however, the redox conditions in the primitive basaltic melts intruding the deep crust exert a first-order control on the behavior of these metals through the speciation of sulfur. The basic causes of the more oxidized character of arc basalts compared with basalts from other tectonic settings is debated between two contrasting hypotheses. One of these postulates that the higher oxidation reflects the mantle source condition, whereas the other attributes the magma oxidation to intra-crustal processes during the ascent. In the first case, the crystallization of magmatic sulfides would be efficiently inhibited given that S+6 (sulfate) is the dominant S specie. This allows to S and metal to be transported to higher crustal levels. Conversely, in the second case, the precipitation of magmatic sulfides in the mid to deep crust would be dominant during the early stages of differentiation (S-2, sulfide), depleting in metal and sulfur the residual magmas that later can reach the high crust. This research explore into the redox conditions of a primitive arc magma at Los Hornitos, a pair of mafic monogenetic vents in the Central-South Andes (35.5°S), where magnesian olivines (Fo90-92; Ni ~3800 ppm) host quenched melt inclusions that were analyzed by a combination of microanalitycal techniques (μ- XANES, EPMA, SIMS & LA-ICP-MS), allowing the determination of Fe speciation (Fe+3/ƩFe) and the major, minor and trace element composition during the early stages of this arc melts in a metallogenetic fertile segment of the Andes. In addition, the examination of textural features and fine-scale zoning of a large amount of crystals, allows to establish the textural and volumetric evolution of v olivine during episodes of rapid growth, where phenocryst-size olivines results of the progressive growth of concentric open structured “crystal frames” that leave behind compositional boundary layers. This challenge the reliability of melt inclusions as indicator of equilibrium conditions as an implicit rule. Results indicate that the primitive basaltic melts (1250°C-1GPa) intruding the deep crust, present a fO2 equivalent to NNO+1 to +1.7, allowing to dissolve up to ~50% of the sulfur cargo as sulfate. This value is considerably higher compared with other arc magmas that only dissolve as much as ~5% of S. Therefore, it is possible to establish that the metallogenetic fertile nature of arc magmas above mature subduction zones relies in the comparatively more oxidized character of the melts intruding the crust from the subarc mantle. In addition, the contribution in volatile and metal of these melts is 3 wt% of H2O, 4000 ppm of S, 1300 ppm of Cl and 200 ppm of Cu. |
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