Santiago Tassara ● Profesor Asistente

The world's transition to using cleaner energy sources to address climate change has led to a sharp rise in the demand for base and precious metals. Consequently, discovering new ore deposits to meet this growing demand and prevent supply shortages has emerged as one of the greatest challenges of the 21st century. Discovery of new magmatic-hydrothermal ore deposits can be improved based on a fundamental understanding of the geological processes that control the flux and focusing of ore-constituting elements in the Earth’s crust, and by identifying the differences between the bulk-rock and mineral chemistry of ore-forming and ordinary—barren—granitoids. Large metal anomalies in the Earth’s upper crust, such as porphyry copper-(molybdenum) deposits (PCDs), occur in intimate association with oxidized and water-rich arc magmatism in subduction zones. However, these deposits occur in restricted crustal domains and form in response to specific tectono-magmatic events, indicating that not all arc magmas have the same ore-forming potential. Understanding why only some magmas produced large PCDs while most other arc magmas remain barren is a fundamental scientific question and key to developing efficient exploration strategies. The volatile element composition of arc magmas, including water, sulfur, and halogens such as chlorine and fluorine, as well as their oxygen fugacity, exert a critical control on their ore-forming potential (i.e., ore fertility). These components are not only key to the complexation and transport of ore metals during hydrothermal activity, but also influence the amount of ore metals transported by magmas and the efficiency to which they are transferred from magmas to exsolved fluids. Magmatic differentiation in lower crustal hot zones beneath thick crustal regions is expected to enhance the volatile element budget and oxygen fugacity of evolving magmas that are discharged to the upper crust. This occurs due to the accumulation of incompatible volatile elements during successive cycles of recharge by mafic magmas and crystallization, facilitated by the deeper and hotter conditions beneath thicker arc crusts. As such, an increasingly recognized hypothesis holds that ore-forming magmas display a particularly increased budget of volatile elements and higher oxygen fugacities when compared to barren arc magmas, and that this is largely influenced by the arc crust thickness. The proposed work will test this hypothesis by focusing on the Miocene to Mio-Pliocene magmatism and associated world-class PCD mineralization in the Andes of central Chile. From the Early Miocene to the Mio-Pliocene, the arc segment located between latitudes ~33–34.5° S in the Andes of central Chile has seen a continued increase in crustal thickness and has evolved from being barren in the Early Miocene to producing some of the largest PCDs of the world in the Mio-Pliocene, such as El Teniente and Rio Blanco-Los Bronces. This geological scenario and the spatial and age distribution of the associated outcropping intrusive rocks offer a unique opportunity to investigate the temporal evolution of the volatile composition of magmas and its consequences for ore fertility. The goal of this proposal is to examine, adopting a regional scale perspective, the evolution in the volatile composition and oxygen fugacity of magmas produced in this arc segment and its relationship to magmatic ore fertility, as well as how this may have been influenced by changes in crustal thickness. To achieve this, I will sample an extensive suite of granitoids that represent a continuum from Early Miocene to Mio-Pliocene magmas, including porphyry-forming intrusions. By combining zircon petrochronology, apatite, biotite, and amphibole mineral chemistry, in conjunction with the bulk-rock composition of intermediate to felsic intrusive rocks, I will be able to constrain relative changes in the hydration state, sulfur contents, halogen and oxygen fugacities, as well as in their associated crustal thickness during the evolution of the selected arc segment. This will be done by implementing a combination of cutting-edge analytical techniques, including synchrotron-based sulfur X-ray absorption near edge structure spectroscopy, electron probe microanalysis, (laser ablation) inductively coupled plasma mass spectrometry, and X-ray fluorescence spectrometry. I aim at (1) testing the differences in the volatile composition of barren and ore-forming intrusive rocks; (2) determining whether there is a gradual change in the volatile systematics of magmas during the evolution of the studied arc segment; and (3) analyzing the relationship between variations in crustal thickness and the volatile composition of associated magmas. The results of this proposal will lead to a better understanding of the magmatic controls underpinning the formation of giant PCDs and will provide valuable insights into identifying the differences between the bulk-rock and mineral chemistry of ore-forming and barren granitoids as tools for vectoring mineralized regions.

Fondequip Mediano EQM230002