Local controls on magma fertility in the Mio-Pliocene metallogenic belt of central Chile: Exploration implications

Abstract

Understanding what produces and how to detect fertile magmas, capable of forming large, economic porphyry-type deposits, is not only a major scientific quest, but it is also relevant economically, as it could lead to better exploration models and higher success rates in mineral exploration. In this work, we use new and previous geochemical and geochronological data from the Mio-Pliocene metallogenic belt of Central Chile to assess the existing tools to detect fertile magmas, study their spatiotemporal variations at different scales and discuss the geological and exploration implications of our findings.Our results suggest that the geochemical ratios currently used to detect fertile magmas are effective, as there is a good correlation between fertile signatures and the presence of known porphyry Cu deposits and prospects, with few false positives, which might be explained by the exhumation and erosion of most of the metal-rich part of the hydrothermal systems.Regarding the genesis of fertile magmas, the spatiotemporal variations we document here are not satisfactorily explained by the processes commonly brought forward to produce this type of intrusions, such as crustal thickening and subduction of sediments, crustal fragments or oceanic fracture zones. These processes operate at regional- or continental-scales in entire arc segments, while magma fertility shows strong contrasts in several coeval intrusions separated only by a few kilometers or tens of kilometers, emplaced within the same arc segment, under the same tectonic regime, crustal thickness and subduction parameters. This implies that there is a strong local control on the production of fertile magmas.Our results support the notion that the most important variable affecting the generation of fertile magmas in subduction margins, is magma residence time at different crustal levels. This variable has a strong local control, as it will depend not only on the overall crustal thickness or the regional tectonic regime, but also on the local orientation of structural pathways controlling magma ascent through the crust, relative to the predominant stress tensor. If magma pathways are oriented at higher angles relative to the maximum stress, magma residence times will be longer, allowing the fractionation of abundant hornblende (giving the residual magma its characteristic fertile or “adakite-like” signature) and the accumulation of magmatic sulfides which can be remobilized by later magmatic pulses; becoming progressively more oxidized due to intra-crustal processes; and concentrating large amounts of incompatible volatiles and metals in the residual melts. This can explain the strong variability observed in magma fertility in nearby coeval intrusions, and also in cross-cutting intrusions separated by very short time periods, which might have followed different ascent pathways.Our results also suggest that the Teno-Maule segment, in the southernmost part of the Mio-Pliocene metallogenic belt, shows striking differences in magma fertility patterns compared with the rest of the belt. There, fertile magmas are scarce and, contrary to the northern part of the belt, there is no evidence of an enhanced magma fertility during the late Miocene – early Pliocene.