Plagioclase records metal fertility in Andean magmatic systems

Subduction zones are home to Earth’s most dangerous volcanoes and the largest deposits of porphyry copper, which supply about three-quarters of the global copper. However, it is still unclear why some magma erupts, while other magma stalls and contributes to crustal growth, or why certain magma formations lead to the creation of economically valuable minerals.

In this study, we examine the plagioclase record within both the volcanic and plutonic formations of the Andean arc, focusing on key components: porphyry Cu-Mo deposits (El Teniente), barren intrusions (within the El Teniente district, Central Chile, and global datasets), and a diverse suite of Central Andean volcanoes (stratovolcanoes, cones, and volcanic fields).

We analyse magmatic plagioclase using high-resolution in-situ microanalytical techniques within a rigorous textural and chemical framework that isolates primary magmatic signals from hydrothermal overprinting in porphyry intrusions coeval with mineralisation. Major and trace elements, together with Sr isotopes, were quantified by electron microprobe and LA-ICP-MS (quadrupole and multicollector), while crystal textures were characterised using petrographic microscopy and X-ray fluorescence microscopy. We found that the anorthite content in plagioclase is an excellent proxy for magmatic copper fertility, defining a narrow range of An30±4 in magmas that host mineralisation from El Teniente, but also in global Cu-Mo deposits. The low An content indicates growth from cold, evolved magma, which sharply contrasts with plagioclase from barren intrusions and volcanic rocks. Interestingly, our results reveal chemical and textural similarities between plagioclase from mineralised porphyries and specific volcanoes located in tectonically anomalous segments of the arc that exhibit petrographic signs of mush development.

Porphyry-related plagioclase also displays significant sharp increases in Sr with minor variations in Fe, along with increasing Sr/Y ratios, indicative of amphibole/garnet fractionation. In contrast, plagioclase from barren intrusions and volcanoes along the main arc show Sr–Fe trends indicative of mafic magma recharge and low Sr/Y ratios, pointing to a lack of deep fractionation. Isotopically, fertile and barren magmas indicate distinctive magmatic pulses, with fertile magmas showing limited isotopic variation.

Thermodynamic modelling of El Teniente magmas indicates that our An_30±4 range crystallised at temperatures below 850°C and immediately after the point of volatile exsolution at low pressure (< 300 MPa). The crystallisation of mineral phases other than plagioclase, such as garnet, orthopyroxene, clinopyroxene, biotite, and alkali feldspar, demonstrates a progression from a more anhydrous magma to a water-rich environment.

These findings indicate a shift in magma dynamics from a barren stage to a fertile stage. During the fertile stage, cold, evolved, calm and hydrous magmatic environments create the conditions necessary to generate porphyry copper deposits. Volcanoes that exhibit similarities to fertile magmas in their plagioclase characteristics may represent systems currently developing the perfect environment to become porphyry deposits.