| Abstrakt: |
Porphyry copper ore‐forming intrusions are distinguished from barren arc magmas by high oxidation state and "adakitic" high Sr/Y and La/Yb. Controversy over petrogenesis of adakites has centered on ambiguities in interpretation of their steep rare earth element (REE) patterns, and on whether garnet participates in their petrogenesis. Lambda (λ) coefficients deconvolute subtle differences in REE pattern curvature, providing a more quantitative method to explore mineral fractionation processes. Here, we use trace element and numerical λ coefficient modeling to assess the relative influence of amphibole, garnet, and plagioclase in the petrogenesis of porphyry ore‐forming intrusions. We find that garnet‐fractionation trends are not evident in REE patterns of many adakitic porphyry‐forming intrusions. Instead, porphyry‐forming intrusions in the western Pacific and Eocene porphyry‐forming intrusions in northern Chile have REE patterns consistent with a garnet‐free, amphibole‐dominated cumulate. Traditional element ratios such at Dy/Dy*, LaN/YbN, and DyN/YbN are poor discriminants for garnet or amphibole fractionation. Plain Language Summary: Copper is a crucial metal for electrification and emission‐free energy generation. Copper is primarily sourced from active and extinct volcanic mountain chains. The minerals forming in the root zones of these volcanoes are crucial for ore formation processes, but identification of the exact minerals involved in these processes is still debated. We use a novel method of detecting small variations in the chemical signature of the rare earth elements, combined with numerical modeling of mineral crystallization to distinguish the participation of two minerals, garnet and amphibole. We show that amphibole is ubiquitous whereas garnet only occurs in some deposits. Therefore, garnet fractionation is not necessary for copper deposit formation as has been previously suggested. Key Points: Analysis of rare earth element pattern curvature distinguishes between amphibole and garnet fractionation better than element ratios like Dy/Yb or Dy/Dy*Polybaric differentiation or plagioclase fractionation is unlikely to overprint a pre‐existing garnet signatureWe find that garnet fractionation is not necessary for porphyry copper deposit formation [ABSTRACT FROM AUTHOR] |
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