Tatnell and 2 co-authors reported in mid-2023 that 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. [Definition of adakites]
Lambda (λ) coefficients deconvolute subtle differences in REE pattern curvature, providing a more quantitative method to explore mineral fractionation processes. Here, the authors 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.
The authors found 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.
The authors concluded that 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. The authors used 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.
The authors also showed 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.
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