Porphyry Copper Deposit Formation: Identifying Garnet and Amphibole Fractionation With REE Pattern Curvature Modeling

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*, La_N/Yb_N, and Dy_N/Yb_N are poor discriminants for garnet or amphibole fractionation.