Tracing the effects of fO2, pressure and H2O on the ore-forming magmas: Perspective from zircon REE composition

Abstract

Efforts have been devoted to explore if there are independent zircon trace element ratios that could be proxy of oxidation state and magmatic water content, and can distinguish between mineralization and barren granites. In this study, I evaluated the difference of the zircon elemental signatures between fertile magmatic suites (which host porphyry Cu-Mo, Mo, Cu-Au, Cu, Cu-Mo-Au, Mo-Cu, and Au deposits) and infertile I-type granites. The “potential” magmatic suites, which means that they are supposed to develop porphyry deposits based on the previously proposed ore-forming criteria (e.g., zircon Eu/Eu* > 0.5) and yet no associated mineralization has been found, were also included in this comparison. Most of the trace elemental signatures cannot differentiate the I-type granite zircons from fertile zircons and “potential” zircons, except for the Eu anomaly (∼0.5 < Eu/Eu* < ∼0.8 for ore-related and “potential” suites; ∼0.2 < Eu/Eu* < ∼0.5 for I-type granites) and middle/heavy rare earth element variation diagrams (MREE and HREE; e.g., Eu/Gd versus Gd/Yb, Lu/Yb versus Gd/Yb, Sm/Ho versus Sm/Lu, and Yb/Lu versus Dy/Lu). The behavior of zircon REE possibly indicate a more MREE-depleted signature for the ore-forming and “potential” suites. Taken together, zircon REE features documented in this study highlight the fractional crystallization of hornblende and garnet, and the delay of plagioclase crystallization, revealing the combined effect of high overlying pressure, high fO2 and high H2O (or H+) concentration on the differentiation of fertile magmas.