Porphyry Cu formation in the middle Jurassic Yerington batholith, Nevada, USA: Constraints from laser Raman, trace element, U-Pb age, and oxygen isotope analyses of zircon

Abstract

Porphyry copper systems provide the majority of global copper resources and are generally formed from highly oxidized magmas. Zircon, a common accessory mineral in granitoid rocks that host porphyry deposits, is well established as an effective tool for assessing timescales and evolution of magmatic conditions. We present new U-Pb ages, trace element concentrations, and oxy­gen isotope ratios of zircon measured by secondary ion mass spectrom­etry (SIMS) from a suite of cogenetic host rocks and ore-bearing porphyry dikes from the Yerington copper mine, western Nevada, USA. Zircons were subjected to chemical abrasion and thermal annealing in order to evaluate Pb loss, and laser Raman analyses were performed to avoid measurements of radia­tion damaged or non-crystalline (potentially metamict) portions of zircon. Weighted-mean U-Pb ages from ore-bearing Yerington porphyry dikes and granitoid host plutons overlap at 2σ uncertainty, ranging from 168.7 ± 1.1 Ma to 170.0 ± 1.4 Ma. Zircon trace element concentrations show fractional crystallization trends, such as decreasing Ti versus increasing Eu anomaly (Eu N /Eu N *) and Yb/Gd. Uranium concentrations range from 90 to 2200 ppm (average is ~320 ppm U), Eu-anomaly ratios range from 0.19 to 1.05, and Ce N /Ce N * values range from 20 to 980. Oxygen isotope compositions range from 4.8 ± 0.7‰ to 5.7 ± 0.8‰ (sample means), with the most depleted composition from the youngest porphyry dike. We find no statistically significant difference in ages, trace elements, or oxygen isotopes from chemically abraded and untreated zircons. Based on variations in magmatic conditions as suggested by Eu anomalies, trace element trends, model zircon crystallization temperatures, and δ 18 O in zircon, we conclude that variable but increasing oxidation and ongoing fractional crystallization were strong controls on elemental partitioning and ore-forming processes in the Yerington system.