ZIRCON COMPOSITIONAL EVIDENCE FOR SULFUR-DEGASSING FROM ORE-FORMING ARC MAGMAS
We report SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element abundances of zircon, a nearly ubiquitous and robust trace mineral in crustal magmas. Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (mostly EuN/EuN* > 0.4). Because europium is sensitive to oxidation (Eu2+ vs Eu3+) and in oxidized conditions is incorporated into zircon, the zircons with small europium anomalies record strongly oxidized conditions in ore-forming plutons relative to those not associated with ore formation. The magnitude of the europium anomaly in zircon likely reflects the amount of sulfur degassing of SO2-rich magmatic-hydrothermal fluids and variations in Fe/S during magma crystallization. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S6+) to SO2 (S4+) upon degassing. This interpretation reinforces the important role of sulfur-rich fluids in porphyry and epithermal mineral deposit formation. Zircon compositions thus may identify ancient magmas that released sulfur-rich gases, and regional surveys of zircon composition are potentially a valuable tool for mineral exploration.