ZIRCON COMPOSITIONAL EVIDENCE FOR SULFUR-DEGASSING FROM ORE-FORMING ARC MAGMAS

Porphyry Cu (±Mo±Au) and epithermal Au-Ag deposits are major sources of mined metals, and are commonly formed by magmatic-hydrothermal fluids derived from hydrous magmas in Phanerozoic convergent margin settings. The igneous rock assemblages associated with these occurrences are also common in modern convergent margin settings, but very few, past or present, have produced the acidic hydrothermal fluids containing metals, chlorine, and sulfur necessary to engender an ore deposit.

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 Eu_N/Eu_N* > 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.