ION MICROPROBE STUDY OF AU AND TRACE METALS IN SULFIDE MELT INCLUSIONS FROM A RHYOLITE PORPHYRY INTRUSION IN THE CAETANO CALDERA, NEVADA

Magmatism at Caetano caldera (34 Ma) was the youngest part of ~40-34 Ma magmatic activity related to the formation of large Carlin-type gold deposits, Cortez and Cortez Hills (~35.7 Ma), located 1-4 km northeast of the caldera margin. To test Caetano magmas as potential sources of Au and other metals, we used secondary ion mass spectrometry (SIMS) to quantify metal abundances in sulfide (pyrrhotite) melt inclusions from a glassy rhyolite porphyry intrusion along the northern caldera margin, selected because of its freshness and relative abundance of magmatic sulfides. Pyrrhotite occurs as small (~5-30 μm) spherical melt inclusions in orthopyroxene, ilmenite and plagioclase phenocrysts, and as isolated blebs in groundmass glass. Electron microprobe analyses indicate molar Fe/S ratios of 0.84-0.89, corresponding to N_FeS=0.91-0.95, and log fS₂= 0 to -3 at magmatic temperatures of 800-850°C. The largest inclusions in ~20 thin sections were analyzed using a SHRIMP-RG ion microprobe, operated with a ~5 nA Cs+ primary beam focused to a ~20 μm spot size. Concentrations were calibrated using a range of synthetic pyrrhotite and pyrite standards. The SHRIMP-RG was operated at a mass resolving power of ~15,000 ΔM/M to eliminate isobaric interferences, achieved by narrowing secondary slits and energy filtering. Homogeneous and low Au concentrations occur in pyrrhotite melt inclusions (0.9±0.6 ppm), corresponding to ~5-9 ppb Au in equilibrium rhyolite (based on K_Dpo-rhyolite=140±40). These results corroborate earlier ion microprobe data on quartz-hosted rhyolite melt inclusions in the Caetano Tuff that contained ppb-level Au. Concentrations of other trace metals (ppm) are more heterogeneous in Caetano pyrrhotites, Cu (100-30,000), As (5-8,000), Ni (200-4,000), Se (40-300), Sb (0-100), Te (0-20). This study demonstrates the utility of ion microprobe techniques to quantify low-level Au contents at the 10s of μm scale required for melt inclusion work. Our results do not support a model in which Caetano magmas were an important source of Au in nearby Carlin-type deposits; however, earlier, non-explosive phases of magmatism (preceding Caetano Tuff eruption and ring-fracture intrusion) may have had higher volatile and metal contents that were degassed during explosive eruption.