LOW SALINITY FLUIDS FROM LARGE OPEN SYSTEM MAGMA CHAMBERS FORM PORPHYRY-CU (MO-AU) DEPOSITS

Porphyry Cu-(Mo-Au) deposits form from cooling and decompression of magmatic volatiles in the upper crust. Our results from fluid inclusions, SIMS oxygen isotopes, cathodoluminescence, and trace elements in quartz suggest that ore metals are supplied from the magma into the hydrothermal system by low salinity (5 wt% NaCl equiv), CO₂-bearing fluids containing up to 1 wt% Cu. In these fluids, where Na/Cl ratios are ~1, in addition to Cl-, S is also likely involved in Cu transport.

Low salinity CO₂-bearing fluid inclusions are trapped in both early quartz-rich veins with potassic alteration that formed at temperatures between 550° and 750°C as well as later pyrite quartz veins with sericitic alteration that formed at temperatures between 380° and 450°C and postdate ore precipitation. In veins from Butte, Montana, d¹⁸O in quartz indicates no influx of meteoric water during the formation of veins with potassic (9.3-10.5‰) or sericitic (11-13‰) alteration. Helium isotope ratios of 2 to 8 R/Ra in fluid inclusions trapped in pyrite in sericitically altered veins from several deposits indicate that mantle volatiles contributed significantly to the formation of these late veins.

These results are contrary to models where hypersaline brines transport Cu from the magma to the site of ore deposition owing to the prominence of copper-chloride complexes. They also contradict models that explain the observed metal and alteration zonation patterns in porphyry copper deposits by calling on changes in fluid chemistry resulting from closed system magma crystallization, coupled with meteoric water influx. Instead, our results suggest that the composition of input magmatic fluids is relatively constant during the formation of a porphyry type deposit, and that continued influx of mantle-derived volatiles (probably in magmas) occurs throughout the lifespan of deposit formation. We therefore conclude that magmas that lie below porphyry deposits behave as open systems allowing the continued input of melts and volatiles from the mantle. Either the abundance of mantle-derived volatiles is large enough or the magmatic systems themselves are large enough that local crystallization below a porphyry center does not significantly affect the bulk composition of the exsolved hydrothermal fluid during the formation of a porphyry-type deposit.