TIMING IS EVERYTHING – HOW O-H-S-CL-C-BEARING MAGMATIC FLUIDS EVOLVE INTO MINERALIZING FLUIDS IN ORE-BEARING PORPHYRITIC STOCKS

The geochemical behavior of volatiles in felsic stocks exerts strong controls on the mineralized versus barren nature of the intrusions. Recent investigations of ore metal-enriched fluid inclusions of mineralized porphyritic plutons measured relatively low Cl concentrations (less than 13 wt% NaClequiv) demonstrating that low-salinity vapor is an efficient medium for transporting and depositing metals including copper and gold. A critical question bearing on these fluids is: at what stages of magma evolution did the fluids exsolve and/or last sequester metals and other components, and therefore, what stages of magma evolution do these fluids actually represent?

Addressing this question requires accurate knowledge of the concentrations of magmatic H2O, S, Cl, and CO2 in mineralizing and barren intrusions, and this information is obtained from methods that are somewhat wanting. Volatile abundances in felsic melt inclusions of subduction-related volcanic systems are measured, but many felsic inclusions represent partially degassed magma so these data have little bearing on maximum volatile enrichments in melt and on the earliest stages of the exsolution of magmatic fluids that ultimately evolve into mineralizing fluids. A related and perhaps more fundamental concern is if the volatile abundances of eruptive systems are actually equivalent to those of potentially mineralizing plutonic systems, and unfortunately, constraints from volatiles in melt inclusions from plutonic rocks are far less abundant. Another approach is to determine these components in apatite of plutonic rocks, but these apatite compositions may not reflect early stage magmatic values but rather late-stage reactions with fluids at sub-solidus conditions.

Given these limitations, we apply the volatile abundances of melt inclusions in mafic subduction-related volcanic rocks, geochemical modeling, and experimental partition coefficients for Cl and S for equilibria involving melt and O-H-S-C-Cl-fluids with or without apatite to demonstrate that the first fluids to exsolve are the most saline and highly efficient at scavenging ore metals from magma. It is also shown that closed-system (batch-fluid) evolution of these magmas ultimately generates low-salinity vapors equivalent to those of fluid inclusions of mineralized porphyries.