METAL CHLORIDES TO METAL ALLOYS: AN INVESTIGATION OF MAGMATIC CL ABUNDANCES FROM DEPTH TO QUENCH AND ASSOCIATED PRECIOUS METAL-BEARING MINERALS

Research on newly discovered metal alloys (Co-Cu-Sn-Ag) hosted by vesicles of basaltic samples from several volcanoes around the world indicates that both the composition and abundance of alloys changes greatly. Some flows are almost absent of alloys while other flows or bombs from the same volcano host many alloys. The largest and most abundant alloys are formed in the vesicles of “cow pie” bombs from Stromboli, Italy. Data we will present suggests that alloy metals are dominantly transported as volatile chlorides to the vesicles where they are deposited as either sulfides and/or metal alloys (perhaps as a function of T and Cl/S). The size and abundance of these metal alloys could simply be related to the amount of Cl in the magma. This possibility is being investigated by looking at variations in Cl in the magma before eruption, before open-system degassing, and then after eruption. Surveys of alloy abundance of other Cl-rich mafic volcanoes such as Etna and Vesuvius suggest that it is not simply a function of the original Cl content of the magma. Pre-eruption concentrations of Cl and other volatiles are being evaluated by analyzing melt inclusions in olivine and pyroxene of cow pie bombs. The behavior of the Cl after eruption is being investigated by looking at Cl gradients in quenched matrix glasses and examining the abundance of vesicles and the volume of glass between vesicles. Effects of cooling are also being examined by comparing volatile concentrations of glass from bomb rims to cores (and perhaps lavas from the same eruptive episode). Our hope is that by understanding the role of Cl in the formation of these metal alloys, the processes involved in forming ore deposits on a larger scale can be better understood.