A fundamental role of carbonate-sulfate melts in the formation of iron oxide-apatite deposits

Iron-rich carbonate-sulfate melts are fundamental to the formation of iron oxide-apatite ore deposits, according to a detailed fluid-inclusion study that characterized the mineralizing fluids for two mineralizing systems in the United States. Genetic models for iron oxide-apatite deposits are controversial and span a spectrum from orthomagmatic to hydrothermal endmembers. This lack of consensus is rooted in uncertainties as to the nature and origin of ore-forming fluids in these systems. Here, we present a fluid-inclusion study of mineralizing fluids at two iron oxide-apatite deposits (Buena Vista, Nevada and Iron Springs, Utah). We found that the inclusions in both systems comprise both aqueous brine and ubiquitous iron-rich carbonate-sulfate melts. These melts were found throughout the paragenesis of both deposits and show a tremendous capacity to transport ferric iron. Hence, we argue that orthomagmatic fluids played a role in mineralization at both Buena Vista and Iron Springs, and that the main ore-forming fluid was an iron-rich carbonate-sulfate melt formed by the assimilation and anatexis of evaporite-bearing carbonate rocks. The geological conditions that give rise to carbonate-sulfate melts are also a common feature of other classic iron oxide-apatite systems worldwide. Hence, we argue that the process of assimilation, anatexis and immiscibility of carbonate-sulfate melts is fundamental to iron oxide-apatite formation and provides a common link between iron oxide-apatite systems in different geological settings.