CARBONATE-SULFATE MELTS AND THEIR ROLE IN MAGNETITE-APATITE FORMATION WORLDWIDE (Invited Presentation)

The origins of magnetite-apatite deposits are contentious and proposed genetic models vary widely. Central to this debate, the chemical compositions of the ore forming fluids are still poorly understood, and even recent studies have invoked a wide variety of possible fluid types, from hydrothermal brines through to iron-rich silicate or oxide melts. To address this longstanding debate, we have analyzed fluid and melt inclusions in samples from numerous magnetite-apatite deposits around the world. We find that the most common and consistent inclusion type – virtually ubiquitous amongst all systems we have studied so far – is polycrystalline melt inclusions composed predominantly of calcium carbonate and sulfate minerals (calcite and anhydrite) and containing high modal proportions of Fe-oxides (magnetite, hematite, ilmenite) and silicates (feldspars and quartz). Importantly, inclusions of this type occur at magnetite-apatite deposits throughout the SW USA, South American Cordillera, China and Iran. The inclusions consistently contain 10’s of wt% Fe and re-melt at igneous temperatures ≥700°C. Hence, our results point to a critical role played by hitherto unrecognized carbonate-sulfate melts in the formation of magnetite-apatite deposits. We surmise that these inclusions have been mostly overlooked by previous workers because they differ markedly from fluid and melt inclusions in other deposit types and represent a rather “exotic” and unexpected liquid composition. That said, the widespread and consistent occurrence of these inclusions implies that carbonate-sulfate melts are much more common than has been previously recognized. These results have major implications regarding genetic models for magnetite-apatite deposits, and how they relate to occurrences of magnetite-apatite veins in other settings such as alkalic porphyries and carbonatites. Overall, the key implication of these results is that genetic models for these deposits should be revisited with explicit attention to the widespread evidence for a key role of carbonate-sulfate melts.