MELT INCLUSION EVIDENCE FOR SULFATE-CARBONATE-CHLORIDE MELTS IN IRON OXIDE-APATITE DEPOSIT

The type of fluids involved in the iron oxide–apatite (IOA) deposits are the crux of the debate about their origins. We report the ubiquitous occurrence of high-temperature, polycrystalline melt inclusions in IOA deposits from around the world, revealing a direct genetic links between molten salts and these deposits. We studied the petrographic characteristics of thousands of inclusions at room temperature, identified their phases by Raman spectroscopy and EDS, and conducted microthermometry experiments to re-melt these inclusions in the lab. These polycrystalline inclusions show complex compositions of chlorides, sulfates, carbonates, silicates, calc-silicates, and metal sulfides and oxides. Four main types of polycrystalline inclusions with end-member components are summarized based on their compositions: (i) chloride melt inclusions (including sylvite, halite, hibbingite, etc.); (ii) sulfate melt inclusions (including anhydrite, barite, cesanite, and gypsum); (iii) calc-silicate melt inclusions (including diopside, wollastonite, garnet, and tremolite/ actinolite); and (iv) carbonate melt inclusions (including calcite, dolomite, natrite and trona). Although some examples of end-member type inclusions are reported, most studied inclusions are transitional between different types, underscoring that all four types are intimately related and represent different flavors of a broader category of salt melts. Polycrystalline inclusions across all studied deposits melt between 585 °C and 1200 °C, and thus represent high-temperature ionic liquids. These high-temperature inclusions are widely distributed in all deposits studied, and seem to be a fundamental feature of IOA systems. Although the detailed characteristics, types, and distribution of inclusions vary between different deposits, taken together they suggest a key role of magma contamination and immmiscibility. Therefore, we argue that these results reveal consistent involvement of hitherto unexpected molten salts in forming magnetite-apatite rocks worldwide.