Sodic(-calcic) alteration in Fe-oxide-Cu-Au districts: an origin via unmixing of magmatic H2O-CO2-NaCl +/- CaCl2-KCl fluids

Iron-oxide-Cu-Au deposits, particularly those formed in deeper level (pluto nic) environments, are commonly characterized by regional scale sodic(-calc ic) alteration, which typically formed pre- or syn-Cu-Au mineralization. Th e sodic(-calcic) assemblages include albite, scapolite, pyroxene, actinolit e, apatite, titanite, epidote and calcite. The consistent presence of coexi sting hypersaline aqueous and CO2-rich fluids in minerals from sodic(-calci c) alteration and associated Fe-oxide-Cu-Au deposits is the result of unmix ing of H2O-CO2-NaCl +/- CaCl2-KCl magmatic fluids. Experimental evidence in dicates that the Na/(Na + K) ratio of fluids in equilibrium with two alkali feldspars in CO32--bearing parent fluids would be significantly higher tha n in unmixed chloride-bearing aqueous fluids. Therefore, fluid unmixing cau sed by decreases in temperature and/or pressure, will result in albitizatio n of wall rocks, as is observed in most deeper level Fe-oxide-Cu-Au deposit s. This alteration style may be succeeded by K-feldspathization with decrea sing temperature because of the increase in equilibrium Na/(Na + K) in chlo ride-bearing fluids buffered by alkali feldspars.