Proterozoic iron oxide (Cu-U-Au-REE) deposits: Further evidence of hydrothermal origins

Variations in the structural setting and mineralogy of Proterozoic iron oxide Cu-U-Au-REE (rare earth element) deposits, particularly regarding apparent depth of formation, have resulted in models ranging from direct injection of an Fe- oxide melt to formation via hydro-thermal replacement. The Emmie Bluff Fe-oxide deposit, South Australia, is unequivocally of hydrothermal origin and exhibits overprinting relations, visible at both the meso- and microscopic scale, that strongly support a model involving two temporally distinct fluids, as previously proposed for the enormous Olympic Dam deposit. The evidence indicates that the first stage of magnetite-rich ore formation was associated with high-temperature fluids from a major mid-Proterozoic felsic intrusive event. This was followed by infiltration of lower temperature fluids that produced the hematite-rich mineralization. Whole-rock stable isotope analyses are consistent with this model and suggest that the initial fluids were magmatic, with a δ¹⁸O value of 8‰-9‰, whereas the later mineralizing fluids had lower values, 0‰- 2‰, perhaps indicative of a meteoric fluid component.