PROTEROZOIC IRON-OXIDE (CU-U-AU-REE) DEPOSITS - FURTHER EVIDENCE OF HYDROTHERMAL ORIGINS

Variations in the structural setting and mineralogy of Proterozoic iro n oxide Cu-U-Au-REE (rare earth element) deposits, particularly regard ing apparent depth of formation, have resulted in models ranging from direct injection of an Fe-oxide melt to formation via hydrothermal rep lacement. The Emmie Bluff Fe-oxide deposit, South Australia, is unequi vocally of hydrothermal origin and exhibits overprinting relations, vi sible at both the meso- and microscopic scale, that strongly support a model involving two temporally distinct fluids, as previously propose d for the enormous Olympic Dam deposit. The evidence indicates that th e first stage of magnetite-rich ore formation was associated with high -temperature fluids from a major mid-Proterozoic felsic intrusive even t. This was followed by infiltration of lower temperature fluids that produced the hematite-rich mineralization. Whole-rock stable isotope a nalyses are consistent with this model and suggest that the initial fl uids were magmatic, with a deltaO-18 value of 8 parts per thousand-9 p arts per thousand, whereas the later mineralizing fluids, had lower va lues, 0 parts per thousand-2 parts per thousand, perhaps indicative of a meteoric fluid component.