INDIAN OCEAN HYDROTHERMAL MINERAL DEPOSITS: CU-RICH AND EPITHERMAL AU-/AG-ENRICHED OCCURRENCES

Up to now, two extinct hydrothermal massive sulfide fields have been discovered in the Indian Ocean. In both fields, hydrothermalism has ceased between 11.000 and 13.000 years ago. The first occurrence (Sonne Field, 2850 m water depth) is located in the 4th segment of the Central Indian Ridge, north of the Rodriguez Triple Junction. The second one (Mount Jourdanne, 2940 m water depth) is located in the 11th segment of the South West Indian Ridge.

The Sonne Field appears to be a typical mid-ocean ridge massive sulfide occurrence with black and white smoker mineralizations; it is now, however, in a state of physical and chemical disintegration, and therefore depleted in anhydrite. The chimney structures were formed by multiple hydrothermal events. Three types of sulfide mineralizations can be distinguished: (1) Cu-rich massive sulfides, (2) pyrite-markasite massive sulfides, both of which originate from the black smoker activity, and (3) sphalerite- and barite-bearing jasper breccia, this latter obviously having been formed at epithermal temperature conditions. In contrast, the Mount Jourdanne deposit shows a much more complex mineralogy. Based on the macroscopic features, the samples can also be subdivided into three groups: (1) tube-like, small massive sulfide chimneys, (2) mound massive sulfides, and (3) clastic ore. All three contain the following minerals: sphalerite, pyrite, markasite, chalcopyrite, silica and barite, and to a lesser degree, galena and cubanite; on fissures and cracks even realgar, boulangerite, and Pb-As-sulfosalts were observed. The latter three mineral phases, together with galena, are quite unusual for massive sulfides in a basaltic environment. The Pb- and As-rich samples also contain surprisingly high concentrations of Au and Ag.

In order to explain these specific mineralization patterns, a very extreme fractionation process must have taken place, possibly in combination with a felsic source rock in the deeper underground. Since elements like Pb, Ag, Zn, and As like to form highly soluble chloride and chloro- complexes, we suggest that phase separation and the respective development of a brine has caused this unusual metal assemblage . Similar genetic conditions can be completely excluded for the Sonne Field massive sulfides.