Contribution of sulfur isotope to metallogenic studies in the SE Afar Rift, Republic of Djibouti

The Republic of Djibouti, located at the SE part of the Afar volcanic Triangle, is characterized by intense tectonic and bimodal volcanic activity. It is emplaced over an earlier magmatic rift system, as old as 25-30 Ma. Hydrothermal mineralization were recently discovered in this area. Here, we present the first isotope study on sulfides and sulfates minerals associated with these mineralization. Twenty samples from hydrothermal quartz, sulfides, gold, ± carbonate veins and breccias were studied from 8 different sites representative of 4 major volcanic events ranging in age from early Miocene up to Present. Sulfides and sulfates were analyzed by ElementalAnalyzer-Isotope Ratio Mass Spectrometry (EA-IRMS) methods. Values of δ34S for all samples range between 9.20‰ to 21.32‰. Three groups of δ34S values are identified (i) Gypsum with δ34S values are 21.32‰; (ii) Gypsum with δ34S values ranging from 1.08 to 16.33‰; (iii) Pyrite grains with δ34S values from –9.20‰ to 5.8‰. For the first group, δ34S values indicate gypsum from marine origin (sulfate in the modern ocean is 21‰). In the second group, the lower gypsum δ34S values (1.08 to 16.33‰) are not common (lower than seawater sulfate). They are likely due to the disproportionation of magmatic SO2 leading to enrichment of 34S in the sulfate [1, 2]. For the third group, the δ34S values of pyrite close to 0‰ are typically reported to volcanic rocks. However, the negative δ34S values of sulfides can be explained by the disproportionation of magmatic SO2 that causes the enrichment of 32S in the sulfides [1, 2]. These values classically indicate a source of sulfur from magmatic fluids. The wide ranges of δ34S values observed in the continental rift system at Djibouti are similar to those related to epithermal gold deposits [3]. The discovery of epithermal mineralization in a recent-active continental rift system is unique and supplies new insights about hydrothermal processes associated with volcanic activity in a back-arc system.