| Authors |
Moussa, N.; Fouquet, Y.; Le Gall, B.; Bohn, M.; Etoubleau, J. ; Grassineau, N.; Caminiti, A. M.; Rolet, J.; Jalludin, M. |
| Abstract |
Epithermal gold mineralization were recently discovered in the SE part of the Afar volcanic Triangle, Republic of Djibouti. Mineralization generally occur as veins and are mainly associated with acidic volcanic intrusions along the faults at the edges of graben structures established during the last 4 Ma. Sixty samples were analyzed from 4 different sites representative of 4 major volcanic events. Mineralogical analyses based on optical reflected light microscopy, X-Ray diffractometry, X-Ray fluorescence, inductively coupled plasma mass spectroscopy, electron microprobe and scanning electron microprobe, led us to identify different types of gold mineralization (i) native gold, electrum, hessite (Ag1,98Te1,02), tetradymite (Bi1,99Te2,13S0,88) and sulfides (chalcopyrite, pyrite, sphalerite, galena, bornite, digenite, marcasite) in massive quartz breccias and banded chalcedony, (ii) gold, electrum, pyrite, hematite, magnetite, traces minerals (argentite) and adularia in banded chalcedony. Gold-telluride associations are generally regarded as characteristics of low sulfidation epithermal gold deposits. Te is found in three different forms (i) Te occurs as invisible or submicrometer-size inclusions of hessite in pyrite where a high content (up to 41 wt %) of Te were measured; (ii) as visible independent telluride grains (hessite (Ag1,98Te1,02) and tetradymite (Bi1,99Te2,13S0,88)) disseminated in quartz and (iii) as grains of hessite up to 50 ìm with pyrite inclusions (10 ìm). Tellurium is preferentially transported in a gas phase and may result from condensation of magmatically derived H2Te(g) and Te(g) into deep-level of chloride waters [1]. At Hes Daba site, sulfur isotopic ratios of pyrite indicate values of ä34S between –9.20‰ to 1.45 ‰. The ä34S values of pyrite close to 0‰ are typically reported to volcanic rocks and the negative ä34S values classically indicate a source of sulfur from magmatic fluids. Thus in our samples telluride mineralization and sulfur isotopic studies of sulfides suggest that gold can be related to a magmatic fluid contribution and is precipitated in the deep high temperature part of the epithermal system. |