Sulfur and strontium isotopic study of epithermal mineralization: A case study from the SE Afar Rift, Djibouti

Abstract

Epithermal mineralization was recently described in synrift volcanic rocks from the SE Afar Rift in Djibouti. To infer fluid sources in this system, coupled sulfur and strontium isotopic analyses were performed on mineralized samples from hydrothermal veins and stockwork zones. Mineralization includes gypsum and chalcedony, and/or quartz ± carbonate containing gold and sulfides. Sulfur isotopic compositions of sulfides and sulfates were determined using Multicollector-Inductively Coupled Plasma Mass Spectrometer (MC ICPMS); Laser Ablation MC ICPMS; and Continuous Flow Elemental Analyzer-Isotope Ratio Mass Spectrometry while strontium isotopes were determined by Thermal Ionisation Mass Spectrometry (TIMS) methods. Sulfur isotopic composition of sulfides (mainly pyrite) varies from − 9.2 to + 6.6‰. These values are classically reported for hydrothermal sulfides resulting from the mixing of variable S sources such as seawater sulfate and magmatic source. In addition, individual pyrite grains within the same sample show both positive and negative δ34S values suggesting multi-phase mineralization processes. Coupled sulfur and strontium isotopic compositions of seven sulfate samples hosted in (i) gypsum mound, (ii) evaporites, (iii) stockwork veins in sediments or in volcanic rocks were also investigated to characterize the sources of Sr and S. The δ34S values of the sulfate ranged from − 0.8 to + 14.3‰ while the 87Sr/86Sr ranged from 0.70389 to 0.70639 consistent with variable mixing ratios of Sr and S derived from volcanic sources and seawater-derived sulfate. This study shows that, although precious-metal deposition is related to low-sulfidation epithermal environment, both acidic fluid of magmatic and saline origins contributed to the hydrothermal system in the SE Afar Rift. Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Mineralogy, fluid inclusions and stable isotope study of epithermal Au-Ag-Bi-Te mineralization from the SE Afar Rift (Djibouti)

“…In our hydrothermal sites, pyrite and chlorite have been described at Ali Addé and Hes Daba sites. Therefore, it is here suggested that gold could have been transported as bisulfide complexes in Hes Daba and Ali Addé areas although multiple sources of fluids are documented in these sites (Moussa et al, 2017). Furthermore; Moussa et al, 2012 reported two different stages of hydrothermal ore deposits: i) the first stage corresponds to massive quartz -containing sulfide minerals; ii) the second stage is characterized by the formation of chalcedony gangue containing gold and sulfide minerals.…”

“…As such, it has been the focus of intense geological and geophysical investigations that resulted in a number of propagation rift models principally devoted to the last myrs rift history in SE Afar (Manighetti et al, 1997;Audin et al, 2001;Daoud et al, 2011). These tectono-magmatic models have been recently completed by integrating hydrothermal aspects that emphasized the role of dominantly magma-related fluids in the occurrence of epithermal ore deposits (Moussa et al 2012;2017). These occurrences are mainly represented by base metal and precious metals-bearing quartz/carbonate ±adularia veins (Moussa et al, 2012).…”

“…4-5). In addition,Moussa et al (2017) identified the source of mineralization with sulfur and strontium isotopes and they further suggested that two major types of fluids contributing to the formation of ore deposits were i) hydrothermal fluids enriched in magmatic volatiles and leading to significant leaching of the underlying volcanic rocks (Da'asbiyo, Ali Addé, Hes Daba, Arta and Babba Alou sites) and ii) hydrothermal fluids probably initially enriched in magmatic SO 2 , but that underwent significant dilution with seawater and/or contamination with evaporite layers that later dissolved during the circulation of the hydrothermal fluids (e.g. Babba Alou, Arta Plage and Asa Leyta sites).In this work, we have investigated the fluid inclusions (Ali Addé and Hes Daba) and stable isotopes (Arta, Hes Daba and Asa Leyta) of the most mineralized sites.…”

Mixed carbonate-siliceous hydrothermal chimneys ahead of the Asal propagating rift (SE Afar Rift, Republic of Djibouti)

“…Measured 87 Sr/ 86 Sr ratios of Sakallol-Harallol chimneys are consistent with Sr isotope compositions of hot spring waters in this area (0.70365 -0.70626; Awaleh et al, 2017) and Djibouti basalts (0.70309 -0.70664; Vidal et al, 1991;Barrat et al, 1993;Deniel et al, 1994;Faure, 2001). More recently, Moussa et al (2017) investigated Sr isotopes of chalcedony-quartz veins (0.7032 and 0.7072), suggesting that Sr was derived from different sources of fluids, such as magmatic, hydrothermal and seawater.…”

“…1A) (Stieljes, 1973;De Chabalier and Avouac, 1994;Deniel et al, 1994;Pinzutti et al, 2013). Over the last decades, the discovery of many hot springs and fumaroles in this region has fostered major interest in geothermal exploration (Fontes et al, 1989;San Juan et al, 1990;D'Amore et al, 1998;Awaleh et al, 2015a, b;2017). Since the 1970s, many hydrothermal structures have also been identified in the lakes that are aligned along the rift system (e.g.…”