A New Low Sulphidation Epithermal Gold District in the African Rift Valley of the Republic of Djibouti

The association of gold occurrences, bimodal volcanism, hot spring activity and
rifting within the Afar Depression of the African Rift Valley showed potential for low
sulphidation epithermal gold deposits. Recent exploration there has defined a new
epithermal gold district within the Assal-Gaggade-Hanle grabens in the Republic of
Djibouti.
Gold mineralization is associated with the compositionally bimodal, basalt–rhyolite suite and extreme extension. Low sulphidation gold-bearing epithermal veins
hosted by basalts, and vein and disseminated gold mineralisation within rhyolite
domes, are located along faults bounding major NW-SE grabens. To date some ten
outcropping epithermal systems have been discovered during field work with three
of them tested by diamond drilling. The epithermal systems original palaeosurface,
defined by recent sinters and active fumaroles, is preserved, exposed and, clearly evident.
The Pandora vein system, within the Gaggade graben, has been mapped for ten
strike kilometers and drill tested over two kilometers length with potentially economic gold grades confirmed over 1,200 m of strike and to a vertical depth of 150 m
below outcrop. The veins comprise chalcedonic quartz commonly showing multiple
phases of hydrothermal brecciation. The Pandora system is partially covered by postmineral basalts and extends northwest to the Hesdaba prospect, where at least five
mineralized structures are associated with rhyolite intrusives. At Hesdaba, there is no
sinter, or any other indication of an eroded palaeosurface, but smectite is the main
alteration mineral suggesting that mineralization was emplaced at shallow depths.
Southeast at Pandora, sinter is present indicating that the hydrothermal system migrated southward, as the graben evolved.
The Assaleyta prospect is located on the south-west side of the Assal graben.
Limited drilling has defined gold mineralization, over three strike kilometers, that
is associated with veins and disseminated within hydrothermal breccias in coalesced
rhyolite domes. These domes are found astride graben-controlling faults.
Gold mineralization was initiated at the time when the grabens were forming
and being infilled by large deep lakes that subsequently drained as the rift system
developed. Mineralization is associated with late rhyolite domes that cut the so-called
Afar Stratoid series (2.8–0.76 Ma) when the grabens were active, starting from Hanle
(SW) to Assal (NE) between 2.7 and 1.0 Ma. Gold mineralization is less than 1 Ma old.
The rate of gold discoveries worldwide has declined because, of necessity, exploration for deeper blind mineralization is slower and more costly than surface discovery.
The Afar Depression discoveries demonstrate that potentially economic outcropping mineralization based on solid geological models can still be found, with persistence, in
new geologically favourable environments. In this case, low sulphidation epithermal
gold occurs in a bimodal rift environment.
Key Words: Djibouti, bimodal, low sulphidation, new discoveries