Download Epithermal gold mineralization in a young emerged oceanic axis:

Epithermal gold mineralization in a young propagating rift axis: the Asal-Tadjoura
accretionary system, SE Afar, Republic of Djibouti
Moussa, N.[1,2,3]*, Fouquet, Y.[1], Caminiti, A. M.[3], Bohn, M.[1], Etoubleau, J.[1], Le
Gall, B.[2], Jalludin, M.[3], Rolet, J.[2], Delacourt, C.[2]
[1] IFREMER, Centre de Brest, BP 70, 29280 Plouzané, France
[2] UMR 6538 Domaines océaniques, UBO-IUEM, Place Nicolas Copernic, 29280, Plouzané,
France
[3] IST, Centre d’Etudes et de Recherches de Djibouti, B.P. 486, Djibouti
* Email: [email protected]
The Afar Triangle results from the interaction of a number of actively-propagating tectonomagmatic axes. One of them encompasses the western extremity of the Gulf of Aden
acretionnary ridge and its emerged tip zone along the Asal rift. During the last 3 Ma, this
rifting event was accompanied by a bimodal (mafic and acidic) volcanic activity, emplaced
over an earlier magmatic rift system, as old as 25-30 Ma. The entire volcanic sequences
display clear evidence for successive hydrothermal processes. During the last 3 Ma
mineralization is preferentially express both by intense weathering and veins mineralizations
in acidic facies close to magmatic intrusions. Eighty samples from hydrothermal veins were
studied on 8 different sites representative of 5 main volcanic events ranging in age from early
Miocene up to Present. Gold was found in excess of 200 ppb in 30 per cent of the samples,
indicating an efficient hydrothermal process to transport and concentrate gold. Mineralogical
analyses based on optical reflected light microscopy, X-Ray diffractometry, X-Ray
fluorescence, inductively coupled plasma mass spectroscopy and electron microprobe, led us
to identify two types of gold mineralization (i) native gold and sulfides (chalcopyrite, pyrite,
bornite and ± sphalerite, galena) in massive quartz breccias, and, (ii) gold, electrum, hematite,
magnetite, trace minerals (argentite) and adularia in banded chalcedony. Another group
without gold is characterized by quartz, pyrite ± goethite. Secondary minerals are
characterized by goethite, native silver and native copper. Arsenic is enriched in pyrite in
samples with a high gold content. Field studies show that mineralized zones are associated
with late felsic intrusions and intense hydrothermal alteration. The acid volcanism, the
occurrence of adularia and the native gold and electrum in banded silica veins, are classically
observed in neutral epithermal systems on island arcs. The discovery of this type of
mineralization in a recent-active continental rift system supplies new insights about
hydrothermal processes associated with volcanic activity in a spreading context.