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Springer Nature Switzerland
Springer
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| Abstract: |
Serpentinization of ophiolitic serpentinites in the
Neoproterozoic Nubian Shield of Egypt is commonly
associated with the formation of listvenites, talc-carbonate
rocks and magnesite veins along fault planes and
shear zones. The serpentinized ultramafics and associated
listvenites in the Nubian Shield have regional
significance in relation to the evolution of the Arabian-
Nubian Shield (ANS) and are globally recognized as a
key example of the processes that played a role in shaping
Proterozoic continental crust. Furthermore, listvenitization
of ophiolitic ultramafic rocks has attracted special
attention due to its spatial and temporal association with
gold mineralization. In general, listvenites represent
the end products of different alteration processes with
variable degrees of carbonatization, silicification and/
or potassium metasomatism. During the listvenitization
process, primary ferromagnesian silicate minerals in
ultramafic rocks undergo replacement by carbonate minerals,
resulting in the release of silica that subsequently
form quartz and/or fuchsite (Cr-rich mica). Listvenite
bodies of different shapes and sizes are observed in the
Egyptian Nubian Shield, particularly in the central and
southern sectors of the Eastern Desert (ED). They occur
as dyke-like bodies or lenses of reddish-brown color,
spatially associated with highly tectonized, allochthonous
ophiolitic ultramafics, especially along the plane
of shear zones. Based on mineralogy and bulk chemistry,
listvenites in the ED can be classified into three
varieties; carbonate-rich listvenite (>70 vol% carbonate,
SiO2 < 35%), silica-carbonate listvenite (40–60 vol%
quartz and SiO2˃38 wt.%) and birbirite (>70 vol.%
quartz and SiO2˃80 wt.%). These different varieties of
listvenites were formed during successive stages of fluiddominated
replacement reactions. They can form before,
during or subsequent to the obduction of ophiolites onto
the continental crust. The carbonate-rich listvenite seems
to have developed simultaneously with the process of
serpentinization, while the silica-carbonate listvenite and
birbirite formed at a later stage. The presence of serpentine
and fresh Cr-spinel relics in listvenites, along with
their occurrence along shear zones, indicate their development
through metasomatism of the ultramafic protolith
by hydrothermal fluids that circulated along the
thrust faults. The fresh Cr-spinel cores within the listvenites
have high Cr# (> 0.60), comparable to fore-arc
Cr-spinels of ophiolites in the Arabian-Nubian Shield
(ANS). Significant gold concentrations in listvenites,
especially in the silica-carbonate type, have been documented
in various regions in the Eastern Desert (ED) of
Egypt, e.g., El-Barramiya, El-Anbat, Um Khashila-Um
Huweitat, Gabal Sirir, Gabal Shilman, Malo Grim,
Balamhindit and Abu Fas. Gold deposition in listvenite
is triggered by rock interaction with Au-bearing
CO2-rich fluid. A combination of CO2 originating from
the mantle and that derived from the surface has been
broadly recognized as a likely source for these CO2-rich
hydrothermal fluids. The infiltration of these fluids in
serpentinites causes breakdown of Au-hosting minerals
and liberates gold and other fluid-mobile elements forming
Au-bearing CO2-rich fluid phase. Carbonatization
process can potentially increase gold concentration up
to 1,000 times relative to the original ultramafic protolith,
e.g., Abu Fas, Gabal Shilman and Malo Grim.
Also, a temporal and genetic relationship connects gold
mineralization in listvenites with some granitic intrusions,
e.g., El-Barramiya, El-Anbat, Gabal Shilman
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