Listvenitization of Ophiolitic Serpentinites and Related Gold Mineralization in the Neoproterozoic Nubian Shield of Egypt
摘要
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 fluid-dominated 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 and Gabal Balamhindit. Furthermore, the distinction in rheology between listvenite and granites stimulates the precipitation of gold at their contacts. This is consistent with the existence of listvenite-lode associations in the greenstone belts of the ED at fault intersections of major thrust structures, particularly in areas where granitoid massifs are found, e.g., El-Barramiya, El-Sid and El-Anbat gold deposits. However, barren listvenites or listvenites with lower gold contents than their parent rock are also recorded in association with auriferous listvenites (e.g., Malo Grim, Um Khasila, El-Barramiya) or in other separate occurrences (e.g., El-Sid and Um Halham). The wide variation of gold content in listvenites among separate occurrences, and even within the same occurrence depends on lots of factors including: (1) the original gold concentration of the ultramafic protolith, (2) pressure–temperature (P–T) conditions, (3) origin and composition of the hydrothermal fluids, (4) shear deformation, (5) the duration and intensity of fluid-rock interaction and (6) the timing of listvenite formation.