Formation of gold-bearing listvenite in the mantle section of the Neoproterozoic Bir Umq ophiolite, Western Arabian Shield, Saudi Arabia

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Serpentinized peridotite and associated listvenites of Neoproterozoic age outcrop in the Bir Umq area of western Saudi Arabia. The mantle section of the Bir Umq ophiolite is extensively serpentinized. Serpentinite host samples are low in Al2O3 (0.48–0.75 wt%) and CaO (0.28–1.24 wt%) and have high Mg# (0.90–0.92), indicating a strongly depleted mantle protolith, typically associated with supra-subduction zone environments and more specifically with fore-arc settings. Listvenite bodies of various shapes and sizes developed by alteration of serpentinite. Listvenites occupy the hanging walls of a stack of thrust faults, while serpentinite dominates the footwalls. Based on mineralogical composition and whole-rock geochemistry, the listvenites of Bir Umq are distinguished into carbonate listvenite and silica-carbonate listvenite; the latter is further divided into mineralized and non-mineralized samples. Carbonate listvenite is high in MgO, Fe2O3, and CaO, but depleted in SiO2. Silica-carbonate listvenite is characterized by the presence of rhythmic banding of quartz and carbonate minerals and by the presence of fuchsite. The Bir Umq listvenites preserve various stages of the progressive alteration and metasomatic transformation of their ultramafic protoliths due to interaction with hydrothermal fluids enriched in CO2, SiO2, Au, K, and other fluid-mobile elements. The association with thrusting suggests that faults acted as conduits for fluids derived from metamorphism of the underlying units during subduction and obduction. Schistosity and deformation fabrics in carbonate listvenite imply that initial listvenitization took place at conditions similar to the conditions of serpentinization. On the other hand, the absence of deformation fabrics in silica-carbonate listvenite suggests that it postdates serpentinization and therefore represents a separate and later fluid infiltration event. Finally, the mineralized silica-carbonate listvenite is highly enriched in fluid-mobile elements Zn, Pb, Cu, Ag, and most notably Au; this enrichment is not correlated with silica content. This suggests that yet a third fluid infiltration event is responsible for the mineralization and Au enrichment. Au concentrations are 0.84–2.31 ng g−1 in host serpentinite, 26–403 ng g−1 in carbonate listvenite, 152–545 ng g−1 in non-mineralized silica-carbonate listvenite, and 2286–3712 ng g−1 in mineralized silica-carbonate listvenite.