Abstract
The variably serpentinized mantle peridotites of the Late Neoproterozoic Ess ophiolite (Western Saudi Arabia) are highly altered along shear zones and thrust planes to form erosion-resistant listvenites. The listvenites are distinguished petrographically and geochemically into three types: carbonate, silica-carbonate and silica (birbirite) listvenites. Geochemical analyses are consistent with expectations from petrography: carbonate listvenite is low in SiO (sub 2) content but high in MgO, Fe (sub 2) O (sub 3) , and CaO relative to silica-carbonate and birbirite, which is remarkably high in SiO (sub 2) at the expense of all other components. The total REE contents are low in silica-carbonate and carbonate listvenites but highly enriched in birbirite, with a large positive Eu anomaly. The host serpentinites have all the characteristics typically associated with highly depleted mantle harzburgite protoliths in supra-subduction fore-arc settings: bulk compositions are low in Al (sub 2) O (sub 3) and CaO with high Mg# [molar Mg/(Mg+Fe)], relict Cr-spinel has high Cr# [molar Cr/(Cr+Al)] and low TiO (sub 2) , and relict olivine has high Mg# and NiO content. The Cr-spinel relics are also found in the listvenites; those in serpentinite and carbonate listvenites have significantly higher Mg# than those in silica-carbonate and birbirite, suggesting re-equilibration of Cr-spinel in the later phases of listvenitization. The varieties of listvenite capture successive stages of fluid-mediated replacement reactions. The carbonate listvenite appears to have developed syn-contemporaneously with serpentinization, whereas silica-carbonate listvenite and birbirite formed later. The listvenite formation resulted in leaching and removal of some components accompanied by deposition of others in the solid products, notably CO (sub 3) , SiO (sub 2) , REE (especially Eu), Au, Zn, As, Sb and K. Our data show that listvenitization concentrated gold at sub-economic to economic grades; measured gold concentrations in the host serpentinite are 0.5-1.7 ng/g, versus 4-2569 ng/g in carbonate listvenite, 43-3117 ng/g in silica-carbonate listvenite and 5-281 ng/g in birbirite. The listvenite deposits in the Jabal Ess area merit further exploration for gold.