Gold in mantle peridotites from upper Proterozoic ophiolites in Arabia, Mali, and Morocco

Abstract

An ultramafic rock source has been suggested for many Archean gold deposits related to carbonatized komatiites. Similar gold mineralization and alteration mineralogy are known in gold-bearing carbonatized peridotites (i.e., listwaenites) from various ophiolites. Carbonatized ultramafic rocks are abundant in upper Proterozoic ophiolites in three areas (Arabia, Mali, and Morocco) that were investigated. There, the average gold contents in serpentinized mantle peridotites range between 3 to 5 ppb. Most of the gold is located in accessory magnetites (10 ppb). Magnetite crystallization and dissolution during serpentinization and shearing, followed by carbonatization of the ultramafic rocks, resulted in concentration of gold in carbonatized serpentinites. Fluid inclusions, trace elements, and isotopic investigations indicate that gold was mobilized at low temperature (150 degrees -250 degrees C), by seawater-derived heterogeneous H 2 O-CO 2 -CH 4 hydrothermal fluids during serpentinization and carbonatization. Gold concentration in listwaenites, which may reach economic grades of 1 to 10 ppm, are closely associated with sulfides, sulfarsenides, or arsenides, suggesting gold transport by sulfur or arsenic complexes in the hydrothermal fluids. Gold was precipitated with these minerals and with silica by an increase in pH as the acidic gold-bearing hydrothermal solutions interacted with the carbonate lenses. Thus, an upper mantle rock source is suggested for gold concentrations in ophiolite complexes.