A metamorphosed, stratabound-epigenetic origin for a gruneritic Archean gold deposit, Barberton, South Africa

Abstract

The Lily mine, located some 24 km northeast of Barberton in South Africa, occurs stratabound within carbonaceous Fe- and Mn-rich metapelites containing abundant grunerite. It is interpreted to be a metamorphosed epigenetic Archean gold deposit. The host rocks form part of a graywacke-shale sequence close to a major regional structure known as the Lily Fault. They are intensely deformed close to the fault with strain partitioned into finely laminated, carbonaceous phyllosilicate-rich units. The bulk geochemistry of the host rocks is dominated by pelitic, ferruginous and quartz-rich fractions mixed in varying proportions. Mineralization is concentrated in rocks composed largely of the pelitic and ferruginous fractions that have prograde metamorphic almandine, biotite, grunerite, magnetite and Fe carbonates. Magnetite-rich rocks were produced by progressive metamorphism of greenalite and/or siderite and do not reflect `oxide facies iron formation'. Much of the quartz-rich fraction occurs in deformed and metamorphosed veins associated with the mineralization. Gold is concentrated in the `Main Reef Zone' immediately adjacent to the Lily Fault. Native gold occurs with arsenopyrite, pyrrhotite, minor chalcopyrite, sphalerite, galena and stibnite in association with abundant deformed and metamorphosed veins composed of quartz–albite–grunerite–ankerite–sulfides in graphite-depleted, biotite–grunerite–tourmaline schists and massive grunerite–ankerite rocks. Ultramafic schists in the footwall of the Lily Fault exhibit intense K–CO 2 alteration (phlogopite±dolomite±calcite) and quartz veining but are essentially barren of gold. Primary gold concentration probably occurred by destabilization of bisulfide complexes through reactions with reduced iron silicates and carbonaceous matter during channelized fluid influx along the Lily Fault. Subsequent metamorphism occurred at T 500–550°C and P<3.5 kbar (transitional greenschist–amphibolite), possibly in association with the emplacement of the Nelspruit batholith at ≈3105 Ma.