Volcanogenic Indium in Massive Sulfide Deposits of the Murchison Greenstone Belt, S. Africa

摘要

The Archean Murchison greenstone belt, Northern Province, South Africa, represents a rifted volcanic arc sequence hosting the largest volcanogenic massive sulfide (VMS) district in Southern Africa. The VMS deposits of the "Cu-Zn line" developed during the initial phases of highly evolved felsic volcanism between 2971 and 2965 Ma and are closely associated with quartz-porphyritic rhyolite domes. Elevated heat supply along slow-spreading rift segments ensured regional hydrothermal convection along the entire rift axis. Recurrent volcanism resulted in frequent disruption of hydrothermal discharge and relative short-lived episodes of hydrothermal activity. Stable thermal conditions favoured the local development of mature hydrothermal vent fields from focused fluid discharge and sulfide precipitation in thin layers of volcaniclastic rocks. Ore mineralogy and geochemical composition attest to hydrothermal activity at moderate temperatures of <= 250 deg C for the entire rift axis, with short-lived pulses of higher temperature (approx 300 deg C) fluid upflow. Major and trace metal composition of the deposits attests to the highly differentiated felsic source rocks. Fluid and host rock compositions constrained favourable conditions for the enrichment of indium in the massive sulfide deposits defining the "Cu-Zn line" as a type example for the enrichment of indium in the volcanogenic environment.