Broken Hill, Australia and Bergslagen, Sweden - Why God and Mammon bless the Antipodes!

Abstract

The sulphide deposits of the Broken Hill and Bergslagen areas occur in Lower-Middle Proterozoic sequences of variably deformed low to high metamorphic grade rocks. The number of events of coeval deformation and metamorphism, intensity of deformation and grade of metamorphism at Broken Hill are higher than in the Bergslagen area. A long history of retrograde metamorphism has been recognised at Broken Hill. Furthermore, granitic rocks in Bergslagen are coeval with volcanicity associated with ore deposition or are post-tectonic whereas at Broken Hill all plutonic rocks can be related to events of metamorphism, specially retrogression. Bergslagen is characterised by abundant metavolcanics (especially explosive acid volcanics) and moderately shallow water sequences probably deposited in a rift whereas metasediments deposited in a deep rift predominate at Broken Hill. Mineralization at Broken Hill is at least two orders of magnitude larger than at Bergslagen, is intimately associated with three events of paired volcanism and is associated with a great volume of extremely diverse exhalite deposited in a moderately reducing environment. The Bergslagen mineralization was probably deposited from numerous small seawater-dominated convective geothermal systems leaching metals from porous, permeable hot acid volcanic rocks. These geothermal systems produced regional alteration and localised intense footwall Mg-metasomatism associated with egress and ore deposition. In contrast, mineralization at Broken Hill formed in a deep rift every time there was a massive invasion of basaltic magma into faulted wet sediments. This increase in the geothermal gradient resulted in lower crustal melting, paired volcanism, initiation of geothermal systems, fault-bounded fluid ascent at the graben margin and with the resultant hydrothermal precipitation above the fault and the lack of widespread hydrothermal alteration. It is suggested that the principal reason for the striking differences between Broken Hill and Bergslagen is that the proposed deep rift in thin crust at Broken Hill allowed tapping of anomalous metalliferous mantle fluid and the leaching of crustal rocks whereas at Bergslagen, there were possibly numerous small graben in an extenion zone hence heated seawater did not have access to a large volume of hot porous permeable crust for leaching, egress of the ore fluid was not focussed and there is no evidence for a mantle contribution to the ore fluid.