Archean abiogenic and probable biogenic structures associated with mineralized hydrothermal vent systems and regional metasomatism, with implications for greenstone belt studies

Abstract

Abiogenic structures, formed by fluid emission in subaerial mud pools and nearby stromatolites, from stratigraphically related carbonate sediments, occur in the southern part of the approximately 3.3 to 3.5-b.y.-old Barberton greenstone belt. The mud pool structures are associated with ferruginous shales, banded iron-formations, and in places barite deposits; they relate stratigraphically to transgressive pods of ironstone. The pods probably represent buried, mineralized hydrothermal channels and chimneys. Soft sediment deformation structures adjacent to these pods suggest fossil fluid circulation patterns in the sediments with horizontal flow over distances greater than 300 m and vertical motions an order of magnitude less. Associated metasomatism has significantly altered the original chemistry of the host rocks on a regional scale, prior to 13.8 b.y. This is described with specific reference to silicification. Silicification patterns in the igneous rocks underlying the sediments also indicate fluid circulation patterns an order of magnitude greater than those in the sediments. Thin but regionally extensive and originally horizontal alteration planes were probably zones of high fluid pressure along which large-scale, gravity-induced mass transportations occurred. Such transportations greatly influenced the subsurface mineral precipitation and the surface geologic processes within the extensive area of hydrothermal activity, in which the inferred high geothermal gradient was probably related to shallow-level igneous activity. At least minor mineralization, including local gold concentration several orders of magnitude greater than that found in average crustal rocks (e.g., ppb to ppm), occurred simultaneously with these processes of metasomatism during sedimentation, igneous activity, and tectonism within the belt, and prior to the intrusion of the greenstone belt by surrounding granitoids.