The role of magmatic fluids in the ~3.48 Ga Dresser Caldera, Pilbara Craton: New insights from the geochemical investigation of hydrothermal alteration

Abstract Hydrothermal fluids played a key role in establishing the environmental conditions in which ancient stromatolites grew within the North Pole Chert of the ~3.48 Ga Dresser Formation (Pilbara Craton, Western Australia). However, there has been uncertainty as to the physicochemical conditions of the hydrothermal system in relation to (i) the distribution of hydrothermal alteration, (ii) the relative contribution of seawater and/or magmatic volatiles to the hydrothermal fluids, and (iii) the origin of some of the major elements mobilized in the hydrothermal fluids. This study examines the hydrothermal alteration of the underlying North Star Basalt in order to better understand the nature of the circulating fluids and to determine the processes responsible for the transport and accumulation of metals and metalloids to the near surface environment. Detailed geological mapping reveals a complex distribution of alteration mineral assemblages that is controlled at all stratigraphic depths by the distance to the major fluid pathways that are now represented by hydrothermal silica veins. With increasing distance from the vein margins, alteration assemblages change from argillic (kaolinite–quartz) to phyllic (illite–quartz), and then to propylitic (chlorite–albite–epidote) and actinolitic (actinolite–albite–chlorite–epidote) at more distal positions. Illite Ar–Ar dating of argillic–altered basalt proximal to major hydrothermal veins immediately below the North Pole Chert confirms a syn–depositional hydrothermal origin of alteration, and demonstrates that the mineralogical and chemical features developed through the circulation of hydrothermal fluids were largely preserved after subsequent thermal overprints at 3.25, 3.06, and 2.29 Ga. The spatial distribution of the alteration mineral assemblages indicates that the fluids circulating in the hydrothermal system were highly acidic (pH