Growth fault control of Early Archaean cherts, barite mounds and chert-barite veins, North Pole Dome, Eastern Pilbara, Western Australia

Abstract

This paper describes the results of detailed mapping and combined structural and sedimentologic investigation of the ∼ 3.5 Ga old North Pole Chert along the eastern and southern flanks of the North Pole granitoid dome of the Pilbara craton. The following relationships have been found. (1) Stepwise and repeated thickening over north block-down listric normal growth faults, now tilted to 120 E. Apart from the thickness distribution, the growth fault character has been inferred from progressive upward decrease of throw towards gentle flexuring, and the presence of roll-over anticlines, in a restored pre-tilt cross-section. The faults are unrelated to the actual geometry of the dome, and overprinted by pre-doming compressive deformation. (2) A reconsideration of the sedimentary model of the chert-barite unit with emphasis on vertical grainsize distribution and facies architecture shows direct synsedimentary fault control. (3) Black chert veins, clustered in the hangingwall blocks of the growth faults follow a conjugate fracture pattern in and immediately beneath the North Pole Chert, symmetrical with respect to the synsedimentary fault geometry. Upwards, the veins terminate in the lowermost chert-barite unit of the North Pole Chert; downwards, the majority of the veins converge towards centres at the growth faults in the underlying basalt. (5) Barite occurs: in veins, in association with black chert; as large synsedimentary mounds (15 m high, 50 m wide), formed on the original shallow subaqueous basin floor; as commonly silicified sinters and sedimentary or early diagenetic evaporite associated with littoral and/or stromatoloidal facies. The barite mounds are directly connected with chert-barite veins, and are uplapped by banded sedimentary chert and diamictite with barite clasts. Field relationships suggest precipitation of barite, particularly in the mounds, may have been primary and synsedimentary. Vein barite at or near the original depositional surface of the chert is associated with base metal sulphide at depth. The tectono-sedimentary relationships between normal faults, sedimentation patterns and thickness distribution, primary and secondary chert, synsedimentary barite mounds, stratiform barite and chert-barite veins, support a tensional fault-controlled, hydrothermal model with Ba, Si and sulphide emanation in ca 50 m depth of water. Such a model envisages boiling and/or degassing above vents to locally mix a normally stratified water body, causing instantaneous oxidization into sulphate. The sedimentary model of the North Pole Chert indicates repeated uplift and subsidence, with development of faults and fractures, the intensity of which diminished from the lowermost chert-barite unit of the North Pole Chert to the uppermost, fifth unit.