Mineralogy and geochemistry of pattern formation in print stone from the Pilbara, Australia

Abstract ‘Print stone’ is an iron-banded siltstone from the Pilbara Province of Western Australia that bears partial resemblance to iconic East Kimberley ‘zebra rock’ in both pattern morphology and mineralogical composition. Using a combination of mineralogy and elemental geochemistry, this investigation examines the mechanisms underlying the formation of periodic iron-oxide banding in print stone. We demonstrate that print stone patterns probably arose from the periodic deposition of hydrothermal pyrite during the early Palaeoproterozoic, as evidenced by the distinctive cuboid morphology of the hematite pigment, the deposition of iron oxides along fluid-transport pathways, the presence of extensive hydrothermal pyrite elsewhere in the formation, and the presence of a positive europium anomaly. Through spatial analysis of the iron-oxide banding, we further show that print stone adheres to the Liesegang spacing law with a spacing coefficient of 0.018. This suggests that the periodic deposition of pyrite in print stone arose due to the Liesegang phenomenon, which was probably triggered by the infiltration of near-neutral, sulfidic hydrothermal fluids into a ferruginous, feldspathic shale. Altogether, the present findings demonstrate the opportunity for iron-oxide Liesegang bands to develop in hydrothermal systems, providing additional insight into the mechanisms underlying the formation of the East Kimberley zebra rock and other banded geological material.