Aligned hydrothermal fluid‐flow pathways in Middle Permian near‐shore marine sediments beneath a basaltic lava flow

Tubular and curviplanar structures, outlined by the occurrence of haematite/goethite, chlorite, quartz and albite, are developed in the Middle Permian Broughton Formation in the southern Sydney Basin, New South Wales, Australia. These structures are interpreted as fluid‐flow pathways resulting from the ejection of heated pore fluids as a thick (>30 m) basaltic lava (the Bumbo Latite Member) was emplaced rapidly on top of near‐shore, unconsolidated, wet, sandy marine sediments (the 53 m thick Kiama Sandstone Member). Evidence of fluid‐flow pathways in the sandstone is observed for ~20 m below the basalt. Subhorizontal and parallel, elongate tube‐like flow structures tend to be between 5 cm and 30 cm in diameter and are exposed laterally for a few tens of metres on the shore platform, although their full extent cannot be determined. Fluid‐flow pathways are marked by intense mineralisation and include enclosed tubes as well as unenclosed sheets, which may be flat or locally highly curved. Cross‐cutting relationships reveal several generations of the tube‐like features and imply the passage of hydrothermal fluids through the tubes. However, while significant alteration is apparent in the tube rims, little or no alteration is visible inside tubes at the hand specimen scale, but on a microscopic scale, the tube cores show alteration and extensive development of laumontite. Fluid‐flow features are grouped in moderately to well‐sorted sand beds and are underlain by layers with a higher silt component and exhibiting extensive bioturbation. These features imply that bedding‐controlled sediment porosity and permeability played a large role in determining the location of fluid‐flow conduits. The hot lava heated and pressurised the pore water, inducing horizontal hydrothermal fluid flow away from the lava and producing the mineral assemblage defining the fluid‐flow pathways.