Structural controls on sulphide deposition at the dyke–lava boundary, slow-spreading ocean crust, Macquarie Island

Ocean Drilling Program hole 504B revealed an ocean crust hydrothermal sulphur anomaly on the dyke–lava transition, with implications for global sulphur sinks. Here we confirm the presence of the anomaly sporadically along 7.5 km of dyke–basalt contact on the Macquarie Ridge at Macquarie Island, a 39–9.7 Ma slow‐spreading setting. Background contact‐zone pyrite S contents average 1845 p.p.m. across ～50 m. However, zones of small‐scale brittle faulting that commonly occur on and above the dyke–basalt contact average between 5000 and 11 000 p.p.m. S (20–30 m widths). These consist of steep ridge‐parallel faults and fault splays on the contact, overlain by up to 50 m of linked pyritic fault trellis. The contact zone faults are haloed by disseminated pyrite–chlorite, cross‐cut by quartz–chlorite–sphalerite and epidote‐cemented breccias, containing evidence of turbulent flow. The structural control on sulphur deposition is attributed to the active extensional slow spreading setting. With increasing extension, diffuse mixing across the contact was replaced by channellized flow and dynamic mixing in fault arrays. The magnitude of the dyke–lava transition sulphur sink must be reassessed to take account of this heterogeneity.