Structural controls on sulphide deposition at the dyke–lava boundary, slow-spreading ocean crust, Macquarie Island Garry J. Davidson, 1 Rick Varne, 2  Anthony V. Brown 3 and Robert Connell 4 1 Centre for Ore Deposit Research, and 2 School of Earth Sciences, University of Tasmania, Private Box 79, Hobart, Tasmania, 7001, Australia; 3 Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania, 7018, Australia; and 4 Tasmanian Aquaculture and Fisheries Institute, Private Bag 49, Hobart, Tasmania, 7001, Australia Introduction Macquarie Island, south-east of Aus- tralia in the Southern Ocean, consists of oceanic crust that formed by slow- spreading rifting, and was uplifted to sea-level by transpression after the spreading ridge became a strike-slip margin. It is an ideal place to evaluate hydrothermal behaviour in a slow- spreading setting, and here we specif- ically focus upon the behaviour of sulphur. Improving our knowledge of sulphur is important because it plays key roles in metal sulphide transport, sulphide deposition, buffering of hydrothermal oxidation states and even potentially the maintenance of some subsurface microbial communi- ties. Our work was possible because extensive cross-sections of oceanic crust occur on the island (Varne et al., 1969), which justified its World Her- itage listing in 1997. The study site (Sandell Bay) includes a dyke–lava transition in a 7.5-km-long ridge-par- allel section, previously interpreted as an off-axis dyke swarm (Goscombe and Everard, 2000), as well as the initiating edge of the sheeted dyke complex. We establish: (1) the nature and distribution of subseafloor alter- ation in this environment; and (2) the extent and controls upon a sheeted dyke–basalt sulphur anomaly, docu- mented for the first time in three dimensions in oceanic crust. Background Macquarie Island, located 1500 km south-east of Australia in the South- ern Ocean, formed as oceanic crust along the NE–SW Proto-Macquarie Spreading Ridge (PMSR), which was active from 39 Ma to 9.7 Ma (Duncan and Varne, 1989; Sutherland, 1995). During this time the spreading zone changed from mainly extensional magmatic to mainly transcurrent, with concommitant shortening of ridge segments to 25–40 km, reorientation from NE- to E-striking ridges, and slow spreading rates (20 mm yr )1 ; Lamarche et al., 1997; Varne et al., 2000). After 9.7 Ma the plate mar- gin became a dextral transcurrent fault zone (Molnar et al., 1975) (see Fig. 2). Transpression associated with 450 km of offset since 5 Ma (Mas- sell et al., 2000) produced extensive uplift (Varne and Rubenach, 1972) and deformation, forming the modern submarine Macquarie Ridge Complex mountain chain. This includes the exhumed Macquarie Island landmass rising at up to 1.45 mm yr )1 (Selkirk et al., 1983) 5.5 km east of the main plate boundary (Massell et al., 2000). The northern 9 km of the island exposes deep oceanic lithosphere, from ultramafic rocks, through gab- bros, to sheeted dykes (Fig. 1), whereas the southern section consists of pillow basalts and subordinate sheeted dykes (Blake, in Mawson, 1943; Griffin and Varne, 1980). The published tectonic history comprises: (D1) N–S extension and main crust formation, including up to 2.5 km of differential uplift to expose gabbroic lower crust; (D2) NE–SW extension, featuring off-axis dyke swarms, faults and veins; and (D3) formation of transcurrent and transpressional structures (Goscombe and Everard, 2000). The island consists of the youngest known PMSR crust. This can be explained if strike-slip motion initiated close to the site of final spreading activity, at about 9.5 Ma (Massell et al., 2000). In the study area, the Sandell Bay Sheeted Dykes (SBSD) (Zone SVIII of Goscombe and Everard, 1999) are exposed in shore platforms and in a 200–350-m-high coastal plateau wall (Figs 1 and 2). The SBSD and adjoin- ing basalts were transpressively uplif- ted in a discrete fault block that is bound by the NW-striking Sandell ABSTRACT 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 spread- ing 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. Terra Nova, 16, 9–15, 2004 Correspondence: Dr Garry J. Davidson, CentreforOreDepositResearch,Schoolof Earth Sciences, University of Tasmania, Private Box 79, Hobart, Tasmania, Austra- lia. E-mail: Garry.Davidson@utas.edu.au   Deceased July 2001. Ó 2003 Blackwell Publishing Ltd 9 doi: 10.1046/j.1365-3121.2003.00518.x