Geological and structural controls on magnetism in the Tanami Mine corridor, Tanami Desert, N.T.

Gold mineralisation was first discovered at the Tanami Mine in August 1900. However, it was not until the late 1980's that large-scale open pit mining and systematic gold exploration was carried out within and around the Tanami Mine. The Tanami Mine Corridor, a 15 km long, NNE-striking arcuate package of magnetic Mount Charles Beds on the western margin of the Frankenia Granite dome, is host to more than 1 Moz. of contained gold in more than 30 individual deposits. Gold mineralisation is hosted in 350-000°, 020-040° and 060-070°-striking, east- to SEdipping brittle shear zones cross-cutting stratigraphy. Host rocks include subgreenschist facies basalt and turbiditic sedimentary rocks which dip and face 30-70° west. Regional-scale, low-magnetic alteration zones within the Tanami Mine Corridor are the result of chlorite-carbonate hydrothermal alteration along syn-mineralisation shear zones. Chlorite-carbonate alteration patterns delineated from detailed 1VD magnetic images include: (i) regional alteration domains, which are 0.5-3 km long and up to 200 m wide haloes around strike-continuous (>10 km), 030-040°-striking, eastdipping shear zones; (ii) stratabound haloes, which are 350-450 m long and up to 150 m wide within autoclastic and pillow basalt flows; and (iii) cross-cutting haloes which have developed in brittle fracture zones adjacent to 060-090° trending shear zones. The cross-cutting haloes are commonly 50-100 m wide and up to 600 m long. Chlorite-carbonate alteration is the result of CO 2 -H20-rich hydrothermal fluids channelling up brittle structures and causing alteration of titanomagnetite in basalt and magnetite-bearing interflow sedimentary units to chlorite and sphene. The most intensely altered zones have resulted in 100% replacement of titanomagnetite, whereas weak to strongly altered zones have partial replacement of titanomagnetite of between 10 and 90%. Gold mineralisation and related sericite+quartz±pyrite alteration (SQ±P) overprints chlorite-carbonate alteration and forms intense but restricted (<10 m), magnetitedepleted alteration haloes around mineralised veins. SQ±P alteration haloes are not readily recognised from airborne or total magnetic intensity (TMI) magnetic profiles as they are often too narrow and masked by the broader chlorite-carbonate lowmagnetic haloes. SQ±P haloes are characterised by an inner halo (