Long structural zones in the Archaean Yilgarn Craton, Western Australia

Eight structural zones from 250km to 800km long, traverse the granite-greenstone terrane of the Eastern Goldfields and Murchison districts of the Yilgam Craton. The structural zones, herein called LSZ, (long structural zones) have been traced using 1 :500,000 geological compilations and AGSO (Australian Geological Survey Organisation) aeromagnetic and gravity data. LSZ are complex linear deformation zones, resulting in the reactivation of structures formed prior to a craton wide shortening episode. Some of the pre-existing structures are interpreted as low angle extensional features, post dating the formation of much of the supracrustals; others as still earlier structures controlling original volcanic and sedimentary greenstone belt architecture. The structural corridors reactivated by the shortening event are overprinted by strikeslip faults which formed relatively narrow shear zones within the corridors. Most of the LSZ contain or abut linear belts of relatively coarse, immature, locally derived elastic sediments, which have been in turn overprinted by shear zones. Carbonate and alkali metasomatism is commonly strongly developed within the local sediment troughs. Numerous alkalic syenitic plugs have been intruded within and adjacent to the LSZ. At I larger scales, several LSZ are associated with K-Th-U rare metal (W-Sn-Be-Li)rich granitoids, or alkali rich syenites in belts up to 250km long. LSZ contain a record of the youngest events related to the evolution of the greenstonegranite terrane; the youngest sediments, the belts of elastics; the youngest shear zones; and the youngest intrusive suites. LSZ are direct hosts to only a few large Yilgarn gold deposits. Several major deposits · occur within structures directly connected to LSZ. Although major, deeply penetrating crustal scale structures, LSZ are not a necessary or sufficient condition for the formation of major gold deposits in the Yilgarn.