Regional geology of the southern Furneaux Group

The lower Palaeozoic Mathinna Beds are intruded and contact metamorphosed by sixteen Devonian, granitoid plutons in the southern Ferneaux Group, northeastern Tasmania. Contact metamorphic aureoles range from 200m to 2km wide and on Clarke Island the metamorphic assemblage cordierite-andalusite-biotite-quartz-potash feldspar is preserved.<br>Approximately 70 per cent by area of the granitoid terrain is composed of garnet and/or cordierite-bearing biotite granites (eight plutons), 20 per cent biotite granites (five plutons) including two altered plutons, and 10 per cent hornblendebiotite granodiorites (three plutons). The granitoids were intruded by widespread dolerite dykes of pre-Tertiary age. Cainozoic sedimentary sequences are thin and irregularly distributed. Mid-Tertiary sediments formed in lagoonal and shallow water marine environments have been reworked during the Quaternary by alluvial and aeolian processes In the Rooks River tinfield the cassiterite, which was eroded from pervasively altered Rooks River granite, is thought to be a lag deposit derived by reworking of the Tertiary sediments. Potential areas of exploration include southern end of the Lee River valley near the strongly altered RooksRiver granite and the alluvial fans north and south of the altered Hogans Hill granite pluton.