Geology and mineralisation of the White Devil deposit, Tennant Creek, Northern Territory, Australia

Investigations into the textures in the ironstones, relationships between Au-Bi-Cu mineralisation and associated chlorite, metal zonation and distribution and paragenesis have been made on the White Devil Deposit in the Proterozoic Tennant Creek district of the Northern Territory. The textures within the ironstone revealed that they consist of intergrown and interlocking masses of magnetite laths pseudomorphed after hematite. The consistent textures and overgrowth of primary magnetite on the laths and in fractures indicates the ironstones existed in entirety and were deformed prior to magnetite alteration. The open porous lath texture of the ironstones has implications for ore deposition. The mineralising stage was accompanied by the deposition of chlorite. The gold, bismuth sulphosalts and chalcopyrite are intergrown with coarse grained chlorite in the spaces between the magnetite laths and around magnetite stringers. The chlorite associated with ore and altered sediments ranges from low temperature green Mg-rich pycnochlorite to high temperature blue Fe-rich ripidolite. Geothermometry has shown that the chlorite has a temperature range from 230 to 330°C. Talc alteration of Mg chlorite occurred during the waning stage of the hydrothermal event. The metal zonation within the deposit is not clear, even within the orebodies studied. The initial hematite ironstone formed in chloride sediments within the shear. The fluids became more reduced and interacted with the hematite, altering it to magnetite. The mineralising stage saw the introduction of higher temperature fluids which interacted with the magnetite causing the deposition of gold and chlorite followed by bismuth and copper with associated chlorite at higher temperatures. As the fluids cooled, alteration of Mg chlorite to talc, martitisation and quartz flooding of the Pinter Ironstone occurred.