Some Aspects of the Magnetic Signature of the Bottle Creek Gold Deposit, Western Australia

The Bottle Creek gold deposit lies on a north-trending structural break between two major terrains in the Ularring greenstone belt, north of Kalgoorlie. Aeromagnetic data in the vicinity of the deposit suggest several structures which appear to have controlled gold mineralisation. The mineralised horizon lies at a structurally discordant, north-northwest-trending internal junction within a thick mafic sequence in the eastern terrain. Lithological variations within this sequence have been inferred to represent distinct upper and lower groupings, and are structurally determined. Interpreted dips within the eastern terrain are very steep and can be contrasted with much shallower regional dips in the western terrain. Mineralisation within the Emu Formation host, which occupies a sheared zone, contains pyrrhotite but the observed magnetic responses along the structure appear to reflect magnetite-constructive alteration near the boundary. There are two shears; the largest separates western and eastern sequences, which meet acutely but are not mineralised along the shear. There is a lesser parallel structure further east. The structures are separated by a mafic unit which generates a chain of isolated magnetic responses along this structure. The Bottle Creek deposits are associated with sheared and altered junctions along the, apparently, lesser shear. Major deposits appear to be localised where this shear is intersected by large northeast-trending fractures.