GROUNDWATER GEOCHEMISTRY IN EXPLORATION - AN INVESTIGATION IN THE BLACK-FLAG DISTRICT, WESTERN-AUSTRALIA

The Black Flag area is a typical Yilgarn Block gold district with mult iple occurrences of structurally controlled primary mineralisation, fo r the most part hosted by mafic volcanics and intrusives of Archaean a ge. Exploration and mining since the 1980s has largely focused on supe rgene ores, which are developed within the deeply weathered regolith a bove and adjacent to the primary mineralisation. Economic gold deposit s also occur in a buried Tertiary palaeodrainage system, which is as y et only partially explored. Much unexplored prospective ground is conc ealed by deep transported overburden or has been extensively leached, reducing the effectiveness of existing geochemical exploration techniq ues. Comprehensive groundwater geochemistry has identified widespread groundwater Au anomalies that could have been detected by widely space d reconnaissance sampling (10 x 10 km). Anomalous groundwater As broad ly coincides with important bedrock Au deposits, whereas waters associ ated with palaeochannel Au deposits, have background As concentrations . Groundwater As thus distinguishes between the two major styles of Au deposits. Groundwaters are hypersaline, salinity increasing toward th e south, and range from near-neutral pH values in the northwest to dis tinctly acid in the southeast. Chemical modelling of each water sample identifies a carbonate terrain predominantly in the north and barite saturation of many groundwaters from the northwest to the southeast. S aturation of groundwaters with respect to magnesite and/or chlorite an d overall high concentrations of Ni, Co, Cr and Li indicate mafic lith ologies in many parts of the study area; saturation with respect to K- minerals reflects equally wide areas of felsic or K-altered mafic lith ologies. Using a portion of the log(act(K)+/act(H)+) vs log[(act(Mg)2)/(act(H)+)(2)] diagram, each sample was assessed in terms of the stab ility of both Mg and K alteration minerals [kaolinite, Mg-chlorite, bi otite and muscovite (sericite)]. All samples are closely correlated (r = 0.98), plotting on a trend line that passes from the kaolinite fiel d through the kaolinite-muscovite-chlorite triple point into the chlor ite field. This correlation and spread suggests that all groundwaters in the study are associated with variously weathered versions of a min eral assemblage that is common to different lithological units of the Black Flag sequence. Further work is needed to determine the extent to which this assemblage reflects the alteration associated with mineral isation, the regional greenschist metamorphism, or both. However, the potential to map alteration mineral assemblages by means of groundwate r composition has important implications for future exploration.