Groundwater geochemistry in exploration: An investigation in the Black Flag district, Western Australia
Titel:
Groundwater geochemistry in exploration: An investigation in the Black Flag district, Western Australia
Auteur:
Giblin, A. Mazzucchelli, R.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 44 (1997) nr. 4 pagina's 433-443
Jaar:
1997-08
Inhoud:
The Black Flag area is a typical Yilgarn Block gold district with multiple occurrences of structurally controlled primary mineralisation, for the most part hosted by mafic volcanics and intrusives of Archaean age. Exploration and mining since the 1980s has largely focused on supergene ores, which are developed within the deeply weathered regolith above and adjacent to the primary mineralisation. Economic gold deposits also occur in a buried Tertiary palaeodrainage system, which is as yet only partially explored. Much unexplored prospective ground is concealed by deep transported overburden or has been extensively leached, reducing the effectiveness of existing geochemical exploration techniques. Comprehensive groundwater geochemistry has identified widespread groundwater Au anomalies that could have been detected by widely spaced reconnaissance sampling (10 × 10 km). Anomalous groundwater As broadly coincides with important bedrock Au deposits, whereas waters associated 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 the south, and range from near-neutral pH values in the northwest to distinctly 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. Saturation of groundwaters with respect to magnesite and/or chlorite and overall high concentrations of Ni, Co, Cr and Li indicate mafic lithologies in many parts of the study area; saturation with respect to K-minerals reflects equally wide areas of felsic or K-altered mafic lithologies. Using a portion of the log(actK+/actH+ ) vi log[(actMg2+)/(actH+)2] diagram, each sample was assessed in terms of the stability of both Mg and K alteration minerals [kaolinite, Mg-chlorite, biotite and muscovite (sericite)]. All samples are closely correlated (r = 0.98), plotting on a trend line that passes from the kaolinite field through the kaolinite-muscovite-chlorite triple point into the chlorite field. This correlation and spread suggests that all groundwaters in the study are associated with variously weathered versions of a mineral 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 mineralisation, the regional greenschist metamorphism, or both. However, the potential to map alteration mineral assemblages by means of groundwater composition has important implications for future exploration.
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