PIXE analysis of hydrothermal fluids in the Wernecke Mountains, Canada

Fe-oxide Cu-Au deposits in the Wernecke Mountains, Canada, and the Cloncurry District, Australia, allow the study of fluids from deposits which are similar in character, but which also display some key differences. The deposits from the two regions are similar in that they were formed by large hydrothermal systems, exhibit significant hydrothermal brecciation, and have similar ages. The Wernecke breccia has been dated at 1595 Ma, whereas dates obtained for the Cloncurry deposits lie in the range 1500 to 1540 and 1595 Ma. The main contrast between the two areas is the conspicuous absence of syn-mineralisation granitic intrusions in the Wernecke region. Studies in the Cloncurry district suggest a significant magmatic component to the fluids, in part consistent with the spatial and temporal association of economic deposits with the abundant intrusions in the region. Economic deposits are yet to be identified in the Wernecke region, and the absence of syn-mineralisation intrusions leaves a question mark over the source of the fluids that formed the extensive Fe-oxide CuAu breccias. The study of these two areas therefore provides an ideal opportunity to compare fluids of potentially quite different origins and thus improve our understanding of the types of fluids that can form Fe-oxide Cu-Au deposits and help in identifying the critical ingredients in ore deposit formation. Geological Setting The dominant rocks in the Wernecke Mountains belong to the Proterozoic Wernecke Supergroup (1840-1725 Ma). The sequence is divided into three main components, the basal unit being the Fairchild Lake Group, which consists of mainly siltstone. Layers of scapolite within a zone of sodic alteration (albite) are observed in the upper Fairchild Lake Group (Hunt et al., 2002) and may be an indication of pre-existing evaporite beds. Overlying this is the Quartet Group, made up of black carbonaceous shale, with minor siltstone and fine-grained sandstone, while uppermost is the Gillespie Lake Group dominated by dolostone. Minor intrusions are observed (dykes with ages of 1720 and 1270 Ma), and rare volcanic rocks (Thorkelson, 2000). Numerous breccia zones are present throughout the region and are associated with hydrothermal Fe-oxide-Cu-Au mineralisation. The brecciation in the region postdates the volumetrically minor Bonnet Plume River intrusions and has been dated at 1595 Ma (Thorkelson, 2000). Figure 1 represents the stratigraphy of the Wernecke Supergroup and illustrates the relative positions of each of the prospects sampled for this study.