Structure of the highly mineralised late-Archaean granitoid- greenstone terrain and the underlying crust in the Kambalda- Widgiemooltha area, Western Australia, from the integration of geophysical datasets.

Abstract

The highly mineralised (Au, Ni) Kambaida-Widgie- mooltha area comprises typical late-Archaean granitoid- greenstone sequences, and is situated within the Kalgoorlie Terrane of the southern Norseman-Wiluna Belt in the Yilgarn Craton of Western Australia. In the Kambalda- Widgiemooltha area, high-resolution aeromagnetic data, two deep seismic reflection profiles and gravity data are available to examine the geophysical signature and three- dimensional structure of this important region of the Kalgoorlie Terrane.The Kambaida-Widgiemooltha area has previously been divided, from west to east, into three fault-bounded tectono-stratigraphic domains, the Coolgardie, Kambaida and Parker Domains. Lithological associations and structural features defined by the aeromagnetic data substantiate the division of the study area into these three north-northwesttrending domains, and indicate that each domain consists of two, approximately equal-width, sub-domains. A four-stage deformation sequence, which is recognised from the aeromagnetic data, includes early south-to-north directed recumbent folding and thrust repetition (Dl), regional north-northwest folding and imbrication (D2), transcurrent movement on major north-northwest shear zones (D3), and the formation of late north-northeast and northwest-trending lineaments (D4).Two deep (20s TWT) seismic reflection profiles across the Coolgardie and Kambaida Domains reveal a threelayered crust beneath the study area. The upper layer (0-10 km), which includes the greenstone belt, is generally unreflective in the upper 5 km. The lower part of this layer (7.5 to 10 km) is more reflective and comprises a felsic unit which could be a continuation of the greenstone belt stratigraphy, attenuated basement, or rift-related sedimentary rocks. Domain-bounding faults are interpreted as listric structures, which have allowed the partial imbrication of the domains during regional deformation.The middle layer (10-27 km) is characterised by moderate to strong, variably dipping reflections. It is interpreted as an imbricate sequence of granitoids and gneisses. A more homogeneous, but weakly reflective, character is indicative of the lower crustal layer (27 - 35 km). East-dipping truncating reflectors at the western margin of the study area suggest that this layer is younger than the upper and middle layers and may, therefore, represent the late-tectonic external granitoids. The Moho is not imaged, but a general decrease in the number of reflections indicates that the crust is about 35 km thick.