Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potential

Abstract

Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite–greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.

Introduction

The Archaean Yilgarn Craton of Western Australia contains numerous world class gold deposits located within a complex granite–greenstone succession. It has been recognized that these gold deposits are spatially associated with major structures in the region (e.g. the Bardoc Shear and the Boulder-Lefroy Shear, Kalgoorlie region; Witt, 1993). The three-dimensional geometry of these major structures, however, is poorly understood. In addition, the relationship between the Eastern Goldfields Province and overlying Proterozoic and Phanerozoic sedimentary basins is also poorly understood but is of interest to both the mineral and petroleum exploration industry. Critical information on the three-dimensional crustal architecture is needed to understand the geodynamic processes responsible for the evolution, size and other characteristics of the gold mineral systems.

Current knowledge of the three-dimensional nature of the Eastern Goldfields Province was largely restricted to the Kalgoorlie region where a series of deep seismic reflection profiles allowed development of detailed 3D geological models (Archibald, 1998, Goleby et al., 2002). These models illustrate the topology of the controlling major structures and demonstrate that the greenstones are 5–7 km thick and lie on a low-density, low velocity basement assumed to be gneiss or felsic granulite. The models show a series of subhorizontal to low-angle shear zones at depths ranging between 6 and 9 km. Conceptual and fluid-flow numerical models for the development of gold mineral systems (Hall, 1998, Sorjonen-Ward et al., 2002) have largely been developed from these 3D geological models.

In late 2001, Geoscience Australia (GA) and the Geological Survey of Western Australia (GSWA), in conjunction with the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC), acquired over 430 km of deep seismic reflection data across the northeastern Eastern Goldfields Province portion of the Yilgarn Craton (Fig. 1) to constrain the three-dimensional crustal architecture of this region and assist in the development of improved mineral and petroleum exploration models.

The main regional traverse was 01AGSNY1, a 380-km long approximately east-west oriented traverse (Fig. 1). It was recorded to obtain information on the crustal architecture of the Eastern Goldfields Province. The western end of this traverse was located south of the township of Leonora while its eastern end was located in the Yeo Lake region (Fig. 1).

This regional deep seismic reflection traverse crossed two of the major mining centres of the Eastern Goldfields Province, the Leonora mining centre (Fig. 1) that includes the Sons of Gwalia gold deposit, and the Laverton mining centre (within the Laverton Tectonic Zone) that includes the Wallaby and nearby Granny Smith gold deposits and the Sunrise Dam gold deposit further to the south as well as smaller deposits along nearby shear zones.

A short traverse, 01AGSNY3 extended the regional traverse 52 km northeastwards from Lake Yeo across the western margin of the Officer Basin and further into the basin (Fig. 1). This short traverse tied to the mineral exploration drill hole NJD1 that was interpreted to bottom in Proterozoic rocks that overlie the Yilgarn Craton. NJD1 went through the Neoproterozoic–Palaeozoic Officer Basin then into a succession consisting of Neoproterozoic to Mesoproterozoic sedimentary rocks (Apak and Tyler, 2003).

In addition to the above seismic survey, an additional seismic survey was undertaken within the mineralized Laverton Tectonic Zone. This survey was undertaken in partnership with two gold exploration companies. Two traverses were recorded: Traverse 01AGSNY2 was recorded through the Wallaby Gold Mine (Fig. 1), whereas Traverse 01AGSNY4 was recorded near the Sunrise Dam Gold Mine (Fig. 1).