The Thomson Orogen, which underlies sedimentary rocks of the Great Australian Basin in northwest NSW, is one of the state's major greenfields regions. The orogen has potential for orogenic, magmatic arc and oceanic crust -related gold and base metal deposits. Variable cover over the region has previously restricted exploration. However, the New Frontiers initiative will significantly reduce exploration risk by acquiring and interpreting seismic, magnetic and radioelement data, gravity, regolith mapping, regional geochemistry, drilling, depth-to-basement maps and 3D modelling of mineral systems.

Depth of Basement Map of the Thomson Orogen

Thomson-Lachlan Seismic Project

The Lachlan and Thomson orogens form part of the Tasmanides of eastern Australia, which is part of the interaction between the growing Australian craton (part of Gondwana) and the proto-Pacific plate from the Neoproterozoic through to the Mesozoic. The boundary between the orogens lies in the far northwest of NSW and its curved east-west trend is highly unusual in terms of Tasmanide tectonics.

Figure 1. Seismic Line 2 form the Thomson-Lachlan seismic
reflection project showing the basic interpretation

The Thomson-Lachlan seismic reflection project was designed to test the scientific and metallogenic nature of the boundary, and, in particular, the reason for the prominent gravity anomaly that lies parallel to, and just north of, the boundary. Results to date indicate that the boundary is a major north-dipping thrust fault that offsets the Moho. Latest movement on the fault was in the Carboniferous (Fig. 1). The seismic data also indicate the possibility of Mississippi Valley-type mineral systems along the margins of the Mt Jack High that marks the thrusted southern edge of the Nelyambo Trough.

Thomson Orogen regional geochemistry

Figure 2. Project area and sample sites for the low density
baseline geochemical survey of the Thomson Orogen

The surface geochemistry of the NSW Thomson Orogen is now better constrained due to results of the combined work of NSW DPI, (former) CRC LEME, Geoscience Australia, University of Adelaide, Australian National University and MMI Technologies. A low density baseline geochemical survey was conducted across the Thomson Orogen (Fig. 2). The technique involved collecting 700 samples from 99 sites in catchment areas, using a variety of mediums, including overbank soil, vegetation, lag and NITON (portable XRF) soil sampling. The results of the baseline survey showed that there was a negative correlation of metal content with depth to (base of Cretaceous) basement. The survey also indicated elevated base metal values in areas of base metal mineralisation in the Bourke area. Other techniques such as lag and vegetation were shown to have good vectoring capability in gold and copper rich areas in the Tibooburra-Milparinka area in the western area of the Thomson Orogen.

The baseline geochemical report can be downloaded from the CRC LEME website.

Thomson Orogen regolith mapping

Regolith mapping and associated studies in the NSW Thomson Orogen by NSW DPI, (former) CRCLEME and University of Adelaide, has helped to elucidate its landscape history since the Cretaceous.

A number of maps at various scales are available for free download from the CRC LEME website.

Thomson Orogen Explorers Guide

A significant piece of work to come out of the Thomson Orogen Project is the 'Guide for mineral exploration through and within the regolith in the southwestern Thomson Orogen, NSW', by Steve Hill (University of Adelaide), John Greenfield, Phil Gilmore and Bill Reid (NSW DPI). It represents a concise summary of the results of over ten years of research in northwestern NSW.

The Thomson Region Explorers' Guide can be downloaded free from the CRC LEME website.

For more information on the mapping project please contact the Geological Mapping Team.

(Last updated on 24/02/2009)