Ian Roach

Drill rig used for the 2017 drilling program

Seven drill-holes successfully sampled basement rocks of the Thomson Orogen, beneath the Eromanga Basin, in the Bourke-Hungerford area of remote northwest NSW (with another five holes completed within Queensland). The drilling program was part of the cross-border collaborative Southern Thomson NCF Project between GSNSW, the Geological Survey of Queensland and Geoscience Australia. The project sits within the national Uncover Initiative, which aims to reverse the decline in Australia's known mineral reserves by providing new information to explorers about undercover regions.

The rocks of the Thomson Orogen are potentially prospective for copper, lead-zinc, gold and other useful metals, however mineral systems are masked by younger sedimentary rocks of the Eromanga Basin. The Southern Thomson Project initially acquired and analysed airborne and ground-based geophysical surveys and undertook surface geochemical sampling, field mapping and satellite image analysis to define areas of interest within this vast region (>300 km by >300 km).

This geophysical work helped inform the stratigraphic drilling program that was conducted between July and September 2017 under four Mineral Allocation Areas (MAAs) and coincident Exploration Licences (ELs) held by the Secretary, NSW Department of Planning and Environment, on behalf of the Crown. These MAAs have now been repealed and the coincident ELs cancelled (10 August 2018), releasing the areas for future work.

Preparation for drilling included:

  • landholder and community consultation
  • consideration of the basement depth and nature of basin cover
  • cultural protection preparations involving local Aboriginal groups
  • selection of areas of lowest environmental sensitivity
  • liaising with groundwater agencies to assess groundwater conditions and sensitive areas
  • ground geophysical surveys to measure and predict the thickness of cover sequences

Drilling results

The drilling program tested a variety of distinctive basement signatures evident in airborne magnetic and electromagnetic data. Drilling methods involved a combination of rotary mud-drilling through cover sequences, and diamond drilling of underlying basement rocks to provide around 50 m of representative core samples from each site (see examples below). Wireline geophysical logs were run in the holes prior to casing. The drilling results are summarised below:

Site ID Total Depth Basement lithologyU-Pb age (zircon). From Cross et al. (in prep.)
Milcarpa 1 290.9 m Rhyodacite 396 Ma (magmatic)
Euroli 1 153.7 m Metasedimentary schist 431 Ma (youngest of 5 grains)
Tongo 1 312.8 m Granodiorite 425 Ma (magmatic)
Laurelvale 1 386.8 m Siliclastic meta-siltstone 456 (MDA)
Janina 1 222.2 m Granite/quartz monzonite 421 Ma (magmatic)
Congararra 1 119.6 m Mixed metasedimentary and altered intrusion, gneissic textures Not analysed
Congararra 2 317.9 m Muscovite-biotite-cordierite granite 421 Ma (magmatic)

Congararra 1 (mixed metasedimentary and altered intrusion, gneissic textures)

Congararra 2 (muscovite-biotite-cordierite granite)

Euroli 1 (metasedimentary schist)

Janina 1 (granite / quartz monzonite)

Laurelvale 1 (silicic meta-siltstone)

Milcarpa 1 (rhyodacite)

Tongo 1 (granodiorite)

Examples of drillcore from each of the seven drillholes

All basement cores have been comprehensively sampled using petrography, geochemistry and geochronology, as well as being scanned by the HyLoggerTM. Note that for each of the drill holes, the overlying sedimentary cover material (Eromanga Basin rocks) was sampled every 1 m depth as rotary mud chips. The drillcores and rotary mud chips are stored at the WB Clarke Geoscience Centre in Londonderry for future studies by industry, researchers and other organisations.

Results of the stratigraphic drilling are publicly available through the GSNSW project webpage and presented in the relevant borehole completion reports (in collaboration with colleagues from Geoscience Australia) for each drillhole:

Location of stratigraphic drillholes within four exploration licences (now cancelled) in the Bourke-Hungerford region.Background image is isostatic Bouguer gravity over TMI RTP tilt-filter.