Lachlan Fold Belt Project

This project concluded 30 June 2000.

Introduction

The Lachlan Fold Belt National Geoscience Mapping Accord (NGMA) project, a collaboration between Geoscience Australia, the New South Wales Department of Trade and Investment predecessor, the Department of Mineral Resources, and the Victorian Department of Primary Industries predecessor, the Department of Natural Resources and Environment, concluded in June 2000. The Lachlan Fold Belt has a long history of mineral production including gold (Au), copper (Cu), lead (Pb), zinc (Zn), silver (Ag) and tin (Sn). The potential for new discoveries in the Lachlan Fold Belt is high, and the scientific input from the project played a leading role in focussing exploration effort.

The project commenced in 1991 with the following major objectives:

• develop a modern framework for exploration for Au and base metals in the Lachlan Fold Belt which will enhance exploration potential and reduce exploration risk in highly prospective areas
• develop knowledge and understanding of the geological and geodynamic evolution and metallogenesis of the Lachlan Fold Belt with particular focus on Au-Cu mineralisation associated with the Ordovician volcanic suite and base metals associated with Silurian volcanic sedimentary successions
• define the nature and distribution of regolith materials in the region and develop an understanding of the landscape evolution and implications for mineral exploration and land management.

Major activities included:

• acquisition of new high resolution airborne magnetic and gamma ray spectrometric data
• geological mapping and field data acquisition and release of geological maps at 1:100 000 and 1:250 000 scale
• consequent geochemical, petrological and mineral deposit studies
• developing comprehensive digital geological datasets
• mapping regolith components and developing models for landscape evolution
• obtaining high precision U-Pb zircon geochronology using a Sensitive High Resolution Ion Microprobe (SHRIMP).

Project highlights

Bathurst 1:250 000 sheet

Following the completion of field mapping on the Bathurst sheet in 1994, a field conference based in Orange was held to present preliminary research results and maps. The conference excursion guide is available from the New South Wales Department of Minerals and Petroleum. A comprehensive digital GIS package for the Bathurst 1:250 000 sheet was released in March 1997. The package includes layers for outcrop geology, derived geology (interpretive pre-Cenozoic geology, cross-sections), geochemistry, site observation and structural data, petrology and mineral occurrence as well as fossil, geochronology and photographic databases. The second edition Bathurst 1:250 000 scale geology map and a comprehensive explanatory notes volume were released in 1998. The six component 1:100 000 scale geology maps and complementary regolith landform maps are also available.

Dubbo 1:250 000 sheet

The second edition 1:250 000 scale Dubbo geological map and explanatory notes were released in 1999. As with the Bathurst sheet, the explanatory notes were the first published for the Dubbo area and contain detailed descriptions of more than 200 units as well as summary chapters on tectono-stratigraphic history, structure, metallogenisis and coal geology. A field conference was held in April 1997 to mark release of the preliminary second edition of the Dubbo 1:250 000 sheet. A conference excursion guide and digital GIS package of the Dubbo 1:250 000 sheet is available from the New South Wales Department of Industry and Investment.

Forbes 1:250 000 sheet

The most recent project activity centred on the Forbes 1:250 000 sheet. Field mapping was completed in 1998, and a field conference, with accompanying excursion guide, was held in April 1999. A new second edition Forbes Geological Sheet 1:250 000 S155-7 Explanatory Notes, along with regolith and basement geology maps, were released in 2000. In addition, a new style of map was released which illustrates the depth to bedrock in a region which is substantially covered by Quaternary alluvial sediments. This map is of use to mineral explorers in planning exploration programs and to hydrologists for delineation of groundwater flow in the Quaternary sediments, which are up to 150 metres thick.

The new map sheet is accompanied by comprehensive explanatory notes and GIS data package on CD. A major SHRIMP U-Pb zircon geochronology study was included in the Forbes project, greatly enhancing interpretation of the age of volcanic and associated sedimentary successions, the timing of plutonic and deformation events as well as their association with mineralisation.

Goulburn 1:250 000 sheet

Airborne geophysical data for the Goulburn 1:250 000 sheet area were acquired and released in 1997.

Ballarat 1:250 000 sheet

Final editions of the Willaura and Skipton 1:100 000 geological maps in central Victoria were completed in June 2000. The maps, and an explanatory notes record for the Willaura sheet, are available through Geoscience Australia and GeoScience Victoria. Airborne geophysical data also were acquired for the Ballarat 1:250 000 sheet.

Cootamundra 1:250 000 sheet

Airborne geophysical data for the Cootamundra 1:250 000 sheet area were acquired in 1998. A 1:250 000 scale basement geology interpretation of the geophysical data and previous outcrop mapping was released in 1999.

Wagga Wagga 1:100 000 sheet

Airborne geophysical data and a preliminary geological map were released in 1993.

Australian Geodynamics Cooperative Research Centre (AGCRC) Seismic and Regional Crustal Framework Studies

1997 Survey - The AGCRC completed a deep seismic reflection acquisition program in the eastern Lachlan Fold Belt in 1997. Three reflection lines were acquired (total line length of about 105 kilometres). A north-south line through the Lachlan Transverse Zone was designed to image north-dipping structures which escaped the subsequent strong east-west shortening. This line is located near the Cadia and Forest Reefs deposits and the Forest Reefs area. The other two lines (one through Molong and the other to the east of Orange) form part of an east-west section designed to provide structural details from the Cowra Trough in the west, across the Molong volcanic belt to the Mumbil Shelf and Hill End Trough in the east. Line 2 is near the Mount Bulga and Lewis Ponds deposits and Line 3 is just south of the Copper Hill deposit.

The AGCRC also carried out a crustal refraction experiment in the eastern Lachlan Fold Belt in July 1997. The more than 300 kilometre north-south profile from near Gundaroo to a point east of Dubbo followed the Molong volcanic belt and was designed to provide information on the distribution of mafic volcanic and sub-volcanic rocks in the crust, particularly comparing the nature of the crust in the Lachlan Transverse Zone with the crust to the north and south of this zone.

Around 3000 samples were collected at half drill rod intervals (2.25 metres) along all three reflection traverses. Vital information was obtained on regolith profiles over varying bedrock types in a range of geomorphic environments. Evidence for postulated paleodrainage was gained, as was an indication of the pre-Mesozoic sedimentation over this part of the Lachlan Fold Belt.

1999 Survey - The AGCRC completed a second survey in 1999 involving three lines across the Gilmore Fault Zone and Lake Cowal Volcanic Complex. Preliminary results of the survey were presented at the 15th Australian Geological Convention in Sydney in July 2000, highlighting steep east-dipping faults linked to a major shallow west-dipping thrust surface in the Gilmore Fault Zone.

A workshop attended by around 50 industry, university and government geologists was held in Orange in March 2001 to present the final seismic interpretations and a crustal architecture synthesis covering the area from the Nyngan sheet in the north-west to Goulburn in the south-east.