Stuart, Dr Cait1, Ricketts, Mel; Gilmore1, Phil1
1Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia
The Riverina region of New South Wales is well known for agriculture, but the region also has a long history of gold mining, dating back to the late 19th century. The Riverina has seen relatively little modern mineral exploration. The five-year (2014 to 2019) East Riverina Mapping Project undertaken by the Geological Survey of NSW has highlighted the prospectivity of the region for structurally-controlled, low-sulfide (orogenic) gold, as well as intrusion-related gold and porphyry copper–gold mineralisation.
Structurally-controlled, low-sulfide gold mineralisation within the eastern part of the Riverina region, which extends from West Wyalong to Albury, is hosted by Ordovician to Devonian units and largely located adjacent to the Gilmore Fault Zone. The area has a current endowment (past production and identified resources) of 33.45 t (1.4 M ounces) of gold in 23 identified goldfields situated along the Gilmore Fault Zone and other major, parallel structures.
For each goldfield, a review of historical data, host lithologies, endowment, structural setting, style and timing of mineralisation was undertaken. Each goldfield was considered in a regional structural context. In particular, the structural setting and timing of mineralisation were related to movement along the Gilmore Fault Zone and associated structures.
Structurally-controlled, low-sulfide gold mineralisation was found to occur in three main settings:
- In narrow vein arrays in orientations that fit within a Riedel shear model
Veins are typically found in more competent rock types adjacent to second or third order splay faults of the Gilmore Fault Zone or major parallel structures. Mafic dykes are commonly associated with shear zones and may have caused desulfidation of gold-bearing fluids, resulting in mineralisation. Examples of this setting include Adelong, West Wyalong and Sebastopol.
2. Along the contacts between intrusions and metasedimentary country rock, or in pressure shadows of intrusions
Competency contrast between intrusions and metasedimentary rocks is thought to have focussed fluids along these contacts or around competent bodies. Pressure changes during fluid migration likely played a role in gold mineralisation. Examples include Yalgogrin, Weethalle and Grong Grong.
3. Gold found in fold hinges and fold limbs adjacent to the Gilmore Fault Zone
Barmedman is the only recognised example of this style in the area.
Geochronological data for gold mineralisation in the project area are rare and further work is required. However, field relationships indicate the majority of structurally-controlled, low-sulfide gold mineralisation occurred during the Devonian, in the Bindian Event or the Tabberabberan Contraction, with minor mineralisation also potentially occurring during the Ordovician Benambran and Carboniferous Kanimblan contraction events.
Cait Stuart was a Graduate Geoscientist with the Geological Survey of NSW and is now a Geologist with the Northern Territory Geological Survey, where she undertakes mapping and mineral systems projects on NT geology. She has experience in structural and metamorphic geology and has BEnvSc and PhD degrees.v