Project status

Geological mapping for the six 1:100 000 sheets is complete and provisional (print on demand) maps are available. The 1:250 000 Cargelligo map sheet and the Cargelligo Explanatory Notes are now available (see below).

Geology

The Cargelligo 1:250 000 map sheet is located in central New South Wales, over the western part of the Lachlan Orogen and eastern edge of the Murray Basin. Dominant physiographic features over the project area include Lachlan River, the Ural Ranges, the Lachlan Range, ranges associated with the Cocoparra Syncline and interspersed extensive aeolian and fluvial plains. The eastern region of the project areas consists of low, rolling hills that are deeply weathered and variably covered by transported and in-situ regolith.

Historical mining of vein-hosted gold occurred around Lake Cargelligo and in the Yalgogrin-Weethalle area. Significant deposits of alluvial tin in the Gibsonvale-Kikoira area have also been mined.

Palaeozoic sequences of the Cargelligo map area are considered to have significant mineral potential. Much of the Ordovician through to Early Devonian stratigraphy, which hosts mineralisation in the Cobar area, extends south into the Cargelligo project area.

Extensional tectonism in the Silurian-Early Devonian led to the development of meridional depositional troughs. To the north, the Early Devonian Cobar Basin contains the Cobar group of mines (Endeavour, The Peak, CSA, and McKinnon’s Tank) and the Mount Hope Trough contains the Comet-Great Central deposits. Immediately to the north-east of the project area lies the Mineral Hill Mine. The prospective Rast, Melrose and Mt Hope Troughs were interpreted from aeromagnetic data to extend south into the Cargelligo sheet area.

The Cargelligo region contains evidence of at least four structural events: (1) Early Silurian development of tight NNW-trending folds with associated axial planar cleavage and localised development of imbricate fault systems; (2) Middle to Late Silurian regional sinistral transtension linked with the opening of the Rast Trough; (3) regional dextral transpression in the Middle Devonian which caused inversion of the Silurian-Devonian Rast Trough with associated regional faulting, folding and cleavage development; and (4) Early Carboniferous deformation which resulted in open NNW-W trending folds in the Late Devonian Cocoparra Group along with reactivation of major faults.

Recent mapping of Ordovician slates and turbidites in the eastern third of the sheet has revealed approximately 50 new graptolite localities. The Ordovician sequence was intruded by numerous fractionated, S-type, megacrystic Early Silurian (c. 432 Ma) granites of the Koetong Supersuite. The exposed Silurian-Devonian Rast Trough sequence consists of quartz turbidites of the Crossleys Tank Formation and the conformably overlying felsic Ural Volcanics. The Browns Reef deposit (stratabound Zn-Pb-Cu-Ag) lies in steeply-dipping interbedded shallow marine volcaniclastic and fossiliferous sedimentary rock of the Preston Formation. The Woorara Fault zone is thought to have exerted a regional structural control on mineralisation. New S isotope data suggest that S was sourced from reduced seawater sulphate and from a second source, possibly metamorphosed basement. The Pb isotope signature (CSIRO) suggests mixing of Pb from both basement-metamorphic and basinal sources, with the system formed during the Early Devonian.

Recent mapping has identified a likely felsic volcanic centre in the Ural Ranges, 19km south-west of Lake Cargelligo. A plethora of subaqueous (-?subaerial) facies have been mapped in the Ural Volcanics, including rhyolitic lavas, intrusions, pyroclastic rocks and volcaniclastic rocks with sparsely interbedded turbidites. Late stage monzodiorite dykes intrude the sequence. Rare reworked marine shelly fossils have been found at the base of the sequence, near the poorly exposed contact with the underlying Crossleys Tank Formation. The Ural Volcanics contain minor gold occurrences, and traces of sulphide have been noted in coherent facies, mafic xenoliths and dykes. The coeval Gurragong Volcanics to the south contain common ignimbrites and are thought to represent a more emergent volcanic pile.

New geochemical data suggest that the Ural Volcanics are rhyolitic, borderline alkalic and part of an A-type igneous province. The inferred tectonic setting of eruption is a continental intra- or back-arc extensional basin. New SHRIMP U/Pb dates of 424 to 413 Ma (mean) indicate the period of volcanism.

A thick sequence of fluvial (and rare shallow marine) quartz sandstone and minor conglomerate of the Yar Sandstone (Walters Range Group) were deposited during the Early Devonian on the Walters Range Shelf, between the Rast and Mount Hope Troughs. At the same time, fluvial and alluvial fan conglomerate, sandstone, rare slate and phyllite of the Square Head Formation were deposited on the Hillston Shelf. Sulphides have been intersected at the buried Tara Prospect, on the Hillston Shelf, and old workings and vein gold occurrences have been noted along the Bootheragandra Fault zone.

The Late Devonian Cocoparra Group, which consists of fluvial sandstone and conglomerate, is a probable correlative of the Mulga Downs Group to the north. Recent mapping has identified an interval of marginal marine influence in the Rankin Formation, based on new fossil evidence.

Widespread Cainozoic aeolian and fluvial deposits, along with minor lacustrine and colluvial deposits and leucitite lava, have been mapped based on geomorphic and lithological character. Newly identified outcrops along the Lachlan River have assisted hydrogeologists in identifying groundwater recharge areas.

Regolith-hosted mineralised systems include: deep lead/placer tin deposits at Kikiora and Erigolia, tin-gold placer at Narriah, magnesite-rich accumulations of regolithic carbonate at Cargelligo, and dolomite-rich regolith carbonate accumulations at Weja near Tullibigeal. Small dry lakes are mined for gypsite/copi in the Goolgowi region.

Radiometric imagery has proven to be vital in mapping regolith and identifying geomorphic features.