Evolution of the Lachlan Orogen in the East Riverina region, NSW: Insights from 100 new SHRIMP dates

Bodorkos S1, Gilmore PJ2, Eastlake MA2, Bull KF2, Blevin PL2, Trigg SJ2, Campbell LM2 and Waltenberg K1

1Minerals, Energy and Groundwater Division, Geoscience Australia (GA), 2Geological Survey of New South Wales (GSNSW), Department of Regional NSW

The southern part of the central Lachlan Orogen in NSW is prospective for intrusion-related tin-tungsten (e.g. Ardlethan), porphyry-style copper-gold, and orogenic gold mineralisation; however, the regional geological framework has long been poorly understood. GSNSW’s 2014–2018 East Riverina mapping project (spanning the area between West Wyalong and the Murray River, east to Cootamundra and Adelong, and west to Narrandera and the Murray Basin) updated large areas of 1960s–1990s geological mapping, to support mineral prospectivity studies. The project included a collaborative GA–GSNSW geochronology program that generated nearly 100 new U–Pb SHRIMP zircon dates, to establish a regional chronology of felsic and intermediate magmatism, and to understand the depositional history of the intervening basin successions. Some highlights include:

  1. Sheared 493 Ma and 489 Ma granites of the Belimebung Igneous Complex confirm previous LA-ICPMS dating of the first Cambrian igneous rocks identified within the Gilmore Fault Zone. They were intersected in drillcore beneath the early Silurian Gidginbung Volcanics north of Barmedman, and indicate a possible latest Cambrian age for NW-trending magnetic lineaments near the Gilmore Fault.
  2. The propylitically-altered 439 Ma Cooba Monzonite southeast of Junee extends the known distribution of ‘Phase 4’ Macquarie Volcanic Belt rocks (prospective for porphyry copper mineralisation) to the south.
  3. Tholeiitic andesites of the 432 Ma Junawarra Volcanics and related rocks host gold mineralisation at Dobroyde, northeast of Junee. These are geochemically distinct from, and significantly younger than, the shoshonitic 439–436 Ma Gidginbung Volcanics to the northwest, and establish two separate associations of Silurian gold-bearing volcanic rocks in the region.
  4. More than 20 S-type plutons of the 432–427 Ma Koetong Supersuite have been dated, as far north as Nymagee, east to Young and Tumut, and west under Murray Basin cover to Barellan and Howlong.
  5. The predominantly mid-Silurian ‘Wagga Batholith’ is transected and flanked by NW-trending belts of Early Devonian (420–412 Ma) plutonic rocks linked to intrusion-related mineralisation. S-type granites associated with tin-tungsten mineralisation include the 418 Ma Burrandana Granite and the de-silicated 417 Ma Ryan Granite in the south, and fractionated 415–413 Ma granites near Ardlethan in the north. Contemporaneous I-type rocks include the cassiterite-bearing Yithan Rhyolite at Ardlethan, diorite associated with gold mineralisation at Mount Adrah, and intrusions east and south of Tarcutta and Holbrook.
  6. Regional Early Devonian volcanism encompasses the expanded 419–416 Ma Gurragong Group in the Cargelligo-Ardlethan area, and the newly-defined Culcairn Group (comprising the 415–413 Ma S-type Budginigi Ignimbrite and the 413–411 Ma Wallandoon Ignimbrite and Hickory Hill Diorite) in the Culcairn-Henty-Walbundrie area. These volcanic units constrain the timing of pre-eruptive siliciclastic sedimentation and influence the detrital signatures of post-eruptive sediments.
  7. I-type plutonism is youngest in the southwestern East Riverina. Early Devonian (414 Ma and 411 Ma) Leeton Igneous Complex granites stitch the unexposed northwestern extension of the Kancoona Fault. Northeast and north of Albury, the 407–402 Ma Mullengandra Monzodiorite and Jindera Granite appear to be associated with Central Victorian granites of similar age.

Biography

Simon Bodorkos has worked in Geochronology at Geoscience Australia since 2007, and has conducted the U-Pb SHRIMP dating program supporting the Geological Survey of New South Wales’ East Riverina project since 2013. His co-authors include GSNSW’s East Riverina mapping team and mineral systems specialists.

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