Basin provenance and its control on mineralisation within the Early Devonian Cobar Basin, western Lachlan Orogen, eastern Australia

Abstract

The Cobar Basin, in central New South Wales, is an Early Devonian extensional basin that formed in the western Lachlan Orogen. The basin was filled with shallow- to deep-water sequences of the Cobar Supergroup and hosts small to large polymetallic deposits. Detrital zircon geochronology and whole-rock geochemical data collected from the Amphitheatre Group of the Cobar Supergroup provide constraints on the history of basin fill and illustrate the dynamic interplay between basin provenance and mineralisation, corresponding to the evolving tectonic regime. Data reveal provenance dissimilarity between the southern and northern parts of the basin. In the south, units of the Amphitheatre Group received abundant detritus from ca 430–410 Ma magmatic rocks situated to the southwest and southeast of the basin. By contrast, the northern successions were predominately sourced from recycled Ordovician basement found to the northwest, north, northeast of the basin, along with contributions associated with the Macquarie Arc. This spatial provenance variation, however, is less significant in the younger formations: the northern and southern sequences both exhibit an increase in older recycled detritus upwards with time. This reflects a progressive modification of basin paleogeography, during the transition from rift phase to sag phase. The rift-phase basin geography is characterised by fault-restricted deposition with predominant sediments derived from local proximal sources. The subsequent sag-phase subsidence exhibits a uniform depositional system with more homogenised basin input. This provenance variation is coeval with the stratigraphically controlled mineralisation features within the Amphitheatre Group successions, implying a provenance influence on mineralisation. Data suggest the different sediment source regions have produced distinct detrital mineral compositions between the major mineral-hosting and mineral-barren formations. The enrichment of some detrital minerals in the mineral-hosting units, such as feldspar, muscovite, Ti-minerals (and carbonate), is suggested to be an important factor for mineralisation in the basin.

KEY POINTS

1. Detrital zircon geochronology and whole-rock geochemistry illustrate the history of basin fill for the Cobar Basin.

2. Spatial and temporal variation of basin provenance reflects a modification of basin geography, corresponding to the evolving tectonic regimes.

3. The change in basin source regions is one of key controls on the mineralisation within the Cobar Basin.

Keywords:

• Cobar Basin
• detrital zircon geochronology
• whole-rock geochemistry
• provenance analysis
• tectonics
• mineralisation

Acknowledgements

The work has been supported by the Mineral Exploration Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Program. Steven Trigg, Liann Deyssing and Lorraine Campbell are thanked for their assistance during fieldwork and sample collection. GSNSW staff at the WB Clarke Geoscience Centre—Londonderry Drillcore Library are thanked for their help with drill core sampling. We thank Sun Yanyan for help with thin-section preparation. Chris Fergusson and Bill Collins are thanked for their helpful reviews of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the Supplemental data (Appendices 1 and 2).

Additional information

Funding

This research was funded by MinEx CRC and the Geological Survey of New South Wales (GSNSW). This is MinEx CRC Document 2023/323.