3D modelling of subsurface ancient volcanic successions: a workflow developed during regional reconstructions of the Cowal Igneous Complex in the Ordovician Macquarie Arc of NSW, Australia

Mineral deposits are commonly associated with and hosted in volcanic successions. Volcanic facies analysis as applied in mineral exploration is powerful for characterising host stratigraphy to understand volcanological and eruption processes, depositional setting, and tectonic processes before, during and after ore-forming events. However, the specialist experience required in studying ancient, deformed successions impedes the uptake of facies analysis as a routinely applied technique in mineral exploration. This challenge is reflected in the literature with a paucity of examples where 3D models are developed directly from facies analysis and is worth addressing, as stratigraphic approaches are commonly inadequate for understanding and modelling the complexities within ancient volcanic terrains. We address this gap by demonstrating a technique that uses digitised graphic logs to construct correlation charts in a graphics editor to interpret ‘restored’ volcanic successions within fault blocks. This approach is validated with concurrent cross-sections that depict the structural-stratigraphic framework. The work is iteratively revised until the interpretation is internally consistent across both the correlation chart and the cross-section. This technique is presented within a four-phase workflow that guides the reader from initial scoping, through to graphic logging, interpretation of volcanic successions by locality (or fault block) and finally a synthesis and 3D model. This workflow was developed from our experiences in the buried and deformed Ordovician Cowal Igneous Complex that hosts world-class gold deposits that are currently being mined at E42 (open pit) and GRE46 (underground mine). A case study of the Cowal Igneous Complex is presented to demonstrate the value and real-world application of this workflow.