Facies analysis, sequence stratigraphy, and carbon isotope chemostratigraphy of a classic Zn-Pb host succession: The Proterozoic middle McArthur Group, McArthur Basin, Australia

Abstract

The McArthur Basin is part of a Proterozoic basin system on the North Australian Craton that represents a world-class Zn-Pb province. Ore bodies are typically stratiform and hosted by pyritic, organic-rich, and dolomitic siltstones deposited in local depocenters and sub-basins. The mineralization is characterized by syngenetic and/or diagenetic textures. These characteristics highlight the need to understand the sedimentological and structural evolution of the basin for mineral exploration. Here we report a facies analysis of the middle McArthur Group (Tooganinie to Lynott formations) in the southern McArthur Basin, distinguishing four facies associations and 19 lithofacies. Depositional environments range from slope and deep subtidal settings to supratidal sabkhas. The middle McArthur Group records a systematic ∼3.5‰ shift in the carbon isotope ratio of carbonates (δ13Ccarb) that can likely be used for basin-wide or even global correlation. The Barney Creek Formation, the main Zn-Pb host unit, was mostly deposited under deep subtidal to slope conditions, although shoaling to shallow subtidal environments locally occurred on paleohighs. Together with the overlying Reward Dolostone, it comprises two 3rd-order transgressive-regressive sequences, which distinguishes it from the younger and less prospective but lithologically similar Caranbirini Member, which only comprises one incomplete sequence. The HYC Pyritic Shale Member of the lower Barney Creek Formation, which hosts most of the known mineralization, is lithologically similar across the studied area, and reflects significant deepening of the entire basin. A maximum flooding surface in the HYC Pyritic Shale Member represents the most pyritic and organic-rich interval and can be developed as a black shale in sub-basin depocenters. It represents an ideal chemical trap for base metals in syngenetic models for mineralization; however, lithification and compaction would convert this black shale interval into a physical trap in diagenetic models. Regardless of the preferred model, sequence stratigraphy integrated with facies maps can be used for targeting.