Tectonically-induced neptunian dykes and breccias of the Paleoproterozoic Teena Dolomite: Significance to stratiform zinc deposits in the McArthur Basin, Australia

Abstract

A widespread system of breccias and neptunian dykes is present within the Paleoproterozoic Teena Dolomite of the McArthur Basin. Most breccia types within the unit are infilled by early fibrous marine cements and dark grey sediments. We interpret the dominant breccia types as in-situ synsedimentary fracture systems (neptunian dykes) while some matrix-supported breccias are likely to be mass flow deposits. Fibrous marine cements filling the neptunian dykes have several different textures and all now consist of dolomite. Petrographic evidence indicates the cements were likely precipitated as calcite and dolomitized during early diagenesis. The trace and rare earth element chemistry of the marine cements is typical of Paleoproterozoic marine cements and indicate relatively anoxic marine conditions.

The breccias in the Teena Dolomite appear to be the result of a basin-wide tectonic event that occurred during Teena Dolomite to early Barney Creek time at ∼1640 Ma. Early marine-cemented neptunian dykes likely formed by tectonic fracturing and gravitational collapse on tectonic highs. Many large stratiform Zn-Pb deposits in northern Australia (HYC, Teena, Lady Loretta, Mt Isa) are hosted by shales that are coeval with this tectonic event, suggesting a genetic link between tectonism and mineralization. We suggest that this ∼1640 Ma tectonic event is responsible for both the release of mineralization fluids (via faulting and/or dewatering), and the deposition of deeper water organic-rich fine-grained sediments that host the mineralization (via tectonic subsidence). This tectonic event appears to be an important component of the sedimentary exhalative mineral system in the Carpentaria Zinc Belt of northern Australia.