Tectono-sedimentary setting of the Lady Loretta Formation: synrift, sag or passive margin?

The Proterozoic stratiform sediment-hosted base-metal deposits of the north Australian zinc belt are often assumed to share a common tectono-sedimentary setting related to riffing. Rift models have varied from basin-scale to isolated half-grabens. The sedimentary rocks that host mineralisation have been interpreted as either synrift or part of a rift-sag phase. The Lady Loretta Zn-Pb-Ag deposit was believed to have formed within the Paradise Graben. However, synrift models previously proposed for the Lady Loretta deposit can be challenged by structural and sedimentological evidence. There are no unusual facies or thickness changes that might indicate growth faulting. A distinctive stromatolitic package in the underlying formation and the highly pyritic facies that host mineralisation can be traced outside of the supposed graben. Palaeocurrent data and the distribution of inter-ore breccias are not consistent with synsedimentary fault activity. In terms of the present understanding of the basin evolution, a passive margin setting for the Lady Loretta Formation is preferred to simple rift-sag.