Dating Layered Intrusions and Their Ore Deposits

English Abstract:

Mafic layered intrusions preserve stunning rock records of the processes by which magma evolves in crustal magma chambers. They host major resources of Cr-Ni-Cu-V and platinum group elements (PGE), including the two world-class PGE reef-type deposits, the Merensky Reef in the Paleoproterozoic Bushveld Complex and the J-M Reef in the Neoarchean Stillwater Complex. Accurate and precise dates are needed to establish how different parts of layered intrusions are related to each other, including defining genetic relations with their contained ore bodies. Surprisingly, the classic layered intrusions in petrology (e.g., Stillwater, Bushveld, Muskox, Skaergaard) do not yet possess robust geochronological frameworks. Dateable accessory minerals, predominantly zircon, are typically present in minute quantities (10s to 100s of grains per 10-20 kg of sample), which is sufficient to provide high-precision geochronologic results. Heterogeneous textures and macroscopic evidence for interstitial minerals that crystallized from evolved interstitial melt at near-solidus temperatures are key criteria for selecting prospective samples. As part of larger chronostratigraphic studies of the Bushveld and Stillwater complexes, we have successfully determined the ages of a wide range of mineralized samples, including basal Ni-sulphide mineralization, chromitite horizons, and the PGE reefs. Our U-Pb geochronological results indicate that (1) these two major layered intrusions formed incrementally over millions of years, (2) distinct time gaps or magmatic hiatuses are recorded in their respective stratigraphic successions of cumulates, and (3) ages do not systematically decrease with increasing relative stratigraphic position (i.e., some older units are bounded by younger rocks). These extended magmatic time spans for large mafic-ultramafic layered intrusion and their associated ore deposits are remarkably similar to those recognized in intermediate to felsic plutonic and volcanic systems.