Van Tuyl Lecture: Wolfgang Maier, Cardiff University at Wales
September 6 @ 4:00 pm - 5:00 pm
Wolfgang Maier, Cardiff University at Wales
241 Berthoud Hall, 4PM
Topic: Formation of the Bushveld Complex and its Ore Deposits
Abstract: The formation of the Bushveld ore deposits remains controversial, despite a century of research. Here it is proposed that sulphide and oxide deposits in the Bushveld Complex and other layered intrusions form when large, incompletely solidified magma chambers undergo central subsidence in response to crustal loading, resulting in slumping of semi-consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides, oxides, olivine and pyroxene and less dense layers enriched in plagioclase. The most economic PGE, Cr and V reefs form in large, multiple-replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification. In smaller intrusions, cooling rates are faster, subsidence is less pronounced and, where it occurs, the cumulate may be largely solidified, resulting in insignificant mush mobility and mineral sorting. Layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower. An additional factor controlling the PGE, Cr and V prospectivity of intrusions is their location within cratons. Intracratonic environments offer more stable emplacement conditions that are more amenable to the formation of large, layered igneous bodies. Furthermore, intrusions sited within cratons are more readily preserved because cratons are underlain by thick, buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust.