Secular Changes in Lava/Magma Channelization and the Volcanic/Subvolcanic/Plutonic Settings of Magmatic Ni-Cu-PGE and Chromite Mineralization on Earth

Abstract:

The degree of channelization in magmatic systems is controlled by magma density, viscosity, and flux; environment of emplacement (subaerial/submarine) and substrate topography if volcanic; and country rock density/rheology/structure and degree/orientation of differential stress if subvolcanic/plutonic. Lower viscosity Archean komatiites ascended rapidly, producing lava channels (e.g., Kambalda) and subvolcanic feeder sills (e.g., Mt. Keith). Higher viscosity Proterozoic komatiitic basalts/ferropicrites/picrites had more difficulty ascending through the crust and produced subvolcanic feeder dikes (e.g., Eagle-Tamarack, Voisey’s Bay) and sills (e.g., Thompson, Pechenga) and fewer lava channels (e.g., Raglan). Phanerozoic picrites/basalts produced mainly dikes (e.g., Kalatongke) and feeder sills (e.g., Noril’sk, Jinchuan). All were capable of eroding S-rich substrates/wall rocks and generating magmatic Ni-Cu-PGE deposits. Some chromite deposits appear to have formed in channelized magmatic systems, and although typically finer-grained chromite is much easier to transport than typically larger sulfide melt droplets, almost all known volcanic Ni-Cu-PGE deposits formed during lava emplacement and no volcanic chromite deposits have yet been identified. This suggests that Fe-Ni-Cu-(PGE) sulfides and chromite are more easily transported horizontally within sills and lava channels, but less easily transported vertically. If magmatic Fe-Ni-Cu-(PGE) deposits can form by partial melting of Fe sulfide-rich sediments underneath lava/magma channels and dynamic upgrading of sulfide xenomelts by reaction with the magma (the prevailing model), then magmatic chromite deposits can form by partial melting of Fe oxide-rich sediments underneath lava/magma channels and dynamic upgrading of oxide xenocrysts by reaction with the magma. The anomalously thick (up to 100m) Black Thor-Blackbird, Inyala, Ipueira-Medrado, Kemi, Nkomati, and Sukinda chromite deposits may be examples of this process.