Publication Type:

Book Chapter


AGU 2015 fall meeting, American Geophysical Union, Washington, DC, United States, Volume 2015 (2015)


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.


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