Trace-element geochemistry and petrogenesis of barren and ore-associated komatiites

Most komatiite-associated magmatic Ni-Cu-(PGE) sulfide deposits formed from sulfide-undersaturated magmas and are interpreted to have formed in dynami c lava channels or magma conduits by incorporation of crustal sulfur. They commonly exhibit geochemical and isotopic evidence of crustal contamination and chalcophile element depletion on the scale of individual cooling units or parts of individual cooling units that appear to be associated with the err-forming process. It is possible, therefore, to discriminate between ro cks generated during ore-forming processes and rocks generated during norma l igneous processes by identifying signatures characteristic of crustal con tamination (e.g., Th-U-LREE enrichment, negative Nb-Ta-Ti anomalies) or sul fide segregation (e.g., Co-Ni-Cu-PGE depletion) (or both) and distinguishin g them from signatures characteristic of normal igneous fractionation or ac cumulation of crystals. The amounts of contamination and chalcophile elemen t depletion produced during the ore-forming process depend on several facto rs: 1) the stratigraphic architecture of the system (e.g., thickness and ph ysical accessibility of the contaminant), 2) the fluid dynamics and thermod ynamics of the lava or magma, 3) the physical, chemical, and thermal charac teristics of the contaminant, 4) the amount of contaminant melted and incor porated (e.g., amount of silicate partial melt), 5) the sulfur and metal co ntent of the contaminant, 6) the initial saturation-state of sulfide in the magma, 7) the assimilation:crystallization ratio, 8) the amount of lava re plenishment, and 9) the effective magma:sulfide ratio (R factor) of the sys tem. Because these processes vary independently from deposit to deposit, fr om area to area within a deposit, and within a single area with time, there are many opportunities to decouple contamination from chalcophile element depletion.