TRACE-ELEMENT GEOCHEMISTRY AND PETROGENESIS OF BARREN AND ORE-ASSOCIATED KOMATIITES

Abstract

Most komatiite-associated magmatic Ni-Cu-(PGE) sulfide deposits formed from sulfide-undersaturated magmas and are interpreted to have formed in dynamic 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 ore-forming process. It is possible, t here- fore, to discriminate between rocks generated during ore-forming processes and rocks generated during normal igneous processes by identifying signatures characteristic of crustal contamination ( e.g., Th-U-LREE enrichment, negative Nb-Ta-Ti anomalies) or sulfide segregation ( e.g., Co-Ni-Cu-PGE depletion) (or both) and distinguishing them from signatures characteristic of nor- mal igneous fractionation or accumulation of crystals. The amounts of contamination and chalcophile element depletion pro- duced during the ore-forming process depend on several factors: 1) the stratigraphic architecture of the system ( e.g., thickness and physical accessibility of the contaminant), 2) the fluid dynamics and thermodynamics of the lava or magma, 3) the physical, chemical, and thermal characteristics of the contaminant, 4) the amount of contaminant melted and incorporated ( e.g., amount of silicate partial melt), 5) the sulfur and metal content of the contaminant, 6) the initial saturation-state of sulfide in the m agma, 7) the assimilation:crystallization ratio, 8) the amount of lava replenishment, and 9) the effective magma:sulfide ratio (R fact or) of the system. Because these processes vary independently from deposit to deposit, from area to area within a deposit, and with in a single area with time, there are many opportunities to decouple contamination from chalcophile element depletion.