The Conduits of Magmatic Ore Deposits
Résumé
The magmatic plumbing system plays a crucial role in the formation of ore deposits such as the Ni-Cu-PGE deposits of the Noril'sk-Talnakh region and the chromite-PGE deposits in the Bushveld Complex. The Noril'sk-Talnakh deposits are hosted by small intrusions with an unusual form; they contain central thickened segments, differentiated into gabbroic and olivine-rich cumulates, and flanked by thinner apophyses. The central segments are only 100-380 m thick but individual intrusions have been traced along strike for up to 17km. In the model developed by Naldrett and co-workers, (Naldrett, 2004 and references therein) these intrusions are interpreted as conduits that linked deeper staging chambers to overlying volcanic sequences. The ore sulfides formed when the magma assimilated granitoid and/or anhydrite-bearing sediments; these sulfides then accumulated from rapidly flowing magma that passed through the conduits to feed lava flows at the surface. This model has been criticized on three grounds: (1) Latypov (2002) has argued the intrusions and the lava flows are not comagmatic, because the former are olivine normative and the latter Si-saturated. (2) Czamanske et al. (1995) maintain that differences in trace element and isotope compositions indicate distinct origins for the intrusions and lava flows. (3) Ripley et al. (2003) have shown that the volcanic rocks lack the heavy S-isotope signature acquired by the ores during anhydrite assimilation. These criticisms can be countered as follows. The difference in the extent of Si-saturation between intrusions and lavas is due largely to the presence of olivine cumulates in the intrusions: their bulk compositions are olivine enriched and not those of magmatic liquids. The sulfur-isotope and trace-element compositions of the lavas have been affected by magma-rock interaction after the magmas left the ore-bearing intrusions, as these magmas flowed laterally in extensive sills within the sedimentary and volcanic strata. This interaction effaced part of the geochemical signature of ore formation. Finally, because of the complexity of the plumbing systems, individual magmas followed independent paths to the surface. On these paths, the extent and type of wall-rock interaction differed considerably and upon intrusion or eruption, the magmas had contrasting compositions reflecting the particular path that they followed. Geochemical data provide evidence that the first tholeiitic magmas to enter the system assimilated granitic crust in a deep magma chamber. Although sulfide segregated as a result of this interaction, this sulfide may have been left in the deep chamber and may not form part of the ore deposits. The ores themselves formed when other, less contaminated, magmas assimilated anhydrite-rich sedimentary rocks as they flowed through a near-surface sill complex in the upper levels of the sediment pile. The specific stratigraphic location of deposits such as Noril'sk-Talnakh, Raglan and Penchega is related to the dynamics of magma flow in this near-surface sedimentary setting, which influenced how magma interacted with sedimentary rocks. Complex magmatic plumbing also influenced the formation of the deposits in the Bushveld Complex. This intrusion was fed by plume-derived magmas (not boninite or partial melt of lithospheric mantle) that had assimilated crustal material in lower to mid-crustal magma chambers. The ore minerals segregated, at least in part, in the lower chambers and were transported by flowing magma into the main Bushveld Chamber.
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