International Geologiical Congress - Oslo 2008

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MRD-15 Ni-Cu-PGE sulphide deposits

 

Genesis of mineralized and barren magmatic rocks in the Noril'sk district Likhachev

 

Alexander Likhachev, TSNIGRI (Russian Federation)
 

 

Analysis of data on the composition of rocks, ores, and minerals from magmatic bodies in the Noril'sk district and adjacent territories (Likhachev, 2006) led us to the following conclusions:

The cumulus phase settling during the crystallization differentiation of tholeiitic magmas of the Siberian flood basalts was ferrous olivine (Fo75-50). More magnesian olivine of the picritic basalts and picritic gabbro-dolerites of the mineralized intrusions (Fo82-75) was not in equilibrium with the accompanying silicate and sulfide minerals. Together with chromite, they show features of residual, but not cumulus, phases of undepleted mantle material. Coarse-grained aggregates found in picritic basalt display traces of partial melting and may be fragments of the mantle material from which the mafic magmas were derived that produced the tholeiitic basalts of the Noril'sk district and their comagmatic intrusions.

Fine-grained ("granulated") domains of picrite composition found in the taxitic and picritic units of the mineralized intrusions may have been chilled fragments of the parental high-temperature melt of a thermal plume. The basalts of the Nadezhdinskaya suite, which occur in the area and are depleted in ore components, may have been melting products of the Ni- and Cr-poor pyroxene constituent of the mantle material.

The results of our experiments indicate that the partial melting and dissolution of sulfide and sulfide-silicate associations is accompanied by a significant fractionation of S isotopes (up to 8.4‰). For the Cu-Ni associations, this was pronounced in the enrichment of the low-melting (i.e., Cu-richer) products of the heavy 34S isotope, which was possible in natural processes during the generation of ore-bearing magmas of the Noril'sk type. The source of the ore material of the Noril'sk deposits was the S-oversaturated melt of the thermal plume, which contained sulfide material enriched in the 34S isotope. Chilled fragments of this melt occur in the form of the aforementioned fine-grained "granulated" domains of picritic composition. The barren mafic magmas were generated via the partial (without involvement of the sulfide fraction) melting of the mantle material under the effect of the thermal plume. Away from the plume head, the degree of partial melting of the surrounding rocks decreased, and this gave rise to magmas of various compositions, which occur in the vertical section of the regional magmatic sequences.

Neither the Noril'sk district itself nor its neighboring territories contain large volcanic edifices, which are indicators of intermediate crustal magmatic chambers. Because of this, the magmas could hardly differentiate and be contaminated in intermediate chambers.
Reference: Likhachev A.P. Platinum-nickel-copper and platinum deposits. M. Aslan. 2006. 496 pp.

 

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