MULTI-STAGE MAGMATIC TO METAMORPHIC FORMATION OF NI-CO-CU(PD) MINERALIZATION IN A SUBDUCTION-RELATED SETTING: A CASE STUDY OF THE ILDEUS-LUCHA INTRUSIVE COMPLEX (STANOVOY MOBILE BELT, SE SIBERIA)

The Ildeus-Lucha Complex (ILC) intrudes a 900 m thick amphibolite-schist unit formed in a back-arc basin setting within the Stanovoy accretion/collision zone. The Triassic (233 Ma) U-Pb zircon ages reflect northward subduction of Paleozoic ocean floor beneath the southern edge of Siberian craton along the Uda-Murgal magmatic arc. This arc was built upon mature continental crust (similar to modern Kamchatka) as evidenced from the Archean (2514 Ma) relic zircons. The Cretaceous (143 Ma) ages of apatites from ILC possibly reflect thermal event contemporaneous with the onset of Okhotsk-Chukotka volcanic arc. ILC is associated with very strong (500-3000 nTl) magnetic signature and large (40-60 km²) high-grade soil anomalies of Ni (0.05-0.6 wt.%) and Co (0.005-0.34 wt.%). Intrusive rocks include dunites, pyroxenites, wehrlites, and gabbro-norites variably metamorphosed under the greenschist facies conditions. Trace element compositions of ultramafic protoliths and magmatically-textured marginal gabbros, especially Ti, Nb and Ta depletions coupled with LREE enrichments indicate derivation from a subduction-related mantle source. Preserved igneous minerals include orthopyroxene, clinopyroxene, pigeonite and Ca-plagioclase. Sulfides in serpentine-tremolite-actinolite-chlorite matrix include pentlandite, Co-pentlandite (Co=1.53-14.79 wt.%), pyrrhotite, chalcopyrite, bornite, heazlewoodite, talnakhite, digenite, mackinawite and pyrite along with Ni-arsenide and Ni-wurtzite. Some Cu-sulfides contain magnetite exsolutions. Pentlandite and magnetite includes sobolevskite (PdBi). Subduction-related metamorphism hosting sulfide mineralization is associated with saline fluids as evidenced from ubiquitous presence of F-Cl-apatite and Br-Cl salts. We propose that base metal mineralization at the ILC was formed by either 1) emplacement of mantle wedge-derived melts at crustal levels followed by subduction-related metamorphism, scavenging and redeposition of base metals and Pd in presence of saline fluids or 2) assimilation of C-S-rich metasediments (abundant xenoliths of reworked pyrite schists coupled with ³⁴S isotope enrichment in marginal gabbro and pyroxenite) followed by low-grade subduction-related metamorphism of ILC intrusions.