Mineralium Deposita (v.51, #8)

Geological structure and ore mineralization of the South Sopchinsky and Gabbro-10 massifs and the Moroshkovoe Lake target, Monchegorsk area, Kola Peninsula, Russia by Pavel V. Pripachkin; Tatyana V. Rundkvist; Yana A. Miroshnikova; Alexey V. Chernyavsky; Elena S. Borisenko (973-992).
The South Sopchinsky massif (SSM), Gabbro-10 (G-10) massif, and Moroshkovoe Lake (ML) target Monchegorsk area, Kola Peninsula, are located at the junction of the Monchepluton and Monchetundra layered intrusions. The intrusions were studied in detail as they are targets for platinum-group element (PGE) mineralization. The rocks in these targets comprise medium- to coarse-grained mesocratic to leucocratic gabbronorites, medium-grained mesocratic to melanocratic norites and pyroxenites, and various veins mainly comprising norite, plagioclase-amphibole-magnetite rocks, and quartz-magnetite rocks. The veins contain Ni-Cu-PGE mineralization associated with magnetite and chromite. In all targets, the contacts between gabbronorite and norite-pyroxenite are undulating, and the presence of magmatic (intrusive) breccias suggests that these rocks formed through mingling of two distinct magmatic pulses. In places, the gabbronorites clearly crosscut the modal layering of the norites and pyroxenites. Trace element data indicate that the gabbronorites have similar compositions to rocks of the upper part of the Monchetundra intrusion, whereas the norites and pyroxenites resemble rocks from the lower to intermediate stratigraphic levels of the Monchepluton, such as in the Nude-Poaz and Sopcha massifs. Sulfide mineralization in the studied targets principally consists of secondary bornite, millerite, and chalcopyrite. In contrast, the primary sulfide assemblage within the layered sequence of the adjacent Monchepluton is characterized by pentlandite, chalcopyrite, and pyrrhotite. Therefore, the mineralization in the studied targets is interpreted to be of a contact style. We argue that the studied area represents the contact zone between gabbronorites of the Monchetundra intrusion and norites and pyroxenites of the Monchepluton. In addition, the rocks were overprinted by postmagmatic veining and remobilization of contact style sulfide and PGE mineralization.
Keywords: Layered intrusions; Kola Peninsula; Monchegorsk area; South Sopchinsky massif; PGE mineralization

Cu–Ni–PGE fertility of the Yoko-Dovyren layered massif (northern Transbaikalia, Russia): thermodynamic modeling of sulfide compositions in low mineralized dunite based on quantitative sulfide mineralogy by Alexey A. Ariskin; Evgeny V. Kislov; Leonid V. Danyushevsky; Georgy S. Nikolaev; Marco L. Fiorentini; Sarah Gilbert; Karsten Goemann; Alexey Malyshev (993-1011).
The geology and major types of sulfide mineralization in the Yoko-Dovyren layered massif (northern Transbaikalia, Russia) are presented. This study focuses on the structure, mineralogy, and geochemistry of poorly mineralized plagiodunite and dunite in the lower part of the intrusion. Assuming these rocks contain key information on the timing of sulfide immiscibility in the original cumulate pile, we apply a novel approach which combines estimates of the average sulfide compositions in each particular rock with thermodynamic modeling of the geochemistry of the original sulfide liquid. To approach the goal, an updated sulfide version of the COMAGMAT-5 model was used. Results of simulations of sulfide immiscibility in initially S-undersaturated olivine cumulates demonstrate a strong effect of the decreasing fraction of the silicate melt, due to crystallization of silicate and oxide minerals, on the composition of the intercumulus sulfide liquid. Comparison of the observed and modeled sulfide compositions indicates that the proposed modeling reproduces well the average concentrations of Cu, Cd, Ag, and Pd in natural sulfides. This suggests the sulfide control on the distribution of these elements in the rocks. Conversely, data for Pt and Au suggest that a significant portion of these elements could present in a native form, thus depleting the intercumulus sulfide melt at an early stage of crystallization.
Keywords: Yoko-Dovyren layered massif; Average sulfide composition; Mineralized dunite; COMAGMAT; Modeling S saturation; Precious metals

The 1858 ± 17 Ma Chineysky layered anorthosite-gabbronorite massif is located in the southern part of the Siberian platform, within the Kodaro-Udokan metallogenic province of Northern Transbaikalia. The Chineysky Massif outcrops over approximately 130 km2 and contains Russia’s largest V ore resources, hosted within titanomagnetite-rich layers, concentrated in the Magnitny and Etyrko deposits. The titanomagnetite ore reserves were estimated at 2 billion tons with 30 % Fe and 10 % TiO2 on average. In addition, two large Cu-PGE deposits—Rudny and Kontactovy—are hosted in the contact rocks between the intrusion and the sandstone floor rocks. A distinctive feature of the Chineysky sulfide ore is their Cu-enriched composition with much lesser amounts of nickel and cobalt (Cu/Ni/Co ~ 76:7:1). The sulfide ore contains up to 355 ppm PGE and 30 ppm Au + Ag. Three types of sulfide mineralization have been distinguished: (1) endo-contact disseminated sulfides within gabbronorite, (2) exo-contact impregnations in sandstone, and (3) veins of massive sulfides in the exo-contact sandstone. The ore consists predominantly of chalcopyrite, with less abundant pentlandite, pyrrhotite, Co-Ni arsenides and sulfoarsenides, linneite-group minerals, sphalerite, cubanite, and millerite. In addition, many rare minerals were recognized in the ores, including PGM (sperrylite, michenerite, merenskyite, etc.). Using the latest version of the COMAGMAT-5 program, the parental magma temperature (~1150 °C), its composition (~55 wt.% SiO2, 5.8 % MgO), and the most primitive olivine (Fo77) and plagioclase (An69) compositions were calculated. According to the model, titanomagnetite starts to crystallize at T < 1133 °C (fO2 = NNO + 0.5), triggering sulfide liquid immiscibility when the silicate magma had ~0.15 to 0.1 wt.% S.
Keywords: Northern Transbaikalia; Layered intrusion; Cu-PGE and V-Ti-Fe deposits; COMAGMAT-5; Chineysky; Udokan

Multiple sulfur isotope and mineralogical constraints on the genesis of Ni-Cu-PGE magmatic sulfide mineralization of the Monchegorsk Igneous Complex, Kola Peninsula, Russia by A. Bekker; T. L. Grokhovskaya; R. Hiebert; E. V. Sharkov; T. H. Bui; K. R. Stadnek; V. V. Chashchin; B. A. Wing (1035-1053).
We present the results of a pilot investigation of multiple sulfur isotopes for the Ni-Cu-PGE sulfide mineralization of the ∼2.5 Ga Monchegorsk Igneous Complex (MIC). Base Metal Sulfide (BMS) compositions, Platinum Group Element (PGE) distributions, and Platinum Group Mineral (PGM) assemblages were also studied for different types of Ni-Cu-PGE mineralization. The uniformly low S content of the country rocks for the MIC as well as variable Sm-Nd isotope systematics and low-sulfide, PGE-rich mineralization of the MIC suggest that S saturation was reached via assimilation of silicates rather than assimilation of sulfur-rich lithologies. R-factor modeling suggests that the mixing ratio for silicate-to-sulfide melt was very high, well above 15,000 for the majority of our mineralized samples, as might be expected for the low-sulfide, PGE-rich mineralization of the MIC. Small, negative Δ33S values (from −0.23 to −0.04 ‰) for sulfides in strongly metamorphosed MIC-host rocks indicate that their sulfur underwent mass-independent sulfur isotope fractionation (MIF) in the oxygen-poor Archean atmosphere before it was incorporated into the protoliths of the host paragneisses and homogenized during metamorphism. Ore minerals from the MIC have similar Δ33S values (from −0.21 to −0.06 ‰) consistent with country rock assimilation contributing to sulfide saturation, but, also importantly, our dataset suggests that Δ33S values decrease from the center to the margin of the MIC as well as from early to late magmatic phases, potentially indicating that both local assimilation of host rocks and S homogenization in the central part of the large intrusion took place.

Mantle source of the 2.44–2.50-Ga mantle plume-related magmatism in the Fennoscandian Shield: evidence from Os, Nd, and Sr isotope compositions of the Monchepluton and Kemi intrusions by Sheng-Hong Yang; Eero Hanski; Chao Li; Wolfgang D. Maier; Hannu Huhma; Artem V. Mokrushin; Rais Latypov; Yann Lahaye; Hugh O’Brien; Wen-Jun Qu (1055-1073).
Significant PGE and Cr mineralization occurs in a number of 2.44–2.50-Ga mafic layered intrusions located across the Karelian and Kola cratons. The intrusions have been interpreted to be related to mantle plume activity. Most of the intrusions have negative εNd values of about −1 to −2 and slightly radiogenic initial Sr isotope compositions of about 0.702 to 0.703. One potential explanation is crustal contamination of a magma derived from a mantle plume, but another possibility is that the magma was derived from metasomatized sub-continental lithospheric mantle. Samples from the upper chromitite layers of the Kemi intrusion and most samples from the previously studied Koitelainen and Akanvaara intrusions have supra-chondritic γOs values indicating some crustal contamination, which may have contributed to the formation of chromitites in these intrusions. Chromite separates from the main ore zone of the Kemi and Monchepluton intrusions show nearly chondritic γOs, similar to the coeval Vetreny belt komatiites. We suggest that the Os isotope composition of the primitive magma was not significantly changed by crustal contamination due to a high Os content of the magma and a low Os content of the contaminant. Modeling suggests that the Os and Nd isotope compositions of the Monchepluton and Kemi intrusions cannot be explained by assuming a magma source in the sub-continental lithospheric mantle with sub-chondritic γOs. A better match for the isotope data would be a plume mantle source with chondritic Re/Os and Os isotope composition, followed by crustal contamination.
Keywords: Chromite; Os isotopes; Nd isotopes; Mantle plume; Layered intrusions; Paleoproterozoic; Fennoscandian Shield