Geodynamic Setting and Cu-Ni Potential of Late Permian Xiwanggou Mafic-Ultramafic Rocks, East Kunlun Orogenic Belt, NW China

Several magmatic Cu–Ni sulfide deposits have recently been explored along the deep Middle Kunlun fault related to the extension of the East Kunlun orogenic belt in Qinghai Province, NW China. The Xiwanggou mafic–ultramafic rocks associated with Cu–Ni sulfide mineralization are first to be dated as late Permian compared to most of the deposits formed during late Silurian–early Devonian in this region. The Xiwanggou complexes located in the junction area between the East Kunlun and West Qinling belts, are composed of gabbros, olivine-gabbros, pyroxenites, olivine-pyroxenites, and peridotites. The Cu–Ni mineralization are mainly hosted in the olivine-pyroxenites and pyroxenites, whereas the sulfide-poor mineralization distributed in gabbros and olivine-gabbros. Zircon LA-ICP-MS U–Pb dating of the gabbro and olivine-pyroxenite revealed their crystallized ages of 250.8 ± 0.8 Ma and 257.3 ± 0.7 Ma, respectively. The trace element characteristics of the Xiwanggou fertile mafic-ultramafic rocks shows the enrichments in Sr, Rb, Th, Ba and light rare earth elements, and depletions in Nb and Ta, which are associated with the slab derived fluid input and dehydration melting of amphiboles. Meanwhile, Sr–Nd and Hf isotopic compositions of the gabbro [εNd(t) = 0.66–1.18; εHf(t) = 5.2–12] and olivine-pyroxenite [εNd(t) = −1.09 – −0.43; εHf(t) = 5.4–17.7] show that the magma was mainly derived from the metasomatized portions of subcontinental lithospheric mantle (SCLM) source in the mantle wedge. The magma primarily experienced dehydration melting processes of amphiboles and subsequently underwent hydrated melting in the overlying mantle wedge and relatively reduced background. The cool subduction process of the Anemaqen oceanic lithosphere maybe trigger large melting in the mantle wedge resulting in a relative low-Ni content in the melt. The transpressional windows formed by the right-lateral strike-slip shearing action of the Wenquan and South Kunlun faults in the South Kunlun forearc belt created a significant conduit for the magma ascending. The thermometer of Fe and Ni exchange between coexisting olivine and sulfide melt indicates the magma were yielded in a temperature range of ca. 1200–1300°C and an oxygen fugacity range of ca. –10.57 to –8.98 (log unit), which suggested that the parental magma of the Xiwanggou complex derived from a relatively reduced source favoring Ni relative to Fe in the melt. The intermediate sulfide segregation from the melt resulted in a medium tenor potential for the Xiwanggou complex.

Predicting Missing Seismic Velocity Values Using Self-Organizing Maps to Aid the Interpretation of Seismic Reflection Data from the Kevitsa Ni-Cu-PGE Deposit in Northern Finland

We use self-organizing map (SOM) analysis to predict missing seismic velocity values from other available borehole data. The site of this study is the Kevitsa Ni-Cu-PGE deposit within the mafic-ultramafic Kevitsa intrusion in northern Finland. The site has been the target of extensive seismic reflection surveys, which have revealed a series of reflections beneath the Kevitsa resource area. The interpretation of these reflections has been complicated by disparate borehole data, particularly because of the scarce amount of available sonic borehole logs and the varying practices in logging of borehole lithologies. SOM is an unsupervised data mining method based on vector quantization. In this study, SOM is used to predict missing seismic velocities from other geophysical, geochemical, geological, and geotechnical data. For test boreholes, for which measured seismic velocity logs are also available, the correlation between actual measured and predicted velocities is strong to moderate, depending on the parameters included in the SOM analysis. Predicted reflectivity logs, based on measured densities and predicted velocities, show that some contacts between olivine pyroxenite/olivine websterite-dominant host rocks of the Kevitsa disseminated sulfide mineralization—and metaperidotite—earlier extensively used “lithology” label that essentially describes various degrees of alteration of different olivine pyroxenite variants—are reflective, and thus, alteration can potentially cause reflectivity within the Kevitsa intrusion.

Cognate gabbroic xenoliths from a tholeiitic subvolcanic sill complex: Implications for fractional crystallization and crustal contamination processes

Intruded into the Palaeogene lava field and underlying Moine (Neoproterozoic) crystalline basement rocks around Loch Scridain, Isle of Mull, Scotland, is a suite of high-level, inclined, xenolithic sheets, ranging in composition from basalt, through andesite and dacite, to rhyolite. These sheets, associated with the Mull central volcano, were emplaced post 55 Ma. As well as numerous crustal xenoliths, the more basic members of the complex contain a diverse suite of ultrabasic and basic xenoliths. Xenolith types include feldspathic peridotite with cumulus olivine, pyroxenite, gabbro with cumulus plagioclase and cumulus clinopyroxene, and pure anorthosite. Mineralogical data, coupled with whole-rock major- and trace-element data from a small number of the xenoliths suggest that the xenoliths represent early-formed cumulates cognate with their host basalts. Sr and Nd isotope data from the xenoliths confirms the cognate origin, and also shows that the basic magmas suffered crustal contamination at an early stage.