Volume 60, Nº 1 (2018)

The tungsten distribution in rocks of the Kukulbei Complex in eastern Transbaikal region results in a high potential of rare-metal peraluminous granites (RPG) for W mineralization and displays a different behavior of W in Li–F and “standard” RPG. These subtypes differ in the behavior of W in melt, spatial localization of mineralization, and the timing of wolframite crystallization relative to the age of the parental granitic rocks. The significant of W concentration is assumed to be due to fractionation of the Li–F melt; however, wolframite mineralization in Li–F enriched granite is not typical in nature. The results of experiments and our calculations of W solubility in granitic melt show that wolframite hardly ever crystallizes directly from melt; it likely migrates in the fluid phase and is then removes from the magma chamber to the host rocks, where secondary concentration takes place in exocontact greisens and quartz–cassiterite–wolframite veins. At the same time, the isotopic age of accessory wolframite (139.5 ± 2.1 Ma) within the Orlovka massif of Li–F granite is close to the formation age of the massif (140.6 ± 2.9 Ma). A different W behavior is recorded in the RPG subtype with a low lithium and fluorine concentration, exemplified by the Spokoininsky massif. There is no significant W gain in the melt. All varieties of wolframite mineralization in the Spokoininsky massif are derived from greisens, veins, and pegmatoids yielding the same crystallization ages (139.5 ± 1.1 Ma), which are 0.9–1.8 Ma later (taking into account the mean-square weighted deviation) than the Spokoininsky granite formation (144.5 ± 1.4 Ma). Perhaps this period corresponds to the time of transition from the magmatic stage to hydrothermal alteration. Comparison of the isotope characteristics (Rb–Sr and Sm–Nd isotope systems) of rocks and the associated ore minerals (wolframite, cassiterite) from all examined deposits shows a depletion in ε_Nd values for ore minerals relative to the rock and the opposite behavior for the intial Sr isotope ratios. This may indicate the specific nature of ore matter, where the effect of the juvenile component is definitely expressed. Our geochronological results show that tantalum and tungsten mineralization took place within a narrow age interval, almost synchronously with the crystallization of associated granites. The coeval development of peraluminous magmatism enriched in lithophile rare elements and volatiles with ore complexes located in different structural settings and separated by a considerable distance from each other (up to 500 km) suggests a regional and deep-seated magma source. Rifting and increased thermal flux from the mantle, manifestations of which have been recorded during this period in the territory, may be a deep-seated process.

The paper presents pioneering data on the composition, texture, and crystal structure of molybdenite from various types of molybdenum mineralization at the Bystrinsky Cu–Au–Fe porphyry–skarn deposit in the eastern Transbaikal region, Russia. The data were obtained using electron microprobe analysis (EMPA), laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS), and high-resolution transmission electron microscopy (HRTEM). Molybdenite found at the deposit in skarn, sulfide-poor quartz veins, and quartz–feldspar alteration markedly differs in the concentrations of trace elements determined by their species in the mineral, as well as in its structural features. Molybdenite-2H from skarn associated with phyllosilicates occurs as ultrafine crystals with uniform shape and texture; no dislocations or inclusions were found but amorphous silica was. The molybdenite composition is highly contrasting in the content and distribution of both structure-related (Re, W, and Se) and other (Mn, Co, Ni, Cu, Zn, As, Ag, Cd, Sb, Te, Ag, Pd, Au, Hg, Pb, and Bi) metals. In the sulfide-poor quartz veins, highly structurally heterogeneous (2H + 3R) molybdenite microcrystals with abundant defects (dislocations and volumetrically distributed inclusions) are associated with illite, goethite, and barite. Some single crystals are unique three-phase (2H + 3R polytypes + amorphous MoS₂). The mineral has a low concentration of all trace elements, which are uniformly distributed. However, individual domains with uniquely high Pd, Te, Ni, Hg, and W concentrations caused by mineral inclusions are found in some grains. Molybdenite from quartz–feldspar alteration is characterized by low concentrations of all trace elements except for Re and Se, which enrich some domains of the grains. Our data indicate that the compositional and structural heterogeneity of molybdenite from the Bystrinsky deposit are its crucial features, which obviously correlate with the types of Mo mineralization.