Fukalite: an example of OD structure with two-dimensional disorder

The real crystal structure of fukalite, Ca4Si2O 6(OH)2(CO3), was solved by means of the application of order-disorder (OD) theory and was refined through synchrotron radiation diffraction data from a single crystal. The examined sample came from the Gumeshevsk skarn copper porphyry deposit in the Central Urals, Russia. The selected crystal displays diffraction patterns characterized by strong reflections, which pointed to an orthorhombic sub-structure (the "family structure" in the OD terminology), and additional weaker reflections that correspond to a monoclinic real structure. The refined cell parameters are a = 7.573(3), b = 23.364(5), c = 11.544(4) Å, β = 109.15(1)°, space group P21/c. This unit cell corresponds to one of the six possible maximum degree of order (MDO) polytypes, as obtained by applying the OD procedure. The derivation of the six MDO polytypes is presented in the Appendix1. The intensity data were collected at the Elettra synchrotron facility (Trieste, Italy); the structure refinement converged to R = 0.0342 for 1848 reflections with I > 2σ(I) and 0.0352 for all 1958 data. The structure of fukalite may be described as formed by distinct structural modules: a calcium polyhedral framework, formed by tobermorite-type polyhedral layers alternating along b with tilleyitetype zigzag polyhedral layers; silicate chains with repeat every fifth tetrahedron, running along a and linked to the calcium polyhedral layers on opposite sides; and finally rows of CO3 groups parallel to (100) and stacked along a

Biostratigraphy versus isotope geochronology: Testing the Urals island arc model

Formation of the Urals volcanic-hosted massive sulphide (VHMS) deposits is considered to be related with the intra-oceanic stage of island arc(s) development in the Upper Ordovician–Middle Devonian based on the biostratigraphic record of ore-hosting sedimentary rocks. However, the direct Re-Os dating of four known VHMS systems in the Urals gives significantly younger Re-Os isochron ages ranging from 355 ± 15 Ma up to 366 ± 2 Ma. To address this discrepancy, we performed SHRIMP U-Pb dating on zircons extracted from rhyodacites (Eifelian biostratigraphic age of 393–388 Ma) from the footwall of the Alexandrinka VHMS deposit which has a Re-Os isochron age of sulphides of 355 ± 15 Ma. New 206Pb/238U mean age of 374 ± 3 Ma (MSWD = 1.4 and probability = 0.11) is considered to be the crystallisation age of the host volcanic rock. This age is ca. 15 Ma younger than the Eifelian (393–388 Ma) biostratigraphic age and overlaps the Frasnian–Famennian boundary (372 ± 2 Ma), characterised by the final stages of Magnitogorsk Arc – East European continent collision. Such an inconsistency with geochronological age may be due to a reburial of conodonts during resedimentation as a result of erosion of older rocks in younger sedimentary sequences

Rhenium distribution in molybdenite from the Vosnesensk porphyry Cu ± (Mo,Au) deposit (Southern Urals, Russia)

Molybdenite from the Voznesensk porphyry Cu ± (Mo,Au) deposit, southern Urals, Russia, displays high Re content and a well-documented oscillatory zoning for this mineral. The molybdenite forms part of a quartz-molybdenite-pyrite-chalcopyrite assemblage cemented at low-temperatures by pumpellyite and tobermorite. Oscillatory zoning occurs as micro-bands up to 200 μm wide and 200-600 μm long oriented parallel to the basal cleavage, as well as within basal planes of molybdenite sheets. The micro-bands are Re-enriched to different degrees (0.3-1.0 wt.%, typically they contain 0.6-0.8 wt.% Re). Outside these structures the Re content in molybdenite is up to 0.25 wt.%, usually <0.10-0.15 wt.%. It is suggested that the Re in the Voznesensk molybdenite retains its original growth pattern, marked by sharp concentration variations, and is not the result of leaching through post-crystallization diffusion. Variations in the number, width, and Re-content in individual micro-zones observed within the oscillatory zones may suggest involvement of extrinsic mechanisms of micro-zone formation. However, the Re content of the Voznesensk molybdenite (ICP-MS and EPMA data) is almost uniform along the studied vertical and lateral interval of the mineralization, suggesting no major variation in Re concentration in the fluid and favoring uniform, selforganization processes causing Re enrichment and oscillatory zoning in molybdenite. Despite the strong degree of epigenetic post-crystallization deformation of the molybdenite flakes and their cementation by low-temperature tobermorite, there is no essential change of the primary micro-zoning in the distribution of Re and any evidence of its epigenetic migration

Trace-element geochemistry of molybdenite from porphyry Cu deposits of the Birgilda-Tomino ore cluster (South Urals, Russia)

Mineralogical, electron microprobe analysis and laser ablation-inductively coupled plasma-mass spectrometry data from molybdenite within two porphyry copper deposits (Kalinovskoe and Birgilda) of the Birgilda-Tomino ore cluster (South Urals) are presented.† The results provide evidence that molybdenites from these two sites have similar trace-element chemistry. Most trace elements (Si, Fe, Co, Cu, Zn, Ag, Sb, Te, Pb, Bi, Au, As and Se) form mineral inclusions within molybdenite. The Re contents in molybdenite vary from 8.7 ppm to 1.13 wt.%. The Re distribution within single molybdenite flakes is always extremely heterogeneous. It is argued that a temperature decrease favours the formation of Re-rich molybdenite. The high Re content of molybdenite observed points to a mantle-derived source.© The Mineralogical Society 2018. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. The attached file is the published version of the article

Systematization of u-pb zircon ages of granitoids from the copper porphyry deposits on the urals

There is a generalization of U-Pb age of zircons from the copper-porphyry deposits of the eastern slope of the Urals. Approved reserves of the largest ones are about 1.4-1.8 Mt of Cu (at an average content of 0.4-0.6 wt % of Cu). Porphyry mineralization is confined to the small massifs of quartz-diorite composition, localized exclusively within sub-meridional volcanic areas of island-type separated by sialic zones. U-Pb ages were determined by LA ICP-MS, Goethe University Frankfurt (Germany), by SHRIMP-II, VSEGEI (St.Petersburg, Russia) and by SHRIMP-IIe/mc, IBERSIMS, Granada University (Spain). In the South Urals lateral section from east to west (approximately 160 km) the age of some quartz diorite porphyry deposits decreased from D1-2 (390 and 380 Ma, the Gumeshky and small Voznesensk deposits in Tagilo-Magnitogorsk Megazone) to D2-C11 (362 and 356 Ma, major Mikheyevsk deposit Tarutinsk deposit in the eastern part of the East-Ural volcanic megazone) and C12 (336 and 335 Ma, Benkalinsk, Zhaltyrkol’sk deposits in Valeryanovka zone). In addition, in the western part of the East-Ural volcanic megazone (in Uvelka allochthonous tectonic structure) there are S1-2 ore-bearing porphyry quartz-diorite massifs. They include the large industrial Tomino-Bereznyaki ore cluster with epithermal and porphyry mineralization (427-429 Ma) and of Zelenodolsk porphyric deposit (418 Ma) located at the distance of 25 km to the South. In indicated direction, ore specificity also changеs: Cu-(Au)- and Au-Cu-porphyric deposits are replaced by Cu-(Au, Mo)-porphyric ones. Within the Magnitogorsk zone from the early- to the late island-arc stage, the age of ore-bearing granitoids decreases (390, 381, 374 and 362 Ma), at that time their composition changes from diorite to shoshonite. Isotopic and petrogeochemical data suggest that considered island-type diorite is perhaps the result of selective melting of metabasalts of low crust or of depleted mantle (mantle wedge). This melting occurred repeatedly according to the displacement in time of its source from the west to the east of the Urals