Mineralogical and geochemical characteristics of scheelite from the Vostok-2 au-bi-cu-w skarn deposit (Primorsky krai)

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Abstract

The scheelite (CaWO4) is main ore mineral from the Vostok-2 reduced skarn type deposit, which located in Primorie region (Russia) and connected with the Sikhote-Alin central fault. By mineral composition and geochemical characteristics ore rocks were divided into two most common types: skarns and quartz-veins. This study presented results of complex (mineragraphy, cathodoluminescence (CL), electron-probe microanalysis (EPMA), laser-ablation inductively-connected plasma mass-spectrometry (LA-ICP-MS)) researching of scheelite. These parameters for scheelite like inner structure, illumination character in CL and UV, contents of micro- and macroimpurities, Eu/Eu* value, REE spectral shape are crucial indicators of the conditions of mineralization. These signs allowed to identify different mechanism of REE entering in scheelite from skarns and quartz-veins (3Ca2+ ↔ 2REE3+ + □ and Ca2+ + W6+ ↔ REE3+ + Nb5+ where □ is vacancy in the Ca site). By distribution specific of REE three types of scheelite were identified and their temporal relationships were established. Because scheelite inherits REE characteristics from mineral-forming environment some conclusions were shown: evolution of the ore-forming fluid, pulsation nature of the substance’s intake and its single source, and reductive conditions of mineralization were proved for the deposit as a whole.

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About the authors

A. E. Keshikov

VS Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences

Author for correspondence.
Email: keshikovae@igm.nsc.ru
Russian Federation, Novosibirsk

P. A. Nevolko

VS Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences

Email: keshikovae@igm.nsc.ru
Russian Federation, Novosibirsk

D. V. Bondarchuk

LLC “Nornickel Technical Services”

Email: keshikovae@igm.nsc.ru
Russian Federation, Saint Petersburg

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2. Fig. 1. Geographical location of the Vostok-2 deposit (after Khanchuk et al., 1997; Soloviev et al., 2017). 1 – Paleozoic and Mesozoic accreted terranes; 2 – Cretaceous-Paleogene Eastern Sikhote-Alin volcanic belt; 3 – Early-Late Cretaceous granite intrusions; 4 – faults (I – Central Sikhote-Alin; II – Eastern Sikhote-Alin); 5 – tungsten deposits and ore occurrences; 6 – tin-tungsten ore occurrences; 7 – rare metal tin-tungsten deposits and ore occurrences; 8 – gold deposits and ore occurrences.

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3. Fig. 2. Position of Cretaceous-Paleogene volcanogenic and plutonogenic formations within the eastern Sikhote-Alin volcanic belt (after Gladkov et al., 1988; Rub et al., 1982; Soloviev et al., 20171; etc.). 1 - Paleogene andesites, basaltic andesites and basalts; 2 - Upper Cretaceous andesites; 3 - Upper Cretaceous andesitic tuffs, lava breccias and agglomerates; 4 - Lower Cretaceous tuffs, tuffites and tuffaceous siltstones; 5 - Lower Cretaceous tuff conglomerates and gravelites; 6 - Triassic-Lower Cretaceous cherts; 7 - Middle Jurassic-Lower Cretaceous carbonate-siliceous sandstones; 8 – large lenses of limestones and dolomites in Middle Jurassic-Lower Cretaceous deposits; 9 – Late Cretaceous granite, granite-porphyry, diorite and dolerite dikes; 10 – Late Cretaceous granite plutons; 11 – Early-Late Cretaceous granodiorite stocks; 12 – Early-Late Cretaceous granite stocks; 13 – Early Cretaceous monzonite-granodiorite-granite pluton Dalnensky; 14 – faults; 15 – structural unconformity between Triassic-Lower Jurassic and Middle Jurassic-Lower Cretaceous deposits.

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4. Fig. 3. Scheelite-sulfide ores of the Vostok-2 deposit. a, b – limestone skarn with scheelite-sulfide mineralization in daylight (a) and ultraviolet (b) light; c, d – quartz vein with scheelite-chalcopyrite-pyrrhotite mineral association in daylight (c) and ultraviolet (d) light; d, e – quartz vein with arsenopyrite and rare scheelite grains in daylight (d) and ultraviolet (e) light.

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5. Fig. 4. Scheelite-sulfide skarn and mineralized quartz veins of the Vostok-2 deposit, micrographs. a – regular arrangement of rare silver-bearing minerals in ores of the first type; b – position of scheelite in the massive texture of the first ore type; c – structures of solid solution decomposition and the relationship of sulfides in ores of the second type; d – veinlet-disseminated texture and the relationship of ore minerals in ores of the second type. Mineral designations: Apy – arsenopyrite, Bi – native bismuth, Ccp – chalcopyrite, Dys – dyscrasite, Gn – galena, Hes – hessite, Pyh – pyrrhotite, Sch – scheelite, Stn – stannite, Sp – sphalerite, Ttr – tetrahedrite.

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6. Fig. 5. Heterogeneous internal structure of scheelite from the Vostok-2 deposit, CL images. The dotted line indicates the boundaries of zones, Roman numerals in circles indicate zone numbers.

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7. Fig. 6. CL images of scheelite grains with marked LA-ICP-MS analysis points. The color indicates the scheelite type by the nature of the Eu anomaly in the rare earth spectra: green – negative, blue – absent or weakly expressed, red – positive. The numbers in circles are analysis points (Electronic Appendix 2). a, b – scheelite from calcareous skarns, c–e – scheelite from quartz sulphide veins.

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8. Fig. 7. Chondrite-normalized (Sun, McDonough, 1989) rare-earth spectra of scheelite of the first (a, b), second (c, d) and third (d, e) types. a, c, d – scheelite from calcareous skarns; b, d, e – scheelite from quartz sulfide veins.

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9. Fig. 8. Correlation diagram ΣREE-Nb for scheelite of the Vostok-2 deposit, demonstrating the mechanisms of rare earth element incorporation. The colors indicate the values ​​corresponding to different types of scheelite, distinguished by the nature of the rare earth spectra and Eu anomaly: green - the first type, blue - the second type, red - the third type. Circles indicate values ​​for scheelites from skarns, squares - for scheelites from quartz veins.

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10. Fig. 9. Correlation diagrams of impurity components of scheelite from ores of the Vostok-2 deposit, demonstrating the absence of mixing of fluid from different sources (Liu et al., 2019): a – Ho-Y diagram, b – Y/Ho-Eu/Eu* diagram.

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11. Fig. 10. Correlation diagrams of impurity components of scheelite from ores of the Vostok-2 deposit, demonstrating the fractionation of rare earth elements: a – LREE/TREE-Eu/Eu*, b – LREE/TREE-Y.

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12. Fig. 11. REE fractionation and change in the Eu-anomaly value: a – diagram of REE – Eu/Eu* ratio; b – SRZE diagram – Eu/Eu* ratio; c – HREE diagram – Eu/Eu* ratio.

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13. Fig. 12. Change in the Eu/Eu* value in scheelite from the ores of the Vostok-2 deposit: a – Eu-Eu/Eu* diagram, b – Y + ∑REE-Eu/Eu* diagram.

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