2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

The typomorphic charactristics of zircons from Verhneurmijsky granite massif (Far East) in connection with ore potential

Machevariani М.М., Alekseev V.I., Marin Yu.B.

St. Petersburg State Mining University, St. Petersburg, Russia

wia59@mail.ru

Having used electron microscopy permits and microanalysis systems researched typomorphic features of the zircons from Verchneurmijsky granite massif in Far East. Crosscutting study of the zircon crystal morphology and ultimate composition to predict peculiar properties of mineragenesis conditions and afford opportunity to draw sharp disjunction of zircon features from different intrusive phases. Features of the zircons can be used at regional partition of intrusive formations and searches of rare-metal granites.

Granites of the Verhneurmijsky massif (located in the interfluve of the rivers Amur and Amgun, Russian Far East) belong to the myao-chanskiy complex, which is traditionally connected with stannane mineralization of the Komsomolskiy and Badjalskiy areas (Grigoryev, 1997). Bedrock exposures of the latest subalkaline Li-F granites were founded in the east exocontact. This exposures turn up controlling the location of the unique Pravourmiyskoye wolframium- stannum minefield (Brusnitsyn et al., 1993).

For the purpose of development of mineralogic-geochemical ore content criterions of the intrusive, we investigated typomorphic zircon features from rocks of both complexes – early biotite and subalkaline zinnwaldite granites. After the complex optical supervision we can summarize that yellow and light-brown prismatic zircons with fine oscillatory zoning are dominated in the biotite granites. Rare-metal zinnwaldite granites contain dark-brown subisometric coarse zoning zircon. Kristallmorphology analysis show contrast morphology difference between the zircons from biotite and zinnwaldite granites (fig. 1). Process of biotitic granites crystallization occurs in the low-hydrous and high temperature (850–900°С) conditions, wide range of zircon morphotypes (I.A. axis) testifies source melt heterogeneity. Formation of rare-metal granites concerned with fluid saturated melt at a 650–750°С temperature, but terminated in the hydrothermal conditions. Interruption of the vertical temperature zircon morphotype trend, apparently shows imposition of the mineralizes fluids (detached in the course of emanational differentiation) on the already crystallized rocks. Revealed practically uninterrupted morphotype “stock” (I.A. = 700, I.T. = 300–800°С) is typical for potentially ore-bearing granite of subalkaline-leucogranite formation (Brodskaya et al., 1986).

Fig. 1. The distribution of morphotypes of zircon in the diagram J. Pupin (Brodskaya et al., 1986) for biotite (a) and zinnwaldite (б) granites from Verhneurmijsky granite massif.

Electron microscopy (BSE, SE, CL) and X-ray spectroscopic analysis gave an option of zircon anatomy and elemental composition investigating. Low crystallization velocities and stable melt conditions bring about formation of homogeneous zircons with oscillatory zoning in biotitic granites. As a consequence of accelerated crystallization of Li-F granites porous skeletal-formed zircons with strained crystalline grid were generated. This zircons are notable for coarse zoning, heterometry cracks and abundant inclusions of “minerals-prisoners”. Reticulate texture of zircons from rare-metal granites could be caused by dissolution as a consequence of rapid change of crystal growth conditions.

Elemental composition of studies zircons show a steady isomorphic impurity of iron (0,4–1,0 wt.%), which should be regarded as their regional geochemical feature. The probable mechanism of the isomorphic substitution: Fe²⁺ + 3(Y,REE)³⁺ + P⁵⁺ = 3Zr⁴⁺ + Si⁴⁺; Fe²⁺ + 4(Y,REE)³⁺ + P⁵⁺ = 4Zr⁴⁺ + Si⁴⁺. At the same time, the composition of trace elements in this zircon are not constant and depends on the type of the source granite. Zircon from biotite granites – (Zr_0.97Fe_0.02U_0.01)Si_0.99O₄ – are usually devoid of impurities; zircons from zinnwaldite granites – (Zr_0.65 U_0.12Th_0,09Hf_0,04Fe_0.02)Si_0.99O₄ – characterized by elevated concentrations of trace elements: Hf, Th, U, REE, especially in the peripheral parts of the grains (Fig. 2). The most contrasting difference of zircons be founded in Hf content: Zr/Hf ratio in zircons from zinnwaldite granites and biotite granites are 19 and 58, respectively.

Fig. 2. The difference in elemental composition of the zircons from biotite (1) and zinnwaldite (2) granites from Verhneurmijsky granite massif.

Thus, in granites from Verhneurmijsky granite massif were fixed two types of zircon. The first type is significant for the biotite granites – it is typical magmatic zircons, specific for initial members of the rare-metal granitic series – granodiorites and normal granites. Properties of the second type zircons, identified during the study of zinnwaldite granites, allow us to link them with the crystallization of rare-metal lithium- fluoric melt occurring in the final stages of evolution of granitoid series. Hf-containing second type zircon formed in the fluid saturated melt, and at later stages of its growth was subjected to intense hydrothermal effects.

The complex of typomorphic zircon features from granitoids expands the possibilities of petrological correlations and identification of rare-metal granites of subalkaline-alkaline granites formations in the Amur region (Marin, 2004). A detailed investigation of revealed features will help to use them to assess the extent of potential ore content of the granites.

The study was supported by Ministry of Education and Science of Russian Federation, State Contract № 14.740.11.0192.

References:

Brodskaya R.L. Marina Е.Yu., Shnay G.К., Saminina I.А. Restoration of conditions and kinetics of formation of rare-metal granite formations based on the crystal morphology of accessory zircon / Zapiski VMO. 1986. № 1. P. 50–62 (in Russian).

Brusnitsyn А.I., Panova E.G., Smolenskiy V.V. Discovery of granites of lithium-fluoric geochemical type within the Verhneurmijskiy ore cluster// Izvestiya VUZov. Geology and exploration. 1993. № 6. P. 150–153 (in Russian).

Grigoryev S.I. Material composition features of late mesozoic granitoids from Komsomolskiy and Badjalskiy ore areas, their petrogenesis and links with mineralization // Regional geology and metallogeny. 1997. № 6. P. 103–115 (in Russian).

Marin Yu.B. Accessory minerals in granitoid series of stannane and molybdenum provinces // Zapiski RMO. 2004. № 6. P. 1–7 (in Russian).