Allanite as a mineral-indicator of the tin-bearing granites in Badzhalsky area (Priamurye)

With the help of complex electronic and optical microscopy with various resolution and microanalysis systems in granites from Badzhalsky area (Russian Far East) revealed two types of allanite: allanite-(Ce) and allanite-(Y), specific to the biotitic and zinnwaldite granites respectively. Typomorphic features of allanite in granites from Badzhalsky area were investigated and gave advanced capabilities of the regional division of the granitoids and search of rare-metal granites of subalkaline-alkaline granite formation in the Amur region.

The study of accessory mineralization of granitoid series, terminating in the formation of rare-metal granites, allowed to establish the existence of two types of ore-bearing rocks: “magnetite granites” from the molybdenum provinces have relatively high alkalinity, and low-alkalinity “ilmenite granites” from stannane provinces (Ishihara, 1977). Granites of these types are also known as orthite and monazite granites (Marin, 2004). However, we shuld acknowledge that deviations from this classification exists. Thus, the biotite granites from Verkhneurmiyskiy massif, which controls the location of the cognominal wolframium-stannum ore cluster in the Far East, contains allanite (orthite) as one of the main accessory minerals. It stands to mention that this ore cluster includes one of the stannum supergiant – mainfield Pravourmiyskoye (Geodynamics..., 2006). There is a need to clarify typomorphic properties of allanite in granitoids from Badzhalsky area.

In the biotite granites allanite characterized by high idiomorphism, prismatic habit and thick brown color, reaching a length of 1–2 mm. Its association with biotite is distinctive. Optical properties show absence of metamict traces. Electron microprobe analysis revealed that the studied mineral is allanite-(Ce) – (Ce_1,01La_0,55Nd_0,33Th_0,01)_1,9(Al_0,6Fe_0,78)_1,38Si_4,02O₁₂OH. Its elemental composition shows presence of such trace elements as Th, Ti, Nd, La (table). Allanite from the biotite granites is characterized by compositional zoning: there is reduction in Th content and increased in Ce, La content from core to the edge of the crystal.

Table. Representative chemical compositions of allanite in granites from Badzhalsky area

Component	Biotite granite				Zinnwaldite granite
Al₂O₃	13.74	13.03	13.08	13.49	6.49	–	–	–
SiO₂	20.49	19.90	20.17	21.19	16.89	17.38	17.24	18.27
CaO	8.23	7.55	8.16	11.22	14.02	–	–	2.03
TiO₂	–	–	–	6.41	–	–	–	–
MnO	–	3.27	–	–	4.22	–	–	–
FeO	14.28	13.81	18.30	13.13	1.88	11.51	11.49	7.16
Y₂O₃	–	–	–	–	27.47	26.38	24.49	24.20
La₂O₃	11.21	10.76	7.61	10.50	–	–	–	–
Ce₂O₃	18.03	19.03	15.19	14.01	4.4	8.63	8.56	7.37
Nd₂O₃	8.75	8.37	7.51	10.06	–	9.84	10.49	7.73
Sm₂O₃	–	–	–	–	–	–	8.58	5.92
Gd₂O₃					5.11	8.21	8.44	6.71
Dy2O₃	–	–	–	–	6.99	9.60	10.73	8.38
Er₂O₃	–	–	–	–	5.85	8.44	–	6.37
Yb₂O₃	–	–	–	–	6.67	–	–	5.82
ThO₂	5.27	4.27	9.98	–	–	–	–	–
Note. Space – element is not set

An interesting discovery was made in the subalkaline zinnwaldite granites from Pravourmiyskoye ore field, for which stannane mineralization genetically related to (Brusnitsyn et al., 1993). In this granites, as far as in the earlier biotite granites, allanite was identified. This type of allanite is distinct in a very small size (10–50 mm), sharp xenomorphism and sophisticated skeletal crystal forms. Moreover it is associated with feldspars from host granite. Discovered mineral is allanite-(Y): (Ca_1,85Y_1,83Dy_0,13Yb_0,11Er_0,08)_2,15(Al_0,63Fe_0,08)_0,71Si_3,26O₁₂OH. Its chemical composition shows isomorphic impurities of HREE, having zoning distribution in crystals (table). It is different from allanite in biotite granites by low Si, Al, Fe content and the absence of Th impurities (fig.).

Thereby, in granites from Badzhalsky area were revealed two types of allanite: allanite-(Ce) and allanite-(Y), specific to the biotite and zinnwaldite granites respectively. Established a certain contradiction between the widespread presence of allanite and host granites stanniferous "monazite" granitoids type belonging. Allanite-(Y) was firstly described in zinnwaldite granites. It reflects the specific REE-specialization of the source melt.

Figure. The allanite chemical composition differences in biotite (1) and zinnwaldite (2) granites from Badzhalsky area.

Typomorphic features of allanite in granites from Badzhalsky area were investigated and gave advanced capabilities of the regional division of the granitoids and search of rare-metal granites of subalkaline-alkaline granite formation in the Amur region (Marin, 2004).

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

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).

Geodynamics, magmatism and metallogeny of the Russian East: in 2 books. 2 books / ed. by A.I. Khanchuk. Vladivostok: Dalnauka, 2006. Book 2. P. 573–981 (in Russian).

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

Ishiara S. The magnetite-series and ilmenite-series granitic rocks // Miner. and Geol. 1977. Vol.27. P. 293–306.