MESOZOIC GRANITOID MAGMATISM OF THE DAURIAN DOME, EASTERN

TRANSBAIKALIA: PROBLEMS OF FORMATION CLASSIFICATION AND ORE POTENTIAL

Kozlov V.D.*, Spiridonov A.M.*, Bydnikov S.V.**, Ponomarchuk V.A.***

*Institute of Geochemistry SB RAS, Irkutsk, Russia, kvd@igc.irk.ru

**Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Moscow, Russia aprel@igem.ru

***Institute of Geology and Geophysics SB RAS, Novosibirsk, Russia, ponomoar@uiggm.nsc.ru

The Daurian (Khentei-Daurian) dome of the Eastern Transbaikalia is characterized by abundant granodiorite occurrences of the main intrusive phase (MPh) of batholith intrusions, subdivided into the Daurian (PZ₃) and Kyra (T-J₂) complexes. They are intruded by the subvolcanic granodiorite-granite intrusions of Sokhondo and leucogranites of the Kharalga complexes (J_2-3), associated with deep faults. The considered Bylyra-Khapcheranga region (being transversal to the general NE strike of the Daurian dome) contains industrial tin mineralization, consisting of cassiterite placers in Bylyra district, cassiterite-sulfide deposit in Khapcheranga district as well as gold-quartz deposits in Lubava district. Earlier petrologic geochemical investigations indicate that the most productive tin-sulfide mineralization in the region is related to sparse rare-metal leucogranite intrusions of the J_2-3 Kharalga complex which are enriched with granitophile rare including ore-forming elements (B, F, Li, Rb, Cs, Be, Sn, W, Mo, Ta, Pb, Th, U). Such intrusion is associated with Khapcheranga tin-sulfide deposit. Granite pegmatites and non-commercial greisen mineralization, associated with fissure bodies of leucogranites of additional intrusion phase (AIPh) of the Kyra complex were sources for the cassiterite placers in the Bylyra district (Kozlov, Svadkovskaya, 1977).

The Bylyra-Khapcheranga region includes large massifs of MPh granitoids (from SSE to SSW): Khamaro-Tyrin of the Kyra complex which intrudes T₁ schist (250-245 Ma), vast Levo-Bylyra massif of the Daurian complex on margins of the dome and large Bylyra massif of the Kyra complex in inner part of the dome, comprising the Bylyra placers (Geologic map 1:500 000 1997 by I.Y. Rutstain and N.N. Chaban in Russian). The Khamaro-Tyrin and Levo-Bylyra massifs comprise medium-grained porphyry-like amphibole-biotite granodiorites demonstrating large (1-3 mm) hornblende grains. The Bylyra massif involves coarse-grained porphyroblastic amphibole-biotite granodiorites-granites with large grains of quartz and porphyroblasts of potash feld spar. The petrochemical difference is insignificant: SiO₂ concentration varies within 66-68.5%; there is a gradual decrease in Na₂O and increase in K₂O concentrations from marginal Khamaro-Tyrin massif towards the inner part of the dome (Table 1). Granitoids of the MPh of all studied massifs demonstrate similar REE spectra that suggest their genetic similarity.

Table 1. Average petrochemical parameters and sum degree of depletion-enrichment by granitophile elements (IC) of granitoids of the Bylyra-Khapcheranga Region

Note: * - ## plutons by (Kozlov, Svadkovskaya, 1977, scheme, Fig. 2, p.18-19).

The geochronological ⁴⁰Ar/³⁹Ar dating from amphibole of typical granitoids of the Daurian and Kyra complexes (Levo-Bylyr and Bylyr massifs, two samples) show average age values for all four samples as 169-176± Ma (J₂). Being corrected the age of granitoids is estimated as 170-180 Ma (J_1-2).

The distribution of 12 granitophile volatile and rare elements in MPh granitoids is illustrated by the concentration index (CI). The index shows the sum degree of depletion (−) and enrichment (+) relative to the Clarke level of granitoids of the studied massifs. It is evident from the table that the MPh granodiorites of the periphery Khamaro-Tyrin massif demonstrated the lowest concentrations of the majority of granitophile elements (## 6, 6a), while MPh granites of the Arashentui site (#4b), including the tin placer district are marked by the maximum (+4.3 Clarke) concentrations. Sn (3,5 Clarke), Cs (2,4 Clarke) and U (1.8 Clarke) are most intensively concentrated. For comparison we give CI values in AIPh granites of the massif 20a (+6.4) which is associated with non-commercial greisen Sn deposit, and CI of leucogranites of the Khapcheranga stock (+17, Khapcheranga deposit).

On the whole, change of CI values, considering petrochemical data and age dating indicate the evolutionary development of MPh granitoids, as a united intrusive system. The concentrations of granitophile including ore-forming elements increase from marginal massifs of main phase towards the inner area of the system which is characterized by the greatest thickness (Menaker, 1972) that could provide a more intensive magmatic differentiation and formation of the vast granite dome of the main phase enriched by rare elements. That resulted in pegmatite and pneumatolytic tin mineralization. Thus, massifs of the Daurian and Kyra complexes can be regarded as subphase of the main intrusive phase (MPh₁ and MPh₂).

References

Geological Map of Chita Region, 1:500 000. Ed. By I.G. Rutshtein, N.N. Chaban, Chita: GGUP “Chita Geological Survey”, 1997.

Kozlov V.D., Svadkovskaya L.N. Petrochemistry, geochemistry and ore-bearing of granitoids of Central Transbaikalia. Novosibirsk: Nauka, 1977. 252p.

Menaker G.I. Structure of the Earth crust and conformity of ore deposits placed in Central and East 1972 Transbaikalia // Geology of ore deposits, 1972, N. 6, P.3-16.