SOURCES OF CENTRAL CHUKOTKA RARE METAL GRANITOID METALLOGENIC

SPECIALIZATION

Efremov S.V.

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

The suggested report is dedicated to the main problem of rare metal granitoid genesis, i.e. identification of rare element source. The idea formulated in the late XX century and postulating that metasedimentary rocks of the upper continental crust are the source of both rare elements and rare metal granitoids themselves (Kovalenko, 1977; Tauson, 1977; Foster, 1999). However, It became evident that concentrations of rare elements in metasedimentary rocks were not high enough for rare metal granitoid formation with the accumulation of the fact material (Schuiling, 1967; Kozlov 1985). The new hypotheses about the rare metal granitoid relation to geochemically specialized sources in the continental crust (Schuiling, 1967) and Earth’s mantle (Rub, 1982; Troshin, 1983; Shcheglov, 1987) appeared in this respect.

One of the most productive ideas was suggested by Yu.P. Troshin who associated rare metal granitoids of the East Transbaikalia with geochemically specialized mantle basalt magmatism of the same age. The idea was supported in studying the Central Chukotka granitoids. As a result, the genetic link of rare metal granites with partial potassium and ultrapotassium magmas was proved. The results obtained allowed to consider the source of basaltoid magmas (metasomatically transformed lithospheric mantle) as the main source of rare elements, and magmas themselves – as the conductors of this specialization (Dudkinsky, 1993; Efremov, 2007). The present paper is the continuation of these papers and dedicated to contribution estimate of mantle and crust components to balance of rare metal granitoid rare elements.

Further, the sources of rare metal granitoid substance are implied as sources of metallogenic specialization. This methodical procedure can be applied to granitoids. The genetic link with the accompanying rare metal mineralization was proved was proved for them. Tin-bearing granitoids of Central Chukotka are just such granitoids.

Geological formations of the Early Cretaceous postorogenic volcano-plutonic association were chosen as objects of study by us. The volcanic covers, dyke belts and granitoid intrusions composed by rocks of potassium (shoshonite-latite) and ultrapotassium series are included into this association. The greatest attention is paid to granitoid intrusions, where a gradual transition from partial mantle magmas to rare metal granitoids is realized (Efremov, 1996; Efremov, 2007). The reasons of these variations are of particular interest and have the key alue in understanding the processes that result in formation of rare metal granites.

It was believed in initial stages of the study that these variations were caused by processes of of primary mantle magma differentiation (Efremov, 1996). However, the subsequent papers showed that the possibility of rare metal granitoid formation as a result of primary magma contamination by continental crust matter was not less likely (Efremov, 2007).

To solve this dilemma, we used the isotope data (Sr, Nd, Pb, O) that allowed to discriminate more reliably the processes of mixture and crystallizational differentiation. It is caused by the fact that isotopes do not fractionate in chemical reactions, and most geochemical reservoirs have rather contrast “isotope marks”.

The obtained results demonstrated that various extent of primary ultrapotassium magma hybridization by acid crust melt, with the crust component ranging from 40 to 70%. From this, there are god reasons for us to consider that both continental crust of the region and geochemically specialized dominant within the mantle part of continental lithosphere were the sources of the Chukotka rare metal granitoid specialization.

Taking into consideration that melts are the mixing components, we can evaluate of this or that source contribution to balance of rare elements. The crust and mantle source contribution to balance of rare elements calculated for various proportions of the mixing components is presented in Table 1. We see that shares of crust component, its contribution to balance of rare elements rarely exceeds 50%. It is most brightly observed for Sn, main profiling element of the Chukotka ore province. The Part of mantle component exceeds 75% in its balance.

The obtained results show quite convincingly that rare metal granitoids have a complex source of metallogenic specialization. In addition, the geochemically specialized dominant in the mantle part of continental lithosphere.

Table 1.

Part of crust component in mixture	Part of crust component in balance of rare elements
	Rb	Li	Cs	F	Sn	U	Th	W	Be
70 %	54	67	44	29	24	32	54	32	35
50 %	34	47	25	15	12	17	33	17	18
40 %	25	37	18	10	8	12	25	12	13

References

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Efremov S.V., Kozlov V.D. (2007) Ultrapotassium basites of Central Chukotka and their role in understanding the genesis of tin-bearing granitoids // Geology and Geophysics. V.48. N2. P.283-286. (in Russian)

Efremov S.V., Dudkinsky D.V., Kozlov V.D. (1996) New data on the West Chukotka rare metal plumasite granites origin // Doklady of RAS. V.349. N5. P.664-665. (in Russian)

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