The Behavior of Rare Earth Elements During the Alteration of the Siberian Platform Basalts

Alexander G. Polozov Institute of Geochemistry SB RAS, P.O.Box 4019 Irkutsk, 664033, Russia

Alexander E. Vorontsov Institute of Geochemistry SB RAS, P.O.Box 4019 Irkutsk, 664033, Russia

In the given contribution the attention is focused on the metasomatic transformations of basic rocks, resulting from the influence of hydrothermal solutions, genetically associated with brines of sedimentary covers of the Siberian platform. These rock are found both in iron ore deposits of Angara-Ilim type (an.3, 4), and in sedimentary cover of platform.

Metasomatic transformations of basalt are expressed in formation of the next main guest minerals: garnet (R-1386),
hornblende (Ya-3660), feldspar and calcite (Ya-6953, Ya-6957), that are reflected in the supply of Mg, Ca, and the removal of the most major elements, Ba and Sr in samples R-1386, Ya-3660, or in the supply of K, CO2, Rb, Ba, Sr and the removal of some major and trace elements (Ya-6953, Ya-6957).

Feldspar and calcite formations are responsible for both the reduction of SREE, and the its increase, that depends on the composition of veinlets and associated metasomatic zones, while for such close to sulfate veins the reduction of SREE is as a whole characteristically. The La/Yb value is increased in metasomatic rocks nearly in all cases, while Eu/Eu* and Ce/Ce* (the redox condition indicators of metasomatic and hydrothermal solutions) are decrease together, or decrease (Eu/Eu*) and increase (Ce/Ce*), and reflect the inversion of these parameters in the veins.

The work was supported by the Russian Foundation for Basic Research, grant 94-05-17362-a.

Table 1.

NN Samples REE+Y La/Yb Eu/Eu* Ce/Ce*

1 Ya-6953/5 128.74 4.8 0.71 0.69

.2 Ya-6953/1 92.14 8.2 0.59 0.97

3 R-1386B 105.00 3.9 0.60 0.87

4 R-1386A 163.05 4.8 0.99 1.04

5 Ya-6957/2 120.90 3.9 0.86 0.87

6 Ya-6957/1 146.46 5.7 0.66 0.67

7 Ya-6957 298.30 5.5 0.34 0.73

8 Ya-3660/3 58.43 3.6 1.16 0.85

9 Ya-3660/1 61.35 6.5 0.82 1.06

10 Ya-3660 26.22 14.0 1.04 0.81

Note: the analyses 1,3,5,8 - basalts; 2,4,6,9 - metasomatic rocks; veins with sulfides (an.7) and with sulfates (an.10).

Figs. 1-4: REE normalized patterns of the alterated basic rocks of the Siberian platform.