『Abstract
The Bayan Obo mine in North China contains the largest rare-earth
element (REE) resources in the world. The mechanism of giant REE
enrichment in such a restricted region has been the focus of several
studies. Carbonatites are known to contain high concentrations
of REE. Three types of carbonatite dykes occur around the Bayan
Obo deposit, including dolomite, calcite and calcite-dolomite
carbonatite varieties. The contact relations show that the intrusion
of calcite carbonatite dykes post-date the dolomite dykes during
the process of evolution of the carbonatite magma. The geochemical
data show that the content of Sr and light (L) REE in these dykes
gradually increases from dolomite [(La/Yb)N
values range from 1.6 to 3.8], through calcite-dolomite [(La/Yb)N ranging from 51 to 57], to calcite type [(La/Yb)N ranging from 85 to 4617]. This evolutionary
trend suggests the crystal fractionation of the carbonatite magma,
which might have played a critical role for the giant REE accumulation
in the Bayan Obo region. The εNd(t) values
of the carbonatitic dykes show a tight cluster between -0.47 and
0.65, whereas the initial Sr isotope values show a broad range
from 0.703167 to 0.708871. The massive ore-hosting dolomite marbles
show comparable element content and Nd isotope composition, and
a Sm-Nd isochron age similar to that of the carbonatite dykes,
implying a close relationship during their magmatic origin. This
interpretation is also supported by the intrusive contact between
the ore-hosting dolomite marble and Mesoproterozoic Bayan Obo
group, as well as the presence of wall-rock xenoliths in the dolomite
marble.
Keywords: Carbonatite; REE deposit; Petrology; Geochemistry; Mesoproterozoic;
Bayan Obo; China』
1. Introduction
2. Geological setting
3. Sampling and field relation
3.1. Carbonatite dykes
3.2. Ore^hosting dolomite marble
4. Analytical methods
5. Analytical results
5.1. major elements
5.2. Trace element and REE
5.2.1. Carbonatite dykes
5.2.2. Ore-hosting dolomite marble
5.2.3. Sedimentary carbonate rocks
5.3. Sr-Nd isotope and Sm-Nd isochron age
5.3.1. Sr-Nd isotope
5.3.2. Sm-Nd isochron age
6. Discussion
7. Conclusions
Acknowledgements
References