『Abstract
Mineralization with exchangeable rare earth element (REE) and
yttrium (MEX-REY) has been recognized in the weathering profiles
in South China since the early 1970's. This type of REY mineralization
occurs in weathering profiles of parent rocks ranging in composition
from granite to acidic volcanic rocks and lamprophyre. The majority
of the known resources occurring in the weathering profiles of
granitic rocks. Total resources of this type of REY amount to
millions tons of rare earth oxides, and therefore represent one
of the most important types of rare earth resources in China,
particularly for heavy rare earth elements (HREE) and yttrium.
Accumulation of REY in the weathering profiles of granitic rocks
is strongly controlled by the resistance to weathering of the
principal REY-bearing accessory minerals in the parent rocks;
only a limited proportion of total REY (<30%) is incorporated
in the rock-forming minerals. MEX-REY more commonly occur in weathering
profiles developed on granitic rocks within which most of the
REY are incorporated in accessory minerals weakly resistant to
weathering (doverite, parisite, etc.). For the well-developed
weathering profiles, three horizons can be distinguished from
surface downwards: the lateritic horizon (A), the weathered horizon
(B), and the weathering front (C). Continuous leaching, coupled
with low rate of denudation, results in the accumulation of REY
in the subsurface horizons (the B and C horizons), and thus results
in REE differentiation within the well-developed, layered, and
mature weathering profiles. Exchangeable REY, which can be replaced
by cations like NH4+ and Na+ etc. in electrolyte
solutions and can be removed by complexing agents such as EDTA,
are commonly the major form of REE occurrence in the B horizon.
Cerium is enriched in the top layer (A horizon) and depleted in
the subsurface horizons of the weathering profiles, most likely
due to the oxidation of Ce(III) to Ce(IV) followed by cerianite
formation or absorption onto clays and/or Fe and Al oxyhydroxides.
Keywords: Weathering profiles; Ion-exchangeable rare earth element
mineralization; Granitic rocks; South China』
1. Introduction
2. Main geological characteristics of the parent rocks
2.1. Xinxiu granite
2.2. Heling porphyry
2.3. Huashan and Guposhan granites
2.4. Accessory mineral controls on REE enrichment in the weathering
profiles
3. REE distribution patterns
3.1. REE accumulation and differentiation in the weathering
profiles
4. Discussion
4.1. Factors controlling the differentiation of REEs in the
weathering profile
4.2. Cerium anomalies in the weathering crusts
5. Summary
Acknowledgements
Appendix A. Methods and procedures for REE analysis
References