Distribution and enrichment of rare metal elements in the basement rocks of South China: Controls on rare-metal mineralization

The southeastern part of the South China Block (SCB) is composed of the Nanling-Yunkai and Wuyi terranes and the Jiangnan Orogenic Belt (JOB), of which the Nanling and the JOB are two world-famous rare-metal metallogenic belts. Rare-metal deposits in both areas are closely associated with crustally-derived granites. However, rare-metal abundances and enrichment mechanisms in Precambrian basement rocks, which probably are sources of the ore-forming granites in the SCB, have not been well understood. Bulk compositions of more than 800 basement rock samples are used to unravel the abundances, variation and enrichment mechanism of the rare-metal elements in Precambrian basement rocks of the SCB and their genetic links with rare-metal mineralization. The basement meta-sedimentary rocks of the westernmost section of the JOB have the highest Sn (10.2 ppm), W (7.7 ppm), Be (3.46 ppm) and Cs (17.1 ppm) abundances, whereas those of the eastern JOB have low Sn (2.4-3.2 ppm), W (1.2-2.1 ppm), Nb (12.9-13.9 ppm) and Be (2.22-2.34 ppm), but relatively high Li (53.3-57.0 ppm) contents. The eastern Nanling basement rocks show high Sn (4.5 ppm), W (3.0 ppm) and U (3.67 ppm) relative to upper continental crust (UCC), similar to the western Nanling basements (4.2 ppm, 3.0 ppm and 4.03 ppm, respectively), but have significantly higher Rb/Sr, Li, Be, Cs and lower CaO/Na2O and Ba than the western Nanling basements, implying the presence of more metapelitic rocks in the eastern Nanling basements. The Wuyi terrane is characterized by low rare-metal abundances, e.g Sn (2.1 ppm), W (1.3 ppm), Cs (5.7 ppm) and Li (36.4 ppm). Rare-metal enrichment mechanisms are also distinct in the basement rocks of different terranes of the SCB. Four types of enrichment mechanism are identified: magmatic recycling, chemical weathering, source enrichment and alternative redox depositional environments. The enrichment of Sn, W, Cs and Be in the westernmost basement of the JOB is attributed to a higher proportion of old and multi-cycled materials and alternating depositional environments, and the enrichment of Sn, W, REE and U in the Nanling basement is ascribed to the higher proportion of recycled detritus and an enriched source. Chemical weathering (clay adsorption) is an important initial enrichment mechanism in the eastern JOB basement. Comparisons of rare-metal mineralization in the SCB with the basement compositions suggest that rare-metal mineralization of the granites is not only related to the rare-metal contents of the source, but also is closely related to the lithology of the source. Stronger W mineralization in the eastern JOB and eastern Nanling than in their western parts is probably related to their more pelite-rich basements.