Multiple-staged granite evolution and TaNb mineralization in South China

Abstract

The Mesozoic post-orogenic granites in South China are widespread. Hundreds of tungsten and tin mineral deposits are closely associated with these granites. However, the number of TaNb deposits including those of the granite-type and the pegmatite-type, are relatively less. On the basis of geology, petrology, geochemistry and mineralization data from 8 ore deposits and related granites, we suggest that the TaNb mineralized granites are the special products of well-evolved granite magmas. The most important W and Sn deposits are clustered in post-Caledonian uplift and adjacent Hercynian-Indosinian depression of the South China orogenic belt. Most of the TaNb mineralizations are found within 20 km from the boundary faults surrounding the South Jiangxi post-Caledonian uplift. The paragenetic features of rare metal elements show that TaNb are accompanied by W in the uplift region, and by Sn in the depression region. The general intrusive sequence of a rare metal-bearing granite complex is: rare metal barren (porphyrytic) biotite granite—W and/or Sn ore-forming granite—TaNb (Sn) mineralized granite. The geological and geochemical data from eight mineralization districts indicate that the TaNb mineralizations are always developed in the last stage of a multiple-stage granite evolution. Albite-rich granite is the most common rock type of the TaNb ore-bearing granite, while the maximum albite contents in different deposits vary from more than 60% to less than 30%. Quartz with “snow ball” structure, topaz, and Li-micas (lepidolite, zinnwaldite, Li-muscovite and protolithionite) exist as common typomorphic minerals. The typical TaNb host are Mn-rich columbite-Tantalite and sometimes microlite and Ta-cassiterite. The pegmatoid crust (stockscheider) can be used as one of the most distinctive indicators for the degree of rare metal-tearing granite evolution based on its thickness and zonation. Compared with the normal granites, the TaNb mineralized granites have very special geochemical compositions. The TaNb mineralized granites are characterized by high Na 2O, Na 2O/K 2O ratios (>1 wt%), Al 2O 3, Li and F and very low TiO 2, MnO, MgO and CaO. Most typically, nearly all the TaNb mineralized granites bear very low total REE contents (less than 50 ppm), low Sr and Zr (both usually less than 30 ppm) and very high Rb (more than 700 to 3000 ppm). Geochemical features, excluding the contribution of post-magmatic alteration, change gradually from the earlier to the later phases in a granite complex, showing some trends of magmatic evolution. For example, two good negative exponential correlation curves on the RbSr diagrams (or a correlogram for other elements) of Yichun ad Dengfuxian multiple-stage granite plutons indicate a fractional crystallization mechanism which might have been an effective way to enrich the rare metal elements. The granite-type TaNb deposits, where Ta is economically more important than Nb, will be the only TaNb mineralization type discussed in this paper.