Formation of a hydrothermal kaolinite deposit from rhyolitic tuff in Jiangxi, China

The Longmen kaolinite deposit is one of the largest hydrothermal clay deposits of Ganxi volcanic basin (northern Wuyi Mountain area, China). The pristine host rocks are rhyolitic crystal-vitric tuff and minor lapilli tuff from the Late Jurassic Ehuling Formation. The ore consists of kaolin-group minerals (kaolinite, dickite), pyrophyllite with minor quartz, sericite, pyrite, etc.. From the host rocks to the transition zones (altered rocks) then to the vein ores, contents of SiO2 and TFe2O3 decrease, whereas Al2O3 and LOI increase, consistent with the contents increase of kaolin minerals and pyrophyllite in the samples. The total REE abundances of the ores are much lower than that of the host and altered rocks, Rb, Nb, Nd, Zr, Ti and Y are significantly depleted. Apparent zoning features of bulk geochemistry and mineral component reflect that the kaolinite deposit occurred at the expense of the host rock by ascending hydrothermal fluids with distinct removal of SiO2, TFe2O3, Na2O, K2O. According to the mineral assemblage, the formation temperature of this deposit falls within the range of 270-350 A degrees C. With regard to the industrial applications, the kaolinized ores are suitable for use in ceramics and gemologic materials crafted for seal stones. Moreover, in mineralogical terms, this deposit is also proved to be an excellent example for studying channeled hydrothermal alterations of rhyolitic tuff.