Global Occurrence, Geology and Characteristics of Hydrothermal-Origin Kaolin Deposits

Abstract

Kaolin-group minerals occur in nature as the result of high-sulfidation acid sulfate, sulfur-poor HCl-, HF- and H2CO3-rich acidic fluid-related hydrothermal alterations and in situ geochemical weathering. These minerals possess different crystallographic and chemical properties that determine their application areas, mainly in the ceramic and paper industries, and as nanocomposite materials. The physicochemical properties of hydrothermal kaolin deposits are the result of the type of parent rock, the effect of the regional tectonism-associated magmatism, and the chemical features of hydrothermal fluids that interact with the deep basement rocks. However, understanding these geothermal systems is one of the most challenging issues due to the rich mineralogical assemblages, complex geochemistry and isotopic data of hydrothermal alteration zones. This study evaluates the formation of hydrothermal-origin kaolin-group minerals by considering their characteristics of hydrothermal alteration, isotopic compositions and differences in characteristic properties of low- and high-sulfidation occurrences; this paper also addresses mineralogical and structural differences between hypogene and supergene kaolin formations, and kaolin–alunite–pyrophyllite association, and it provides examples of worldwide occurrences. The study of the mineralogical assemblages, geochemistry and isotopic data of the hydrothermal alteration zones is one of the most challenging subjects in terms of gaining a detailed understanding of the geothermal systems. Silicification processes are subsequent to late-stage alteration after the completion of kaolinization processes, erasing existing hydrothermal mineralogical and geochemical traces and making interpretation difficult. In the early stages involving magmatic–hydrothermal-origin acidic geothermal fluids, the latter comes from the disproportionation of SO2 (+H2O) and H2S oxidation to H2SO4 in hydrothermal environments. In the later stages, due to spatial and temporal changes over time in the chemistry of geothermal fluids, the system comes to have a more alkali–chloride composition, with neutral pH waters frequently saturated with amorphous silica which characteristically precipitate as siliceous sinter deposits containing large amounts of opal-A.