A study of clay mineralogy and illite Kübler index with respect to hydrothermal alteration in the Yinsan polymetallic deposit, South China

The Yinshan polymetallic deposits is a hydrothermal deposit spatially associated with Late Jurassic felsic to intermediate volcanic-subvolcanic rocks. Illite is the dominant, clay mineral formed via hydrothermal alteration of feldspar during fluid-rock interaction. The Kubler indes of illite has been used to map paleo-fluid flow associated with hydrothermal mineralization. In the Pb-Zn-Ag minerlizing episode 9136–130 Ma), fluid/rock ratios were relatively low, fluid flow was mainly by pore osmosis, and the solute transported slowly. Illite formed at this time and contains some swelling layers. In the second Cu-Au mineralizing episode (125–122 Ma) fluid/rock ratios were relatively higher, and fluid flow was mainly through fractures or channels. Illite associated with stage 2 hydrothermal activity has no swelling layers and a lower Kubler index. The third hydrothermal episode (104 Ma) has no economic mineralization, and produced only minor illite. The clay mineral assemblages, their distributions, and the micro-structural features and variations in the Kubler index of illite are indicators of temperature changes, and the type and intensity of hydrothermal alteration. Consequently, the Kubler index can be used to aid exploration for Cu, Au, Pb, Zn and Ag mineralization in the district.