Modern Closed-Basin Potash Evaporites and Brine Evolution, Qaidam Basin, Qinghai Province, China

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The origin of ancient potash evaporites, in terms of parent waters, depositional environments, and timing of formation is controversial largely because no modern potash-bearing basins have been described in detail. The goal of this study is to document the geochemical and sedimentological conditions of potash mineralization in the Qaidam Basin of western China, where modern bedded potash salts (carnallite and sylvite) are widely developed. The project consists of three interrelated parts: (1) chemistry of inflow waters and their evolution into brines, using a computerized specific ion interaction solution model, (2) field and petrographic study of modern potash salt deposits and geochemistry of associated brines, and (3) Holocene-Pleistocene history of brine evolution and potash salt deposition from study of two cores (fluid inclusions in halite, pore fluid chemistry, and petrography). Many ancient potash deposits lack the MgSO4-salts that are characteristic of a seawater evaporite sequence. Hardie (1984, in review) challenged the marine origin of ancient MgSO4-free potash evaporites and proposed a new model involving hydrothermal inflow waters, especially for those formed in tectonically- active basins. Study of the Qaidam Basin, with recent tectonic activity, MgSO4-free potash evaporites, and major spring inflow, will allow testing of new hypotheses on the origins of ancient potash evaporites.