Apatite and fluorite control the transport of tungsten in calcium-bearing hydrothermal fluids

Minerals associated with ores (gangue minerals) record the physicochemical conditions of ore-forming fluids. Whether and/or how they affect the solubility of ore minerals, however, is rarely evaluated. Here, we present results of an experimental study of the solubility of scheelite (responsible for >70% of the global supply of tungsten). They show that phosphate and fluoride sequester calcium from scheelite to form apatite and/or fluorite. These gangue minerals, which are common in tungsten deposits, vary in abundance depending on pH and the phosphorus/fluorine content of the fluid. Because of their precipi-tation, scheelite dissolution releases hundreds of ppm of tungsten to the fluid, especially at near neutral pH. This suggests that the presence of phosphorus/fluorine in a tungsten-bearing hydrothermal fluid may prevent the deposition of scheelite due to the formation of apatite/fluorite, or lead to the dissolution of scheelite and, in turn, enrich the fluid in tungsten. Models based on our data show that magmatic-hydrothermal phosphorus-and fluorine-rich fluids are enriched in tungsten by a factor of up to 220 dur-ing high-temperature alteration, and form high-grade deposits of ferberite on cooling, or of scheelite because of their interaction with calcium-enriched rocks. Thus, the concentration of fluorine and phos-phorus, as well as pH, are key factors affecting the transport and deposition of tungsten in calcium -bearing hydrothermal systems that, if they act synergistically, can efficiently remobilize early low-grade tungsten mineralization and enrich the ore-forming fluids in tungsten, leading to the formation of giant deposits. Such competition between ore and gangue minerals for components of the mineralizing fluid is also probably a feature of hydrothermal systems involving other metals and should be considered in future studies of the genesis of the corresponding ores. (c) 2023 Published by Elsevier Ltd.