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

Abstract

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 precipitation, 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 during 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 phosphorus, 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.