Tungsten mineralization is typically associated with reduced granitic magmas of crustal origin. While this type of magmatism is widespread, economic tungsten deposits are highly localized, with ∼90% produced from only three countries worldwide. Therefore, the occurrence of reduced magmatism, while necessary for tungsten enrichment, seems to be insufficient to form such rare deposits. Here we explore the mechanisms that lead to wolframite precipitation and evaluate whether they may exert a decisive control on tungsten global distribution. Tungsten differs from other rare metals enriched in magmatic-hydrothermal ore deposits because it is transported as an anionic species. Precipitation of the main tungstate minerals scheelite, CaWO4, and wolframite, (Fe, Mn)WO4, thus depends on the availability of calcium, iron, or manganese. We demonstrate quantitatively that magmatic fluids at Panasqueira, Portugal, provide tungsten in solution, whereas the host rock contributes the iron required to precipitate wolframite. The combination of special source conditions with specific reactive host rocks explains why major wolframite deposits are rare and confined to a few ore provinces globally.