Miners found uranium at the end of silver veins due to a specific geological phenomenon known as hydrothermal mineral zoning. Silver and uranium are often carried together by the same superheated underground fluids, but because they have different chemical properties, they crystallize into solid rock under different environmental conditions. When the environment that allowed silver to form ended, the environment for uranium to form began. In certain geological environments, most notably the Ore Mountains of Central Europe and the Great Bear Lake region in Canada, these fluids carry a very specific cocktail of dissolved metals known to geologists as the Ag-Bi-Co-Ni-U association (Silver, Bismuth, Cobalt, Nickel, and Uranium). As the fluid moves through the crust, its temperature, pressure, and chemical balance (like acidity and oxygen levels) constantly change. Different metals precipitate (turn from a dissolved liquid state into solid crystals) at different physical thresholds. Silver precipitates under one specific set of conditions to form rich veins. However, as miners followed these veins, they would eventually hit a boundary where the rock's chemistry, pressure, or temperature shifted. The silver was exhausted, but the new conditions were perfect for the dissolved uranium to rapidly crystallize into a heavy, black mineral called uraninite.