The fluid path
Heat (from a cooling magma body, deep burial, or both) drives groundwater through cracks in the rock. The water dissolves metals and other elements from surrounding rock as it migrates. When the fluid hits a temperature drop, pressure drop, or chemistry change (often a different host rock), the dissolved load precipitates as crystals on the fracture walls.
Open-space growth
Crystals grow inward from the fracture walls into an open void. With nothing to push against, they develop large euhedral (well-formed) faces. This is why vein and vug specimens have such clean crystal terminations compared to rock-bound minerals from metamorphic environments. Crustification — sequential layered growth as the fluid chemistry shifted — records the geological history of the vein in cross-section.
Famous vein associations
Sulfide-quartz veins: galena, sphalerite, chalcopyrite, pyrite, quartz, often capped by calcite. Tungsten-tin veins (Yaogangxian, Xuebaoding): wolframite, cassiterite, scheelite, fluorite, quartz. Antimony veins (Lengshuijiang): stibnite, quartz, calcite. Each combination tells you something about the temperature and chemistry of the parent fluid — the same toolkit professional mineralogists use to interpret economic deposits.