Quantitative analysis of microscopic fluid inclusions by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) has greatly improved our understanding of fluid–rock interactions and ore deposit formation. Spatially resolved analysis can track the chemical evolution of fluids within texturally complex veins and along fluid pathways. LA–ICP–MS makes it possible to analyze chemical (e.g. Br/Cl) and isotopic tracers (e.g. Pb), and to identify fluid sources and timescales of transient fluid flow. LA–ICP–MS analysis has demonstrated that selectively metal-enriched fluids control the formation of magmatic-hydrothermal and sediment-hosted ore deposits and that sulfur decisively influences the partitioning, transport, and precipitation of metals in crustal fluids.