Physical properties and composition of ore-forming fluids in magmatic-hydrothermal systems are mostly investigated by conventional fluid inclusion studies in gangue minerals that are assumed to be “cogenetic” with the mineralization. However, ore-precipitating fluids can be directly studied by analyzing fluid inclusions in ore minerals by using near-infrared (NIR) petrography and microthermometry in combination with LA-ICP-MS analysis. The first part of the thesis presents new data on the applicability and limits of NIR fluid inclusion studies in ore minerals, while the second part tests this method by comparing the properties and composition of inclusion fluids trapped in associated ore and gangue minerals, revealing that fluid inclusions hosted in gangue minerals are not always representative of the ore-forming fluid. Our method is finally applied to the study of the polymetallic "Main Stage" veins at Butte (USA), producing a new vein formation model explaining the district-scale metal zonation.