Simulation of hydrothermal systems by numerical methods permits computation of the initial positions (sources) of all fluids in the system. The locus of these positions, which are sourcelines, defines an essential link between the theories of irreversible mass transfer and those of thermally driven fluid flow. The concept of a fluid sourceline is introduced to facilitate the quantitative description of the initial composition of fluids which ultimately circulate through rocks in the vicinity of hot plutons.The mathematical definition of fluid sourcelines and pathlines for hydrothermal systems permits the quantitative prediction of the initial position and, hence, initial compositions of all fluids which circulate through rocks in the vicinity of an igneous intrusion and the variations in temperature, pressure, and rock type along flow paths and, hence, the changes in composition of the fluid as it flows from its source to a position of interest in the system.Examples of fluid sourcelines, sourceregions, and pathlines defined for inert fluids in an idealized hydrothermal system indicate the predominant source of fluids flowing through permeable hot plutons is from host rock environments adjacent to and above the pluton. These sourceregions in two dimensions include rocks from a 50-km 2 region as far as 5.5 km away from a 2.7-km-wide and 4.5-km-tall pluton.

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