ratio ultimately whether the system is rock- buffered or fluid-buffered, temperature, and pressure. If the aim of studying hydrothermal alteration is to constrain better some or all of these variables the detailed field and petrographic observations, summarized in steps 1 4, should be augmented by other data. For example, the composition of the hydrothermal minerals should be determined quantitatively by electron microprobe in order to compute shifts in mineral-fluid stability boundaries as a function of mineral composition usually depicted on activity ratio, activity-temperature, or pH-fo. diagrams. Similarly, high quality geochemical analyses. generally involving a combination of methods, are required to determine the material transfer involved m alteration. Finally, fluid inclusion and stable isotope data should be acquired in order to assess the composition and rigin of the hydrothermal fluid