Fluid-inclusion measurements and textural relationships demonstrate that coarse-grained quartz and calcite from the upper part of The Geysers thermal sytem formed during the transition from liquid-dominated conditions to the modern vapor-dominated regime. The most diagnostic characteristics of these transition assemblages are: 1) evidence of quartz dissolution; 2)contemporaneous trapping of high-and low-salinity waters representing liquids dominated by boiled reservoir fluid and condensate respectively; 3) the presence of solid inclusions in quartz of bladed, but commonly corroded, calcite; and 4) the common occurrence of multiple generations of primary, C02-bearing vapor-rich inclusions. Corrosion of the quartz and calcite occurred as condensate, generated at the top of the developing heat pipe, percolated down-ward. These minerals were subsequently precipitated when the descending fluids boiled.

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