A quadrupole mass spectrometer was used to measure bulk samples of conservative gas species N2, Ar, and He in fluid inclusions from a variety of hydrothermal systems. Analyses of these tracer elements help determine (1) if gases extracted by bulk inclusion analyses can provide accurate measurement of N2-Ar-He in active and fossil geothermal systems, (2) if hydrothermal fluids associated with paleogeothermal systems in a continental setting follow N2-Ar-He systematics similar to' those in the western Pacific Rim active geothermal systems, specifically New Zealand, and (3) whether different deposit types systematically vary with regard to N2-Ar-He. The N2-Ar-He ratios of fluid inclusion volatiles released from recently deposited minerals from the Valles system are similar to those of present day Valles thermal waters. Those inclusion samples from deep within the Valles system, below a regional aquitard, increase in N2. Compositions for inclusions from the Questa and Copper Flat-porphyry deposits are N2-rich, similar to those of arc-related volcanic gases, whereas those from Taylor Creek Sn deposit appear to be mixtures of magmatic and crustal components. N2-Ar-He ratios of the Precambrian Tribag deposit suggest a basalt source, but significant levels of self-generated He from U and Th in the inclusion fluids are also possible. Inclusions from two epithermal deposits with low-salinity inclusions have N2-Ar-He ratios trending towards air-saturated meteoric waters (ASW), and those inclusions with higher salinities indicate minor to no ASW component. The N2-Ar-He ratios in Fresnillo and Cochiti inclusions, which have magmatic helium isotopic ratios, indicate additions of magmatic gases to meteoric fluids. Inclusions from sediment-hosted deposits that contain hydrocarbon-bearing brines are He-rich, as are meteoric waters with a long residence time in the crust. At relevant pressure-temperature-composition conditions, Henry's Law constants of N2, Ar. and He are similar. and thus, the relative amounts of these species trapped under boiling conditions will not vary appreciably from solubility-controlled amounts in coexisting liquid. Furthermore, there is no evidence of He loss or gain in inclusions by diffusion process. Data from the various systems that were examined indicate that fluid inclusion N2-Ar-He compositions may be related to magmatic fluid, sedimentary brine, deep circulating meteoric fluid, and shallow circulating meteoric fluid sources, which we modified from those proposed by GIGGENBACH (1986). Relationships between the N2-Ar-He tracer gases and other measurable quantities in fluid inclusions are indicated to be helpful in understanding fluid mixing processes in paleogeothermal systems.
