Argon isotopes from a number of sources are present in quartz veins and chert; in situ decay of potassium, parentless 40Ar leached or outgassed from surrounding rocks, atmospheric argon dissolved in ancient hydrothermal fluids, and, modern atmospheric argon. The manner in which the 40Ar- 39Ar techniques can be used to unscramble these components, by way of correlations with Cl and K, is described in relation to two parallel studies of vein quartz and the Gunflint chert. Information concerning several processes can be inferred, including: contributions of surface waters to trapped fluids (from the concentration of atmospheric argon in the fluids), degree of water-rock interaction (from the concentration of parentless 40Ar and/or the ratio of parentless to atmospheric argon), effects of dilution of primary hydrothermal brines by meteoric water (which results in a decrease in the ratio of both Cl and parentless 40Ar to atmospheric argon), effects of boiling (from low concentrations of atmospheric argon and an increase in Cl 40Ar ). The K-Ar chronology of deposition can also be inferred in suitable circumstances. In principle, the ( 40Ar 36Ar ) ratio of the ancient atmosphere can be determined from fluids free of parentless 40Ar. However, a ( 40Ar 36Ar ) ratio determined for a low salinity end member in Gunflint chert probably reflects the presence of modern meteoric water.