Brine history indicated by argon, krypton, chlorine, bromine, and iodine analyses of fluid inclusions from the Mississippi Valley type lead-fluorite-barite deposits at Hansonburg, New Mexico

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Argon, krypton, chlorine, bromine, and iodine were measured in a homogeneous population of high-salinity hydrothermal fluid inclusions from the Tertiary-age Mississippi Valley-type (MVT) lead-fluorite-barite deposits at Hansonburg, New Mexico to establish new types of evidence for the history of both the fluid and the major dissolved salts. Noble gases and halogens in fluid inclusions containing 10 −10–10 −9 L of brine ( Cl= 3 molal) were analyzed by laser microprobe noble-gas mass spectrometry ( lmngms) on neutron-irradiated samples. The concentrations of 36Ar ( 4.7 × 10 −8 molal) and 84Kr 1.8 × 10 −9 molal) in the fluid inclusions are equal to those of fresh surface waters in equilibrium with air at approximately 20 ± 5°. The mole ratios of Br/Cl ( 1.2 × 10 −4 ) and I/Cl ( 1–2 × 10 −6 ) are among the lowest measured in any natural waters, similar to those of modern brines formed by dissolution of Permian NaCl-bearing evaporites in southeast New Mexico. 40Ar/ 36Ar ratios (600) are twice that of air, and indicate that the fluid inclusions had excess radiogenic 40Ar ( 1.4 × 10 −5 molal) when trapped. The amount of excess 40Ar appears to be too large to have been acquired with Cl by congruent dissolution of halite-bearing evaporites, and possibly too small to have been acquired with Pb by congruent dissolution of granitic basement rocks with Proterozoic K Ar ages. From the lmngms data, combined with published Pb and S isotope data, we infer the following sequence of events in the history of the Hansonburg MVT hydrothermal brine: (1) the brine originated as relatively dilute meteoric water, and it did not gain or lose atmospheric Ar or Kr after recharge; (2) the originally dilute fluid acquired the bulk of its Cl and sulfate in the subsurface after recharge by dissolving halite-bearing Permian? marine evaporites; (3) the high salinity brine then acquired most of its Pb and excess radiogenic 40Ar from interactions with aquifer rocks other than evaporites, possibly clastic sedimentary rocks or basement rocks with Phanerozoic K Ar “ages”; and (4) the brine deposited fluorite without having boiled or degassed.