FLUID INCLUSION ACTIVE AND NOBLE GASES IN COARSE-BANDED ALUNITE VEINS AT ALUNITE RIDGE, MARYSVALE, UTAH, AND IMPLICATIONS FOR CRUST-MANTLE MAGMATIC VOLATILE EVOLUTION

The spectacular (≤ 20 m) coarse-banded alunite veins at Alunite Ridge are likely associated with a pulse of mid-Miocene bimodal basalt-rhyolite magmatism in the ENE-trending Pioche-Marysvale mineral belt. To trace volatile evolution in this porphyry?-epithermal system we extracted inclusion fluids from 76 samples of sequenced alunite bands as well as earlier vein quartz and platy calcite. We analyzed them for H₂O, H₂, CH₄, CO₂, SO₂, H₂S, N₂, He, Ar, C_nH_m, HCl, HF by quadrupole MS and for isotopes of He, Ne, Ar by high resolution static sector MS. New 3-D plots of gas compositions relative to δD-δ¹⁸O of alunite yield surprising insights into time-space fluid evolution.

Helium R/Ra ranges from 0.002 to 28.4, and inversely correlates with ⁴He/⁴⁰Ar*. Solubility-diffusion-gas partitioning led to He>>>Ne>Ar with the most radiogenic crustal He ≈10⁵ times in excess of Ar. Alunite fluids are low-density volatiles that during most of the paragenesis remain single phase X_H2O ≈ 0.77 (0.30 to 0.96), carry significant X_H2 ≈ 0.046 (0.001 to 0.275), and correlate low-density volatiles with mantle He. SO₂ correlates with CO₂ and HCl; both correlate inversely with HF. Median H₂S/SO₂ is ≈ 0.07. CO₂/³He* ranges from 3.9x10⁷ to 1.7x10¹⁵ supporting the evolving contribution of both mantle and crustal volatile components. The CO₂-H₂S reaction "blocking" temperature T°C_avg ≈ 468° and log ƒO₂ ≈ -43 to -16 are compatible with alunite deposition. Fluids with the greatest amounts of organics are characterized by atmospheric active and noble gas and meteoric water stable isotope compositions. Alunite fluid compositions vary slightly between bands and vein locations.

These data define magmatic hydrothermal processes beginning with intrusion of mantle-derived basalts to mid-crust levels and melting of crustal material to form felsic magmas. A homogeneous shallow crust “reservoir” of supercritical volatiles forms by mixing of SO₂- and CO₂-rich volatiles exsolved from basalt magma with HF- and HCl-bearing low-density supercritical magmatic volatiles from rhyolite magma. This magmatic fluid episodically ascends, entraining meteoric water, organic matter, and radiogenic He from crustal sources. Near surface fluid decompression in open veins partitions volatiles, sequentially forming steam-pipe calcite, quartz and banded alunite.