AR/AR GEOCHRONOLOGY OF MAGMATIC-STEAM ALUNITE FROM ALUNITE RIDGE, MARYSVALE VOLCANIC FIELD, UTAH: TIMING OF MAGMATIC FLUID DISCHARGE AND AGE OF CONCEALED INTRUSIONS

Previous K-Ar dating and stable isotopic studies of coarsely banded epithermal alunite veins from Alunite Ridge within the Marysvale volcanic field, Utah indicate they formed at about 14 Ma from magmatic steam discharged during cooling of near-surface, water saturated, igneous intrusions. The alunite filled open space in fractures that cut Oligocene volcanic rocks. Each cm-scale band of crystalline alunite in the veins is believed to record the episodic release of volatile-rich acidic steam exhaled from a cooling magma. High precision ⁴⁰Ar/³⁹Ar dating of seven successive alunite bands in one large vein and single bands from other localities were conducted to constrain precisely the timing and duration of magmatic fluid discharge and the age of presumed subjacent intrusions.

High precision ⁴⁰Ar/³⁹Ar geochronology of alunite from seven bands from one sample yield ages of 13.91 +/- 0.13 Ma, consistent with a short-lived magmatic event with episodic vapor discharge ocurring on timescales that cannot be resolved by ⁴⁰Ar/³⁹Ar geochronology, perhaps recording exhalation frequencies similar to those presently observed at Yellowstone. Vein alunite from four separate mines and prospects centered around Alunite Ridge and Deer Trail Mountain yield reproducible ⁴⁰Ar/³⁹Ar ages of 15.5 Ma and 14.0 Ma that require the existence of two intrusions. The 14 Ma ages on alunite from Deer Trail Mountain and Alunite Ridge are similar to published K-Ar ages on sericite from the Deer Trail polymetallic replacement deposit. The 15.5 Ma magmatic pulse at Alunite Ridge was previously unrecognized, but is consistent with geological data that define a spatial and temporal southward migration of regional magmatism within the Marysvale volcanic field that may possibly be related to a localized tear and/or associated rollback of the underlying slab. Published stable isotope data on vein alunite from Alunite Ridge require the presence of at least one intrusion at depth. This investigation demonstrates that high precision ⁴⁰Ar/³⁹Ar geochronology of epithermal magmatic steam alunite veins is a viable method to determine both the timing of magmatic fluid discharge and the age of intrusive phases in porphyry-epithermal systems where intrusive rock is not exposed.