PINE GROVE MO PORPHYRY AND NG ALUNITE AREA, UTAH – DEEP AND SHALLOW PRODUCTS OF NEIGHBORING SILICIC CUPOLAS ABOVE A LARGE PLUTONIC COMPLEX

The Pine Grove Mo porphyry (PG) and the NG Alunite Area (NG) occur ~10 km apart on the W and E side of the E-tilted Wah Wah Range in SW Utah. PG is the 9th largest Climax-type Mo resource (112.5 Mt @ 0.22 wt% Mo) and NG is the largest alunite resource (669 Mt @ 15 wt% Al₂O₃) in the US. They are two examples of a string of early Miocene silicic magmatic centers and hydrothermal systems that formed after the transition from contraction to extension and calcalkaline to bimodal volcanism in the ENE-trending Pioche-Marysvale belt.

PG formed ~1.7 km below the paleosurface and is now ~1 km below the surface after uplift and erosion. ASTER-derived mineral maps reveal a ~12 km diameter halo of advanced argillic alteration (pyrophyllite, dickite, alunite) in Neoproterozoic quartzites around exposures of QSP-altered rhyolite plugs (SiO₂ up to 77.2%), pebble dikes, and small polymetallic deposits. NG is comprised of four, 1-3 km sized areas of replacement alunite that formed at, and up to 300 m below, the paleowater table. They locally contain disseminated pyrite or coarse banded alunite veins and are distributed around three rhyolite plugs (SiO₂ up to 76.1%). ASTER-derived mineral maps show the areas of quartz-alunite and kaolinite±hematite alteration form a ~12 km halo around QSP alteration in the central rhyolite plug and adjacent volcanic rocks where a quartz vein contains ~400 ppm Mo.

The PG Mo deposit formed after eruption of the 22.9 Ma Tuff of PG, likely after 22.7 and 21.8 Ma rhyolite domes, and before crosscutting 19.1 Ma rhyolite dikes. Part of NG is hosted in the Tuff of PG. Alunite altered rocks are cut by a fresh 22.2 Ma rhyolite plug. Dates on vein (22.5 Ma) and replacement alunite (20.6 Ma) span the range of new ⁴⁰Ar-³⁹Ar plateau dates on 3 widely separated alunite veins of 21.65±.15, 21.54±.13, 20.93±.26. The new dates require episodes of magmatic steam discharge from at least 2 different intrusive phases.

At PG, alunite on fractures in quartzite and pyrite from polymetallic deposits have d³⁴S values of 2.4-11.0‰. Across NG, replacement and vein alunite, pyrite, and jarosite have d³⁴S values of 2.4-6.6‰. The 42 Mt of S in alunite at NG and 16 Mt of S in MoS₂ at PG may be derived from the same magmatic source.

The common attributes and zoning in these neighboring systems suggest that Climax-type Mo deposits (e.g. PG) may exist below advanced argillic lithocaps (e.g. NG).