THE AGE OF ZIRCONS AND COMPOSITION OF MELT INCLUSIONS IN QUARTZ PHENOCRYSTS FROM AN ALTERED DIKE INDICATE THE ~14 MA ALUNITE RIDGE - DEER TRAIL SYSTEM OVERLIES A 30.7 MA SILICIC INTRUSION

The shallow coarse-banded magmatic steam alunite veins at Alunite Ridge and deeper Pb-Zn-Ag-Au-Cu manto deposits below Deer Trail Mountain in the Marysvale mining district of Utah are dated at 14 Ma and are likely related to a concealed porphyry. Only a few small igneous dikes of unknown age are present and all are altered obscuring their original compositions. The middle Miocene rift setting with bimodal magmatism, presence of MoS₂ and CaF₂ in mantos, and high F in alunite veins permit the presence of a concealed Climax-type Mo porphyry.

At the Mineral Products [MP] mine, a quartz-alunite-pyrite altered dike and subsequent coarse banded alunite vein are localized by a N30W fault. The textures of relict minerals suggest the dike originally contained about 50 % phenocrysts of 1 mm quartz, feldspar, and biotite as well as ~0.1 mm zircon.

Quartz phenocrysts contain 5-25 micron melt inclusions (~ 80% glass, 20% vapor, 0-5% daughter minerals) that were analyzed to ascertain the chemical composition, degree of differentiation and metal content of the dike. Major and minor oxides were determined by EMPA at 15kV and 20nA with a 1 micron beam diameter and peak counting times of 20 to 400 seconds. Trace elements were determined by LA-ICP-MS using NIST 610 and 612 as calibration standards with normalization to the EMPA Al concentration. EMPA avg. wt. %: SiO₂ 75.8, Al₂O₃ 12.8, K₂O 5.7, Na₂O 1.8, CaO .74, FeO .35, MgO ≤ .27, TiO₂ .15, Cl .14, F ≤.05, S≤.007, P₂O₅ <.005, Total 98.0. LA-ICP-MS avg. ppm: Rb 190, Sr 180, Zn 86, La 35, Th 32, Nd 18, U 11, Mo 10, Y 6.8, Sm 2.8, Dy 1.8, Ta 1.4, Yb 1.3, Eu 0.5.

The MP dike is hosted in the Three Creeks Tuff Member of the 34-22 Ma Bullion Canyon Volcanics which erupted episodically from different stratovolcanoes. To establish the age of the MP dike, zircons were separated for SHRIMP-RG U/Pb dating. Euhedral magmatically zoned zircons yield a mean ²⁰⁶Pb/²³⁸U age of 30.7±0.4 Ma with some ~1715 Ma inherited zircon cores.

The juxtaposition of 30.7 Ma peraluminous high silica rhyolite and 14 Ma magmatic steam alunite in the same fault system suggests the district is underlain by two intrusive complexes associated with different calc-alkaline and bimodal volcanic suites. The low F, low Rb/Sr and small Eu anomaly of the MP dike are unlike the highly differentiated rhyolites that produce Climax-type Mo porphyry and related deposits.