USING ZIRCON TRACE ELEMENTS TO CONSTRAIN MAGMA OXYGEN FUGACITY AND ESTIMATE THE TUNGSTEN MINERAL RESOURCE POTENTIAL OF PLUTONS IN NORTHEASTERN WASHINGTON

Global demand for tungsten continues to increase, but there are few known deposits in the U.S., making the U.S. economy dependent on imported tungsten as domestic exploration efforts are limited by a lack of research into tungsten ore-forming processes. The oxidation state of a magma, measured in oxygen fugacity, fO₂, controls what reactions are favored during crystallization and what minerals are concentrated and precipitated, making it a key factor in ore formation. Previous workers have assessed the mineralization potential of copper porphyry deposits through analysis of zircon trace elements to constrain oxidation state, as zircon is common in crustal igneous rocks and is resistant to weathering and alteration. We have applied these zircon trace element methods to several plutons and a dike swarm in the region surrounding the now-inactive Germania tungsten mine in the Cedar Canyon mining district in northeastern Washington in order to constrain a magma oxygen fugacity associated with tungsten mineralization. The intrusions range from granite to granodiorite, and zircon U-Pb age calculations gave two age populations, ranging from 100.3 to 104.8 Ma and 71.8 to 74.7 Ma respectively, with the ore-bearing pluton associated with the Germania mine belonging to the older population. Zircons from thirteen samples of ore-bearing and barren intrusions of both age populations were analyzed by laser ablation ICP-MS to constrain magma oxygen fugacity and zircon crystallization temperature. Ti-in-zircon temperatures were calculated with an assumed TiO₂ activity of 0.7. Zircons from the older, tungsten-bearing middle Cretaceous units have fO₂ values relative to the quartz-fayalite-magnetite (QFM) buffer of -0.6 to 0.7 and estimated Ti temperatures of 607 to 640 °C; zircons from the younger, barren late Cretaceous units have slightly higher QFM fO₂ values of 1.3 to 2.2 and estimated temperatures of 655 to 738 °C. This distinct difference in magma fO₂ and zircon crystallization temperature coincides with tungsten mineralization, suggesting that zircon chemistry can be used as a tool to quantify the potential for tungsten mineralization in a crystallizing magma chamber. By using the results provided by the natural laboratory of the Cedar Canyon mining district, tungsten exploration can be conducted more efficiently.