ASSESSING THE ORE MINERALIZATION DEPTH AND CRUSTAL HISTORY OF PLUTONS IN NORTHEASTERN WASHINGTON USING THERMODYNAMIC MODELING OF WALL ROCK ASSEMBLAGES

Critical minerals are essential for advanced technologies across all economic sectors, but domestic deposits are poorly constrained, leaving the U.S. economy vulnerable to disruption. In the hangingwall of the Kettle Detachment Fault (KDF) in northeastern WA, several ore-bearing plutons are present that may be a significant resource. However, the deposits’ formation depths, a component responsible for ore genesis, are unknown. To constrain regional ore formation depths, the mineral assemblages of calc-silicate and pelitic rocks from the contact aureoles of two Cretaceous plutons in the Cedar Canyon Mining District (CCMD) were quantitatively constrained with scanning electron microscopy and XMapTools. These assemblages were then compared to thermodynamic models created with GeoPS, a Gibbs free energy minimization calculator that uses whole rock geochemistry to predict the P-T stability of possible sample mineral assemblages. Comparisons between modeled and observed assemblages of four samples indicate that andalusite + cordierite-bearing aureoles of the 103.5 ± 0.2 –100.1 ± 0.4 Ma Germania pluton, the host a significant W deposit, formed at 540 – 610 °C and 0.9 – 1.8 kbars, corresponding to a depth of 3.5-7 km. The andalusite + corundum (pelite) and diopside + biotite-bearing (calc-silicate) contact aureoles of the 74.0 ± 0.2 – 73.7 ± 0.3 Ma, weakly mineralized Fruitland pluton, formed between and 680 – 870 °C and 0.3 – 0.6 kbars, corresponding to a depth of 1.2-2.4 km. These results are consistent with accepted W deposit formation depths and suggest an average exhumation rate of ~0.1 km/My between 100 Ma and 74 Ma. Eocene extension then brought both plutons to the surface, where they were directly overlain by Eocene volcanics. Future work to acquire better temperature constraints with trace element thermometry can improve the resolution of pluton pressure and depth constraints. This, coupled with the modeling of additional Cretaceous intrusive aureoles associated with ore deposits in the CCMD, can help to identify the formation conditions of ore bodies in the region, helping to contribute to exploration efforts for domestic critical minerals.