Assessing the Ore Mineralization Depth and Crustal History of Plutons in Northeastern Washington Using Thermodynamic Modeling of Wall Rock Assemblages

Author

David William MillerFollow

Document Type

Thesis

Date of Degree Completion

Spring 2024

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Chris Mattinson

Second Committee Member

Hannah Shamloo

Third Committee Member

David Hernández Uribe

Abstract

Critical minerals are essential for advanced technologies across all economic sectors, but domestic deposits are unconstrained and underutilized, leaving the U.S. economy vulnerable to disruption. In northeastern WA, several ore-bearing plutons may be a significant resource; however, their 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, located in the Adams Mountain and Hunters (AMH) quadrangles in the hangingwall of the Kettle detachment fault (KDF), were quantitatively constrained with scanning electron microscopy and XMapTools and compared to thermodynamic models created with GeoPS. Comparisons between modeled and observed assemblages of four samples indicate that andalusite and cordierite-bearing aureoles of the 104 – 100 Ma Germania pluton, the host of a significant tungsten (W) deposit, formed at depths 3.7 – 13 km, assuming a metasedimentary overburden density of 2600 kg/m³. The andalusite and corundum (pelite) and clinopyroxene and biotite-bearing (calc-silicate) contact aureoles of the 74 – 71 Ma, weakly mineralized Fruitland pluton, formed at depths between 0.1 – 2.1 km. These results are consistent with global W deposit formation depths and suggest an average exhumation rate of ~0.2 km/My between 100 Ma and 74 Ma. Thermodynamic models indicate that the Eocene Kettle dome’s garnet and sillimanite gneiss, located in the footwall of the KDF, was metamorphosed at depths of 18 – 28 km between 59 – 52 Ma and exhumated at rates between 7.6 – 8.6 km/My after 52 Ma. These results indicate that mid-Cretaceous plutons exposed at the surface in the KDF’s hangingwall formed at greater depths favorable for W mineralization and underwent exhumation before the shallower intrusion of late-Cretaceous plutons, which have the same exposure level and less mineralization potential. These findings indicate that older, mid-Cretaceous plutons are a superior target for future W exploration in northeastern Washington. Future work to improve the resolution of pluton and core complex pressure, temperature, depth, and exhumation rate constraints, coupled with modeling additional intrusive aureoles in the AMH and Orient quadrangles, can further constrain formation conditions of deposits in the region, aiding future exploration for domestic critical minerals.

Recommended Citation

Miller, David William, "Assessing the Ore Mineralization Depth and Crustal History of Plutons in Northeastern Washington Using Thermodynamic Modeling of Wall Rock Assemblages" (2024). All Master's Theses. 1939.
https://digitalcommons.cwu.edu/etd/1939