EVALUATING THE ROLE OF DEEP EARTH PROCESSES IN MINERAL DEPOSIT FORMATION, NORTH CASCADES, WASHINGTON

Sulfide minerals are the source of many economically significant elements and these elements are thought to be concentrated by a combination of both deep and shallow processes. However, the role of processes deep in Earth’s crust is less well known compared to shallower processes. The Chelan Migmatite Complex (CMC) in the North Cascades near Chelan, Washington is a deep-crustal unit that preserves a multistage magmatic history, and was used to evaluate the role of deep Earth processes in sulfide formation. We evaluated sulfide minerals present at each stage of the geologic history (deep vs. shallow), and which stages host minerals with the highest concentrations of sulfide minerals. Sulfides appear to be primarily associated with the quartz diorite gneiss unit that formed deep in the crust at depths of 25-30 km, and represents the earliest magmatic stage associated with the mid-Cretaceous migmatite-forming event. Based on sample textures documented by a combination of reflected light imaging and scanning electron microscope (SEM) backscattered electron + Energy Dispersive X-Ray Spectroscopy images, there were likely multi-stage mineral growth and replacement events involving pyrite, chalcopyrite, and iron oxides (goethite and magnetite) in the quartz diorite gneiss. The pyrite crystallized first, then began being partially replaced by chalcopyrite; chalcopyrite also formed small individual grains in the matrix. Both sulfides were subsequently partially replaced by rims of iron oxides, except where the sulfides were included inside silicate minerals. Further work involving continued SEM analysis of other sulfides present in the quartz diorite gneiss will yield a better understanding of the replacement processes occurring within this unit. Comparing these results to observations from samples of the later magmatic and hydrothermal stages will allow us to compare sulfide formation and alteration processes at different crustal levels. Our results will provide context for the resource exploration needed to support advancing technologies.