CRITICAL MINERALS AND GEOLOGIC MAPPING IN WASHINGTON STATE

A statewide critical-mineral favorability model is developed for Washington that combines published mineral-systems tracts from the USGS with existing statewide mineral-occurrence data. These results are winnowed using land ownership, geologic maps, and lidar availability to highlight areas with critical minerals that lack sufficiently detailed geologic mapping. On this basis, the Adams Mountain and Hunters 7.5-minute quadrangles in northeastern Washington were selected for study in 2022.

New geologic mapping, geochemistry, geochronology, and compilation of existing maps improve our understanding of the area’s >1.3 billion-year geologic history. Metasedimentary and metavolcanic rocks of the Deer Trail Group and Windemere Group are the oldest in the area (~1.3–0.7 Ga). These rocks are typically foliated or cleaved, and contain evidence for the rifting of Rodina. The Addy Quartzite straddles the Precambrian–Paleozoic boundary and underlies a thick sequence of metacarbonate and shale, which are exposed in a now-vertical sequence of thrust sheets. Paleozoic metasedimentary and metavolcanic rocks of the Roberts Mountains allochthon are found in the northern part of the map area. Two voluminous granitic plutons intrude the Paleozoic and older rocks: a 100–105 Ma zoned pluton near the Germania mine, which hosts tungsten and molybdenum mineralization, and a more-widespread 71–74 Ma pluton near Fruitland, which is locally associated with mineralized skarn. Eocene andesitic to rhyolitic flows and tuffs were rapidly emplaced 51.5–52.2 Ma on the exhumed 71–74 Ma pluton and are found in fault-bounded half grabens that presumably developed above the Kettle detachment fault. The volcanic rocks are similar to the Sanpoil Volcanics farther west; both are adakites and likely formed by partial melting of continental crust and not from subduction. Much of the map area was repeatedly covered by continental glaciers during the Pleistocene.

A regional geochemical assessment shows that mineralization is greatest in late-stage dikes and veins within the 100–105 Ma pluton (high As, B, Bi, Mn, and W; minor Ag, Cu, Ga, Mo, Pb, Sn, and Zn). Calc-silicate skarn associated with the 71–74 Ma pluton is also mineralized (high Mo; minor Cu, Sn, and W). The Eocene volcanics do not show any notable mineralization.