The Okanogan Highlands province is situated east of the Cascade Range and north of the Columbia Basin. To the east and north, the highlands extend into northern Idaho and southern British Columbia, respectively. They are characterized by rounded mountains with elevations up to 8,000 feet above sea level and deep, narrow valleys. The Columbia River divides the Okanogan Highlands into two geographic regions: to the east of the river are the Selkirk, Chewelah, and Huckleberry Mountains; to the west are the Kettle, Sanpoil, and other mountains.
The eastern portion of the Okanogan Highlands contains the oldest sedimentary and metamorphic rocks of the state. Precambrian Belt Supergroup, Windermere Group, and Deer Trail Group metasedimentary rocks extend from British Columbia south to the Columbia River. The nation's second largest magnesium operations are located near Addy, in Stevens County. Dolomite and magnesite are mined from the Stensgar Formation dolomite of the Deer Trail Group. Precambrian dikes and sills cut these ancient rocks. In the vicinity of Spokane, mountains such as Mica Peak consist of Precambrian high-grade metasedimentary rocks.
Since the Precambrian, huge amounts of crust have been accreted to the North American continent. Everything west of the Precambrian rocks in northeastern Washington has been created through complex processes along an active continental margin or transported from elsewhere and stuck onto the continent of this region.
In the eastern highlands, Precambrian metasedimentary rocks are overlain by marine rocks representing each of the Paleozoic geologic time periods. The Cambrian record starts with sandstone (now quartzite) followed by shales and then limestones that grade into rocks of the Ordovician Period. Cambrian trilobites indicate relatively shallow seas. During the Ordovician, dark shales full of graptolites indicate deeper water conditions. All these rocks were subjected to metamorphism during Jurassic through Eocene time.
Cambrian rocks, in particular, are important sources of mineral wealth. Near Metaline Falls, Pend Oreille County, large Mississippi Valley-type zinc deposits were mined by room-and-pillar mining methods. Closed in 1977, the mines were noted for large calcite crystals and palygorskite. At Metaline Falls, the argillaceous Cambrian-Ordovician Metaline Formation was mined by open-pit methods for the manufacture of portland cement. All Paleozoic and some younger rocks have been repeatedly folded into a northeast-trending regional structure called the Kootenay Arc, which extends northeastward for 150 miles into British Columbia and contains numerous lead-zinc mines. In Stevens County, Cambrian Addy Formation quartzite is mined for its silicon content, and near Northport, Devonian argillite includes interbedded barite.
Two-mica granites rich in uranium host formerly productive uranium mines on the Spokane Indian Reservation. Mount Spokane consists of foliated Cretaceous two-mica granite that intruded Precambrian paragneiss (high-temperature metamorphosed sedimentary rocks). Weathering of such granites has released uranium into younger sediments, including a small peat bog near Colville. Radon gas is also generated by the two-mica Cretaceous granites and poses a hazard to humans.
To the west, the Okanogan Highlands are separated from the Cascades and the fold-thrust belt of the Methow terrane by a structure called the Pasayten fault zone. In contrast to miogeosynclinal (formed from deposition of sediments near a craton and lacking a volcanic component) metasedimentary rocks in the eastern portion of the Okanogan Highlands, the western part contains eugeosynclinal (formed from deposition of clastic sediments away from a craton and containing volcanic units) metasedimentary rocks. These two geosynclinal sequences were compressed and juxtaposed by Jurassic-Cretaceous thrust faulting. Permian and Triassic volcanic flows and sedimentary rocks in the western Okanogan Highlands were subject to diverse intrusive events during the Jurassic and then again in the Cretaceous.
The Eocene Epoch had a profound effect on the geology of the land. It was during this time that the Okanogan Highlands were subject to tectonism, plutonism, volcanism, sedimentation, development of gneiss domes, and epithermal precious metal deposition. Overprinted on the Okanogan Highlands are massive gneiss domes and north-south trending grabens. Stresses due to Tertiary extension and strike-slip faulting were relieved by concurrent ductile deformation in the form of spectacular domes. Extensional fracturing led to the introduction of dike swarms and the development of the regional Republic graben. Associated Eocene volcanic activity filled the Republic, Keller, and Toroda Creek grabens with volcaniclastic rocks. In later stages, the basins received fluvial and lacustrine sediments. Famous fossil lake beds at Republic contain Eocene plants, insects, and fish.
The western Okanogan Highlands is an important mineral-producing area. The center of gold mining is the Republic District in the Republic graben. By January 1, 1989, the district had produced more than 2.5 million ounces of gold and 14 million ounces of silver. Ore deposits occur in the Eocene Sanpoil Volcanics. These deposits are considered to represent fossil hot spring (epithermal) systems related to the final stages of Eocene calc-alkaline volcanism. Other significant gold deposits, such as those near Cooke Mountain, were formed by replacement of Permian and Triassic metasedimentary rocks. Associated minerals are magnetite, pyrrhotite, pyrite, and chalcopyrite. At the Buckhorn property, Okanogan County, gold mineralization is present in a skarn that contains bismuth and cobalt minerals (bismuth, bismuthinite, hedleyite, joseite, cobaltite, and erythrite). Porphyry copper-molybdenum deposits with chalcopyrite and molybdenite have been outlined by drilling at Oroville and Keller. The Keller Mount Tolman deposit contains the third or fourth largest molybdenum reserve in the United States, with 2.4 billion tons of ore averaging 0.093% MoS2 and 0.09% Cu.
The Okanogan Highlands were covered by great ice sheets during the Pleistocene Epoch. As the ice sheets retreated to the north, lakes formed in the valleys of the Columbia and Pend Oreille Rivers. Along the Canadian boundary, terrace deposits indicate lake levels 2,000 feet above current sea level. Melt waters filled these lakes with sand, silt, and clay. In Okanogan County, undrained depressions have accumulated crystallized salts and brines with the following minerals: trona, burkeite, natron, gaylussite, thermonatrite, thenardite, mirabilite, epsomite, gypsum, halite, and hydromagnesite.
The above text is modified from the following article: Lasmanis, Raymond, 1991, The geology of Washington: Rocks and Minerals, v. 66, no. 4, p. 262-277. © Copyright Heldref Publications (Helen Dwight Reid Educational Foundation). Used with permission.