Notes from the Metaline Formation Lead/Zinc District

Easy walking in the Selkirk Range

I was doing some mineral assessment work in northeast Washington State earlier this summer. The open woodlands makes for easy going in the field compared to the brush wresting I engage in on the west side of the Cascades. However, drinking water is a bit bigger of an issue when the temperatures are in the upper 90s like the day I took this picture in the southern part of the Selkirk Range.

The northern half of Washington State is nearly entirely mountainous from the Northwest Cascades and North Cascades in the west to the Selkirk Range in the east. Except for a few of the valleys and highest peaks these ranges are mostly forested. Continental glacial ice covered all of these ranges such that the drainages have been defined by deep glacial carved valleys. The part of the Selkirk Range I was working in was fairly gentle with lots of exposed rock from the glacial scraping and open forest. To the north and east the mountains get higher and the forest starts taking on a decided wet aspect that looks remarkably like western Washington.
The rocks I was inspecting had unfortunately been given very similar names: the Maitlen Formation and the Metaline Formation. The Maitlen includes Early Cambrian (520 million years) limestone and dolomite. The Metaline Formation is Middle Cambrian to Middle Ordovician (510 to 470 million year) and includes bedded limestone. Both have been metamorphosed in the area I was working such that I really could not distinguish one from the other without relying on other unit relationships.

Maitlen Limestone

Gypsy Quartzite

Stoffel and others (1991) interpret both of these units as well as associated formations of similar age to have been deposited within a passive margin basin adjacent to the former edge of North America. The formations were subsequently deformed and faulted within the Kootenay Arc during the late Triassic (210 million years). Morton (1992) has interpreted the margin deposits of the Metaline and overlying Ledbetter Formation to have been deposited along a more tectonically active North American margin with faulting and laterally discontinuous depositional environments during deposition and compaction of the Metaline and Ledbetter Formations.
The Metaline Formation has been a rich mining unit in the history of northwest Washington. The primary mining that has been done in the area has been lead and zinc deposits. The mineralization of the Metaline Formation is considered to be a combination of Mississippi Valley type and Irish type ore deposits (Morton (1992). These types of deposits are formed by conate water (water that was present at the time of deposition) within the formation being pushed out of the formation during and after deposition as the sediment compressed. As water moves through the deposit, sources of zinc and lead are leached and deposited in concentrated areas due to changing water chemistry as the water moves through the formation. Minerals associated with the ores in Mississippi Valley type and Irish type lead-zinc deposits are typically sphalerite, galena and pyrite with other less common sulphite minerals. The largest open pit mine in the state is located in the Selkirk Range is a now abandoned lead/zinc mine. A number of lead/zinc mines are scattered throughout the Metaline Formation. Perhaps the best exposures are within the deep narrow gorge of the Pend Orelle River (pronounced pond-or-ay) just south of the Canadian border.

Mine workings north of Metaline Falls within the Metaline Formation

Drowned mine entrance along reservoir shore

Mineralized zones within the Metaline Formation

Exposure of Ledbetter Formation, likely source of lead and zinc

The lovely Peewee Falls across the Ledbetter Formation