Wednesday, August 25, 2010

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

4 comments:

Brian Green said...

Thank you so much for explaining this. My family had four generations down in the Pend Oreille Mine and not a single one of them told me that Metaline and Metaline Falls (where we lived) were named for the Metaline rock formation! I also now know, thanks to you, that the Metaline formation is where the limestone is, and the layer above it is the Ledbetter formation and is where they were going to get lead and zinc (though the mine went far deeper than that layer). This stuff fascinates me. Thanks for sharing!

Brian Green said...

Oh, and that mine entrance you have a picture of is only about twenty feet deep. I've pulled my kayak in (16' kayak) and barely had enough room. You have to be careful though because the fluctuations of the river can be significant and swift and can cause you serious problems if you get stuck inside.

Keep up the great work on these posts. I'll be back often now that I know of this blog.

Dan McShane said...

Thanks Brian.
The mines are within the Metaline where the zinc and lead were concentrated at three different levels. The source material from which the lead and zinc leached are thought to be the Ledbetter Formation, but the area where the minerals are found are in the Metaline. It is a complex story and these rocks are very old and have been faulted and folded.
Thanks for the info on the shaft. It sounds like an exploratory shaft.
I'll being doing more posts about the Selkirk Range and the Pend Orelle. I also hope to get over there agian soon.

Milena said...

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