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Underground at the Pend Oreille Mine

by Vince Scartozzi

Editor’s note: The Pend Oreille Mine, owned by Teck Cominco American, Inc., is located north of Metaline Falls. It reopened in January 2004 after being closed since 1977. Ore-bearing rock is extracted by room and pillar mining and transported to the surface. The targeted production rate of 2200 tons/day of concentrates was achieved this March. The underground mine extends below the Pend Oreille River, and thousands of gallons of water are pumped out of the mine and discharged, after treatment, into the river each day. Many people have various ideas of what the term “ore” means. In the simplest terms ore refers to any naturally occurring material from which minerals of economic value can be extracted at a reasonable profit. At the Pend Oreille Mine, the minerals which make up ore are Sphalerite (ZnS) and Galena (PbS). Therefore, we are mining ore that contains profitable concentrations of the metals Zinc (Zn) and Lead (Pb). The use of these metals in modern society is far reaching, and ranges from pharmaceuticals and galvanizing steel (from zinc), to fishing weights, bullets, and the battery in your car (from lead), just to name a few.

Ore at the Pend Oreille Mine occurs in a carbonate rock (limestone/dolomite) called the Metaline Formation, which formed around 500 million years ago in shallow ocean waters. Later in geologic time, hydrothermal fluids (hot water with dissolved metals) from within the earth ascended through cracks or faults upward until it encountered the Metaline rocks at which time the zinc and lead minerals formed in cracks, cavities and fractures of the rocks. These broken rocks are termed “Breccia”.

The Pend Oreille Mine has the unique distinction of hosting two differing ore bodies at the same location. These “stacked’ horizons differ in ore types and grades, ore distribution geometries and stratigraphic occurrences. Historical mining at Pend Oreille, from 1929 to 1977, focused on what is called the “Josephine Horizon” which is located at the top of the Metaline Formation. It is at or near the contact with the overlying Ledbetter Slate and is associated with multiple episodes of solution-collapse brecciation. The overlying Ledbetter Slate represents a carbonaceous basinal environment. The Ledbetter appears to have formed a barrier or trap to hydrothermal fluid migration in the Metaline. These fluids ponded up under the Ledbetter and eventually deposited zinc and lead in the Josephine Horizon. Approximately 14 million tons were mined from the Josephine with ore grades of 2.3% zinc and 1.1% lead. The majority of Josephine ore has been mined out, or deemed too low grade economically, and we are now mining the “Yellowhead Horizon”.

The Yellowhead occurs approximately 900 feet below the Josephine, at a stratigraphic contact between the Upper Metaline and the Middle Metaline formations. This horizon represents a transition from a shallow water environment to a peritidal depositional environment. The horizon may represent an ancient evaporitic bed. Yellowhead ore, unlike Josephine ore, is associated with abundant iron sulfide occurrences. The iron-zinclead sulfides are hosted in stratabound solution breccia and coarse sparry dolomite. Current proven and probable minable reserves in the Yellowhead stand at close to 6 million tons with diluted ore grades of 7.5% zinc and 1.2% lead. A further 2.8 million tons has been identified as an inferred resource. The zinc grades at Pend Oreille are moderate to low, compared to other deposits, and will require an efficient and cost-conscious operation to be profitable. Recent exploration is attempting to identify additional resources under Washington Rock, and we feel that there is very good potential to find additional ore in the area around the mine that would extend the mine life beyond the current ten-year plan. The Pend Oreille Mine staff are proud to be part of the Metaline Falls, Metaline, and Ione areas, as well as Pend Oreille County, and hope that our presence will help the community prosper.


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