Tanco Mine

Tanco Mine or Bernic Lake mine is an underground caesium and tantalum mine, owned and since 2019 owned and operated by Sinomine Resource Group on the north west shore of Bernic Lake, Manitoba, Canada. The mine has the largest known deposit of pollucite and is also the world's largest producer of caesium.

The mine has the largest tantalum reserves in Canada having estimated reserves of 2.1 million tonnes of ore grading 0.22% tantalum. The mine also has additional reserves amounting to 7.3 million tonnes of ore grading 2.76% lithium. The mine also produces caesium brine, which is converted into caesium formate which is used mostly as additive for drilling fluids to increase the fluid's density. A concentrated solution of caesium formate has a density of 2.3 g/cm3.

History
The pegmatite ore body now mined by the Tanco Mine was discovered in the late 1920s and the first mining started in 1929. Several times the mine was closed, reopened and closed, until in 1969 when it was reopened as a tantalum mine.

Cabot Corporation bought the mine in 1993, and began the production of caesium brine from pollucite in 1996.

Tanco Mine was purchased by Sinomine in June 2019. Sinomine paid $135 million with an additional 10-year lithium royalty consideration. Tax impact was unexpected.

Geology
The pegmatite found at the north west shore and below the lake floor of Bernic Lake is a granitic igneous rock enriched in the incompatible elements, for example caesium, lithium, tantalum and beryllium. Pegmatite forms if magmatic rock slowly crystallizes, and the incompatible elements are concentrated in the residual molten magma. Examples of minerals found in the mine are the lithium-containing spodumene and amblygonite, caesium-containing pollucite, beryllium-containing beryl and tantalum- and niobium-containing simpsonite and tantalite.

Deposit
The pollucite (chemical formula (Cs,Na)2Al2Si4O12*2H2O) deposit associated with the pegmatite is the largest known deposit of this mineral and with 350,000 tons, it accounts for two thirds of the known resources. With an estimated average global use of 30,000 kg/year the reserves of the mine would last for 2,000 to 3,000 years. The mined pollucite contains approximately 24% Cs2O. The ore body is 1400 m long, 600 m wide and 100 m deep. For several decades the pegmatites at Bernic Lake have supplied the world with the needed caesium.