Keystone Quarry, Chewelah Mining District, Stevens County, Washington, USAi

Regional Level Types
Keystone Quarry	Quarry
Chewelah Mining District	Mining District
Stevens County	County
Washington	State
USA	Country

The Keystone Quarry is located in sec. 9, T31N, R39E and was first opened in the 1890's as a marble quarry and until about 1905 was operated by the U.S. Marble Co. as a source of ornamental stone. As late as 1945, a closely spaced row 10 feet long, of drill holes in the brucite body, which was a unique feature of the Keystone deposit, could still be seen as evidence of the size of blocks that were quarried and of the care taken in breaking the material free. Published analyses (Shedd, 1903) showing the high magnesia content of the "marble" led to its "rediscovery" as a magnesite deposit in 1916, and it was from the Keystone that the first magnesite shipments from Stevens County were made. Discovery and development of the Finch deposit, which was larger and more favorably located and was owned by the same company (Northwest Magnesite), put the Keystone in a standby position for many years. Production was resumed in 1946 after completion of an aerial tramline to the deposit and has continued on a limited basis. Total production is estimated to have been around 500,000 tons of crude magnesite. The ore was rather high grade, averaging only 3 percent silica and 3 percent lime.

The magnesite at the Keystone was for the most part dark gray to almost black. The deposit was roughly triangular and had a maximum width of 250 feet and a length of about 500 feet in a northeasterly direction. The exposed surface was about 63,000 square feet, excluding a wedge of dolomite in the northeastern part. The floor of the deposit lay at about 3,600 feet, or roughly about 250 feet below the highest outcrop. Owing to the very steep slope, the deposit was wedge shaped and had a blunt edge toward the southeast.

Toward the northern end of the deposit there occurred a lens of rather massive almost translucent gray-green brucite, prized by the early "marble" workers because of its attractive color and the comparative ease with which it could be worked (hardness of brucite = 2½ of calcite marble = 3). Unfortunately, objects d'art and ornamental stone carved from the brucite showed in the course of time an unpleasant tendency to develop a white powdery coating (hydromagnesite) not characteristic of the better quality of marbles. This was indeed one of the features that led to the curtailment and eventual closing of the early quarrying operations at the Keystone. The brucite, probably, was formed as a hydrothermal alteration, controlled by a fault channelway, of earlier magnesite. In turn, the brucite was locally altered to hydromagnesite. The brucite body was about 20 feet wide at its maximum, and extended some 100 to 150 feet in length. Chemical analysis and microscopic study of the material by Clarke (1903) led him to conclude that, although brucite was the dominant mineral, it was variably intermixed with serpentine, chlorite, hydromagnesite, and dolomite.