Recently I came upon a copy of Geological Survey Circular 186, 1952, F.B. Moore and C.R. Butler, Pitchblende Deposits at the Wood and Calhoun Mines, Central City Mining District, Gilpin County, Colorado. Like many Geological Survey documents, it contains a pocket with neatly folded scale drawings of the mine workings. These drawings chart the location and elevation of the shafts and drifts and give a best estimate as to the extent of the formation.

Vein Structure in the Central City District. (From Geologic Survey Circular 186, 1952.)

What is interesting about the map above is not so much the minute detail of the locations, but rather the obvious trend of the veins (solid lines with dots).  They are all east-northeast trending.  The country rock is largely precambrian granite gneiss and quartz biotite schist according to the survey. It doesn’t take long to figure out that the mine locations correlate with the veins.

Geological Survey maps of 3 levels of the Wood Mine, Gilpin County, Colorado (Geological Survey Circular 186, 1952).

The second figure was generated by Moore and Butler in 1950 and shows the locations of pitchblende occurrences in three levels of the Wood Mine near Central City, Colorado. The red dots indicate the location of pitchblende along the three drifts at the 135, 197, and 275 foot levels of the mine.

Profile of the Wood Mine, Central City District, Illustration from Geological Survey Professional Paper 371, 1963.

 The Wood Mine was an early and prolific source of pitchblende, though presumably it began as a gold /silver operation. The workings reportedly reached a depth of 600 ft.  The Wood vein is in a fault fissure that shows extensive alteration from hydrothermal action. The width of the vein varies with the type of country rock in which it is found, but ranges from from 1 to 18 inches and has been followed for a lateral distance of nearly 1000 ft.

The productivity of uranium mines is commonly expressed in terms of equivalent weight of U3O8 rather than weight of ore, given the large variety of mineral forms uranium is found to occur in. The figure above is from Geological Survey Professional Paper 371, P.K. Sims and collaborators, Geology of Uranium and associated Ore Deposits, Central Part of the Front Range Mineral Belt, Colorado. Extensive stoping has been done in an attempt to vertically intercept the vein. This was a common practice in hard rock mining- let gravity bring the muck to you.

Pitchblende was discovered on the dump of the old Wood shaft in 1871. Circular 186 reports that by the end of 1872, 6,200 lbs of ore containing 3,720 lbs of U3O8 had been removed. By 1916, 102,600 lbs of ore bearing 30,040 lbs of U3O8 equivalent had been recovered.  The high grade pitchblende was hand sorted and that below ca 10 % was discarded or lost in gold and silver processing.

In Circular 371, Sims observes that “Pitchblende occurs as small, discontinuous lenses and streaks on the footwall of the Wood Vein, which are separated by nonuraniferous vein material.”

What is intriguing is that pitchblende was apparently an item of commerce in the early 1870’s. Radium, extracted from pitchblende, was not discovered until 1898 by the Curies.  A procedure for the preparation of sodium diuranate, Na2U2O7 6H2O, was reported as early as 1849 (Patera, J. pr Chem. 1849, [i] 46, 182. Early uses of uranium yellow were in paints and stains for glass and porcelain. This pigment has also been used for the production of fluorescent uranium glass.