The red rocks that occur along faults and shear zones in the Precambrian granitic ter-rane north of Lake Athabaska are more radioactive than others in the area. They are mylonites ranging from protomylonite to ultramylonite, the latter generally forming anastomosing veinlets in the less intensely crushed types.

In the protomylonites the feldspar is deformed albite colored with hematite dust, especially along the cleavages; unstrained minerals fill the interstices. In the more intensely crushed types the hematite dust permeates most of the rock; the unstrained minerals present form scattered aggregates or may locally replace the mylonite completely.

The first of these late minerals is clear albite, which forms rims on the red albite relicts. Then follow, in variable order and with repetitions, clear subhedral quartz and albite, specularite, and penninite containing radioactive anatase grains. The latest veins contain a]so pitchblende, calcite, and specularite. Oxidation products of pitchblende are locally abundant in solution cavities.

The early iron and later radioactive elements were introduced probably by solutions that rose along the mylonite zones. The other elements were probably present before mylonization; and all the late minerals, excepting pitchblende and calcite, may be products of recrystallization.

The dusty hematite was probably not formed either by radioactive bombardment of ferrous minerals or by unmixing of iron-feldspars.

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