Radioactive tertiary porphyries in the Central City district, Colorado, and their bearing upon pitchblende deposition

Author / Creator

Phair, George

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Physical

Summary

Results of analyses of 117 samples indicated that the Tertiary porphyry sequence in the middle part of the Front Range ranks among the most radioactive igneous series in the world, according to the...

Results of analyses of 117 samples indicated that the Tertiary porphyry sequence in the middle part of the Front Range ranks among the most radioactive igneous series in the world, according to the present literature. With the intrusion of nonporphyritic, lime-poor, quartz bostonite dikes in the western half of the Central City district, the magmatic enrichment in both uranium and thorium reached a peak of more than 20 fold over the best available averages for granitic rocks.

The sequence of events in the Central City district is thought to be as follows: (1) intrusion of slightly ot moderately radioactive monzonite throughout the eastern half of the district, (2) intrusion of excessively radioactive, nonporphyritic varieties of quartz bostonite in the western half of the district north of what was to become the area of pitchblende deposition, (3) intrusion of the highly radioactive quartz bostonite porphyry dikes with which 15 of the 17 known occurrences of pitchblende are now associated (within 500 feet), (4) deposition of pitchblende as a local and unusual variant in the regional pyritic-gold ore deposition near, but not in, the quartz bostonite porphyry dikes. The implication of the field and chemical evidence is that uranium-rich solutions given off by a cooling quartz bostonite mass at depth became further enriched by leaching uranium from the quartz bostonite channelways while en route to higher levels. Zircon, the probably host for much of the uranium and part of the htorium in the rocks, separated in reduced amounts from the youngest quartz bostonite liquids - a change which, in effect, tended to throw uranium into the residual liquid. Possible mechanisms by which uranium became concentrated with respect to thorium in the derived aqueous solutions are considered. In this connection the late magmatic introduction of fluorite and of ferric oxides may be of special significance.

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