DISTRIBUTION OF U, TH, AND K IN GRANITES OF THE AUE AND EIBENSTOCK MASSIFS, ERZGEBIRGE, GERMANY

Samples from mine workings and drill holes were used to study the dist ribution of U, Th, and K in the Aue granitic massif over a vertical ex tent of nearly 2.5 km and in some granites of the Eibenstock massif do wn to a depth of 250 m from their gently dipping roof. In the inner zo nes of the granites, at a depth of 1-1.5 km below the present-day surf ace, the original content of U and Th is 35-40 ppm, and the U/Th ratio in the unaltered granites is close to unity. Toward the surface, the granites become increasingly oxidized (as indicated by the Fe oxidatio n coefficient), the primary distribution of the radiogenic elements is disturbed (according to fission-track studies), and the abundances of U and Th decrease sharply. For example, in the Aue massif, the averag e U content decreases nearly eightfold upward, and the Th content decr eases nearly sevenfold. Enormous amounts of U and Th-hundreds and thou sands of tons-were leached from the granite of the Aue dome (whose top is exposed) under the influence of Neogene and Paleogene epigenetic p rocesses. These amounts are much greater than the uranium reserves hos ted within the large Schlema deposit, which is marginal to this massif . In the weaker oxidized granites of the eastern slope of the Eibensto ck massif, which are separated from the surface by a 500-1000-m layer of gneissic schists, the epigenetic losses of U and Tn are less signif icant. Computer modeling indicates that Fe oxidation of granites influ ences the scale of U scavenging at 150 and 25 degrees C. The K content (its initial value is estimated at 5.4 wt % K2O) does not vary with t he approach to the surface, but substantially depends on the distance from the contact of the granites, which was intersected by pathways of mass exchange between the melt and the environment during the emplace ment of the granitic massifs. The original abundances of U, Th, and K in the deep zones of the Erzgebirge pluton enable the radiogenic heat generation of the granites to be evaluated at similar to 15 HPU, while most intrusions of granitoid composition are characterized by a value of 4.8 HPU.