RARE-EARTH MINERALS IN CARBONACEOUS SHALE OF THE KUMAK GOLD DEPOSIT, SOUTH URALS, RUSSIA

The article presents the results of the study of carbonaceous shales from the Kumak deposit. The analysis of their geochemical features reveals their shallow water-near shore sedimentation conditions. The high-aluminous terrigenous sedimentary material underwent minimal transport and was formed mainly due to destruction of basic rocks. The sediments experienced a high degree of weathering typical of humid climates and deposited under conditions of oxidation and pre-oxidation. It is found that rare-earth elements (REE) practically do not accumulate in carbonaceous shales; their contents are correlated directly with the amount of terrigenous admixture, Normalization with respect to Post-Archean Australian shales (PAAS) shows that REE contents are in the range from 0.1 to 1 units. The carbonaceous shales are characterized by uniform normalized curves-preferential accumulation of light REE as against heavy REE, with a distinct europium minimum in a number of samples. The main minerals-concentrators for light REE are Monazite-(Ce) and Rhabdophane-(Ce), and for heavy REE-Xenotime-(Y). Xenotime is an yttrium mineral, with the content of Y2O3 up to 47.61 wt.%. Among the other REE which regularly replace yttrium, dysprosium, gadolinium and erbium prevail. Monazite is a cerium mineral, with the content of Ce2O3 up to 37.07 wt.%. The ratio of Ce2O3 to the sum of the other light REEs is stable and equals 1. In contrast to xenotime, monazite contains a significant thorium admixture reaching 13.32 wt.% of ThO2. Georceixite, bastnasite and pietersite-(Ce), which is an agardite-(Na) mineral, are represented by single grains. A large number of Th-REE compounds of complex composition and an unidentified As-Ce mineral in zircon are also observed.