PORPHYRIC ALKALINE-ULTRABASIC POTASSIC ROCKS OF THE CENTRAL TOMTOR MASSIF (ARCTIC SIBERIA): CARBONATIZED LAMPROITES

Abstract

We have discovered almost unaltered porphyric alkaline-ultrabasic potassic rocks in the central Tomtor Massif and have first carried out an X-ray microanalysis of the hosted minerals (phlogopite, leucite, clinopyroxene, amphibole, K-feldspar, etc.). Using modern analytical methods (neutron activation and X-ray fluorometry), we determined the contents of 43 major, trace, and radioactive elements and CO2 in 16 samples of differently carbonatized porphyric rocks and estimated the content of CaO in the unaltered rocks. A correlation analysis showed that during carbonatization, the rocks received Ca, Nb, REE, Sr, and Ba, and lost Si, Ti, Mg, and Na, thus transforming into rocks compositionally similar to kamafugites. The obtained structural data on concentric faults, horizontal separations, phenocryst orientation, etc. suggest that the studied alkaline-ultrabasic potassic rocks are volcanics that occur in a subsidence caldera 6-10 km across. Based on the classification diagrams of Holmes, Foley et al., and La Roche, we have established that the porphyric rocks are dominated by lamproites. The Tomtor Massif includes a unique "upper ore horizon" placer deposit and a "lower ore horizon" deposit. The latter is made up mainly of volcanic rocks intruded and metasomatically replaced by carbonatites and enriched in Nb and REE as a result of hydrothermal processes. Some porphyric rocks from dikes and volcanic pipes of the Tom tor Massif were referred to kamafugites according to their larnite-normative composition.