A FIRST FINDING OF ANOMALOUSLY Cs-RICH ALUMINOSILICATE MELTS IN ONGONITE: EVIDENCE FROM MELT INCLUSION STUDY

Abstract

The chemical properties of cesium allow accumulation of this element in the late silicic derivatives of igneous complexes, in particular, in rare-metal granites, pegmatites, and related metasomatic rocks. However, the Cs content can be high enough only in pegmatites to form its own mineral (pollucite), which occasionally occurs in considerable amounts. In other rocks, Cs concentrates largely in micas and feldspars. The possible maximal level of Cs accumulation in the melt remains poorly studied. Important new information has been obtained from the study of volcanic glass and melt inclusions (MI) in minerals. The rhyolitic glass concentrates 220–870 ppm Cs, occasionally up to 3770 ppm. To date, the highest Cs2O content (1.2–5.5 wt %) has been detected in MIs captured by quartz from miarolitic pegmatite of the Malkhan field of the central Transbaikal region and the southwestern Pamirs (Leskhozovskaya and Vezdarinskaya veins).