Magmatic vs. hydrothermal origin of fluorites from Vlastějovice, Bohemian Massif

Abstract

Several aspects of fluorite mineralization in barren pegmatites are studied and discussed in order to explain their magmatic or hydrothermal origin. The barren pegmatites are intruded in skarns at Vlastějovice in the northern part of the Moldanubian Zone. Abundant violet to colourless fluorite with hornblende and garnet were formed during pegmatite contamination by the country-rock skarn. The process included import of calcium and fluorine from the skarn body and formation of reaction zones along the pegmatite margins. Optical microscopy and cathodoluminiscence study show that fluorites are very homogeneous, without any visible growth zones. The REE patterns show a strong HREE depletion in fluorites from contact zones in contrast to those from central zones of pegmatites. The HREE contents in fluorite from contact zones are interpreted as a result of REE diffusion between fluorite and garnet at high temperatures. Pseudosecondary fluid inclusions found in fluorite (FS1) probably represent evolved primary pegmatitic fluid saturated by calcium (CO2 loss followed by calcite precipitation) at the pegmatites margins. Minimal temperature of fluorite crystallization can be estimated from the homogenization temperature of early pseudosecondary inclusions (FS1A: 308-372 °C). Thus, we can assume, that fluorites probably formed during the late stages of pegmatite consolidation under magmatic-hydrothermal transition conditions.