THE ROLE OF MAGMATIC PROCESSES IN THE FORMATION OF BANDED Li,F-ENRICHED GRANITES FROM THE ORLOVKA TANTALUM DEPOSIT, TRANSBAIKALIA, RUSSIA: MICROTHERMOMETRIC EVIDENCE

Abstract

Quartz- and topaz-hosted melt, fluid, and mineral micro-inclusions have been studied to shed light on the origin of the massive and banded Li,F-enriched granites that host the Orlovka tantalum deposit, in Transbaikalia, Russia. Certain quartz and topaz grains, similar to most of the others in their morphology and structure, contain primary or secondary melt inclusions (or both), suggesting that these rocks are of magmatic origin. Their textural features are assumed to stem from different regimes of cooling of parental melts, as indicated by morphological peculiarities of rock-forming minerals. From thermometric and analytical studies of the melt and fluid inclusions, it follows that the Li,F-enriched granites were formed from melt that was enriched in F (~4 wt%) and H 2 O (~6 wt%), contained CO 2 in addition to H 2 O (mole fractions are ~0.08 and ~0.92, respectively), and had unusually low viscosity (~50 Pa·s at 660°C). The existence of quartz crystals that contain melt inclusions and columbite–tantalite microcrystals in the same growth-zones suggests that the melt became tantalite-saturated during early stages of crystallization at the top of the intrusion and late in the crystallization sequence at lower levels. With regard to results of model calculations, the uppermost position of the most Ta-rich melt in the pluton is considered to be caused by the removal of interstitial residual melt from deeper parts of the magmatic body and emplacement into a previously solidified crystalline carapace rather than by crystal settling.