Formation temperature, REE contents and optical spectra of fl uorite from the Mikhalkovo deposit (Central Rhodopes, Bulgaria): genetic implications and practical signifi cance

摘要

The Mikhalkovo fl uorite deposit is a suitable example of a deposit where the temporal and spatial crystal-morphological evolution of the fl uorite crystals and aggregates can be traced, as well as its link with the temperature of formation and zonal distribution of the REE during the ongoing mineral-forming process. Fluorite fl uid inclusions data indicate that the fl uorite in the Mikhalkovo deposit crystallized from 190 ℃ to 110 ℃ and a salinity ≤ 1 eq. wt. % NaCl in a variety of textural and morphological types. Detailed study of the patterns of temperature change with progressive growth of the different crystal zones shows that fl uorite precipitation took place during rising (for octahedral crystals) and falling (for cubic crystals) temperatures, and that the pH controlled the process of crystallization and the formation of one or another crystal face. REE data indicate that the deposition was multistage and the composition of sequential impulses of hydrothermal fl uid changed. Thus, the implications of the REE data are that fl uorine activity decreased with time and with successive batches of hydrothermal fl uid. In the Mikhalkovo deposit there is a strong correlation between the morphology of fl uorite crystals and its composition. The statistical treatment of the content of REE has revealed that the main fl uorite varieties differ considerably also in the distribution of REE. There is a tendency for the LREE to be concentrated in earlier, whereas the HREE in later mineralogical varieties as well as an increase of their content in the uppermost levels in the deposit. While the photoluminescence (PLS) and optical (OAS) spectra reveal different impurities such as Ce, Sm and Eu, the thermoluminescence (TLS) and X-ray luminescence (XRLS) spectra are dominated by signals from Dy ions with only traces of emission from the other impurities. The considered examples reveal that part of the characteristic colour and luminescence centers and their combination in natural fl uorites are reliable as geochemical and genetic indicators. The analysis of the spectral results makes it possible to evaluate theconditions of formation of crystals and aggregates of fl uorite, the evolution of the composition in the mineral-forming medium andto interpret the nature of the zoning in the crystals, individual bodies and the deposit.Natural fl uorite crystals contain information not only about the environment of their origin, but also about the conditions that are needed for growing optical grade synthetic crystals. This makes them suitable for both genetic studies and technological purposes. The comparative analysis between the optical properties of the raw fl uorite and artifi cially grown monocrystals is particularly rewarding in revealing that (1) the TLS are informative about their prospective radiation stability, (2) the OTS show the contents of the structural admixtures (REE) that would hinder their prospective application, (3) the OAS are also used to control the homogeneity of the raw material as well as the process of crystal growth, (4) the infrared (IRS) are characteristic for every fl uorite variety and thus are used for the preparation of the raw material for crystal growth.