Graupner T.1/2, Kempe U.2, Dombon E.3, Paetzold O.4, Leeder O.2
Melt- and gas-liquid inclusions in magmatic quartz, apatite and zircon in different types of granites and in hydrothermal quartz, fluorite and beryl from veins of the Ulaan uul, Buraat uul and Tsunkheg deposits were investigated by microthermometry and Raman spectroscopy. The homogenization temperatures (Th) of crystalline silicate melt inclusions range from 735 to 1030oC. Two generations of fluids are distinguishable in the ore veins according to Th, salinity and the gross compositions of fluid inclusions at room temperature. The Th values of aqueous inclusions in veins are between 180 and 433oC (first generation) and 55 and 225oC (second generation). Using fluid inclusions in vein fluorites I and II (core and rim of the same fluorite crystal) a temperature increase during crystallization of fluorite II in Ulaan uul is indicated.
Fluid inclusion data indicate that there are different trends in the development of fluid circulation (salinity, homogenization temperatures) within the occurrences of Ulaan uul and Buraat uul. These trends are correlated with phase separation in the fluids. In Ulaan uul a repeated opening of the system resulted in losses of considerable amounts of CO2 and significant mineral deposition. In Buraat uul there is no evidence for an intensive opening of the system, leading to the formation of low-grade ores. Fluid inclusion gases (first phase inclusions) in veins are CO2-dominated. The amounts of N2 and CH4 are notably lower with N2/CH4 ratios >1. Inclusions of the second phase contain no or only traces of gases (only indicated by clathrate melting) in most samples.
Fluorite I from Kyzyltau shows a strong enrichment of HREE and a strong negative Eu anomaly common for rare metal-bearing ore systems. Fluorite II from Buraat uul and Tsunkheg is characterized by a dramatic change in the incorporation of REE: decreasing HREE-contents with a decreasing europium anomaly. At the same time, the REE distribution pattern in fluorite II from Ulaan uul remained unchanged despite a strong increase in the total REE content.
The W(-Y-Be) deposits of Kyzyltau are characterized by high Y fractionation relative to REE, low-iron metasomatism (albitization, muscovitization) and the lack of notable tin mineralization despite the tin potential of the ore systems (e.g., high tin contents in greisens and wolframites). Fluid inclusion data and geochemical and geological indications suggest formation of the tungsten deposits near the tops of Li-F subvolcanic intrusions. Ore deposition occured at significant distances from the intrusions during repeated boiling and possible escape of gases.
A generalization of the reviewed data from the literature shows that the evolution of tin-bearing fluid-magmatic systems in a subsurface environment may lead to phase separation into two or three subsystems. >From high-temperature, Fe-rich, K-dominated brines cassiterite ores can precipitate in quartz veins together with Fe-chlorite and Fe-tourmaline (Bolivian type). If phase separation of a gas-rich (CO2, F2, N2), low-salinity and high-temperature fluid occurs, cassiterite-wolframite or wolframite ores are deposited during repeated boiling of the system with escaping CO2 (Cornwall/Devon and Kyzyltau type). The interaction of both fluid types with the wall rocks can result in a breakdown of plagioclase and the formation of late low-temperature brines with increasing Ca content.
According to this interpretation, tin mineralization may be found in a more favourable setting in the vicinity of the Kyzyltau deposits.