Fluid regime and ore formation in the tungsten(-yttrium) deposits of Kyzyltau (Mongolian Altai): evidence for fluid variability in tungsten-tin ore systems

The Kyzyltau ore field is located in the northern part of the Mongolian Alt ai Mountains. W(-Y-Be-Mo) mineralization occurs in veins and stockworks in granites, felsic volcanics, basaltic flows, and conglomerates. The main ore minerals in the veins are wolframite, fluorite, beryl and minor molybdenit e. Inclusions in hydrothermal quartz, fluorite and beryl from veins, as wel l as in magmatic quartz in vein-hosting granites of the Ulaan uul, Buraat u ul and Tsunkheg deposits were investigated using microthermometry and laser Raman spectroscopy. Pseudosecondary/primary inclusions in the Kyzyltau ore veins are predominantly liquid-rich aqueous inclusions, containing variabl e amounts of CO, as the main gas component, and showing N-2/CH4 ratios > 1. Secondary inclusions contain no or only trace concentrations of gases, as indicated by clathrate melting. The T-h values of pseudosecondary/primary i nclusions in veins are between 180 and 433 degrees C, Fluid inclusion data indicate that phase separation processes leading to fluid immiscibility occ urred in vuggy quartz II and green fluorite II in the Ulaan uul and Buraat uul ore veins. Phase separation pressures of 100-350 bars were estimated. F luorite I from the Kyzyltau ore field shows a strong enrichment of HREE and a strong negative Eu anomaly common for rare metal-bearing ore systems. Fl uorite LI from Buraat uul and Tsunkheg is characterized by a change in the incorporation of rare earth elements (REE) with decreasing HREE contents an d a decreasing Eu anomaly. The REE distribution patterns in fluorite LI fro m Ulaan uul remained unchanged compared with fluorite I despite a strong in crease in the total REE content. A review of the literature shows that from high-temperature, Fe-rich, K-dominated brines cassiterite ores can precipi tate in quartz veins together with Fe-chlorite and Fe-tourmaline (Bolivian type). If phase separation of a gas-rich, low-salinity and Fe-rich fluid oc curs, cassiterite-wolframite ores may be deposited (Cornwall/Devon type). T he deposits of Kyzyltau are characterized by low-Fe alteration assemblages, a predominance of sodium over potassium, high contents of REE and Y, and a lack of extended tin mineralization despite the tin potential of the ore s ystems. Fluid inclusion data as well as geochemical and geological indicati ons suggest formation of the tungsten deposits near the tops of Li-F-rich s ub-volcanic intrusions. We interpret the pH of the mineralizing fluid to be the main factor controlling wolframite precipitation in the Kyzyltau ore f ield. Fluid-wall rock interactions, a lowering of the temperature and unmix ing processes in the ore fluid generated contributions to neutralization an d buffering of the acid CO2-bearing fluid into a pH range where tungstates were precipitated. It can be inferred that formation of tungsten ores witho ut precipitation of extended tin mineralization is possible in deposits cha racterized by high potential tin and tungsten. According to this inference, tin mineralization may be found in a more favourable setting in the vicini ty of the Kyzyltau deposits. (C) 1999 Elsevier Science B.V. All rights rese rved.