Abstract:

The liquid immiscibility and hydrothermal fluid exsolution from magma are the important separation mechanisms of ore-forming fluid from granite magma. At present, the understanding of the two mechanisms is mainly from researches of melt and fluid inclusions, or none-in-situ experiments. The newest hydrothermal diamond anvil-cell (HDAC) is used to observe the two processes in situ. In exsolution experiments with the initial sample of silicate and pure H₂O or LiCl dissolved aqueous fluid, the H₂O-rich fluid separated from H₂O-saturated silicate melt when H₂O is saturated in silicate melt. In the experiment of liquid immiscibility with the initial samples of NaAlSi₃O₈-LiAlSiO₄-SiO₂-LiCl-H₂O, the silicate melt balls can be separated into a H₂O-rich melt and H₂O-poor melt during cooling from homogeneous silicate melt. The sharp fall of pressure can induce the liquid immiscibility. These experimental results show that the hydrothermal fluid exsolution occurred in H₂O-saturated silicate melt is the second boiling of magma, and plays a great role in the mineralization of granite type deposit. The H₂O-rich melt separated from granite magma by liquid immiscibility can crystallize big and oriented crystals, suggesting that liquid immiscibility of granite magma is an important formation mechanism for granite pegmatite. The differences between the above two type experiments suggest that the element Lithium is an important factor for liquid immiscibility in granite magma. In the further research, we should combine the HDAC with microbeam analysis in order to analyze the enrichment process of metal element at high temperature and pressure.

Key words: hydrothermal diamond anvil-cell, in-situ observation, liquid immiscibility of granitic magma, granitic magma boiling, granitic pegmatite