The fluid immiscibility at magmatic to hydrothermal transition and its bearing on ore-forming process in the Mianning REE deposit, Sichuan, China: Evidence from fluid inclusions

摘要

The Mianning REE deposit hosts the third world's largest known REE resource. Three types of ore body i.e., aegirine-augite-barite pegmatite, carbonatite and veinlet types, have been recognized. Fenitizations is well developed around the carbonatite complex which is composed of various sizes of cabonatite, alkali basic pegmatite, lamprophyre, granite porphyry and diabase dikes. Bastnaesite constitutes the major REE ore mineral in association with gangues of aegirine, augite, microcline, perthite, calcite, quartz, barite and fluorite. As evidenced by hundreds of fluids inclusions studies, the great bulk of magmatic-related ore deposits were formed from dense aqueous fluids, in part with a significant mole fraction of CO_2 and other gas, at temperatures of well under 500 ℃( ROEDDER, 1992). The dense aqueous fluids is a high-density, high-salinity phase and is typically referred to as either "hydrosaline liquid/melt" or "brine"(HALTER, WEBSTER, 2004). Hydrosaline liquid inclusions with daughter minerals, along with different type of fluid inclusions, have been found in fluorite and quartz in the early stage of the aegirine-augite-barite pegmatite vein in Mianning REE deposit. These fluid inclusions pppulation provides a unique opportunity of investigating the composition of the fluid during the magmatic to hydrothermal transition. The present study focuses on the nature of the fluids involved in REE mineralization in the deposit, and on the possibility that fluid immiscibility at magmatic to hydrothermal transition of an original H_2O-CO_2-BaSO_4-NaCl-(F-REE) fluids may have contributed to ore genesis for Mianning REE deposit.