Fluid inclusion studies of the Changpai area in Zhuguang Mountain, northern Guangdong Province

The Changpai area, located within the southern part of the Zhuguang Mountain, is an important prospecting area in South China. In this paper, the authors present new fluid inclusion and stable isotopic data of the study area to discuss the nature of the oreforming fluids and uranium mineralization. Two types of fluid inclusions of the mineralization stage were recognized by petrographic observations:three-phase CO₂-rich (type Ⅰ) and two-phase vapor-liquid (type Ⅱ) fluid inclusions. Type Ⅱ fluid inclusions include two sub-types, i.e. liquid-rich (type Ⅱ-1) and vapor-rich (type Ⅱ-2). Based on petrographic features and the results of microthermometry, the fluid inclusions from the mineralization stage can be classified into two suites:(1) type Ⅰ and Ⅱ-2 fluid inclusions with homogenization temperatures of 291~388℃ and salinities of 3.23%~7.87% NaCl equivalent; (2) type Ⅱ-1 fluid inclusions with homogenization temperatures of 140~260℃ and salinities of 1.74%~10.24% NaCl equivalent. The fluid inclusions of suite 1 and 2 represent the early and late sub-stages of fluids during the mineralization stage, respectively. Raman analyses of fluid inclusions show that the vapor ingredient of the ore-forming fluid inclusions contains CO 2, CH₄ and H₂. S isotopic compositions (δ³⁴S=-10.2‰~-3.2‰) of the pyrites associated uranium mineralization are similar to those from other U deposits in South China. The fluid inclusions of suite 2 are characterized by wide range of salinities and have lower homogenization temperatures than those of suite 1, indicating that fluid mixing process took place. Combined with geological features, the authors hold that U was derived from the granite which was associated with partial melting of uranium-rich ancient crust, and that temperatures declining and fluid mixing played a key role in U precipitation.