北淮阳成矿带东段东溪低硫型浅成热液金矿床:来自蚀变矿物、流体包裹体和氢氧同位素的证据

The Dongxi low-sulfidation type epithermal gold deposit,in the eastern section of the Beihuaiyang metallogenic zone:Evidence from alteration mineralogy,fluid inclusion and H-O isotopes

北淮阳成矿带东段早白垩世晓天火山盆地产出戴家河金矿、东溪金矿、隆兴金矿等一系列浅成低温热液型金矿床.主要针对晓天火山盆地中规模最大的东溪金矿开展详细的围岩蚀变观察、X射线衍射矿物鉴定、流体包裹体显微测温、激光拉曼探针和氢氧同位素分析.X射线衍射矿物鉴定证实东溪金矿发育有低硫型浅成低温热液型金矿中特征性蚀变矿物-冰长石的存在,结合普遍发育的硅化-碳酸盐化,推断东溪金矿形成于偏碱性的环境.流体包裹体岩相学研究表明,东溪金矿主要发育富液相的气液两相水溶液包裹体,成矿期流体的均一温度范围为216~254℃,盐度为0.2 wt.%~1.7 wt.%NaCl eq.,属中低温度、低盐度的流体.成矿后期流体的均一温度明显降低,其盐度也略有降低,显示了更广泛的大气降水的参与.氢氧同位素数据显示,东溪金矿的成矿流体远离岩浆水的范围,而靠近大气降水演化线.氢氧同位素数据,结合中低温度、低盐度的流体特征,表明循环的大气降水广泛参与了成矿作用.对蚀变矿物、流体包裹体和氢氧同位素分析结果表明,东溪金矿是典型的低硫型浅成低温热液金矿床.

The Early Cretaceous Xiaotian volcanic basin in the eastern section of the Beihuaiyang metallogenic zone hosts a group of epithermal gold deposits such as Daijiahe,Dongxi and Longxing. In this study,we focused on the Dongxi gold deposit,which is the largest gold deposit in the Xiaotian volcanic basin,to perform detailed observation of wall rock alteration,X-ray diffraction(XRD) mineral identification,fluid inclusion microthermometry,Raman spectroscopy and hydrogen-oxygen analysis. The XRD mineral identification confirms the existence of adularia in the Dongxi gold deposit,which is one of the characteristic altered minerals in the low-sulfidation type epithermal gold deposits. In combination with the commonly developed silicification and carbonatization,we deduce that the gold deposit was developed in a relatively alkaline environment. Fluid inclusion petrographic study shows that liquid-rich aqueous fluid inclusions were ubiquitous presence in the Dongxi gold deposit. Fluid inclusion microthermometric results show homogenization temperatures(Th) of 216~254 ℃and salinities of 0.2 wt.% ~1.7 wt.% NaCl equivalent,indicating that the ore-forming fluids are moderate-low temperature and low salinity. The homogenization temperature further decreased in the post-ore stage,and its salinity also decreased slightly,indicating the involvement of the meteoric water in the post-ore stage. Hydrogen and oxygen isotopic data are plotted nearby the line defined by meteoric water,but far from the range of magmatic water. Such hydrogen and oxygen isotopic data,combined with moderate-low temperature and low-salinity fluid characteristics,indicate that a large amount of circulated meteoric water was involved in mineralization. Based on the evidence from alteration mineralogy,fluid inclusion and H-O isotopes,we suggest that the Dongxi gold depoist is a low-sulfidation type epithermal deposit.