个旧西凹铜-锡多金属矿床地球化学特征及地质意义

Geochemical Characteristics and Geological Significance of the Xi'ao Cu-Sn Polymetallic Deposit in the Gejiu Area

西凹铜-锡多金属矿床位于个旧东区老卡岩体内侧边缘的蚀变带中,矿化与钾长石化、萤石化和黄铁矿化等蚀变密切相关。成矿作用可分为矽卡岩阶段、锡石-石英阶段、石英-硫化物阶段和碳酸盐岩阶段。对该矿床未蚀变花岗岩、绿帘石-绿泥石化花岗岩及钾化花岗岩矿石的主量、微量及稀土元素,矿化期黄铁矿的硫、铅同位素特征进行了分析。研究发现,元素W、Bi、Sn、Cu、Zn、Ag具有共同成矿的特征。钾化花岗岩矿石及绿帘石-绿泥石化花岗岩的稀土元素特征与未蚀变花岗岩相似,表明成矿与花岗岩关系密切。硫同位素特征暗示玄武岩可能为Cu的成矿提供了物源;铅同位素表明成矿物质具有混合源的特征,表明花岗岩也为成矿提供了部分来源。Eu异常、较高的Y/Ho比值和Y、Ho发生分馏的特征表明早期成矿流体温度较高且富含氟,结合钾长石化、萤石化等围岩蚀变以及流体包裹体研究,认为该矿床成矿流体具有高温、富CO2、富氟富钾的特征,具典型的岩浆热液矿床特征,可能形成于后碰撞的伸展构造背景中。

The Xi＇ao Cu-Sn polymetallic deposit is located in the alteration zone at the margin of the Laochang-Kafang granite in Gejiu, Yunnan province. The mineralization is closely associated with K-feldspathization, fluoritization and pyritization, which can be divided into four ore-forming stages： skarn, quartz-cassiterite, quartz-sulfide and carbonate. Major, trace and rare earth element characteristics of the least altered granites, chloritized-epidotized granites and K-feldspathized granite ores, as well as the sulfur and lead isotopic features of the pyrites from this deposit were systematically studied. The results show that the ores are rich in W, Bi, Sn, Cu, Zn and Ag. REE characteristics of the granite ores and chloritized-epidotized granites are similar to those of the least altered granites, indicating a close relationship between the mineralization and granite. Sulfur isotopic features of the pyrites are similar to the basalt and basalt ores developed in Kafang Cu-Sn polymetallic deposit adjacent to the Xi＇ao deposit, implying that the basalt probably was a source of Cu. Lead isotopic characteristics indicated a mixing origin of ore forming elements. Eu anomalies, fraction of Y and Ho, and high Y/Ho ratios suggested that the ore forming fluid was high temperature and fluorine rich. Combined with the widespread fluorite and K-feldspar, and previous results on fluid inclusions, we suggest that the ore-forming fluid was of high temperature and riched in CO2 and potassium and fluorine, which is similar to the intrusion-related hydrothermal deposits developed in the collisional orogeny, implying the deposit was formed in the post-collisional extensional tectonic setting.