西藏亚贵拉矿区两期岩体LA-ICP-MS锆石U-Pb定年及地质意义

摘要

亚贵拉Pb-Zn-Ag矿床是冈底斯北侧Pb-Zn多金属成矿带内的一个大型矿床.钻孔编录发现,亚贵拉矿区内棕红色花岗斑岩和灰白色石英斑岩均被大量的含硫化物石英脉穿切.同时,与这两套斑岩直接接触的灰岩地层内同样伴有强烈的硫化物矿化和矽卡岩化.本文采用LA-ICP-MS锆石U-Pb测年技术,对矿区含矿棕红色花岗斑岩和灰白色石英斑岩分别进行了年代学研究,得到两者锆石的加权平均年龄分别为132.1±1Ma和62.4±0.6Ma.在钻孔编录过程中未发现典型沉积喷流成因的矿体,而绝大部分矿体以含有大量矽卡岩矿物为特征,表明亚贵拉Pb-Zn-Ag多金属矿床应主要受岩浆热液控制.锆石U-Pb定年结果显示,该矿区首先经历了早白垩世与棕红色花岗斑岩有关的岩浆事件,然后于古新世又受到了与灰白色石英斑岩有关的含矿热液富集沉淀.结合已有研究资料分析可知,冈底斯北带的成矿作用从白垩纪一直持续到中新世,且均以Pb-Zn-Ag等贱金属矿化为特征,显示冈底斯北带古老基底对矿化金属类型的强烈控制.%The Yaguila Pb-Zn-Ag deposit is a large-size ore deposit on the north margin of Gangdise. Some experts considered that the ore deposit was formed by marine jet flow superimposed by magmatism, whereas others thought that it was formed only by magmatism. Detailed drill hole investigations into the Yaguila ore deposit show that both the light brown porphyry and the gray quartz porphyry contain a large volume of ore-bearing stockwork quartz veins. Meanwhile, large amounts of massive ores are developed along the contact zone between the two types of porphyries and limestone. Therefore, both the light brown porphyry and the gray porphyry must have played an important role in mineralization of the Yaguila ore deposit. In the present study, the authors analyzed zircon U-Pb ages for the two kinds of porphyries by using LA-ICP-MS. Mean ages of the concor- dant points for the two types of porphyries are 132.1 ± 1 Ma and 62.4 ± 0.6 Ma, respectively. Typical sedimentary ore bodies were not found during the field survey, but large volumes of skarn ores were found in most ore bodies, indicating that metals of the Yaguila Pb-Zn-Ag deposit were most likely derived from magmatic hy-drothermal fluids related to both the light-brown porphyry and the gray porphyry. Therefore, two stages of magma events were found in this ore deposit. The first magma event occurred in the early Cretaceous, related to the light-brown porphyry, which was superimposed by the second mineralization event related to the gray porphyry in the Paleocene. Combined with the results of previous researchers, the authors hold that the ore-forming process dominated by Pb-Zn-Ag mineralization on the north margin of the Gangdise belt occurred during Cretaceous-Miocene period. Intense Pb-Zn-Ag mineralization implies that the paleo-basement of northern Gangdise belt might have played an important role in controlling the metal types of the mineralization in this area.