西藏冈底斯成矿带斑岩铜矿的成岩成矿年龄

DIAGENETIC AND MINERALIZATION AGES FOR THE PORPHYRY COPPER DEPOSITS IN THE GANGDISE METALLOGENIC BELT, SOUTHERN XIZANG

近年来在西藏开展的资源评价和研究工作初步揭示出冈底斯成矿带斑岩铜矿床具有巨大的资源潜力。本文通过多种方法对冈底斯成矿带中驱龙、厅宫、冲江等斑岩铜矿进行了同位素定年研究 ,确定出冈底斯斑岩铜矿成岩年龄为 (1 7.5 8± 0 .74 )Ma(锆石离子探针 (SHRIMP) ) ,成矿年龄为 (1 5 .99± 0 .32 )Ma(Re Os等时线法 ) ,蚀变年龄介于 1 2 .0 0～ 1 6 .5Ma之间 (K Ar法 ) ,成矿年龄与蚀变年龄较为一致。根据同位素定年结果 ,冈底斯斑岩铜矿的成岩成矿年龄明显晚于冈底斯碰撞期花岗岩质侵入岩的年龄。因此笔者认为 ,西藏冈底斯斑岩铜矿形成于碰撞后的伸展构造背景 。

Recent examination and assessment about the porphyry copper deposits in Gangdise metallogenic belt, southern Xizang have revealed that these porphyry copper deposits are highly prospective. Several methods have been used for the isotopic dating of the Qulong, Tingong and Chongjing porphyry copper deposits, which gives out a diagenetic age of 17.58±0.74Ma (single-zircon dating of SHRIMP), a mineralization age of 15.99±0.32Ma (Re-Os isochron dating) and an alteration age ranging between 12.00Ma and 16.5Ma (K-Ar dating).The mineralization age is in general agreement with the alteration age. It can be seen that the diagenetic and mineralization age for the porphyry copper deposits in Gangdise metallogenic belt is noticeably later than the age for the collisional granitic intrusive rocks in this belt. The authors contend that the porphyry copper deposits in the study area were formed in a post-collisional extensional tectonic setting, and closely related to the delamination of the mountain roots of the orogenic belts and the uplifting of the Qinghai-Xizang Plateau.