班公湖-怒江带西段尕尔穷矽卡岩型铜金矿含矿母岩成岩时代的重新厘定及其地质意义

The Redefinition of the Ore-forming Porphyry’s Age in Gaerqiong Skarn-type Gold—Copper Deposit, Western Bangong Lake—Nujiang River Metallogenic Belt, Xizang(Tibet)

尕尔穷矿床为近几年在班公湖—怒江带西段取得找矿新突破的产于斑岩—碳酸盐岩接触带的矽卡岩中和构造破碎带中的铜金(铁)矿床,该矿床位于班公湖—怒江缝合带南缘的措勤—申扎火山岩浆弧中。本文在前人研究的基础上,进一步开展成岩成矿年代学研究,拟解决前人资料中成岩与成矿相差22Ma以上的疑惑,重新厘定矿床的形成时限。在详细了解矿床地质特征的基础上,确定了矽卡岩型铜金矿的成矿母岩为石英闪长岩,利用LA-ICP-MS锆石U-Pb手段对石英闪长岩进行定年。阴极发光(CL)图像显示锆石振荡环带清晰,长轴和短轴之比在1.5∶1～4∶1之间。锆石上15个有效测点给出206Pb/238U年龄加权平均值为87.1±0.4Ma(MSWD=0.88),代表石英闪长岩的结晶年龄,结合矿石中辉钼矿Re-Os同位素年龄(86.87±0.50Ma),显示矿床由岩体侵位结晶至矿质沉淀经历了1Ma左右的演化时间。矿床可能形成于班公湖—怒江特提斯洋关闭后冈底斯—念青唐古拉板片和南羌塘—三江复合板片的弧—陆碰撞阶段。

Abstract： Gaerqiong deposit is a new found copper--gold （iron） deposit in Western Bangong Lake （ Bangong Co）--Nujiang River metallogenic belt, which is hosted in the skarn belt in the contact zone of porphyry and carbonatite and in the fault fracture zone. It is located in the Coqen--Xainza magmatie arc in the south edge of Bangong Lake--Nujiang River suture zone. Based on the previous research, this paper is to solve the puzzle that porphyry emplacing is older 22Ma or more than ore-forming, and to redefine the ore-forming time range of the deposit by chronology research. A detailed survey of the deposit characteristics was carried out, and quartz diorite is determined as the ore-forming porphyry. The LA-ICP-MS U-Pb dating and cathodeluminescence was carried out for the zircon grains from quartz diorite. The cathodeluininescence images show that zircon grains have clear Inaginatic zonation, with the ratio of long axis to short axis range from 1.5 to 4 ： 1. The analytical results indicate that the weight average 206pb/238U age of 15 points on the grains is 87. 1 ± 0.4Ma （MSWD = 0. 88） , which represents the porphyry＇ s forming age. As the Re-Os isoehron age of molybdenite from ore is 86.87 ± 0.50 Ma in the deposit, it is concluded that it took 1 Ma for fluid evolving and orebody forming after the crystallization of porphyry. We inferred that the deposit was formed during the arc--landmass collision between Gangdise-- Nyainqentangglha plate and Southern Qiangtang--Sanjiang composite plate after the Bangong Lake--Nujiang Thetis Ocean closed. Key word： Gaerqiong ; quartz diorite ; LA-ICP-MS Lake--Nujiang River inetallogenic belt, Xizang（Tibet） zircon U-Pb dating; arc--landmass collision; Bangong