新疆西天山智博铁矿成矿时代——来自锆石U-Pb年龄和黄铁矿Re-Os年龄的证据

Zircon U-Pb and Pyrite Re-Os Geochronology of the Zhibo Fe Deposit in Western Tianshan,Xinjiang,China

智博铁矿是新疆西天山阿吾拉勒铁成矿带的大型富铁矿床,在区域成矿规律研究和找矿勘查方面极具代表性。本文选择能限定铁矿体时代的围岩火山岩、侵入围岩地层的石英闪长岩开展锆石U-Pb测年,并挑选与磁铁矿共生的黄铁矿开展Re-Os同位素定年,试图厘定其成矿时代。本文获得5件铁矿石样品中黄铁矿的Re-Os等时线年龄为307±12Ma,围岩绿帘石化安山岩的锆石U-Pb年龄为312.0±3.4 Ma,安山质集块岩年龄为317.8±2.5 Ma,两件石英闪长岩的年龄分别为317.0±2.2 Ma和315.4±0.9 Ma,据此认为智博铁矿成矿于晚石炭世,该时期火山喷发-岩浆侵入-铁矿近乎同时形成,是同一岩浆-热液事件的不同体现,并推测“晚石炭世火山机构+岩浆侵位+热液蚀变”区是铁矿就位的场所,应当作为重要勘查目标。

The Zhibo iron deposit is a large iron-rich deposit located in the Awulale iron metallogenetic belt(AIMB)in Western Tianshan,Xinjiang,China.Owing to its huge resources and high grade(Fe^(T)>50%),the Zhibo iron deposit has attracted much attention and is regarded as a representative example for the study of regional metallogeny and mineral exploration.In this study,zircon U-Pb and pyrite Re-Os isotopic dating methods were used to determine the mineralization age.Pyrite,which is closely associated with magnetite,was selected from five massive magnetite ores in the eastern segment of the deposit.Re-Os isotopic dating of the five pyrite samples yielded an isochron age of 307±12 Ma.Meanwhile,zircon U-Pb dating of the least altered wall-rock andesite yielded a weighted average age of 312.0±3.4 Ma,further confirming that the iron deposit was formed during the Late Carboniferous.The andesitic agglomerate representing explosive facies near the volcanic center in the Zhibo deposit has a zircon U-Pb weighted average age of 317.8±2.5 Ma.Two quartz diorite samples from the deposit have zircon U-Pb weighted average ages of 317.0±2.2 Ma and 315.4±0.9 Ma,respectively.It can be concluded that the volcanic eruption,magmatic intrusion,and iron ore formation were almost simultaneous during the early Late Carboniferous,which may have been different manifestations of the same magmatic-hydrothermla event.This paper speculates that the area of“Late Carboniferous crater+magmatic emplacement+hydrothermal alteration”may be the site where the iron ores are located,and should be an important exploration target for the AIMB.