青海东昆仑造山带夏日哈木超大型岩浆铜镍钴硫化物矿床成因及地质背景研究

Cu-Ni sulfide deposits within orogenic zones are very important in China. The discovery of the Xiarihamu super-large Cu-Ni-Co (about 1 million tonnes Ni metal with average grade of 0.68 wt%) at the east Kunlun orogenic zone changes the Ni resource distribution in China. It is very significant for addressing the geological background, condition and formation mechanism of the Ni-Cu sulfide deposits in orogenic belt as the largest Ni-Cu deposit in the orogenic zones of the world. Comparing with the Early Permian Cu-Ni deposit in the northern Xinjiang, the Xiarihamu deposit is higher in minerallization proportion and average metal grades and ~98% ores are hosted by the M1 ore body. The lastly obtained U-Pb age (406±3Ma）of the zircon seperated from the Xiarihamu olivine websterite and recent regional magmatism studies permit us to speculate that the formation of the deposit is associated with a early Devonian collisional or post collisional magmatic event (rather than mantle plume), which resulted in mafic magma intrusions and eruption in the region. The objective of this project is to reveal the processes, mechanism and key controlling factors of magma evolution and sulfide segregation in the magma plumbing system at Xiarihamu and the exploration indicators. The tectonic beckground of the origin of the mantle derived magma resulting the magmatic sulfide minerallization will be addressed as well based on the geochronology and geochemistry studies of the early Devonian magmatism in eart Kunlun orogenic zone. This study is significance for theoretically better understanding the formation of the Cu-Ni sulfide deposit in orogenic zone.

造山带铜镍硫化物矿床在我国具有重要地位，东昆仑造山带夏日哈木超大型铜镍钴矿床（镍金属储量近百万吨，平均品位0.65wt%）的发现不仅改写了我国镍资源分布格局，也是全球造山带最大的铜镍矿床，对于深入剖析造山带铜镍硫化物成矿的背景、条件和机制具有非常关键的意义。相对新疆北部早二叠世铜镍矿床，夏日哈木矿床矿化率和平均品位更高，约98%储量集中于M1矿体中。根据最新锆石U-Pb年龄（406±3Ma），结合区域岩浆作用研究结果，推测该矿床与东昆仑造山带早泥盆碰撞或后碰撞幔源岩浆事件（而非地幔柱）有关。本项目拟通过夏日哈木和附近含矿岩体的岩石、矿床、矿物、地球化学和年代学研究，系统探讨岩浆通道系统中岩浆演化和硫化物熔离-聚集的过程、机制、关键控制因素及找矿标志；同时，通过东昆仑同时代岩浆作用的年代学和地球化学研究，探讨与成矿有关的幔源岩浆起源的构造背景和诱发机制，深化和充实我国造山带岩浆硫化物成矿理论

本项目于对夏日哈木矿床、东昆仑造山带晚古生代其它多个含岩浆硫化物矿化的镁铁-超镁铁岩体（包括夏日哈木矿床周围的其它三个岩体、石头坑德岩体、水仙南岩体等），以及同时代的火山岩剖面进行了实地考察和采样。完成了大量岩石和矿石常微量元素及铂族元素、Sr-Nd同位素、S同位素、Mg同位素、锆石U-Pb同位素定年及Hf-O同位素、造岩矿物常微量元素的电子探针和LA-ICPMS分析、矿石三维结构CT扫描及元素分布面分析等分析测试。明确了夏日哈木矿床形成于早泥盆世，年龄为408Ma作用，形成与洋盆闭合之后，并建立了成岩成矿模式：即认为昆仑洋盆闭合之后，俯冲洋壳在大约100km的深度撕裂-断离导致软流圈上涌，引发遭受交代的地幔楔中的辉石岩部分熔融形成富Ni贫Cu的原始岩浆；这种原始岩浆进入地壳后，同化地壳中富含硫化物的地层导致深部硫化物熔离，硫化物被岩浆携带进入夏日哈木岩浆房，在岩浆房低洼处以及岩浆通道变宽的部位卸载-沉淀-成矿，携带硫化物的岩浆多次补充是形成超大型矿床的关键。我们还认为岩浆通道的入口在岩体的西端，因此，向西深部仍有很好的找矿潜力。.与新疆黄山-镜儿泉成矿带比较发现，夏日哈木矿床的原始岩浆起源深度更大，东昆仑其它几个含铜镍硫化物矿化的镁铁超镁铁岩体的形成年龄与夏日哈木岩体由较大差异。这些现象说明与大火成岩省铜镍硫化物成矿相比，造山带岩浆硫化物矿床的形成背景和成矿机制具有多样性和复杂性，不能一概而论，还需要更深入的研究工作。