东昆仑志留纪辉长岩地球化学特征及与铜镍成矿关系探讨

The geochemistry characteristics of Silurian gabbro in East Kunlun Orogenic Belt and its mineralization relationship with magmatic Ni-Cu sulfide deposit

东昆仑造山带新发现的夏日哈木超大型岩浆铜镍矿床、石头坑德大型岩浆铜镍矿床及冰沟南小型岩浆铜镍矿床,其矿体均赋存于橄榄辉石岩内,而辉长岩又是该含矿橄榄辉石岩的直接围岩,并且辉长岩的形成时代相近(夏日哈木辉长岩431Ma、石头坑德辉长岩425Ma、冰沟南辉长岩427Ma),产出位置属于同一大的构造单元,均邻近昆北及昆中断裂。通过对辉长岩的地球化学特征研究,发现明显富集轻稀土元素和明显的Nb-Ta负异常,亏损高场强元素。Sr-Nd同位素研究,表明东昆仑夏日哈木和石头坑德辉长岩岩体的母岩浆来自一个曾经被交代富集的地幔源区,可能揭示了由于从洋壳释放出的流体交代地幔楔的岩石成因。结合区域构造演化和辉长岩形成时代,认为东昆仑夏日哈木和石头坑德辉长岩形成于碰撞后伸展环境。辉长岩岩浆源区性质与赋矿辉石岩存在明显不同,并非同一岩浆活动的产物。辉长岩在东昆仑造山带地区直接充当了含矿辉石岩的直接围岩,与岩浆铜镍硫化物矿体的形成没有关系,富含橄榄石的超镁铁质岩石更有利于形成具有较大经济价值的铜镍矿体。这为指导东昆仑找矿实践和岩浆铜镍矿床成矿理论研究提供了基础。

The ore bodies of the magmatic sulfide deposits found in the East Kunlun Orogenic Belt,the Xiarihamu giant Ni-Cu sulfide deposit, Shitoukengde large Ni-Cu deposit, and the Binggounan small Ni-Cu deposit, are hosted in olivine-websterite surrounding by gabbro. The gabbro age of these three sulfide deposits almost is same（ Xiarihamu gabbro was formed in 431 Ma,Shitoukengde gabbro is 425 Ma,and the Binggounan gabbro is 427 Ma）. Moreover,all these three deposits are adjacent to the North Kunlun Fault or the Central Kunlun Fault. The gabbros from these three magmatic sulfide deposits show the following features：（ 1）enriched in light rare earth elements;（2） depleted in high field strength elements;（3） obvious Nb-Ta negative anomaly. Based on the above features and the Sr-Nd isotopes,we suggest that the parent magma of gabbro from the Xiarihamu Ni-Cu sulfide deposit and the Shitoukengde were derived from an enriched mantle source,the mantle wedge metasomatized by the fluid released from the oceanic crust. Combined with the regional tectonic evolution and the gabbro U-Pb ages,we suggest that the gabbro from the Xiarihamu Ni-Cu sulfide deposit and the Shitoukengde were formed in a post-collisional setting. The age and the origin of the olivine websterite hosting the sulfide ore are very different from those of the gabbro. The olivine websterite and the gabbro were the products from different magmatism activities. The gabbro acts as a surrounding rock for the ore-bearing ultramafic lithofacies. We found that the ultramafic lithofacies containing olivine are more favorable for the formation of the magmatic sulfide ore body. This is of great significance for the magmatic sulfide Cu-Ni deposit prospecting in the East Kunlun Orogenic Belt.