东天山香山铜镍-钛铁成矿与两类岩浆演化关系探讨

香山矿床是东天山唯一一个铜镍矿体与钛铁矿体共存的复合型矿床,目前普遍被认为是同源岩浆高度演化的产物。但为何区内众多同时代镁铁-超镁铁质岩体中只有香山岩体产出有大型钛铁矿床?这一问题仍有待进一步探讨。本文通过系统对比香山与其相邻的黄山东、黄山和黄山南等典型铜镍矿的地球化学特征、母岩浆性质及岩浆演化过程,认为香山岩体母岩浆相对其相邻典型铜镍矿床并不富集Fe和Ti,且其母岩浆早期演化过程及分异演化程度相似。模拟计算表明:岩浆体系早期都处于较还原的环境(f_(O_2)QFM+1),而且都发生了大量橄榄石、斜方辉石和单斜辉石的分离结晶和硫化物的熔离,由此消耗了岩浆体系中大量的Fe,导致残余岩浆中Fe含量降低;另外在岩浆演化的各阶段,尤其到岩浆演化后期,大量角闪石的结晶进一步降低了残余岩浆中的Fe和Ti的含量,不能造成Fe和Ti的逐渐富集,据此我们认为同源岩浆演化不具备形成钛铁矿床的物质基础。通过初步对比东天山地区典型铜镍矿床和钛铁矿床与该地区早二叠世两类玄武岩的地球化学特征及演化趋势,我们初步推测东天山地区铜镍成矿与钛铁成矿是两类不同性质岩浆独立演化的产物,铜镍成矿可能与拉斑玄武质岩浆演化密切相关,而钛铁矿则可能与碱性玄武岩演化密切相关。

The relationship between NiCu-FeTi deposits and two magma series, Eastern Tianshan, NW China

The Xiangshan complex is an unique complex which hosts both Ni-Cu sulfide and Ti-Fe oxide orebodies in Eastern Tianshan. Many researchers consider these orebodies belong to comagmatic evolution products. However, why only do the Xiangshan deposit show the coexistence of magmatic Ni-Cu sulfide and Ti-Fe oxide deposits among the contemporaneous mafic-ultramafic introsions? To answer this question, this article systematically compares the geochemical characteristics, parental magma properties and the evolution processes of the Xiangshan Ni-Cu-bearing intrusion with those of the other contemporaneous Ni-Cu deposits, including the Huangshandong, Huangshan and Huangshannan deposits. We find that the parental magma of the Xiangshan Ni-Cu-bearing intrusion does not has higher Fe-Ti contents than other typical Ni-Cu deposits, and their evolution processes and differentiation degrees are similar. The crystallization differentiation of minerals (olivine, orthopyroxene and clinopyroxene) and the sulfide segregation consumed abundant Fe element in the melt, resulting in the decrease of Fe content in the residual magma reduce. Moreover, the crystallization of hornblende at each stage of the magma evolution, especially at the late stage, reduced the Fe and Ti contents in the residual magma. Thus, the residual magma does not have the potential to form Fe-Ti orebodies. So we infer that the Ni-Cu and Fe-Ti orebodies were not the products of the same comagmatic evolution. Meanwhile, by comparing the geochemical characteristics and evolution trends of the Xiangshan deposit with those of the two types of the Early Permian basalts (the tholeiitic and alkaline basalts) in this area, we infer that mineralization of the Ni-Cu sulfide and Fe-Ti oxide orebodies belongs to the evolution products of these two types of magma series, respectively.