埃达克质岩的金属成矿作用

The Metalliferous Mineralization Associatied with Adakitic Rocks

介绍了"埃达克质岩"的术语、与成矿有关的埃达克质岩的分布、成矿背景,讨论了埃达克质岩有利于成矿的控制因素。"埃达克质岩"是指那些具有与俯冲洋壳熔融形成的"埃达克岩"类似地球化学特征,如SiO2≥56%,Al2O3≥15%,亏损Y(≤18×10-6)和重稀土元素(如Yb≤1.9×10-6),高Sr(很少样品的Sr含量低于400×10-6),无-正Eu,Sr异常,贫高场强元素等,但可以形成于不同构造背景并可有不同成因的岩浆岩。埃达克质岩具有重要的金属成矿意义,其有利成矿背景主要包括岛弧、大陆板内伸展和大陆活动碰撞造山带环境。世界上许多(包括三个最大的)斑岩铜矿都与埃达克质斑岩密切共生,因此埃达克质岩的成矿潜力巨大。在岛弧和大陆板内伸展环境中,来自俯冲玄武质洋壳或洋壳沉积物或拆沉的大陆地壳产生的熔体或释放的超临界流体与地幔的相互作用,一方面可能导致熔体被地幔橄榄岩混染,另一方面可能导致高Fe2O3含量的熔体或超临界流体对地幔的交代作用,地幔氧逸度升高,地幔金属硫化物被氧化分解,有利于铜、金等的矿化。

Adakitic rocks have important implications for the metalliferous mineralization. This paper introduces the nomenclature of ＂adakitic rocks＂, the distribution and tectonic settings of the adakitic rocks associated with metalliferous mineralization. The factors that the adakitic rocks are favorable for the metalliferous mineralization are discussed in detail at last. Similar to a subducted oceanic crust-derived adakite, the ＂adakitic rocks＂ are characterized by the following geochemical characteristics： SiO2 ≥ 56 wt%, Al2O3≥ 15 wt%, depleted Y （≤ 18 × 10^-6 ） and heavy rare earth element contents （Yb≤ 1.9 × 10^-6）, high Sr （rarely 〈 400× 10^-6 ）, negligibly positive Eu and Sr anomalies, and depleted high field strength element contents. They can be generated in different tectonic settings and by different mechanisms. The favorable tectonic settings for the metalliferous mineralization include are, within-continent extension, and active collisional orogenic belt between different continents. Many large and giant are closely associated with adakitic rocks, indicating that they have a huge metalliferous potential. In the are and within-continent extension settings, the interaction between subducted basaltic oceanic crust, or sediments, or delaminated continental crust-derived melts, or released super critical fluids with mantle causes contamination of melts by mantle peridotites on the one side, and metasomatism of mantle peridotites by the melts or fluids with high Fe2O3 on the other side. Thus, the raised oxygen fugacity in the mantle causes metal sulfides to be decomposed, which is favorable for the Cu-Au mineralization.