长江中下游地区鄂东南和九瑞矿集区成矿岩体特征及其识别标志

长江中下游地区是我国一条重要的铜多金属成矿带,成矿与燕山期岩浆活动密切相关,矿床类型主要有斑岩型和矽卡岩型。在长江中下游成矿带西段的鄂东南和九瑞地区产有该带中几个十分重要的大型铜多金属矿床,如铜绿山、鸡冠嘴、铜山口、城门山、武山等。通过对该区岩体的系统对比研究表明,成矿岩体和不成矿岩体的矿物组成,主微量元素成分及成岩年龄上并无明显差异。总体而言,九瑞地区岩浆岩的形成年龄集中在141～148Ma之间,略早于鄂东南地区与铜矿相关的岩浆岩(集中在137～140Ma)。对岩浆岩全岩的Sr-Nd同位素和锆石Hf同位素研究表明,一些成矿岩体具有比不成矿岩体更高比例的幔源物质贡献。对岩浆岩中主要造岩矿物,如角闪石和黑云母的详细研究,可以区分它们的不同结晶历史,从而揭示岩浆从早至晚演化过程中,相容元素和不相容元素、成矿元素和挥发份元素的变化规律,判别岩浆分离结晶过程、流体出溶过程,指示成矿与否。通过对角闪石和黑云母温压计的应用,估测了九瑞和鄂东南地区成矿岩体与不成矿岩体的侵位压力和深度,发现成矿岩体一般具有较低的压力(4kbar)和较浅的侵位深度。九瑞和鄂东南地区成矿岩体均具有较高的氧逸度。成矿岩体演化到晚期,氧逸度显示升高的趋势,岩浆中的挥发分/成矿金属含量较高。而不成矿岩体就位前岩浆贫化Cu、S、Cl等元素,不能分异出含足够成矿元素的成矿热液。因此,通过详细的矿物学、特别是造岩矿物角闪石和黑云母以及副矿物锆石和磷灰石的主微量元素和同位素组成的研究,以及由其计算出的温度、压力、氧逸度、流体成分等参数,可以区分成矿与不成矿岩体,从而为长江中下游成矿带乃至其他类似地区的深部找矿工作提供理论指导。

Geological characteristic and discrimination criteria of the ore-related granitoids from the Edong and Juirui districts in the Middle-Lower Yangtze Metallogenic Belt

The Middle-Lower Yangtze Metallogenic Belt is an important copper-polymetallic ore belt in China. The types of ore deposits related to Mesozoic Yanshanian magmatism are mainly porphyry and skarn orebodies. In the southeastern part of this metallogenic belt, namely the Edong and Juirui districts, there occur several important large copper-polymetallic deposits, including the Tonglvshan, Jiguanzui, Tongshankou, Chengmenshan and Wushan deposits. A systematic comparative study of the granitoids in the Edong and Juirui districts shows that there is no significant difference in the mineral compositions, the major and trace element compositions and emplacement ages of the ore-related and ore-barren rocks. In general, the formation age of magmatic rocks in the Jiurui district is mostly concentrated between 141Ma and 148Ma, which is slightly earlier than the magmatic rocks associated with copper deposits in the Edong district which concentrated mainly at 137~140Ma. The whole-rock Sr-Nd isotope and zircon Hf isotope studies of the granitoids show that some ore-related rocks have a higher proportion of mantle-derived material contributions than the ore-barren rocks. A detailed study of major rock-forming minerals in the granitoids, such as amphibole and biotite, can be used to distinguish their different fractional crystallization history, and to reveal the characteristics of the compatible and incompatible elements, the ore-forming elements, and the volatile elements during the evolution of magma from early to late stages. These characteristics can be used to discriminate the magma fractionation and crystallization process, the fluid exsolving process, and to identify their mineralization potential. Through the application of amphibole and biotite thermometers, the emplacement temperature, pressure and depth of the ore-related and ore-barren rocks in the Jiurui and Edong districts are estimated. It is found that the ore-related rocks generally have lower pressure (< 4kbar) and shallower emplacement depth. The ore-related granitoids in the Jiurui and Edong districts have high oxygen fugacity. Along with the evolution from early to late stages, the oxygen fugacity showed an increasing trend for the ore-related granitoids, and the volatile and metallogenic metal contents in the magma also show an increasing trend. Due to the depletion of elements such as Cu, S and Cl in the ore-barren granitoids, these rocks cannot produce the ore-forming hydrothermal fluid with sufficient ore-forming metals. In conclusion, through detailed mineralogy, especially the major and trace elements and isotopic compositions of the amphibole and biotite and the accessory minerals such as zircon and apatite, as well as the calculated parameters of temperature, pressure, oxygen fugacity and fluid composition, it is possible to distinguish between ore-related and ore-barren granitoids, providing theoretical guidance for deep prospecting in the Middle-Lower Yangtze Metallogenic Belt.