牟定郝家河铜矿床的沉积-改造成因

THE SEDIMENTARY-REFORMATION ORIGIN OF THE HAOJIAHE COPPER DEPOSIT,MOUDING COUNTY, YUNNAN PROVINCE

关于牟定郝家河矿床的成因,前人主要有沉积同生说、沉积成岩说和后生成矿说等论点。笔者根据近年来研究获得的新资料,认为矿床为沉积-改造(?)成因。早白垩世晚期—晚白垩世早期,来自东部古陆的富铜岩石和铜矿床长期风化形成的富铜风化产物,在高能氧化辫状河环境形成紫色矿源层。喜马拉雅早期褶皱运动,形成了矿区构造,矿源层中Cu活化迁移到背斜鞍部附近等低压空间浅色层中沉淀,从而形成了工业矿体。

The Haojiahe copper deposit in Mouding County, Yunnan Province was previously considered to be of syndepositional, syndiagenetic or deuterogenic origin. Through the investigation made in recent years, however, the authors hold that the ore deposit is genetically of sedimentary-reformation type. The ore district is located in the eastern part of the middle sector of the Central Yunnan Mesozoic interior basin, and the exposed strata comprise in upward succession purple mudstone of Puchanghe Formation (Lower Cretaceous), sandstone of Matoushan Formation and mudstone of Jiangdihe Formation (Upper Cretaceous). The major structures include the south-north Xiangduoji anticline, the NW-trending Shizishan anticline and the east-west F_1 and F_(51) faults. Orebodies lenticular and podiform in shape occur in light-colored sandstone of Haojiahe Member of lower Matoushan Formation on the saddle of the Shizishan anticline, obviously controlled by concealed faults on the saddle of the anticline. Within the orebodies barren 'windows' and specimen ore veins are frequently observed. Mineral associations are simple, and copper minerals are dominated by chalcocite. Ore fabrics of hydrothermal filling-metasomatic origin are well developed, and there commonly exist postdepositional veinlike, spotted and cloud dry asphalt, with organic carbon showing positive correlation with Cu and Ag content. Light-colored beds encircle the saddle of the Shizishan anticline and are in erratic contact with the purple bed; hence, penetrating bedding is commonly seen. These facts cannot be explained by the syndiagenetic viewpoint;rather, they can serve well as evidence for late reformation. Geochemical studies of the strata show that in Mesozoic strata of the region copper abundance tends to increase whereas organic carbon abundance tends to decrease from the lower part upward.Original copper content of the strata in the ore district is(55.61～171.9)×10^(-6).Of these strata, the Puchanghe Formation and the lower purple bed of the Matoushan Formation are relatively enriched in copper and silver (Cu 119×10^(-6)～171.9×10^(-6), Ag 0.51×10^(-6)～2.09×10^(-6)), and thus seem to be the major source bed. The close spatial relationship and the inheritance and consistency of minor elements between the orebodies and the purple bed all support the argument of late reformation. Geochemistry of the strata in the ore district also indicates the presence of a NW-trending copper-silver low-value zone in the overlying purple bed on the southwestern side of the Haojiahe ore belt. This, combined with zonation of minerals and elements, suggests that copper and silver in the source bed once migrated upward and northeastward, i.e., they traveled from the deep high-pressure area at the southwestern limb of the anticline to the shallow low-pressure area at the saddle of the anticline to form the ore deposit. Hydrogen, oxygen, carbon, sulfur and lead isotopic analyses and mineral inclusion measurements show that sulfur and lead came mainly from the strata, and ore-forming fluids were deep-circulatory underground hot brines characterized by low (somewhat intermediate) temperature, medium-low salinity, Medium-high-density and rich CO_2. Late Yanshanian to early Himalayan folding formed the Shizishan anticline. Copper in the source bed was remobilized and ascended to the saddle of the anticline, forming orebodies in the light-colored beds.