中文摘要：

龙桥铁矿床是庐枞火山岩盆地中的一个大型的铁矿床,多年来对其矿床成因的认识存在较大的争论。文章在野外地质研究工作的基础上,通过对矿床中菱铁矿的岩矿分析鉴定和电子探针测试,确定了矿床纹层状矿石中的菱铁矿为沉积成因。通过对菱铁矿的产出特征分析,并结合龙桥铁矿床的部分地质地球化学研究成果,认为在该矿床形成过程中,早期沉积形成了纹层状的菱铁矿层,在燕山期的岩浆热事件中,部分沉积菱铁矿被交代形成了磁铁矿和具有残余骸晶结构等一系列矿石交代组构特征的矿物。纹层状矿石既具有沉积特征,也具有热液改造特征,证实了矿床的形成存在早期（三叠纪）的沉积成矿（菱铁矿）作用和晚期（燕山期）的热液成矿（磁铁矿）作用。菱铁矿的研究为进一步确定龙桥铁矿床的成因提供了新的佐证。

英文摘要：

The Longqiao iron deposit is one of the most important large ore deposits in Luzong basin, one of the ore-concentrated area in the Middle and Lower Yangtze Valley metallogenic belt. The genesis of this ore deposit remains a problem of much controversy. Based on careful and detailed field investigation and mineral association research, this paper has reported some new achievements concerning siderite minerals in this deposit. There exist two types of siderites in the ores： one occurs in the lamellar ores, and the other assumes the veined form. Petrographic and electron probe analyses as well as C-O, H-O isotope geochemical characteristics of the siderites in the lamellar ore all show that siderites of this kind were formed earlier than magnetite and other minerals associated with magnetite, and that they occurred in a marine sedimentary environment. Some siderites were transformed into magnetite by later hydrothermal activities, resulting in the formation of residual and metasomatic textures in the ores. The lamellar iron ores are composed of sedimentary siderite and hydrothermal minerals such as magnetite, garnet, pyrite and phlogopite as a result of the water-rock interaction between the hydrothermal fluids evolved from the Mesozoic magmatic intrusion and the siderite-bearing Early Mesozoic （Middle Triassic） marine sedimentary rocks. Most of the magnetites in the ore deposit, especially in the massive iron ores, were precipitated directly from the magmatic water-dominated hydrothermal system. The metallogenic process of the Longqiao iron deposit can be divided into the early （Middle Triassic） sedimentary ore-forming stage characterized by the deposition of siderite-bearing rocks and the late （Early Cretaceous or Yanshanian） hydrothermal ore-forming stage characterized by the formation of magnetites associated with other hydrothermal minerals.