陕西大西沟喷流沉积型菱铁矿矿床地质特征及矿床成因

Geological characteristics and origin of Daxigou SEDEX siderite deposit in Shaanxi Province

陕西大西沟铁矿床位于秦岭造山带山-柞盆地西北部,与银洞子大型银铅矿床毗邻。矿体主要赋存在泥盆系青石垭组中上段,容矿岩石为一套海相复理石碎屑岩和碳酸盐岩建造。矿床的金属矿物主要有菱铁矿、磁铁矿、黄铁矿,其次为黄铜矿、磁黄铁矿等;非金属矿物主要是重晶石、石英、铁白云石,其次为方解石、绢云母、绿泥石、黑云母、斜长石、钠长石、堇青石等,局部地段由于表生氧化和次生富集作用而形成针铁矿、赤铁矿、蓝辉铜矿等。与矿化有关的围岩蚀变较弱,主要有硅化、绢云母化、绿泥石化和碳酸盐化。基于野外地质观察、矿物共生组合和矿石结构构造的系统研究,将大西沟铁矿床的形成划分为3期6阶段,分别是:①喷流沉积期:硅质岩-黄铁矿-菱铁矿阶段(Ⅰ)、重晶石-磁铁矿阶段(Ⅱ);②热液改造期:堇青石-黄铁矿-磁铁矿阶段(Ⅲ)、石英-碳酸盐阶段(Ⅳ)、碳酸盐-硫化物-磁铁矿阶段(Ⅴ);③表生氧化期(Ⅵ)。流体包裹体显微测温结果表明,喷流沉积期Ⅰ阶段菱铁矿完全均一温度和盐度w(NaCl_(eq))峰值区间分别为230～270℃和13%～14.5%,Ⅱ阶段重晶石中流体包裹体的完全均一温度和盐度w(NaCl_(eq))峰值区间分别为220～290℃和9%～13%;热液改造期Ⅳ阶段菱铁矿和石英中气液两相包裹体均一温度峰值区间为240～300℃,盐度w(NaCl_(eq))为2.6%～15.7%;热液改造期Ⅴ阶段菱铁矿与石英中流体包裹体,除大量气液两相包裹体外,还发育有含子矿物多相包裹体,其中,气液两相包裹体均一温度峰值区间为290～340℃,盐度w(NaCl_(eq))为5.1%～13.4%,含子矿物多相包裹体均一温度峰值区间为380～440℃,盐度w(NaCl_(eq))为40.6%～59.7%。含子晶流体包裹体可能是流体不混溶或/和高盐度流体加入的反映。矿区内不同产状碳酸盐矿物的C、O同位素组成比较均一,δ^(13)C_(PDB)值集中在-3.58‰～-1.15‰之间,δ^(18)O_(SMOW)值为21.22‰～21.82‰,均表现出海相碳酸盐或海底喷流热液溶解海相碳酸盐的特征。大西沟矿床的地质、矿化和流体特征与海底热液喷流沉积型矿床一致,可能属于典型的喷流沉积型菱铁矿床,但后期受到秦岭造山作用的影响及热液叠加改造并形成磁铁矿和少量硫化物。大西沟菱铁矿矿床与邻近的银洞子铅锌矿组成一个较完整的喷流沉积成矿系统,两者可能分别代表了喷流沉积的边缘相和中心相。

The Daxigou iron deposit is located in the northwest of Shan-Zha Basin in the Qinling Orogen, adjacent to the Yindongzi Ag-Pb deposit. The orebodies are hosted in the upper section of Devonian Qingshiya Group, which is a set of flysch clastic-carbonate sedimentary formations of marine facies. The metallic minerals consist mainly of siderite, magnetite, pyrite, and chalcopyrite with minor pyrrhotite, hematite, goethite, and digenite. The non-metallic minerals are dominated by barite, quartz, and ankerite, with a little calcite, sericite, chlorite, biotite, plagioclase, albite, and cordierite. Supergene oxidation has formed some iron and copper supergene minerals, such as goethite, hematite, and digenite. Wall rock alteration associated with mineralization is weakly developed, and is mostly characterized by silicification, sericitization, chloritization, and carbonatization. On the basis of a systematic study of field geological features, mineral paragenesis, ore texture and structure, the mineralization process can be divided into six stages: SEDEX period: the silicolite-pyrite-siderite stage (Ⅰ) and the barite-magnetite stage (Ⅱ);hydrothermal alteration period: the cordierite-pyrite-magnetite stage (Ⅲ), the quartz-carbonate stage (Ⅳ) and the carbonate-sulfide-magnetite stage (Ⅴ), and supergene period (Ⅵ). Fluid inclusion study shows that the temperature and salinity w(NaCleq) of siderite from stage Ⅰ focus on 230~270℃ and 13%~14.5%, respectively;the temperature and salinity w(NaCleq) of barite from stage 2 are mainly distributed in 220~290℃ and 9%~13%, respectively. The fluid inclusions in siderite and quartz from stage Ⅳ in hydrothermal alteration period mostly are gas-liquid two-phase inclusions, with the temperature and salinity w(NaCleq) distributed in 240~300℃ and 2.6%~15.7%, respectively. The siderite and quartz from stage Ⅴ have many gas-liquid two-phase inclusions and polyphase inclusions containing daughter minerals. The temperature of gas-liquid two-phase inclusions ranges from 290 to 340℃, with the salinity w(NaCleq) distributed in 5.1%~13.4%;the temperature and salinity w(NaCleq) of polyphase inclusions are distributed in 380~440℃ and 40.6%~59.7%, respectively. The polyphase inclusions containing daughter minerals in the siderite and quartz may represent the nonmiscibility of fluids or the participation of high salinity fluids. Carbon and oxygen isotopes of different types of carbonate minerals are relatively homogeneous. The δ13CPDB and δ18OSMOW of the carbonate minerals are -3.58‰^-1.15‰ and 21.22‰~21.82‰, respectively, showing the characteristics of marine carbonates or marine carbonate dissolved by seafloor hydrothermal fluids. The geology, mineralization and fluid characteristics of the Daxigou deposit are consistent with features of SEDEX (Sedimentary Exhalative) deposit, and hence it may belong to SEDEX siderite deposit. Subsequently, parts of magnetite and sulfide minerals might have been formed under the influence of Qinling Orogeny and hydrothermal modification. The Daxigou siderite deposit and its adjacent Yindongzi lead and zinc deposit have formed a whole sedimentary deposit system, and may represent the edge phase and the center phase of the sedimentary exhalative system, respectively.