西秦岭洛大式菱铁矿矿床的成矿年龄及其成因

Metallogenic Age and Genesis of the Luoda-Type Siderite Deposits in West Qinling

铁矿是我国的战略性矿产资源。本次研究以西秦岭洛大式菱铁矿矿床为研究对象,在矿床成矿期次研究的基础上,采用黄铁矿Re-Os同位素等时线法确定成矿年龄;结合黄铁矿硫同位素组成、区域地质背景和矿床地质特征,探讨成矿物质来源,判断矿床成因;同时采用排除法和与矿石特征相关联的方法,探讨黄铁矿Re-Os同位素测试结果出现过度散布点的原因。结果表明:洛大式菱铁矿矿床成矿年龄为136±20 Ma(2σ),对应中晚侏罗世-早白垩世早期汇聚构造体系与中晚白垩世大陆伸展构造体系的转换阶段。锇同位素组成初始值(^(187)Os/^(188)Os)_(i)为0.04±0.22(MSWD=1.11),具有幔源特征;硫同位素组成具有岩浆硫与地层硫混合硫及无机还原硫的特征;铁可能主要来源于幔源流体及其对深部晚印支期花岗岩类岩体的萃取;沿矿石裂隙分布的黄铁矿中的裂隙被杂质充填,导致黄铁矿Re-Os同位素测试结果出现过度散布点。研究认为,洛大式菱铁矿矿床是与幔源流体作用有关的浅成中-低温热液型矿床,寻找同类型铁矿需要遵循构造-流体耦合成矿规律;用于测试Re-Os同位素的黄铁矿样品,在采集制备过程中需要排除沿后期裂隙带分布的伴生氧化物比例较大的黄铁矿。

Iron ores are critical ore resources in our country.Taking the Luoda-type siderite deposits in West Qinling as examples,this work analyzed the metallogenic stages and determined the metallogenic age using Re-Os isochrons of pyrite.Combined with sulfur isotopic composition of pyrite,regional geological background,and deposit geology,the material sources of the deposits were discussed,and their genesis was determined.In addition,this work explored the reasons for overspread points of Re-Os isotopes of pyrite by the exclusion method and also associated with ore characteristics.Results show that the Luoda-type deposits were formed at about 130±20 Ma(2σ),corresponding to the transitional period from the Middle-Late Jurassic to early Early Cretaceous continental convergence to Late Cretaceous continental extension.The initial osmium isotopic composition of 0.04±0.22(MSWD=1.11)was indicative of a mantle source of ore-forming materials.The sulfur isotope was characterized by a mixed source of magmatic sulfur with sedimentary sulfur as well as by inorganic reduction sulfur.Iron may be derived from mantle fluid and its leaching of late Indonesian granitoids.The fissures in pyrite along ore fracture were filled by impurities that caused the overspread points of pyrite Re-Os isotopes.It is inferred that the Luoda-type siderite deposits should be attributed to meso-epithermal deposits related to mantle fluids,and the prospecting of this type of deposits should focus on the metallogenic regularities of coupling tectonism and fluids.For sampling and preparing pyrite samples for testing Re-Os isotopic tests,the pyrites with high proportion of iron oxides along late fissures should be removed.