基于低温热年代学的宁芜盆地玢岩铁矿床剥蚀作用与保存潜力研究

Ningwu basin, located in the Yangtze River Valley metallogenic belt, is one of the most productive ironstone producer in China, with numerous porphyritic iron deposits widely distributed. Most of the found iron deposits in Ningwu basin are exposed on the surface, indicating regional exhumation happened within the whole basin after mineralization. The deep structure and petrographic characters of the diorite porphyrite show diverse exhumation probably happened among the different sections of the basin. Such regional exhumation and diverse exhumation among different sections will significantly influence the preservation potential of the deep-burial porphyritic iron deposits. However few attentions have been paid for this aspect so far, especially lack of the quantitative research. This project will use the zircon (U-Th)/He, apatite (U-Th)/He, and apatite fission-track thermochronologies to establish the low-temperature thermal history for three representative porphyritic iron deposits in the Ningwu basin, namely the Meishan deposit in the northern section, the Taocun deposit in the middle section, and the Gushan deposit in the southern section of the Ningwu basin. By rebuilding the low-temperature histories, the exhumation process, exhumation rate and thickness for these three deposits will be quantitatively determined. On the basis of three representative deposits and combined with the geological features and ore-forming depth, the exhumation degree and preservation potential of deep-burial iron mineralization of the three sections in the Ningwu basin will be comprehensively evaluated.

长江中下游宁芜盆地产出众多的玢岩铁矿床，是我国重要的铁矿石生产基地。盆地内所发现的矿床绝大部分已出露地表，说明成矿后盆地经历了区域性剥蚀作用；盆地深部构造及辉石闪长玢岩的岩相学特征进一步表明，盆地不同地段之间还可能存在差异性剥蚀作用。这种区域性及各个地段之间可能存在的差异性剥蚀作用，会对盆地不同地段的深部玢岩铁矿床的保存潜力产生重要影响，但目前尚未对此开展过深入研究，尤其缺乏对剥蚀作用的定量研究。本课题计划在盆地北、中、南三个地段分别选择梅山铁矿床、陶村铁矿床、姑山铁矿床为代表，联合使用锆石(U-Th)/He、磷灰石(U-Th)/He、磷灰石裂变径迹三种低温热年代学方法，构建三个矿床成矿后的低温热历史，借此精确反演成矿后的剥蚀历史，获得剥蚀速率和剥蚀量等关键参数。在评定典型矿床剥蚀程度的基础上，结合地质特征及成矿深度，综合评价宁芜盆地北、中、南三个地段的剥蚀程度和深部玢岩铁矿床保存潜力。

本项目选择宁芜盆地凹山铁矿和陶村铁矿开展了热年代学研究，对这两个矿床的成矿辉长闪长玢岩进行了锆石和磷灰石的(U-Th)/He年龄测定，在此基础上对比分析了这两个矿床的剥露历史和剥蚀程度。研究发现凹山铁矿和陶村铁矿在自23Ma左右开始剥蚀冷却，与中国东部的新生代构造反转事件相联系。结合成矿年龄，发现长江中下游地区在早白垩世开始的长期拉张构造状态不仅有利于早白垩世矿床的大规模形成，还有利于这些矿床的保存。但是这种拉张状态在新近纪开始时宣告结束，代之以大规模的东西向挤压应力，称之为新生代构造反转，导致了这些中生代矿床的剥露。在此基础上，结合前人已发表的关于宁芜盆地内中段和南段分割断裂的地震剖面解译结果，评价了宁芜地区中段和南段的隆升剥蚀差别，指出宁芜地区中段具有进一步寻找姑山式玢岩铁矿的潜力。在项目研究过程中，还对宁芜盆地北段梅山铁矿床的成矿辉长闪长玢岩和附近的蒋庙辉长岩进行了岩石学及矿物学研究，发现这两种岩浆均为高氧化性岩浆，具有磁铁矿早结晶的特征，体现重要的成矿和构造意义。