江西赣中铁矿“红绸带”式铁矿体形态成因及其找矿预测

摘要

江西赣中新余铁矿是我国重要的铁矿类型之一，产于震旦纪火山‒沉积浅变质岩系硅铁建造中。铁矿体普遍呈现“红绸带”式形态，前人认为是多期次褶皱叠加的结果。本文通过野外调查，在整个铁矿区发现，区域透入性的拉伸线理、A型褶曲十分发育，局部地段甚至出现鞘褶皱；系统测量表明，区域拉伸线理、A型褶皱的脊线走向稳定在295°~320°之间，倾角一般小于25°。推测赣中铁矿经历了强烈的塑性流变，“红绸带”式铁矿体是塑性流变，而非多期褶皱叠加变形的结果；整个铁矿区的原始形态应是一个鼻端向南封闭的巨形鞘褶皱，但变形期后不均匀的构造抬升和剥蚀，导致了不同铁矿区现今地表出露了原始形态的不同部位。结合褶皱构造对铁成矿物质的控制作用和矿区的地层出露状态分析，认为大陂‒陂头、寨口‒太平山‒良山一带皆处于鞘褶皱的前缘部位，具有寻找富大厚矿体的找矿前景，松山‒杨家桥处于鞘褶皱的西翼，平剖面上都发育小型鞘褶皱和红绸带式重叠矿体，因而也具有良好的找矿前景。%The Xinyu iron deposit, located in central Jiangxi province, is one of the most important BIFs-type deposits in China. It is hosted in the Late Proterozoic volcanic-sedimentary rocks. Ore-bearing layers, especially quartz veins or bands in hosting rocks in most mining areas, are strongly deformed, showing complex Z-, S-type folds, and folded lens. As a result, the “red-ribbon” style folded iron-bodies are common. Multi-phases of deformation and folding were proposed to interpret the origin of the “red-ribbon” style ore-bodies, which suggests that they were produced by fold superimposition. Our field investigation revealed that regional penetrative mineral stretching lineation and A-type folds were well developed throughout the entire Xinyu iron deposit. Sheath folds were identified abundantly in the Yangjiaqiao, Pitou mining areas in the southern part of the Xinyu deposits. Systematic measurements indicated that the mineral stretching lineation and hinge lines of A-type fold varied from NW295° to NW320°, with plunging angles of less than 25°. The quartz-feldspar pebble, the so-called pseudo-pebble in metamorphic pebble-bearing siltstones was strongly elongated in the lineation direction. Thus, it is suggested that the Xinyu iron-ore deposits have experienced strongly regional plastic deformation subsequent to the sedimentation and metamorphism of BIFs. The ductile fabrics such as regional mineral stretching lineation, A-type folds and sheath folds, as well as the “red-ribbon” style of folded iron-ore-bodies, are regarded as the products of plastic deformation. It was also suggested that the original form of the Xinyu iron-ore deposit area could be a giant sheath fold, generated from differential southward viscous flow. Regional non-uniform tectonic uplifting and erosion after the plastic deformation resulted in varied preservation of the regional sheath fold, exhibiting various structure patterns in different mining areas at the surface. The Dapi-Pitou and Zhaikou-Taipingshan-Liangshan areas, both located in the front of the regional giant sheath fold, are favorable locus for ore-forming material (iron) to be settled through differential flow, forming large, thick, and high-grade ore bodies. And thus, they are the most favorable areas for ore prospecting. The Songshan-Yangjiaqiao mining area, which is located in the west limb of the large sheath fold with abundant ptygmatic folds and small sheath folds, is also a favorable ore prospecting area.