宁乡式沉积铁矿床的时空分布和演化

TIME-SPACE DISTRIBUTION AND EVOLUTION OF THE NINGXIANG TYPE SEDIMENTARY IRON DEPOSITS

宁乡式铁矿是我国最重要的沉积型铁矿床 ,广泛分布于我国南部的鄂、湘、赣、川、滇、黔、桂诸省以及甘南地区。含矿建造主要赋存于中上泥盆统 ,而以上泥盆统为主。可划分出 7个成矿区 ,其中最重要的是鄂西—湘西北成矿区。在 1个矿床中通常有 1～ 4层铁矿 ,其中有 1个是主矿层。矿石主要由鲕状赤铁矿组成 ,次有菱铁矿、鲕绿泥石和褐铁矿 ,含铁品位一般为 30 %～ 4 5% ,含磷通常偏高 ,介于 0 .4 %～ 1.1%。中泥盆世和晚泥盆世沉积铁矿在分布范围、矿床规模、赋矿围岩建造和矿石特征等方面有一定差异。文章对鄂西—湘西北成矿区的矿床时空演化作了重点论述 ,对铁矿的分布与岩相古地理的关系及矿床生成条件进行了讨论 ,指出含矿建造大多产于海侵程序的沉积岩系中 ,在湿热环境下较封闭或半封闭的古海盆、古海湾或潮坪中的浅海 -滨海相沉积组合是有利的成矿古地理条件 ,提出要解决这类高磷铁矿的进一步开发利用问题的关键是加强冶炼试验研究。

Ningxiang type iron deposits constitute a most important type in sedimentary iron deposits of China.The iron bearing formations mainly occur in the Middle and ,especially,Upper Devonian strata.This type of iron deposits is widely distributed in such southern and central areas of China as Hubei,Hunan,Jiangxi,Sichuan,Yunnan,Guizhou,Guangxi and south Gansu.According to their distribution and geological characteristics,seven metallogenic regions may be distinguished,of which the west Hubei northwest Hunan region is of the greatest importance.The iron deposits generally have one to four ore beds,mostly occurring at the transition from silty to shaly or calc shaly facies.Iron ore consists chiefly of oolitic hematite and also siderite associated with chamosite at many localties.Nonmetallic minerals are generally quartz and calcite with minor kaolinite and collophane.The ore contains TFe 33%～45%,often with quite high phosphorus content(0 5%～1 1%).Locally,a certain amount of high grade iron ore may be present. In this paper,a comparison between Upper an Middle Devonian sedimentary iron deposits in such aspects as distribution,size,wall rock formation and characteristics of iron ores has been presented.The time space evolution of Ningxiang type iron deposits in the west Hubei northwest Hunan metallogenic region has been described in detail. Iron bearing formations occur mainly at the sedimentary rock facies of transgressive sequence.The favorable paleogeographic lithologic conditions for the formation of Ningxiang type iron deposits include moist tropic climate,relatively isolated marine basin,gulf or tidal flat close to old land,and littoral neritic sedimenatry rock facies.The authors consider that the key to further exploitation and utilization of Ningxiang type iron ores lies in the strengthening of smelting experimental investigation.