铜陵叶山铁矿赤铁矿微尺度矿物学研究及地质意义

Study on the micro-mineralogy of hematite from the Yeshan iron deposit in Tongling,and its geological significance

铜陵隆起区内广泛分布着铜-铁-金多金属矿床,也存在众多小型铁矿,其中叶山铁矿是典型代表之一。矿体主要产于石炭系黄龙组内,具有层控特征,铁矿化以镜铁矿/赤铁矿为主。为了深入探讨该铁矿的形成机制及与沉积环境的关系,本文对该矿中不同产状的赤铁矿进行了详细野外地质观察和显微结构分析,并将赤铁矿划分为3类(Hem 1、Hem 2、Hem 3)。激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)分析表明Hem 1具有高Si、Al、Ti的特征,微量元素V、Cr、Co、Ni、Cu、Zn、Sr、Ba、Sn、Pb含量相对较高,Co/Ni比值小于1。Hem 1酸不溶物的扫描电镜(SEM)观察显示石英粒径为2～600μm,表面有风成的断面、孔洞、长条状"V"型凹痕等微结构和形貌,同时粘附有纳米硅质小球,表明石英为潮间带环境下的碎屑。以上结果表明该区黄龙组地层存在沉积赤铁矿型铁质建造,发生重结晶作用。Hem 2和Hem 3在主量元素上以低Ti含量,Si、Al、Mn含量变化大为特征;化学组成及元素相关性图解表明Hem 2、Hem 3并非单一成因,可分为两类:高W,低Mn、Si、Co/Ni比值(<1. 5)为沉积-热液叠加改造成因(Hem 2_(SH)、Hem 3_(SH));低W,高Mn、Si、Co/Ni比值(> 1. 5)为热液成因(Hem 2_H、Hem 3_H)。因此,微量元素含量上,Hem 2_(SH)与其共生磁铁矿差异大,Hem 2_H与其共生磁铁矿差异小; Hem 2_(SH)/(Hem 3_(SH))相对Hem 2_H/(Hem 3_H)显示亏损(W元素除外); Hem 2_(SH)、Hem 3_(SH)、Hem 2_H和Hem 3_H各自之间的波动变化,反映了热液改造过程元素的继承性和热液组分演化过程及其对前驱体的叠加影响,但元素相关性图解排除了Hem 1作为Hem 2_(SH)、Hem 3_(SH)前驱体的可能。综上分析结果表明:叶山铁矿可能属于沉积-热液叠加改造型矿床,黄龙组地层存在以菱铁矿为主的沉积铁建造并提供了该矿床的主要成矿物质,受到燕山期岩浆活动驱动的热液流体叠加改造。赤铁矿的多成因特征不仅为认识叶山铁矿的富集过程提供了重要依据,同时也为进一步了解铜陵地区热液改造型铁矿与黄龙组同生沉积环境之间的关系提供了重要启示。

Copper-gold-iron polymetallic deposits are widely distributed in the Tongling uplift area,however,many small-scale iron deposits are also existed in this area,such as Yeshan iron deposit,one of the typical iron deposits. This deposit is characterized by the strata-bound orebodies and mostly hosted in the Carboniferous Huanglong Formation. The iron ore mineral in this deposit is dominant by specularite/hematite. In order to further investigate the forming mechanism of the iron deposit and its relationship with the sedimentary environment,detailed field and microscope observations had been focused on various locations and occurrence of hematite ores in this paper,and three types of hematite were identified( Hem 1,Hem 2,Hem 3). In-situ LA-ICP-MS analyses of hematite showed that Hem 1 had high Si,Al,Ti contents and relative high contents of V,Cr,Co,Ni,Cu,Zn,Sr,Ba,Sn,Pb,as well as low Co/Ni ratio( < 1). SEM observations showed quartz( acid-insoluble components of Hem 1) was 2 ~ 600μm in size,microstructures and micromorphologies,such as aeolian fracture,holes and long striped V-shaped impressions were existed on the surface,at the same time,some quartz had nanometer silica globules adhered,indicating these quartz had a detrital origin in the intertidal environment.Above analysis also manifested that sedimentary iron formation consisted of hematite was existed in the Huanglong Formation and recrystallized. Hem 2 and Hem 3 had low Ti content,variable contents of Si,Al,Mn in major elements. According to the chemical components and plots of elements,both Hem 2 and Hem 3 were from mixed origins of which two groups can be divided:( 1) the sedimentary-hydrothermal overprinted origin( Hem 2SH,Hem 3SH),this group was featured by high W and low Mn,Si contents with Co/Ni ratio < 1. 5;( 2) the hydrothermal origin( Hem 2H,Hem 3H),this one had low W,high Mn,Si contents with Co/Ni ratio> 1. 5. Hence,Hem 2SH showed great differences to its associated magnetite in trace elements,on the contrary,Hem 2H had similar contents of trace elements to the associated magnetite. Hem 2SH/( Hem 3SH) showed depleted in trace elements( except W) compared to Hem 2H/( Hem 3H),and variations of trace elements in Hem 2SH,Hem 3SH,Hem 2H or Hem 3H reflected the inheritance of elements in the hydrothermal overprinted process and the evolution process of hydrothermal components and its impacts to the precursor,but Hem 1 was not the precursor of Hem 2SH,Hem 3SHaccording to the plots of elements. The above results indicated Yeshan iron deposit might belong to a sedimentary-hydrothermal overprinted deposit,the sedimentary iron formation in the Huanglong Formation was dominant by siderite,and provided most of the metallogenic material for this deposit,then it was overprinted by the hydrothermal fluid derived from Yanshanian magmatism. Poly-genetic hematite not only provides important evidences for the concentration process of Yeshan iron deposit,but also provides important enlightenments for the relation between hydrothermal overprinted iron deposits and syngenetic sedimentary environment of Huanglong Formation in the Tongling area.