新疆西天山查岗诺尔铁矿床矿物学特征及其地质意义

摘要

Located in the eastern Awulale metallogenic belt of Western Tianshan Mountains, the large-size Cha-gangnuoer iron deposit is hosted in andesitic volcaniclastic rocks or andesitic tuff of the Lower Carboniferous Da-halajunshan Formation, with a lenticular marble beneath the main ore body. The ore bodies occur as lamellar, stratoids and lenses, controlled by NW-, NWW- and NEE- striking faults and circular faults. Mineralizations occur along the fissures and fractures in the wall rocks. This ore deposit is composed of two major ore bodies, I. E., FeI and FeII. The FeI ore body strikes NE-SW, about 2 900 m in length, 63 m in average thickness, with Fe average grade of 36.87% and ore reserves of more than 190 million tons. Wall rock alterations includechiefly garnetization, actinolitization, chloritization, epidotization etc. Ore minerals consist mainly of magnetite and subordinated pyrite and chalcopyrite while gangue minerals are composed of garnet, actinolite, chlorite, epidote, tremolite, and calcite. Electron microprobe analyses show that the end member of garnet is andradite (Adr) with an average content of 61.58 %, grossularite (Grs) with an average content of 35.45 %, and alman-dite (Alm) with an average content of 2.48%. The composition of pyroxene is dominated by diopside (Di, 54.22% on average) and hedenbergite (Hd, 44.22% on average), with a small amount of johannsenite (Jo, less than 5.53%). Components of garnet and pyroxene are Adr37.97~97.89Grs0.19~57.21 (Aim + Sps)0.84~4.38, Di28.68~87.46Hd10.46~70.13.Jo0.24~5.53, which indicates that 0 characteristics of these two skam minerals are quite similar to those in calcic skam from the major large iron deposits, suggesting that they probably resulted from skarnization; (2) their wide and different compositions reveal that skarns were not formed under a wholly closed equilibrium condition. Epidote is rich in Al and Ca but poor in Fe and Mg, implying that Fe and Mg were separated from each other at the time when pyroxene was converted into epidote. The chemical compositions of magnetite are rich in Mg, Mn, Ca, Si but poor in Al, Ti, Ni, exhibiting a similar feature to those formed in calcic skarn iron deposits. In the Ca + Al + Mn versus Ti + V discriminant diagram showing spot analyses of magnetite and hematite, almost all data from the Chagangnuoer ore deposit fall into the region of the skarn type iron deposit. In addition, in the ternary plot of TiO2- Al2O3- MgO of magnetite, many data from the Chagangnuoer ore deposit tend to be seated in the sedimentary metamorphogenic and contact meatasomatic region while less data drop into magmatic mafic-ultramafic region. These two diagrams may suggest that the formation mechanism of magnetite from this ore deposit may be similar to that of magnetite from skam iron deposits. Combining the geological characteristics with the study of mineralogy, the authors hold that iron mineralization was related to skarnization, and that skarnization might have played an quite important role in the formation of the Chagangnuoer iron ore deposit.%查岗诺尔大型磁铁矿床位于西天山阿吾拉勒东段,矿体赋存于下石炭统大哈拉军山组安山质火山碎屑岩或凝灰岩中,主要呈层状、似层状、透镜状,受NW、NWW、NE断裂及环形断裂构造控制.矿区发育石榴石、透辉石、方柱石、阳起石、钾长石、绿帘石、绿泥石、方解石等蚀变矿物,矿石矿物主要为磁铁矿和赤铁矿,伴生的金属矿物以黄铁矿和黄铜矿为主.电子探针分析结果表明,石榴石和辉石分别为钙铁榴石-钙铝榴石系列和透辉石-钙铁辉石系列,其化学组成可表示为Adr37.97～ 97.89Grs0.19～57.21 (Alm+ Sps)0.84～4.38和Di28.68～87.46Hd10.46～70.13Jo0.24～5.53,与典型的矽卡岩型铁矿中石榴石和辉石的端员组分相似.在磁铁矿和赤铁矿的Ca+Al+Mn-Ti+V图解中,多数样品落入矽卡岩型铁矿的区域;在磁铁矿的TiO2-Al2O3-MgO图解中,多数样品落入或趋近于沉积变质-接触交代磁铁矿区域.结合矿床地质特征和矿物学研究,认为该矿床的形成与矽卡岩化紧密相关,矽卡岩化对铁成矿有重要的贡献.