北山南部敦煌岩群铁矿化磁铁石英岩的变质成因①

北山南部地区敦煌岩群广泛分布着铁矿化磁铁石英岩,这些铁矿化磁铁石英岩沿片岩顺层分布,长度几米到一百多米不等,宽几十厘米到两米左右,浅灰色到肉红色,具强磁性。为了研究这种石英岩的成因,笔者对新疆与甘肃的交界处白山西南侧这套地层的斜长角闪片岩中具有代表性的磁铁石英岩完整采集样品,样品具石英、磁铁矿、白云母、黑云母和绿泥石五种矿物,TFe为12.60%~12.70%。根据野外磁铁石英岩产出地质特征、室内显微镜薄片鉴定结果和电子探针检测各矿物特征,结合化学简项分析和岩石化学分析结果,发现本文研究的磁铁石英岩与国内外BIF(Banded iron formation)型磁铁石英岩不同,不具有沉积特征,可能为单纯变质成因。北山地区敦煌岩群这套古老变质地层可能经历了晚太古代、古元古代两次重要的变质事件,本文研究的铁矿化磁铁石英岩可能是在其中一次变质事件过程中形成的。这套变质地层是铁、铜、金等金属元素富集的含矿地层,具有极其重要的研究意义和找矿意义。

Metamorphic formation of magnetite quartzite with iron mineralization in Dunhuang Group in southern Beishan Mountain, China

Magnetite quartzite with iron mineralization has been formed widely in the Dunhuang group of southern Beishan region. It is distributed along bedding schist, with length ranging from a few meters to more than 100 meters and width ranging from tens of centimeters to about 2 meters, pale gray to flesh red in color, specially, and has strong magnetism. In order to study the genesis of this magnetite quartzite, this work collected complete typical samples from plagioclase-hornblende schist in the Dunhuang group of southwestern Baishan area between Xinjiang region and Gansu Province to conduct analysis. The samples comprise of five minerals of quartz, magnetite, muscovite, biotite and chlorite, with TFe content of 12.60%~12.70%. The field geological characteristics of quartzite, indoor microscope thin-section identification results, electron probe analysis, chemical analysis and petrochemical analysis indicate that the studied magnetite quartzite is different from BIF-type ones, which has no sedimentary features but of pure metamorphic genesis. This set of ancient metamorphic strata, which agrees well with other domestic regions by previous studies, has also experienced two important Neo-Archean and Paleo-Proterozoic metamorphic events, and the magnetite quartzite with iron mineralization in this study may be formed in one metamorphic event of the two processes. This set of metamorphic strata contains abundant iron, copper, gold and other metal elements, and thus has extremely important research and prospecting significance.