山西袁家村矿区前寒武纪铁矿的形成条件

山西省吕梁山地区早元古宙袁家村铁矿以变质程度浅、地层剖面完整和地质构造简单为特征。厚约1200m 的袁家村组含铁岩系可以分成三个段,它们分别代表了三个沉积-成矿旋回。其中每一个旋回都是以粗碎屑岩开始,经过粉砂岩和泥质岩逐渐过渡为较纯的胶体化学沉积物即硅质岩。矿区出现的铁矿物相沿剖面自下而上依次为黄铁矿、菱铁矿、铁硅酸盐(包括铁绿泥石、铁滑石和黑硬绿泥石)、磁铁矿和赤铁矿。这些铁矿物相是沉积阶段、成岩阶段至初级变质阶段的产物。它们在地层剖面上的空间分布,反映各旋回铁矿物相的形成环境,在时间上自早至晚,空间上自下而上,其氧逸度逐渐增高。控制铁矿物相类型和其中铁价态形式的主要因素,是沉积时泥砂质碎屑组分和铁硅质胶体化学组分的性质和相对比例,以及其中有机质的存在与否及其多寡。

The Formation Condition Of 'Rhe Precambrian Iron Ores In Yuanjiacun Ore Deposit, Shanxi Province

The Yuanjiancun deposit is characterized by low grade metamorphism, well exposed stratigraphic section, and simple tectonics. It therefore offers an ideal locality in studying formation environment and metamorphic processes of Precambrian banded iron ores. Developed in shallow water basin of eugeosyncline, the Yuanjiancun foimation some 1200 m thick comprises three sedimentation-ore formation cycles, each of which begins with coarse to medium-grained clastic rocks (quartz sandstone or arkose), passing through fine-grained siltstone and pelitic rocks, and grades into pure colloidal chemical deposits represented by thin-bedded ferruginous quartzites. The iron mineral facies present in the ore district are diverse. From the bottom of each cycle upwards, there generally occur in succession pyrite, siderite iron silicates (composed of Fe-chlorite, minnesotaite and stilpnomelane), magrtetite and hematite. These iron mineral facies are products of sedimentation, diagenesis and metamorphism of very low grade. The spatial distribution of iron mineral facies in stratigraphic section shows that the sedimentation-ore formation environment had an increasing oxygen fugacity in the course of each cycle. The main factors controlling the types of iron mineral facies and the valence state of iron are considered to be the proportion of pelitic-arenaceous components to colloidal chemical materials during deposition as well as the relative amount of organic substance they contain. The existence of abundant organic materials results in the formation of a reducing environment and the facies dominated by ferious minerals, whereas lack of organic materials leads to the development of oxidizing media and the formation of ferric or ferric-ferrous mineral facies.