用Y／Ho比值指示俄罗斯乌拉尔南部晶质菱镁矿矿床的成因
Y/Ho ratio as geneticindicator of sparry magnesites in south Urals, Russia
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摘要: 乌拉尔省南部赋存有两种类型的晶质菱镁矿:1)白云岩地层中的层状矿体;2)白云质灰岩中的透镜状矿体。晶质菱镁矿矿体位于Riphean系列中下层的白云岩中,而在上层的白云岩单元中缺失。这两种类型的菱镁矿可通过矿体形态、晶体大小、石英和白云石含量不同来进行区分。第一种类型的菱镁矿储量巨大,菱镁矿呈粗粒结构,晶体粒径>10mm(最大达150mm);一般来说,矿体与白云岩围岩界限清楚,这种类型矿床以产在Riphean序列下部为特征。第二种类型的菱镁矿由于菱镁矿矿体穿插进入到白云岩围岩中,矿体很不规则,菱镁矿晶体也相对较小(1-5mm),这种类型的矿体主要产在Riphean中部层位中。这两种矿体都显示了交代成因的特征。但这两种菱镁矿矿石在一些主量元素和稀土元素的分布上具有不同的特征:与第二种类型相比,第一种菱镁矿具有较低的FeO,CaO和SiO2含量,与白云岩围岩(La／Lu>1)相比,具La／Lu<1的轻稀土亏损特征。第二种菱镁矿稀土分馏度较低,在稀土分配方面与白云岩围岩有差别。本文还特别讨论了Y／Ho值的重要性,因为该比值在菱镁矿和围岩中的类似性使得划分菱镁矿形成中的热液和成岩交代过程成为可能。因此我们认为,第一种类型菱镁矿,如具有高Y／Ho比值的Satka和Bakal矿床的形成属于沉积盆地发育过程中的早期成岩阶段;第Abstract: There are two types of sparry magnesite deposits in the Southern Urals province: 1) layer-like bodies in the stratigraphic dolostone horizons; 2) lens-like bodies in the dolomitized limestones. The sparry magnesite deposits are located in the dolostones of the Lower and Middle Riphean series and are absent in dolostones units of the Upper Riphean. These two types of magnesite are distinguishing by the shape of ore bodies and size of crystals, quantity of quartz and dolomite admixtures. The resources of the first type are huge; magnesite has a coarse-grained structure with crystal size of more than 10 mm (up to 150 mm) , as a rule, sharp boundaries of ore bodies with host dolomites. This type of deposits is typical for the Lower Riphean sequence. The second type of deposits has a very irregular shape of ore bodies with impregnation of magnesite crystalls into host dolomites, with relatively small size of magnesite crystals (1-5 mm). The second type predominates in the Middle Riphean sections. All the deposits possess the features of metasomatic origin. The two types of magnesite ores show distinct features of some major elements and REE distributions. Magnesites of the first type have lower contents of FeO, CaO and SiO2 in comparison with magnesites of the second type. The first type magnesites are LREE depleted with La/Lu 1) . Magnesites of the second type have a low degree of fractionation and differences in REE distributions between magnesites and country rocks of dolomites. In this paper the importance of Y/Ho ratio is discussed, because the resemblance of this ratio in magnesites and host rocks makes it possible to separate hydrothermal and early diagenetic metasomatic processes of magnesite formation. We suggest the first type magnesites, such as the Satka and Bakal deposits with high Y/Ho ratios belong to early diagenetic stage of development of the rock-forming basin. The second type magnesites, such as the Ismakaevo and Katav-Ivanovsk deposits with low Y/Ho ratios, are related to hydrothermal processes of fluid migration.
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Key words:
- Riphean /
- Precambrian /
- Sparry magnesite /
- Metasomatic fluids /
- REE
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