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我国辽东地区早元古代大石桥组镁质碳酸盐-泥质岩建造中赋存有多个超大型菱镁矿矿床。在这些矿床中,菱铁矿矿体均只限定在大石桥组三段岩层中。该层岩石主要由白云质大理岩、菱镁质大理岩、菱镁矿和少量泥质板岩薄层组成。赋矿层位之下地层为大石桥组二段的云母片岩;其下为大石桥组一段的白云质大理岩与云母片岩夹层。在大石桥组一段中未见菱铁矿体产出。研究表明,菱铁矿的δ18O值为5.2‰~13.8‰,低于围岩大理岩的δ18O值(11.2‰~22.8‰)。但两音的δ13C值大多接近零值,其中菱镁矿δ13C值变化为-1.4‰~1.2‰,大理岩δ13C值变化为-4.5‰~4 4‰。在菱镁矿层位中发现有石膏成层和脉状产出,其δ34S值为23.9‰~26.5‰,显示海相蒸发沉积特征。菱镁矿的稀土元素分析表明存在三种不同页岩标准化配分模式。类型Ⅰ显示中稀土富集特征,类型Ⅲ显示重稀土富集和正铕异常特征,它们可能反映了不同时期成矿热液的特征。而类型Ⅱ显示与围岩大理岩相同的平坦型,反映继承了原岩沉积碳酸盐岩的特征。本文认为,辽东地区的镁质碳酸盐岩(镁方解石和白云石)可能是从蒸发的泻湖盆地中沉积的,而菱镁矿石则主要是沉积后富镁卤水下渗交代原岩碳酸盐岩形成的。由于大石桥组二段云母片岩渗透率低隔水性强,因此菱铁矿的矿化交代作用只发
There are several superlarge magnesite deposits in the magmatic carbonate-argillaceous rocks of the Early Proterozoic Dashiqiao Formation in Liaodong area of China. In these deposits, siderite ore bodies are confined to the third member of the Dashiqiao Formation. This layer of rock is mainly composed of dolomitic marble, magnesian marble, magnesite and a small amount of shale thin layer composition. The strata below the ore-bearing layer is mica schist of the second member of the Dashiqiao Formation, and a dolomitic marble-mica schist interlayer beneath the first member of the Dashiqiao Formation. There is no output of siderite in the section of Dashiqiao Formation. The results show that the δ18O value of siderite is 5.2 ‰ ~ 13.8 ‰, which is lower than the δ18O value of the surrounding rock (11.2 ‰ ~ 22.8 ‰). However, the δ13C values of the two tones are mostly close to zero, of which the δ13C value of magnesite varies from -1.4 ‰ to 1.2 ‰, and the δ13C value of marble varies from -4.5 ‰ to 44 ‰. Gypsum stratigraphic and vein-like outputs were found in the magnesite horizon with δ34S ranging from 23.9 ‰ to 26.5 ‰, indicating marine evaporites. Analysis of rare earth elements in magnesite shows that there are three different shale normalization patterns. Type I shows the characteristics of rare earth enrichment, Type III shows the characteristics of heavy rare earth enrichment and the positive europium anomalies, which may reflect the characteristics of hydrothermal fluids in different periods. While Type II shows the same flat pattern as the surrounding marble, reflecting the inherited characteristics of the original sedimentary carbonate. This paper argues that the magnesian calcites (calcite and dolomite) in the Liaodong area may have been deposited from the evaporized lagoon basin, while the magnesite is mainly deposited after the enrichment of magnesium-rich brine infiltration of protolith carbonate Rock formation. Due to the low permeability of mica schists in the second section of Dashiqiao Formation, the mineralization of siderite can only be explained