中国辽东地区超大型菱镁矿矿床的地球化学特征和成因模式

摘要

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,菱镁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镁矿体产出.研究表明,菱镁矿的δ18O值为5.2‰～13.8‰,低于围岩大理岩的δ18O值(11.2‰～22.8‰).但两者的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,因此菱镁矿的矿化交代作用只发生在其上的大石桥组三段镁质碳酸盐岩地层之中.在区域变质作用过程中,菱镁矿层曾发生变质重结晶作用.在更晚期的热液活动中可能也有一些菱镁矿脉形成.%A number of giant magnesite deposits occur in the Early Proterozoic Dashiqiao Formation of magnesian carbonate and argillite rock sequences in the eastern Liaoning province, northeastern China. The magnesite orebody is hosted only in the third section of the Dashiqiao Formation with rock associations of dolomitic marble, magnesitic marble, magnesite and phyllite thin layer. Below the orebody is the 2nd section of the Dashiqiao Formation of mica schist. In the 1st section of the Dashiqiao Formation occurs interbedded dolomitic marble and mica schist without any magnesite orebody. The magnesite ores show lower δ18O values (5.2 to 13.8 per mil relative to V-SMOW) than the marbles (11.2 to 22.8 per mil), but both the magnesite ores and marbles have similar around zero δ13C values with variations of - 1.4 to 1.2 per mil and - 4.5 to 4.4 per mil, respectively. Gypsum strata and veins have been found associated with the magnesite ores with δ34 S values of 23.9 to 26.5 per mil (relative to V-CDT), indicating a marine facies sedimentary evaporatic origin. Magnesites display three types of shale-normalized REE patterns, with type Ⅱ similar to the host rock marbles, and type Ⅰ of middle REE enrichment and type Ⅲ of heavy REE enrichment and positive Eu anomaly, which may reflect fluiddominated features. We suggest that the magnesian carbonate rocks (Mg-rich calcite and dolomite) were precipitated from an evaporitive lagoon, whereas the magnesite ores were mainly formed by subsequent replacement of Mg-rich brine penetrated down from a brine pool. Due to the low permeability of the 2nd section of the Dashiqiao Formation, the magnesite mineralization took place only above this low-permeability layer of mica schist, i. e. , constrain in the 3rd section of the Dashiqiao Formation. The magnesite orebody has been recrystallized during late regional metamorphism and some magnesite veins may have formed during later hydrothermal event.