中国叠生型铁矿床成矿特征探讨

叠生成矿作用主要是指早期成矿作用被晚期成矿作用叠加、复合和改造。晚期成矿作用的性质常与早期成矿作用不同,也就是说,在早先己有矿床(或矿体、矿源层)的基础上,叠加复合了晚期成矿作用,即成矿时间上有先后、空间产出上有重叠、并对早先形成的矿床进行复杂的复合、叠加和改造,使成矿作用具有多样性、复杂性,并可形成大矿、富矿。铁矿床中叠生成矿作用广泛发育。按矿床或矿体产出和形成的地质特征,中国叠生型铁矿床可分为风化淋滤型、热液叠加改造型和热液叠加复合型3个亚类。风化淋滤型铁矿床在中国分布有限,规模不大,工业利用价值不大,因而中国的叠生型铁矿床主要是指热液叠加改造型和热液叠加复合型两个亚类。热液叠加改造型主要是指早期的铁矿床(或矿体、矿源层)经后期热液叠加改造,使早期的较贫铁矿床(或矿体、矿源层)成为较富铁矿床(或矿体),这是中国BIF型铁矿床中最重要的富铁矿类型,以鞍本地区弓长岭二矿区为典型代表。弓长岭二矿区铁矿,早期在新太古代形成条带状磁铁石英岩(2528 Ma,贫矿石),后期在古元古代,含矿热液交代改造贫铁矿形成富铁矿(1840 Ma)。热液叠加复合型主要是指后期脉型铁(或稀土元素等)矿床叠加在早期(沉积或其他成因)铁等矿床上而形成的矿床,如白云鄂博铁-铌-稀土元素矿床和黔西菱铁矿矿床。白云鄂博铁-铌-稀土元素矿床的形成与火成碳酸岩有关,在中元古代(1.3 Ga)左右,区内火成碳酸岩的侵位,在早期主要形成以岩浆熔离作用为主的铁-铌-稀土元素矿,晚期叠加了加里东期稀土-铌矿化热液脉。古陆边缘构造带或陆内活化带是形成叠生型铁矿床的有利构造空间,较大的地球化学块体,为形成多期、多成因的矿床提供物质来源,叠生型铁矿床的形成明显受构造的控制。叠生成矿是复杂地质过程的一种具体表现。热液叠加改造型和热液叠加复合型的叠生型铁矿床的形成是因中国独特的大地构造环境决定的。叠生成矿作用的研究,尚处在初步阶段。加强对叠生成矿作用的研究,了解其形成的地质背景、成矿机制、作用过程、控矿因素等,对发展矿床学研究,认识区域成矿特征和指导地质找矿具有重要的理论和实际意义。

Metallogenic characteristics of superimposed iron ore deposits in China

Superimposed mineralization means that early mineralization experienced superimposition, compounding and transformation, and late mineralization included one or several periods, with late mineralization usually different from early ones in characteristics. In other words, the earlier ore deposit (orebody or source bed) was subjected to superimposition of late mineralization, and hence there is a sequence of mineralization time and superposition in space. In addition, early formed deposit experienced superimposition, compounding and transformation, which led to diversity and complexity. Sometimes large-size and rich deposits can be formed. Superimposed mineralization is widely developed in iron ore deposits. According to the occurrence and origin, the type of superimposed iron ore deposits in China can be subdivided into three subclasses, i.e., weathering-infiltration, hydrothermal superimposed reworking and hydrothermal superimposed compounding. Superimposed iron ore deposits in China mainly refer to the last two types, because weathering-infiltration iron ore deposits are less distributed and are characterized by small scale and minor industrial value. Hydrothermal superimposed reworking type means that early lean iron ore deposit (orebody and source bed) becomes rich one by hydrothermal superimposed reworking. Hydrothermal superimposed reworking deposits are the most important type of BIF type ore deposits in China, represented by the No. 2 mining area of the Gongchangling iron deposit in Anben area. In No. 2 mining area of Gongchangling, early stage banding magnetite quartzite iron deposit (lean ore) was formed in Late Archean ((2528±10)Ma), and late stage rich ore deposit was formed in Palaeoproterozoic ((1840±7) Ma) through ore-bearing hydrothermal metasomatism. The deposit type of hydrothermal superimposed compounding was formed by late stage vein type iron ore deposit (or REE) superimposed upon early stage iron ore deposit (sedimentary or other origin). The typical deposits are the Baiyan Obo Fe-Nb-REE deposit and the western Guizhou siderite ore deposit. The origin of the superlarge Baiyan Obo Fe-Nb-REE deposit is related to igneous carbonatite. At about 1.3 Ga in Mesoproterozoic, igneous carbonatite was emplaced in Baiyan Obo; iron deposit and part of Nb-REE deposit were formed at the early stage of the evolution of igneous carbonatitee magma. In Caledonian the deposit experienced the superimposition of Nb-REE hydrothermal mineralization. Ancient land margin or intra-continental mobile belt seem to have been favorable tectonic space; large geochemical blocks provided material sources to form deposits of multiple genesis and stages. The formation of superimposed iron ore deposit is obviously controlled by structure. Superimposed mineralization is a concrete manifestation of complex geological processes. The formation of hydrothermal superimposed reworking and hydrothermal superimposed compounding iron ore deposits resulted from the unique tectonic environment of China. The study of superimposed mineralization is still in the preliminary stage. Therefore, it is of theoretical and practical significance to study superimposed mineralization, including ore deposit geological setting, mechanism, process and mineral control factors, with the purpose of further understanding regional metallogenic chasacteristics and providing guidance for geological prospecting.