Carboniferous ore-controlling volcanic apparatus and metallogenic models for the large-scale iron deposits in the Western Tianshan, Xinjiang

Abstract

新疆西天山阿吾拉勒成矿带发育一系列大型海相火山岩型铁矿床，是我国十大重要金属矿产资源接替基地之一，备受关注。该成矿带石炭纪火山活动强烈，前人已经识别出该成矿带东段的区域艾肯达坂破火山口，且查岗诺尔、智博、敦德、备战等大型铁矿床均赋存于下石炭统大哈拉军山组火山岩中，其形成受火山机构控制。然而，迄今为止，这些矿床尚未确立具体的火山机构，矿床与矿区发育的火山机构关系有待查明；此外，与查岗诺尔、智博、备战等矿床包含单一的铁元素不同，敦德矿床具有Fe-Zn-Au元素组合，其原因尚不清楚。在前人研究基础上，通过野外地质剖面观察和测量，结合室内研究和遥感解译工作，我们认为阿吾拉勒成矿带东段的4个大型铁矿床均独自发育石炭纪与成矿有关的火山机构，且不同矿床，其火山机构的特点及其演化各不相同。其中，查岗诺尔矿区发育圆形破火山口，面积约为10km²，矿床位于破火山口北缘，矿体赋存于破火山口环状断裂系中；智博矿区发育椭圆形破火山口，面积约为15km²，矿床位于破火山口西南部，矿体赋存于经多次塌陷的破火山口环状断裂系中；敦德矿区发育椭圆形破火山口，面积约为6km²，矿床位于破火山口的中部，矿体赋存于火山通道及其附近的环状和放射状断裂系中；备战矿区发育椭圆形火山口，面积约为4km²，矿床位于火山口北缘，矿体赋存于环状断裂系中。可见，只有敦德矿床的矿体赋存于火山通道断裂系中，其余矿床的矿体均赋存于火山机构环状断裂系中；高渗透性的火山机构断裂系为成矿流体迁移和金属沉淀提供了有利的空间，是铁矿体形成的关键控矿因素和赋矿部位。此外，这些铁矿床的成矿年龄（319~304Ma）滞后于容矿火山岩的年龄（328~319Ma），矿体与围岩界线多为渐变关系；成矿作用以热液作用为主，其中，敦德矿区发育单一的热液成矿作用，具有Fe-Zn-Au元素组合，其它矿床成矿作用复杂，包括热液成矿作用、沉积成矿作用和可能的岩浆熔体成矿作用，仅发育有Fe元素。基于此，可将阿吾拉勒成矿带东段的海相火山岩型铁矿床进一步划分为两个亚类，即火山通道型铁多金属矿床（敦德）和火山边缘型铁矿床（查岗诺尔、智博、备战）；成矿模式包括火山通道相热液富集铁多金属成矿模式和火山边缘相沉积-热液富集铁成矿模式。;Numerous large-scale submarine volcanic iron deposits occurred in the Awulale metallogenic belt in the Western Tianshan, Xinjiang, which is one of the ten most important alternative reserves in China, and it has attracted much attention. The Awulale belt is characterized by intensively Carboniferous submarine magmatic activities. Previous studies have recognized a huge Aikendaban oval caldera which occurs in the eastern Awulale belt, and several large-scale iron deposits, including Chagangnoer, Zhibo, Dunde and Beizhan, are found hosting in the volcanic rocks of the Lower Carboniferous Dahalajunshan Formation, which are controlled by this volcanic apparatus. However, so far, the volcanic apparatus within each iron deposit has not been recognized, the relationship between the deposit and the volcanic apparatus is not identified. In addition, unlike the Chagangnoer, Zhibo and Beizhan deposits which contain single Fe metal, the Dunde deposit is characterized by a Fe-Zn-Au assemblage, and its genetic reason is still unclear. On the basis of previous studies, through field geological profile observation and survey, combined with indoor research and remote sensing interpretation, we believe that some Carboniferous volcanic eruption cycles occurred in each iron deposit, forming itself volcanic apparatus which is closely related to iron mineralization. These volcanic apparatus have different characteristics and evolutionary processes for each other. A circular caldera, with an area of about 10km², occurs at Chagangnuoer, and it experienced the collapse of the cratering. The Chagangnuoer deposit is located in the north of this caldera, and the ore body occurs in the ring fault system of the caldera. An oval caldera, with an area of about 15km², occurs at Zhibo, and it also experienced collapsing; the Zhibo deposit is located in the southwest of the caldera, and the ore body occurs in the ring fault system. An oval caldera, with an area of about 6km², occurred at Dunde; the Dunde deposit is located in the central part of the caldera, and the ore body occurs in the volcanic channel and its surrounding ring and radial fault system. The half crater occurs at Beizhan and the whole crater has an area of about 4km²; the Beizhan deposit is located in the north of the crater, and the ore body occurs in the ring fault system. It is clear that the iron orebodies in all deposits are hosted in the ring fault systems of a caldera, except for the orebodies in the Dunde deposit which are hosted in the volcanic channel and adjacent fault system. The high permeability fault system of the volcanic apparatus acts as channels for the migration of the ore-forming fluids and deposition of iron oxides and is the key ore-controlled factor and important ore-hosted site for the iron orebody. In addition, the published geochronological data show that the ore-forming age of these deposits (319~304Ma) is slightly later than that of their host volcanic rocks (328~314Ma). The orebodies and the host rocks are gradual. The mineralization is characterized by predominant hydrothermal mineralization. The Dunde deposit is characterized by single hydrothermal mineralization with Fe-Zn-Au assemblage; whereas, the Chagangnoer, Zhibo and Beizhan deposits have complicated mineralization, including sedimentary, hydrothermal and possible magmatic melt mineralizations with single Fe metal. Thus, marine volcanic-hosted iron deposits in the Awulale belt can be divided into two sub-types: one is volcanic channel phase-hosted Fe multimetal deposit; and the other is volcanic margin phase-hosted Fe deposit. Based on these, two models are proposed: one is volcanic channel phase-hosted Fe multimetal model with single hydrothermal mineralization; and the other is volcanic margin phase-hosted Fe model with sedimentary-hydrothermal mineralization.