庐枞盆地与A型花岗岩有关的磁铁矿-阳起石-磷灰石矿床——以马口铁矿床为例

Apatite-actinolite-magnetite deposit related to A-tpye granite in Luzong basin:Evidence from Makou iron deposit

庐枞中生代火山岩盆地位于长江中下游断陷带内,地处扬子板块的北缘。庐枞盆地内的火山岩和侵入岩分布广泛,包括龙门院、砖桥、双庙和浮山四组火山岩以及34个侵入岩体。最近在庐枞盆地南部正长岩中发现一种新类型铁矿床——马口铁矿床。本次工作通过详细野外地质和室内研究,系统开展了马口铁矿床矿床地质特征、成矿年代学和成矿流体特征研究。马口铁矿床的矿体沿石英正长斑岩体中构造破碎带产出,产状严格受构造破碎带控制,矿床围岩蚀变晕范围较为局限。矿床的成矿作用可分为碱性长石阶段、磁铁矿阶段、石英-硫化物阶段和碳酸盐阶段,其中磁铁矿阶段是主要成矿阶段,形成与宁芜玢岩型铁矿床相类似磁铁矿-磷灰石-阳起石三矿物组合。矿床中与磁铁矿共生的磷灰石流体包裹体均一温度范围为252.2～322.6℃,反映其成矿温度略低于典型的玢岩铁矿床。通过赋矿岩体锆石LA-ICP-MSU-Pb定年和金云母Ar-Ar方法,确定马口铁矿床的成矿时代为127.3±0.8Ma,赋矿岩体的形成时代为129.4±1.4Ma,结合矿床地质特征,可以得出矿区内石英正长斑岩只是提供了赋矿空间,矿床的形成与其无直接成因联系,而可能与黄梅尖A型花岗岩体有关。马口铁矿床是庐枞盆地成矿作用最后阶段的产物,也可能是长江中下游成矿带中最晚一次成矿事件的代表,具有显著的特色。

The Luzong Mesozoic volcanic basin is located in the Middle-Lower Yangtze River faultdepression zone, which is located in the northern margin of the Yangtze Block. The volcanic rocks and intrusive plutons occur widely in Luzong basin, including four groups of volcanic rocks, such as Longmenyuan Group, Zhuanqiao Group, Shuangmiao Group and Fushan Group and 34 intrusive plutons. A lot of metallic and nonmetallic deposits such as iron, copper, gold, lead, zinc, uranium, alunite and anhydrite deposits occur in the basin. A new type of iron ore deposit, Makou iron deposit, related with syenite has been discovered in the southern part of Luzong basin. Based on detailed field investigations and laboratory study, we have carried out study of geological characteristics geochronology and oreforming hydrothermal fluid characteristics of this iron deposit. The orebodies occur along tectonic fracture zone in quartz syenite porphyry and the range of alterated halo is limited. Four stages of hydrothermal oreforming processes are recognized, including alkali-feldspar stage, magnetite stage, quartzsulphide stage and carbonate stage. The predominant mineralization associates with magnetite stage. The magnetiteactinoliteapatite mineral assemblage of iron ore is similar to Ningwu porphyrite iron deposits. The homogenization temperatures of fluid inclusions in apatites range from 252. 2℃ to 322. 6℃, which indicates the temperature of mineralization is slightly lower than typical porphyrite iron deposit. LA-ICP-MS U-Pb zircon dating of quartz syenite porphyry is 129. 4 ±1.4Ma and 40Ar/39Ar dating of phlogopite is 127. 3 ±0. 8Ma. Combining with geological characteristies of ore deposit, it is concluded that the quartzsyenite intrusion has merely provided orebearing space and the mineralization is not directly related with the quartz syenite porphyry but probably relates to the Huangmeijian Atype granite. The Makou iron ore deposit is the product of the final stage of mineralization in Luzong basin and probably represents the latest metallogenic event in the MiddleLower Yangtze River Valley metallogenic belt.