滇西北衙多金属矿田矿床成因类型及其与富碱斑岩关系初探

北衙金多金属矿田是与金沙江-哀牢山新生代富碱斑岩有关的成矿作用的典型代表之一,近年来在矿产勘查方面又有重大突破,金已达到超大型矿床,伴生铁、铜、银、铅、锌也达到了大-中型矿床规模。本文基于野外观察与室内研究,结合前人研究成果,对北衙多金属矿的成因类型,富碱斑岩与成矿作用的关系及成矿机制进行了系统总结,对与成矿相关的富碱斑岩进行了主量元素及锆石LA-ICP-MS的测试,探讨了铁矿的成因。研究表明,矿田原生金属矿床可分为:斑岩型铜金矿化,夕卡岩型铁、金、铜、铅、锌矿化,爆破角砾岩筒中的铁、金、铅、锌矿化以及热液型金、银、铅、锌矿化。其中夕卡岩型和热液型矿床是该区最主要的成矿类型。新生代富碱斑岩(石英正长斑岩)的年龄分别34.92±0.66Ma和36.24±0.63Ma。属于钾质碱性岩系列。它不仅为含矿流体的上升提供了动力和热能,而且还是成矿物质和成矿流体的主要来源,因此形成以斑岩体为中心,由斑岩型、夕卡岩型、热液型等矿床构成的一个连续的成矿系统。钾质碱性岩及矿床是在碰撞造山走滑构造系统深部壳幔相互作用的产物。本区岩体接触带中发育大量由菱铁矿和磁铁矿组成的铁矿体,其中大部分的磁铁矿是一种具有赤铁矿的板状晶或聚片双晶假象的穆磁铁矿。对磁铁矿和菱铁矿形成条件的分析表明,磁铁矿和菱铁矿主要是在碱性环境下交代含铁夕卡岩矿物形成的。当热液中H+的浓度降低时,赤铁矿被还原为磁铁矿,但仍保留了赤铁矿的晶形,于是成为穆磁铁矿。由此推测,本区成矿作用是在成矿流体及夕卡岩化交代作用长时间反复持续进行的条件下发生的,这可能是本区得以形成巨量金属堆积的重要原因之一。

Genetic types and the relationship between alkali-rich intrusion and mineralization of Beiya gold-polymetallic ore field, western Yunnan Province, China

The Beiya gold-polymetallic ore field is one of the typical deposits related to the Jinshajiang-Ailaoshan alkali-rich intrusion of the Cenozoic Period, and it is also a magmatic-hydrothermal metallogenesis system related to alkali-rich intrusion during the Himalaya epoch. Based on the recent extensive researches and field investigations, this article systematically studies and concludes the genetic of various ore types, the relationship between mineralization, alkali-rich porphyries and the genesis of iron ore. In the ore field, four types of gold-polymetallic deposits have been recognized, there are porphyry-type Cu-Au mineralization, skarn-type Fe-Au-Cu-Pb-Zn mineralization, breccia-type Fe-Au-Pb-Zn deposits and hydrothermal Au-Ag-Pb-Zn mineralization in wallrock. Among the fours type of gold-polymetallic deposits, skarn and hydrothermal type are the most important metallogenesis. The whole-rock geochemical dating of quartz syenite porphyry shows that it belongs to potassic alkali-rock. And the LA-ICP-MS U-Pb age are defined as sample WDS-1 (31.5±1.1Ma), sample HNT-1 (31.34±0.73Ma). The quartz syenite porphyry which is shoshonitic series, not only provides a rising force and heat, but also acts as the main source and carrier for the ore-forming material and mineralization fluid. As a result, it builds a porphyry body-centered continuous porphyry-hydrothermal system, which is made of porphyry-type, skarn-type, hydrothermal-type, and various weathered-sedimentary-type deposits. The progress of forming the quartz syenite porphyry and ores was accompanied by the strong magmatic activity in the Sanjiang metallogenic belt, and the crust-mantle reaction made an important contribution to the mineralization and diagenesis in Beiya area. A large number of iron ore are produced along the contact between the alkali-rich porphyry and surrounding rock. It is found that the native iron ore accounts for 30.08% of magnetite and siderite accounts for 24.68%. Most of the magnetites which assume tabular forms and polysynthetic twin are the illusion of hematite. According to research, magnetite and siderite is the metasomatism product of the iron-bearing skarn in the alkaline environment. When the H⁺ concentration in Hydrothermal is decreases, hematite will be reduced to magnetite. It is the illusion of hematite when magnetite retains the crystal forms of hematite. The analyses show that the ore-forming fluids and the skarn mineralization last for a long time, which may be one the most important reason for the accumulation of that abundant metal.