大兴安岭南段维拉斯托锡多金属矿岩浆-热液过渡过程：微区矿物学和流体演化记录

The Weilasituo Sn-polymetallic deposit is a large deposit that has been recently discovered in the southern Great Xing’an Range. This deposit shows a typical “up veins + underneath mineralized granite” mineralization zoning. Besides, the ore cluster area shows horizontal mineralization zoning, from the inner granite body outward, consisting of high-T Sn-W-Mo mineralization, middle-T Cu-Zn mineralization and peripheral low-T Pb–Zn-Ag mineralization. The magmatic-hydrothermal transition process (MHTP) of Sn-polymetallic deposits is an important scientific issue that needs to be resolved. Taking the Weilasituo Sn-polymetallic deposit as the major study object and using In-situ mineralogy and individual melt/fluid inclusion analysis techniques as well as stable isotope and helium-argon isotope tracer methods, this study aimed to explain the distribution behaviors and controlling factors of ore-forming elements and volatiles in coexisting phases during MHTP, identify the physical and chemical conditions restriction and the mineralization precipitation mechanism during evolution processes, and define the sources of ore-forming materials/fluids. On the basis of evidence of mineralization distribution regularities, mineralization ages, fluid P–T–X evolutionary track and material sources of tin and lead–zinc–silver mineralization alteration zoning, the genetic relationship between tin mineralization and peripheral vein–type lead–zinc–silver mineralization was recognized and a reasonable metallogenic model was established. This study not only enriches the metallogenic theory in Central Asian Orogenic Belt but also provides a scientific basis of prospecting exploration for tin deposits in the Great Xing’an Range.

大兴安岭南段维拉斯托锡多金属矿是近年来新发现的大型矿床，具有“上脉下体”的垂向矿化分带特征，同时矿集区范围内表现出以花岗岩体为中心的高温锡钨钼矿化、中温铜锌矿化、外围低温铅锌银矿化的水平分带。锡多金属矿床岩浆-热液过渡过程是一个有待查明的重要科学问题。本项目以维拉斯托锡多金属矿为重点研究对象，以微区矿物学和单个熔融/流体包裹体分析技术为主线，辅以稳定同位素和氦-氩同位素示踪研究，试图阐明岩浆-热液过渡过程中成矿元素和挥发份在共存相间的分配行为及其控制因素，确定演化过程中物理化学条件制约和矿化沉淀机制，厘定成矿物质和成矿流体来源。基于锡矿化和铅锌银矿化的蚀变分带、矿化空间展布规律、形成时代、流体P-T-X演化轨迹和物质来源等证据，揭示锡矿化与外围脉状铅锌银矿化之间的内在成因联系，建立合理的成矿模型。本项目的开展不仅丰富了中亚造山带的成矿理论，而且将为大兴安岭地区锡矿找矿勘查工作提供科学依据。

大兴安岭南段是我国北方最重要的锡钨成矿带，近年来在热液脉型银铅锌矿深-边部陆续探明多个大型锡矿，其成矿特征与全球著名的南美安底斯锡银成矿带类似。目前针对大兴安岭南段锡矿的成矿机制及锡矿化与外围铅-锌-银矿化的成因关系尚不清楚。本次研究以维拉斯托锡多金属矿为重点，通过岩相学、矿物学、年代学和地球化学研究，查明了岩浆-热液过渡过程中的成矿元素的富集机制和控制因素，确定了成矿流体演化过程和物质源区，明确了深部锡矿化与外围脉状铅锌银矿化的内在成因联系。同时，全面总结了大兴安岭南段早白垩世锡矿床成矿规律，建立区域成矿模型。取得了以下主要认识：1、维拉斯托锡矿发育多期岩浆-热液成矿作用，锡-钨-锂矿化与外围脉状铅-锌-银-铜矿化属同一岩浆-热液成矿系统，不同金属元素在不同的矿化空间内的选择性沉淀是造成元素分带性的直接原因；2、锡矿化过程中发生了强烈的水-岩反应和流体混合作用（岩浆水、变质流体与大气降水）。围岩黑云母斜长片麻岩中黑云母与岩浆出溶流体的反应可能导致含Sn流体中HCl的损耗和铁锂云母的形成，从而促进Sn的进一步富集和锡石的沉淀；3、在锡的迁移-卸载沉淀中，Sn (II)主要存在于成矿早期，在成矿晚期，由于氧化还原作用或Cl-活性的增强，Sn逐渐转变成Sn (IV)配体形式；4、石英温-压计估算的初始岩浆熔体的温度和压力分别为~700℃和1.5 kbar，为浅成高分异岩浆。随着温度的不断降低和SO2的歧化反应，流体体系中的fO2和硫饱和度不断升高。同时，岩浆脱气过程中产生的SO2增加了流体的活动性，促进了晚期金属硫化物的富集沉淀；5、大兴安岭南段早白垩世锡矿形成于活动大陆边缘弧后伸展和软流圈上涌的构造背景，流体的温-压下降、不同来源流体混合、超临界流体不混溶作用、流体多次沸腾和水-岩反应等是金属元素超常富集的主要机制。