敦德大型铁锌矿床成因研究

It occurs a number of rich iron ore deposits in the Awulale iron-copper metallogenic belt in western Tianshan, Xinjiang autonomous region, China. These iron deposits are characterized by large reserves, high ore grade, hosted in volcanic or volcaniclastic rocks, developing a mass of skarn but without related intermediate-acid intrusive rocks. However, these characteristics cause long-standing controversy concerning the genesis of these iron deposits. The Dunde large iron-zinc deposit is one of these rich iron ore deposits. This application chooses it as the research object and determines its genesis through the following research: (1) detailed research on the genesis of skarn including its distribution, zoning, mineral assemblage, the processes of prograde metamorphism and retrograde metamorphism; (2) systematic study on the geological characteristics of the ore body, mineral assemblages, ore texture and structure, ore-forming stages; (3) systematic study on the relationship between skarn formation, zinc mineralization and iron mineralization; (4) trace element and stable isotope compositions tracking the sources of ore forming materials; (5) determining the age of iron ore and the ore-bearing volcanic rocks using advanced isotopic dating techniques. This project will help to understand the genesis of rich iron ore deposits hosted in marine volcanic and pyroclastic rocks, and provide important information for further prospecting of rich iron ore deposits in the Awulale iron-copper metallogenic belt in western Tianshan.

新疆西天山阿吾拉勒铁铜成矿带发育大量富铁矿床，这些富铁矿床以规模大、品位富、矿体产在火山岩或火山碎屑岩中、未见相关中酸性侵入岩却大量发育矽卡岩为特色，因而，对其成因长期存在争议。本项目拟选择其中的敦德大型铁锌矿作为研究对象，以矽卡岩、锌成矿与铁成矿的关系研究为切入点，通过详细解剖矽卡岩的分布、分带、矿物组合、进化蚀变和退化蚀变过程；解剖矿体的地质特征、矿物组合、矿石组构、成矿期次；研究矽卡岩矿物和闪锌矿与磁铁矿的成生关系，从而确定或约束磁铁矿矿物的成因。再辅以先进的同位素定年技术测定赋矿火山岩、矿石的时代和微量元素、稳定同位素多元示踪成矿物质来源研究，最后达到厘定敦德大型铁锌矿成因类型的目的。本项目有助于深化对以敦德铁矿为代表的海相火山岩、火山碎屑岩中富铁矿成因的认识，并为新疆西天山阿吾拉勒铁铜成矿带富铁矿的继续找矿勘查提供理论基础。

火山岩型铁矿是我国现阶段铁矿成因分类体系中的重要一员。火山岩型铁矿可依容矿火山岩的地质环境，进一步分为陆相与海相两个亚类型。海相火山岩型铁矿多具有富矿石含量高、矿体产在海相火山岩或火山碎屑岩中、未见相关中酸性侵入岩却大量发育矽卡岩的特色，导致对其成因长期存在矿浆成矿、气水热液成矿和火山沉积成矿的争议。针对此，申请人选择被前人认为是矿浆成矿的敦德铁矿进行成因解剖。研究以矽卡岩和铁、锌成矿关系为切入点，通过详细的矿床地质观察、显微镜下鉴定、稳定同位素多元示踪和同位素测年，查明了铁、锌成矿关系，厘定了铁的成因机制。该矿床主要有块状、条带状、角砾状、脉状和斑杂状等几种矿石类型。近矿体围岩发育较强矽卡岩化蚀变，远离矿体则发育绿帘-绿泥石化和碳酸盐化蚀变。蚀变和矿石矿物的形成被划分为四个阶段：（1）矽卡岩化阶段，形成了钙铝（Gr39-80）-钙铁（Ad15-58）榴石和透辉石（Di63-97）；（2）磁铁矿阶段，沉淀了大量磁铁矿，另有少量赤铁矿、磷灰石、绿帘石、铁韭闪石等；（3）硫化物阶段，主要沉淀了砷化物（斜方砷铁矿、毒砂）和硫化物（闪锌矿、磁黄铁矿、黄铁矿、黄铜矿、方铅矿），及少量石英和方解石；（4）绿泥石-碳酸盐阶段，沉淀了方解石、绿泥石和少量绢云母。磁铁矿在第二个阶段形成，闪锌矿在第三个阶段形成，铁、锌矿化为一次成矿事件先后两个阶段的产物，均由热液成矿作用形成。从铁成矿到锌成矿，热液系统氧逸度显著降低。硫化物阶段的闪锌矿内部多见乳滴状、叶片状黄铜矿离溶体，一般黄铜矿从闪锌矿中离溶的温度在350°C~400°C，因此早于硫化物阶段的磁铁矿结晶温度可能在400°C以上。方解石中发育有丰富的包裹体，其中气液两相包裹体均一温度变化于147.3～367.4°C，冰点温度变化于-21.6°C～-1.4°C，盐度为2.4～23.4wt% NaCl equiv。硫化物的δ34S为3.8～8.1‰，平均6.8‰。2件磁黄铁矿样品获得Re–Os模式年龄306.8±3.5Ma和307.8±3.6Ma。矿区英安岩锆石LA-ICP-MS U-Pb年龄为316.0±1.7Ma，钾长花岗岩为295.75±0.71Ma。成矿环境可能为伸展环境。综合分析，认为敦德铁锌矿床可能由中性侵入岩体派生的岩浆热液交代和充填火山岩、火山碎屑岩而成。另外，发现两个新的重要科学问题。