三江中段夏塞银铅锌矿床微量元素富集机制及对成矿过程的指示：硫化物微区分析

The in situ laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) can be used to precisely determine the concentrations of trace elements of sulfides, and provide containts for studying occurrence and enrichment mechanism of these elements. The Xiasai Ag-Pb-Zn deposit is in hydrothermal veins and related to the late Early Cretaceous granite, in central Sanjiang metallogenic belt, SW China. Some trace elements such as Cd, In, Sb, Bi are enriched in sulfides from the Xiasai deposit. In this proposal, based on systematic field work for geology of the Xiasai Ag-Pb-Zn deposit and analysis of thin and polished sections of alterational rocks and ores, four mineralizational stages, Stage 1 to Stage 4, are recognized by our preliminary work. We will invegate the micro-morphology characteristics of main sulfides from different space using a Scanning electron microscope and Energy Dispersive Spectrometer (SEM-EDS). After these, an Electron probe micro-analyzer and LA-ICPMS are used to precisely determine the concentrations of trace elements of sulfides from the Stage 2 to Stage 4. Conbining with the geology of the Xiasai deposit, homogenization temperature, pressure and geochemistry of fluid inclusions, Re-Os age of arsenopyrites, H-O, B, S, He-Ar, and Sr-Nd-Pb-Hf isotopic geochemistry of ores and rocks, we try to study the occurrence and enrichment mechanism of trace elements enriched in sulfides and its inplications for mineralizational process. In addition, in order to establish macroscopic and microscopic integrated discriminant index to distinguish ore genesis of hydrotheral vein-type Pb-Zn deposits, a contrast in this proposal on combination and characteristic ratios of trace elements of sulfides will been pose between hydrotheral vein-type Pb-Zn deposits and other Pb-Zn deposits, i.e. VMS, MVT, Sedex and Skarn types.

LA-ICPMS原位微区分析能精确测定硫化物微量元素含量，为元素的赋存状态和富集机制研究提供微区证据。夏塞银铅锌矿床为三江成矿带中段与早白垩世晚期花岗岩体有关的热液脉型矿床，硫化物富集Cd、In、Sb、Bi等微量元素。本申请拟在全面野外基础地质、室内岩相学、矿相学和硫化物SEM-EDS形貌特征研究基础上，采用EPMA和LA-ICPMS原位微区技术对主成矿期S2-S4三个成矿阶段不同空间部位典型硫化物组合开展精确微量元素测试。结合矿床地质特征、Re-Os同位素年代学、流体包裹体温压及气液相成分、H-O, B, S, He-Ar, Sr-Nd-Pb-Hf等多元同位素示踪，阐明微量元素赋存状态、富集机制及对成矿过程的指示，刻画微量元素在成矿过程中的精细地球化学过程。对比分析VMS型、MVT型、Sedex型、矽卡岩型铅锌矿床微量元素组合及特征比值，建立热液脉型铅锌矿床成因宏观、微观综合判别指标。

三江成矿带中段的夏塞银铅锌矿床富集Cd、In、Ge、Bi等微量元素，为硫化物LA-ICPMS微区微量元素测试及对铅锌矿床成因的指示提供了良好的研究对象。银铅锌矿体以脉状形式赋存在NNW向断裂构造中，从南往北发育Sn-Cu→Cu-Zn→Ag-Pb矿化分带。成矿过程划分为锡石、毒砂-磁黄铁矿、闪锌矿-黄铜矿、含银矿物-方铅矿、黄铁矿-碳酸盐五个成矿阶段。.锆石U-Pb定年厘定成矿岩体年龄为103±1 Ma，为A型花岗岩，形成于弧-陆碰撞造山后伸展构造背景。流体包裹体测温显示为浅成低温热液型矿床。δDH2O为-137.5‰~ -105.2‰，δ18OH2O为-3.7‰~8.4‰，成矿流体为岩浆水与大气降水混合。闪锌矿S同位素为-9.7‰~ -3.1‰，方铅矿为-10.5‰~ -4.9‰，闪锌矿-方铅矿同位素对计算出含矿热液总硫δ34S∑s为-8.48‰，与花岗岩硫同位素一致，硫主要来源于岩浆。硫化物与花岗岩钾长石Pb同位素组成相同，成矿与岩浆活动具有密切的成因关系。.硫化物电子探针及能谱等测试发现夏塞银铅锌矿床中发育自然铋和红锑镍矿等矿物，并系统对银矿物及含银矿物进行了测定。自然铋以包裹体形式赋存于方铅矿中，而Ni主要以矿物包裹体形式存在于磁黄铁矿中。银以深红银矿、黝锑银矿、银黝铜矿、金银矿等微细脉状充填和显微包裹体形式的含银矿物为主。.对黄铜矿、黄铁矿和闪锌矿开展了LA-ICPMS微区微量元素测试。黄铜矿富集Cr、Mn、Zn、Se、Ag和Pb等元素及微量的Ga、Ge、Cd、In和Bi，这些元素与成矿元素Cu呈负相关关系，以类质同象形式置换Cu。黄铁矿以富Mn、Cu、As、Pb、Sb贫Co、Ni为特征，Co/Ni比值为0.56-5.62, 显示为岩浆热液成因。闪锌矿以富Mn、Cu、Cd为特征，另含有较高含量的Ag（9.96-526.7ppm）和In（10.71-16.30ppm）及微量Ge（2.15-3.72ppm）和Ga（低于检出限-6.31ppm）。黄铁矿和闪锌矿中富集的微量元素组合、特征比值等表明夏塞银铅锌矿床为岩浆热液型矿床，与流体包裹体、H-O-S-Pb同位素研究结果相吻合。这为矿床成因研究的微观证据提供了一个典型案例。