沉积岩成岩及变质过程中硫化物的结构和组成变化与造山型金矿床的成因：以西秦岭八卦庙金矿床为例

There is a consensus that orogenic gold deposits formed largely due to metamorphic devalotization of Au-rich sedimentary and volcanic rocks. Core scientific issues in the genesis of orogenic gold deposit are how Au and associated trace elements occur in the source rocks, how they were liberated, transported, and gradually enriched to form ore-fluids. These questions, however, remains not well understood. To further address these issues, we propose an integrated textural, compositional, and isotopic study of the Baguamiao giant sediment-hosted orogenic gold deposits in the West Qinling orogen. We first characterize the mineralogy, paragenesis, and textures of variopus pyrite and pyrrhotite from the ores, alteration assemblages, and unmineralized coutry rocks of the Baguamiao gold deposit. The reuslts will provide significant insights into the transformation and evolution of the sulfide minerals during diagenesis, regional metamorphism and associated hydrothermal activities. We further use laser ablation ICP-MS to analyze gold and associated trace elements in various pyrites and pyrrhotites to document their geochemical behavior during recrystallization and dissolution-reprecipitation of pyrite and replacement of pyrite with pyrrhotite, processes critically important to form auriferous fluids. In-situ S and Pb isotope analysis of distinct generations of pyrite and pyrrhotite and contrast areas of within single grains are cthen onducted to provide additional evidence supporting a metamorphic source of gold, sulfur and other elements for the Baguamiao orogenic gold deposit. This study will yield significant new insights into the ore genesis and refine the genetic model of the Baguamiao gold deposit, enabling a bettter understanding of gold metallogeny of the entire West Qinling orogen and other sediment-hosted orogenic gold deposits in the world's Phanerozoic orogenic belts.

多数造山型金矿床的形成可能与沉积岩或火山-沉积岩的变质脱流体作用有关。金在各类沉积岩和火山-沉积岩中的赋存状态及其在区域变质和相关热液活动过程中的释放、转移、搬运、富集机制是造山型金矿床成因研究的关键科学问题。本项目以西秦岭八卦庙超大型金矿床为研究对象，首先开展金矿石、蚀变岩和未蚀变围岩中不同产状黄铁矿-磁黄铁矿的矿物学研究，厘定从沉积-成岩到区域变质和热液成矿过程中硫化物的结构变化和成因机制，进而对不同类型黄铁矿和磁黄铁矿的微量元素组成进行微区原位分析，揭示黄铁矿结构变化及黄铁矿向磁黄铁矿转变过程中微量元素的释放、迁移和富集机制及其对形成含金流体的贡献。在此基础上开展各类黄铁矿-磁黄铁矿硫-铅同位素组成的原位分析、含水蚀变矿物羟基的氢-氧同位素组成分析和高精度的成矿年龄测定，阐明八卦庙金矿床金的来源、转化和富集过程及其控制因素，完善八卦庙金矿床成因模式，促进西秦岭造山型金矿床的研究。

西秦岭造山带八卦庙金矿床的容矿地层和矿石中发育5类黄铁矿（Py1~Py5）和3类磁黄铁矿(Po1~Po3)，其中Py1- Py2和Py3分别为沉积和成岩成因，Py4-Py5即Po1-Po3为热液成因。LA-ICP-MS微区原位分析结果显示，大部分微量元素(Co、Cu、Zn、Mo、Ag、Sb、Te、Ba、Au、Tl、Pb、Bi)在沉积-成岩期黄铁矿中更为富集；从早期黄铁矿到晚期黄铁矿，金的平均含量逐渐降低。Ag、Sb、Pb、Cu、Zn、Te、Bi、Ba的含量也具有从早期黄铁矿到晚期黄铁矿逐渐下降的趋势（最多可下降2个数量级）。三类磁黄铁矿具有类似的微量元素组成特征，金 (0.06-0.28 ppm Au）和其他大部分微量元素的含量均很低。分析结果显示，金等微量元素在沉积-成岩期黄铁矿中有显著的预富集，这些黄铁矿向磁黄铁矿转变的过程中释放了大部分微量元素，并为八卦庙金矿床的成矿提供了重要的成矿物质来源。.对各类黄铁矿和磁黄铁矿的离子探针（SIMS）硫同位素分析结果显示：沉积成岩期黄铁矿的δ34S变化范围较大(-15.97~16.67‰），与海相沉积岩的硫同位素组成一致。热液成因黄铁矿δ34S变为 0.70~16.45‰，暗示晚期热液黄铁矿和磁黄铁矿的硫可能主要来自古生代海相沉积地层的硫酸盐还原或直接来自早期沉积-成岩成因黄铁矿分解释放的硫。.NW-NWW向矿脉中4个云母样品的40Ar/39Ar坪年龄为214.36 ± 0.88~210.13 ± 0.86 Ma，NE向脉体的坪年龄分别为209.60 ± 0.82 Ma和209.58±0.70Ma，暗示矿区存在两期成矿作用，但均形成于晚三叠世，与秦岭造山带的碰撞造山作用时间一致。.基于上述分析结果提出，中-晚泥盆世时期金及其他微量元素在碳酸盐和碎屑岩中的沉积成岩成因黄铁矿中得到初步富集。泥盆纪热水喷流活动沿凤-太同生盆地边缘和内部深大断裂将深部成矿物质带入盆地，在海底形成含铅、锌、银、金等多金属的矿源层，形成早期沉积成岩成因富金黄铁矿。三叠纪的陆陆碰撞造山和区域绿片岩相变质作用使沉积-成岩成因黄铁矿发生重结晶并向磁黄铁矿转变，在此过程中将早期黄铁矿中的金、硫和其他微量元素释放出来形成含金热液并沿各种脆韧性断离迁移，在有利的构造部位形成含金矿脉和蚀变，形成八卦庙金矿床。