江南造山带黄金洞金矿床成矿机制:矿物形成环境与金成矿物理化学条件制约

Metallogenic mechanism of the Huangjindong gold deposit,Jiangnan Orogenic Belt: Constraints from mineral formation environment and physicochemical conditions of metallogenesis

黄金洞超大型金矿床位于江南造山带中段,赋存于新元古界浅变质岩系中,受控于NNE-NE向长平断裂带,金资源储量达100吨。该矿床可由南至北分为金福、金塘、杨山庄和曲溪矿段,主要矿化类型有石英-硫化物脉型、构造角砾岩型和黄铁毒砂绢英岩型。金属矿物主要发育有自然金、毒砂、黄铁矿、黄铜矿、闪锌矿、方铅矿、磁黄铁矿等;非金属矿物有石英、绢云母、方解石和菱铁矿等,其中金主要以自然金与不可见金形式存在。根据野外与镜下观察,金成矿作用分为Ⅰ石英-金-毒砂-黄铁矿、Ⅱ石英-金-多金属硫化物-白钨矿和Ⅲ石英-辉锑矿-绿泥石3个阶段,前二者为主要成矿阶段。曲溪矿段Ⅱ阶段毒砂相对不发育、而磁黄铁矿和自然金显著发育,绿泥石主要发育于Ⅲ阶段中,与辉锑矿及闪锌矿共生。根据不同矿段各阶段毒砂与Ⅲ阶段绿泥石成分,计算其温度、lgf(S_2)与lgf(O_2),可见Ⅰ阶段成矿温度与硫逸度高于Ⅱ阶段:杨山庄矿段两阶段成矿温度分别为300～378℃、260～300℃,lgf(S_2)分别为-11～-7.2、-11.9～-10.1;金塘两阶段成矿温度为240～311℃、245～298℃;金福Ⅱ阶段成矿温度上限为297℃;曲溪矿段成矿温度为268～368℃,Ⅱ阶段毒砂lgf(S_2)与Ⅲ阶段绿泥石lgf(O_2)分别为-13.2～-8.7、-50.9～-40.1。根据不同阶段矿物之间的相互关系及成矿温度与硫逸度演化特征,推断Ⅰ、Ⅱ成矿阶段伴随强烈的硫化作用,金以类质同象方式进入毒砂和黄铁矿中,形成不可见金;其中Ⅱ阶段由于成矿流体压力骤降,含金流体发生相分离作用,H2S等气体大量逃逸,导致成矿流体中硫含量骤降,加以硫化作用持续消耗流体中的硫,促进了含金络合物分解与自然金的沉淀。

The Huangjindong gold deposit is located in the middle part of Jiangnan Orogenic Belt,and is a large gold deposit with a gold reserve of 100 t. It is hosted in the Neoproterozoic epimetamorphic terranes and controlled by NE-NNE trending Changping fault.The Huangjindong gold deposit comprises Jinfu,Jintang,Yangshanzhuang and Quxi ore blocks from south to north. The ores of the gold deposit are mainly composed of sulfide-quartz vein,quartz breccia and pyrite-arsenopyrite-sericite-quartz altered slate. Natural gold,arsenopyrite,pyrite,chalcopyrite,sphalerite and pyrrhotite are mainly metallic minerals. Non-metallic minerals contain quartz,calcite,siderite,and so on. The occurrences of gold include visible and invisible gold. It is considered that gold metallogenic stages could be divided into auriferous quartz-arsenopyrite-pyrite stage（ Ⅰ）,auriferous quartz-polymetallic sulfide-scheelite stage（ Ⅱ）,quartz-stibnite stage（ Ⅲ） based on field and microscopic observation. The first two stages are the main metallogenic stage. Mineral assemblage of Quxi ore block is slightly different from that of other ore blocks. There are less arsenopyrite and more pyrrhotite with visible gold developing in quartz vein of stage Ⅱ. Chlorite is contemporaneous with sphalerite and stibnite in stage Ⅲ. Based on arsenopyrite thermometer and stage Ⅲ chlorite thermometer from each stage in different ore blocks,it is concluded that temperature of stages Ⅰ and Ⅱ in Yangshanzhuang ore block are 260 - 378℃,240 - 300℃,while the lgf（ S2） are-11 --7. 2 and-11. 9 --10. 1,respectively. They are 215 - 311℃,170 - 298℃ and-11 --7. 2,-11. 9 --10. 1 in lgf（ S2） in Jintang. The maximum temperature of stage Ⅱ in Jinfu is 297℃,and the mineralization temperature of Quxi is 268 - 368℃. lgf（ S2） of Jinfu is about-13. 2--8. 7,and the lgf（ O2） is-50. 9 --40. 1 in Quxi. Based on the correlations between the minerals in different stages and the evolution characteristics of the ore-forming temperature and lgf（ S2）,it is referred that Au get into arsenopyrite of stages Ⅰ and Ⅱthrough isomorphism with strong sulfidation to form the invisible gold. In the stage Ⅱ,a large amount of gas such as H2 S escapes due to the phase separation of the gold-bearing fluid,which results from the sudden pressure drop of the ore-forming fluid. The escaping of gas leads to the reduction of the sulfur content in the ore-forming fluid and the sulfur content is expanded by sulfofication. They accelerate the decomposition of gold-bearing complex and precipitation of natural gold.