江南造山带曲溪金矿床载金黄铁矿地球化学特征

Geochemical Features of Gold-Bearing Pyrite at the Quxi Gold Deposit, Jiangnan Orogen

湘东北曲溪金矿床位于江南造山带中段黄金洞金矿田,其赋存于新元古界冷家溪群浅变质岩系中,受NNE-NE向长平断裂及其次级断裂控制;主要发育石英–硫化物型、破碎蚀变角砾岩型和蚀变岩型三种矿化。金属矿物主要为自然金、黄铁矿、毒砂、黄铜矿、方铅矿和闪锌矿等;非金属矿物主要为石英、方解石、绿泥石与绢云母等。金以自然金和不可见金两种形式产出,其中不可见金主要为赋存于金属硫化物中的“固溶体金”和“纳米级自然金”。根据脉体穿插关系和矿物共生组合,将曲溪金矿床成矿作用划分为:I. 石英–毒砂–黄铁矿–绢云母阶段;II. 石英–金–多金属硫化物–绢云母阶段;III. 石英–绿泥石–方解石阶段。电子探针分析表明,曲溪金矿床黄铁矿中As含量达0.008~2.92 wt%,为含砷黄铁矿。砷与硫具有一定负相关关系,表明少量砷取代硫进入黄铁矿晶格,这增加了Au进入黄铁矿的机率。除此以外,铜与金具有较弱的正相关性,银含量偏低。热液黄铁矿中钴、镍含量及其比值波动较大,部分Co/Ni比值小于1,说明在热液运输或沉淀过程中,流体与沉积地层发生了强烈的物质交换。曲溪金矿床地质、硫化物组构特征以及元素地球化学数据指示:硫化作用导致流体中硫氢络合物分解是不可见金的主要沉淀机制;流体压力波动导致的流体不混溶是可见金的主要形成机制。

Quxi gold deposit of the Huangjindong goldfield is located in the northeast Hunan province in the central part of the Jiangnan orogenic belt. It is hosted by the Neoproterozoic slate of the Lengjiaxi Group and controlled by the Chang-Ping Fault and its secondary faults. Three mineralization styles include quartz-sulfide veins, altered breccia, and disseminated and stockworks. Ore minerals are mainly native gold, pyrite, arsenopyrite, chalcopyrite, galena, and sphalerite. Gangue minerals are mainly quartz, calcite, chlorite, and sericite. Gold occurs in the form of native gold and invisible gold. The invisible gold is in the form of “solid solution gold” and “nanometer native gold” in sulfides. Based on the cross-cutting relationships and assemblage associations, mineralization stages at the Quxi deposit could be divided into: I. quartz-arsenopyrite-pyrite-sericite stage;II. quartz-gold-polymetallic sulfide stage;III. quartz-chlorite-calcite stage. The electron probe analysis shows that the pyrite at Quxi gold deposit is arsenic bearing pyrite with arsenic content of 0.008~2.92 wt%. The arsenic in pyrite shows a negative relationship with sulfur, which means minor arsenic replaced the sulfur in pyrite by isomorphism. This improved the opportunity that gold incorporates into pyrite. Besides, gold and copper show a weak positive correlation, and silver content is very low in pyrite. The values of Co and Ni content and their ratio fluctuate remarkably, and the Co/Ni ratio is <1 in the hydrothermal pyrite, which indicates intense material exchange occurred between fluid and sedimentary rocks. Together with the geological features, textures of pyrite and its elemental compositions, it is suggested that sulfidation resulted in the instability of the sulfur hydrogen complex and deposition of invisible gold while the fluid immiscibility caused by fluid pressure fluctuation resulted in the deposition of native gold.