广西金牙金矿载金硫化物化学成分特征及其对成矿物源的指示

Composition of gold-bearing sulfides and their implication for ore-forming materials in Jinya Gold Deposit,Guangxi Province

金牙金矿是桂西北地区最早发现、也是国内较典型的卡林型金矿之一,矿体受桂西—百色断褶带北东侧近南北向次级断裂控制,赋存于三叠系百逢组细砂—粉砂岩、泥质粉砂岩中。地质、岩相学和载金硫化物电子探针分析研究表明,金牙金矿至少发育两个阶段的热液矿物组合,多数热液矿物具有多世代的特征;黄铁矿和毒砂中As与S呈负相关关系,Au与As相关关系不明显,但不同阶段的黄铁矿Au与As表现出一定的规律性,沉积期大部分测试点Au、As含量较低,成岩期Au、As含量明显升高;成矿期各阶段热液黄铁矿均含有较高Au、As含量;其包裹的交代残余黄铁矿中Au、As变化较大,这可能是由于受后期热液作用使Au、As发生了活化迁移的结果。黄铁矿Co、Ni分析表明,矿石中黄铁矿主要为热液成因,围岩中黄铁矿主要为沉积成因,蚀变围岩中黄铁矿两种成因兼有;环带状黄铁矿核部(Py2?)既有沉积成因又有热液成因特征,可能是由于沉积成岩期的沉积成因型黄铁矿受后期热液作用叠加改造变化的结果;其Au、As含量均低于中部和边部,推断认为可能是由于沉积—成岩阶段的黄铁矿在后期构造或热液作用过程中,将其Au、As熔解并搬运、迁移,使核部表现出Au、As含量降低的现象。新鲜围岩中黄铁矿和毒砂均含有一定量的Au,且成矿热液的S来自其流经地层中的S,推断成矿物质可能主要来源于围岩地层。矿区内虽未发现岩浆岩,而据前人分析不排除有含Au质岩浆岩沿深大断裂上升迁移贡献金源的可能性,但岩浆岩是否参与成矿,有待我们进一步研究。根据以上分析,推断成矿作用可能主要分为三个阶段:沉积成岩期Au预富集→成矿期萃取先期富集成矿物质形成含矿流体上升迁移,在有利的场所卸载成矿并使围岩矿化蚀变→成矿期后受风化和淋滤作用形成氧化矿石。

The Jinya Gold Deposit is a typical Carlin-type gold deposit, and it is one of the firstly discovered Carlin-type gold deposits in China. The gold deposit is mainly hosted in the Middle Triassic fine-sandstones, siltstone, argillacoue siltstone. The deposit is obviously controlled by the NS-trending secondary fault in the northeast of NE-trending Guixi-Baise fault. Field work, petrography, electron microprobe data indicate that hydrothermal minerals in Jinya Gold Deposit could be classified into at least two forming stages. Most of these minerals have characteristic of multiple generations. There is a negative correlation between As and S content in the pyrites and arsenopyrite. The correlation between As and Au is not obvious, but found a positive correlation between them in a certain range. Au and As contents in the pyrite formed in different stage have regular patterns. The contents of Au and As in the sedimentary pyrite are low. In contrary, these contents in the diagenesis pyrite are much higher. Hydrothermal pyrite developed in every stage of ore-forming period has abundant Au and As. The varying contents of Au and As in the relict pyrites in the core of banded pyrite which indicate that some Au and As have been activated and migrated in some test areas during the later hydrothermal activity. Analysis of Co and Ni in pyrite suggests that the pyrites in ore bodies are hydrothermal pyrite, the pyrites in adjacent rocks are sedimentary pyrite, the pyrites in alteration wall-rock comprising both of them. The pyrite（Py2？） with the characteristics of hydrothermal pyrite and sedimentary pyrite in the ore of banded pyrite probably due to the pyrites in the stage of sedimentary and diagenesis derived from hydrothermal-superimposing in later hydrothermal activity. Its Au and As contents are less than the other parts. It is possibly shown that Au and As in the sedimentary pyrite resolved and migrated during the later hydrothermal activity. That caused the lower content of Au and As in the core of the pyrite. The pyrite and arsenopyrite in the fresh wall-rock contained some Au, and S of the ore fluid derived from the strata that it flows through, which suggests the ore-forming material major derived from the country rocks. Igneous rocks have not been found at surface in the mine district. However, according to the previous analysis, there may be igneous rocks containing Au rise and migration along the deep fracture. The relationship between mineralization and magmatism needs to be further studied. Based on the above analysis, the mineralization may be divided into three stages ： The Au pre-enriched in the diagenetic stage→The fluid of the mineralization period extracted the ore-forming material which pre-enriched in the first stage, and it rose and migrated along the deep fault and fracture zone. The ore bearing fluid unloads in syn-sedimentary faults, secondary fault and favorable lithology sites, and made the wall rock altered→After the metallogenic epoch, the ore bodies form the oxidized ore by weathering and leaching under acidic conditions.