华北克拉通若干脉状金矿的黄铁矿标型特征与流体成矿过程

TYPOMORPHIC CHARACTERISTICS OF PYRITE FROM THE LODE GOLD DEPOSITS IN NORTH CHINA CRATON: IMPLICATIONS FOR FLUID MINERALIZATION

通过对华北克拉通若干典型脉状(造山型)金矿流体成矿过程的研究分析发现:其可分为3个阶段,对比不同阶段黄铁矿的矿物学、微量元素和热电性等方面的特征,显示中阶段细粒黄铁矿(烟灰状黄铁矿)微量元素成分复杂,含量较高,金含量也明显较高,相应的金矿石品位较高;中阶段黄铁矿多呈灰绿黄绿色,晶形常为五角十二面体,自形程度低(他形半自形),粒度细小,晶体表面平滑度差,凹入角发育,表面自由能高,吸附金的能力强,载金能力增强;中阶段黄铁矿热电性较强。结合理论和实验研究成果,认为杂质元素加入导致黄铁矿晶格畸变,电荷不平衡,热电性增强等一系列现象;使黄铁矿的结晶学特征、成分和热电性构成了"三位一体"的标型特征,可作为有效的金矿勘查的直接标志。富金的烟灰状黄铁矿快速沉淀于热液瞬时过饱和或物理化学性质瞬时巨变的条件下,流体沸腾和混合是最重要机制。鉴于黄铁矿标型特征的演化和形成机制与陆陆碰撞体制流体成矿作用的3阶段模式(即CMF模式)完全吻合,借助CMF模式进一步分析烟灰状黄铁矿在脉状造山型金矿床中的发育时间和空间,为CMF模式补充了矿物学方面的支持,同时,赋矿断裂带的脆韧性转变带、挤压向伸展转变期最有利于流体沸腾和混合,是高品位脉状金矿床形成的最佳空间和时间。

This paper classifies the hydrothermal mineralization of lode gold deposits in North China craton into three stages and summaries the mineralogical, compositional and thermoelectric characteristics of pyrites of different metallogenic stages. It has been stated that the middle stage pyrites, usually called ash-like pyrites, are dark to yellow-green colored,fine to micro-grained, pentagonal dodecahedron-shaped,hypidiomorphically-allotriomorphically crystallized, and roughly surfaced with indecent angles.Therefore,the middle stage pyrites have high surficial energy to absorb gold from the ore-forming fluids, and then have high content of gold. In addition,the pyrites in middle stage show stronger thermoelectricity than those of the early and late stages. Through results obtained from theoretical and experimental studies, it is interpreted that input of trace elements resulted in lattice aberration, electric charge imbalance and thermoelectric enhancement in pyrite. Hence the trinity of the mineralogical, compositional and thermoelectric characteristics of pyrite could be employed as gold exploration indicator.The ash-like pyrites were rapidly formed from a fluid suddenly supersaturated or changed in physicochemical nature. Boiling and mixing of fluids are two most important possibilities.Since the temporal changes in typomorphical characteristics and geneses of pyrites of different stages are entirely in accord with the three-stage model of collisional orogeny, metallogeny and fluid flow (CMF model), the CMF model is employed to understand the developing position and time of ash-like pyrites in the orogenic-type lode gold systems. This provides mineralogical evidence for the CMF model, and shows the accuracy of the typical characteristics established for gold-enriched pyrites. The brittle-ductile transition level of a ore-hosting fault and the transition regime from regional compression to extension are most favorable for fluid-boiling and fluid-mixing,and therefore, are the best site and time to form high-grade gold ores.