Typomorphic characteristics of pyrite from lode gold deposits in the North China craton: Implications for fluid mineralization

Abstract

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.