Correlations between trace elements in pyrite and gold mineralization of gold deposits on the North China platform

Abstract

By studying both the microscopic physical and chemical typomorphic characteristics of typical mineral pyrite samples associated with representative gold deposits on the north-central margin of the North China Platform, this paper seeks to identify macroscopic metallogenic mechanisms of gold deposits and to reveal the formation mechanism of lattice gold in pyrite. Typomorphic characteristics of pyrite reveal that pyrite grain size has a negative correlation with gold content. Cubic pyrite, as the dominant crystal form, contains more gold than pentagonal dodecahedral pyrite. Both pyrite crystal forms and chemical compositions indicate that the replacement style of gold deposit formed in a low saturability, low sulfur fugacity, and at temperatures either much higher or much lower than its best forming temperature; comparatively, that of the quartz vein style of gold deposit occurred under conditions with the best temperature, rich in sulfur, and with high sulfur fugacity. The Au/Ag ratios of the pyrites show that both the replacement and quartz vein styles of deposits are mesothermal and hypothermal, while the Co/Ni ratios of the pyrites indicate that the quartz vein style is of magmatic-hydrothermal origin. The X-ray diffraction intensity of pyrite rich in gold is lower than that of pyrite poor in gold at the quartz vein style. In general, with an increase in gold content in pyrite, the total sum intensity ΣI decreases. The pyroelectricity coefficient has a negative correlation trend with the values of (Co + Ni + Se + Te)-As and (Co + Ni + Se + Te)/As. The pyrite pyroelectricity of the replacement style is N-type, indicating that it formed under low sulfur fugacity, while that of the quartz vein style is a mixture of P-N types, indicating that it formed under high sulfur fugacity. On the pyroelectricity-temperature diagram, pyrite of the replacement style is mainly distributed between 200 and 270 °C, while that of the quartz vein style varies between 90–118 and 274–386 °C, demonstrating a multistage forming process. In contrast to previous researchers’ conclusions, the authors confirm the existence of lattice gold in pyrites through the use of an electron paramagnetic resonance (EPR) test. Au in the form of Au⁺, entering pyrite as an isomorph and producing electron–hole centers, makes the centers produce spin resonance absorption and results in EPR absorption peak II. The intensity of auriferous pyrite absorption peak II has certain direct positive correlations with pyrite gold content. The #I and #III absorption peaks of pyrites possibly result from the existence of Ni²⁺ and/or Cu²⁺. γ₁, γ₂, and γ₃ are the strongest and most typical absorption peaks of the infrared spectra of the pyrites. Generally, with the increase in gold content in the pyrite samples, γ₁, γ₂, and γ₃ tend to shift to higher wavenumbers, and the gold content in the pyrite samples has a positive correlation with their relative absorbance.