The genesis and gold mineralization of the crypto-explosive breccia pipe in the Yixingzhai gold region, central North China Craton

The Yixingzhai gold deposit is a quartz-vein-type deposit accompanied by skarn iron and porphyry molybdenum mineralization. There are four crypto-explosive breccia pipes which provide an important ore-controlling structure. Here we investigate the composition and compositional zoning of the native gold, garnet, and epidote in the Tietangdong skarn breccia pipe through detailed petrography, electron probe microanalysis, and elemental mapping. The native gold compositions show that they were formed in a relative higher temperature condition. Based on their composition and optical characteristics, the zoning in garnet can be divided into three types. The garnets of Generations 1 and 3 show high contents of spessartine (2.91-10.17 mol%) and grossular (20.94-54.71 mol%) and anomalous birefringence, indicating that they formed under relatively higher pressure and temperature and lower oxygen fugacity. The garnets of Generation 2 are mainly composed of andradite (75.70-99.98 mol%) and show compositional homogeneity, with formation under relative lower pressure and temperature and higher oxygen fugacity conditions. Magmatic-hydrothermal pulses and multiple crypto-explosions could be the main geological processes that caused the formation of the zoning textures. Zoning in epidote is more complex than that in garnet, and from core to the rim, the iron content shows an abrupt increase and aluminium content shows decrease. There is also decrease in iron and increase in aluminium in the inner part. These features might record multiple crypto-explosion processes. Combining the complex replacement features of iron oxides and sulphides in the breccia pipe, we suggest that the Tietangdong breccia pipe formed through several crypto-explosions. The fluid system in the breccia pipe activated during multiple crypto-explosions, and oxygen fugacity varied greatly, which was disadvantageous to the accumulation and precipitation of ore-forming elements. Crypto-explosions destroyed the ore bodies formed during the early stage, resulting in their scattered distribution throughout the breccia pipe. Cone fractures formed eventually with the shrinking of the cooling breccia pipe after the magmatism ceased. Those fractures provided important ore-hosting structures for the gold mineralization during the late stage. Our research indicates that there is high prospecting potential for porphyry Mo, Cu, and Au deposits in the deeper domains of the breccia pipes.