Footprints of ore fluid pathway and implications to mineral exploration in the Shihu Gold Deposit, North China: Evidence from short wave infrared spectroscopy of illitic alteration rocks

Abstract

Most hydrothermal ore deposits have experienced multiple stages of hydrothermal activities and the overprinting makes the footprints of ore fluid pathway difficult to identify, which prevents us from targeting the mineral resource effectively. In this paper, the altered wall-rocks from all accessible underground tunnels of No.101 lode in the Shihu Gold Deposit, North China were investigated by the combined methods of petrographic microscopy and short wave infrared spectroscopy (SWIR). The results show that there are three types of alteration in the Shihu Gold Deposit, i.e., the proximal illitic alteration, the transitional chloritic alteration and the distal propylitic alteration. The wavelength (wvl) of the 2200 nm absorption features (2200wvl) of the illitic alteration rocks around ore bodies are shorter than those from the barren areas, suggestive of the compositional variation of illites derived from the multiple stages of hydrothermal fluids. The domains with higher absorption depth at 1400 nm, 1900 nm and 2200 nm (1400D, 1900D and 2200D) are spatially coincident with the economic ore bodies, which is well in accordance with the field truth that the gold mineralization in the Shihu mining district is intimately related to illitic alteration. The band ratio of 2200D/1900D, which is regarded as the proxy to illite crystallinity, is positively related to the temperature of fluid inclusions. The coincidence between low crystallinity and gold mineralization reflects the fact that fluid temperature during the main mineralization stage was relatively low compared to those of previous stages. Detailed investigation indicated that the economic ore bodies mainly occurred in the structural dilation domains, i.e., the widened portions of the ore-controlling structure were more suitable for the fluids migration and probably acted as the fluid up-flow zones. Therefore, SWIR spectroscopy of illite is a powerful tool to trace the ore fluid pathway, which is crucial to the understanding and exploration of gold mineralization.Based on the geological significance of the SWIR signatures and the investigation of the ore-forming conditions, it can be concluded that there are two fluid up-flow zones in No.101 lode, one of which lies on the north of No.13 cross section and centered at No.7 cross section, whereas the other on the south of No.17 cross section and centered at No.27 cross section. As a result, two steeply dipped ore bodies were formed along the up-flow zones. According to this ore-forming analysis, the location centered at No.47 cross section with elevation below 150 m would be an ideal site for the formation of a third ore body. This has recently been verified by underground drillings in the Shihu Gold Deposit (Yougang Zhang, pers. commun. 2020).