Short wavelength infrared (SWIR) of white micas as a vectoring exploration tool for the Jiaojia gold deposit, Jiaodong Peninsula: implications for district- to deposit-scale gold exploration

Abstract

The disseminated-style gold deposit, as the primary mining deposit-type in the Jiaodong Peninsula, has the current exploration depth exceeding 4000 m. These deposits/orebodies are primarily hosted within the Mesozoic granitic intrusions adjacent to the regional faults. The Jiaojia gold deposit, as the most famous disseminated-style gold deposit in Jiaodong, presents widely white micas alteration assemblages comprising distal potassic zone, intermediate sericitization, and proximal pyrite-muscovite-quartz zone. The mineral chemistry of white micas from three alteration zones was analyzed using short-wave infrared spectroscopy (SWIR) and electron microprobe analysis, along with an evaluation of the physicochemical conditions governing their formation. From distal to the proximal alteration zone, the K/(K + Na + Ca) ratio of muscovite decreases, while the Fe + Mg content first decreases and then slightly increases. This variation indicates that, from the distal to the proximal alteration zones, the temperature of the hydrothermal fluid progressively decreased, while the oxygen fugacity evolved from oxidizing to reducing and then to weakly oxidizing conditions. In each alteration zone, there is no linear correlation between the Pos2200 (the Al-OH absorption feature at 2200 nm) and its composition (e.g., Si, Al_tot, Fe + Mg). Our thermodynamic simulations reveal that during each alteration stage, muscovite of different end-members can form, leading to the formation of mixed peaks in the Pos2200 region of the spectrum. In contrast, Illite crystallinity values (IC values), reflecting the crystallinity of the white mica, is controlled by fluid/rock ratio and can be used as vectors towards potential mineralization. Based on 3D mapping of IC values, the southwest part of Jiaojia gold deposit could represent the most potential mineralization location for deeper drilling. Integrating geological mapping and spectrum data, 180 ∼ 350 m proximal to potential ore bodies can be outlined using the criteria of IC values (IC > 1.3), which could be predicted to gold endowment of disseminated deposits over the entire Jiaodong gold province, East China.