Application of short-wave infrared spectroscopy to exploration of an intermediate-sulfidation epithermal deposit –A case study of Yueyang silver polymetallic deposit, Zijinshan mining field, Fujian Province, China

Abstract

The Zijinshan ore district in Fujian Province, South China, is a major porphyry-epithermal deposit system containing the Luoboling porphyry deposit, Zijinshan HS (high high sulfidation) epithermal deposit, Yueyang IS (intermediate sulfidation) epithermal deposit, and multiple centers of subeconomic mineralization. Yueyang deposit consists of vein-type ore bodies that are strongly controlled by high permeability structures, primarily contacts between different lithologies and fractures within the host rock. The ore body morphology is mainly breccias and veins, and the mineralization is zoned silver – copper – gold from bottom to top. Systematic SWIR mapping has revealed that the mine area has pervasive illite alteration, with a small amount of chlorite and kaolinite alteration at shallow depths. The mineralization evolution can be divided into four stages, with gold and silver minerals mainly precipitated during stage 3.Illite is the most widespread alteration mineral, and therefore was selected for systematic SWIR and microprobe analyses. The wavelength position of the Al-OH (wAlOH) of illite in Yueyang deposit increases from the lower part of the ore body (proximal to fluid source) to the upper part (distal to fluid source), and the illite crystallinity (SWIR-IC) gradually decreases from deep to shallow. The low wAlOH (<2,200 nm) and high SWIR-IC (>1.3) illites are associated with high-grade orebodies.Microprobe analysis results show that the wAlOH of illite is positively correlated with the Mg + Fe content, which may be related to the fluid pH at the time of illite formation. The SWIR-IC is responsive to fluid temperature at the time of alteration. The combination of these two parameters can be used to determine the location along the hydrothermal fluid path, and can point to the heat source that is likely to be most proximal to the porphyry deposit that provided the fluids that evolved to produce the IS epithermal deposit.