## 1 Introduction  
Hydrothermal alteration is a common phenomenon in mineral deposits, which is of great significance to the metallogenic stage analysis and mineral exploration [1]. The wall rock alterations have zoning characteristics, where the mineral assemblage can be used to recognize the mineralization characteristics and element migration rules [2]. In general, the scope of the alteration far exceeds the size of the deposit, which reduces the scale of the exploration effectively [3]. Based on the observations in the field, the different alteration mineral assemblages and the ACF-AKF (A = Al2O3 + Fe2O3 - (Na2O + K2O), C = CaO, F = FeO + MgO + MnO, K = K2O) diagram, a limited number of alterationsoccurring in nature can be seen [4], which can be used to infer the physicochemical conditions of the alteration reactions. The traditional methods for identifying wall rock alterations include using a magnifying glass, using a polarizing microscope, X-ray diffraction (XRD), electron probe microanalysis (EPMA), whole rock geochemistry, etc. However, the composition always changes regularly in the same mineral, which contains vital indications at the micro scale for the processes of alteration and mineralization. In recent years, with the rapid development of analysis methods, some methods, such as inductively coupled plasma mass spectrometry (ICP-MS), produce an energy spectrum that can provide greatly accurate information about mineralogical composition, texture and polytype information but they are known as expensive and time-consuming. Hyperspectral technology is widely used in the geology field due to its advantages in indication at the micro scale for processes of alteration and mineralization and for being non-destructive [5; 6; 7; 8; 9]. The wavelength positions of spectral absorption features characterizes the differences of specific chemical bonds or special elements in the altered minerals [10; 11; 12]. [PERSON] et al. performed calibration of instrument and equipment errors in 2015 [13]. [PERSON] et al. applied HySpex SWIR-320 m to uranium ore prospecting and exploration in 2017, which reduced working time and improved working accuracy due to its high resolution [6]. [PERSON] et al. obtained the hydrothermal mineralization model and analyzed volcano and lakes on Mars, based on the HySpex image and continuous composition data [14]. [PERSON] et al. observed that clinochlore is related to white mica with a longer wavelength at the scale of hand specimens and in the field, which is concerned with the mineralization of porphyry copper [15]. If mineral species are determined [16], the spectral absorption depth is usually positively related to mineral content [17; 18; 19; 20]. [PERSON] et al. predicted the relative abundance of clay minerals by absorption depth [21]. [PERSON] et al. calculated the degree of crystallization of alteration minerals in hand specimens, used to identify the grade of minerals [22]. This method is also widely used to obtain the alteration information in porphyry Cu-Au deposits, skarn-type deposits and non-metallic deposits due to its characteristics of high resolution and effectivity [23; 24; 25; 26]. The Gongchangling iron deposit--Algoma-type banded iron formations (BIFs)--has the most high-grade iron ore in Anshan-Benxi area [27]. Previous studies have focused on the origins of high-grade iron ore, and the major opinion accepted by most scholars is that the magnetite quartzite is modified to form high-grade iron ore by a hydrothermal solution, accompanied by chlorite, garnet and other altered minerals [28; 29; 30; 31]. At present, the source of hydrothermal fluid is argued as either hydrothermal from magma [32; 33], or the metamorphic hydrothermal fluid in the regional metamorphism process [34; 35; 36; 37]. Anyway, there are hydrothermal (gas) altered rocks around the high-grade iron ore. The altered rocks are vein or layer-like, and several meters to dozens of meters thick [38], indicating that they are closely related to the genesis of high-grade iron ore [39; 40].