Hydrothermal alteration mapping using ASTER data in Baogutu porphyry deposit, China

Remote sensing plays an important role in mineral exploration. One of its proven applications is extracting host-rock lithology and alteration zones that are related to porphyry copper deposits. An Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) was used to map the Baogutu porphyry deposit alteration area. A circular alteration mineral zoning pattern was clearly observed in the classification result of potassic, phyllic, argillic, propylitic zones. The potassic is characterized by biotite and anhydrite with an absorption feature centered at 1.94 and 2.1um. The phyllic zone is characterized by illite and sericite that indicates an intense Al-OH absorption feature centered at 2.20um. The narrower argillic zone including kaolinite and alunite displays a secondary Al-OH absorption feature at 2.17 um. The mineral assemblages of the outer propylitic zone are epidote, chlorite and calcite that exhibit absorption features at 2.335um.The performance of Principal Component Analysis(PCA), Minimum Noise Fraction (MNF),band ratio(BR) and Constrained Energy Minimization(CEM) has been evaluated. These techniques identified new prospects of porphyry copper mineralization in the study areas. These results indicate that ASTER is a powerful tool in the initial steps of mineral exploration. Remote sensing instruments provide an opportunity to improve initial steps of ore deposits exploration as which providing detailed information on the mineralogy and rock types[1-7]. The porphyry ore deposits were generated by hydrothermal fluid processes which alter the mineralogy and chemical composition of the country rocks[8]. The altered rocks have diagnostic spectral absorption features due to its mineral assemblages[9]. Remote sensing technology has been used to detect hydrothermal alteration mineral assemblages for reconnaissance stages of porphyry copper and gold exploration[6,10-12]. In the past 20 years, new spaceborne multispectral and hyperspectral remote sensing instruments have provided higher spectral resolution data that can be used for mineral exploration[8]. As well, the availability of laboratory-measured spectral data for rocks and minerals allows geologists to identify unique lithologies. Remote sensing technology is becoming an increasingly important tool for mineral exploration. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a multispectral imager that was launched on board NASA's Terra spacecraft in December, 1999 which covers the visible, near infrared (VNIR), short wave infrared (SWIR), and thermal infrared (TIR) spectral regions with 14 channels with high spatial, spectral and radiometric resolution[13]. ASTER sensor allows the discrimination and identification of 2 To whom any correspondence should be addressed. 35th International Symposium on Remote Sensing of Environment (ISRSE35) IOP Publishing IOP Conf. Series: Earth and Environmental Science 17 (2014) 012174 doi:10.1088/1755-1315/17/1/012174 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 hydrothermal alteration minerals in the SWIR region. The ASTER bands in the SWIR were specifically selected to highlight the presence of spectral absorption features of minerals, such as clay, carbonates, sulfates and other hydrous phases due to overtones and combination tones of fundamental absorptions of Al-O-H, Mg-O-H, Si-O-H, and CO3 2. Geology of the study area [14,15]. The object of the study was to use ASTER data to map the hydrothermal alteration mineral zones associated with porphyry copper mineralization in Baogutu porphyry Cu–Mo deposit. The major part of this belt has well-exposed and sparse vegetated surface which is ideal for remote sensing investigations. The lithologic information and alteration zones associated with porphyry copper mineralization was extracted by the methods of Minimum Noise Fraction (MNF), Principal Component Analysis (PCA) ,band ratio and specialized Constrained Energy Minimization(CEM). The Baogutu porphyry Cu–Mo deposit in Toli County is the largest porphyry copper deposit in the Junggar region of Xinjiang[16] locating at the southern side of the Dalabute fault in the West Junggar and tectonically along the margin of the Devonian–Carboniferous Dalabute–Kalamaili remnant ocean basin[17]. There are Lower Carboniferous Xibeikulasi, Baogutu, and Tailegula Formations which exposed in the Baogutu ore district. The Xibeikulasi Formation consists of a suite of tuffaceous pebbly graywacke and tuffaceous siltstone. Eight copper-bearing porphyry intrusions have been found in the Baogutu ore district and its surroundings and Baogutu porphyry Cu–Mo deposit occurs in the No. V pluton. Wall-rock alteration in the Baogutu deposit can be divided into four zones: the potassic alteration zone mainly includes K-feldspathization and biotitization; the main mineral assemblage of the quartz–sericite alteration zone is sericite + quartz + pyrite ± K-feldspar; the mineral assemblage of the propylitic alteration zone is chlorite +zoisite + calcite + albite ± zeolite; and the mineral assemblage of the zeolite-carbonate alteration zone is epistilbite + laumontite +calcite + quartz ± albite[17]. Figure 1. Geological map of the Baogutu porphyry copper belt (after Fuquan Yang et al.2012) 3. Data set and Methods 3.1. ASTER data ASTER image was acquired on September 21, 2002. The image was pre-georeferenced to UTM zone 45 north projection using the WGS-84 datum. Atmospheric correction was applied by using the Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes (FLAASH) algorithm. RGB color composite of Band 3, Band2 and Band 1 is shown in figure2. 35th International Symposium on Remote Sensing of Environment (ISRSE35) IOP Publishing IOP Conf. Series: Earth and Environmental Science 17 (2014) 012174 doi:10.1088/1755-1315/17/1/012174