Abstract:One of the most important characteristics of porphyry copper deposits (PCDs) is the type and distribution pattern of alteration zones which can be used for screening and recognizing these deposits. Hydrothermal alteration minerals with diagnostic spectral absorption properties in the visible and near-infrared (VNIR) through the shortwave infrared (SWIR) regions can be identified by multispectral and hyperspectral remote sensing data. Six Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) bands in SWIR have been shown to be effective in the mapping of Al-OH, Fe-OH, Mg-OH group minerals. The five VNIR bands of Landsat-8 (L8) Operational Land Imager (OLI) are useful for discriminating ferric iron alteration minerals. In the absence of complete hyperspectral coverage area, an opportunity, however, exists to integrate ASTER and L8-OLI (AO) to compensate each other's shortcomings in covering area for mineral mapping. This study examines the potential of AO data in mineral mapping in an arid area of the Duolong porphyry Cu-Au deposit(Tibetan Plateau in China) by using spectral analysis techniques. Results show the following conclusions: (1) Combination of ASTER and L8-OLI data (AO) has more mineral information content than either alone; (2) The Duolong PCD alteration zones of phyllic, argillic and propylitic zones are mapped using ASTER SWIR bands and the iron-bearing mineral information is best mapped using AO VNIR bands; (3) The multispectral integration data of AO can provide a compensatory data of ASTER VNIR bands for iron-bearing mineral mapping in the arid and semi-arid areas.

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The Qulong deposit in Tibet is one of the largest porphyry copper-molybdenum deposits in China. We used short -wave infrared (SWIR) spectroscopy to examine the spectral characteristics of the extensively developed chlorite in this deposit. X-ray diffraction and electron microprobe analyses were used for phase identification and to obtain the chemical composition, ion substitution relationships, and formation environment of the chlorite. SWIR spectral parameters were applied to detect the hydrothermal centers. The results indicate that the wavelength of the absorption feature for Qulong chlorite Fe-OH (Pos2250) range from 2240 to 2268.4 nm; the chlorite substitution relationships are dominated by Mg-Fe substitution at the octahedral sites together with Al Ⅳ -Si substitution at the tetrahedral sites; the chlorite formation temperatures range within the medium-low temperature hydrothermal alteration range from 164 to 281°C, with an average value of 264°C; the wavelength of the chlorite peak position for Fe-OH (2250 nm) absorption and its chemical composition are positively correlated with Al Ⅵ , Fe + Al Ⅵ , Fe/(Fe + Mg), Fe, and Fe + Al Ⅳ but negatively correlated with Mg and Mg/ (Fe + Mg); and the wavelength associated with the chlorite Fe-OH (2250 nm) absorption feature is positively correlated with the temperature at which the chlorite formed. These correlations indicate that more Fe and Al Ⅵ ions and fewer Mg ions at the octahedral sites of chlorite lead to a longer the wavelength of the chlorite Fe-OH (2250 nm) absorption feature and a higher chlorite formation temperature. The wavelength of the Qulong chlorite Fe-OH (2250 nm) absorption feature (>2252 nm) can thus serve as an exploration indicator to guide the detection of hydrothermal centers in porphyry copper deposits. The results of the study indicate that the mineralogical and SWIR spectral characteristics of chlorite are significant indicators for locating hydrothermal centers within porphyry deposits.