Lithological Mapping Of The Mafic-Ultramafic Complexes In Beishan Area,northwest Gansu Province,China:Using ASTER Data

A surge in remote sensing applications in the recent years has lead to the development of highly advanced sensors that are utilized for multispectral and hyperspectral data acquisition.These data exhibit enhanced spatial and spectral resolutions.Hyperspectral systems is confirmed useful in the production of detailed lithological maps,and the principles of spectroscopy of minerals or mineral assemblages is used in the interpretation of remotely sensed imagery.Electronic and vibrational processes generate absorption features or spectral signatures of different minerals,thus different minerals and rocks can be identified from the reflectance and absorption patterns that occur on the spectral wavelength.The Beishan area has more than seventy mafic-ultramafic complexes sparsely distributed in the area and is of a big importance in mineral resource exploration related to maficultramafic intrusions.Many mafic-ultramafic intrusions,which are mostly in small sizes,have been omitted by geological maps.This research focuses on Huitongshan as the study area,which is a major district for mafic-ultramafic occurrences in Beishan,Gansu Province,China.The objective of this research is to explore the feasibility of using Advanced Spaceborne Thermal Emission and Reflection(ASTER)data and field data to map the lithological units especially the mafic-ultramafic igneous rocks in Huitongshan area.Rock samples collected from the study area were measured using the SR-3500 Full Range Spectrometer in-situ.Spectra were collected from the rock specimens to determine the spectral features of maficultramafic units.It was confirmed that gabbro and pyroxene display characteristic reflectance and absorption features due to differences in electronic transit,charge transfer and conduction of minerals.Analysis of the spectra also indicates that the degree of weathering that the host rock is exposed to effects the intensity and the presence of characteristic absorption features.The ASTER imageries were processed using different spectral analytical techniques such as band ratio,principal component analysis(PCA)and mafic index.Band ratios(6/8,5/4,2/1)in RGB is applied on the visible near infrared and shortwave infrared ASTER data,and as a result a false colour composite output image highlighting the mafic-ultramafic units was generated.On the output image generated,gabbro appeared red in colour,basalts appeared in yellow to yellow-green,diabase appeared light green,diorites appeared yellowgreen to light,granites appeared blue and migmatite and gneiss appeared dark blue.PCA was applied on the VNIR+SWIR and TIR ASTER bands and PC1,PC2 and PC3 were selected in order to generate RGB colour composite output images as they contain the dominant spectralinformation.In the VNIR+SWIR PCA output image,basalt,gabbro and diabase are difficult to discriminate from each other as they all appear pink in colour.But granite appears light green to light blue.Diorite appears pink to light green.And migmatite and gneiss appear light green.In the TIR PCA output image discriminated between the Basalts light pink in colour.Whereas gabbro are dark pink to red in colour.It is however difficult to distinguish between diabase,granite and diorite as their colours range from dark green to pink,green,and light green to pink respectively.Finally,Mafic Index(MI)was applied to the TIR data and RBD13 used in order to generate an MI output image.RBD13 ranges from white(light)to dark(black),with white representing the highest index and black representing the lowest index,thus discriminating the mafic-ultramafic rocks from the other rocks.The lithology in areas with the lowest index delineated the presence of granites,and migmatite and gneiss.Diorite displayed moderate MI index.Areas with the highest MI values delineated the presence of diabase,basalt and gabbro.The results of the remote sensing techniques applied to the ASTER data were integrated with the results from the field investigation,thus producing an output map that successfully discriminated the major lithological units,and most specifically distinguished the mafic-ultramafic rocks from the other rocks units in the study area.