WE4.T06: Geology and Solid Earth V, Wednesday, July 27, 16:00 - 16:20 , #1725 Mineral index maps of the southern Namibia using HyMap and ASTER data Shoko Oshigami1*, Tatsumi Uezato1, Yasushi Yamaguchi1, Yessy Arvelyna2, Atsushi Momose2, Yuu Kawakami2, Taro Yajima2, Shuichi Miyatake2, Anna Nguno3 1Nagoya University 2Japan Oil, Gas and Metals National Corporation 3Geological Survey of Namibia, Ministry of Mines and Energy Introductions • Objectives – An development of rock and mineral identification method using hyper-spectral sensor data – Extraction of hydrothermally-altered minerals and pegmatite in southern Namibia • Hydrothermally-altered minerals (alunite, dickite, kaolinite, pyrophyllite) – having diagnostic absorption features in shortwave infrared (SWIR) regions • Pegmatite – rich in mica group minerals (muscovite, lepidolite) and quartz – Mica group also has diagnostic absorption features in the SWIR regions Data • HyMap reflectance data – 32 bands in the SWIR regions: band 95 (1.95 mm) –band 126 (2.48 mm) Sensor HyMap ASTER (SWIR) (TIR) • ASTER surface Area Southern Cuprite emissivity (2B04) data Namibia – 2 bands in thermal Spatial 5 3.5 90 infrared (TIR) regions: resolution band 12 (8.9-9.3 mm), (m) band 13 (10.3-11.0 mm) Spectral ~16 ~700 resolution • Reference spectra (nm) – The USGS Digital Spectral Library Data processing flows ASTER surface HyMap reflectance data USGS reference spectra emissivity data band 13/ band 12 Continuum removal [Ninomiya and Fu, 2002] Modified Spectral Angle Mapper SiO2 content index map (MSAM) Color composite map of mineral indices “Continuum-removal MSAM” method Modified Spectral Angle Mapper (MSAM) • SAM measures the degree of similarity between reference (T1) and image spectra (T2) by calculating the angle between these spectra (q), treating them as vectors in n-dimension [Kruse et al., 1993] • Instead of T1 and T2, MSAM uses the difference vectors (T1’, T2’) which are derived by subtracting the average image vector (Tfl) from T1 or T2 [Kodama et al., 2010]. • MSAM has an advantage over SAM because it is insensitive to the grain Basic concept of SAM and MSAM size [Kodama et al., 2010]. methods [Fig. 5 in Kodama et al., 2010]. Continuum removal Continuum-removal spectrum • “Continuum” means a convex background of the reflectance spectra • Removing of continuum is effective for mineral identifications [e.g., Green and Graig, 1985; Yamaguchi and Lyon, 1986] continuum • The ratio of original reflectance spectrum to the continuum is defined as a continuum-removal spectrum. • We applied MSAM to continuum- removal HyMap spectra using USGS reference spectrum of continuum-removal reference alunite and its continuum, spectra. continuum-removal spectrum. Validation • Test site – Cuprite, Nevada, USA • Method – Comparing our mineral index maps with the reference map • Reference map – Mineral map derived by using AVIRIS data and “Tricorder” software tool [Clark and Swayze, 1996] • Test minerals – Alunite, calcite, chlorite, dickite, kaolinite, montmorillonite, high- and low-Al muscovite, pyrophyllite Color composite maps of Alunite : Calcite : Chlorite Continuum removal + MSAM MSAM Background: HyMap band 5 (0.4982 mm) image Color composite maps of Dickite : Kaolinite : Montmorillonite Continuum removal + MSAM MSAM Background: HyMap band 5 (0.4982 mm) image Color composite maps of Low- : High-Al muscovite : Pyrophyllite Continuum removal + MSAM MSAM Background: HyMap band 5 (0.4982 mm) image Threshold of each index Index Threshold • Lepidolite – Determined by examining the Alunite 0.6 features in the reflectance Calcite 0.6 spectra of extracted image pixels corresponding to each Chlorite 0.6 threshold value Dickite 0.8 • Others Kaolinite 0.7 – Determined by comparing with Lepidolite 0.7 the reference mineral map of Cuprite [Clark and Swayze, Montmorillonite 0.7 1996] High-Al Muscovite 0.7 • Assumption Low-Al Muscovite 0.7 – Threshold values of mineral indices determined in Cuprite Pyrophyllite 0.7 region are also applicable to southern Namibia region. Study area • Porphyry copper deposits Hydrothermal alteration • Pegmatite-type deposits • Pegmatite-type deposits Mosaic image of ASTER Level 1B data (band 1). [Groenewald et al., 1997; Becker et al., 1999] Color composite map of mineral indices: Haib • Mineral index map – Alunite was not extracted in the whole study area although its existence has been expected in Haib • Field survey (black arrow) – Silicified and oxidized rock • X-ray analysis of rock sample (black arrow) – Pyrophyllite > kaoline (Dickite was not included on 1km the list of x-ray analysis) Dickite : Kaolinite : Pyrophyllite Background: HyMap band 5 (0.4982 mm) image Color composite map of mineral indices: Tantalite Valley • Mineral index map – low-Al muscovite areas highest SiO2 content – high-Al muscovite areas slightly higher SiO2 content • Field survey – Pegmatite (black arrows) – Silicified rock with quartz dykes (white arrow) • X-ray analysis of rock sample – Sericite (black arrows) 1km – No data (white arrow) Lepidolite : low-Al Muscovite : high-Al Muscovite Background: SiO2 content index map HyMap spectra of (1) Hydrothermally-altered minerals 2.17 mm 2.33 mm 2.37 mm 2.21 mm 2.17 mm 2.33 mm Dickite 2.21 mm 2.33 mm Pyrophyllite • Bold lines: HyMap spectra • Thin lines: reference spectra • Dotted lines: continuum-removal spectra Kaolinite HyMap spectra of (2) Mica group minerals 2.19 mm 2.21 mm 2.34 mm 2.34 mm Lepidolite 2.23 mm 2.36 mm 2.21 mm Muscovite (high-Al) • Bold lines: HyMap spectra • Thin lines: reference spectra • Dotted lines: continuum-removal spectra Muscovite (Low-Al) Summary • We developed “continuum-removal MSAM” method using HyMap reflectance data in the SWIR regions to extract minerals related to hydrothermal alteration and pegmatite. • Accuracy of this approach was confirmed by comparing our mineral index maps to a previously published mineral map of Cuprite. • The continuum-removal MSAM method successfully identified hydrothermally-altered and mica group minerals in southern Namibia, and the results are consistent with those of x-ray analyses and field survey. • The spectral pattern of the extracted pixels is mostly consistent with each reference spectrum. • Combination of SiO2-content index from ASTER data and high-Al muscovite index from HyMap data seems to be help for searching pegmatite. This work is a part of mineral exploration renovating program conducted by Japan Oil, Gas and Metals National Corporation (JOGMEC) and is fully funded by the Ministry of Economy, Trade and Industry, Japan. Continuum removal • The way of “continuum” determination 1. Calculating slopes of the lines through band 95 and all other bands (96 to 126). The band with largest slope in a positive direction is defined as band A. 95 A B 2. Calculating slopes of the lines through band A and the 126 subsequent bands (A+1 to 126). The band with positively largest slope is defined as band B. 3. Repeating this calculation USGS reference spectrum of and connecting the bands 95, alunite and its continuum, A, B, … , 126 derives continuum-removal spectra. “continuum”. SiO2 content index • SiO2 content is possibly one indicator of pegmatite. • In silicate rocks, absorption peak in thermal infrared (TIR) emissivity spectra moves to longer wavelength as the rock type changes from felsic to ultramafic [Walter and Salisbury, 1989]. • The emissivity in ASTER band 12 (8.9-9.3 mm) is lower than in band 13 (10.3-11.0 mm) for felsic rocks, and higher for ultramafic rocks. • Using this spectral feature, SiO2 content index is defined as follows; SiO content index = (ASTER) band 13/ band 12 2 [Ninomiya and Fu, 2002] Color composite map of mineral indices: Sandfontain-Ramansdrif • Location – Western part of Sandfontain- Ramansdrif area close to Haib area • Mineral index map – Low-Al muscovite areas highest SiO2 content – Lepidolite, high-Al muscovite areas slightly higher SiO2 content • No field survey, no rock 1km sample Lepidolite : Low-Al muscovite : High-Al muscovite Background: SiO2 content index map Color composite map of mineral indices: Sandfontain-Ramansdrif • Mica group minerals are coexistent with hydrothermally- altered minerals. 1km Dickite : Kaolinite : Pyrophyllite Background: HyMap band 5 (0.4982 mm) image .