Mineral index maps of the southern Namibia using HyMap and ASTER data

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1 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 Oshigami 1*, Tatsumi Uezato 1, Yasushi Yamaguchi 1, Yessy Arvelyna 2, Atsushi Momose 2, Yuu Kawakami 2, Taro Yajima 2, Shuichi Miyatake 2, Anna Nguno 3 1 Nagoya University 2 Japan Oil, Gas and Metals National Corporation 3 Geological Survey of Namibia, Ministry of Mines and Energy

2 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

3 Data HyMap reflectance data 32 bands in the SWIR regions: band 95 (1.95 mm) band 126 (2.48 mm) ASTER surface emissivity (2B04) data 2 bands in thermal infrared (TIR) regions: band 12 ( mm), band 13 ( mm) Reference spectra The USGS Digital Spectral Library Sensor Area Spatial resolution (m) Spectral resolution (nm) Southern Namibia HyMap (SWIR) Cuprite ASTER (TIR) ~16 ~700

4 Data processing flows HyMap reflectance data USGS reference spectra ASTER surface emissivity data Continuum removal band 13/ band 12 [Ninomiya and Fu, 2002] Modified Spectral Angle Mapper (MSAM) SiO 2 content index map Color composite map of mineral indices Continuum-removal MSAM method

5 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 size [Kodama et al., 2010]. Basic concept of SAM and MSAM methods [Fig. 5 in Kodama et al., 2010].

6 Continuum removal 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] The ratio of original reflectance spectrum to the continuum is defined as a continuum-removal spectrum. We applied MSAM to continuumremoval HyMap spectra using continuum-removal reference spectra. Continuum-removal spectrum continuum USGS reference spectrum of alunite and its continuum, 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

7 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, highand low-al muscovite, pyrophyllite

8 Color composite maps of Alunite : Calcite : Chlorite Continuum removal + MSAM MSAM Background: HyMap band 5 ( mm) image

9 Color composite maps of Dickite : Kaolinite : Montmorillonite Continuum removal + MSAM MSAM Background: HyMap band 5 ( mm) image

10 Color composite maps of Low- : High-Al muscovite : Pyrophyllite Continuum removal + MSAM MSAM Background: HyMap band 5 ( mm) image

11 Threshold of each index Index Threshold Alunite 0.6 Calcite 0.6 Chlorite 0.6 Dickite 0.8 Kaolinite 0.7 Lepidolite 0.7 Montmorillonite 0.7 High-Al Muscovite 0.7 Low-Al Muscovite 0.7 Pyrophyllite 0.7 Lepidolite Determined by examining the features in the reflectance spectra of extracted image pixels corresponding to each threshold value Others Determined by comparing with the reference mineral map of Cuprite [Clark and Swayze, 1996] Assumption Threshold values of mineral indices determined in Cuprite region are also applicable to southern Namibia region.

12 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

13 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 1km X-ray analysis of rock sample (black arrow) Pyrophyllite > kaoline (Dickite was not included on the list of x-ray analysis) Dickite : Kaolinite : Pyrophyllite Background: HyMap band 5 ( mm) image

Color composite map of mineral indices: Tantalite Valley Mineral index map low-al muscovite areas highest SiO 2 content high-al muscovite areas slightly higher SiO 2 content Field survey Pegmatite

14 Color composite map of mineral indices: Tantalite Valley Mineral index map low-al muscovite areas highest SiO 2 content high-al muscovite areas slightly higher SiO 2 content Field survey Pegmatite (black arrows) Silicified rock with quartz dykes (white arrow) 1km X-ray analysis of rock sample Sericite (black arrows) No data (white arrow) Lepidolite : low-al Muscovite : high-al Muscovite Background: SiO 2 content index map

15 HyMap spectra of (1) Hydrothermally-altered minerals 2.17 mm 2.33 mm 2.21 mm 2.37 mm 2.17 mm Dickite 2.21 mm 2.33 mm 2.33 mm Pyrophyllite Kaolinite Bold lines: HyMap spectra Thin lines: reference spectra Dotted lines: continuum-removal spectra

16 HyMap spectra of (2) Mica group minerals 2.19 mm 2.21 mm 2.34 mm 2.34 mm 2.23 mm 2.21 mm Lepidolite 2.36 mm Muscovite (high-al) Muscovite (Low-Al) Bold lines: HyMap spectra Thin lines: reference spectra Dotted lines: continuum-removal spectra

17 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 SiO 2 -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.

18 The way of continuum determination Continuum removal 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. 2. Calculating slopes of the lines through band A and the subsequent bands (A+1 to 126). The band with positively largest slope is defined as band B. 95 A B Repeating this calculation and connecting the bands 95, A, B,, 126 derives continuum. USGS reference spectrum of alunite and its continuum, continuum-removal spectra.

19 SiO 2 content index SiO 2 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 ( mm) is lower than in band 13 ( mm) for felsic rocks, and higher for ultramafic rocks. Using this spectral feature, SiO 2 content index is defined as follows; SiO 2 content index = (ASTER) band 13/ band 12 [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 SiO 2

20 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 SiO 2 content Lepidolite, high-al muscovite areas slightly higher SiO 2 content 1km No field survey, no rock sample Lepidolite : Low-Al muscovite : High-Al muscovite Background: SiO 2 content index map

21 Color composite map of mineral indices: Sandfontain-Ramansdrif Mica group minerals are coexistent with hydrothermallyaltered minerals. 1km Dickite : Kaolinite : Pyrophyllite Background: HyMap band 5 ( mm) image

Extraction and Integration of Mineralogical and Topographic Information Derived from ASTER and DEM Data

Extraction and Integration of Mineralogical and Topographic Information Derived from ASTER and DEM Data Yasushi Yamaguchi, K. Kurata, R. Hirai (Nagoya Univ.) S. Noda (JOGMEC), and S. Kodama (GSJ) GRSG