This study developed a "continuum-removal MSAM" method to extract minerals from HyMap hyperspectral data in southern Namibia. The method accurately identified hydrothermally altered and mica group minerals when validated against a reference Cuprite map. Mineral index maps showed distributions consistent with field surveys and analyses. Combining HyMap and ASTER data helped locate pegmatites, indicated by variations in a silicon dioxide content index. The method provides a tool for mineral exploration in southern Namibia.

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 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

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) 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

4. 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

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

6. 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.

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, high-
and low-Al muscovite,
pyrophyllite

8. Color composite maps of
Alunite : Calcite : Chlorite
Continuum removal + MSAM MSAM
Background: HyMap band 5 (0.4982 mm) image

9. Color composite maps of
Dickite : Kaolinite : Montmorillonite
Continuum removal + MSAM MSAM
Background: HyMap band 5 (0.4982 mm) image

10. Color composite maps of
Low- : High-Al muscovite :
Pyrophyllite
Continuum removal + MSAM MSAM
Background: HyMap band 5 (0.4982 mm) image

11. 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.

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]

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
• 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

14. 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

15. 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

16. 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)

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 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.

18. 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.
USGS reference spectrum of
3. Repeating this calculation
and connecting the bands 95, alunite and its continuum,
A, B, … , 126 derives continuum-removal spectra.
“continuum”.

19. 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;
SiO2 content index = (ASTER) band 13/ band 12
[Ninomiya and Fu, 2002]

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 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

21. 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