Characterization of Gold Mineralization in
Northern Pakistan Using Imaging Spectroscopy
Shuhab D. Khan, Ünal Okyay, Laeiq Ahmad, and Mohammad T. Shah
Abstract
This study applied an integrated imaging spectroscopy and
geochemical approach to evaluate gold mineralization in
northern Pakistan. The study analyzed several rock samples
for gold concentrations. These analyses found gold concentra-
tions of up to 96 ppm in sulfide-mineralization zones in the
Astore area. Several rock samples were scanned using visible-
near and shortwave infrared hyperspectral sensors. Compari-
son of different datasets provided a method for evaluating
spatial distribution of minerals in the samples. Dominant
minerals in the alteration zone included galena, chalcopyrite,
pyrite, and azurite. The high-resolution imaging spectroscopy
data were evaluated using the Spectral Angle Mapper (
SAM
)
and Support Vector Machine (
SVM
) classifiers. Although
SVM
yielded fewer unclassified pixels,
SAM
gave superior classifica-
tion results and distribution estimates, particularly for sulfide
minerals, which exhibit lower overall reflectance profiles. The
surface mineral distribution estimates obtained from
SAM
are
in a very good agreement with electron microscope data.
Introduction
The geology of northern Pakistan is a superb example of
continental collision tectonics; this area experienced two
major collisions. The first was between an arc system, the
Kohistan-Ladakh with the Indian plate, and the second was
a collision of India-Kohistan with Asia at 50 Ma (Khan
et al.
,
2009). The arc system was ultimately trapped between the In-
dian and the Karakoram blocks. Understanding the sequence
of tectonic events that resulted in this remarkable episode of
crustal growth is not only very interesting scientifically, but it
has significant potential to contribute to locating deposits of
economically valuable minerals.
Detailed geological work has been carried out in the
northern part of Pakistan (Jan and Howie, 1981; Coward
et
al.
, 1982; Butler and Prior, 1988; Khan
et al.
, 2009) where
various tectonic domains were identified, including continen-
tal margins, mélanges, island arcs, and back-arc basins, and
potentially holds deposits of economically valuable minerals
(Figure 1). Worldwide, the occurrence of economically signifi-
cant gold deposits has been reported in these types of tectonic
environments (Groves
et al.
, 1998), in which gold deposits
occur as magmatic, hydrothermal, epithermal, porphyric,
Carlin-type, and placer-type deposits. The occurrence of
placer gold is reported in the upper reaches of the Chitral and
Gilgit areas, and along the Indus River in northern Pakistan
(Figure 1) (Austrominerals, 1978).
However, little work has been done on mineral exploration
in northern Pakistan. Tahirkheli (1974) worked on alluvial gold
prospects in the Chitral and Gilgit areas and along the Indus
River in this region. Investigations of placer-mineral deposits
along the Indus, Gilgit, Hunza, and Chitral Rivers were conduct-
ed by workers from the Geological Survey of Pakistan (Ahmad
and Chaudhry, 1976). In addition, Pakistan Mineral Develop-
ment Corporation (
PMDC
, 2001) conducted some investigations
of placer gold. Furthermore, gold-exploration and mineral-anal-
ysis projects were conducted during the geochemical surveys
of stream sediments in the Chitral, Gilgit, and Skardu regions
of northern Pakistan from 1992 to 1997 (
PMDC
, 2001). These
surveys collected and analyzed 4,096 geochemical stream sedi-
ment samples covering an 80,000 km
2
area of northern Pakistan.
This work identified 168 gold anomalies (
PMDC
, 2001) and four
target areas. Halfpenny and Mazzucchelli (1999) also carried
out a reconnaissance survey while conducting the regional,
multi-element drainage geochemistry in this region. Recently,
high concentrations of gold in stream sediments from major
streams in northern Pakistan was reported (Ali
et al.
, 2015).
The present study focused on previously identified gold
anomalies and carried out detailed fieldwork, imaging spec-
troscopy, and geochemical work to identify which rocks con-
tained gold deposits. Except for a very few sites, most of the
target zones showed no, or very low, concentrations of gold.
This paper summarizes our work for the northern areas and
provides details on one such site near Astore Valley, which is
located near the Indus suture zone (Figure 1).
The goal of this work is to use laboratory-based hyperspec-
tral imaging techniques for mapping textures and mineral
associations of mineralized rock samples to assess potential of
ground-based remote sensing for imaging vertical cliff faces of
the high Himalayas in areas where traditional satellite or air-
borne data cannot be used because of steep slopes and clouds.
Materials and Methods
Field Work
Between 2011 and 2014, ten field trips were conducted in ma-
jor valleys of northern Pakistan to identify mineralized zones,
to map and assess mineral potential, and to collect rock
samples for detailed petrographic and geochemical analyses
(Figure 2). Sites for detailed field work were selected based on
high gold concentrations that were reported by previous stud-
ies (
PMDC
, 2001). These earlier investigations were based on
geochemical analyses that were conducted on samples mainly
collected from stream sediments.
Laboratory-Based Imaging Spectroscopy
The laboratory-based imaging spectroscopy data were acquired
using visible near-infrared (
VNIR
) and shortwave infrared (
SWIR
)
Shuhab D. Khan and Ünal Okyay are with the Department
of Earth & Atmospheric Sciences, University of Houston,
Houston Texas 77204 (
).
Laeiq Ahmad is with the Department of Geology, University
of Swabi, Anbar, Khybar Pakhtunkhwa, Pakistan.
Mohammad T. Shah is with FATA University, Akhurwal,
Khyber Pakhtunkhwa, Pakistan.
Photogrammetric Engineering & Remote Sensing
Vol. 84, No. 7, July 2018, pp. 425–434.
0099-1112/18/425–434
© 2018 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.84.7.425
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
July 2018
425
