THE
, Alteration.
t model, the
ical geologic
tion strategy
the program
eposit scale,
Both spatial
€ reasons, it
)n of remote
legy that is
evelopment
and spatial
yrocess and
er consider
The deposit
an effective
ram follows
cal geologic
ensing. The
f alteration
and resolu-
of explora-
and spatial
al coverage
st common
oration and
stage from
cause of a
termediate
h coverage,
lific remote
throughout
te with the
> commonly
large scale)
project or
ts for these
Table 1. Spatial resolution requirement in mineral exploration and development.
STAGE DESCRIPTION COVERAGE (km2) SCALE SPATIAL RESOL.(m)
Reconnaissance Rapid exploration over 5000-20000 Small 20 - 80
large area 1: 100,000
Regional Exploration within known 500-5000 Intermediate 10 - 30
mineral belt or trend, or 1: 24,000
individual volcanic field
or mountain range
District Exploration and mapping 10-500 Large 6 - 10
within a mining district 1: 12,000
or hydrothermal center
Deposit Detailed mapping from 0.1-10 Very large 3-7
early project phase to 1: 2400
feasibility drilling
categories are about 20-80m, 10-30m, 6-10m, and 3-7m,
respectively (Table 1). Spatial resolution requirements
also vary with both ore deposit models and the geologic or
detectable features that are sought (Tables 2-10). For these
reasons it is important to outline exploration objectives and
establish a thorough remote sensing strategy on which to
base the selection of remote sensing instruments and
imagery, keeping in mind that detectability of a feature
depends on its spectral contrast with the surrounding
surface cover as well as the size of the ground resolution
cell. For the purpose of the spatial resolution requirements
presented here, it is assumed that a feature must be one-half
the width of the ground resolution cell in order to be
detected. Geologists do not typically record objects less
than 1/32 inch wide on a map. This parameter also
constrains practical spatial resolution requirements and is
taken into account in Table 1.
At the reconnaissance exploration stage, space-based re-
mote sensing is most applicable to areas with minimal
geological knowledge, especially inaccessible regions that
are poorly mapped. The exploration geologist may be
interested in exploring for favorable structure, lithology,
or alteration. Regional exploration, as considered here, is
larger scale than reconnaissance exploration and com-
monly focused within confined target areas such as a
mountain range, mineral belt, structural corridor, volcanic
field, intrusive belt, or other tectonic, magmatic, or
metallogenic zone. At this intermediate scale, the
explorationist has usually identified empirical geologic
Criteria that relate spatially to mineralization and is ca-
pable of more selective, detailed remote sensing decisions.
Within confined target areas at the regional scale, litholo-
gies, intrusive complexes, and ore-controlling faults may
be important guides. Although threatening the economic
barrier at most projects today, airborne scanner imagery
could be quite effective at this intermediate scale.
639
3.0 SPECTRAL RESOLUTION REQUIREMENTS
The position of band passes and the spectral resolution of
a scanner are important considerations in the design of
remote sensing strategies. In general, the smaller the area
of interest, the larger the scale requirement and the greater
the demands on spectral resolution. The multispectral
scanners with band passes that range upward from about
100 nanometers are more applicable to small scale pro-
grams from reconnaissance and regional stage into district
stage. Modern hyperspectral scanners are capable of spec-
tral sensitivities resembling laboratory spectrometers and
provide a tool for detailed alteration mapping and differen-
tiation of rock types and plutonic rock phases. Figure 1
shows the important spectral intervals for some of the key
alteration minerals and other deposit related features.
The unique spectra of many of the secondary minerals that
comprise alteration suites offer potential for direct mineral
identification with narrow-band hyperspectral sensors.
Spectra of alteration minerals and rock types have been
published by Hunt and Ashley (1979), Lee and Raines
(1984), and Christianson et al. (1986). Narrow absorption
maxima or troughs in the SWIR are fortuitous features in
the spectra of many of the important hydrothermal and
supergene alteration minerals. Propylitic assemblage min-
erals (chlorite, epidote, and calcite), argillic minerals
(kaolinite, dickite, and montmorillonite), phyllic alter-
ation minerals (sericite, illite), advanced argillic alteration
(alunite, pyrophyllite), amorphous varieties of silica, su-
pergene clays, and both potassic zone and contact meta-
morphic biotite absorb energy in the SWIR. Clark et al.
(1993) used an advanced spectral mapping algorithm to
actually map degrees of kaolinite crystallinity, Na/Ca
variation in montmorillonite, and Na-K solid solution in
alunite with airborne AVIRIS data at Cuprite, Nevada.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
