ralogy
lution.
osit Scale
-20
-8
4-8
1-8
Table 8. Spectral and spatial requirements for the alkalic/rift-related gold deposits.
CHARACTERISTIC DESCRIPTION SPECTRAL RANGE(um) SPATIAL RESOL.(m)
Multispec. Hyperspec.* Recon Deposit Scale
Primary Alteration** Weak silica, calcite, K- 2.15-2.35 9.7, 10.0, 10-30 8-12
feldspar, kaolinite, illite, TIR 2.33. 22
pyrite
Secondary Alterat. Hematite, goethite, 0.6-0.7, 0.85, 0.95 10-30 8-12
jarosite 0.8-1.0 0.92
Host Rocks Syenitic plutons, trachytic VNIR,TIR 9.7, 10.0, 20-30 10-20
volcaniclastics and flows, SWIR TIR
diatremes
Zoning*** Silica, calcite-illite-montm.- SWIR,TIR 9.7,.2.33, 10-20 8-10
chlorite; jarosite-goethite, 2.36; 0.92
hematite 0.95
Structure Steep normal faults, shears, VNSWIR,TIR, Spect. feat. 20-80 10-20
grabens, small depression s Microwave above
Vegetation Subtle changes directly NIR 0.7-0.8 10-20 8-12
over mineralization
* Position of key absorption maxima or emission minima. ** Alteration extensive, but typically weak.
*** Iron oxides zoned from central jarosite to goethite. Central silica and calcite zoned out toward clay
minerals and chlorite.
Spatz (1996a and b) has described the remote sensing
features of the sediment-hosted and volcanic-hosted
subtypes. The reader is referred to these earlier works for
fundamentals regarding mineral and alteration spectra
and instrument parameters.
4.2 Porphyry Copper and Porphyry Molybdenum
Deposit Models
Porphyry ore systems have common characteristics which
are important to aerospace remote sensing detection.
Porphyry copper deposit models have been outlined by
Titley and Hicks (1966), Lowell and Guilbert (1970), Rose
(1970), Sillitoe (1973), Gustafson and Hunt (1975),
Hollister (1978), Einaudi et al. (1981), Titley (1982),
Titley and Beane (1981), Schroeter(1995), and Pierce and
Bolm (1995). Porphyry molybdenum models have been
described by Wallace et al.(1978, White et al.(1981), and
Keith et al.(1993); and the porphyry gold model was
documented by Vila and Sillitoe (1991) and Vila et al.
(1991). Remote sensing characteristics of porphyry depos-
its have been described by Spatz (1992), Spatz and Taranik
(1994), Spatz and Wilson (1995), and Spatz (1995).
Porphyry deposits are commonly subdivided according to
major contained metals: porphyry copper, porphyry cop-
Per/gold, porphyry copper/molybdenum, porphyry molyb-
denumofthe granodiorite type, and Climax-type porphyry
molybdenum deposits. Tables 9 and 10 outline important
geologic features of the porphyry copper and porphyry
molybdenum systems. Porphyry gold systems are treated
largely by the high-sulfidation gold tvpe described above.
645
Porphyry ore deposits may be intrusion hosted, wall rock
hosted, or both. Porphyry copper and porphyry gold depos-
its are characterized by a plutonic rock suite that ranges in
composition from quartz monzonite to diorite. Intrusions at
the Climax-type porphyry molybdenum deposits are often
more felsic and may include sub-volcanic porphyries
(e.g..,rhyolite and dacite porphyry). In some instances the
most proximal ore related porphyritic phases can be traced
in the field to more primitive earlier phases. The TIR is an
important remote sensing interval for this application.
Hypogene alteration that accompanies mineralization con-
sists generally of a central zone of orthoclase and/or biotite
(potassic zone), surrounded by quartz-sericite-pyrite
(phyllic), which in turn as surrounded by chlorite-epidote-
calcite (propylitic). Each of these mineral assemblages is
potentially detectable with remote sensing instruments
chiefly in the SWIR and TIR intervals, and individual
mineral phases can be discriminated with hyperspectral
scanners. Variations of this general zoning scheme include
systems without distinct phyllic zones, wall rock deposits
that may involve calc-silicate skarn alteration, and contact
metamorphic biotite development that could be confused
with the potassic core. Supergene weathering can result in
an extensive iron oxide and argillic cap.
Each of the porphyry deposit types differs in tectonic
setting, alteration assemblage, and plutonic host composi-
tion, characteristics which define the deposit models and
influence choice of remote sensing instruments and imag-
ery in strategic planning. Knowledge of the spectral prop-
erties of the models and the spatial resolution requirements
for feature detection enable the remote sensing user to select
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996