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