Largie1, N.H. Lockett1, R.A. Agar1
Pty Ltd, Perth, Western Australia
Sensing Consultants, Stanford, California
Presented at the Ninth Thematic
Conference on Geologic Remote Sensing, Pasadena, California, USA, 8-11 February 1993.
Gold exploration in Nevada has previously targeted areas
by proximity to known mineralisation, either bedrock or placer; by identification of
prospective alteration signatures using Landsat TM imagery; by recognition of mineralised
structures from both satellite imagery and air photographs or by the identification of
argillic and silicic alteration zones using airborne multi spectral data. In practice,
while satellite data provides broad general targets with insufficient mineral
discrimination to permit reliable and consistent ranking of targets, airborne data alone
can produce a proliferation of mineral alteration targets so that the exploration
geologist is little better off in terms of prioritisation. This work attempted to develop
a systematic approach to exploration in arid terrains, utilising Landsat MSS data in
conjunction with geology and loci of ore deposits to provide a focus for more detailed
study using Geoscan AMSS data.
During the past three summers the 24 channel Geoscan airborne
scanner has operated in the USA, and over 42% of Nevada has been flown with a 6 to 10
metre pixel resolution. A large percentage of this imagery was flown for speculation or
for in house use, but had not been interpreted. In January 1992, a relatively small, low
budget program was instituted to take the interpretation of the imagery through to
"grass roots" exploration, including field sampling of outcrop (as
"chips") and stream drainages (as Bulk Leach Extractable Gold; BLEG), which
would hopefully lead to staking and property acquisition.
The method applied followed the development of a number of
exploration concepts based upon pattern recognition and modelling upon known but
relatively undisturbed mineralisation. A series of parameters were developed which
collectively defined an optimum hypothetical gold target and were applied across the
board. Landsat MSS data were used to identify areas with clay/iron alteration, regional
structural elements such as lineaments and ring fractures, and other spectral anomalies.
The Landsat interpretation was integrated with known geology and mineralisation data which
provided an effective regional prospectivity map against which spectral anomalies from
both the Landsat and Geoscan data could be prioritised.
The Geoscan data was interpreted in two stages. A "first
cut" interpretation was performed in Western Australia. All of the data were
interpreted to identify argillic, propylitic, and sericitic alteration signatures using
the Geoscan Image Processing System (GIPSy). Targets generated were both within and away
from alteration zones previously interpreted from the Landsat data. These
"anomalous" areas were ranked on a scale of 1-5, (5 = highest), by integrating
the Geoscan spectral response of the target with the structural setting, host rock,
interpreted Landsat alteration (if any), proximity to published ore deposits and
physiographic setting. In the process of integrating different data sets and ranking
targets, high quality exploration targets have been generated.
A "second cut" into "A", "B" and
"C" categories was then made utilising another set of "selection
rules" based upon one or more geologic models. These final targets were so indicated
for the field work stage. The sites were examined in the field and sampled geochemically.
Rock chip samples from outcrops (which had been selected by image analysis) and 2-3 kg
drainage basin samples up to several kilometres below the target sites were collected.
These latter were -200 mesh dry sieved samples and were submitted for Bulk Leach
Extractable Gold dissolution (BLEG). This method is used extensively in Australia and has
a sensitivity of 1 part per billion gold.
The field results showed many of the prime targets to be
anomalous geochemically and to consist of the style and mineralogy of alteration
conceptualised. Some of the targets were by this time being drilled, a function of the
time lapse between Geoscan data acquisition and interpretation in this case. The remaining
geochemically anomalous targets were sufficiently encouraging in terms of prospectivity to
be worth securing as claims. The process has produced over one hundred attractive
exploration targets in a mature exploration area. This demonstrates Geoscan data
integration to be a cost effective reconnaissance exploration method for arid or semi-arid
The aim of this project was to identify alteration systems
related to bulk mineable (generally low grade, high tonnage) mineralisation in Nevada,
U.S.A. Exploration targets include volcanic and sediment hosted epithermal precious metal
systems and gold deposits associated with skarn or porphyry style copper mineralisation.
Selecting areas suitable for exploration in Nevada presents a
problem: substantial exploration has taken place in many areas and much of the ground has
probably had at least a cursory investigation. It was the task of this project to achieve
a technological advantage through the use of Geoscan remote sensing imagery integrated
with standard methods of exploration area/target selection. The Geoscan imagery used in
this study has a spatial resolution of between 8.5 and 10 metres per image pixel.
1.2 LOCATION OF PROJECT
The project area is located in central and southern Nevada,
U.S.A. The areas of Geoscan image interpretation on which this report is based are within
the 1:250 000 scale map areas of Walker Lake, Mariposa, Reno, Millett, Tonopah, Goldfield,
Ely and Elko (Fig. 1).
A study by de Largie of Nevada geology, tectonics, epithermal and
bulk mineable ore deposits and ore deposit models assisted in the definition of broad
characteristics suitable for target identification at the scale of this project. The
identification of exploration targets was based on the integration of the study results
with the interpretation of several databases.
- Geoscan MkII 24 band airborne multispectral scanner imagery over
Nevada. Area covered: 33 527 square kilometres. Pixel resolution: 8.5 to 10 metres.
- Huntings' 1:250 000 scale maps of:
(a) Landsat lineaments and circular features with coincident
aeromagnetic and gravity structures identified.
(b) Mineral deposits based on 1982-83 U.S.G.S. MILS data.
(c) Target synthesis maps generated by an interpretation of
Landsat MSS imagery depicting alteration zones, volcanic centres and possible exploration
(d) Geological Synthesis maps.
3. Nevada 1:250 000 scale county geological maps.
4. U.S.G.S. 1:100 000 topographic maps.
Target selection criteria are based on the results of the study
of selected major epithermal and bulk mineable precious metal deposits in Nevada. The
criteria are structural setting, host rock lithology and age, proximity to known
mineralisation and the alteration and structure interpreted from the Geoscan multispectral
imagery. Targets are ranked according to their characteristics with respect to the target
selection criteria. To identify unique targets, it is necessary first to qualify a target
through interpretation of its spectral response. The location of the spectrally anomalous
areas is plotted onto a target synthesis base plan. The structural setting of the target
is noted from a combination of a Landsat lineament study, aeromagnetic data and county
geological maps - all at 1:250 000 scale. The host rock and local geological setting of
targets are gleaned from the Nevada county geological maps. Target proximity to known
mineralisation is established from the 1:250 000 Huntings' database and the 1:100 000
Nevada Topographic series maps.
1.4.1 Geoscan AMSS
The interpretation of Geoscan 24 band AMSS imagery forms the
basis of this project. Basic principles and methods of data acquisition and interpretation
are covered in Sec. 2 of this report. The analysis of AMSS imagery seeks to identify areas
of alteration resulting from mineralisation. Structural, lithological and physiographic
information are also gleaned from the data.
1.4.2 Evaluation of Host rocks with respect to Mineralisation
Most epithermal ore deposits are hosted by silicic to
intermediate volcanics. There is not usually a genetic link between host rock and
mineralisation. Mineralisation is usually younger by more than a million years in low
sulphur epithermal systems and is found in several rock types within a specific deposit.
High sulphur deposits arc hosted mostly by rhyodacite, dacite and quartz latite. There may
be a genetic link between host and mineralisation (Hayba et al., 1985).
One half of the epithermal and bulk mineable precious metal
deposits in Nevada are hosted by Tertiary andesitic, rhyolitic or dacitic breccias, tuffs
and flows. The remainder are hosted by limestones, siltstones and shales which are chiefly
Ordovician to Early Silurian with a subordinate group of Carboniferous to Triassic hosts,
and rarely Cambrian. The major mineralisation event was Tertiary for both the volcanic and
sediment hosted epithermal ore deposit types (Bagby & Berger, 1985).
Care was given to the significance placed on host rock.
Favourable rock types (e.g. acid volcanics) and favourable formations (e.g. Ordovician
Vinini Formation) would rank highly, but the significance of a host not known to be
mineralised would not be discounted if it had a favourable alteration signature, and was
in a suitable structural setting.
1.4.3 Assessment of Proximity to Known Mineralisation
The assessment of the proximity of targets to known ore deposits
or workings was tempered with caution. Specific mineral districts have expanded over the
years to cover very large areas. These districts nucleate about the major mining centres,
e.g. Tonopah, Round Mountain and Carlin. Outside of these major districts are many areas
with strong alteration signatures, in structural settings conducive to mineralisation,
with little or no recorded activity. Thus, a target distal to known deposits or workings
would not be discounted if a strong alteration signature was observed and the structural
setting was favourable. The reliability of information on the location of workings,
particularly very old workings, has at times been questionable.
Interpretation of Structural Setting
Information used to establish the structural setting of
exploration targets were:
1. Lineament interpretation of Landsat MSS imagery at 1:250 000
2. Aeromagnetic interpretation at 1:250 000 scale.
3. Nevada County geological maps.
4. Published data on the location of volcanic centres and caldera
The initial study of precious metal deposits in the western
United States established that major ore deposits are almost ubiquitously in complex
structural settings and/or in association with volcanic complexes. Most deposits are
located at the intersection of a low angle thrust fault and a steeply dipping (usually
normal) fault. Many major deposits are marginal to caldera rims. Acid Sulphate type
epithermal deposits have a genetic relationship with calderas, and in particular ring
fracture volcanic dome complexes at caldera margins. Adularia Sericite types are spatially
related where caldera development has provided ground preparation by enhancing
permeability for later mineralisation (Hayba et al., 1985). Assessment of structural
setting is an integral part of the target ranking process.
2.0 AIRBORNE REMOTE SENSING AND THE GEOSCAN MKII AMSS
ACQUISITION AND EQUIPMENT
The Geoscan airborne multispectral scanner (AMSS) is an imaging
spectrometer used from a Cessna 404 Titan Courier aircraft. The AMSS records reflected or
emitted electromagnetic radiation from the earth's surface. These properties are measured
because rocks, minerals and their derived weathering products differentially absorb or
emit the sun's radiation. Data are recorded by the AMSS in a relative reflectance/radiance
mode in the visible and near infrared (VNIR), shortwave infrared (SWIR) and mid infrared
or thermal infrared (TIR) regions of the electromagnetic spectrum. Data are recorded in 24
of 46 available co-registered channels of which channels 1-10 are in the VNIR, 11-18 in
the SWIR and 19-24 in the TIR.
The system has a field of view of 92 degrees and an instantaneous
field of view of 3.0 milliradians. Each data line has 768 pixels which can be resampled to
produce a 1024 pixel wide image which corrects for the difference in the measured pixel
width at the edges of the flight path compared with the pixel width directly below the
The electromagnetic radiation reaching the scanner is reflected
by a high speed rotating mirror onto twenty four detectors. The Geoscan AMSS differs from
almost all other airborne and satellite remote sensing systems in two ways: Co-registered
thermal data collection and relative reflectance/radiance data measurements. The scanner
records the relative brightness of the surface by setting the offsets of each channel to a
mid-range digital number (DN) of 127. The gain or amplification of each channel is then
set to an 8-bit dynamic range 0-255. The gains and offsets are set during a dummy flight
over the area to be scanned immediately before the AMSS survey. This results in the best
possible raw data being collected which needs no pre-processing prior to interpretation.
The AMSS records imaging data onto 5.25 inch optical disks and
backs up onto exabyte cartridge tapes. The scanner operator views the data as it is
recorded in the aircraft using the Geoscan Image Processing System (GIPSy). Imperfections
in the data are identified in flight and re-recording can be performed immediately if
DISPLAY AND INTERPRETATION
The Geoscan Image Processing System processes the digital data
recorded by the AMSS. A correction can be applied to minimise the effects of atmospheric
backscatter. The data are displayed as coloured images on the GIPSy monitor in
combinations of the three colours red, green and blue. A geometric correction can be
applied to the image to remove edge distortion caused by the differential angle of
incidence of the scanner across the width of the survey area.
Each of the twenty four recorded bands or any permutation thereof
can be represented by one of the three basic colours. The bands of data can be viewed
independently or in combination with other bands. Bands can be combined in a variety of
mathematical combinations to enhance the spectral characteristics of specific groups of
alteration mineralogies, rock types, vegetation and structural anomalies.
Alteration styles targeted during the image interpretation are
propylitic, sericitic or phyllic, and argillic. The alteration types are defined by having
the following indicator minerals in their respective alteration assemblages :
montmorillinite-illite/smectite-quartz-kaolin-chlorite = argillic
quartz-sericite-pyrite = sericitic
Lateral and vertical changes in alteration mineralogy occur in
epithermal systems. The alteration mineralogy is also a function of host rock chemistry.
It is therefore necessary to identify alteration minerals associated with epithermal and
porphyry related systems. The target mineralogy includes silica, adularia, kaolinite,
sericite, montmorillinite, illite, talc, epidote and iron oxides.
The spectral enhancement treatments used to identify possible
targets broadly select argillic, propylitic and silicic areas. It should be noted that
responses are not mutually exclusive between treatments, nor do they exclude responses
from other minerals with similar spectral characteristics (Figs. 2, Figs 3 and Table 1).
The treatments of Table 1 were tested by de Largie over several mining areas and were
found to give a moderate to strong spectral signature with treatments 1 and 2, and a
variable weak to very strong response from treatments 3 and 4. In line with the result, a
typical high priority target (rank 5 out of 5) should have a moderate to strong spectral
response from either combination of treatments 1, 2 and 4 or 1, 2 and 3. In practice, rank
5 targets often have a strong response from treatments 1, 2 and 4 with a variable response
from treatment 3. Further treatments were used to help distinguish propylitic alteration:
R,G,B = 13-18, 13-17, 13-16; silicification: R,G,B = 23-19, 23-20, 23-21; and
sericitization: R,G,B = 13-15, 13-14, 14-15 (Plates 1 - 10).
||Sericite, Kaolinite, Alunite, Pyrophyllite (spectral
response is white in image)
||Epidote, Chlorite, Talc, Calcite, Jarosite,
Sericite, Kaolinite, Pyrophyllite, Montmorillonite (spectral response is white in image)
||Limonite, Jarosite, Goethite (spectral response is
white in image)
||Silica as Silica Sinters, Jasperoids and Breccias
(spectral response is blue in image)
(spectral response is yellow in image)
||Silica (spectral response is white in image)
Spectral curves derived from laboratory based scanning systems
are used to gain an understanding of the relationship between the location of AMSS bands
and particular spectral characteristics of individual minerals or rock types. By knowing
the relative strength of absorption or reflection in different AMSS bands, a theoretical
spectral curve can be established. A comparison of the theoretical curve with the known
spectra will assist in identifying the minerals or rock types giving the AMSS response
(Figs. 2 & 3).
Mineral mapping is possible in situations where the concentration
and real distribution of the mineral species is sufficient. The first stage of this
project has targeted alteration assemblages rather than specific individual minerals. The
spectral treatments were therefore designed to detect broad spectrum alteration. The
targeted ore deposit would be anticipated to have a moderate size hydrothermal system and
would be expected to have had a large alteration halo with several alteration types. To be
sure this strategy was correct, the treatments of Table 1 were tested by de Largie over
several mining areas. These mining areas gave a moderate to strong spectral signature with
treatments 1 and 2, and a variable, weak to very strong response from treatments 3 and 4.
In line with the result, a typical high-priority target, (rank 5 out of 5), should have a
moderate to strong spectral response from either combination of treatments 1, 2 and 4 or
1, 2 and 3. In practice, rank 5 targets often have a strong response from treatments 1, 2
and 4 with a variable response from treatment 3 (Table 1).
Three hundred and thirty eight exploration targets have been
identified and ranked according to their structural setting, host rock, alteration
signature, geophysical setting and proximity to known ore deposits. They were generated
through an analysis of 33 500 square kilometres of 24 band AMSS imagery using the Geoscan
Image Processing System. Spectral anomalies identified by the image interpretation were
correlated with Landsat MSS imagery, mapped 1:250 000 scale geology, topographic data,
gravity data, aeromagnetic data and the known location of volcanoes, volcanic complexes,
calderas, mines and old workings. One hundred and twenty eight of these have silicic,
argillic and propylitic alteration signatures, and are in structurally complex
environments. Most of these are associated with aeromagnetic and/or gravity anomalies.
There are one hundred and twenty eight high priority targets
generated by the initial data integration and interpretation. These high priority targets
have argillic, propylitic and usually silicic Geoscan spectral signatures and are located
in structurally complex terrains.
Most targets in this study are associated with regional scale
faults or fault intersections at basin range margins. This agrees with the tectonic
setting of known epithermal deposits which are often in environments of multi generation,
multi directional faulting. High ranking targets (rank 4 and rank 5) have moderate to
strong widespread alteration haloes. Alteration type is often sericitic, silicic, argillic
and propylitic. All high ranking targets are in favourable tectonic settings such as range
margins, caldera margins and contrasting fault type intersections.
A comparison of the characteristics of known ore deposits in
Nevada with the characteristics of the high ranking targets of this project indicates that
a technological advantage has been achieved by the integration of multi source geological
information with a consistent Geoscan data interpretation.
3.1 GEOCHEMISTRY RESULTS
Encouraging geochemistry results have been obtained from several
targets. The field programme is still in its infancy and only thirteen targets have been
visited so far. Rock chips have been assayed using fire assay and stream sediment/drainage
samples. Drainage samples were assayed using the BLEG technique with a sensitivity of one
part per billion. The location of sample sites was chosen using image analysis and the
Selected results are:
||Bleg: 1.2ppb Au
||Rock: 86ppb Au
||Bleg: 2.5ppb Au
||Rock: 84ppb Au
||Bleg: 99.8ppb Au, 57.6ppb Au
||Rock: 404ppb Au
||Rock: 66ppb Au
||Bleg: 65.4ppb Au
||Rock: 55ppb Au
||Rock: 6llppb Au
||Bleg: 9.6ppb Au
||Rock: 584ppb Au
GIPSy screen images of targets WL20/1 and WL14/5
are represented in figures 4 to 10. Sample locations and assay values are shown. The
alteration systems are large and image analysis is used to identify the best sample sites.
DISCUSSION AND CONCLUSIONS
Remote sensing interpretation generally produces a very large
number of spectral anomalies. The task of this project was to select valid high quality
exploration targets from the many features which although are spectrally anomalous, are
probably not valid exploration targets.
Data integration was performed to assist in the target validation
process. This resulted in 128 high ranking targets being selected from the total of 338
targets. The study area is 33 500 square kilometres. Thus, there is approximately one
target per 100 square kilometres and one high rank target per 260 square kilometres.
Nevada is a mature exploration area and it is therefore likely that ground availability
will be tight. Thus the number of targets on open ground will be considerably less than
the number initially identified.
There has been limited feedback of field information into this
study. Ideally, a field study of targets selected early in the project should be done and
the information fed back into the interpretation process. In this way the reliability of
the spectral interpretation is significantly increased.
Spectral resolution is a limiting factor in satellite borne
remote sensing systems. Geoscan data can be collected with a pixel size of between two and
twenty metres. For exploration purposes, an optimal range of pixel sizes is eight to
twelve metres. All the data from this project have a pixel resolution in this range. The
measurement of relative reflectance benefits the interpretation geologist as no data
preprocessing or massaging is needed before data use. This is a limitation on almost all
other remote sensing MSS systems.
Known ore deposits were ranked according to the same target
selection criteria used for this study and fifty percent were of rank 4, thirty percent
were of rank 5 and twenty percent were of rank 3. This is a similar distribution to the
ranking of targets from this project where thirty eight percent are of rank 5, thirty five
percent are of rank 4 and nineteen percent are of rank 3. Rank 4 and rank 5 targets have
silicic, argillic and propylitic alteration of various intensities, and are in favourable
structural settings, such as thrust faults and/or calderas and/or major faults or
lineaments. The most common structural position of rank 5 targets is at major fault
intersections. Seventy eight percent of rank 5 targets are Tertiary volcanic hosted and
the remainder are sediment hosted or mixed volcanic/sediment hosted. The most common host
rock combinations are Silurian-Ordovician carbonaceous sediments and Tertiary volcanics or
The distinction between rank 4 and rank 5 targets is usually
based on a weakness in silicic or argillic alteration signature in rank 4 targets.
Considering the results from the study of known deposits with regard to alteration, rank 4
targets remain a high priority.
There are sixty three rank 3 targets. Many have moderate
alteration, but have been discounted because of the relatively small size of the inferred
alteration system. There are some rank 3 targets which display poor or weak alteration
signatures, but are located in favourable structural settings. They represent a medium
The preliminary field program of thirteen targets has delivered
eight targets which have significantly anomalous gold or gold and silver values.
The comparison of characteristics of known deposits with
characteristics of exploration targets generated with this project, combined with the
(relatively very few) results of the ground follow up program, indicates that the method
adopted is successful and reliable. In using the Geoscan technology in conjunction with
the 'standard' methods of target/area selection, a technological advantage has been
achieved both in terms of the quality of generated exploration targets and the man hours
taken to identify them.
Bagby, W.C., and Berger, B.R., 1985, Geologic characteristics of
sediment-hosted, disseminated precious-metal deposits in the western United States, in
Berger, B.R., and Bethke, P.M. (eds), Geology and Geochemistry of Epithermal Systems,
Reviews in Economic Geology v.2, pp.169-202.
Hayba, D.O., Bethke, P.M., Heald, P., and Foley, N.K., 1985,
mineralogic, and geochemical characteristics of volcanic-hosted
epithermal precious metal deposits, in Berger, B.R., and Bethke, P.M. (Eds) Geology and
Geochemistry of Epithermal Systems, Reviews in Economic Geology v.2, pp. 129-167.
|Plate 1: Spectrally
impressive acid volcanic hosted target at a caldera rim. An east west arcuate structure
can be seen in the upper half of the area.
||Plate 2: Argillic
alteration is indicated by the bright white areas with R,G,B = 11-13,11-14,11-15. The
alteration zone extends several kilometres beyond the field shown here.
|Plate 3: A large and
area of intense silicification is indicated by the blue areas in the treatment R,G,B =
||Plate 4: Magnesian
rich clays are identified using the treatment R,G,B = 11-16,11-17,11-18. Propylitic and
argillic mineralogies give a positive response with this treatment and are identified by
the bright white areas.
|Plate 5: This target
is a rank 4 target. It is a silica sinter with anomalous gold values. The strong silica
response (blue) is easily identified to the upper left of the sample sites shown. More
anomalous samples have been collected along the alteration trend.
||Plate 6: Propylitic
alteration is indicated by the white response along the significant structural trend
striking approx. 045 degrees.
|Plate 7: A northwest
trending argillic alteration belt is indicated by the white response with treatment R,G,B
= 11-13,11-14,11-15. Assay values are shown.
||Plate 8: Propylitic
alteration is indicated by bright areas with treatment R,G,B = 13-18,13-17,13-16.
|Plate 9: The yellow
areas indicate sericitic alteration with treatment R,G,B = 13-15,13-14,14-15.
||Plate 10: Iron oxides
are represented by the white areas with the treatment R,G,B = 6-7, 6-8, 6-9.