TECHNICAL PAPERS

AN INTEGRATED LANDSAT-MSS, AEROMAGNETIC. AIR-PHOTOGRAPHIC AND GEOSCAN AMSS APPROACH TO GOLD EXPLORATION IN NEVADA

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 terrains.

1.0 INTRODUCTION

1.1 OBJECTIVES

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 AREA

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).

Figure 1.

1.3 SCOPE

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.

1. Geoscan MkII 24 band airborne multispectral scanner imagery over Nevada. Area covered: 33 527 square kilometres. Pixel resolution: 8.5 to 10 metres.
2. 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 target areas.

(d) Geological Synthesis maps.

3. Nevada 1:250 000 scale county geological maps.

4. U.S.G.S. 1:100 000 topographic maps.

1.4 TARGET SELECTION METHODOLOGY

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 imagery

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.

1.4.4 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 scale.

2. Aeromagnetic interpretation at 1:250 000 scale.

3. Nevada County geological maps.

4. Published data on the location of volcanic centres and caldera margins.

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

2.1 DATA 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 aircraft.

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 necessary.

2.2 IMAGE 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 :

gypsum-chlorite-epidote-K-feldspar-calcite-albite-adularia = propylitic

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).

Figure 2.	Figure 3.

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).

3.0 RESULTS

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 topographic base.

Selected results are:

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.

4.0 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 intrusives.

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 priority target.

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.

5.0 REFERENCES

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, Geologic,

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.

6.0 PLATES

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 = 20,21,22.	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.