Remote Sensing and GIS for Mineral Exploration
Content
Series ARC-SDSU-002-97
Affiliated Research Center
FINAL REPORT
Integrated Use of Remote Sensing and GIS for Mineral Exploration
La Cuesta International, Inc.
La Cuesta International, Inc.
San Diego State University Commercial Remote Sensing Program, National Aeronautics and Space Administration
Affiliated Research Center Final Report
Integrated Use of Remote Sensing and GIS for Mineral Exploration: A Project of the NASA Affiliated Research Center at San Diego State University
Project conducted by:
La Cuesta International, Inc. 1805 Wedgemere Road El Cajon, California 92020
Report prepared by:
Mr. W. Perry Durning La Cuesta International, Inc. and Mr. Stephen R. Polis and Dr. Eric G. Frost, Department of Geologic Sciences, San Diego State University and Mr. John V. Kaiser Department of Geography, San Diego State University
Report prepared for:
Dr. Douglas A. Stow, Principal Investigator San Diego State University Department of Geography San Diego, California 92182-4493 and Commercial Remote Sensing Program Office National Aeronautics and Space Administration John C. Stennis Space Center, Mississippi 39529
January 20, 1998
EXPORT ADMINISTRATION REGULATIONS NOTICE
This document contains information within the purview of the Export Administration Regulations (EAR), 15 CFR 730-744, and is export controlled. It may not be transferred to foreign nationals in the U.S. or abroad without specific approval of a knowledgeable NASA export control official, and/or unless an export license/license exception is obtained/available from the Bureau of Export Administration (BXA), United States Department of Commerce. Violations of these regulations are punishable by fine, imprisonment, or both.
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Table of Contents
EXPORT ADMINISTRATION REGULATIONS NOTICE .................................................. iii Executive Summary ................................................................................................................. vi 1.0 Introduction..........................................................................................................................1 2.0 Structural Mapping ..............................................................................................................2 3.0 Alteration Mapping..............................................................................................................3 4.0 Radar ....................................................................................................................................5 5.0 Results and Conclusions ......................................................................................................5 6.0 References............................................................................................................................6 Appendix A. Technical Proposal .............................................................................................20 Appendix B. Commercial Proposal .........................................................................................24 Appendix C. Schedule .............................................................................................................25
Figures
Figure 1. Tertiary dip domain map of the southern Basin and Range Province. ......................8 Figure 2. Generalized geologic map of the northern part of the Colorado River Trough and adjacent region. ..........................................................................................................................9 Figure 3. Diagrammatic representation of opposite polarity tilt patterns in extensional terranes as separated by a strike-slip or transfer fault (A) or an accommodation zone (B). ....10 Figure 4. Geometric and kinematic characteristics of Neogene extensional deformation, Colorado River extensional corridor, NV, AZ, and CA. (Frost and Heidrick, 1996)..............11 Figure 5. Detachment fault-fold geometry and deep-crustal structure, Colorado River extensional terrane, as based on CALCRUST and reprocessed industry seismic lines...........12 Figure 6. Diagrammatic model of crustal extension showing truncation of upper-plate normal faults at depth into a gently inclined detachment fault. ...........................................................13 Figure 7. A SPOT-Landsat TM ratio threshold merge of the area around the southern Chocolate Mountains illustrating extensional antiforms and areas of potential hydrothermal alteration highlighted in yellow. ..............................................................................................14 Figure 8. A SIR-C radar Landsat TM threshold merge of the area around the Mesquite mine.15 Figure 9. An over-simplified structural model depicting the Mesquite mine located within a newly interpreted accommodation zone...................................................................................16
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Figure 10. Landsat TM 741 color composite image of the southern Colorado River illustrating extensional faults and the newly interpreted accommodation zone. .....................17 Figure 11. A SIR-C radar color composite with interpreted Tertiary upper-plate transport directions and accommodation zone structure illustrated. .......................................................18 Figure 12. A Landsat TM ratio color composite of the area around the Mesquite mine. .......19 Figure A-1. Diagrammatic representation of opposite polarity tilt patterns in extensional terranes as separated by a strike-slip or transfer fault (A) or an accommodation zone (B). ....23
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Executive Summary
The Affiliated Research Center (ARC) program was conducted with La Cuesta International, Inc. (LCI) and supported by San Diego State University (SDSU). The purpose of the program was to develop the procedures and demonstrate the feasibility of using broad-band and hyperspectral, remotely sensed data to identify extensional geologic structures (accommodation zones) associated with precious/base metal deposition. In most cases, current mineral exploration concepts have failed to recognize the association of mineralization with unique extensional structures called accommodation zones. These zones show little obvious deformation, yet focus fluid migration and mineralization into predictable regions of the crust. The Mesquite gold mine, located in southeastern California, approximately 60 km east of the Salton Sea, was studied to determine if it resides within an unrecognized accommodation zone. Landsat Thematic Mapper (TM), Satellite Pour l'Observation de la Terre (SPOT), and radar data were observed both separately and in a merged format to extract spectral and spatial information using ER Mapper software. A variety of images were produced to highlight important structural features along with areas of hydrothermal alteration. Images produced include Landsat TM and Shuttle Imaging Radar-C (SIR-C) radar color composites, color ratio composites, principal components, thresholds and Landsat TM-SPOT and Landsat TM-SIR-C radar merges. Hyperspectral data (Advanced Visible/Infrared Imaging Spectrometer (AVIRIS) and Airborne Terrestrial Applications Sensor (ATLAS)) where obtained though not processed, because the data did not cover the study area. The ARC project proved to be extremely beneficial in training LCI with remote sensing procedures that resulted in the following: • The recognition of an accommodation zone within which the Mesquite gold mine resides. • The establishment of a template to identify hydrothermally altered areas from Landsat TM data. Prior studies in a non-ARC area showed less than 10% of TM anomalies were related to hydrothermal alteration. Using the ARC template greater than 50% of the TM anomalies checked in the field showed hydrothermal alteration. • The discovery of two virgin mineral systems in another non-ARC exploration area as a direct result of Landsat TM data interpretations using the template described above. Thus, this study has provided an immediate positive impact for LCI by providing a template to process TM imagery to successfully evaluate large areas of interest for mineral potential, focus field evaluations, and ultimately provide a higher probability for exploration success.
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1.0 Introduction
Recognition of major gold deposits formed in association with Tertiary crustal extensions in the western U.S. has been established and similar occurrences are now being recognized globally. In most cases, current mineral exploration concepts have failed to recognize the association of mineralization with unique extensional structures called accommodation zones. These zones, described below, show little obvious deformation, yet focus fluid migration and mineralization into predictable regions of the crust. Integrating remotely sensed data with existing geologic data provides a unique opportunity to identify the location of these previously unrecognized zones. Guided by an understanding of accommodation zones, Landsat TM, SPOT, and radar data were utilized to locate an unrecognized accommodation zone in which the Mesquite gold mine is located. Further remote sensing efforts made possible by insights developed through this ARC effort support the association between accommodation zones and precious/base metal deposition.
Accommodation Zones
In areas of crustal extension, the crust breaks along a multitude of normal faults, commonly termed an "array," with different segments having different transport directions as shown in Figures 1, 2, and 3. These segmented sections occur at a scale of 50 - 500 km along strike. Within these extensional terranes, transport of major regions is in one uniform direction (Bosworth et al., 1986; Lister et al., 1986). However, zones with opposite transport generally exist in adjacent domains. Between these zones of opposite transport, a zone of deformation must exist to allow opposite motion to occur during the same deformational phase. These zones have been called "accommodation zones," and have only recently been recognized within extensional systems on a worldwide basis. Accommodation zones link the normal fault arrays of opposing transport directions. These regions are often zones of little obvious deformation, appearing not as strike-slip faults, but brecciated “null” zones because the entire volume of rock has been affected (Anderson, 1971; Bosworth et al., 1986). Because accommodation zones represent areas of vergence reversal within extensional terranes at the up-dip tips of the regional faults (Figures 3 and 4), they focus fluid migration and mineralization into predictable regions of the crust. The Nelson District in southern Nevada is an excellent example where alteration and mineralization occur within an accommodation zone (Faulds et al., 1987; Frost and Heidrick, 1996). This zone is between two opposite facing regions of extensional transport that can be discerned on the regional tectonic maps and appears to point directly to the major gold mineralization. The three-dimensionality of extended crust has been well documented by researchers in the petroleum industries through high-quality, three-dimensional seismic investigations. These seismic studies have demonstrated the presence of accommodation structures in nearly all the extensional terranes in the world. Within these zones, fluids flow toward and saturate portions of the accommodation zones. Knowledge of how accommodation-zone tectonics localize fluid flow allows researchers to target discrete areas for detailed exploration within the much larger extensional terrane.
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The association of accommodation zones with mineralization is due to three main factors: 1. Accommodation zones are deep-crustal breaks that often become syntectonic volcanic centers because they localize the magmatic material, thus becoming an elongated volcanic and plutonic center that intrudes existing fault zones and provides the thermodynamic energy to drive mineralized fluids. 2. There is an increase in brecciation and the number of faults with a net decrease in the average fault slip within the accommodation zone, making the faulting more subtle but opening much larger volumes of extended rock. This provides excellent long-term permeability for multigenerational fluid flow and subsequent mineralization. 3. The geometry is such that a localized compressional stress regime forms anticlinal culminations located structurally up-dip from the normal faults it separates or "accommodates" on either side. This extensional geometry provides a flowpath from multiple normal faults toward the accommodation zones where mineralization can occur repeatedly as the faults continue to break through time. Combining optical and radar remote sensing and image processing provides a powerful approach to search for accommodation zones. Because the areas of opposite dip domains are so large, interpreting field data and large scale maps easily misses the location of accommodation zones. Since the recognition of opposite dip domains has not been part of field investigation methodologies before, many of the geologic maps and their synthesis are simply inadequate to discern accommodation zone structures. Optical and radar data show geologic structure and enable the geologist to synthesize the tectonics and also discern the potential locations of hydrothermal alteration. This image analysis, coupled with an understanding of the tectonic processes and significance of the localized alteration, provides a powerful tool for exploration.
2.0 Structural Mapping
The antiformal-synformal character of the detachment fault system is one of the best ways of finding unrecognized large-scale normal faults and determining where accommodation zones might be found. Because of the regional perspective provided by the images and the display of spectral and topographic data with optical and radar images, the antiformal-synformal character of ranges can be readily discerned. In most areas, the long axis of the antiforms is parallel to the upper plate transport direction, much like megamullion structures elongated in the transport direction (Figures 5 and 6). These mullion structures appear to have a fairly consistent orientation on a regional scale and appear to be more pronounced as more relative motion has taken place on the fault structures. An obvious cause of this relationship is that the once moderate angle faults with their fluted fault patterns have tilted over more and more, making the mullions into whale-like, antiformal highs and trough-like synformal lows. Due to the regional nature of these distinctive antiformal-synformal features, optical and radar imaging provides feature recognition for these targets, and enables potential hydrothermally saturated antiforms to be highlighted. Figure 7 is a SPOT- Landsat TM ratio
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threshold merge of the area around the southern Chocolate Mountains. The image illustrates the ability to map and target extensional antiforms and areas of potential hydrothermal alteration highlighted in yellow. The strongest alteration signature in the image is along the detachment fault antiform located closest to the Mesquite mine and the Mount Barrow pluton responsible for the Mesquite gold mineralization (Frost, 1990). By changing the SPOT backdrop to radar (Figure 8) and keeping the Landsat TM alteration data, the topography is readily observed as it highlights antiformal-synformal geometry, as well as dipslopes and fracture patterns. By mapping a larger area, the Mesquite mine was discovered to be located within an accommodation zone (Figure 9). The southern Chocolate Mountains dip to the southwest, and are interpreted to have had a Tertiary northeast upper-plate transport direction, while the Trigo Mountains, located to the northeast across the Colorado river in Arizona, have had an opposite or southwest Tertiary upper-plate transport direction. The Mesquite mine is positioned at the southwest termination of the accommodation zone between these two dip domains, which is characterized by a northeasterly striking topographic low. This accommodation zone was first observed through a Landsat TM Bands 7-4-1 color composite image where volcanic dipslopes, extensional faults and dikes, and the general geometry of the ranges were mapped (Figure 10). Radar data used to image the structure highlighted the linear topographic low of this accommodation zone that trends through the Mesquite mine and continues northeasterly for more than 60 km (Figure 11).
3.0 Alteration Mapping
The goals for producing alteration images for the ARC were to optimally depict all spectral properties that may be related to alteration and then prioritize all targets for field evaluation. There are generally two common types of images used to map hydrothermal alteration: ratios and select principal components analysis (Loughlin, 1991). In this study, ratio images were combined with SPOT or radar data to enhance the structural geology that ultimately controls the areas of mineralization.
Landsat TM Ratios
Band ratioing is a technique that has been used for many years in remote sensing to effectively display spectral variations (e.g., Goetz et al., 1975). Properly computed band ratio images display little topographic or geomorphic information because the ratio of reflectivity of any two bands for a given material is not a function of illumination. Thus, the distinction between foreslopes and backslopes is lost, while spectral contrasts are enhanced. There are many types of band ratio images, though a “threshold-modified” four-component technique (Crippen, 1989) provided the best results of any ratio combination used for alteration mapping in the arid to semi-arid terranes of this study (Figure 7). Crippen’ four-component s technique uses three band-ratio images (one each for the red, green, and blue output channels) for the chromatic components of the image (Crippen et al., 1990). The technique then reintroduces the spatial detail using an achromatic SPOT image or TM band 4 that contains
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spatial detail. Thus, in the final image, colors display spectral information while intensity primarily displays topographic and geomorphic information. The three ratios, 3/1, 5/7, and 5/4 of the four-component technique are selected for their sensitivity to lithologic variables, as previously described, and for their lack of statistical redundancy (Crippen, 1989, Crippen et al., 1988). In the arid to semi-arid regions in which we studied (Mojave desert of California and Arizona, NE Baja California, and Durango, Mexico), these ratios generally are directly related to the presence of ferric iron (3/1), ferrous iron (5/4) and clays, carbonates and hydroxyl-bearing minerals, and vegetation (5/7). Adjustment of the data for atmospheric factors is suggested prior to calculation of the ratio images, otherwise significant distortions of the data, many of which are difficult to detect, may result (Crippen, 1989). Application of noise-removal routines, such as destriping (Crippen, 1989), is also beneficially applied to ratio images. The result of this processing is an image that depicts variations in iron content as variations in red, (3/1) and blue (5/4), and variations in hydroxyl-bearing minerals (and/or carbonates) as variations in green, (5/7). Typically, water is black, vegetation is green, desert varnish is blue, cinder cones are magenta, playa deposits are green if clay rich or red if silty, and hydrothermally altered areas are yellow. Many other rocks are depicted in blue, green, magenta, or white (Figure 12). Although this image contains both lithologic and alteration information that is extremely useful in geological reconnaissance, it is not the best image for either independent alteration or lithologic mapping. We have found that a threshold modified, four-component image (Figure 7) provides the best ratio alteration images, while a 7-4-1 color composite (Figure 10) is the best for general lithologic mapping. Assigning the highest digital numbers to three separate images performs the threshold modification, while pixels with intermediate to low values are nullified. A 5/7, 3/1, and a 5/7+3/1 combination was used, and was intended to highlight areas of hydroxyl-bearing minerals, iron-oxides and anomalous concentrations of both hydroxyls and iron-oxides respectively. These three images, 5/7, 3/1, and a 5/7+3/1 were then classified into green, red, and yellow opaque colors and draped over a SPOT or radar image (Figures 7 and 8). The specific areas of hydrothermal alteration are easily observed in the threshold images due to the sharp boundaries generated in Figures 7 and 8, as compared with Figure 12. Yellow pixels in Figures 7, 8, and 12 are anomalous concentrations of both hydroxyls and ironoxides that may be indicative of limonite, and/or pyritized sericite and/or pyritized argillic alteration. The yellow signature circled in red in Figures 7 and 8 is a hydrothermally-altered gold bearing breccia.
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4.0 Radar
Synthetic Aperture Radar (SAR) differs from optical sensors in that optical systems, such as Landsat TM, are passive and rely on the electromagnetic energy generated from the sun to image the earth’ surface. Since optical data are collected at frequencies similar to what the s human eye perceives, they are unable to “see” in darkness or cloud cover. SAR alternatively is an active system that sends its own microwave energy down to earth. Microwaves allow for atmospheric penetration and, under certain conditions, the penetration of very dry sand or soil, ice, and vegetation canopies, allowing for exploration not otherwise attainable. This ability to penetrate clouds and vegetation canopies has established radar as a viable exploration tool around the mid-latitudes where near constant cloud cover exists and outcrops are few due to jungle cover. However, this study shows that radar can be very beneficial in arid to semiarid terranes due to its ability to highlight subtle structures unobservable by optical data. Figure 11 is a color composite SIR-C image of the southeastern California, southwestern Arizona area. It was produced by assigning red, green, and blue to C band (6-cm wavelength) horizontally transmitted and horizontally received, C band horizontally transmitted and vertically received, and L band (24 cm wavelength) horizontally transmitted and horizontally received, respectively. The look angle is to the northeast with an incidence angle of 44 degrees. This highlights topographic and roughness features that are northwest striking, and inclined toward southeast or the look angle. The color differences are a consequence of topographic changes, moisture content, and surface roughness. The most important feature in this image is the northeast-trending topographic low between the red arrows. This feature is interpreted to be a transfer fault related to the late stage development of an accommodation zone structure that trends for approximately 60 km and cuts between the two major orebodies that comprise of the Mesquite mine. Figure 8 is an L band horizontally transmitted and horizontally received SIR-C gray scale image with a Landsat alteration drape. The radar-alteration merge provides an effective way to locate structurally controlled hydrothermal fluids associated with mineralization.
5.0 Results and Conclusions
The use of digitally enhanced optical and radar data has proven to provide profound exploration insights when interactively used by the field geologist. This integration of interactively used imagery data with a regional understanding of extensional terranes and ore genesis has already provided new opportunities for LCI, and, potentially in time, the entire mining industry as a result of this much valued ARC study. This study has provided a new method for LCI to efficiently inspect large areas of interest for mineral potential by using a straightforward yet sophisticated procedure developed by the highly knowledgeable members involved from the SDSU Geology and Geography Departments. The computer and remote sensing training provided by SDSU and NASA were recognized to be extremely beneficial to LCI whereby they were immediately adopted and integrated into
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ongoing exploration programs conducted concurrently with the ARC program. This proved to be extremely advantageous and resulted in multiple business accomplishments: • A greater than 50% success rate in the recognition of hydrothermal systems through Landsat TM alteration mapping was accomplished as compared to a previous less than 10% success rate from prior “hard copy interpretations” of the same general area. • In a separate area of interest, Landsat TM interpretations resulted in the discovery of two virgin mineral systems that where targeted from the mapping of five full Landsat TM scenes. • These successes have furthered an established relationship between LCI and SDSU, and many students have expressed an enthusiastic interest in working with remotely sensed data in an exploration mode. Besides Steve Polis, who represented LCI in this study and completed his thesis on the same topic ultimately leading to a related career, other SDSU graduate students are now working with LCI with similar aspirations. One of these students has already demonstrated the utility of multispectral thermal infrared imagery from the NASA Stennis ATLAS system in discriminating mineral alteration. This is viewed as a positive relationship whereby LCI can benefit from ongoing related academic research, while SDSU students and faculty can stay abreast with industry needs to provide future geoscientists to find the much needed natural resources that the world demands.
6.0 References
Anderson, R. E., 1971, Thin skin distension in Tertiary rocks of southeastern Nevada: Geological Society of America Bulletin, v. 82, p 43-58. Bosworth, W., Lambiase, J., and Keislar, R., 1986, A new look at Gregory’ rift: The s structural Style of continental rifting: EOS (American Geophysical Union Transactions), v. 67, p. 577- 583. Crippen, R. E., 1989, Development of remote sensing techniques for the investigation of neotectonic activity, eastern Transverse Ranges and vicinity, southern California, Ph.D. thesis, Univ. of Calif., Santa Barbara, 304p., 1989b. Crippen, R. E., R. G. Blom, and J. R. Heyada,1988, Directed band ratioing for the retention of perceptually-independent topographic expression in chromaticity-enhanced imagery, International Journal of Remote Sensing, 9, 749-765. Crippen, R. E., E. J. Hajic, J. E. Estes, and R. G. Blom,1990, Statistical band and band-ratio selection to maximize spectral information in color composite displays, in preparation for submission to International Journal of Remote Sensing.
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Faulds, J. E., Mawar, C. K., and Gaisaman, J. W., 1987, Possible modes of deformation along "accommodation zones" in rifted continental crust: Geological Society of America Abstracts with Programs, v.19, p.659-660. Frost, D.M., 1990, Gold ore has distinctive lead isotopic "fingerprint": Geological Society of America Abstracts with Programs, v.22, no.3, p.24. Frost and Heidrick, 1996, Tertiary extension and mineral deposits, Southwestern United States: Society of Economic Geologists, v. 25, p. 26-37. Goetz, F. H., F. C. Billingsley, A. R. Gillespie, M. J. Abrams, R. L. Squires, E. M. Shoemaker, I. Lucchitta, and D. P. Elston,1975, Application of ERTS images and image processing to regional problems and geological mapping in northern Arizona, JPL Technical Report 32-1S97. Lister, G. S., Etheridge, N. A., and Symonds, P. A., 1986, Detachment faulting and the evolution of passive continental margins: Geology, v. 14, p. 246-250. Loughtin, W. P., 1990. Geological exploration in the western United States by use of airborne scanner imagery. ERIM Conference: Remote Sensing, an Operational Technology for the Mining and Petroleum Industries. London, 29-31 Oct., pp. 22~241.
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Figure 1. Tertiary dip domain map of the southern Basin and Range Province. This map shows the strike and dip of tilted mid-Tertiary (35-15 Ma) sedimentary and volcanic rocks as summarized by Rebrig and Heidrick (1976, Fig. 4). Superimposed on the data is the tilt-block domain terminology proposed by Spencer and Reynolds (1989). The Province is divided somewhat proportionally into three mega-domains including the Lake Mead, Whipple, and San Pedro. Each of these domains can be traced along strike for 250-300 kilometers and covers between 30,000 and 35,000 square kilometers. These domains are separated along complex lateral transfer and accommodation zones. (Frost and Heidrick, 1996)
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Figure 2. Generalized geologic map of the northern part of the Colorado River Trough and adjacent region. The area of the Colorado River trough is surrounded to the west, north, and east by large zones showing only minor amounts of extension at exposed crustal levels. Within the Colorado River extensional corridor, however, stretching factors (B) vary between 1.5 and 2.5. The boundary separating the WSW-tilted Whipple domain from the ENE-tilted Lake Mead domain is referred to as the Whipple-Lake Mead “Accommodation Zone.” Data modified after Faulds et al. (1988). (Frost and Heidrick, 1996)
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Figure 3. Diagrammatic representation of opposite polarity tilt patterns in extensional terranes as separated by a strike-slip or transfer fault (A) or an accommodation zone (B). (A) is a drawing of the model of Liggett and Ehrenspeck (1973), which was developed for this region to explain the interrelationship between extension, tilts, and strike-slip faulting. (B) shows how opposite polarity tilt domains can be produced using opposite-tilted detachment faults separated by an accommodation zone, which is a model suggested for African rifts by Bosworth (1985). Domains in this model are linked by the accommodation zone, which is almost a null zone of apparent surface deformation rather than a strike-slip fault. (Frost and Heidrick, 1996)
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Figure 4. Geometric and kinematic characteristics of Neogene extensional deformation, Colorado River extensional corridor, NV, AZ, and CA. (Frost and Heidrick, 1996)
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Figure 5. Detachment fault-fold geometry and deep-crustal structure, Colorado River extensional terrane, as based on CALCRUST and reprocessed industry seismic lines. Multiple normal faults descend into middle-crustal ductile zone and offset early-formed mylonitic zone. Active mylonitic zone remains sub-horizontal (parallel to earth's surface). As normal faults offset ductile fabric, exhumation of once middle-crustal rock is a product of the offset on the normal faults and tilting over of the bounding normal faults. Extensional fabric traced westward from the Whipple terrane extends, perhaps somewhat discontinuously, to the Central Mojave detachment terrane mapped by workers such as Roy Dokka and Allen Glazner. (Frost and Heidrick, 1996)
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Figure 6. Diagrammatic model of crustal extension showing truncation of upper-plate normal faults at depth into a gently inclined detachment fault. Just as the faults are truncated at depth, they are truncated along strike by the wave-like, or fluted detachment surface. Such truncation of the upper-plate fault panels is readily visible on TM and radar images and can identify the presence and geometry of the major detachment faults. (Frost and Heidrick, 1996)
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Figure 7. A SPOT-Landsat TM ratio threshold merge of the area around the southern Chocolate Mountains illustrating extensional antiforms and areas of potential hydrothermal alteration highlighted in yellow.
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Figure 8. A SIR-C radar Landsat TM threshold merge of the area around the Mesquite mine. This image highlights the hydrothermal alteration from the TM data, as well as the antiformal-synformal geometry, dipslopes, and fracture patterns from the radar.
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Figure 9. An over-simplified structural model depicting the Mesquite mine located within a newly interpreted accommodation zone.
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Figure 10. Landsat TM 741 color composite image of the southern Colorado River illustrating extensional faults and the newly interpreted accommodation zone.
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Figure 11. A SIR-C radar color composite with interpreted Tertiary upper-plate transport directions and accommodation zone structure illustrated. The composite is produced by assigning red, green, and blue to C band (6-cm wavelength) horizontally transmitted and horizontally received, C band horizontally transmitted and vertically received, and L band (24-cm wavelength) horizontally transmitted and horizontally received, respectively. The look angle is to the northeast with an incidence angle of 44 degrees.
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Figure 12. A Landsat TM ratio color composite of the area around the Mesquite mine. The image depicts variations in iron content as variations in red, (3/1) and blue (5/4), and variations in hydroxyl-bearing minerals (and/or carbonates) as variations in green, (5/7).
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Appendix A. Technical Proposal
National Aeronautics and Space Administration ARC PROJECT SUMMARY
Project Title: Integrated Use of Remote Sensing and GIS for Mineral Exploration.
Technical Abstract La Cuesta International is a San Diego, California-based mineral exploration firm specializing in precious metal ore deposit exploration in the United States, Mexico, and Latin America. The proposed project is to develop the procedures and demonstrate the feasibility of using broad-band and hyperspectral remotely sensed data to identify extensional geologic structures associated with precious metal deposition. The resulting procedure will provide the basis for making available a new exploration service for the mining industry. Recognition of major gold deposits formed in association with Tertiary crustal extension in the western U.S. has been established and similar occurrences are now being recognized globally. Current mineral exploration concepts have failed to recognize the association of mineralization with unique extensional structures called accommodation zones. These zones, described below, show little obvious deformation, yet focus fluid migration and mineralization into predictable regions of the crust. Integrating remotely sensed data with existing geologic data provides a unique opportunity to identify the location of these previously unrecognized zones. Guided by an understanding of accommodation zones, remotely sensed data would be utilized to locate appropriate structural targets, which would then be inspected with hyperspectral data and ground verification, to establish the viability of the target area. This procedure is not a simple cookbook process for companies like La Cuesta who are familiar with the geology, but unfamiliar with remote sensing and spatial information technologies. La Cuesta is highly motivated to work with remote sensing and recognizes the vast potential it offers to the mineral exploration profession. Accommodation Zones: In areas of crustal extension, the crust breaks along a multitude of normal faults, commonly termed an “array,” with different segments having different transport directions as in Figure A-1. These segmented sections are at a scale of ~50 to 300 km. Within these extensional terranes, transport of major regions is in one uniform direction. However, zones with opposite transport generally exist in adjacent domains. Between these zones of opposite transport, some zone of deformation must exist to allow opposite motion to occur during the same deformation phase. These zones have been called “accommodation zones,” and have only recently been recognized within extensional systems on a worldwide basis.
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Accommodation zones link the normal fault arrays of opposing transport directions. These regions are actually zones of little obvious deformation, appearing not as strike-slip faults but brecciated zones because the entire volume of rock has been affected. Because accommodation zones represent areas of vergence reversal within extensional terranes at the up-dip tips of the regional faults (Figure 1), they focus fluid migration and mineralization into predictable regions of the crust. The Nelson District in southern Nevada is an excellent example where alteration and mineralization occur within an accommodation zone. The zone is between two opposite facing regions of extensional transport which can be discerned on the regional tectonic maps and point directly to major gold mineralization. The three dimensionality of extended crust has been well documented by petroleum industries through high-quality, three-dimensional seismic investigations. These seismic studies have demonstrated the presence of accommodation structures in nearly all the extensional terranes in the world. Within these zones, mineralized fluids flow toward and saturate portions of the accommodation zones. Knowledge of how accommodation zone tectonics localizes fluid flow processes allows researchers to target discrete areas for detailed exploration within the much larger extensional terrane. The association of accommodation zones with mineralization is due to three main factors; 1. Accommodation zones are deep-crustal breaks that often become syntectonic volcanic centers because they localize the magmatic material, thus becoming an elongate volcanic and plutonic center that intrudes out from existing fault zones and provides the thermal dynamic energy to drive mineralized fluids. 2. There is an increase in brecciation and the number of faults with a net decrease in the average fault slip within the accommodation zone, making the faulting more subtle but opening much larger volumes of extended rock. This provides excellent long-term permeability for multi-generational fluid flow and subsequent mineralization. 3. The geometry is such that a localized compressional stress regime forms anticlinal culminations located structurally up-dip from the normal faults it separates or “accommodates” on either side. This extensional geometry provides a flow-path from multiple normal faults toward the accommodation zones where mineralization can occur repeatedly as the faults continue to break through time. Broad-band remotely sensed image processing provides a powerful method to search for accommodation zones. Because the aerial extent of opposite dip domains is so large, the location of accommodation zones is easily missed by traditional methods of looking only at the field data and large-scale maps. Because recognition of opposite dip domains has not been part of traditional field investigation methods, many of the geologic maps and their syntheses are simply inadequate to discern accommodation zone structures. Remote sensing literature documents the capability of broad-band airborne and satellite imagery to detect geologic structure and in some instances, hydrothermally altered areas. Broad-band imagery shows geologic structure and enables the geologist to synthesize the tectonics and also discern the locations of hydrothermal alteration. By studying the linkage
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between alteration and accommodation zones, exploration targets for analysis can be identified. Unfortunately, broad-band data cannot distinguish individual “indicator” minerals required to further evaluate target areas. Hyperspectral sensor resolution allows the identification of many indicator minerals based upon their characteristic narrow absorption bands. However, by combining broad-band and hyperspectral data into an integrated collection and analysis method, broad-band data can be used to identify structural features which in combination with traditional geologic data can indicate the presence of accommodation zones, while hyperspectral data can provide the ability to detect hydrothermal alteration and indicator minerals. Image analysis coupled with an understanding of the tectonic processes and significance of the localized alteration provides a powerful tool for exploration. The suggested program would involve several stages. First, existing geologic and geochemical data would be assembled and entered into a geographic information system (GIS) as required. Broad-band remotely sensed imagery would be acquired and in conjunction with GIS-processed geologic data be analyzed to define regional areas likely to contain accommodation zones. Hyperspectral data would be acquired for accommodation zone target areas and analyzed to determine the presence of hydrothermal alteration and orebody indicator minerals. Field surveys may be required to refine remote sensing discrimination signatures to improve detection and refine spatial distribution. Geologic, geochemical, fault, gravity and stream-sediment maps will be obtained from state and Federal sources by La Cuesta. Broad band imagery will be acquired from SDSU archival sources. Selected hyperspectral data from existing NASA (JPL) archives will be requested from NASA ARC. GIS and remote sensing software and computing support will be provided by SDSU. Technical guidance and assistance in developing the data integration and analysis procedures will be provided by SDSU with occasional consultations with NASA remote sensing specialists. La Cuesta will commit a full-time geologist to work with SDSU. La Cuesta principals and technical specialists will be available to participate with SDSU staff as appropriate.
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Figure A-1. Diagrammatic representation of opposite polarity tilt patterns in extensional terranes as separated by a strike-slip or transfer fault (A) or an accommodation zone (B). (A) is a drawing of the model of Liggett and Ehrenspeck (1973), which was developed for this region to explain the interrelationship between extension, tilts and strike-slip faults. (B) shows how opposite polarity tilts domains can be produced using opposite-tilted detachment faults separated by an accommodation zone, which is a model suggested for African rifts by Bosworth (1985). Domains in this model are linked by the accommodation zone, which is almost a null zone of apparent surface deformation rather than a strike-slip fault.
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Appendix B. Commercial Proposal
National Aeronautics and Space Administration ARC PROJECT SUMMARY Project Title: Integrated Use of Remote Sensing and GIS for Mineral Exploration Commercial Applications Remotely-sensed imagery is a powerful tool for mineral exploration when properly utilized. Unfortunately, many geologists are skeptical of the use of remote sensing products because of previous false “positive” indicators which resulted in “chasing” spectral anomalies. This is largely due to the gap in information integration between geologists and the remote sensing community. The need for understanding regional geology and structure is critical for remote sensing to be a fully effective exploration tool. La Cuesta feels that the existing resistance to using remote sensing products by geologists provides an excellent business opportunity to synthesize geologic understanding with image-processing and provide an improved and valuable exploration service for mining companies. Today’ computer technology has provided a method by which geologists can use remote s sensing and GIS software to integrate field knowledge, structural geology, and remote sensed imagery. NASA’ Affiliated Research Center (ARC) program provides an excellent s opportunity for La Cuesta to demonstrate the practical application of the approach as an improved method of mineral exploration and a basis for developing future exploration contracts with the mining industry worldwide.
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Appendix C. Schedule
Integrated Remote Sensing and GIS VIP Project Schedule
1997
Project Tasks
Prepare MOA
2
Jun
Jul
Aug
Sep
Oct
Nov
Dec
12
Data Collection
8 17
Software Training
8 19
Broad Band Analysis
30 31
Progress Assessment
6
Hyperspectral Training
23 30
Data Integration & Analysis
20 31
Final Report Preparation
25 22
Project Evaluation
4
25
Affiliated Research Center
FINAL REPORT
Integrated Use of Remote Sensing and GIS for Mineral Exploration
La Cuesta International, Inc.
La Cuesta International, Inc.
San Diego State University Commercial Remote Sensing Program, National Aeronautics and Space Administration
Affiliated Research Center Final Report
Integrated Use of Remote Sensing and GIS for Mineral Exploration: A Project of the NASA Affiliated Research Center at San Diego State University
Project conducted by:
La Cuesta International, Inc. 1805 Wedgemere Road El Cajon, California 92020
Report prepared by:
Mr. W. Perry Durning La Cuesta International, Inc. and Mr. Stephen R. Polis and Dr. Eric G. Frost, Department of Geologic Sciences, San Diego State University and Mr. John V. Kaiser Department of Geography, San Diego State University
Report prepared for:
Dr. Douglas A. Stow, Principal Investigator San Diego State University Department of Geography San Diego, California 92182-4493 and Commercial Remote Sensing Program Office National Aeronautics and Space Administration John C. Stennis Space Center, Mississippi 39529
January 20, 1998
EXPORT ADMINISTRATION REGULATIONS NOTICE
This document contains information within the purview of the Export Administration Regulations (EAR), 15 CFR 730-744, and is export controlled. It may not be transferred to foreign nationals in the U.S. or abroad without specific approval of a knowledgeable NASA export control official, and/or unless an export license/license exception is obtained/available from the Bureau of Export Administration (BXA), United States Department of Commerce. Violations of these regulations are punishable by fine, imprisonment, or both.
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Table of Contents
EXPORT ADMINISTRATION REGULATIONS NOTICE .................................................. iii Executive Summary ................................................................................................................. vi 1.0 Introduction..........................................................................................................................1 2.0 Structural Mapping ..............................................................................................................2 3.0 Alteration Mapping..............................................................................................................3 4.0 Radar ....................................................................................................................................5 5.0 Results and Conclusions ......................................................................................................5 6.0 References............................................................................................................................6 Appendix A. Technical Proposal .............................................................................................20 Appendix B. Commercial Proposal .........................................................................................24 Appendix C. Schedule .............................................................................................................25
Figures
Figure 1. Tertiary dip domain map of the southern Basin and Range Province. ......................8 Figure 2. Generalized geologic map of the northern part of the Colorado River Trough and adjacent region. ..........................................................................................................................9 Figure 3. Diagrammatic representation of opposite polarity tilt patterns in extensional terranes as separated by a strike-slip or transfer fault (A) or an accommodation zone (B). ....10 Figure 4. Geometric and kinematic characteristics of Neogene extensional deformation, Colorado River extensional corridor, NV, AZ, and CA. (Frost and Heidrick, 1996)..............11 Figure 5. Detachment fault-fold geometry and deep-crustal structure, Colorado River extensional terrane, as based on CALCRUST and reprocessed industry seismic lines...........12 Figure 6. Diagrammatic model of crustal extension showing truncation of upper-plate normal faults at depth into a gently inclined detachment fault. ...........................................................13 Figure 7. A SPOT-Landsat TM ratio threshold merge of the area around the southern Chocolate Mountains illustrating extensional antiforms and areas of potential hydrothermal alteration highlighted in yellow. ..............................................................................................14 Figure 8. A SIR-C radar Landsat TM threshold merge of the area around the Mesquite mine.15 Figure 9. An over-simplified structural model depicting the Mesquite mine located within a newly interpreted accommodation zone...................................................................................16
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Figure 10. Landsat TM 741 color composite image of the southern Colorado River illustrating extensional faults and the newly interpreted accommodation zone. .....................17 Figure 11. A SIR-C radar color composite with interpreted Tertiary upper-plate transport directions and accommodation zone structure illustrated. .......................................................18 Figure 12. A Landsat TM ratio color composite of the area around the Mesquite mine. .......19 Figure A-1. Diagrammatic representation of opposite polarity tilt patterns in extensional terranes as separated by a strike-slip or transfer fault (A) or an accommodation zone (B). ....23
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Executive Summary
The Affiliated Research Center (ARC) program was conducted with La Cuesta International, Inc. (LCI) and supported by San Diego State University (SDSU). The purpose of the program was to develop the procedures and demonstrate the feasibility of using broad-band and hyperspectral, remotely sensed data to identify extensional geologic structures (accommodation zones) associated with precious/base metal deposition. In most cases, current mineral exploration concepts have failed to recognize the association of mineralization with unique extensional structures called accommodation zones. These zones show little obvious deformation, yet focus fluid migration and mineralization into predictable regions of the crust. The Mesquite gold mine, located in southeastern California, approximately 60 km east of the Salton Sea, was studied to determine if it resides within an unrecognized accommodation zone. Landsat Thematic Mapper (TM), Satellite Pour l'Observation de la Terre (SPOT), and radar data were observed both separately and in a merged format to extract spectral and spatial information using ER Mapper software. A variety of images were produced to highlight important structural features along with areas of hydrothermal alteration. Images produced include Landsat TM and Shuttle Imaging Radar-C (SIR-C) radar color composites, color ratio composites, principal components, thresholds and Landsat TM-SPOT and Landsat TM-SIR-C radar merges. Hyperspectral data (Advanced Visible/Infrared Imaging Spectrometer (AVIRIS) and Airborne Terrestrial Applications Sensor (ATLAS)) where obtained though not processed, because the data did not cover the study area. The ARC project proved to be extremely beneficial in training LCI with remote sensing procedures that resulted in the following: • The recognition of an accommodation zone within which the Mesquite gold mine resides. • The establishment of a template to identify hydrothermally altered areas from Landsat TM data. Prior studies in a non-ARC area showed less than 10% of TM anomalies were related to hydrothermal alteration. Using the ARC template greater than 50% of the TM anomalies checked in the field showed hydrothermal alteration. • The discovery of two virgin mineral systems in another non-ARC exploration area as a direct result of Landsat TM data interpretations using the template described above. Thus, this study has provided an immediate positive impact for LCI by providing a template to process TM imagery to successfully evaluate large areas of interest for mineral potential, focus field evaluations, and ultimately provide a higher probability for exploration success.
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1.0 Introduction
Recognition of major gold deposits formed in association with Tertiary crustal extensions in the western U.S. has been established and similar occurrences are now being recognized globally. In most cases, current mineral exploration concepts have failed to recognize the association of mineralization with unique extensional structures called accommodation zones. These zones, described below, show little obvious deformation, yet focus fluid migration and mineralization into predictable regions of the crust. Integrating remotely sensed data with existing geologic data provides a unique opportunity to identify the location of these previously unrecognized zones. Guided by an understanding of accommodation zones, Landsat TM, SPOT, and radar data were utilized to locate an unrecognized accommodation zone in which the Mesquite gold mine is located. Further remote sensing efforts made possible by insights developed through this ARC effort support the association between accommodation zones and precious/base metal deposition.
Accommodation Zones
In areas of crustal extension, the crust breaks along a multitude of normal faults, commonly termed an "array," with different segments having different transport directions as shown in Figures 1, 2, and 3. These segmented sections occur at a scale of 50 - 500 km along strike. Within these extensional terranes, transport of major regions is in one uniform direction (Bosworth et al., 1986; Lister et al., 1986). However, zones with opposite transport generally exist in adjacent domains. Between these zones of opposite transport, a zone of deformation must exist to allow opposite motion to occur during the same deformational phase. These zones have been called "accommodation zones," and have only recently been recognized within extensional systems on a worldwide basis. Accommodation zones link the normal fault arrays of opposing transport directions. These regions are often zones of little obvious deformation, appearing not as strike-slip faults, but brecciated “null” zones because the entire volume of rock has been affected (Anderson, 1971; Bosworth et al., 1986). Because accommodation zones represent areas of vergence reversal within extensional terranes at the up-dip tips of the regional faults (Figures 3 and 4), they focus fluid migration and mineralization into predictable regions of the crust. The Nelson District in southern Nevada is an excellent example where alteration and mineralization occur within an accommodation zone (Faulds et al., 1987; Frost and Heidrick, 1996). This zone is between two opposite facing regions of extensional transport that can be discerned on the regional tectonic maps and appears to point directly to the major gold mineralization. The three-dimensionality of extended crust has been well documented by researchers in the petroleum industries through high-quality, three-dimensional seismic investigations. These seismic studies have demonstrated the presence of accommodation structures in nearly all the extensional terranes in the world. Within these zones, fluids flow toward and saturate portions of the accommodation zones. Knowledge of how accommodation-zone tectonics localize fluid flow allows researchers to target discrete areas for detailed exploration within the much larger extensional terrane.
1
The association of accommodation zones with mineralization is due to three main factors: 1. Accommodation zones are deep-crustal breaks that often become syntectonic volcanic centers because they localize the magmatic material, thus becoming an elongated volcanic and plutonic center that intrudes existing fault zones and provides the thermodynamic energy to drive mineralized fluids. 2. There is an increase in brecciation and the number of faults with a net decrease in the average fault slip within the accommodation zone, making the faulting more subtle but opening much larger volumes of extended rock. This provides excellent long-term permeability for multigenerational fluid flow and subsequent mineralization. 3. The geometry is such that a localized compressional stress regime forms anticlinal culminations located structurally up-dip from the normal faults it separates or "accommodates" on either side. This extensional geometry provides a flowpath from multiple normal faults toward the accommodation zones where mineralization can occur repeatedly as the faults continue to break through time. Combining optical and radar remote sensing and image processing provides a powerful approach to search for accommodation zones. Because the areas of opposite dip domains are so large, interpreting field data and large scale maps easily misses the location of accommodation zones. Since the recognition of opposite dip domains has not been part of field investigation methodologies before, many of the geologic maps and their synthesis are simply inadequate to discern accommodation zone structures. Optical and radar data show geologic structure and enable the geologist to synthesize the tectonics and also discern the potential locations of hydrothermal alteration. This image analysis, coupled with an understanding of the tectonic processes and significance of the localized alteration, provides a powerful tool for exploration.
2.0 Structural Mapping
The antiformal-synformal character of the detachment fault system is one of the best ways of finding unrecognized large-scale normal faults and determining where accommodation zones might be found. Because of the regional perspective provided by the images and the display of spectral and topographic data with optical and radar images, the antiformal-synformal character of ranges can be readily discerned. In most areas, the long axis of the antiforms is parallel to the upper plate transport direction, much like megamullion structures elongated in the transport direction (Figures 5 and 6). These mullion structures appear to have a fairly consistent orientation on a regional scale and appear to be more pronounced as more relative motion has taken place on the fault structures. An obvious cause of this relationship is that the once moderate angle faults with their fluted fault patterns have tilted over more and more, making the mullions into whale-like, antiformal highs and trough-like synformal lows. Due to the regional nature of these distinctive antiformal-synformal features, optical and radar imaging provides feature recognition for these targets, and enables potential hydrothermally saturated antiforms to be highlighted. Figure 7 is a SPOT- Landsat TM ratio
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threshold merge of the area around the southern Chocolate Mountains. The image illustrates the ability to map and target extensional antiforms and areas of potential hydrothermal alteration highlighted in yellow. The strongest alteration signature in the image is along the detachment fault antiform located closest to the Mesquite mine and the Mount Barrow pluton responsible for the Mesquite gold mineralization (Frost, 1990). By changing the SPOT backdrop to radar (Figure 8) and keeping the Landsat TM alteration data, the topography is readily observed as it highlights antiformal-synformal geometry, as well as dipslopes and fracture patterns. By mapping a larger area, the Mesquite mine was discovered to be located within an accommodation zone (Figure 9). The southern Chocolate Mountains dip to the southwest, and are interpreted to have had a Tertiary northeast upper-plate transport direction, while the Trigo Mountains, located to the northeast across the Colorado river in Arizona, have had an opposite or southwest Tertiary upper-plate transport direction. The Mesquite mine is positioned at the southwest termination of the accommodation zone between these two dip domains, which is characterized by a northeasterly striking topographic low. This accommodation zone was first observed through a Landsat TM Bands 7-4-1 color composite image where volcanic dipslopes, extensional faults and dikes, and the general geometry of the ranges were mapped (Figure 10). Radar data used to image the structure highlighted the linear topographic low of this accommodation zone that trends through the Mesquite mine and continues northeasterly for more than 60 km (Figure 11).
3.0 Alteration Mapping
The goals for producing alteration images for the ARC were to optimally depict all spectral properties that may be related to alteration and then prioritize all targets for field evaluation. There are generally two common types of images used to map hydrothermal alteration: ratios and select principal components analysis (Loughlin, 1991). In this study, ratio images were combined with SPOT or radar data to enhance the structural geology that ultimately controls the areas of mineralization.
Landsat TM Ratios
Band ratioing is a technique that has been used for many years in remote sensing to effectively display spectral variations (e.g., Goetz et al., 1975). Properly computed band ratio images display little topographic or geomorphic information because the ratio of reflectivity of any two bands for a given material is not a function of illumination. Thus, the distinction between foreslopes and backslopes is lost, while spectral contrasts are enhanced. There are many types of band ratio images, though a “threshold-modified” four-component technique (Crippen, 1989) provided the best results of any ratio combination used for alteration mapping in the arid to semi-arid terranes of this study (Figure 7). Crippen’ four-component s technique uses three band-ratio images (one each for the red, green, and blue output channels) for the chromatic components of the image (Crippen et al., 1990). The technique then reintroduces the spatial detail using an achromatic SPOT image or TM band 4 that contains
3
spatial detail. Thus, in the final image, colors display spectral information while intensity primarily displays topographic and geomorphic information. The three ratios, 3/1, 5/7, and 5/4 of the four-component technique are selected for their sensitivity to lithologic variables, as previously described, and for their lack of statistical redundancy (Crippen, 1989, Crippen et al., 1988). In the arid to semi-arid regions in which we studied (Mojave desert of California and Arizona, NE Baja California, and Durango, Mexico), these ratios generally are directly related to the presence of ferric iron (3/1), ferrous iron (5/4) and clays, carbonates and hydroxyl-bearing minerals, and vegetation (5/7). Adjustment of the data for atmospheric factors is suggested prior to calculation of the ratio images, otherwise significant distortions of the data, many of which are difficult to detect, may result (Crippen, 1989). Application of noise-removal routines, such as destriping (Crippen, 1989), is also beneficially applied to ratio images. The result of this processing is an image that depicts variations in iron content as variations in red, (3/1) and blue (5/4), and variations in hydroxyl-bearing minerals (and/or carbonates) as variations in green, (5/7). Typically, water is black, vegetation is green, desert varnish is blue, cinder cones are magenta, playa deposits are green if clay rich or red if silty, and hydrothermally altered areas are yellow. Many other rocks are depicted in blue, green, magenta, or white (Figure 12). Although this image contains both lithologic and alteration information that is extremely useful in geological reconnaissance, it is not the best image for either independent alteration or lithologic mapping. We have found that a threshold modified, four-component image (Figure 7) provides the best ratio alteration images, while a 7-4-1 color composite (Figure 10) is the best for general lithologic mapping. Assigning the highest digital numbers to three separate images performs the threshold modification, while pixels with intermediate to low values are nullified. A 5/7, 3/1, and a 5/7+3/1 combination was used, and was intended to highlight areas of hydroxyl-bearing minerals, iron-oxides and anomalous concentrations of both hydroxyls and iron-oxides respectively. These three images, 5/7, 3/1, and a 5/7+3/1 were then classified into green, red, and yellow opaque colors and draped over a SPOT or radar image (Figures 7 and 8). The specific areas of hydrothermal alteration are easily observed in the threshold images due to the sharp boundaries generated in Figures 7 and 8, as compared with Figure 12. Yellow pixels in Figures 7, 8, and 12 are anomalous concentrations of both hydroxyls and ironoxides that may be indicative of limonite, and/or pyritized sericite and/or pyritized argillic alteration. The yellow signature circled in red in Figures 7 and 8 is a hydrothermally-altered gold bearing breccia.
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4.0 Radar
Synthetic Aperture Radar (SAR) differs from optical sensors in that optical systems, such as Landsat TM, are passive and rely on the electromagnetic energy generated from the sun to image the earth’ surface. Since optical data are collected at frequencies similar to what the s human eye perceives, they are unable to “see” in darkness or cloud cover. SAR alternatively is an active system that sends its own microwave energy down to earth. Microwaves allow for atmospheric penetration and, under certain conditions, the penetration of very dry sand or soil, ice, and vegetation canopies, allowing for exploration not otherwise attainable. This ability to penetrate clouds and vegetation canopies has established radar as a viable exploration tool around the mid-latitudes where near constant cloud cover exists and outcrops are few due to jungle cover. However, this study shows that radar can be very beneficial in arid to semiarid terranes due to its ability to highlight subtle structures unobservable by optical data. Figure 11 is a color composite SIR-C image of the southeastern California, southwestern Arizona area. It was produced by assigning red, green, and blue to C band (6-cm wavelength) horizontally transmitted and horizontally received, C band horizontally transmitted and vertically received, and L band (24 cm wavelength) horizontally transmitted and horizontally received, respectively. The look angle is to the northeast with an incidence angle of 44 degrees. This highlights topographic and roughness features that are northwest striking, and inclined toward southeast or the look angle. The color differences are a consequence of topographic changes, moisture content, and surface roughness. The most important feature in this image is the northeast-trending topographic low between the red arrows. This feature is interpreted to be a transfer fault related to the late stage development of an accommodation zone structure that trends for approximately 60 km and cuts between the two major orebodies that comprise of the Mesquite mine. Figure 8 is an L band horizontally transmitted and horizontally received SIR-C gray scale image with a Landsat alteration drape. The radar-alteration merge provides an effective way to locate structurally controlled hydrothermal fluids associated with mineralization.
5.0 Results and Conclusions
The use of digitally enhanced optical and radar data has proven to provide profound exploration insights when interactively used by the field geologist. This integration of interactively used imagery data with a regional understanding of extensional terranes and ore genesis has already provided new opportunities for LCI, and, potentially in time, the entire mining industry as a result of this much valued ARC study. This study has provided a new method for LCI to efficiently inspect large areas of interest for mineral potential by using a straightforward yet sophisticated procedure developed by the highly knowledgeable members involved from the SDSU Geology and Geography Departments. The computer and remote sensing training provided by SDSU and NASA were recognized to be extremely beneficial to LCI whereby they were immediately adopted and integrated into
5
ongoing exploration programs conducted concurrently with the ARC program. This proved to be extremely advantageous and resulted in multiple business accomplishments: • A greater than 50% success rate in the recognition of hydrothermal systems through Landsat TM alteration mapping was accomplished as compared to a previous less than 10% success rate from prior “hard copy interpretations” of the same general area. • In a separate area of interest, Landsat TM interpretations resulted in the discovery of two virgin mineral systems that where targeted from the mapping of five full Landsat TM scenes. • These successes have furthered an established relationship between LCI and SDSU, and many students have expressed an enthusiastic interest in working with remotely sensed data in an exploration mode. Besides Steve Polis, who represented LCI in this study and completed his thesis on the same topic ultimately leading to a related career, other SDSU graduate students are now working with LCI with similar aspirations. One of these students has already demonstrated the utility of multispectral thermal infrared imagery from the NASA Stennis ATLAS system in discriminating mineral alteration. This is viewed as a positive relationship whereby LCI can benefit from ongoing related academic research, while SDSU students and faculty can stay abreast with industry needs to provide future geoscientists to find the much needed natural resources that the world demands.
6.0 References
Anderson, R. E., 1971, Thin skin distension in Tertiary rocks of southeastern Nevada: Geological Society of America Bulletin, v. 82, p 43-58. Bosworth, W., Lambiase, J., and Keislar, R., 1986, A new look at Gregory’ rift: The s structural Style of continental rifting: EOS (American Geophysical Union Transactions), v. 67, p. 577- 583. Crippen, R. E., 1989, Development of remote sensing techniques for the investigation of neotectonic activity, eastern Transverse Ranges and vicinity, southern California, Ph.D. thesis, Univ. of Calif., Santa Barbara, 304p., 1989b. Crippen, R. E., R. G. Blom, and J. R. Heyada,1988, Directed band ratioing for the retention of perceptually-independent topographic expression in chromaticity-enhanced imagery, International Journal of Remote Sensing, 9, 749-765. Crippen, R. E., E. J. Hajic, J. E. Estes, and R. G. Blom,1990, Statistical band and band-ratio selection to maximize spectral information in color composite displays, in preparation for submission to International Journal of Remote Sensing.
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Faulds, J. E., Mawar, C. K., and Gaisaman, J. W., 1987, Possible modes of deformation along "accommodation zones" in rifted continental crust: Geological Society of America Abstracts with Programs, v.19, p.659-660. Frost, D.M., 1990, Gold ore has distinctive lead isotopic "fingerprint": Geological Society of America Abstracts with Programs, v.22, no.3, p.24. Frost and Heidrick, 1996, Tertiary extension and mineral deposits, Southwestern United States: Society of Economic Geologists, v. 25, p. 26-37. Goetz, F. H., F. C. Billingsley, A. R. Gillespie, M. J. Abrams, R. L. Squires, E. M. Shoemaker, I. Lucchitta, and D. P. Elston,1975, Application of ERTS images and image processing to regional problems and geological mapping in northern Arizona, JPL Technical Report 32-1S97. Lister, G. S., Etheridge, N. A., and Symonds, P. A., 1986, Detachment faulting and the evolution of passive continental margins: Geology, v. 14, p. 246-250. Loughtin, W. P., 1990. Geological exploration in the western United States by use of airborne scanner imagery. ERIM Conference: Remote Sensing, an Operational Technology for the Mining and Petroleum Industries. London, 29-31 Oct., pp. 22~241.
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Figure 1. Tertiary dip domain map of the southern Basin and Range Province. This map shows the strike and dip of tilted mid-Tertiary (35-15 Ma) sedimentary and volcanic rocks as summarized by Rebrig and Heidrick (1976, Fig. 4). Superimposed on the data is the tilt-block domain terminology proposed by Spencer and Reynolds (1989). The Province is divided somewhat proportionally into three mega-domains including the Lake Mead, Whipple, and San Pedro. Each of these domains can be traced along strike for 250-300 kilometers and covers between 30,000 and 35,000 square kilometers. These domains are separated along complex lateral transfer and accommodation zones. (Frost and Heidrick, 1996)
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Figure 2. Generalized geologic map of the northern part of the Colorado River Trough and adjacent region. The area of the Colorado River trough is surrounded to the west, north, and east by large zones showing only minor amounts of extension at exposed crustal levels. Within the Colorado River extensional corridor, however, stretching factors (B) vary between 1.5 and 2.5. The boundary separating the WSW-tilted Whipple domain from the ENE-tilted Lake Mead domain is referred to as the Whipple-Lake Mead “Accommodation Zone.” Data modified after Faulds et al. (1988). (Frost and Heidrick, 1996)
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Figure 3. Diagrammatic representation of opposite polarity tilt patterns in extensional terranes as separated by a strike-slip or transfer fault (A) or an accommodation zone (B). (A) is a drawing of the model of Liggett and Ehrenspeck (1973), which was developed for this region to explain the interrelationship between extension, tilts, and strike-slip faulting. (B) shows how opposite polarity tilt domains can be produced using opposite-tilted detachment faults separated by an accommodation zone, which is a model suggested for African rifts by Bosworth (1985). Domains in this model are linked by the accommodation zone, which is almost a null zone of apparent surface deformation rather than a strike-slip fault. (Frost and Heidrick, 1996)
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Figure 4. Geometric and kinematic characteristics of Neogene extensional deformation, Colorado River extensional corridor, NV, AZ, and CA. (Frost and Heidrick, 1996)
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Figure 5. Detachment fault-fold geometry and deep-crustal structure, Colorado River extensional terrane, as based on CALCRUST and reprocessed industry seismic lines. Multiple normal faults descend into middle-crustal ductile zone and offset early-formed mylonitic zone. Active mylonitic zone remains sub-horizontal (parallel to earth's surface). As normal faults offset ductile fabric, exhumation of once middle-crustal rock is a product of the offset on the normal faults and tilting over of the bounding normal faults. Extensional fabric traced westward from the Whipple terrane extends, perhaps somewhat discontinuously, to the Central Mojave detachment terrane mapped by workers such as Roy Dokka and Allen Glazner. (Frost and Heidrick, 1996)
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Figure 6. Diagrammatic model of crustal extension showing truncation of upper-plate normal faults at depth into a gently inclined detachment fault. Just as the faults are truncated at depth, they are truncated along strike by the wave-like, or fluted detachment surface. Such truncation of the upper-plate fault panels is readily visible on TM and radar images and can identify the presence and geometry of the major detachment faults. (Frost and Heidrick, 1996)
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Figure 7. A SPOT-Landsat TM ratio threshold merge of the area around the southern Chocolate Mountains illustrating extensional antiforms and areas of potential hydrothermal alteration highlighted in yellow.
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Figure 8. A SIR-C radar Landsat TM threshold merge of the area around the Mesquite mine. This image highlights the hydrothermal alteration from the TM data, as well as the antiformal-synformal geometry, dipslopes, and fracture patterns from the radar.
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Figure 9. An over-simplified structural model depicting the Mesquite mine located within a newly interpreted accommodation zone.
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Figure 10. Landsat TM 741 color composite image of the southern Colorado River illustrating extensional faults and the newly interpreted accommodation zone.
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Figure 11. A SIR-C radar color composite with interpreted Tertiary upper-plate transport directions and accommodation zone structure illustrated. The composite is produced by assigning red, green, and blue to C band (6-cm wavelength) horizontally transmitted and horizontally received, C band horizontally transmitted and vertically received, and L band (24-cm wavelength) horizontally transmitted and horizontally received, respectively. The look angle is to the northeast with an incidence angle of 44 degrees.
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Figure 12. A Landsat TM ratio color composite of the area around the Mesquite mine. The image depicts variations in iron content as variations in red, (3/1) and blue (5/4), and variations in hydroxyl-bearing minerals (and/or carbonates) as variations in green, (5/7).
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Appendix A. Technical Proposal
National Aeronautics and Space Administration ARC PROJECT SUMMARY
Project Title: Integrated Use of Remote Sensing and GIS for Mineral Exploration.
Technical Abstract La Cuesta International is a San Diego, California-based mineral exploration firm specializing in precious metal ore deposit exploration in the United States, Mexico, and Latin America. The proposed project is to develop the procedures and demonstrate the feasibility of using broad-band and hyperspectral remotely sensed data to identify extensional geologic structures associated with precious metal deposition. The resulting procedure will provide the basis for making available a new exploration service for the mining industry. Recognition of major gold deposits formed in association with Tertiary crustal extension in the western U.S. has been established and similar occurrences are now being recognized globally. Current mineral exploration concepts have failed to recognize the association of mineralization with unique extensional structures called accommodation zones. These zones, described below, show little obvious deformation, yet focus fluid migration and mineralization into predictable regions of the crust. Integrating remotely sensed data with existing geologic data provides a unique opportunity to identify the location of these previously unrecognized zones. Guided by an understanding of accommodation zones, remotely sensed data would be utilized to locate appropriate structural targets, which would then be inspected with hyperspectral data and ground verification, to establish the viability of the target area. This procedure is not a simple cookbook process for companies like La Cuesta who are familiar with the geology, but unfamiliar with remote sensing and spatial information technologies. La Cuesta is highly motivated to work with remote sensing and recognizes the vast potential it offers to the mineral exploration profession. Accommodation Zones: In areas of crustal extension, the crust breaks along a multitude of normal faults, commonly termed an “array,” with different segments having different transport directions as in Figure A-1. These segmented sections are at a scale of ~50 to 300 km. Within these extensional terranes, transport of major regions is in one uniform direction. However, zones with opposite transport generally exist in adjacent domains. Between these zones of opposite transport, some zone of deformation must exist to allow opposite motion to occur during the same deformation phase. These zones have been called “accommodation zones,” and have only recently been recognized within extensional systems on a worldwide basis.
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Accommodation zones link the normal fault arrays of opposing transport directions. These regions are actually zones of little obvious deformation, appearing not as strike-slip faults but brecciated zones because the entire volume of rock has been affected. Because accommodation zones represent areas of vergence reversal within extensional terranes at the up-dip tips of the regional faults (Figure 1), they focus fluid migration and mineralization into predictable regions of the crust. The Nelson District in southern Nevada is an excellent example where alteration and mineralization occur within an accommodation zone. The zone is between two opposite facing regions of extensional transport which can be discerned on the regional tectonic maps and point directly to major gold mineralization. The three dimensionality of extended crust has been well documented by petroleum industries through high-quality, three-dimensional seismic investigations. These seismic studies have demonstrated the presence of accommodation structures in nearly all the extensional terranes in the world. Within these zones, mineralized fluids flow toward and saturate portions of the accommodation zones. Knowledge of how accommodation zone tectonics localizes fluid flow processes allows researchers to target discrete areas for detailed exploration within the much larger extensional terrane. The association of accommodation zones with mineralization is due to three main factors; 1. Accommodation zones are deep-crustal breaks that often become syntectonic volcanic centers because they localize the magmatic material, thus becoming an elongate volcanic and plutonic center that intrudes out from existing fault zones and provides the thermal dynamic energy to drive mineralized fluids. 2. There is an increase in brecciation and the number of faults with a net decrease in the average fault slip within the accommodation zone, making the faulting more subtle but opening much larger volumes of extended rock. This provides excellent long-term permeability for multi-generational fluid flow and subsequent mineralization. 3. The geometry is such that a localized compressional stress regime forms anticlinal culminations located structurally up-dip from the normal faults it separates or “accommodates” on either side. This extensional geometry provides a flow-path from multiple normal faults toward the accommodation zones where mineralization can occur repeatedly as the faults continue to break through time. Broad-band remotely sensed image processing provides a powerful method to search for accommodation zones. Because the aerial extent of opposite dip domains is so large, the location of accommodation zones is easily missed by traditional methods of looking only at the field data and large-scale maps. Because recognition of opposite dip domains has not been part of traditional field investigation methods, many of the geologic maps and their syntheses are simply inadequate to discern accommodation zone structures. Remote sensing literature documents the capability of broad-band airborne and satellite imagery to detect geologic structure and in some instances, hydrothermally altered areas. Broad-band imagery shows geologic structure and enables the geologist to synthesize the tectonics and also discern the locations of hydrothermal alteration. By studying the linkage
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between alteration and accommodation zones, exploration targets for analysis can be identified. Unfortunately, broad-band data cannot distinguish individual “indicator” minerals required to further evaluate target areas. Hyperspectral sensor resolution allows the identification of many indicator minerals based upon their characteristic narrow absorption bands. However, by combining broad-band and hyperspectral data into an integrated collection and analysis method, broad-band data can be used to identify structural features which in combination with traditional geologic data can indicate the presence of accommodation zones, while hyperspectral data can provide the ability to detect hydrothermal alteration and indicator minerals. Image analysis coupled with an understanding of the tectonic processes and significance of the localized alteration provides a powerful tool for exploration. The suggested program would involve several stages. First, existing geologic and geochemical data would be assembled and entered into a geographic information system (GIS) as required. Broad-band remotely sensed imagery would be acquired and in conjunction with GIS-processed geologic data be analyzed to define regional areas likely to contain accommodation zones. Hyperspectral data would be acquired for accommodation zone target areas and analyzed to determine the presence of hydrothermal alteration and orebody indicator minerals. Field surveys may be required to refine remote sensing discrimination signatures to improve detection and refine spatial distribution. Geologic, geochemical, fault, gravity and stream-sediment maps will be obtained from state and Federal sources by La Cuesta. Broad band imagery will be acquired from SDSU archival sources. Selected hyperspectral data from existing NASA (JPL) archives will be requested from NASA ARC. GIS and remote sensing software and computing support will be provided by SDSU. Technical guidance and assistance in developing the data integration and analysis procedures will be provided by SDSU with occasional consultations with NASA remote sensing specialists. La Cuesta will commit a full-time geologist to work with SDSU. La Cuesta principals and technical specialists will be available to participate with SDSU staff as appropriate.
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Figure A-1. Diagrammatic representation of opposite polarity tilt patterns in extensional terranes as separated by a strike-slip or transfer fault (A) or an accommodation zone (B). (A) is a drawing of the model of Liggett and Ehrenspeck (1973), which was developed for this region to explain the interrelationship between extension, tilts and strike-slip faults. (B) shows how opposite polarity tilts domains can be produced using opposite-tilted detachment faults separated by an accommodation zone, which is a model suggested for African rifts by Bosworth (1985). Domains in this model are linked by the accommodation zone, which is almost a null zone of apparent surface deformation rather than a strike-slip fault.
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Appendix B. Commercial Proposal
National Aeronautics and Space Administration ARC PROJECT SUMMARY Project Title: Integrated Use of Remote Sensing and GIS for Mineral Exploration Commercial Applications Remotely-sensed imagery is a powerful tool for mineral exploration when properly utilized. Unfortunately, many geologists are skeptical of the use of remote sensing products because of previous false “positive” indicators which resulted in “chasing” spectral anomalies. This is largely due to the gap in information integration between geologists and the remote sensing community. The need for understanding regional geology and structure is critical for remote sensing to be a fully effective exploration tool. La Cuesta feels that the existing resistance to using remote sensing products by geologists provides an excellent business opportunity to synthesize geologic understanding with image-processing and provide an improved and valuable exploration service for mining companies. Today’ computer technology has provided a method by which geologists can use remote s sensing and GIS software to integrate field knowledge, structural geology, and remote sensed imagery. NASA’ Affiliated Research Center (ARC) program provides an excellent s opportunity for La Cuesta to demonstrate the practical application of the approach as an improved method of mineral exploration and a basis for developing future exploration contracts with the mining industry worldwide.
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Appendix C. Schedule
Integrated Remote Sensing and GIS VIP Project Schedule
1997
Project Tasks
Prepare MOA
2
Jun
Jul
Aug
Sep
Oct
Nov
Dec
12
Data Collection
8 17
Software Training
8 19
Broad Band Analysis
30 31
Progress Assessment
6
Hyperspectral Training
23 30
Data Integration & Analysis
20 31
Final Report Preparation
25 22
Project Evaluation
4
25
