Digital Reconnaissance Geological Mapping and Sampling in the Remote, Harsh, Desert Terrain of Northern

Terry Arcuri and George Brimhall Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767, [email protected]

Abstract

Reconnaissance geological mapping and sampling were conducted in the of northern Chile in the summer of 1999. During this expedition, a new digital mapping system consisting of a pen computer with Geomapper software, a differential GPS, and a laser range finder were utilized. The harsh environment dictated that this system be highly portable, weather resistant, rugged, and stable for the collection of geological data and sample locations. Of personal concern were the ergonomics and weight of the system since it would be worn for eight hours at a time while hiking over rough, mountainous terrain. The GPS and laser were conveniently carried in a backpack along with all batteries, while the GPS antenna was set atop the backpack frame. The computer was slung over one shoulder while moving and connected to the GPS, which allowed for continuous mapping while hiking. When necessary, the laser range finder could be connected and used in areas where steep slopes and cliffs made a hiking approach impossible. This enabled the mapping of areas that would have otherwise been inaccessible out to a distance of approximately 300 meters, yielding valuable data. This distance was essentially the limit of the confident visual identification of geological features and the maximum working distance of the laser. Using digital topographic and geological base maps stored in the computer greatly aided in the determination of mapping sites and allowed for the plotting of sample locations directly on composite maps. These base maps were georeferenced prior to mapping by locating readily accessible, discernable positions using a Trimble AG-132 sub-meter GPS with real-time differential corrections provided by Omnistar. A large quantity of data was collected over a short period of time, which allowed a larger area to be mapped than previously expected. After each day of mapping, data was saved to a backup ZIP disk and the systems batteries were recharged. The following day, mapping would commence at the precise point where the previous days’ work had concluded. By careful mapping, specific strata could be correlated over many kilometers and samples could be taken from numerous locations to represent each lithology without concern of stratagraphic errors. This system facilitated the collection of large quantities of spatially accurate data that were crucial in understanding the geological significance of Jurassic-age formations. Location Map Longitude 71 70 69 68 67 66 65 20

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Figure 1. Location map of the study area within northern Chile. Mapping was focused in the region to the north and west of the town of Calama.

2 Introduction

The Jurassic sediments of northern Chile have been the subject of numerous studies examining the depositional environment (Ardill et al, 1994, 1998; Chong and Pardo, 1976; Gröschke et al, 1988; Harrington, 1961; Printz, 1986; and Von Hillebrandt et al., 1986) and the tectonic history of the region (Hartley et al, 2000; Printz et al, 1994). The conclusions of these works are that the Jurassic system consisted of a marine back-arc basin which experienced numerous transgressive-regressive events (Printz et al, 1994; Ardill et al, 1998). Evaporite minerals such as gypsum and halite were deposited in specific parts of this basin at various times throughout the Jurassic,(Ardill et al, 1998; Bell, 1989; Printz et al, 1994) culminating in massive, basin-wide deposition of gypsum in the late Oxfordian (~154 Ma) as a result of a final regressive event (Printz et al, 1994; Gröschke et al, 1986). Recent workers have indicated a potential link between these Jurassic evaporite minerals and economically important copper mineralization at the porphyry copper deposit (Arcuri and Brimhall, 2000). It is this linkage which motivated the current study to locate and sample evaporite mineral occurrences within the Jurassic sequence and correlate these occurrences throughout the region. Samples were collected for geochemical and isotopic analysis in order to examine their stratagraphic homogeneity as well as their genetic relationship to copper mineralization at the Radomiro Tomic porphyry copper deposit. Reconnaissance geological mapping and sampling were conducted in the Atacama desert of northern Chile in the summer of 1999 by T. Arcuri and G. Brimhall. Figure 1 shows the location of the study area in northern Chile with respect to major cities. The large area covered by this study dictated that time was an important factor in obtaining as much pertinent data as possible. In addition, traditional mapping methods would not have allowed for the precise locating of sample points, which was of fundamental importance to this project.

Objective

The objective of this mapping project was to identify and map Jurassic sedimentary sequences for lithologic variations and to correlate these variations between sections. Of particular importance were evaporite bearing strata which represented a potential source of sulfate and chloride. These anions could have affected nearby copper mineralization by being incorporated into a fluid phase interacting with a magmatic system. These specific evaporite lithologies were the targets of sampling efforts designed to investigate their distribution in the study region.

Challenges

Numerous challenges were encountered at the onset of this study: the size of the study area, the harsh environment of the Atacama desert, the need for accurately located sample sites, and the need to perform mapping and sampling in a timely manner. Once in the field it was important for the mapping system to be stable and dependable since most field sites were remote and isolated, generally over 50 kilometers from the nearest town.

3 If any component of the system were to break or the batteries were to lose their charge, an entire day of mapping would be lost. Figures 2 through 7 show the variety of desert terrain covered in the mapping campaign. Of note in these photographs is the complete lack of vegetation. In this region, rainfall is exceedingly sparse, with mean annual precipitation well below 1 cm per year. Isolated areas within the study region have experienced no precipitation throughout recorded human history. The lack of vegetation leads to near 100% exposure of bedrock lithologies, except where a layer of gravel and dust mantles outcrops, at times this layer can reach a thickness up to tens of centimeters.

Figure 2. Photograph looking north of the Cerros de Paqui, 30 Km north of Calama. The volcanos San Pedro and San Pablo can be seen in the distant right of the picture. The paved road in the foreground was built after the generation of the latest version of topographic maps.

Problems of sample location were encountered in areas such as those seen in figures 2 through 5 where the lack of terrain features and vast expanses of desert limited the precision of location determination using traditional topographic map techniques. Many man-made features such as roads and pipelines did not exist at the time the latest generation of topographic maps were generated, (ca.1972) making such features worthless for use in sample locating. Delineation of recently constructed features from those depicted on old topographic maps was not always possible in the field. In areas of rugged topography, (figures 6 and 7) the high relief limited access for sample collection and slowed the acquisition of geological data. In dangerous terrain, personal safety was an important issue which restricted access to certain areas. In these remote locations the ability to receive medical assistance should any accidents occur was highly doubtful.

4 Figure 3. Photograph looking due west from the Pampa Chuquicamata, 60 Km northwest of Calama. Mountains across the plain are Jurassic sediments while those in the far distance are dominantly volcanic. This road was constructed within the past two years and does not appear on any of the regional maps.

Figure 4. Photograph of a sample taken in the Pampa Cere, 40 Km north of Calama. Rock hammer in the foreground marks the location of the sample, note the pickup truck in the distance.

5 Figure 5. Photograph of a sample location in the Quebrada de Paqui, 60 Km northeast of Calama. Rock hammer and sample bag mark location of a sample taken from this large alluvial fan. Note the lack of terrain features that could help establish an accurate sample location.

Figure 6. Photograph taken from the top of the Cerros de Chuquicamata, 25 Km north of Calama. Note the pickup truck in the center of the picture for scale.

6 Figure 7. Photograph looking west at the Jurassic and Cretaceous sediments in the Sierra San Lorenzo, 50 Km northwest of Calama. Photograph shows the northeast dipping limb of an anticline, plunging to the southeast (to the right). The strata on the distant hillsides are dominated by Cretaceous volcanoclastics. This photo shows the location of the field example discussed in the test.

Solution

The harsh environments of the Atacama desert made it necessary that the mapping system be highly portable, weather resistant, and rugged. All of these features needed to be addressed in order to make a stable platform for the collection of geological data and sample locations. Personal concerns about the weight and ergonomics of the system were important because it would be used for eight hours at a time while hiking over rough, mountainous terrain. The solution for these mapping and sampling problems manifested in the utilization of a digital mapping system consisting of a pen computer with Penmap and Geomapper software, a differential GPS, and a laser range finder (figure 8). The GPS and laser were conveniently carried in a backpack along with all batteries, while the GPS antenna mounted to the top the backpack frame. The computer was slung over one shoulder and connected to the GPS, which allowed for continuous position acquisition while hiking. The laser range finder was connected and used in areas where steep slopes and cliffs made a hiking approach impossible or particularly dangerous. Inside the backpack, the GPS receiver, batteries, and laser were stored in an upper compartment, leaving the bottom portion available for carrying collected samples and supplies. With a complete daily supply of batteries and provisions, the entire system weighed between 25 and 30 pounds. To overcome the problems associated with the vast expanses and subdued topography, (figures 2 through 4) features and samples were located using GPS. The incorporation of a Global Positioning System proved to be very effective for mapping in

7 this type of environment. Due to the lack of cloud cover and obscuring terrain, numerous satellites were visible to the GPS antenna at all times, yielding sub-meter accuracy in position location.

Figure 8. Photograph of George Brimhall using the digital mapping system to record a map location in Quebrada Chug chug, 55 Km northwest of Calama. The backpack contains the GPS receiver, batteries, and a laser range finder. The GPS antenna is a 6-inch, yellow and white dome, mounted on the top of the frame. The system is controlled by the handheld, pen computer, which is easily carried in hand, or on a sling.

In areas of high relief and limited access,(figures 6 and 7) the incorporation of the laser range finder became a vital data collection device. To use the laser range finder, first a location was determined and labeled using the GPS, usually on a high visibility hilltop or ridgeline. After a position was chosen and determined, the laser was connected to the computer and configured to use the current location as a base. From that point it was then possible to fire the laser at features up to 300 meters distant and have them accurately displayed on the map as well as entered into the proper GIS database. Although the laser was accurate to 0.1 meters at greater distances, 300 meters was essentially the limit of the confident visual identification of geological features. The use of this method allowed for the accurate collection of data across large expanses of desert and hazardous terrain that would have otherwise been too dangerous or time consuming to access or traverse. The large scale of the structural features that needed to be mapped and sampled required a tremendous amount of data in order for them to be adequately represented in a graphic format. By using the laser strategically located in areas with good visibility, hours of hiking could be eliminated. This time-saving step allowed the operator to focus time in the field on sample collection instead of basic surveying to determine sample

8 locations. Due to the efficiency of this method, adjacent areas were incorporated into the mapping that were not originally intended, increasing the total mapped area by 30%. This expansion allowed for better characterization of the lithologies within the study area and yielded valuable information. The heart of the digital mapping system is a Fujitsu 2300 pen computer running Penmap software with Geomapper visual user interface (Brimhall and Vanegas, 2001). The computer is lightweight and has a transflective screen, which allows for easy viewing in the field. Transflective screens become brighter when viewed in direct sunlight and have the added benefit of using reflected sunlight instead of batteries to power the screen. This power saving is important because it decreases the number of batteries needed to complete a day of mapping and therefore decreases the weight carried by the operator.

Figure 9. Screen capture image of the configuration used in the mapping project in northern Chile. The various buttons allow the user to control many menu functions as well as shift between different GIS layers for plotting data. This configuration has the local geological formation as well as different lithologies as the utilized GIS layers.

The software enables the operator to develop mapping legends and GIS layers that are specific to a particular mapping problem. The various mapping configurations can be modified if the need arises in the field. Figure 9 shows the mapping configuration developed for this particular study area. Each button represents a GIS layer or function that allows the operator to switch between layers rapidly. The configuration in figure 9 shows nine lithology buttons and ten formation buttons as well as standard mapping structural icon buttons. Other buttons displayed in this configuration allow the operator to setup and operate the GPS or the laser, display backmaps, as well as edit the GIS

9 databases. With this configuration, it was possible to map lithologic variations within different formations and have the data saved into defined GIS layers. The map field displayed in figure 9 shows another one of Penmap’s important capability, the ability to display various georeferenced backmaps. These raster and vector backmaps are used to constrain the area of interest and provide quick access to topographic and formation information. Once displayed, specific areas of the map can be enlarge using zoom functions located on the pull-down menu displayed on the left side of the screen. This feature was used to incorporate detailed, small-scale mapping into the same database as the regional, large-scale mapping.

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Using digital topographic and geological base maps stored in the computer greatly aided in the determination of mapping sites and allowed for the plotting of sample locations directly on composite maps. Figures 10 and 11 show two of the maps used during this mapping study. Figure 10 is a digital topographic in a .dxf format and Figure 11 is a geological map of the same region in a .bmp format, both of which were obtained from Codelco Exploration, Calama. These base maps were georeferenced prior to mapping by locating readily accessible, discernable GPS positions such as road

10 intersections and bridges. It was important to choose those features that were originally plotted on the topographic map in 1972, and not those which were constructed at some later time. Since over 30 years have passed since the generation of the topographic maps, many features on the maps were no longer visible on the ground. The GPS used for this mapping project was a Trimble AG-132 sub-meter GPS with real-time differential corrections provided by Omnistar. Once the maps were georeferenced they were run through a program which cut them into smaller sections (tiles), each capable of being viewed independently. These map tiles saved computer memory during mapping by allowing the program to have only a small section of the overall map actively displayed. These georeferenced map tiles could then be displayed one at a time or turned off according to the user’s needs at the time.

Figure 11. Digital file of a geological map in .bmp format obtained from CODELCO Exploration, Calama. This type of file can be directly imported into the pen computer and used as a backmap to guide field work. Data collected can be displayed directly on the underlying map or viewed independently.

At the end of each day of mapping, data was saved to a backup ZIP disk and the system’s batteries were recharged. The following day, mapping would commence at the precise point where the previous days’ work had concluded. Using this method, daily mapping augmented previously compiled information allowing for interpretations in the field to guide future work. This reduced the overall mapping time needed by enabling the operator to view all collected data and determine when sufficient quantities had been obtained to describe a particular area.

11 Field Example

An example of the mapping completed during the 1999 field season using the digital mapping system is displayed in figures 12 through 15. A digital screen capture image,(figure 12) shows a region in the Sierras de San Lorenzo, 50 km northwest of Calama which was mapped in a two day period. A photograph of this area shows the rugged topography encountered (figure 7). The grid spacing in this figure 12 is 1 Km and the geological structure is an anticline, plunging to the southeast at approximately 20°. The blue field delineates the extent of the upper Jurassic Quihuita formation and the pink field shows the location of the lower Cretaceous Estratos Cuesta de Montechristo as named on maps obtained from Codelco Exploration, Calama. These formations are also known as the Jurassic Grupo Caracoles and Cretaceous Formacion San Salvador (Chong and Pardo, 1993). This data is displayed over a 1:50,000 scale topographic backmap of the Cerros de Paqui region.

1 Kilometer

Figure 12. Screen capture image of data collected in the Sierra San Lorenzo, 50 km northwest of Calama. The structure being mapped is an anticline plunging to the southeast. Blue and pink fields represent the Jurassic Quihuita formation and the Cretaceous Estratos Cuesta de Montechristo respectively. Grid lines are at 1 Km spacing.

12 In a closer view of the example area, (figure 13) more detail can be recognized in the major Jurassic and Cretaceous units, as well as the Quaternary gravel deposits which overly certain areas. The contact between the Jurassic and Cretaceous formations is continued well past the delineated fields. This was accomplished by using the laser to map the contact from a high hill with an unobstructed view. Unfortunately many of the access roads were in a state of disrepair, and travel along them was not always possible. Access roads entered the study area from the southwest and ran along the incised channel. In this area, the road became impassable on the far western side of the Jurassic outcrops. Within the core of the fold, an incised channel exposed many of the Jurassic lithologies and allowed for easy access by foot. The north-south lines in the bottom center of the map are a series of andesite dikes which cut the fold from the south. These dikes appear to terminate at the Jurassic- Cretaceous contact, but no direct evidence from outcrop exposures could be found. The two red outlined fields depict the areas that are enlarged in figures 14 and 15.

1 Kilometer

Figure 13. Screen capture image of the Sierra San Lorenzo mapping area. Data window has been enlarged to reveal more detail. Zoom capabilities of the software allow regional views to quickly be changed to detailed, outcrop scale views. Also displayed are the locations of subsequent figures. Overall field of view is 3.5 x 2 Kilometers.

13 At higher magnification, more of the mapping details in the northwest region become visible (figure 14) and individual data points can be examined. Lithologic variations within the Jurassic Quihuita formation can be seen in figure 14 as well as structural measurements, GPS points, and field notes. The transitions from limestone to siltstone, back to limestone, and finally to sandstone can easily be seen by using standard geological fill patterns. Also visible are the GPS nodes that were automatically collected every 5 meters along the traverse path. Although not visible in previous images due to scale constraints, all of the data seen in this image are always present in the particular map and fixed to their respective georeferenced nodes. Field notes and sample numbers were recorded directly on the map during the traverse. Each note is associated with a node, fixing the description to a specific location and allowing the operator to specifically export these observation, independently from the various other data.

10 Meters

Figure 14. Screen capture image displaying the variations in lithology on a traverse across the Jurassic Quihuita formation sediments, the location is displayed in figure 13 . Blue circles with “x” highlights are GPS positions continuously acquired at 5 meter intervals during the traverse. Limestone, siltstone, sandstone, and conglomerate lithologies are displayed as well as sample locations, notes, and structural data.

14 Examining a enlarged image of the upper Jurassic nose of the anticline to the southeast (figure 15) reveals a variety of data points. Lithologic variations can again be seen, including a basal volcanic unit, which is in contact with the Quaternary channel gravel to the south. Structural data indicate strike and dip locations as well as a small scale fold plunging to the northwest. The rock hammer icon indicates the exact point a rock sample was collected. Sample numbers are not legible at this scale, but with the zoom capability of the software, the scale can easily be enlarged until the numbers can be read. It is also possible to turn the infill patterns off, allowing data and text to be seen without obstructing patterns. The text “gypsum” indicates that the lower limestone unit displayed small quantities of disseminated gypsum. GPS nodes mark the path of the traverse taken through the sediments and a 5 meter interval. Structural data is incorporated into specific GIS layers, which allow them to be exported as independent data sets for analysis. For example, strike and dip measurements can be exported as a data file into a separate computer program, which allows for structural analyses such as stereonet projections.

10 Meters

Figure 15. Screen capture image of detailed lithologic variations within the Quihuita formation sediments, the location is displayed on figure 13. Blue circles are GPS points acquired continuously during the traverse. Conglomerate, limestone, sandstone, volcanic, and evaporite lithologies are displayed in this map. Sample locations, (rock hammer icon) and structural data,(strike and dip and fold symbols) are located at individual nodes on the map, making their locations fixed in the GIS database and available for independent analysis.

15 Conclusions

Lithologic variations within the Jurassic Quihuita formation were the primary focus of the study performed in the summer of 1999. To investigate these variations, reconnaissance mapping and sampling excursions were initiated which allowed specific strata to be sampled and correlated over many kilometers. Samples could be taken from numerous locations to represent each lithology without concern of stratagraphic errors and repetition of strata. This sampling technique allowed for sample compilation from various outcrops to characterize the pertinent lithologies within the Jurassic Quehuita formation. Data collected from the mapping excursions was utilized in the investigation of the possible incorporation of volatile elements into the mineralization at the Radomiro Tomic porphyry copper deposit. Samples collected were the subject of independent chemical and isotopic analysis and the results from this work will be the subject of future publications regarding the Radomiro Tomic deposit. The incorporation of this new digital mapping system into field research allowed for increased efficiency and precision in data collection. A large quantity of data was collected over a short period of time, which allowed a larger area to be mapped than previously expected. The ability to increase the study area aided in the characterization of important lithologies by allowing the workers to compare the same strata in various locations simultaneously. The harsh environment in which this study was conducted proved not to be a problem for the digital mapping system. There were no adverse affects from factors such as the heat, direct sunlight, or the wind-borne sand and dust in the Atacama desert. The low precipitation rates which characterize this region also proved beneficial since the computer is water resistant but not waterproof. With this new mapping system, large quantities of spatially accurate data were collected in a timely manner. This proved very important in investigating the geological significance of Jurassic-age formations. The highly portable, robust, and accurate mapping system greatly aided in the collection of a quantity of data which would have otherwise required much more time to collect using traditional mapping techniques.

Acknowledgments

The authors would like to express appreciation to Irene Montero S., Tina Takagi, Abel Vanegas, for helpful discussions and reviews of the proceeding material. Their comments and suggestions have greatly improved this manuscript. Logistical field support in Chile was aided by Enrique Tidy of Codelco, and Patricio Cuadra from the Radomiro Tomic mine. Geologists of the Codelco exploration office in Calama, Chile were a valuable resource of information. Access to the Codelco exploration library in Calama, and corporate archives in Codelco central, Santiago proved extremely beneficial. This project was completed during doctoral research performed at the Earth Resources Center, University of California, Berkeley and made possible by a grant from Codelco, Chile.

16 References

Arcuri, T. and Brimhall, G., “The Radomiro Tomic Porphyry Copper Deposit and the Regional Influence of Mesozoic Sediments on Mineralization.” 2000. CODELCO 6- month progress report. Ardill J, et al., Sequence stratigraphy of the Mesozoic Domeyko basin, northern Chile, 1998, Jrnl Geol Soc London, Vol. 155, p. 71-88. Ardill, J R. et al. "High resolution sequence stratagraphic analysis of the Mesozoic Domeyko Basin, northern Chile". 1994. (Actas - Congreso Geologico Chileno; Vol. 7, Vol. 1; p. 393-396) Bell C M, Saline lake carbonates within an Upper Jurassic-Lower Cretaceous continental red bed sequence in the of northern Chile, 1989, Sedimentology, vol. 36, p. 651-663. Brimhall G. and Vanegas, A. Digital mapping of geology and ore deposits with GeoMapper, 2000, GSA Abstracts with Programs, v. 32, no. 7, p. A-514. Chong, G., Pardo, R., 1993, Geologia del distrito de Chuquicamata, segunda region de , Subgerencia de geologia, superintendencia de exploraciones y desarrollo geologico, Chuquicamata. Chong, G. "Relations of the Jurassic and Cretaceous systems in the pre-Andean zone of northern Chile". 1976. (Actas - Cong Geol Chileno; , no. 1 Vol. 1; p. A21-A42) Gröschke M, et al. “Marine Mesozoic paleogeography in northern Chile between 21°- 26° S”. 1988. in Bahlburg, H, Breitkreuz, C, and Giese, P, eds. The Southern Central Andes, Lecture notes in earth sciences 17, Springer, p. 105-117 Gröschke, M. et al. "Lithology and stratigraphy of Jurassic sediments in the north Chilean Pre-Cordillera between 21 Grad 30' and 22 Grad S". 1986. (9. SYMPOSIUM ON LATIN-AMERICAN GEOSCIENCES; Zentralblatt fuer Geologie und Palaeontologie, Teil I: Allgemeine, Angewandte, Regionale und Historische Geologie; Vol. 1985, no. 9-10; p. 1317-1324) Harrington, H. “Geology of parts of Antofagasta and Atacama , northern Chile”. 1961. Bull. AAPG, vol. 45, no. 2, p. 169-197. Hartley, A. J. et al. "Development of a continental forarc: A Cenozoic example from the Central Andes, northern Chile". 2000. Geology, v. 28, no. 4, p. 331-334 Prinz, P. "Middle Jurassic corals of shallow-water environments in northern Chile".1986. (10. GEOWISSENSCHAFTLICHES LATEINAMERIKA-KOLLOQUIUM; Kurzfassungen Der Beitraege; Berliner Geowissenschaftliche Abhandlungen, Reihe A: Geologie und Palaeontologie; Sonderband; p.168-169) Printz, P., et al. “Sediment accumulation and subsidence history in the Mesozoic marginal basin of Northern Chile”. 1994. in Petroleum basins of : AAPG Memoir 62, Tankard, A., Suarez, S., and Welsink, H., p. 219-232. Von Hillebrandt, A. et al. "The marine Mesozoic of northern Chile between 21 degrees and 26 degrees". 1986. (TEIL 1: FORSCHUNGSBERICHTE AUS DEN ZENTRALEN ANDEN (21 GRAD - 25 GRAD S) UND AUS DEM ATLAS- SYSTEM (MAROKKO) 1981-1985; FORSCHERGRUPPE: MOBILITAET AKTIVER KONTINENTALRAENDER; Berliner Geowissenschaftliche Abhandlungen, Reihe A: Geologie und Palaeontologie; Vol. 66, no. 1; p. 169-190)

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