Digital processing of satellite images applied to the epithermal ore deposits exploration, Santa Cruz province, Argentina.
1Gómez, J.C., 1de Barrio, R.E., 1-2Schalamuk, I. and 1-2Lopez, R. 1. Instituto de Recursos Minerales, F.C.N.yM. (UNLP). 2. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).
Abstract –In the Deseado Massif, southern Argentina there are many gold and silver epithermal deposits, some of them in exploitation. These deposits area associated to basic to acid volcanic and piroclastic rocks, Jurassic in age, belonging to Bajo Pobre, Chon Aike and La Matilde Formations. Epithermal precious metal deposits correspond to low sulfidation type. Hydrothermal alteration processes (silicification, argillization and propilitization) have not generated big halos or are covered by modern sediments. For these reasons such processes can not be identified through traditional methodologies. The digital processing of Landsat 7 images was used with the objective of founding new targets. In this way several tasks were undertaken i.e. geometric correction, HIS enhancement and digital analysis. This methodology has allowed to identified areas with argillic alteration that were not observed in the satellite images without special processing latterly confirmed with field work.
I. INTRODUCTION Fig. 1 Work area location In the Deseado Massif, southern Argentina there are many gold epithermal deposits, which have been Upper Jurassic, whose deposits refer to Bahía Laura partially studied and explored by costeaning and Group, which is made up of the Chon Aike and La Matilde Formations. perforations. The main ore deposit is the Cerro Vanguardia mine, in exploitation since 1998. The first Formation is mainly of acid composition. It
As part of the research of those epithermal deposits, consists of lavas and ash flow tuffs. The lavic facies that are being taken for the Instituto de Recursos (Figure #3) are light colored, characterized by Minerales (Universidad Nacional de La Plata) in the subrounded outcrops which reach hundreds of Deseado Massif Geological Province, it was made meters in diameter, its scarce porphyrism and the digital processing of Landsat 7 images in the mesoscale flow banding. On the other hand, the ash northwestern region (69° 45’ to 70° 30’ W Long., flow tuffs (Figure #4) are red, brown or yellowish and 46° 30’ to 47° 30’ S. Lat., Figure 1) with the colored, forming thick broad units with variable welding degree. objective of founding new targets. In this way several tasks were undertaken i.e. geometric La Matilde Formation rocks are principally tuffs correction, HIS enhancement and ratios that permit and reworked sediments. identified hydrothermal alteration zones latterly Cretaceous sedimentary and pyroclastic rocks from confirmed with support of field work. Chubut Group and Tertiary sediments belonging to
II. GEOLOGICAL SETTING the Golfo San Jorge Basin, outcrop all over the northern studied area. Usually these rocks are In the studied area, the oldest stratigraphical unit covered by tertiary basalts in outcrops affected by belongs to the volcanic rocks of the Bajo Pobre fluvial and eolic erosion. They are withish and Formation (Middle Jurassic). Such unit is composed locally well stratificated. of andesitic and basandesitic lavas and dominant Tertiary basalts begin to spread out towards the grayish, greenish, violet volcanic agglomerates with North of the studied area. Their thickness amounts to a high alteration. The following volcanic unit which a few meters only. They belong to the so-called involves the greatest area, belongs to the Middle- A B
20 Km Km Fig. 2 Digital processing of Landsat 7 images A: Geological image Fusion IHS RGB 742 + Panchromatic. B: Epithermal alteration Image, Fusion Intensity RGB 742 + 5/7 ratio. Arrows show argillic alteration zones and associated lavic lithologies (Chon Aike Formation).
Basalto Cerro Sombrero and its equivalents and fall Such processes can not be identified through within the Eocen-Oligocen age. traditional methodologies.
Tertiary basalts of possible Mio-Pliocen age outcrop Hydrothermal alteration mainly consists of in the southern area. Usually, they appear as silicification, argillization and in less proportion subhorizontal levels, overlying Jurassic as well as propilitization. Tertiary Formations. Their presence in the area is very scarce. IV. METODOLOGY
a) GEOMETRICAL CORRECTION III.. MINERALIZATIONS
Epithermal gold and silver deposits correspond to In order to carry out this work, it was necessary to take basic information spatially refferenciated. For low sulphidation type and have different emplacements such as: veins, veinlets, stockworks this reason Landsat 7 (TM +ETM) Path 229 Row 93 and 94 images were used. A field trip was made to and scarce disseminations. (Schalamuk et al., 1997, take control points, using GPS technology. 1999, 2002)
The vein shaped ones can be up to a few hundreds Before this, the field work was planned taking into meters long and their maximum thickness is from 2 account many points: the particular spatial resolution of the images, the maximum permit error, possibility to 3 meters. They are essentialy made up of siliceous material (quartz, chalcedony, opal) and very scarce of access to punctual places and the spatial distribution of points (Lillesand and Kiefer, 2000, sulphurs. The ore is composed by native gold, Gao, 2001). electrum, native silver and some silver sulphosalts. Hydrotermal alteration processes, which have not Thirty GPS points were also taken using a Garmin generated big halos or which are covered by modern E’trex Submit with a planimetric error between 8 sediments, are associated with these mineralizations. and 15 mts. The points were located using natural Figure 3 Acid lava outcrops , Chon Aike Formation, southem of del Infierno Creek
Figure 4 Rhyolitic ash flow tuff, Chon Aike Formation, southem of del Infierno Creek. and human features as reference, easily visible in the and Campo Inchauspe geodetic datum, with a images. planimetric error (rms) from 25 to 30 m, and an
For the digital processing the software ER Mapper spatial resolution of 15 m (pixel size). To perform V. 6.2 was employed. Using different processing the geometrical correction a first order polynomial images techniques and the information taken in the was used and the resampling system was the cubic field work a geometrical correct image was convolution. generated in a map projection Gauss Krüger Band 2 b) LANDSAT 7 IMAGE ENHANCEMENT AND DIGITAL reasonable to think in a genetic linking even though ANALYSIS PROCESS it rest more research to confirm this.
RGB algorithms were generated matching different - Altered lava outcrops are regionally aligned with a bands, looking for a particular enhance in order to main orientation NW-SE and subserviently WNW- discriminate the different facies of Chon Aike ESE and NE-SW.
Formation. Then using HIS fusion algorithms - The above mentioned method enables to identify (Intensity, Hue, Saturation), RGB combinations argilitic alteration areas which are not visible in were jointed with the 15 m. resolution Panchromatic satellite images with normal vision. Therefore, this image, reaching to different products. method is useful for exploration purposes, since For image enhancement the following steps were these characteristics are truly related to epithermal used: Au and Ag mineralizations in other areas of the a) Directional spatial filters were carried out, using Deseado Massif. the panchromatic band to remark the linear features and then the interpretation and mapping of these VI. ACKNOWLEDGMENTS structures. Statistical treatment has allowed to Authors thank Agencia Nacional de Promoción establish three main fracturation systems: NW-SE, Científica y Tecnológica (FONCyT) and NE-SW and N-S. FOMICRUZ S.E., Research Project BID b) Anaglyphs, overlapping two neighbor images for 802/OC.AR PID 32/98, whose support this 3D interpretation. investigation. c) Digital RGB fusion, making merges between VII. REFERENCES regular and panchromatic bands. J. Gao, 2001. Non-Differential GPS as an alternative source of DIGITAL ANALYSIS planimetric control for rectifying satellite imagery. Photogrammetric engineering & Remote sensing (PE In a second stage, algorithms with bands ratio & RS), 67: 49-56. Maryland. (Ratios algorithms) were made, i.e. RGB 7,4,2 + 5/7 S. Lillesand and R. Kiefer, 2000. Remote sensing and image interpretation. 4th edition. John Wiley & Sons Inc. 724 (Sabine, 1999) with the intention of discriminate the pgs. pattern of known mineralized areas in lavic facies of Ch.Sabine, 1999. Remote sensing strategies for mineral the ignimbritic – lavic Jurassic complex and related exploration. En A. Rencz (editor): Remote sensing for argillic hydrotermally altered zones in other areas. the Earth sciences, pg.: Cap. 8: 375-428. John Wiley & Sons, Inc. New York. In the next step the work was pointed out in I.B. Schalamuk; M.A. Zubia, A. Genini, R.R. Fernández, 1997. extrapolations the small lithological differences Jurassic epithermal Au-Ag deposits of Patagonia, Argentina. Ore Geology Rewiew. (ed. Elsevier) Vol 12, watched to the whole area of study, getting 20 new Nro. 3: 173-186. areas subsequently researched at the field work, I.B. Schalamuk, R. E. de Barrio, M.A. Zubia, A.Genini and H. where the observations allow to confirm that the Echeveste, 1999. Provincia auroargentífera del pointed areas were lavic lithologies with argillic Deseado, Santa Cruz. En Recursos minerales de la República Argentina (Edit. Zappettini, E.), Instituto de alteration corroborated by petrografic and Geología y Recursos Minerales, SEGEMAR, Anales mineralogic studies (XRD). 35: 1177-1188. Buensos Aires. I.B. Schalamuk, R.E. de Barrio, M.A. Zubia, A. Genini and J. V. CONCLUSIONS Valvano, 2002. Mineralizaciones auroargentíferas del Macizo del Deseado y su encuadre metalogénico, The main conclusions are as follows: provincia de Santa Cruz. XV Congreso Geológico Argentino, Relatorio. In press. - The efficiency of the algorithm fusion intensity, RGB 742 + 5/7 ratio was confirmed as an instrument to detect hydrothermal altered zones showing it as areas with high anomalous brightness.
- The use of spatial directional filters has allowed the improvement of the visual interpretation of lineaments. There were detected three main fracturation systems: N-S, NE-SW and NW-SE.
- The main argillic alteration zones are coincident with lavic rhyolitic facies of Chon Aike Formation.
- As the areas with highest frequency of lavic rocks are extremely coincident with a greater concentration of precious metal deposits its