Keywords: AirSAR, lineaments, alteration zones

Abstract
The Airborne Synthetic Aperture Radar (AirSAR) system developed by the National Aeronautics and Space Administration-Jet Propulsions Laboratories (NASA-JPL) is a tool for a multidisciplinary approach in remote sensing. The AirSAR is presently used in many applications. One such application is for geologic mapping and mineral exploration. This paper presents some results of using AirSAR data as a tool for geologic mapping and mineral exploration in the Philippines. This paper is part of the research study within the framework of the Philippines-NASA AirSAR Project.

The study area is a mineral claim in Dumara, Antique, known as the Dumara Gold Prospect by the Philex Gold, Inc. The gold prospect is mapped within the Lumbuyan Formation.

The available AirSAR images of the study area are in 3 wavelength bands, namely: C-band (3.75-7.5 cm), L-band (15-30cm), and P-band (30-100cm). The single band (C-band VV) was enhanced and analyzed for geologic and structural data. Field verifications were conducted to verify the features observed.

New lineaments were identified near and southeast of the prospect area. Also near the prospect site, three alteration zones were identified. Field investigation shows that mineralization is along the Tigotay fault trending N 75 0 E. The alterations along this fault are evident. However, no intrusive body was found. It seems that the mineralization is transported through the fracture system rather than emplaced due to the low dipping of the alteration zone. The extension of this mineralization must be searched. The thrust faults mapped in the field are correlated with previously mapped structures. Further study of the data and the area is necessary to delineate other probable mineralization areas.

1.0 Introduction
This report is the initial synthesis of the available field reports, field investigation and initial image processing of the AirSAR data over Dumara, Antique, Philippines. The field investigation was carried out in Antique last April 06 to 13, 1999. The said investigation is meant to verify features observed in the AirSAR image and aerial photographs of the study area. It is also meant to gather field data for analysis and to document the features observed in the images.

This report is part of the sub-project under the major project entitled “Analysis of AirSAR and TopSAR Datasets for Resource Management and Development Planning Activities”. The major project’s developmental objective is to “promote the use of remote sensing technology for various applications relating to resource management and developmental planning activities. In this case, the application for the sub-project is geologic studies and mineral exploration.

Remote sensing provides data for preparing and improving geologic maps which are fundamental tools for exploration (Sabins, 1987). In addition, enhanced images, such as radar, help to locate fracture intersections, which are possible mineral target areas.

In Antique, the rugged terrain and the long wet season from May to November inhibit detailed exploration surveys. The available geological data are few. They include the 1987 BMG reconnaissance geological map (1:50,000) and the working base map of Philex (1:125,000). Many parts of the area still have to be mapped effectively. The AirSAR data provided an additional input to the geological mapping and mineral prospecting in Antique.

2.0 The Study Area

2.1 Location and Physiography
The study area is within the Philex claim at Dumara, Antique. (fig.1, index map). The study area is generally rugged and mountainous. It has a high relief and v-shaped ravines and gullies. The ridges and spurs are sharp. The highest elevations in the area are from 700m to 2,000m. These high points are along the eastern side of the study area in a roughly N-S pattern. The prominent peaks are the Mt. Baloy and Mt. Nangtud which are 1900m and 2080m above mean sea level, respectively. Mt. Dumara has an elevation of 1370m and located on the NW side of the study area. To the west, the elevation is moderate in the range of 700m to 1000m.

The major river system draining the area is the Paliuan River and its tributaries. This river system has developed a wide flood plain with braided channels. It exits to the Sulu Sea. The headwater tributaries have a sub-dendritic, sub-trellis to angulate patterns. The complex patterns and arrangements of the drainage system can be explained by a combination of lithological and secondary structural controls.

Vegetation growths are varied at the eastern portion of the study area. Cogon grasses, undergrowth, secondary growths and shrubs are observed covering the spurs, ravines and slopes. Forest, which includes pine forest, grows at high elevations.

2.2 Geology
Geologically, the Dumara area consist dominantly of red to purple mudstones and siltstones, turbidites, volcanic wackes and some conglomerates belonging to the Lumbuyan Formation (fig.1). The Mt. Baloy Volcanics, made up of pillow and brecciated basalt, minor turbidites, boulder conglomerate beds, conformably overlies the Lumbuyan Formation. The volcanics are found along peaks and ridges at the west portion of the study area (Geology of the Philippines, 1996).

The other rock formations in the area are the Igsawa Pyroclastics and Maliao Wackes. Both formations are in fault-contact with the underlying Lumbuyan Formation. However, in some areas, they are found to be uncorformably overlying the Lumbuyan formation. The Igsawa Pyroclastics is composed of the andesite flow breccia, buff wacke conglomerate and calcirudite, calcarenite and marl. The Maliao Wackes, on the other hand, consist of turbidite wackes, thin siltstones, minor conglomerate and andesite flow breccia (Geology of the Philippines, 1996). Both formations are mapped along peaks, spurs and headwater tributaries. They trend in the NE direction. However, the exact relationship of the Igsawa Pyroclastics with the Maliao Wackes is unknown.

2.3 Structures
In the study area, two thrust faults trending NE-SW are observed to be the contact of the wackes and the pyroclastics to the mudstone-siltstone turbidite of the Lumbuyan formation -- a fault-contact (fig.1). New lineaments traced from aerial photos and the AirSAR image are oriented NE-SW and NW-SE. They are complimentary structures to each other. Streams usually follow these lineaments which are natural lines of weaknesses. Anticlinal and synclinal fold structures trending NE-SW were mapped along the main Paliuan River. Other fold structures can be traced in the adjoining area.

3.0 Methodology
In this study, the 1996 AirSAR image (C-band, v.v.) was enhanced and visually interpreted for the major structures. The Environment for Visualizing Images (ENVI) software developed by the Research System Incorporated (RSI) was used to process the C-band v.v. image. A contrast stretch (Default Quick Stretch>Quick Linear 2%, Gaussian stretching, interactive) was applied to improve the display of the radar image (fig.3). In addition, available aerial photographs were interpreted for structures. The interpreted image and aerial photos were compared and correlated with the existing maps of the area. These maps include the BMG geologic map (1:50,000), the NAMRIA topographic map (1:250,000) and the Philex Gold, Inc. reconnaissance geological map (1:125,000).

A field investigation was carried out in the study area last April 06 to 13, 1999. The field inspection was meant to verify the features observed in the AirSAR images, gather field data for analysis and to document the features observed in the images.

4.0 Results and Discussion
One use of the radar data is to map the local fractures that may control ore prospects or deposits. Generally, fracture intersections are possible mineral target areas. The presence of volcanic plugs (arcuate features as seen in the image) increases the desirability of the target area because the plugs sometimes bring with them mineralizing fluids.

In the Dumara radar image, several newly mapped lineaments and fold structures are recognized. The lineaments (old and new) and folds are oriented dominantly in the NE-SW direction (fig. 1). Some lineaments trend NW-SE. Most of the major lineaments and fold structures are along the main Paliuan River and its tributaries. These were verified in the field. Many of the tributaries follow these lineaments which are natural lines of weaknesses creating a sub-trellis drainage pattern in some areas.

In the radar image, new lineaments were identified near and SE of the prospect area. Near the prospect area, they are oriented N-S, NE-SW and E-W. They tend to converge near the existing prospect site. To the SE of the prospect area, two sub-parallel lineaments trending NE-SW were mapped. The significance of the sub-parallel lineaments with respect to the mineralization is still to be studied. The thrust faults mapped in the field are correlated with previously mapped structures on the available geological and working maps. The mapped anticlines form aligned ridges trending NE-SW.

Near the prospect site, three alteration zones were identified. The term “alteration” refers to the changes in color and mineralogy of rocks surrounding a mineral deposit that are caused by solutions that formed the deposit. Such alteration minerals occur in zones thus the term “alteration zone”. However, the presence of an alteration zone does not necessarily indicate the presence of a deposit that can be economically mined. These alteration zones are confined along the slope of the major tributaries of the Tigotay, Meycauayan and Maypunong creeks. The creeks are at an average elevation of 800-900m above sea level.

The alteration zones are concentrically developed on the metavolcanics. The maximum thickness of the whole alteration zones reaches up to 3 m. It is low dipping. Silicification and argillization are prominent on the inner zones. Silicification is the introduction of minute crystalline silica into a non-siliceous rock via groundwater or igneous origin (Concise Oxford Dictionary of Earth Sciences, 1991). In these same zones, pyrite disseminations are noted. Outside of this zone, chloritization tapers gradually outward.

The ore minerals present in hand specimen occur as replacement disseminations and as open-space fillings (they fill the open spaces). Paragenesis or the assemblage of minerals that has formed consists of fine-grained chalcocite, bornite, sphalerite, galena and chalcopyrite. Pyrite occurs sparingly with the base metal sulfides. Quartz and calcite stringers are noted running along the mineralized rocks.

During the field investigation, it was noted that the mineralization is along the N75°E trending fault. The alterations along this fault are evident. This fault could be the spillway of the constricted mineralization at the site. However, no intrusive body was found. It seems that the mineralization is transported through the fracture system rather than emplaced due to the low dipping of the alteration zone. The extension of this mineralization must be searched.

5.0 Conmclusion and Recommedation
The 1996 AirSAR data provided an additional input to the geological mapping and mineral prospecting in Antique. The data is useful for mapping the local fractures in the area. New lineaments were identified near and SE of the prospect area. Some lineaments tend to converge near the existing prospect site. Also near the prospect site, three alteration zones were identified. Field investigation shows that mineralization is along the Tigotay fault trending N 75 0 E. The alterations along this fault are evident. This fault could probably be the spillway of the constricted mineralization in the area. However, no intrusive body was found. It seems that the mineralization is transported through the fracture system rather than emplaced due to the low dipping of the alteration zone. The extension of this mineralization must be searched.

The significance of the two sub-parallel lineament SE of the prospect site in relation to mineralization still has to be studied. The thrust faults mapped in the field are correlated with previously mapped structures. Further study of the data and the area is necessary to delineate other probable mineralization areas.

6.0 Acknowlegement
We thank the DOST-PCASTRD and Philex Mining Corp. for the funding support of this project. We also thank our colleagues at the MGB Central Office and Regional Office No.6 and personnel from Philex, NAMRIA and the UP-TCAGP for joining us in the field.

7.0 References

• Bureau of Mines and Geosciences, 1987. Geologic Map of Bugasong Quadrangle. Sheet 3453 III, Scale 1:50,000. Quezon City.
• Concise Oxford Dictionary of Earth Sciences, 1990. Oxford University Press.
• Corpuz, Jr. M. et. al., 1997. Geologic Report on the Dumara Project. Philex Internal Report.
• Geology of the Philippines, 1996. Unedited version. Mines and Geosciences Bureau. Q.C.
• NAMRIA, 1954. Topographic Map of Roxas City. Sheet 2523. Scale 1: 250,000. Fort Bonifacio, Makati City.
• Sabins, Jr.,F.F.,1987. Remote Sensing: Principles and Interpretation, 2ed. W.H. Freeman and Company, New York, USA.