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.
