The Geology of Geothermal Prospect Leyte, Central Philippines
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167 THE GEOLOGY OF GEOTHERMAL PROSPECT LEYTE, CENTRAL PHILIPPINES Lauro F. Bayrante PNOC-Energy Development Corporation ABSTRACT Results o f geologic studies i n the Burauen area pinpoint two areas o f geothermal interest, the southwestern area o f Mt. Lobi and the Mahagnao pro- per. Both areas appear t o be genetically and spa- tially related to the Quaternary volcanic centers o f the area, and t o the Philippine Fault. Thermal manifestations are found associated with altered andesite and are related to to trending joint sets. Alteration studies indicate the pre- sence of slightly acidic to neutral chloride altering fluid in Daguitan graben and that of an acidic tering fluid that produced the teration i n Mahagnao. INTRODUCTION A series of geoscientific studies were conduc- ted i n the Burauen geothermal prospect from the start of 1980 to mid 1982, by the Philippine Natio- nal Oil Company Energy Development Corporation as part of i t s commitment to explore and develop one of the Philippine indigenous non- oil energy re- source, that of geothermal power. This present geological report i s the result of the detailed surface geological exploration made on t h e prospect. The Burauen prospect i s located on the island o f Leyte, and i s one o f two prospects i n the island EXPLANATION which lies approximately 25 a i r kilometers south- ANGLE LINE WHERE INFERRED1 east of the Tongonan geothermal field. Inciden- AXIS PLUNGE tally, the term Burauen geothermal prospect i s used AXIS PLUNOE CLOSE FOLD in the text to embrace a l l areas i n central Leyte THRUST ON nearest the reservation. Figure 1. Map of Leyte Island showing its relation The island of Leyte tectonically rides on the to the Philippine Fault, areas of probable stress Philippine mobile belt and is part of the southern and other structural features. (Bureau of Mines segment o f the Pacific island arc system (Gervasio, Geosciences, 1962) 1966). The Philippine trench lies some 150 km east o f Leyte. The NW-SE Philippine Fault cuts near volcanic centers 3 and 4. through the island. (Fig. Hith regards to its physiography, the area is The island i s further characterized by iso- characterized by flat to gently sloping ground in lated dormant volcanic centers, some o f which still the east, gradually rising towards two dormant vol- display minor solfataric emissions and are seeming- canoes; Mt. Lobi in the north and Mt. Mahagnao in ly aligned along the Philippine Fault. From north- the south. Mt. Lobi notably dominates the northern west t o southeast these are: (1) Biliran centers; sector while Mt. forms domical clusters in (2) Janagdan center; (3) Lobi-Mahagnao centers; the south. Distinct plug domes dot the periphery and (4) Cabalian center (Fig. 1). Biliran and of both centers. (Fig. 2) Tongonan Geothermal Projects, at present under ad- vance exploration and development, respectively, Burauen is drained by two major river systems, are close to centers 1 and 2. The two prospects namely: Daguitan and Marabong Rivers (Fig. 2). in the island, Burauen and Anahawan, are situated Daguitan receives water from the northern side, 168 nte while Marabong drains the southern side. Both ri- pect. The pluton, as identified i n thin section, vers flow towards the east, between Mt. Lobi and i s a biotite- bearing hornblende diorite Mahagnao, and merge into the Daguitan River. Unconformably overlying these i s a thick se- STRATIGRAPHY quence o f well- indurated, hard, volcanic breccia, agglomerate, tuff breccia, pyroclastics with sub- In the light of the detailed work involved, ordinate amount o f pyroxene andesite herein called several modifications i n the tentative stratigraphy the Leyte Central Formation (Lcf). This i s inter- proposed by D. D. Espiritu (1980) are presented. calated and locally intruded by andesite dykes. A succession scheme and geologic map a r e shown i n The formation dips moderately to the west with figures 3 and 4. maximum width of and an observed thickness o f 700 The oldest rock exposed i n the area form the Leyte tramafic Complex (Luc), and are believed A younger unit, the Albuera Formation (Af), to be part of the undifferentiated ultramafic belt composed o f a sedimentary deposit consisting of extending to Mindanao. The rocks generally occur tuffaceous sandstone, boulder conglomerate, mud- as irregular, highly brecciated masses c u t by i n - stone blankets the Leyte Central Formation i n the tricate calcite, dolomite and occasional quartz west . veinlets along fault zones. Conformably overlying this sedimentary forma- Non-conformably overlying this i n the central tion are two distinct pyroclastic deposits, the portion of the mapped region are the Burauen Vol- Daguitan Tuff (Dt) and the Burauen Pyroclastic canics composed o f Miocene effusives. Three Deposit (Bpd). The former is essentially composed distinct facies are recognized; the andesite por- of slightly friable hornblende-pumiceous-daci tic phyry, pyroxene andesite, and oxy-hornblende ande- tuff, whereas the latter is a thick deposit oftuff, site. These volcanicsare found intruded by a plu- tuff- breccia, andesite tephra, lahar and agglome- ton, referred to as Albuera Diorite (Ad), as seen rate. These are intruded and overlain by cumulo- i n an outcrop on the far western side of the pros- form bodies called the Plug domes (Pd), composed 169 Bayrante THICKNESS ROCK UNITS LITHOLOGY ral ization, teration and thermal spring ments, prominent fault scarps and distinct 1 logical variations. The term East Fault i s used as a reference for the major failure observed on the eastern side of LOBI the prospect. Near vertical scarps and minor gas UNKNOWN seepages are noted near or along this fault. South- eastward, i t is characterized by an alignment of ridges and plug domes. In Mahagnao area, i t cuts across the southwest side of the volcano near the discharge mouth of Lake Mahagnao. Furthermore, the unequivocal stream offsets displayed in Daguitan and Marabong Rivers very strongly suggest a sinistral movement along this fault. Finally, i t is believed that the fault is the most active so far. This i s implied by: (1) the 1-pronounced fault scarps of Dagui tan tuff; (2) the presence of andesitic plug domes aligned along its trace; and (3) the distinct linear trace near Lobi and Mahagnao with the latter still pos- sessing solfataric exhalations. The West Fault strikes and has a near vertical dip. Fracture patterns on the west and east blocks, and the relative movements of the creek indicate a similar displacement as the East line. This fault exhibits the least thermal mani- I I I festations. Evidences of i t s presence include fault scarps, intense linear brecciation of the I Burauen volcanics and abrupt topograhic breaks of Figure 3. Stratigraphic sequence found in the the central area relative to the Central Cordillera. Burauen area. The Central Fault refers to structures observed of biotite-bearing pyroxene andesite and hornblende between the East and West Faults. Analyses of andesi te . fracture and joint readings suggest a left lateral sense of displacement. Significant evidences of The youngest volcanic consist of Quaternary i t s presence include the existence of a crushed effusives referred to as the Lobi Volcanics zone of Burauen volcanics, alignment of minerali- and the Mahagnao Volcanics Rock types ob- zation and alteration, fault scarps and the fault- served are pyroxene andesite, crystal-rich horn- controlled Inangahan River. blende dacite, and hornblende andesite. The Abuyugon Fault trends and dips ver- Recent deposits are composed of alluvium beach tically displacing the Burauen pyroclastic deposit. sand, spring and other fluviatile deposits. This It appears to control the adjacent Abuyugon and generally covers the eastern flat land of the Pangdan gas seepages. Relative sense of displace- reg ion. ment is left- lateral. STRUCTURAL FEATURES As a consequence of movement along the East, and Central Faults, block adjustments occur Several structural features were recognized in inducing the formation of Daguitan graben. the prospect as observed from aerial pho- ritu (1980) believes that the structure was formed tographs and as verified in the field. These are from the vertical slip component of these lines. strike-slip faults, volcano-tectonic depressions, Majority of the thermal manifestations occur within a graben, conjugate fault system and fracture-joint this graben. Evidences for the existence of this sets. graben are the distinct 1ithologic variations par- ticularly near Lobi where the downfaul ted block i s The majority of the strike-slip faults domi- capped by argillic tuff, and the topographic ex- nate the central portion of the prospect, preferen- pressions where the centrally depressed area is tially, along the trace of the Philippine fault. bounded by gravity slip components of the major Three major and one minor NW trending lines, res- lines. pectively, designated as East, Central , and West and Abuyugon Fault lines are recognized. The ex- The southern side of Burauen i s characterized tension of these lines to the northwest is yet to by volcano-tectonic depressions. These structures be verified. Manifestations of these structures appear 1ike assymetrical collapse and deep depres- include strong shattering of the Burauen volcanics sions, localized within the Mahagnao area, with and tuff-breccia, unequivocal stream offsets, the biggest pit occupied by Lake Mahagnao (Danao). 170 I .............. Af WEST FAULT LINE ,-CENTRAL FAULT FAULT LINE I 5 4 I . I 2I 3 I I I 4 I 171 Bayrante There i s one thermal manifestation located north- Quaternary volcanic centers. west of the lake which consists of hot spring clusters with moderate gas ebullitions. The tered outcrops sampled in Lobi a r e largely andesite lava flows of the Burauen These volcano-tectonic depressions were pre- nics.