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The Bacon-Manito Geothermal Project (BGP), located in the provinces of Albay and Sorsogon, Philippines, is a producing geothermal field owned and operated by Energy Development Corporation. It is situated in a tectonically active region, bounded on the east by the Philippine Trench and on the west by the left-lateral NW-SE-trending Philippine Fault. BGP is sited on the Pocdol Mountains, which is part of a NW-SE-trending volcanic array in the Bicol Peninsula that formed since Late Miocene. Although inactive, the Pocdol Mountains are still a host to a suite of geothermal manifestations, which include hot springs, altered grounds, fumaroles, and solfataras. The youngest volcanic deposits were previously dated at < 0.04 Ma. The existing structural map of the area was refined using a combination of aerial photographs and recent TerraSAR DEM data. The identified structures observe distinct trends that reflect the transtensional tectonic setting of BGP. The primary trends are N-S, NNE, and ESE while the secondary trends are NW-SE, NE-SW, and E-W. Alignments of volcanic edifices are observed along NNE, ESE, and E-W. The ESE trend appears important for it defines the Bac-Man Fault Zone, which is the main shear zone related to the San Vicente-Linao Fault, a major splay of the Philippine Fault. This zone is notable for hosting hydrothermal manifestations, caldera structures, and volcanic cones, particularly resurgent features. Most of the igneous rocks collected from BGP for petrological and geochemical analyses have intermediate SiO2 compositions, varying from basaltic andesite to dacite. The majority are lavas but some are tuffs and altered rocks. The geochemistry of the rocks indicates a mostly calcalkaline island arc signature. Rock samples from Mts. Pulog, Pangas, and Palayan Bayan in east BGP show an almost tholeiitic (low-K) chemistry. On the other hand, samples from Mt. Sagpon and Lison Dome in west BGP show a high K calc-alkaline signature. Three distinct trends were noted based on K2O vs. SiO2 values, which allowed distinction of volcanic groups. These are the high K calc-alkaline rocks from portions of west BGP; medium K calc-alkaline rocks, which follows the “andesite trend” from west BGP, as well as Cawayan Crater and Mt. Tanawon from east BGP; and medium K calc-alkaline rocks following the “dacite trend,” which only includes rocks from east BGP. |
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