The fluid-rock interaction history of the Tongonan geothermal field, Leyte, Philippines [microform]
- Bib ID:
- 3282967
- Format:
- Book and Microform
- Author:
- Scott, Graeme Logan
- Description:
- 384 p.
- Summary:
-
The Tongonan Geothermal Field (Leyte, Philippines) presently produces an average of 60 MW$\sb{\rm e}$. Chronological development of the field's evolution was as follows. Extensive volcanism occurred along the Philippine Fault in the Tongonan region in the Miocene and produced the Bao Volcanic Formation (BVF) and the Mahiao Plutonic Complex (MPC). The BVF is composed of basaltic andesite lavas and volcaniclastic sediments interbedded with up to three thin horizons of shales and limestones containing Miocene fossils, and is cut by hornblende-dacite dykes. The MPC contains rocks ranging in composition from gabbro to granite. When the mafic magmas first intruded, they contact metamorphosed the overlying volcanic and plutonic rocks to produce hornblende hornfels. A low temperature ($<$250$\sp\circ$C) geothermal system in the Miocene. In the Pliocene, the Philippine fault reactivated and this coincided with intrusion of granodiorite magma into the Malitbog (eastern) sector of the field.
As the granodiorite crystallised, about 2 mole% of mainly CO$\sb2$ and H$\sb2$S gas in geothermal steam dissociated to form a corrosive acidic condensate which intensely altered the host rocks and deposited kaolinite, alunite, pyrophyllite and diaspore to depths of about 500 m below sea level. However, epidote, illite, chlorite, quartz, anhydrite and garnet deposited from boiling, neutral pH, alkali-chloride, waters in the deep reservoir. The water exchanged oxygen isotopes with the reservoir rock. Fluid inclusion evidence shows that boiling has occurred, and the salinity and temperature of the deep fluid did not change significantly. In the Quaternary, calc-alkaline granite magma intruded the basement rocks in the Mahiao (western) sector of the field.
Only minor gas released from this magma accounts for the lack of acid alteration in the Mahiao sector. The deep neutral-pH fluids deposited mainly quartz and epidote during boiling, and exchanged oxygen isotopes with the host rocks at temperatures above about 300$\sp\circ$C. Na enrichment of the MPC occurred at high temperatures ($>$250$\sp\circ$C) in the Mahiao sector, while Ca and K enrichment occurred in the BVF at low temperatures ($<$250$\sp\circ$C). Although the chloride content of Holocene fluids (10,000 mg/kg) has remained constant, the CO$\sb2$ content of the steam discharging through the system is lower ($<$0.4 mole%) now.
- Notes:
-
- (UnM)AAI8904866
- Source: Dissertation Abstracts International, Volume: 49-12, Section: B, page: 5195.
- Thesis (Ph.D.)--The University of Auckland (New Zealand), 1988.
- Reproduction:
- Microfiche. Ann Arbor, Mich.: University Microfilms International.
- Subject:
- Geology
- Other authors/contributors:
- The University of Auckland (New Zealand)
- Copyright:
-
In Copyright
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Copyright status was determined using the following information:
- Material type:
- Literary Dramatic Musical
- Published status:
- Unpublished
- Creation date:
- 1988
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