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Spatial and Temporal Relationships between Hydrothermal Alteration Assemblages at the Palinpinon Geothermal Field, Philippines—Implications for Porphyry and Epithermal Ore Deposits

By
Andrew J. Rae
Andrew J. Rae
Centre for Ore Deposit Research (CODES SRC), University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia
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David R. Cooke
David R. Cooke
Centre for Ore Deposit Research (CODES SRC), University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia
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David Phillips
David Phillips
Argon and Noble Gas Laboratory, School of Earth Sciences, University of Melbourne, Victoria 3010, Australia
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Chris Yeats
Chris Yeats
Heavy Ion Analytical Facility (HIAF), CSIRO Exploration and Mining, P.O. Box 136, N. Ryde, NSW 2113, Australia
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Chris Ryan
Chris Ryan
Heavy Ion Analytical Facility (HIAF), CSIRO Exploration and Mining, P.O. Box 136, N. Ryde, NSW 2113, Australia
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Danilo Hermoso
Danilo Hermoso
Geothermal Division, Philippine National Oil Company-Energy Development Corporation, Merritt Road, Ft. Bonifacio, Makati City, Philippines 1201
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Published:
January 01, 2005

Abstract

The Palinpinon geothermal field (Negros Island, Philippines) is a high-temperature, liquid-dominated geothermal system. Hydrothermal alteration assemblages in the Nasuji-Sogongon region are associated with the Nasuji pluton and include K silicate (biotite, magnetite), calc-silicate (garnet, clinopyroxene), hypogene advanced argillic (andalusite, zunyite), propylitic (tremolite-actinolite, epidote), and distal illite (smectite, illite) and steam-heated advanced argillic (amorphous silica, kaolinite, alunite) assemblages. Biotite alteration and associated veins formed from magmatic-hydrothermal fluids that had temperatures from 267° to >600°C, salinities of 26 to 56 wt percent NaCl equiv and up to 0.2 wt percent Cu. Hydrothermal biotite (40Ar/39Ar = 0.7–0.6 Ma) and alunite (K-Ar = 0.9–0.8 Ma) formed contemporaneous with the Nasuji pluton (40Ar/39Ar = 0.7–0.3 Ma), implying a genetic link between intrusion emplacement and hydrothermal alteration assemblages.

The emplacement of a blind intrusion in the Puhagan area at depths greater than 2.5 km has provided the heat source for present-day geothermal activity. Calc-silicate, biotite, and propylitic alteration zones developed above this intrusion at depths greater than 2 km. Parts of the biotite and propylitic alteration zones are in thermal equilibrium with the present-day geothermal system. The lack of hypogene advanced argillic alteration at Puhagan is interpreted to indicate that magma degassing has been hindered or prevented, possibly due to high lithostatic confining pressures. At <2 km, illite and steam-heated advanced argillic alteration assemblages have overprinted the biotite and hypogene advanced argillic alteration types associated with the Nasuji pluton and are in thermal equilibrium with the present geothermal system.

The intimate spatial and temporal relationships between the intrusion emplacement and the styles of alteration at Palinpinon are characteristic of mineral deposits such as, porphyry, skarn, and high- and low-sulfidation epithermal. At Palinpinon, a coupled porphyry high-sulfidation epithermal alteration system formed at 0.9 to 0.8 Ma with a coupled porphyry low-sulfidation epithermal system forming today, demonstrating that these alteration systems can form simultaneously in a single mineral district. However, assays (<0.02 wt % Cu, <0.03 wt % Pb, <0.01 wt % Zn, <0.01 wt % Mo, <8 g/t Ag and <0.05 g/t Au) show that the alteration zones at Palinpinon are barren. This could be due to insufficient fracture permeability, resulting in a lack of adequate focus for high volume fluid flux. Alternatively, it could relate to an insufficient supply of metals in the magmatic-hydrothermal fluids.

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Contents

Special Publications of the Society of Economic Geologists

Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth

Stuart F. Simmons
Stuart F. Simmons
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Ian Graham
Ian Graham
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Society of Economic Geologists
Volume
10
ISBN electronic:
9781629490342
Publication date:
January 01, 2005

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