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Book Chapter

Fluid-Rock Interaction at the Magmatic-Hydrothermal Interface of the Mount Cagua Geothermal System, Philippines

By
Agnes G. Reyes
Agnes G. Reyes
Institute of Geological and Nuclear Sciences, 30 Gracefield Road, Lower Hutt, New Zealand
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Rodney Grapes
Rodney Grapes
Institut für Mineralogie, Petrologie und Geochemie, Universität Freiburg, Alberstrasse 14a, 79104 Freiburg, Germany
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Vicente C. Clemente
Vicente C. Clemente
Philippine National Oil Company, Merritt Road, Fort Bonifacio, Metro Manila, Philippines
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Published:
January 01, 2005

Abstract

The Cagua geothermal system can be divided into a narrow region dominated by the release of magmatic fluids localized within the Mount Cagua volcanic crater below 1,540 m and a hydrothermal envelope above this depth. Where structures cut through the magmatic-hydrothermal interface, magmatic gases are directly transported to surface fumaroles along a vapor conduit.

The magmatic fluid in Cagua consists of three components: (1) a 40 wt percent NaCl equiv hypersaline brine with a temperature of 450°C and a KCl/NaCl ratio of 0.65; (2) 380°C critical point fluids containing 0.4 wt percent NaCl equiv, 14 μmol/mol dissolved CO2, and dissolved sulfur; and (3) acidic magmatic condensate, contained in water-filled microfractures, derived from the dissociation of HCl, SO2, and H3BO3 in magmatic vapor.

Essentially alteration is initiated in microfractures where magmatic vapor condenses to form solutions so acidic that rock is leached to form residual minerals composed of quartz, corundum, and andalusite with or without alunite. Initial neutralization at the junction between rock and microfracture causes deposition of Mg, Ba, V, Rb, Pb, Cu, Ge, Sn, and Au. From this junction of neutralization, fluids increasingly interact and equilibrate with rock resulting to pervasive neutral-pH alteration. However, some of the epidote, amphiboles, feldspar, and muscovite within the vapor-rich magmatic-hydrothermal region were formed by the interaction of rock with hydrolysis products of the hypersaline brine in the form of NaOH or Ca(OH)2.

Within the hydrothermal envelope, where temperatures are <350°C, magmatic fluids are diluted by mixing with ground water, condensed steam at about 235°C, and shallow acid-sulfate and carbonic acid steam condensate generated at <120°C.

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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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