Fluid-Rock Interaction at the Magmatic-Hydrothermal Interface of the Mount Cagua Geothermal System, Philippines
Agnes G. Reyes, Rodney Grapes, Vicente C. Clemente, 2005. "Fluid-Rock Interaction at the Magmatic-Hydrothermal Interface of the Mount Cagua Geothermal System, Philippines", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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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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To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.