Spatial and Temporal Relationships between Hydrothermal Alteration Assemblages at the Palinpinon Geothermal Field, Philippines—Implications for Porphyry and Epithermal Ore Deposits
Andrew J. Rae, David R. Cooke, David Phillips, Chris Yeats, Chris Ryan, Danilo Hermoso, 2005. "Spatial and Temporal Relationships between Hydrothermal Alteration Assemblages at the Palinpinon Geothermal Field, Philippines—Implications for Porphyry and Epithermal Ore Deposits", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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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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Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth
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.