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Abstract:The Onsen acid-sulphate type of mineralization is located in the Desmos caldera, Manus back-arc basin. Hydrothermal precipitates, fresh and altered basaltic andesite collected from the Desmos caldera were studied to determine mineralization and mobility of elements under seawater dominated condition of hydrothermal alteration.The mineralization is characterized by three stages of advanced argillic alteration. Alteration stage I is characterized by coarse subhedral pyrophyllite with disseminated anhedral pyrite and enargite which were formed in the temperature range of 260-340°C. Alteration stage II which overprinted alteration stage I was formed in the temperature range of 270-310°C and is characterized by euhedral pyrite, quartz, natroalunite, cristobalite and mixed layer minerals of smectite and mica with 14-15 Å XRD peak. Alteration stage III is characterized by amorphous silica, native sulphur, covellite, marcasite and euhedral pyrite, which has overprinted alteration stages I and II.Relative to the fresh basaltic andesite samples, the rims and cores of the partly altered basaltic andesite samples have very low major, minor and rare earth elements content except for SiO 2 which is much higher (58-78 wt%) than SiO 2 content of the fresh basaltic andesite (55 wt%). REE patterns of the partly altered basaltic andesite specimens are variably depleted in LREE and have pronounced negative Eu anomalies. Normalization of major, minor and REE content of the partly altered basaltic andesites to the fresh basaltic andesite indicates that all the elements except for SiO 2 in the partly altered basaltic andesite are strongly lost (e.g. Al 2 O 3 = -8.3 to -10.9 g/100cm 3 , Ba = -2.2 to -5.6 mg/100cm 3 , La = -130 to -200 µg/100cm 3 ) during the alteration process. Abnormal depletion of MgO, total Fe as Fe 2 O 3 , LREE especially Eu and enrichment of SiO 2 in the altered basaltic andesites from the Desmos caldera seafloor is caused by interaction of hot acidic hydrothermal fluid, which originates from a mixing of magmatic fluid and seawater.

Abstract:The Onsen site is an active submarine hydrothermal system hosted by the Desmos caldera in the Eastern Manus Basin, Papua New Guinea. The hydrothermal fluid is very acidic (pH=1.5) and abundant native sulfur is deposited around the vent. The δ 34 S values of native sulfur range from -6.5 to -9.3 ‰. δ 34 S values of H 2 S and SO 4 in the hydrothermal fluid are -4.3 to -9.9 ‰ and +18.6 to +20.0 ‰, respectively. These δ 34 S values are significantly lower than those of the other hydrothermal systems so far reported. These low δ 34 S values and the acidic nature of the vent fluids suggest that volcanic SO 2 gas plays an important role on the sulfur isotope systematic of the Onsen hydrothermal system. Relationship among the δ 34 S values of S-bearing species can be successively explained by the model based on the disproportionation reaction starting from the volcanic SO 2 gas. The predicted δ 34 S values of SO 2 agree with the measured whole rock δ 34 S values. δD and δ 18 O values of clay minerals separated from the altered rock samples also suggest the contribution of the magmatic fluid to the hydrothermal system. Present stable isotopic study strongly suggests that the Onsen hydrothermal site in the Desmos caldera is a magmatic submarine hydrothermal system.

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The Chatree deposit is located in the Loei-Phetchabun-Nakhon Nayok volcanic belt that extends from Laos in the north through central and eastern Thailand into Cambodia. Gold-bearing quartz veins at the Q prospect of the Chatree deposit are hosted within polymictic andesitic breccia and volcanic sedimentary breccia. The orebodies of the Chatree deposit consist of veins, veinlets and stockwork. Gold-bearing quartz veins are composed mainly of quartz, calcite and illite with small amounts of adularia, chlorite and sulfide minerals. The gold-bearing quartz veins were divided into five stages based on the cross-cutting relationship and mineral assemblage. Intense gold mineralization occurred in Stages I and IV. The mineral assemblage of Stages I and IV is characterized by quartz-calcite-illite-laumontite-adularia-chlorite-sulfide minerals and electrum.

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