Acid-sulphate Type Alteration and Mineralization in The Desmos Caldera, Manus Back-arc Basin, Papua New Guinea

摘要

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 deg C. alteration stage II which overprinted alteration stage I was formed in the temperature range of 270-310 deg C and is characterized by euhedral pyrite, quartz, natroalunite, cristobalite and mixed layer minerals of smectite and mica with 14-15 A 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 percent) than SiO_2 content of the fresh basaltic andesite (55 wt percent). 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_2O_3 = -8.3 TO -10.9 g/100CM~3, Ba = -2.2 to -5.6 mg/100 cm~3, La = -130 to -200 #mu#g/100cm~3) during the alteration process. Abnormal depletion of MgO, total Fe as Fe_2O_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.