Acid-sulphate type alteration and mineralization in the Desmos caldera, Manus back-arc basin, Papua New Guinea

The Onsen acid-sulphate type of mineralization is located in the Desmos cal dera, Manus back-are basin. Hydrothermal precipitates, fresh and altered ba saltic 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 al teration. Alteration stage I is characterized by coarse subhedral pyrophyll ite with disseminated anhedral pyrite and enargite which were formed in the temperature range of 260-340 degreesC. Alteration stage II which overprint ed alteration stage I was formed in the temperature range of 270-310 degree sC and is characterized by euhedral pyrite, quartz, natroalunite, cristobal ite and mixed layer minerals of smectite and mica with 14-15 Angstrom XRD p eak. Alteration stage m is characterized by amorphous silica, native sulphu r, covellite, marcasite and euhedral pyrite, which has overprinted alterati on 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 rar e earth elements content except for SiO2 which is much higher (58-78 wt%) t han SiO2 content of the fresh basaltic andesite (55 wt%). REE patterns of t he partly altered basaltic andesite specimens are variably depleted in LREE and have pronounced negative Eu anomalies. Normalization of major, minor a nd REE content of the partly altered basaltic andesites to the fresh basalt ic andesite indicates that all the elements except for SiO2 in the partly a ltered basaltic andesite are strongly lost (e.g. Al2O3 = -8.3 to -10.9 g/10 0cm(3), Ba = -2.2 to -5.6 mg/100cm(3), La = -130 to -200 mug/100cm(3)) duri ng the alteration process. Abnormal depletion of MgO, total Fe as Fe2O3, LR EE especially Eu and enrichment of SiO2 in the altered basaltic andesites f rom the Desmos caldera seafloor is caused by interaction of hot acidic hydr othermal fluid, which originates from a mixing of magnetic fluid and seawat er.