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

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Authors:Gena, Kaul; Mizuta, Toshio; Ishiyama, Daizo; Urabe, Tetsuro
Author Affiliations:Primary:
Akita University, Institute of Applied Earth Science, Akita, Japan
Other:
University of Tokyo, Japan
Volume Title:Resource Geology (Tokyo 1998)
Source:Resource Geology (Tokyo 1998), 51(1), p.31-44. Publisher: Society of Resource Geology, Tokyo, Japan. ISSN: 1344-1698
Publication Date:2001
Note:In English. 35 refs.; illus., incl. 4 tables
Summary: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 SiO2 which is much higher (58-78 wt%) than SiO2 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 SiO2 in the partly altered basaltic andesite are strongly lost (e.g. Al2O3 = -8.3 to -10.9 g/100 cm3, Ba = -2.2 to -5.6 mg/100 cm3, La = -130 to -200 µg/100 cm3) during the alteration process. Abnormal depletion of MgO, total Fe as Fe2O3, LREE especially Eu and enrichment of SiO2 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.
Sections:Geochemistry; Petrology
Subsections:Hydrogeochemistry; Metamorphism: thermal, metasomatic; Minerals and ores; Oceanic petrology
Subjects:Acidic composition; Andesites; Back-arc basins; Basins; Hydrothermal alteration; Igneous rocks; Metal ores; Metallogeny; Metasomatism; Mineral deposits, genesis; Mineralization; Ore-forming fluids; Sulfates; Volcanic rocks; Australasia; Bismarck Sea; Manus Basin; Pacific Ocean; Papua; Papua New Guinea; South Pacific; Southwest Pacific; West Pacific; Desmos Caldera
Coordinates:S034300 S034100 E1515300 E1515100
Abstract Numbers:05M/1098
Record ID:2001065596
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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