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

Abstract

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.

Laboratory and field-based investigations of subsurface geochemical processes in seafloor hydrothermal systems

“…Several outcrops of heavily bleached rocks cemented with native sulfur were observed near active vents, presumably reflecting advanced argillic alteration by acid-sulfate type fluids (Brimhall and Ghiorso, 1983;Gena et al, 2001;Binns et al, 2007). However, no 'white smoker activity' similar to the DESMOS caldera site (Gamo et al, 1997;Gamo et al, 2006) has been observed anywhere on Pual Ridge.…”

“…In contrast to MOR settings, where CO 2 is the dominant volatile degassed, the exsolution of large quantities of H 2 O and CO 2 from water-rich silicic magmas can be accompanied by acid-volatile species such as SO 2 , HCl and HF. Subsequent entrainment of water-rich magmatic fluids into existing hydrothermal circulation cells could substantially lower pH, thereby influencing the transport of dissolved metals and styles of crustal alteration (Gamo et al, 1997;Gena et al, 2001;Gamo et al, 2006;. Moreover, magmatic fluids have long been proposed as an additional source of economically important metals (i.e.…”

Chemical and Isotopic Compositions of Hydrothermal Fluids at Snail, Archaean, Pika, and Urashima Sites in the Southern Mariana Trough

“…Also, a fundamental question about mass flux at Earth's surface is the influence of subducting materials and magmatic volatiles. Extremely acidic fluids affected by magmatic volatiles raise the solubility of metals and mobilize them from rocks to fluids (Gamo et al 1997b(Gamo et al , 2006Gena et al 2001;Yang and Scott 2006). This aspect of the metal supply may have a great influence on Fe-utilizing ecosystems.…”

Geochemical tracers of processes affecting the formation of seafloor hydrothermal fluids and deposits in the Manus back-arc basin

“…Pyrite precipitated within this environment is a product of reaction between H 2 S derived from magmatic SO 2 disproportionation and Fe-bearing minerals within the host-rock. Although pyrite precipitation likely occurs at greater depths beneath the seafloor within more reducing fluids, disseminated and veined pyrite has been described in advanced-argillic alteration assemblages at seafloor at both DESMOS and SuSu Knolls Gena et al, 2001;. shift to more oxidizing conditions (i.e., S0 4 -dominant) can result from continued degassing of SO 2 and alteration of crustal rocks that exhausts the capacity of the rock to buffer H 2 S/SO 4 2 , similar to supergene alteration processes described for continental magmatic-hydrothermal processes (Giggenbach, 1997).…”