Abstract:
In 1991, the PACMANUS hydrothermal field (PACMANUS, Roger's Ruins, Roman Ruins, Satanic Mills, Snowcap, and E foot) and neighboring hydrothermal fields (DESMOS and Susu Knolls) were discovered in the Eastern Manus Basin, Bismarck Sea, southwest Pacific Ocean. A data set of 80 samples and 19 thin sections of fresh and hydrothermally altered andesitic and dacitic rocks dredged from these hydrothermal fields were used to study the petrology and geochemistry of the volcanic host rocks to aid in the investigation of the geology and genesis of these hydrothermal fields. The Eastern Manus Basin consists of Miocene-to-Present age volcanic rocks, of basaltic through rhyodacitic composition. Alteration is ubiquitous in the 19 thin sections. Igneous plagioclase, + or -clinopyroxene, + or -orthopyroxene, and + or -magnetite and hydrothermal clay minerals, pyrite, and + or -natroalunite are observed in these samples. Plagioclase compositions range from An40 to An90 and pyroxene compositions range from DI95 to DI75 for clinopyroxene and EN85 to EN75 for orthopyroxene. Pearce element ratio (PER) analysis of lithogeochemical data from 80 samples confirm that plagioclase, clinopyroxene, orthopyroxene and subordinate magnetite fractionation are responsible for the observed compositional variations in the fresh rocks, and that pyrite and natroalunite are present in hydrothermally altered rocks. A bulk fractionating plagioclase composition of An37 and pyroxene compositions of DI50 and EN50 were estimated using PER's. Due to the unique hydrothermal mineral observed at these seafloor hotsprings, results from this study cannot be used more generally to understand the geology and genesis of felsic volcanic hosted massive sulphide (VHMS) deposits outside the Manus Basin.