HR: 16:00h
AN: V54A-01    [Abstracts]
TI: Magmatic Contribution of Ore Metals to the Conical Seamount Hydrothermal System, Papua New Guinea: a High-Precision Pb Isotope Study.
AU: Perfit, M
EM: mperfit@geology.ufl.edu
AF: University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611
AU: * Kamenov, G
EM: kamenov@ufl.edu
AF: University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611
AU: Mueller, P
EM: mueller@geology.ufl.edu
AF: University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611
AU: Jonasson, I
EM: ijonasso@nrcan.gc.ca
AF: Geological Survey of Canada, Ottawa, Ottawa, ON K1AOE8
AB: During RV SONNE cruise SO-133 in 1998 polymetallic gold mineralization was discovered at Conical Seamount, located on the flank of Lihir island, Papua New Guinea. The seamount and the island are composed mainly of trachybasalts and basaltic trachyandesites, although some monzonites are found on Lihir. Sr isotopic analyses suggest that most of the Sr in the mineralized samples is derived from the local alkaline lavas. Elevated 87Sr/86Sr ratios in some of the samples suggest that during the waning stages of the hydrothermal system, some of the Sr was contributed either from seawater or the thick sequence of marine sediments underlying the island. High-precision Pb isotopic analyses conducted with MC-ICP-MS show that the ores and volcanic rocks share similar Pb isotopic compositions, suggesting that the Pb in the mineralized zones was ultimately derived from local magmatic sources. The Pb isotopic data, however, reveal small, but significant, differences between the mineralized zones and the associated host lavas. Mineralized samples from Lihir have slightly less radiogenic lead isotopic ratios than their host lavas. These lead isotopic compositions are similar, however, to some of the fresh lavas recovered from Conical seamount and to a monzonite intrusion underlying the Ladolam deposit. Lead isotopic ratios in mineralized samples from Conical seamount, however, are slightly more radiogenic than their host lavas and similar to those of fresh lavas recovered from nearby Tubaf and Edison seamounts. Petrographic data reveal a complex magmatic history for the magma chamber inferred beneath Conical seamount. Based on zoning patterns in the Conical clinopyroxenes, it appears that a sub-seamount magma chamber was recharged with a mafic magma similar to the most primitive, volatile-rich, and xenolith-bearing lavas recovered from Tubaf and Edison seamounts. Rapid cooling of this mafic magma accompanied by exsolution of metal-bearing fluids in the relatively shallow magma chamber is proposed as the mechanism that ultimately resulted in the observed gold mineralization. Similarity of the lead isotopic ratios between the ores and the mafic magmas in the area suggest that the ore metals were primarily derived from the recharging magma, not by hydrothermal leaching of the host lavas by seawater-derived hydrothermal fluids.
DE: 8424 Hydrothermal systems (8135)
DE: 3640 Igneous petrology
DE: 3665 Mineral occurrences and deposits
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting