High-precision Pb isotope measurements reveal magma recharge as a mechanism for ore deposit formation: Examples from Lihir Island and Conical seamount, Papua New Guinea

Abstract

Lihir Island, part of the Tabar–Lihir–Tanga–Feni island chain in the SW Pacific, hosts the giant Ladolam gold deposit. Polymetallic gold mineralization also was discovered offshore at Conical seamount in 1998 during RV SONNE cruise “SO-133”. The seamount and the island are composed mainly of trachybasalts and basaltic trachyandesites, although some monzonites are found on Lihir. Sr isotopic analyses of mineralized samples from the island and the seamount suggest that most of the Sr 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, part 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 are slightly more radiogenic than their host lavas and similar to those of fresh lavas recovered from nearby Tubaf and Edison seamounts. Petrographic observations and microprobe elemental analyses reveal a complex magmatic history for the magma chamber inferred beneath Conical seamount. Based on zoning patterns in the Conical seamount 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 led to the formation of the gold mineralization. Similarity of the lead isotopic ratios between the ores and the mafic magmas in the area indicate that the ore metals were primarily derived from the recharging magma, not by hydrothermal leaching of the host lavas, suggesting that ore deposit genesis can be related to a single volatile-rich magmatic pulse within otherwise barren polyphased volcano-magmatic complex.