IS ORE DEPOSIT FORMATION RELATED TO RECHARGE EPISODES OF VOLATILE-RICH MAGMA: EVIDENCES FROM MELT INCLUSIONS AND HIGH-PRECISION PB ISOTOPES

Experiments indicate that any proportion of melt and fluid can coexist as a homogeneous mixture in subduction P-T regimes. Such a homogeneous mixture separates into a hydrous melt and fluid phase with decreasing temperature and pressure. If such a fluid-melt mix ascends quickly and encounters a stalled magma body close the surface, the ascent will stop and rapid exsolution of large quantities of volatile components can occur. This volatile phase will be highly charged with ore-forming metals and has the potential to form large ore deposits. Evidence obtained from melt inclusions and high-precision Pb isotope measurements indicates that such mechanism may be responsible for the formation of large ore deposits like the giant Ladolam gold deposit on Lihir Island (PNG), and the polymetallic gold mineralization on nearby Conical seamount. The seamount and the island are composed mainly of trachybasalts and basaltic trachyandesites, although some monzonites are found on Lihir. Microprobe elemental analyses of Conical clinopyroxenes reveal chemical zoning consistent with a mafic magma recharge episode in an evolving magma chamber beneath the seamount. Some of the zones contain numerous small melt inclusions composed of alkaline alumosilicate glass, a fluid phase, and clusters of needle-shaped apatite crystals. Most of the inclusions contain a bubble that occupies up to 1/3 of the inclusion, indicating the trapped alumosilicate melt was oversaturated with volatile components and low microprobe totals (89 to 97 wt%) are consistent with high volatile content. High-precision Pb isotopic analyses conducted with MC-ICP-MS reveal small, but significant, differences between the mineralized zones and the associated host lavas indicating that the ore metals were primarily derived from the recharging magma, not by hydrothermal leaching of the host lavas or underlying sedimentary rocks. Combined evidence from melt inclusions and Pb isotopes suggest that major ore deposit formation may be related to one or more volatile-rich magmatic pulses within otherwise barren polyphased volcano-magmatic complexes.