Potassic igneous rocks from the vicinity of epithermal gold mineralization, Lihir Island, Papua New Guinea

Many world-class porphyry copper-gold and epithermal gold deposits worldwide are hosted by volatile-rich and oxidized alkaline rocks. This study investigates potassic igneous rocks from the vicinity of epithermal gold mineralization at Lihir Island, Papua New Guinea. The island consists of five Pliocene-Pleistocene stratovolcanoes, one of which hosts Ladolam, one of the largest epithermal gold deposits discovered to date. Petrographically, the rocks range from porphyritic trachybasalts, trachyandesites and latites to rare phonolites and olivine-clinopyroxene cumulates. In some places, these rocks are cut by monzodiorite stocks. According to Al-in-hornblende barometry, the main crystallization of these rocks occurred close to the surface. Titanium-in-hornblende thermometry as well as olivine-spinel geothermometry and oxygen barometry indicate temperatures of 787-965 degreesC at elevated oxygen fugacities (f(O2)) Of 1.4-4.8 log units above that of the FMQ buffer. Although previous studies have suggested high f(O2) of alkaline rocks associated with copper-gold mineralization based on abundant primary magnetite contents, this is the first direct determination of the f(O2) of such rocks. High f(O2) of parental melts commonly delays the early crystallization of magmatic sulphides; this is important because metals such as Au and Cu preferentially partition into sulphide phases resulting in their depletion in the melt during increasing fractionation. Geochemically, the rocks range from primitive to relatively evolved compositions, as reflected by their SiO2 (45.8-55.0 wt.%) and MgO (1.4-15.3 wt.%) contents and variable concentrations of mantle-compatible elements (130-328 ppm V, 1-186 ppm Ni). Their high K2O content (up to 4.7 wt.%), high average K2O/Na2O ratios (0.8) and high average Ce/Yb ratios (14) are typical of high-K igneous rocks transitional to shoshonites. Although these rocks formed by decompression melting related to back-are rifting in the Manus Basin, the high LILE, low LREE and very low HFSE concentrations are typical of potassic igneous rocks from oceanic (island) are settings. The reason for this remarkable composition is the partial melting of subduction-modified lithospheric mantle, which developed in a stalled subduction zone. Mica phenocrysts in the rocks reveal unusually high halogen concentrations. Magmatic phlogopites contain high F (up to 5.6 wt.%) and elevated Cl contents (< 0.08 wt.%). Hydrothermal biotites from rocks that display potassic alteration have low F (< 0.08 wt.%), but very high Cl concentrations (up to 0.15 wt.%). It is suggested that chloride complexing largely controlled the abundances of Au and Cu in the aqueous fluids responsible for the hydrothermal gold mineralization at Ladolam. (C) 2001 Elsevier Science B.V. All rights reserved.