Geology of the Kapit Ore Zone and comparative Geochemistry with minifie and Liennetz Ore Zones, Ladolam gold deposit, Lihir Island, Papua New Guinea

摘要

The 56 Moz Ladolam gold deposit, with an average gold grade of 2.42 g/t, has previouslyudbeen recognised as the world’s largest known epithermal gold deposit. It is located onudLihir Island, in the New Ireland Province of Papua New Guinea, and is part of the Tabar-udLihir-Tanga-Feni chain of Pleistocene alkalic volcanoes. Alkalic magmatism occurred inudan extensional tectonic regime, in a location where initial arc magmatism was related toudsubduction with the Manus-Kilimailau trench, but was later reactivated as a back arc dueudto the subduction of the Solomon Sea plate along the New Britain trench. Lihir island isudmade up of at least five volcanic blocks, and is surrounded by an uplifted Quaternaryudlimestone reef. The Ladolam gold deposit is located on the eastern side of the island, in theudLuise volcanic edifice. Mineralisation is localised along north-dipping structural zones.udThere are two large ore zones (Minifie and Lienetz) and several smaller (Kapit, Coastal,udPacific and Borefields) in a 3 km2 surface area.udThe Kapit ore body is hosted by diverse volcano-sedimentary and hydrothermaludfacies. The coherent facies include basalt, diorite and andesite. The volcaniclastic faciesudcomprises two types of polymict matrix-supported breccia. Hydrothermal breccia faciesudinclude anhydrite-carbonate-quartz-cemented breccia and pyrite-cemented breccia.udThe Kapit ore zone is characterised by five different paragenetic stages. Porphyrystyleudstage 1 features were not observed at Kapit, but are preserved at Lienetz and Minifie.udStage 2 anhydrite-cemented breccias and veins at Kapit are associated with porphyrystyleudbiotite-K-feldspar-magnetite potassic alteration. Stage 3A pyrite-cemented breccia,udstage 3B quartz-chalcedony-pyrite-cemented breccias and veins, and stage 3C carbonateanhydrite-udquartz-cemented breccias and veins are all associated with phyllic alteration. Inudcontrast, stage 4 disseminated pyrite and pyrite veinlets are associated with intermediateudargillic to advanced argillic alteration assemblages.udGold at Kapit, and elsewhere at Ladolam, is contained in refractory pyrite andudmarcasite. The different types of pyrite and marcasite identified from the Kapit, Lienetzudand Minifie ore zones are acicular, banded, colloform, euhedral, framboidal and anhedral.udLA-ICPMS analyses have revealed that the colloform, framboidal, anhedral ± acicularudmarcasite-pyrite grains have the highest contents of gold and other trace elements. Paragenetically, highest gold contents in Fe-sulfides occur in stage 3A (Lienetz), 3B (Kapit)udand 3C (Minifie) quartz-chalcedony-pyrite-cemented breccias and veins, stage 3A (Kapit)udand stage 3B (Minifie) pyrite-cemented breccias, stage 2B (Lienetz) quartz-anhydritebarite-udcemented breccias, and stage 4 (Kapit) disseminated pyrite and pyrite veinlets. Atudthe deposit scale, Au in pyrite is positively correlated with Ag, As, Sb, Cu, Se, Pb, and Tl.udAnalyses of fluid inclusions, coupled with observed mineralisation and alterationudmineral assemblages from stage 2 (Kapit and Lienetz), show that mineralising fluids wereudrelatively hot (>250°C), saline (4-6 wt% NaCl eq. in average) and had near-neutral pH. Inudcontrast, the mineralising fluids from stage 3 (Kapit, Lienetz and Minifie) were significantlyudcooler (150-250°C), more dilute (1-4 wt% NaCl eq. in average), and acidic. Trends in fluidudinclusion data are interpreted to be the result of mixing between magmatic-hydrothermaludfluids (5-10 wt% NaCl eq., >350°C) and steam–heated seawater (~3.2 wt% NaCl eq.,ud150-200°C). Distinct increases of salinity suggests that adiabatic boiling occured duringudmineralisation; most likely during multiple phases of hydrothermal brecciation. Gold isudlikely to have been transported as a hydrosulfide complex (e.g. Au(HS)₂¯) and boiling and/udor mixing destabilised the Au-hydrosulfide complex and triggered gold precipitation.udGold contents in the mineralisisng fluids were up to 8 ppm, based on LA-ICPMS analyses,udmuch higher than detected in the modern geothermal waters (up to 16 ppb). The positiveudcorrelation of Ag, Tl, As and Sb with Au in fluid inclusions imply that Ag, Tl, As andudSb were transported as hydrosulfide complexes such as H₂As₃S₆¯, HSb₂S₄¯, AgHS(aq) andudTlHS(aq).udLadolam is concluded to be a hybrid gold deposit that contains early porphyrystyleudalteration features, semi-massive pyritic ores hosted by hydrothermal brecciasudthat have affinities with shallow submarine VHMS-style gold mineralisation, late-stageudlow sulfidation style epithermal veins and breccias and modern subaerial to submarineudgeothermal features. The history of the Ladolam deposit, including the Kapit, Lienetz andudMinifie ore bodies is composed of a succession of catastrophic events triggering formationudof different mineralised breccias and veins in an environment that evolved from submarineudto subaerial after a major sector collapse event. Mineralising events can be separatedudinto two distinct phases, transitional porphyry-VHMS (pre-sector collapse) and VHMSepithermalud(post-sector collapse).