Geology, Mineralization, and Hydrothermal Evolution of the Ladolam Gold Deposit, Lihir Island, Papua New Guinea
Graham D. Carman, 2005. "Geology, Mineralization, and Hydrothermal Evolution of the Ladolam Gold Deposit, Lihir Island, Papua New Guinea", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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The Ladolam gold deposit on Lihir Island, Papua New Guinea, is one of the world's largest epithermal gold deposits with a gold resource of 37 million oz. Open-pit mining commenced at Ladolam in 1997. Epithermal gold mineralization formed at <0.5 Ma and is telescoped upon a slightly earlier porphyry environment. A remnant geothermal system is still active.
Lihir Island is located within the Tabar-Lihir-Tanga-Feni volcanic island chain, a group of alkaline silica-undersaturated volcanic rocks of Plio-Pleistocene age (<3.5 Ma) in the Province of New Ireland. Tabar-Lihir-Tanga-Feni magmas were generated in a tensional tectonic regime from a mantle that was preenriched with alkali elements following subduction reversal in the late Miocene. Host rocks to the mineralization are a breccia complex consisting mostly of clasts from preexisting mafic volcanic rocks and altered alkaline intrusions. The setting of the Ladolam breccia complex is a volcanic-hydrothermal diatreme that formed during explosive decompression following sector collapse of the volcanic edifice. Host rocks to the shallow ores are polymictic hydrothermal vent breccias and breccia pipes that overprint the porphyry centers.
Three stages of mineralization are observed. Stage I: Biotite-orthoclase-anhydrite ± magnetite with minor copper-gold-molybdenum porphyry mineralization as veinlets and disseminations; stage II: Refractory sulfide gold mineralization associated with pervasive adularia-pyrite (-leucoxene-illite) alteration in near-surface ores, grading downward into barren anhydrite-adularia-pyrite-vermiculite alteration; and stage III: Quartz-calcite-adularia-pyrite-marcasite ± electrum stockwork veins that overprint and replace earlier anhydrite veins. Stage II refractory sulfide mineralization comprises the bulk-mineable shallow ores in the Minifie area. Stage III veins are mostly subeconomic, although a tabular zone of silicic breccia is an important ore type in the Lienetz area. The latter is capped by advanced argillic alteration of steam-heated origin. Propylitic alteration occurs on the margin of the monzonite intrusion centers, overprinting brecciated volcanic rocks.
Fluid inclusion and stable isotope data are used in conjunction with geologic attributes to interpret a four-stage evolution for the Ladolam hydrothermal system. Stage I porphyry: Pervasive biotite alteration of the monzonites predated sector collapse and extensive brecciation. No fluid inclusions suitable for microthermometric study were found. The δ34S values of pyrite (−1.4 to +2.2‰) and δ18O value of hydrothermal phlogopite (+6.2‰) are consistent with mineralization from a magmatic fluid.
Stage II transitional epithermal: Deep brines of magmatic origin (5–10 wt % NaCl equiv) at 200° to 300°C contained dissolved monovalent-divalent ions and significant gases (CO2 + H2S). Wide-ranging salinities in fluid inclusions from deep anhydrite veins (5–>32 wt % NaCl equiv) is explained by open-system boiling and vapor loss. Fine-grained auriferous pyrite and adularia precipitated in breccias at ∼200°C, immediately above an anhydrite-K feldspar alteration zone as fluids vented to the surface. Pyritic ores were deposited by rapid cooling associated with mixing of the magmatic ore fluid (δ18O ∼6‰) with cool ground water (δ18O ≤0‰), and boiling. The δ34S values of pyrite (−7 to +2‰) and deep anhydrite (13 ± 2‰) suggest a magmatic sulfur source.
Stage III epithermal: Quartz and calcite veins formed under hydrostatic boiling point for depth conditions. Upwelling stage III fluids at 230°C were moderately saline (5 ± 0.5 wt % NaCl equiv) and contained low abundances of dissolved gases (up to 3.5 wt % CO2 + H2S). This brine mixed with dilute meteoric ground water (near 0 wt % NaCl equiv at 170° ± 20°C), depositing quartz as it cooled. Isotopic compositions of deep carbonates (δ13C = −4‰, δ18O = 14‰) indicate a magmatic origin for the brine (δ13C(CO2) ∼ −3‰, δ18O(H2O) = +6‰). At Lienetz, an advanced argillic cap formed due to tectonic uplift of this area relative to Minifie. Silicic breccia ores in the Lienetz area contain pyrite-marcasite with lighter δ34S values (−13 to −2‰) compared to pyrite from Minifie (δ34S, −3 to −1‰). Lienetz ores probably formed at temperatures below 200°C as reduced fluids mixed with descending, oxidized acid-sulfate waters beneath the advanced argillic cap. Gold from the earlier transitional stage II event may have been remobilized by the acid-sulfate waters and reprecipitated as electrum with fine-grained quartz in this mixing zone.
Geothermal fluids in the modern system are neutral-pH chloride-sulfate brines that have a salinity similar to stage III (∼5.5 wt % NaCl equiv) but contain few if any magmatic volatiles. The modern system is interpreted to be a remnant of the ore-forming one. The deep brines are of magmatic origin (δ18O = +6‰, δD = −25‰) and are diluted by meteoric waters (δ18O = −6‰, δD = −40‰) within the top few hundred meters. These waters discharge into the Kapit-Coastal and Luise harbor areas, while acid-sulfate hot springs of steam-heated origin are associated with areas of steaming ground to the west and north of the major orebodies.
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To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.