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Veined peridotite xenoliths from the mantle beneath the giant Ladolam gold deposit on Lihir Island, Papua New Guinea, are 2 to 800 times more enriched in copper, gold, platinum, and palladium than surrounding depleted arc mantle. Gold ores have osmium isotope compositions similar to those of the underlying subduction-modified mantle peridotite source region, indicating that the primary origin of the metals was the mantle. Because the mantle is relatively depleted in gold, copper, and palladium, tectonic processes that enhance the advective transport and concentration of these fluid soluble metals may be a prerequisite for generating porphyry-epithermal copper-gold deposits.
The tectonic relationship between subduction-- related magmatism at convergent margins and porphyry copper-gold (Cu-Au) ore formation has long been recognized (1). However, the physical and chemical processes that govern Cu-Au metallogeny and the ultimate sources) of the metals in these ore deposits are poorly understood. The rhenium-osmium (Re-Os) isotopic system (based on the beta^sup -^ decay of ^sup 187^Re to ^sup 187^Os) is a tracer of metallogenic processes at convergent margins because both elements have geochemical properties similar to metals that occur in porphyry ore deposits (2, 3). Because Re is highly concentrated in crustal rocks and Os is concentrated in the mantle (4, 5), this isotopic system is particularly useful for quantifying the flux of ore elements in island arc settings where the two principal reservoirs for metals are subducted crust and mantle wedge peridotite.
Os isotope studies in subduction zones are currently limited because of the rarity of deep-seated suprasubduction samples (rocks overlying a subducted slab). Previous studies (6, 7) have demonstrated that radiogenic Os is introduced into the subarc mantle by hydrous, oxidizing fluids derived during slab dehydration. We report on results from a suprasubduction xenolith locality, the Tubaf seamount in the Lihir island group of the Tabar-Lihir-Tanga-Feni island arc in Papua New Guinea (Fig. 1). This xenolith locality is important for the following reasons: (i) it contains samples that represent a complete section of oceanic lithosphere at an intraoceanic convergent margin, (ii) it is located adjacent to one of the world's largest and youngest volcano-hosted Au deposits, and (iii) it contains metasomatized mantle peridotite xenoliths with Au-enriched vein minerals that crystallized in the mantle from oxidizing, alkali- and sulfur-rich hydrous fluids.
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