Characteristics of the Mantle Source Region of the Ladolam Gold Deposit, Lihir Island, Papua New Guinea

Brent I. A. McInnes1, Noreen J. Evans1, Michel Grégoire2, Chris Ryan1, Suzanne Y. O'Reilly2 and Joanne McCarron1

1.CSIRO EM, 2.GEMOC, Macquarie

Are igneous intrusions that host giant Cu-Au ore bodies derived from "enriched" mantle source regions? This hypothesis has been difficult to test by studying porphyry Cu-Au deposits themselves because these deposits are typically hosted within igneous intrusives which are the products of extensive fractional crystallization of primary arc magmas. Moreover, there is a dearth of knowledge about the composition and structure of subduction-modified mantle at convergent margins because chemically unmodified mantle-derived arc lavas and xenoliths are rare.

Dredge and TV-grab sampling of a 1 km submarine cinder cone (Tubaf volcano, 1280 m BSL; 3°15.25'S, 152°32.50'E), located 14km SW of the giant Ladolam gold mine (+40 M oz contained Au) on Lihir Island returned 130 ultramafic, mafic and sedimentary xenoliths (see Herzig et al., 1994, EOS 75(44), 513-515 for details). Petrological studies of these samples has provided an unprecedented view of the source region of an arc magmatic system with a clear propensity to produce giant ore deposits.

The xenolith assemblage includes spinel lherzolite, harzburgite, websterite, orthopyroxenite, serpentinite, gabbro, hornblende gabbro, plagiogranite, diabase, basalt, pelagic deep-sea sediment and shallow-water volcaniclastic sediment, as well as coralline and coralgal limestone. These lithologies represent a jumbled proxy drill hole with a minimum sampling depth of 17 km (depth to seismically determined Moho) and a maximum depth of 70 km (no garnet-bearing lherzolite). Because these lithologies are distinctly similar to those reported from ophiolite suites, they can be re-assembled into an "ophiolite-type" model of oceanic lithosphere.

The mineralogy of the anhydrous lherzolites (olivine: Mg# 0.87-0.91; orthopyroxene: Mg# 0.92 and average Al2O3=1.7 wt.%; Cr-diopside: En48Wo44Fs3, with average Cr2O3=0.75, Na2O=0.13 and Al2O3=2.0 wt.%; and spinel: Cr# 0.35-0.56) is similar to that of the Cr-spinel lherzolite group (Type I). These mineralogical characteristics and the predominance of harzburgite (87%) over lherzolite (13%) indicates that the mantle source region for the Lihir volcanoes is a depleted peridotite residue from an earlier episode of melting in a mid-ocean ridge environment. The spinel peridotite xenoliths record 2-pyroxene closure temperatures of 850-1050°C and unusually high oxidation states, ranging from +1.5 to +3 log fO2 units greater than similar depleted peridotites sampled from abyssal mid-ocean ridge environments (-1 log fO2 unit less than the FMQ buffer).

Some of the peridotite xenoliths have distinctive vein structures and mineral compositions generated by water-rock interactions within the mantle wedge. The veins are planar, interconnected networks (1 to 6 cm in width) which crosscut earlier ductile deformation. The veins contain fibrous, radiating orthopyroxene with fine-grained Fe-Ni sulfides and minor olivine, clinopyroxene, phlogopite and magnetite. The presence of H2O-rich fluid inclusions in the orthopyroxene and the lack of shear structures in these veins indicates that metasomatism of peridotite occurred via hydraulic fracturing, probably as a result of the influx of slab-derived hydrous fluids into the mantle. Oxidation of the peridotite is synchronous with this event, as evidenced by the occurrence of zoned spinels (Fe2+-rich cores and Fe3+-rich rims), and secondary magnetite in the orthopyroxenite veins.

The xenolith suite sampled from the Tubaf submarine volcano allows a reconstruction of the physical and chemical properties of the oceanic lithosphere below Lihir Island. The mineralogical and geochemical characterisitics of the xenoliths indicates that the source region of Lihir magmatism consists of depleted mantle originally generated at a mid-ocean ridge spreading center, and subsequently modified by subduction-related processes. The peridotitic mantle wedge was subjected to hydro-fracturing and hydration metasomatism at T=700-1000°C and 5-20 kbars pressure as a consequence of dewatering of a subducted slab. Peridotite-water interaction has created a hydrofractured network of oxidized peridotite enriched in H2O, S, Si, Al, Cu, Sr, Pb, U, Th, Na, K and LREE. Work is currently underway to establish to what degree Au and other elements have also been transported. Preferential partial melting of these enriched mantle regions could account for the highly oxidized, sulfur- and metal-rich nature of the high-K calc-alkaline volcanoes of the Tabar-Lihir-Tanga-Feni island arc.

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