The geological evolution of the Kulumadau epithermal gold deposit, Woodlark Island, Papua New Guinea

Woodlark Island is located within the Milne Bay Province 600 km east of Port Moresby, Papua New Guinea. The mid-Miocene volcanic rocks of Woodlark Island host numerous epithermal deposits, the largest being the Kulumadau deposit. The Kulumadau gold deposit contains 3.8 Mt @ 2.3 g/t with Au:Ag = 1. Despite 100 years of exploration and mining on Woodlark Island, no detailed scientific studies have been undertaken on the Kulumadau deposit, nor have previous authors attempted to place the volcanism or mineralisation on Woodlark Island within a regional geological setting. The Okiduse Volcanics (12.92 ± 0.07 Ma) belong to the Maramuni Arc within the Woodlark Rift and are related to diapirism through modified mantle of the relict Aure-Moresby-Pocklington Trough, as a consequence of slab-pull associated with northward dipping subduction of the downward going Solomon Sea lithosphere at the New Britain and South Solomon trenches of the Melanesian Arc.The Maramuni arc was previously considered to be a high-K calc-alkaline suite on the basis of K2O vs. SiO2 contents. Immobile element discrimination diagrams (e.g. Ta/Yb vs. Ce/Yb and Ta/Yb vs. Th/Yb) utilising rocks from Woodlark Island, and other products of the Maramuni Arc, instead suggest that Maramuni arc rocks within the Woodlark Rift are a calc-alkaline package. The host sequence at Kulumadau is a co-magmatic suite, comprising (from oldest to youngest) plagioclase-rich pyroxene-bearing andesite, the lower plagioclase-rich amphibole-bearing tuffs, upper earlier plagioclase-rich amphibole-bearing tuffs, polymictic debris avalanche breccia and upper later plagioclase-rich amphibole-bearing tuffs which are unconformably overlain by Pleistocene sediments of the Kiriwina Formation. Whole-rock XRF and ICP-MS reveal a regular fractionation trend from the pyroxene-bearing andesite to the amphibole-bearing tuffs and an inverse fractionation trend within the amphibole-bearing tuffs; the latter being attributed to replenishment of the magma chamber. These trends are supported by EMPA of plagioclase and amphiboles. Polymictic debris avalanche breccias are a result of debris avalanches associated with growth faulting and are primarily composed of the lower and upper earlier amphibole-bearing tuffs. Polymictic debris avalanche breccias host a mixture of host blocks and mega-blocks of each of the units at Kulumadau.Structural analysis of SRTM and Heli-mag imagery of Woodlark Island identified early N-S trending faults in pre-Okiduse Volcanics (e.g. > 12 Ma) attributed to slab pull related to the onset of the opening of the Woodlark Basin. Continued N-S extension between 12-11 Ma resulted in NE-SW and NW-SE faults. Subsequent to this continental spreading which initiated at 8.4 Ma transitioned to sea-floor spreading at 6 Ma with the opening of the Woodlark Basin to the south. This resulted in E-W trending faults throughout Woodlark Island. At 2 Ma a triple plate junction was established to the immediate east of Woodlark Island reactivating E-W trending faults to become transpressional faults with some sinistral strike-slip movement. This strike-slip movement was also responsible for the development of P-shears.A lack of outcrop at Kulumadau as well as well as extensive argillic alteration prohibited structural analysis at the deposit scale using conventional structural techniques (e.g. field mapping and core-logging). Instead, faults were delineated using the chemostratigraphy developed using pXRF data and high resolution LiDAR. The chemostratigraphy was defined through a combination of petrographic analysis, core-logging and geostatistical analyses including linear discriminant analysis. Early faulting at Kulumadau (approx. 12-11.5 Ma) was oriented N-S. This was followed by NE-trending dextral faults forming a pull-apart basin, of which Kulumadau was situated on the south-western limb. Normal faulting on this structure was responsible for dip-slip motion to the east. Development of NW-trending joints triggered a debris avalanche responsible for the polymictic debris avalanche breccias. Subsequent volcanism resulted in pyroclastic flows of the upper later amphibole-bearing tuffs which draped the debris avalanche deposits. Further N-S extension and E-W compression resulted in the activation of NW-trending joints to sinistral, reverse faults. In the east of the Kulumadau deposit, reverse faulting was responsible for pushing the pyroxene-bearing andesite over the debris avalanche deposits in the west. At 6 Ma seafloor spreading resulted in N-S extension and the development of E-W trending normal faults. The establishment of the triple plate juncture to the east of Woodlark at 2 Ma reactivated E-W faults at Kulumadau as sinistral strike-slip faults. This continued kinematic regime has continued through to recent times and is responsible for as great as 10 m of throw on N-S trending reverse faults at Kulumadau.Mineralisation at Kulumadau is of the low to intermediate sulfidation epithermal style, and primarily hosted in poorly-consolidated hydrothermal breccias with a matrix comprising illite + quartz + chlorite-rich chlorite/smectite + pyrite ± adularia ± calcite ± corrensite ± sphalerite ± chalcopyrite ± galena ± illite-rich illite/smectite. Monomictic hydrothermal breccias and veins are transitional to polymictic breccias and comprise quartz ± calcite ± pyrite ± chalcopyrite ± sphalerite ± galena ± ankerite. The paragenesis in the monomictic breccias and veins was pyrite I pyrite II sphalerite I (with chalcopyrite disease) + chalcopyrite I + galena I sphalerite II chalcopyrite II + galena II galena III + gold I gold II. At the Kulumadau Main and East lodes mineralisation is structurally controlled, following NE and NW-trending faults, respectively. Fluid inclusion analysis shows a large range in fluid salinities (e.g. 0.88 to 14.77 wt % NaCl) and in conjunction with alteration studies, and a lack of evidence for significant boiling, it appears that mixing of meteoric fluids with saline hypogene fluids was the most likely mechanism responsible for the precipitation of ore constituents. At the Adelaide lode and the northern footwall of the Main lode, focused hydrothermal fluid flow was responsible for hydraulic fracturing of the wallrock. The lithologically controlled permeability contrast between the debris avalanche breccias and the pyroxene-bearing andesites appears to have been responsible for ore precipitation.Due to the poorly consolidated nature of the polymictic hydrothermal breccias at Kulumadau, petrographic analysis was rendered difficult. Compositional principal component analysis was carried-out on assay data for Au, Cu, Pb and Zn to assess element and thus ore mineral relationships. As different treatment methods of left-censored data can greatly impact subsequent principal component analysis, numerous treatment methods were applied; namely the substitution, deletion and distribution-based imputation treatment methods. When compared to the available petrographic data, the distribution-based imputation treatment method out-performed the substitution and deletion methods and was extremely repeatable. Wall-rock alteration studies reveal an early phase of albite and albite + epidote alteration at depth within the Kulumadau deposit followed by more K- enriched, adularia dominant alteration. Away from mineralisation alteration is dominated by rock-buffered assemblages while hypgene upflow zones contain fluid buffered assemblages. Thus, the ratio between adularia and clays (i.e. the adularia number) has been used as a proxy for the relative contribution of meteoric and hypogene fluids. The adularia number for the Main and East lodes supports fluid inclusion studies by showing economic grades in zones of mixing, while higher contributions of hypogene fluids are only responsible for elevated, but non-economic grades. In the Adelaide lode and the footwall of the northern Main lode a high adularia number and calcite abundance supports the interpretation that ore minerals were precipitated in response to lithologically controlled permeability contrast between the debris avalanche breccias and the pyroxene-bearing andesites (i.e. boiling). Sulfate minerals at Kulumadau are post-mineralisation and their stable isotopes (O, S) show a trend from magmatic dominant at the deposit margins transitioning to more seawater-derived values towards the centre of the deposit, with collapse of the hydrothermal system.This study describes an abnormal style of low to intermediate sulfidation mineralisation style using a combination of conventional and unconventional analytical and data processing techniques