Woodlark Island is located within the Papuan Islands Terrane, a continuation of the onshore Eastern Fold Belt which includes the Louisiade Archipelago (Misima and Sudest Islands) and D’Entrecasteaux Islands (Goodenough, Fergusson and Normanby Islands). The basement on Woodlark Island comprises Cretaceous to Eocene low-K, tholeiitic ocean ridge basalts believed to be en echelon to the onshore Papuan Ultramafic Belt, and this is overlain by a sequence of Miocene limestones and overlying volcanic/subvolcanic rocks of the Okiduse Volcanics; the latter of which hosts the known primary gold mineralization on Woodlark. Unconformably overlying the Okiduse Volcanics is the Quaternary-Pleistocene sediments of the Kiriwina Formation which cover ~85 % of the island. Woodlark Island has experienced over 100 years of mining and exploration with a total of 231,059 Au ounces mined from alluvial and hard rock sources since 1963.
All hard rock gold sources discovered to date are of a low to intermediate sulfidation epithermal mineralization style. Of these epithermal deposits, the Kulumadau mine produced 80% of the total hard-rock gold mined prior to 1963. The current Kulumadau gold deposit (3.8 Mt @ 2.3 g/t) is completely hosted within a package of calc-alkaline andesitic volcaniclastics situated at the juncture of a regionally significant NE-trending fault (at the margin of a horst block) and small subsidiary faults. The mineralization appears to be Mid-Miocene in age and gold is hosted in sub-vertical lenses of milled-matrix breccia and veins in adjacent wall rocks. The deposit is almost completely covered by the Kiriwina Formation and has been subjected to post-mineralisation faulting.
The primary aims of the current study are to determine an ore genesis model for the Kulumadau deposit, define the stratigraphy at the deposit scale and determine the spatial distribution of epigenetic alteration facies. The second of these two aims is important for delineating fault blocks and determining their relative movement. Defining the spatial continuity of various volcanic rock-types from drill core is not a trivial task and is further complicated by the pervasive nature of hydrothermal alteration, post-mineralization faulting and the extremely humid conditions which have almost completely oxidized sulfide-bearing diamond drill cores recovered as recently as 2012. To help define the stratigraphy of the Kulumadau deposit, chemostratigraphy has been constructed using fpXRF in conjunction with volcanic textures (if present).
The deposit is strongly structurally controlled and immobile-immobile element analysis has helped in the early stages of delineating the volcanic stratigraphy. Immobile-mobile element analysis in conjunction with detailed core-logging, XRD analysis and petrographic analysis suggest that fluid mixing was an important factor in the genesis of the Kulumadau deposit as the highest grades are often restricted to the areas approaching the juncture between rock-buffered and fluid-buffered alteration assemblages.