Summary:
Woodlark Island is located on the northern margin of the Woodlark Basin. The geology of the island consists of basement Palaeocene-Eocene tholeiitic basalt and sediments (Loluai formation) overlain by mid-Miocene calcalkaline to shoshonitic volcanics (Okiduse formation), which hosts most of the known gold mineralisation.
The island is relatively flat, with the highest point, Mt. Kabat at 243 m above sea level. The majority of the island is covered by a Pleistocene age coral reef formation (Kiriwina formation), effectively masking prospective volcanic sequences.
As the second declared gold field in PNG, gold mining on Woodlark Island commenced in 1895 at several alluvial sites in the Suloga Bay. Numerous alluvial prospects were developed in watersheds across the Okiduse formation, ahead of underground mining operations at Busai and Kulumadau in the early 20th century. Kulumadau was the deepest underground mine in PNG until the 1920s.
The three known main deposits, Kulumadau, Busai, and Woodlark King (Boniavat) are interpreted as structurally controlled epithermal gold deposits, and have been the focus of rigorous drill testing (~300,000 m) through targeted campaigns since the 1960s.
The deposits show evidence of complex paragenetic assemblages and variable degrees of structural overprint. Kulumadau and Busai are differentiated by the significant amount of brecciation and cataclasis at Kulumadau. Both deposits remain open at depth and along strike.
Extensive post-mineralisation Kiriwina formation sediments mask much of the prospective geology, resulting in very little regional scale exploration beyond the areas of known mineralisation. A key focus of the GPR exploration team is to identify, through geochemical sampling, mapping and geophysics, new areas for exploration that have the potential to host gold deposits capable of augmenting the existing Project.
The extensive resource drilling database, geophysical surveys, and limited surface exploration all indicate the widespread occurrence of gold mineralisation well outside the defined resources. Major regional structures associated with known gold resources are evident on geophysical images and form the principal targets for exploration.
Geological Setting
Gold mineralisation at Woodlark is principally hosted by the Miocene age volcanic rocks, and associated with lodes, quartz veins, stockwork zones, and breccias developed within alteration envelopes associated with intrusive breccia complexes. Gold mineralisation is consistent with low sulphidation, base metal carbonate, epithermal systems which are typical of the South-West Pacific.
The zones of alteration and the associated mineralisation contained is interpreted to be controlled by lithology, stratigraphy, and structure. High-grade mineralisation exists within the lower-grade envelopes at Great Northern and Wayai Creek and requires additional drilling to assess.
The majority of Woodlark Island is covered by a veneer of post mineralisation limestone (coronus) of variable thickness, with associated marine clays and basal conglomerates. Potential for additional gold discoveries remain under this cover and the along-strike extensions of existing MREs are important targets.
Deposit Geology
The Great Northern gold deposit is located to the northeast of the Kulumadau gold deposit, and is underlain by Miocene Okiduse Volcanics, which form part of the Uvarakoi Caldera Volcanics, and is covered by recent Holocene and Pleistocene Kiriwina Formation.
The Okiduse Volcanics consist of fine to coarse-grained porphyritic andesites, epiclastics, conglomerates, and minor basalt. The Kiriwina Formation consists of a basal conglomeratic unit and coralline limestone. There is little natural outcrop, and most geologic data has been obtained from costeans which penetrate the relatively thin Kiriwina cover to the north of Kweiyau Creek. There are no exposures of volcanics to the south of Kweiyau Creek due to down-faulting as described below.
The dominant structure is the WNW-trending Kweiyau Fault, which down-faults Kiriwina Formation to the south, against volcanics to the north, and therefore has been active in Pleistocene time (Figure 5). This late normal movement is interpreted based on slickensides preceded by a strike-slip component related to the mineralisation. Minor N-S to NE-SW striking splays from the main fault have been mapped as soft blue to greygreen clay shears. Gold mineralisation occurs as lenses within the Kweiyau Fault but truncated by the northsouth to northeast-southwest minor structures. The mineralisation consists of quartz-sulphide (pyrite, chalcopyrite, galena) clay breccias which pinch and swell.
Mineralisation is hosted in Miocene Talpos Creek Formation which form part of the Watou Mountain Eruptive Centre (Lindley, 2016 & 2021) consisting of lavas, tuffs and ash deposit. Mineralisation is inferred to be hosted in tensional quartz veins that have developed due to lateral movement along the regional N-S feature, which has developed jogs at the intersection of northwest and northeast conjugate structures.
Further reactivation of the northwest structure has resulted in mineralised hydrothermal breccias being emplaced in association with mafics and felsic dyke intrusions. Subsequent post mineralisation faulting has disrupted the vein and breccia zones. Legacy surface mapping and sampling conducted by BHP and HGL (1980s & 1990s) and more recent Kula Gold (2014 to 2015) surface trench mapping and sampling is being compiled. This will improve the understanding of the geological and structural controls to the Wayai Creek mineralisation. The Little MacKenzie prospect has been traced for approximately 1 km at surface with soils analysis and mapping and the mineralisation interpreted to be hosted in the Talpos Creek Formation Volcanics has not been adequately tested with drilling to form a mineral resource. The Woodlark King Mining Centre mineralisation may represent apotheosis to larger mineralised system at depth.
Dimensions
Wayai Creek mineralisation trends to the southwest and is about 500 m long, 100 m wide and extends about 100 m below surface. This is not a solid volume of mineralisation and includes waste zones.
Great Northern consists of several west-southwest trending pods, ranging from about 50 m long to 150 m long and about 10 m wide. Vertically the mineralisation is defined over about 150 m and extends from surface down.