Structural analysis of the Busai Au resource, Woodlark Island, PNG
Creators: name: Lennox, PG
name: Graham, IT
name: Newport, R
Publisher: name: UNSW Sydney
Year: 2010

description: The Busai gold prospect consists of the Busai-Vulcan-Federation (BVF) prospects and the along strike Sintapoi-Munasi prospects within the Mid-to-Early Miocene Okiduse Volcanics (a series of predominantly andesitic lavas and volcaniclastics), Woodlark Island, PNG. Gold has been extracted from both alluvial deposits and limited shallow underground workings in the past and Woodlark Mining Ltd. are looking to develop an open-cut gold mine. Analysis of regional aeromagnetic images, Shuttle Radar Topography Mission (SRTM) images and structures measured from diamond drillcore and field data has provided an enhanced understanding of the structural controls exerted on this carbonate-base metal low-sulfidation gold system. The style of mineralisation is not porphyry-related but simply an epithermal low sulfidation system. The gold-silica-carbonate event is later than the base metal event. Woodlark Island is a 70 km x 10-23 km northwest-southeast elongated mainly Cenozoic (Eocene to Recent) volcanic island with Pleistocene fringing reefs located on the Trobriand Plate on the margins of the recently active Woodlark Basin and along strike from the Nubara Fault separating the Solomon Plate from the Woodlark Basin. The fault pattern from the SRTM imagery is consistent with a stepover involving a pair of offset dextral, strike-slip faults oriented NW-SE. The observed approximately east-west faults from the SRTM imagery may represent linking faults between these two major strike-slip faults. This northwest-strike is subparallel to the strike of the southwest-directed subduction zone between the Solomon and Trobriand Plates. As such, this fault would be analogous to the Philippines Fault inboard and onland in the Philippines from the west-directed subduction system offshore. The Central Horst Block on the island which contains the BVF-Sintapoi mineralisation has boundaries consistent with the strike of the Nubara Fault. The regional aeromagnetic map of this horst shows sigmoidally-shaped, low magnetisation bodies which could be interpreted to have formed during sinistral strike-slip movement on the horst-bounding faults. Some of these faults are Recent in age, as shown by their displacing beds of the Pleistocene Kiriwina Formation in places. Fractures measured in the diamond drillcore (DDH) are differently oriented to faults observed in the BVF area and those mapped by Woodlark geologists. Whereas E-W striking faults are dominant in the DDH dataset, the observed faults in the field are more northerly-striking (N-S, NNE-SSW or NNW-SSE). This in part reflects the data gap due to most of the drilling being to the east. It would perhaps be expected that NE-striking faults would predominate due to the orientation of the horst-bounding structures. The lack of these faults and the prevalence of more northerly structures perhaps indicates that these local faults are Riedel to sinistrally-moving bounding faults. Analysis of the faulting between BVF and Sintapoi would be consistent with a model involving two major NW-striking faults undergoing sinistral strike-slip which form a structural corridor. This suggests that mineralisation is open to the south, recently proven by RC drilling by Woodlark Mining. This study shows the importance of understanding the movement history of major faults associated with mineralised volcanic systems and how this can lead to more successful exploration.
description: Specialist Group in Tectonics and Structural Geology; Port Macquarie, New South Wales; 2010-02-01 - 2010-02-05