Cockatoo Island
Location and Access
Cockatoo Island is situated in the Buccaneer Archipelago, approximately 240km north east of Broome in Western Australia.
Access to the island is via commercial flights from Perth to Broome or Derby, followed by a charter flight, or boat, to Cockatoo Island. On the island a network of sealed and unsealed roads and tracks provide access to most areas.
The tenements are prospective for iron ore and contain numerous infrastructure elements which at various times have supported the mining operations, such as the airstrip, offices, water storage and a series of tracks that support the exploration activities. There are accommodation and other facilities on the island, however these are not owned by Pearl Gull. There is mobile telephone coverage on parts of the island.
The Company’s Project at Cockatoo Island provides Pearl Gull with not only high-grade exploration potential but also strategic positioning with respect to infrastructure required for both Pearl Gull’s planned operations, and the operations of third parties looking to conduct mining works or other operations on Cockatoo Island.
Climate and Physiography
The climate is semi-arid monsoonal, with a wet season and a dry season. Almost all the rainfall, for which the average is between 800 – 1200mm, is derived from monsoonal rains, cyclones and thunderstorms between November and April. During the dry season, the days are warm to hot, and the humidity is low. In the wet season temperatures are high with high humidity.
Moist areas, such as narrow valleys and gorges, support small patches of rain forest and along the coast, where silt has accumulated, the shores are fringed by mangroves. The tidal range is large (up to 10m), and fringing coral reefs surround the island, which is separated from its neighbours and the mainland by deep channels. The reefs are narrow, except in bays or other sheltered locations where extensive lateral growth has been possible.
The topography of Cockatoo Island, part of Yampi Sound, consists of low sinuous ridges, rugged hill country and irregular shorelines with numerous rocky headlands. The exposed rocks consist of alternating hard and soft ferruginous beds and the action of tropical weathering has resulted in a rugged topography and an irregular coastline.
Regional Geology
The geology forming the islands and coastline of Yampi Sound consists of Early Proterozoic (c.1840Ma to c.1800Ma) Kimberley Basin sediments. The succession, which represents shallow marine shelf deposits, is made up of two stratigraphic groups: the Speewah Group and the conformably overlying Kimberley Group.
The Kimberley Group consists of a sequence of conglomerate, arkose, quartz sandstone, feldspathic sandstone, siltstone, mudstone, and glauconitic sandstone together with tholeiitic metabasalt, tuffaceous sandstone and agglomerate. The provenance of the Kimberley Group sediments is more complex in the Yampi area than in other parts of the Kimberley Basin. The Kimberley Group consists of six formations, with the most important, in terms of iron mineralisation and the geology of Cockatoo Island, being the Yampi Formation. This unit is the uppermost unit of the Kimberley Group and where it overlies the Pentecost Sandstone the contact is defined as the first appearance of appreciable quantities of haematite.
The principal rock types in the Yampi Formation are quartz sandstone, haematite sandstone, haematite, phyllitic siltstone, and quartz pebble conglomerate. The sandstones are silica cemented, and all the rocks have undergone low-grade metamorphism, with the terms ‘schist’ and ‘quartzite’ having been applied to them. The effects of metamorphism are, however, slight and confined to a small area. The overall appearance of the rocks is sedimentary.
Local Geology
The Yampi Formation comprising a marine clastic assemblage deposited in a near shore, beach, or beach-bar environment forms most of the outcropping geology on Cockatoo Island. The 900m thick formation consists of various sandstones, siltstone, and quartz-pebble conglomerate. Detailed mapping by workers during the 1950’s and early 1960’s delineated approximately 550m of conformable, clastic sediments exposed on Cockatoo Island, most of which contained haematite in varying proportions. The structure on Cockatoo Island is dominated by folding, with three major folds, one anticline and two synclines, being present.
All folds are overturned and have axial planes that run parallel with the long axis of the island. Minor, normal faults, generally dip-slip or oblique, disrupt the beds. The previously mined mineralisation, which is outside the Pearl Gull tenements, occurs at the base of the Cockatoo formation. The study of heavy mineral abundances suggests that the ores have formed through the concentration of detrital haematite by reworking and winnowing on an ancient beach or sand-bar. The ore body which is mined out to approximately 40m below sea level comprises a single haematitic arenite bed approximately 40m wide cropping out pre-mining as a dip slope along the southern side of the island. This bed extends for 2,130m along strike, originally reached 140mRL (averaging 80m above sea level) and has been intersected by drilling at over 210m below sea level. The haematitic arenite is interbedded with, and along strike grades into, haematite poor clastic sediments.
Mineralisation
The Yampi Sound Islands have a long history of iron ore mining dating back to the early 1900s. Haematite ore outcrops at the base of the Yampi Formation, where it unconformably overlies the Elgee Siltstone on Cockatoo Island and Koolan Island. The orebodies occur around an overturned syncline and at Koolan Island an adjacent overturned anticline. The main orebodies originally extended up to 180m above sea level and are known to extend for at least 210m below sea level. The ore consists predominantly of haematite (grading 66-67% Fe). Small amounts of magnetite are present as relict cores enclosed by secondary haematite. Minor quartz-pebble conglomerate with a haematite matrix is present. The ore grades laterally into haematitic conglomerate or haematitic sandstone.
Local upgrading of the deposit has resulted from leaching of quartz pebbles from the conglomerate. The boundary of the leaching is usually sharp. The ore bodies have been interpreted as heavy-mineral concentrations in clastic sediments. Iron minerals were deposited in the Warton Sandstone and were then eroded and redeposited near a beach or offshore bar. Concentration was the result of prolonged reworking during which quartz and tourmaline were removed to leave a deposit essentially consisting of haematite or magnetite sand. The minor element chemistry of haematite from the orebodies suggests primary derivation from a banded iron-formation source.
The orebody was reported to have two key components, a high-grade zone of almost pure steely blue haematite averaging 69% Fe and a lower grade zone in the footwall, comprising clay schists, haematitic schists, haematitic sandstone, and banded haematite in sediments known as the ‘blue band’ zone.
Cockatoo Mining History
In the late nineteenth century pearlers used ironstone from the beaches of Yampi Sound as ballast in their vessels. In 1907 the first mining lease was taken up on Koolan Island (5km to the east of Cockatoo Island) and small-scale mining began. The first large-scale attempt to mine iron ore from Koolan Island came in 1936 when the Yampi Sound Mining Company announced its intention to supply the commodity to Japan. The Commonwealth Government placed an embargo on all exports in 1938 before an export industry could be established. The embargo was brought about by Japan’s invasion of China and reflecting the environment of mistrust in the years leading up to the Second World War.
Significant iron ore production began in 1951 when BHP was granted mining leases over the known deposits at Cockatoo and Koolan Islands. At Cockatoo Island, BHP completed mining and direct ore shipping in 1986. To the north and east of the mine within the area covered by Pearl Gull’s M04/235 several low-grade dumps were created as a result of stockpiling when BHP mined the main Cockatoo Island pit.
In 1991 M04/235 was granted to Nugold. A feasibility study was completed into processing the dumps by beneficiating the material using gravity and magnetic separation, to produce a high-grade fines product. It was reported by NuGold that material at a grade of 40% Fe was contained within the dumps.
In April 1993, a joint venture agreement with Portman Mining Ltd was signed to manage the beneficiation operation. All the drilling reported during this time was to test the grade of the stockpile material, with all holes ending at the natural ground surface. The beneficiation plant was constructed during 1993 and mining commenced. The first shipments of beneficiated ore at 65-66% Fe were made late 1994 to China and continued through to 1999, when they ceased due to an economic downturn. Remnant mining of the Seawall deposit continued between 2000 and 2002 before further pit cutbacks were made by building a seawall to access ore below the water table. Mining below sea level, using a constructed rock seawall, occurred from 2002 until 2012 by Portman, and then by Pluton from 2012 to 2014.
Previous Local Exploration
The principal rock types in the Yampi Formation are quartz sandstone, haematite sandstone, haematite, phyllitic siltstone, and quartz pebble conglomerate. The sandstones are silica cemented, and all the rocks have undergone low-grade metamorphism, with the terms ‘schist’ and ‘quartzite’ having been applied to them. The effects of metamorphism are, however, slight and confined to a small area. The overall appearance of the rocks is sedimentary.
In 2001, Portman, on behalf of the joint venture, commenced the first reconnaissance geology mapping and sampling to define new iron mineral resources. The lenses sampled were considered too thin to be of economic significance.
In 2005, four samples of haematitic sandstone were taken from the Cutting quartzite and Magazine schist to conduct test work to determine whether the sandstones would be amenable to a beneficiation process. The results indicated that a process of crushing and grinding to a top size of 0.1mm was required to liberate the iron and gangue minerals. Separation using wet high-intensity magnetic separation (WHIMS) was able to produce a high-grade product at acceptable recovery levels.
In 2008 and 2009 a larger RC drilling program was conducted to further define material suitable for beneficiation within the Magazine schist and the underlying Cockatoo formation.
Figure on the left shows the location of the exploration drill holes.
On 4 September 2012 Pluton announced it had purchased the Cockatoo Island Assets, including M04/235, continuing with Portman’s seawall cutback mining of the Seawall deposit on mining lease M04/448. NuGold retained a royalty over the project as the tenement holder. On 5 September 2003 NuGold changed its named to Pelican Resources Ltd (Pelican) and transferred the title of M04/235 to Pelican.
Recent Exploration
Exploration has included reconnaissance visits to determine the first stage of proof that the geology is permissive for discovery of extensions to the Seawall deposit, new locations of Cockatoo Formation hosted iron ore along a parallel anticlinal fold hinge position analogous to Koolan Island, and possible beneficiation ore and potential ballast sources.
A total of 25 holes was drilled in 2008 to test iron grade and mineralisation widths of haematitic quartzite horizons within the fold axial plane of the Magazine schist north of the main Seawall pit. Drilling showed that haematitic quartzite bands were much thinner than expected.
In 2008, an exploration target was estimated based on downhole geology and outcrop geology of iron mineralisation which could then be used for beneficiation purposes. No JORC compliant Mineral Resources or Ore Reserves have been estimated. This work has enabled an exploration target of between approximately 20 Mt and 30Mt at between approximately 20% Fe and 25% Fe to be estimated within the Magazine Schist. Pearl Gull does caution that this target is conceptual in nature as insufficient work has been undertaken to allow for the estimation of a Mineral Resource and it is uncertain if additional exploration would result in the estimation of a Mineral Resource. Metallurgical tests completed to date have indicated that beneficiation, via a WHIMS processing plant, can produce a saleable product.
The transfer of the tenements from Pelican to Pearl Gull occurred on 3 October 2018. The transfer included mining lease M04/235, and miscellaneous licence applications L04/102 and L04/103.
Exploration Potential
The potential of the Pearl Gull tenements surrounding the Cockatoo Island Seawall mine is to find new structural positions of the rich haematite unit mined by BHP, Portman and Pluton from 1951 until 2014. In addition, several areas with geology potentially suitable for beneficiation processing to produce an enriched ore have been identified with initial drilling and test work in some areas conducted by previous operators.
