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North Stradbroke Island |
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Queensland, Qld, Australia |
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Ti Zr
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Super Porphyry Cu and Au
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The North Stradbroke Island heavy mineral sand (HMS) rutile - zircon - ilmenite deposits are located on North Stradbroke Island, forming part of the eastern boundary of Moreton Bay, 30 km ESE of Brisbane in SE Queensland, Australia. On the island, mining areas include the current (2008) Enterprise and Yarraman areas, and the earlier Amity, Bayside, Gordon, Vance, Herring, Ibis, Alpha and Kounpee.
The North Stradbroke Island deposits have been the largest producer of rutile, zircon and ilmenite in eastern Australia. The island is some 38 km north-south by 10 km wide in the north, tapering to 4 km in the south. It is the southernmost of several large sand islands composed of transgressive dunes found along the Pacific coast of east Queensland.
The island’s surface is made up of a series of large, vegetated, unconsolidated, parabolic, Pleistocene and Holocene sand dune ridges rising to 240 m above sea level at Mt Hardgrave, five kilometres east of Dunwich on the central western coast. These dunes rest on a sea-cut basement platform, which is up to 40 m below sea level, locally exposed as several small outcrops of basement, specifically Palaeozoic greenstones in the south and Triassic and Jurassic sandstones that form a headland on the western or Moreton Bay side of the island. Several outcrops of Triassic rhyolite form the north-eastern tip of the island at Point Lookout, with peaks rising to over 90 m above sea level. The sand of the island was supplied by northward flowing longshore drift and was stabilised to the south and west of Point Lookout.
The heavy mineral (HM) bearing sands on North Stradbroke Island was derived from the hinterland by rivers and streams and distributed along the coast by the north flowing long-shore currents to form beaches, sand spits and dunes. The HM was concentrated by wave action, particularly swash cycles during storms, to produce high grade beach placers at strandlines that were preserved by burial. Several intense variations in climate related to ice cap developments and periods of worldwide glaciation resulted in regressive cycles when the sea level fell by as much as 140 m below the present level, each time exposing large sand accumulations on the 40 km wide continental shelf east of the island, deposited during that and former regressions.
Periodic strong south-easterly winds and an arid climate killed the natural vegetation cover, allowing the winds to erode the abundant supplies of sand on the beaches and continental shelf. This resulted in the formation of a successive series of high dunes and the inland concentrations of heavy minerals. This onshore transport has produced a system of north-westerly trending transgressive dunes that comprise the bulk of the island. Four phases of Pleistocene and Holocene dune formation have been distinguished, specifically the 'ancient' Pleistocene dune series formed at approximately 0.3 Ma, the 'old' Pleistocene dunes at 0.15 Ma, and the two Holocene series at 70 000 and 20 000 respectively. The Pleistocene dunes are in general, larger and higher, commonly to 100 to 200 m elevation above sea level (asl.), are more degraded and are capped by a well developed podzolic soil profile compared to the Holocene dunes.
Approximately 6 500 years ago, during a short period of slightly elevated sea level, a coastal platform was eroded around the island's margin. The present-day beaches form the margins of this platform and support the fresh water swamps at the edges of the main island aquifer.
Beach HMS deposits occur as intermittent high grade seams southward from a point some 8 km of Point Lookout, while extensive buried strandlines have been encountered beneath dune accumulations from 11 to 26 km south of Point Lookout and below low parallel dunes in the north-eastern part of the island. The Eighteen Mile Swamp, which is 1 to 2 km inland from the eastern or Pacific coastline, largely separates the Holocene dunes along the coast from the Pleistocene dunes further inland, although lesser Holocene dune are also found to the west.
In general the ratio of rutile+zircon:ilmenite in the Holocene dunes is around 1.5:1, whereas in the Pleistocene dunes it is closer to 0.5:1. The variation has been attributed to a change in source. Additional ilmenite in the earlier dunes is believed to have been derived from widespread Tertiary basalts overlying much of the hinterland Mesozoic basins, the main source of heavy minerals. It has been interpreted that much of this basalt had been eroded by the Holocene, with all of the HM subsequently coming from erosion of the Mesozoic basin sediments. Although the immediate source of much of the HM is believed to be the Mesozoic basins of the hinterland (mainly the Moreton Basin), the ultimate source of the rutile and zircon is taken to be the Precambrian shield to the west which supplied detritus through several stages of Palaeozoic and Mesozoic basins.
Earlier mining concentrated on the younger transgressive dunes with a higher average rutile content, occurring within 3 km of the eastern margin of the dune system, while more recently the Pleistocene dunes have been mined. Early mining of beach deposits exploited thin lenses of high grade, up to 50% HM over thicknesses of around 1 m beneath thin cover.
Total production between 1949 and 1987 was 1.695 Mt of rutile, 1.298 Mt of zircon, 1.054 Mt of ilmnite, 2130 t of monazite. In June 1987 the proven + probable reserves were 1.376 Mt of rutile, 1.23 Mt of zircon and 5 Mt of ilmenite (Wallis & Oakes, 1990).
At the end of 2007, reserve and resource figues quoted by Iluka Resources (2008) were:
Total proved + probable reserves - 360.6 Mt of ore @ 0.9% HM for 3.14 Mt HM, with 47% Ilmenite, 11% Zircon, 14% Rutile;
Total measured +indicated + inferred resource - 615.7 Mt of ore @ 0.9% HM for 5.49 Mt HM, with 47% Ilmenite, 11% Zircon, 14% Rutile
The most recent source geological information used to prepare this decription was dated: 2008.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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