Geology and Mineral Resources 131 Geology

Besides the huge mass of ice, several geological peculiarities, such as negligible seismic activity, the local concentration of meteorites on ice ablation surfaces, and subduction processes which led to the formation of West Antarctica through the aggregation of microcontinents (Ricci et al. 2001), distinguish Antarctica from other continents. In contrast with the other continents which are located in plates with constructive and destructive margins, Antarctica is completely surrounded by the sea and is located in a continuously expanding lithospheric plate. During the last 100 Ma, the continent has therefore occupied a quite stable position with respect to the South Pole. This is an important peculiarity, because it means that climate changes in Antarctica mainly reflect global changes.

In spite of difficulties due to the lack of rock outcrops (there are about 331,000 km2 of ice-free areas, corresponding to less than 3 % of the continental area), the geology and evolution of Antarctica is becoming quite well known (e.g. Splettstoesser and Dreschoff 1990; LeMasurier and Thomson 1990; Tingey 1991; Thomson et al. 1991; Stump 1995; Ricci 1997; J.B.Anderson 1999). The broad structure of the continent is related to the amalgamation (about 500-550 Ma ago) and break-up (about 150 Ma ago) of Gondwanaland and earlier continents. Antarctica was once the central keystone of Gond-wanaland, which also included South America, Africa, Madagascar, Arabia, Ceylon, India, Australia and New Zealand. After the fragmentation of Gond-wana, the continental blocks dispersed and Antarctica drifted towards polar latitudes. Due to its central position in the supercontinent, the main geological structures, especially Antarctic orogenic belts such as the Transantarctic Mountains, are an extension of similar structural units in South America, Africa and Australia (Ricci 1991). Much palaeontological and palaeoenviron-mental evidence indicates that the Gondwana continental blocks have a common history. The fossil fern Glossopteris and herbivorous reptile Lystrosaurus are among the organisms which lived on these continents until 180 Ma ago (Crame 1989; Gee 1989; Olivero et al. 1991; Crame 1992). The period of conti nent drifting from Antarctica is well documented by the age of marine sediments in peri-Antarctic oceanic basins (Ricci et al. 2001).

Three main episodes (Storey 1995) have been identified in the break-up of the Gondwana supercontinent (Fig. 2). Initial rifting led to the formation of a seaway between West (South America and Africa) and East Gondwanaland (Antarctica, Australia, India and New Zealand); in a second stage (about 120-130 Ma ago), South America and Antarctica separated from the African-Indian plate and finally, Australia and New Zealand separated from Antarctica (about 60 Ma ago). In the meantime Antarctica had drifted to the polar position; the separation of the Antarctic Peninsula from South America (about 25-30 Ma ago), with the opening of the Drake Passage, was the final event in the break-up of Gondwanaland. This quite recent separation is testified by remains of the southern beech Nothofagus and even of marsupials in Seymour Island (Antarctic Peninsula; Francis 1991). The formation of the Drake Passage led to the isolation of the Antarctic continent, with the establishment

Fig. 2. Schematic representation of the main episodes in the break-up of the Gondwana supercontinent

of westerly circumpolar oceanic circulation and the development of the Antarctic ice cap.

The general tectonic framework of Antarctica is characterised by the stable ancient shield of Precambrian igneous and metamorphic rocks in East Antarctica. This shield consists of several stable cratons, separated by younger mobile belts and flanked on the Pacific side by younger orogenic rocks (Tingey 1991). Rock outcrops in coastal areas of East Antarctica are mostly metamorphosed rocks and subordinate igneous and sedimentary rocks. Analogous Gondwanian sequences, including Devonian to Triassic sedimentary rocks (Beacon Supergroup) and the Jurassic continental Ferrar Supergroup, outcrop in India and in the continents of the Southern Hemisphere. The Beacon sediments contain sandstones, shales and conglomerates with coal-bearing Permian strata, and are characterised by plant, fish and paly-nomorph assemblages correlating with those of eastern Australia and other parts of Gondwanaland (Truswell 1991). Likewise, the Ferrar Supergroup dolerites provide evidence of a link with similar rocks found in Tasmania, Australia and the Karoo dolerites of South Africa. The Napier Complex in Enderby Land comprises very old rocks (about 3,900 Ma) with mineral associations indicating very high temperatures (about 1,000 ° C; Ricci et al. 2001). In the Transantarctic Mountains, subordinate or sporadic metamorphic rocks of igneous and sedimentary origin form the basement or are incorporated into thick pelagic sequences (mainly Precambrian and Palaeozoic turbidites). These rocks were deformed, metamorphosed and intruded by plutonites in the Cambro-Ordovician, during the Ross Orogeny.

West Antarctica consists of several microplates sharing a common history with South America. This history can be referred to the Phanerozoic (<600 Ma), with the oldest igneous and metamorphic basement rocks underlying sedimentary and volcanic sequences of probable Palaeozoic and Meso-zoic age. Two orogenic belts, both sub-parallel to the Pacific margin, can be traced on the basis of radiometric measurements: the Ellsworth Orogen and Andean Orogen. Proterozoic gneisses and schists, and some igneous and metamorphic rocks occur in Ellsworth Land and Marie Byrd Land. Volcanic and plutonic rocks make up most of the Antarctic Peninsula and the southernmost Andes of South America. Volcanism dating from the Middle Tertiary has continued until recently, and volcanic activity has historically occurred in the South Sandwich Islands and very recently in Deception Island and Bridge-man Island, at the northern end of the Antarctic Peninsula. Volcanic activity elsewhere in Antarctica is associated with the major rifting of West Antarctica during the Cenozoic (LeMasurier and Rex 1991). The rift system is bounded on its poleward side by the Pacific flank of the Transantarctic Mountains, and in West Antarctica by a rift running from the western Ross Sea to Ellsworth Land. On the other side of the rift, the Transantarctic Mountains are the product of rapid uplift which began about 50 Ma ago. Volcanism extends from the Balleny Islands to as far south as Mt. Early, at the head of the Scott Glacier. The

Map Mcmurdo Sound
Fig. 3. Schematic map of the main volcanic areas in the McMurdo Sound and northern Victoria Land

Terror Rift in the western Ross Sea extends between the Mount Erebus and Mount Melbourne active volcanoes and volcanic rocks, collectively known as the McMurdo Volcanics (an association of trachytes, olivine basalts, and phonolites; Harrington 1958), occur in the McMurdo Sound region and northern Victoria Land (Fig. 3).

1.3.2 Geochemical Anomalies and Mineral Resources

On the basis of the above-reported arrangement of tectonic plates, it has been hypothesised that zones of Antarctica which were once juxtaposed with the mineral deposits of other continents may contain similar mineral and/or hydrocarbon resources (de Wit 1985). However, most claims of potential mineral wealth are mere speculations because they are based on superficial continental comparisons without adequate knowledge of geological environments and processes (Hansom and Gordon 1998). Comparisons between geological formations and terranes in Antarctica and the rest of the world should consider rocks of the same age, referring to the same continental refit. This is a difficult task because Antarctica was formed by the amalgamation of fragments from supercontinents older than Gondwanaland, and the juxtaposed parts are now widely dispersed in the Southern and Northern Hemispheres. The continental refit is irrelevant not only for Antarctic terranes older than 180-500 Ma (i.e. the age of Gondwanaland) but also in zones, such as the Antarctic Peninsula, which formed more recently. Like other continents, Antarctica has areas with geochemical anomalies and mineral deposits (metals, non-metals, oil, gas, coal) which could be exploited, but their location is largely unknown. Moreover, the Antarctic Treaty System is a unique international agreement, preventing (at least for the near future) the exploitation of mineral deposits.

Several oil companies carried out exploration during the 1970s (Auburn 1982) and claimed the mineral wealth of Antarctica. In 1981, within the framework of the Antarctic Treaty System, a proposal was made to regulate the exploitation of minerals. The Convention on the Regulation of Antarctic Mineral Resource Activities (CRAMRA) was negotiated in 12 sessions plus numerous informal consultations during a six-year period, before being finalised (ATCP 1988). During the negotiations, the possible environmental impact of mining activities, and the sinking and fuel spill of the Bahia Paraiso near Anvers Island raised public concern. Nongovernmental organisations contributed to the decision of some countries to stall CRAMRA and support a ban of mining activities in Antarctica. Although the Convention was never ratified, it provided an important basis and stimuli for negotiating the comprehensive protection of the Antarctic environment (Blay 1992; Francioni 1993). In October 1991 the Consultative Parties adopted the Protocol on Environmental Protection to the Antarctic Treaty (the Madrid Protocol; ATCP

1992) which places an indefinite ban on all mineral activities. This ban may be reviewed after 50 years, or before, if there is consensus. In the future, states seeking Consultative Party status will be required to ratify the Madrid Protocol.

The assessment of metal concentrations in biotic and abiotic components of Antarctic ecosystems is one of the aims of this book, and it therefore seems appropriate to mention areas of the continent affected by geochemical anomalies which may enhance natural concentrations of some elements in fresh-waters, soils and organisms (Fig. 4). In general, it is believed that the gold, nickel, uranium, copper and iron deposits of Western Australia and those of silver, lead, copper and zinc in central Australia continue in East Antarctica (Wilkes Land and Terre Adelie respectively; Willan et al. 1990). A 400-m-thick exposure of a banded-iron rock formation (iron content about 32 %) has been found on Mt. Ruker (Prince Charles Mountains, between Wilkes Land and Enderby Land) in East Antarctica. Aeromagnetic surveys have shown that the iron anomaly extends westwards for about 180 km (Rowley et al. 1991). Abundant erratic rocks of banded iron occur at Vestfold Hills (Ravich et al. 1982), and other iron-bearing rocks occur in Enderby Land (Neumann Nunatak)

Fig. 4. Sketch map of main metal deposits in Antarctica

and Dronning Maud Land (sometimes associated with low copper and lead contents; Rowley et al. 1983).

The Ferrar Supergroup in the Transantarctic Mountains comprises gab-broic intrusions of the Dufek Massif (Pensacola Mountains), which show iron-titanium oxides and copper and iron sulphides in the exposed top layers (Ford 1990). The presence of minerals of the platinum group in the unexposed middle portion of these rocks has also been speculated. Layered igneous intrusions like those in the Dufek Massif have also been reported in Victoria Land (Hamilton 1964).

The Antarctic Peninsula has often been considered one of Antarctica's most important zones for mineral resources because its geologic and tectonic setting is similar to that of the South American Andes, which bear some of the world's largest copper, antimony, tin, molybdenum, silver, lead, iron, tungsten, zinc and gold deposits. Copper mineralisations are known in the north-western portion of the peninsula and in surrounding islands such as the King George, South Shetland and Anvers islands (Fig. 4); however, the potential of these resources is unknown (Pride et al. 1990).

Thick beds of coal with high ash and low sulphur contents are known in the Permian sandstones of the Beacon Supergroup in the Prince Charles Mountains, Victoria Land, the Weddell Sea coast, and especially in the Transantarctic Mountains (Coates et al. 1990; Rowley et al. 1991).

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