Phantom (The Ghost Who Walks) takes his wife Diana to Keela-Wee, a private beach. After swimming, they roll in gold-rich sand before entering the House of Jade. Perhaps Lee Falk, who in 1936 created Phantom might have referred to coastal deposits of gold!
‘Placer’ derived from Spanish ‘placea’, means an alluvial or glacial deposit of sand and gravel. Placer or heavy minerals or black sands form by mechanical concentration and gravity separation of minerals loosened from weathered rocks. Placers consist of ilmenite and rutile (titanium-rich), zircon, garnet, monazite (with rare earth elements), thorium (radioactive), gold, diamond and other minerals and gemstones. Placers occur in mineable quantities in India, Australia, the USA, Canada, South Africa, Sri Lanka, Brazil, Norway, Indonesia and Malaysia.
The formation of placers depends on: climate, rock types, weathering, transport, deposition, concentration, abundance of minerals with high specific gravity (2.88) and their endurance to chemical and mechanical attacks. Placer deposits are termed as: eluvial, residual, fluvial (rivers, streams), beach, offshore, aeolian and glacial.
Climate: Distinct and variable erosion and transport of the weathered rocks occur in the different climatic zones viz, tropical, dry, moderate, continental and polar. High grade placers occur in humid tropical regions, as along the Indian coasts.
Rocks: Most rocks have unique mineral assemblages that contribute to the placers. For example, granites have monazite, tourmaline and zircon while basalts contain ilmenite, magnetite, and rutile.
Erosion: Weathering, abrasion and dissolution of the earth’s surface result in erosion. The broken-down materials are transported and deposited elsewhere.
Weathering: Rocks are weathered by physical, chemical and biological processes. Physical weathering is due to constant action of wind and water on rocks, while chemical weathering alters the rock’s composition.
Transport: The weathered materials are transported by water, wind and glaciers. Water is the most effective and powerful medium, mainly in humid tropical countries. Streams and rivers flowing from mountains and hills break the rocks into boulders and cobbles and during their transport these are ground into different sand sizes and deposited along the course and mouth of the river. The density and settling velocity of the sands control the travel distance and depositional site eg, valleys, river banks, beaches and in the offshore.
Wind carries smaller particles especially in arid and in areas of high velocity wind and can rapidly move huge amounts of sands across vast distances. As the velocity of the sediment-laden wind reduces, sands get deposited as dunes in deserts and along the coasts. In high-latitude areas, eroded rocks are moved by glaciers and on melting there might be placer minerals and gemstones.
Deposition: Sand deposition is influenced by a reduction in the velocity of the transporting medium and nature and composition of the eroded materials, and physiography (hills, mountains, rivers, lakes, seas). Along the coast, deposition depends on winnowing action caused by the incoming (swash) and outgoing (backwash) waves. This leads to offshore transport of lighter minerals leaving behind the heavier ones.
Concentration: The concentration of heavy minerals depends on coastal geomorphology, direction and velocity of winds, waves, currents, seasonal changes, among others.
5Es: The 5Es of placers are exploration, environment, exploitation, enrichment and economics. Exploration involves using satellite and aerial photographs, drones and field works. Analysis of the data and samples assist to demarcate mineral-rich areas.
Considering their areal extent, thickness and tonnage, the placers may be mineable. Enrichment through various processes would result in separation of minerals which would be processed to extract the metals.
Importance: Study of placers help to determine their sources, depositional environments, paleo-climates and so forth. Placers are beneficial in hi-tech industries, nuclear plants, for medical devices, paint and paper manufacturing, jewellery, amongst others. For eg, garnets are used in industrial water filtration, aerospace composites, and for grinding and lapping. Zircon is used to produce zirconia, zirconium chemicals, ceramics, American diamonds, TV glass and power reactors.
Indian placers: Along India’s 7,500 km long coastline, rutile, ilmenite, zircon, monazite, garnet and sillimanite occur on beaches, in dunes and berms. The first four minerals are ‘prescribed’ or ‘atomic’ minerals. In 1909, Schomberg of Germany found exportable quantities of monazite at Travancore (Kerala). For a decade from 1922, India was the main global supplier of ilmenite. In 1950, the Indian Rare Earths Limited (IREL) was established at Alwaye (Kerala). Mining and separation of heavy minerals commenced in 1965 at Chavara (Kerala) and Manavalakurichi (Tamil Nadu, TN). In the mid-1980s, India permitted private players to mine, process and export the garnet sands from the TN coast. In 1986, the IREL commissioned the synthetic plant at Chatrapur (Odisha).
Since the 1970s Indian placers have been studied by the National Institute of Oceanography (NIO, Goa) of the Council of Scientific and Industrial Research (CSIR), New Delhi and Geological Survey of India, Kolkata. During the 10th Five Year Plan (2002-2007), a network project encompassed several aspects of the placers and involved many institutions and universities. The 5Es of the placers along parts of Maharashtra, Kerala, TN, Odisha and West Bengal; resulted in new techniques for exploration, mining systems and enrichment processes.
Coastal mining: Mining proponents argue that since there is no blasting and use of heavy machinery, hence sound and dust pollution would be less, the minimal effluents would not affect the water table and jobs will be generated. Since natural processes replenish the placers, hence the minerals need to be recovered else they would be carried into the sea making them worthless or more expensive to mine.
Goa’s placers: The CSIR-National Institute of Oceanography (Goa) has detailed the spatial and temporal distributions of minerals and available resources along Goa’s 105 km coast. Sands from surface, subsurface and dunes and from low- to high-tide areas have ilmenite, magnetite, garnet, hornblende, tourmaline, zircon, titanite, sillimanite, etc, derived from weathered hinterland and coastal rocks. The eroded materials are transported by Tiracol, Chapora, Sinquerim, Mandovi, Zuari, Sal, Talpona and Galgibag rivers and deposited along the beaches and offshore.
The North Goa beaches have appreciable heavy minerals for eg, Vagator (10-99 wt%), Keri (9.8-78 wt%), Arambol and Morjim (3-90 wt%). The South Goa beaches have low to moderate concentrations (8-10 wt.%) except at Canaguinim (23-82 wt.%), Palolem (30-70 wt%) and Polem (30-90 wt%). The concentrations are influenced by beach morphology, sediment characteristics, wave climate, erosion-accretion processes, waves, currents, rocky headlands, rock types and the rivers.