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Interview study book Earth Science of Roy H. Williams - ISBN: 9781926692579 (Study Notes)
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Placer Deposits --FinalContents:
1. Introduction
2. Classification
3. Formation of placers
4. Conditions favourable for placer deposit development
5. Factors controlling formation of placers
6. Ages of placers
7. Mineral concentration and accumulation
8. World Distribution of placer mineral deposits
9. Indian distribution
10. Conclusions
Objectives: The objectives of learning this module are to understand the placer deposits by studying their
characteristic, physical properties, classification, formation of placers, factors that are controlling the
formation of placers, world distribution and Indian distribution of placer deposits in particular.
1.0 Introduction
Placers are very important economic minerals found in the near shore environment. These are detrital minerals
which have been transported to their sites of deposition, in a particular form. Placers are mineral deposits that
have been formed by the mechanical concentration of detrital mineral particles in subaqueous
environments.They occur, in beaches, rivers, dunes and offshore areas. The minerals that constitute the placer
deposits are resistant to chemical breakdown and have high density and durability. Placer minerals include
elements in their native state such as gold, platinum and diamond, and resistant minerals such as cassiterite,
ilmenite, rutile, zircon, monazite, garnet, magnetite and corundum.
Most of placer minerals occur within a few kilometers of their source. Thus marine placers are confined to
near shore zone. However, placers also occur on ancient submerged beaches or in drowned valleys far away
from shore line or on raised beaches.
The most commonly used definitions of placers refer to the concentration of valuable detrital minerals formed
by sedimentary processes. Theses minerals are distinguished by the relatively high specific gravity that serves
as the principal cause of concentration.
1.1 Mineral sands: Deposits of mineral sands are gaining economic importance on many occasions. Today
people concentrate on mineral placer deposits occurring along beaches. Most of the mineral sands are
exploited for titanium in the form of rutile and ilmenite, with by products of zircon and commonly kyanite,
sillimanitestaurolite, or garnet. At one time, the mineral sands were shipped as concentrates of three
naturally occurring minerals like rutile, ilmenite, and leucoxene.
1.2 Economic importance: The economic mineral in placers are those with a specific gravity greater than
2.65, i.e., that of the main gangue mineral, quartz. Individual placer deposits are exploited for gold, platinum
group minerals, cassiterite and diamonds. Each of these may also occur as an accessory,as by-product
concentration with another dominant mineral. Less commonly, placers also include sheelite, wolframite,
bismuthinite, native copper, cinnabar, magnetite, chromite, anatase, columbite-tantalite and pyrochlore. Since
the 1970’s, Australia has overtaken India and the United States and now dominates the world’s production of
ilmenite, together with rutile.
1.3 Historical importance: Placers have been mined on all continents except Antarctica. Historically, placers
have yielded a very significant part of the world’s total supply of gold, platinum, tin, and diamonds.
Page 1 of 10
, They are presently the source of most of the world’s titanium. With the exception of mineral sands, the
productive role of placers declined considerably during the 20th century and continues to decrease overall as
reserves diminish. Diamond mining especially in the environment, being the only sector exhibiting growth in
recent years.
The history of placer deposits as a source of economic minerals began with the discovery of monazite in the
beach sands of Manavalakurichi (Tipper, 1909). This was first worked in 1911 and rapidly developed with the
establishment of Ti02 (titanium) pigment industry in Europe and America. In early 1930's, India accounted for
80% of the total ilmenite production in the world. However, by the 1950's the ilmenite production marked a
sudden decrease in the country. This owes, in part to the discoveries of new deposits in Australia and Canada
and the presence of undesirable impurity like chromium and ferric iron in Indian ilmenite. Many features of
Placer deposits and their evaluation were known a century ago.
2.0 Classification of placers: Placers may be classified according to the main mineral constituent, the
physical and chemical properties of the mineral, age, size, tectonic history, means of formation, or
geomorphological form. Kazakevich proposed a classification based on placer’s genesis. Lindgren gave due
credits to geological location and tectonic history.
2.1 Emery and Noakes(1968) classified placer minerals into:
i. Heavy- heavy minerals with specific gravities of 6.8 to 21
ii. Light -heavy minerals with specific gravities of 4.2 to 5.3.
iii. Gems with specific gravities of 2.9 to 4.1
2.2Classification Based on physical properties
Gold, platinum and cassiterite are important heavy- minerals; ilmenite, rutile, zircon and monazite are
important light-heavy minerals and diamond is the most important gem placer mineral. The following table
gives the physical properties of some common placer minerals.
Mineral Hardness Specific Resistance to
(Moh’s scale) gravity weathering
Cassiterite 6.7 6.8-7.1 High
Diamond 10 3.52 Very high
Garnet 6.5-7.5 3.5-4.3 Moderate
Gold 2.5-3 19.3 Very high
Ilmenite 5-6 4.5-5 High
Magnetite 5.5-6.5 5.1-5.18 High
Monazite 5 4.9-5.3 High
Platinum 4-4.5 21.46 Very high
(chemically pure)
Ruby and sapphire 9 3.95-4.10 Very high
Zircon 7.5 4.5-4.7 Very high
2.3Folk (1971) has divided the heavyminerals into three groups, based on their physical and chemical
naturenamely, opaques, ultra-stables and meta-stables.
2.4Routhier (1963) classified Placers into a simpler type of subdivisions. Placers are classified into (1)
eluvial, (2) alluvial or fluvial, (3) eolian, (4)beach placers, and (5) fossil placers based on their mode of
transportation and site of deposition.
1) Eluvial placer: A placer deposit that is concentrated near the decomposed outcrop of the source.This is the
embryonic stage in the development of placers.
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