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Ore Geology Reviews 2

Preface

Placer formation and placer minerals

1. Introduction

b Placer Q is defined by Gary et al. (1972) as b asurficial mineral deposit, formed by mechanical con-

centration of mineral particles, from weathering debris.

The mechanical agent is usually alluvial but can also be

marine, aeolian, lacustrine, or glacial, and the mineral is

usually a heavy metal such as goldQ. From this

phraseology, the term bplacerQ also applies to ancient

deposits formed in this manner, thus obviating the use

of the prefix bpalaeo-Q, when referring to such

sediments and rocks. Also implied by this definition

are the ideas of (a) (weathered) primary mineral

sources, and (b) mineral enrichment (concentration) of

primary-source material by placer processes. Placer

minerals are all heavy minerals, with specific gravities

greater than 2.58; this is slightly lower than that of

bromoform (2.9), the standard liquid used for separat-

ing heavy minerals from a heterogeneous mixture. The

definition in question specifically mentions bheavymetalsQ as examples of placer minerals; however,

high-density metal compounds such as sulphides and

oxides are also commonly found.

Several of the world’s important mineral commod-

ities have been obtained from placers, for example gold

from the Witwatersrand sedimentary rocks (South

Africa) and the gravels of the Klondike goldfields

(Yukon, Canada), the Sierra Nevada, (California), and

South Island of New Zealand. An additional precious

commodity extracted from placers is diamond, for

example in the coastal regions of southwestern Africa

and the interior of this sub-continent. A further

important placer mineral is cassiterite, once mined

from fluvial gravels in the Kinta Valley, Malaysia,

which produced 10% of the world’s tin in 1986. Certain

indispensable industrial minerals are readily recovered

from sandy placers informally known as bblack sandsQ.

0169-1368/$ - see front matter D 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.oregeorev.2005.02.001

Examples of this group of placers are the titaniferous

coastal sand dunes of Richard’s Bay in South Africa

and similar deposits to the north of Maputo, Mozambi-

que (bCorridor SandsQ), and rutile–zircon–ilmenite–

leucoxene deposits of the Murray Basin, southeastern

Australia.

Some placers contain more than one economic

mineral; for example, black sands typically include

Ti-minerals and zircon. Additional examples of multi-

mineral placers are those of the Witwatersrand, several

of which contain uraninite and pyrite in addition to

gold.

Placers seem to have two economic advantages over

primary mineral sources. Firstly, in most cases, mineral

concentrations in primary source material (bweatheringdebrisQ) are generally enhanced by placer processes

(refer to above-mentioned definition). Secondly, ex-

traction of minerals from placers is often much simpler

and more cost-effective than from primary igneous and

metamorphic sources, which often yield refractory ores.

Table 1 s hows th e most imp o rta nt pre sen t a nd

potential placer minerals, along with their densities

and Mohs hardnesses, and comments on their chemical

stability in present atmospheric (bsurficialQ) conditions.The principal criterion considered in compiling this

table is the probability of these minerals being

concentrated and preserved in economic placers, given

their monetary value and physical and chemical

characteristics. A similar compilation is provided by

Carling and Breakspear (this issue).

2. Concentration agents and mechanisms

The concept of mechanical concentration of minerals,

as applied to a given volume of primary material, implies

a decrease in the volume of low-density, non-economic

minerals. Such decreases are brought about by selective

8 (2006) 373–375

Table 1

Important characteristics of present and potential placer minerals

Mineral Specific gravity Mohs hardness Chemical stability

Ilmenite 4.7 6 Unstable

Rutile 4.2 6 Stable

Zircon 4.7 7 Very stable

Diamond 3.5 10 Very stable

Cassiterite 7.0 6 Stable

Magnetite 5.2 6 Unstable

Gold 19.3 3 Stable

Platinum 21.5 3.5 Stable

Uraninite 6.0–9.5 5.5 Unstable

Pyrite 5.0 6 Unstable

Scheelite 6.0 5 Stable

Chromite 4.5 5.5 Stable

Os–Ir–Rua Up to 22 6–7 Very stable

a Alloy.

Preface374

sorting of mineral grains by mass and volume. Mechan-

ical agents that could bring about heavy-mineral

concentration in placers are running water and wind,

two of the four transport agents traditionally thought to

be responsible for all movement of sediment. The

remaining two transport agents, ice and gravity, are

unlikely to bring about significant heavy-mineral con-

centration in sediment transported by them.

3. Describable characteristics of placer minerals

3.1. Density

High density/specific gravity is probably the most

important and desirable characteristic of placer miner-

als, because partial separation of such minerals from the

most common non-economic mineral, quartz, depends

primarily upon density differences between this and the

placer minerals.

3.2. Hardness and malleability

Theoretically, hardness of a potential placer mineral

would determine its resistance to abrasion, and conse-

quent loss of mass during bedload transport, according

to the Sternberg principle. For high resistance to such

abrasion, a hardness greater than that of quartz seems

desirable for placer minerals. However, examination of

the hardnesses of known and potential placer minerals

in Table 1 reveals that only diamo nd has a hardnes s

greater than that of quartz. Soft, malleable metals such

as gold and platinum, seem to deform mainly in a

plastic fashion during transport, to form irregularly

shaped grains; a characteristic which often obscures

their detrital origin.

3.3. Chemical stability

Chemical stability, in particular resistance to oxida-

tion, is a most desirable characteristic of potential

placer minerals, especially those that form alteration

products that constitute less optimal ore minerals. Other

diagenetic and metamorphic alterations, too, might

adversely affect the value of original placer minerals.

4. Papers of this issue

The contributions included in this special issue stem

from the 26th International Sedimentological Congress

held at the Rand Afrikaans University, South Africa,

from 8th to 12th July 2002. During an oral session

dedicated to placers, which lasted almost an entire

working day, 12 papers were presented, and a poster on

the placer theme was displayed.

In a paper based upon Paul Carling’s keynote

address, Carling and Breakspear review the important

concepts of placer formation in gravelly rivers, with due

consideration of the principles of hydrodynamic sorting

and internal sedimentary structures typical of such

rivers. They also provide examples of typical deposi-

tional environments of diamond, tin and gold placers.

Carling et al. report on a study of the distribution of

a heavy-mineral bedload tracer (magnetite) across an

evolving point-bar, using magnetic susceptibility as a

measure of magnetite concentration. These authors

found that the bar-head was most conducive to placer

formation, although false bottom placers developed

along bedding planes. The results of mathematical

hydrodynamic modelling of the point-bar environment

were found to be in accordance with the interpretation

of their field data.

Based upon studies of three young foreland basins,

Craw et al. came to the conclusion that economic gold

concentrations are rare in such settings, because of the

paucity of significant gold sources, and a paucity of

sediment recycling processes during filling of such

basins. They suggest special tectonic conditions to

effectuate the sedimentary recycling required for placer

formation in foreland basins that contain economic gold

concentrations, such as the Witwatersrand.

Lowey reviews the deposits of the world famous

Klondike goldfields, which originated from the weath-

ering and erosion of early Cretaceous, quartz veins.

Gold concentrations occur on three levels of strath

terraces of Pliocene to Holocene age, and, in common

with several other gold deposits around the world, are

found preferentially in braided-river gravel bars. This

author addresses the role of climate in the formation of

F Gerhard Els passed away on Saturday 30th October 2004.

Preface 375

terraces, a factor that can seldom be evaluated for

ancient processes and deposits.

The development of quartz–pebble conglomerates

(QPC), common hosts to placer gold, is the topic of

Youngson et al. They ascribe the formation of such

conglomerates in the South Island of New Zealand to

decomposition of labile minerals, especially within the

groundwater table, and point out that repetitive

sedimentary recycling is a fundamental process of

QPC formation, regardless of tectonic or sedimentary

settings.

Spaggiari et al. describe size variations of diamonds

found in coarse-grained Plio-Pleistocene littoral depos-

its along the southwestern coast of Africa. These

diamonds were evidently transported to the coastline

by the palaeo-Orange River and subsequently reworked

by prolonged, vigorous wave, wind and northward

longshore processes. The distribution of diamonds is

related to littoral processes that were operational within

the palaeo-Orange River mouth during a +30 m Plio-

Pleistocene transgression.

The importance of bedrock trapsites for placer

minerals is illustrated by Jacob et al., who document

Quaternary diamond deposits occurring in three types

of gullies formed on marine-cut platforms of Protero-

zoic rocks in the Sperrgebiet along the Namibian coast.

They conclude that the most favourable diamondiferous

trapsites are those developed in deep bedrock gullies on

platforms that represent high sea level stands.

Heavy-mineral studies are most useful stratigraphic

and provenance tools, as illustrated by Ajdanlijsky and

Dotzov in their paper on a little known basin, the

Neogene–Quaternary Blagoevgrad of Bulgaria. These

authors also record previously unknown occurrences of

placer gold in three stratigraphic units demarcated by

them through a very detailed study of the heavy

minerals.

Falconer et al. report on an investigation of gold and

sulphide minerals in Tertiary quartz–pebble conglom-

erate gold placers in Southland, New Zealand. A

significant finding is the occurrence of pyrite of

apparent long-distance detrital origin, a phenomenon

commonly believed to be unlikely, especially in the

modern oxygen-rich atmosphere. These authors also

draw similarities between the gold and sulphide grains

studied, and those of the much older (Archaean)

Witwatersrand placers.

Acknowledgements

We would like to express our sincere gratitude to the

following referees for their indispensable contributions:

B.J. Bluck (University of Glasgow), O. Catuneanu

(University of Alberta, Canada), J.S. Compton (Uni-

versity of Cape Town, South Africa), W. Dickinson

(Victoria University, Wellington, New Zealand), F.R.

Ettensohn (University of Kentucky, USA), P.G. Gresse

(Transhex, South Africa), T. Hoey (University of

Glasgow), R.A. Kuhnle (United States Department of

Agriculture), J. Mauk (University of Auckland, New

Zealand), J.M. Moore (Rhodes University, South

Africa), J.K. Mortensen (University of British Colum-

bia, Canada), A.J. Tankard (Consultant, Calgary,

Canada), A. van der Westhuizen (Transhex, South

Africa), J.D. Ward (De Beers Africa Exploration, South

Africa). We also wish to thank the Elsevier editorial

staff; Nigel Cook and Patricia Massar for their handling

of this volume, and Friso Veenstra for planting the idea

of a special issue through tentative inquiry.

Reference

Gary, M., McAfee, Jr., R., Wolf, C.L. (Eds.), Glossary of Geology.

American Geological Institute, Washington, DC.

Gerhard ElsF

Pat Eriksson4

Department of Geology, University of Pretoria,

Pretoria 0002, South Africa

E-mail address: Pat.Eriksson@up.ac.za.

4Corresponding author.

Tel.: +27 12 4202238;

fax: +27 12 3625219.

23 February 2005