Content uploaded by Kerry Stanaway
Author content
All content in this area was uploaded by Kerry Stanaway on Jan 10, 2019
Content may be subject to copyright.
REVIEW
Ten placer deposit models from five
sedimentary environments
K. J. Stanaway*
Placers deposits are now known from five sedimentary environments; washout, river, aeolian,
beach, and continental shelf. In each environment, the concentration of mineral grains, or sorting,
takes place either by removal of gangue grains (denudation) or by addition of valuable grains
(accumulation). Any given deposit will result from both processes but one will usually
predominate. Denudation placers all sit on or just above erosive scour surfaces. They arise
from a two-step process; initial particle deposition followed by selective removal of gangue
particles. For example, deposits from a waning flood-stage river will include many different size,
shape and density particles but a subsequent lower energy normal river flow might remove only
the smaller, flatter or the less dense particles. The second fluid flow can be quite different from the
first as, for example, when the wind selectively removes sand grains deposited by waves.
Repeating these two steps, transportation from source and selective entrainment of grains results
in a high placer mineral flux allowing denudation placers to achieve high concentrations of
particular minerals. Denudation placers have a small thicknesses or vertical dimension, and so
they are essentially condensed sections. To be economic, they must have a high value mineral, a
large surface area, a long linear dimension, or exceptional grade, and preferably several of these
features. Accumulation placer formation does not involve later partial rework and selective grain
removal. Concentration grade depends on maximum availability of a valuable mineral and
minimal availability of gangue grains capable of being carried with, and deposited from, a given
fluid flow condition. Such placer deposits tend to be lower grade compared to denudation placers
because the placer mineral flux does not focus on a single two-dimensional surface. Instead
repeated favourable flow energy episodes superimpose placer grain enriched-sediment in
situations of accumulation with minimal scour. Their large volume makes them economically
valuable.
Keywords: Placer, Denudation, Accumulation, Scour, Mineral flux
Introduction
Placers are detrital sediment deposits. They contain
economically extractable concentrations of valuable
minerals with a specific gravity or density greater than
quartz (2?65 g cm
23
). Most theories of placer formation
have been developed by people working on alluvial gold
but this is only one of the viewpoints presented here.
Density and the chemical and mechanical durability
of resistate grains are the principal properties that
facilitate sorting in the creation of placer deposits, but
grain size, and grain shape also play critical roles
(Youngson and Craw, 1999) and surface wettability and
magnetism also contribute. ‘The list of minerals which
can be concentrated in placers contains approximately
40 species, including 30 minerals that can make up
individual deposits’ (Patyk-Kara, 2002). Nephrite (green-
stone, or the actinolitic form of jade) for instance, is a
placer mineral occurring in the glacial outwash alluvial
sediments of the west coast of the South Island of New
Zealand. It is durable, has a density between 3 and
3?5gcm
23
and has some commercial value. The mineral
has been recovered from river sediments and sold, but,
unusually for a placer, it has most value when it is in the
form of boulders.
Placer deposit classification
Recognising some important differences between placers
helps not only for understanding their genesis but also to
predict their location and properties during exploration.
When considering a placer exploration programme, the
most useful target distinction is whether the source
proximity or the sorting factor dominates in concentrat-
ing the relevant mineral. Intuitively we seek zircon in
31 Pohutukawa Road, Beachlands 2018, New Zealand
*Corresponding author, email ktstanaway@xtra.co.nz
ß2012 Institute of Materials, Minerals and Mining and The AusIMM
Published by Maney on behalf of the Institute and The AusIMM
Received 19 June 2012; accepted 26 October 2012
DOI 10.1179/1743275812Y.0000000020 Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 143
beach placers, because most zircon occurs in durable
sand size grains easily dispersed over hundreds of
kilometres, and concentrating only where sorting
processes allow. In contrast, because of both size and
density, gold nuggets and nephrite boulders show
preference for sites close to their provenance. Soft, good
cleavage minerals of lesser durability, such as monazite
prefer sites closer to their provenance as well, and
cannot sustain extensive sediment recycling.
The proximity to source and the sorting both affect all
placers, but not equally. For instance, even in deposits
dominated by sorting such as beach sites, higher grade
deposits will tend to occur closer to source where
dispersal is less. In all environments, the background
concentration, or mineral availability, is a key factor in
placer exploration; even the most effective processes
cannot generate economic deposits where mineral
contents are too low, i.e. too far from the source and
too dispersed.
Geologic environment makes up a third factor to
consider in any placer mineral search. Some fluid flow
regimes occur in only one environment, effectively
isolating certain placer types to that environment.
Fluid flow regimes peculiar to specific environments
include oscillatory wave action on a beach, or on the
seafloor, and single direction, but varying velocity
turbulent or laminar flow in a river. Water-charged
high density flows are confined to specific environments.
Deposit volume is a factor of economic concern in
some placer exploration sites, because low economic
mineral value necessitates large scale accumulation in
sediments at tens of metre to hundreds of metre
thickness, whereas a high mineral value permits mining
in essentially two-dimensional denudation environments
where deposits have only decimetre to metre scale
sediment thicknesses. Diamonds were once sought on
wind deflation surfaces in what is now Namibia
(Cornell, 1920). In contrast, magnetite is mined from
huge dunes at Taharoa and Waikato North Head, New
Zealand (Mauk et al., 2006).
It was Kartasov (1971) who first noted the provenance
versus sorting, and accumulation versus degradation
factors in river placer settings of gold. His autochtho-
nous placers arise at the base of river channels, can
occur with any intensity of flow and consist of coarser
gold trapped in channel floor roughness, and not
transported far from source. In contrast, Kartasov’s
allochthonous gold placers form from finer grained gold
as much as 25 km from source and depend on
deposition from particular flow regimes depositing
limited gold grain shapes and sizes, and these form
during river aggradation.
The fundamental distinction between the two placer
types is not distance from source, as implied by the
names Kartasov chose, but the process of formation; in
one case denudation (i.e. scour, erosion) and in the
other, accumulation. Observing reworked black-sand
concentrations at the foot of erosive mini-scarps cut into
the beach profile or wave-cut dunes, Stanaway (1992)
saw that autochthonous placers were erosive lags from
prior accumulative black-sand enrichments. These accu-
mulations resulted from a process that reworked an
earlier deposit. In terms of exploration modelling,
knowledge of the immediate process of formation and
source site is more useful than knowledge that the
minerals were derived from a certain rock hundreds of
kilometres away. Distance from ultimate source rock is
thus not the prime discriminant for the two placer
processes, and hence the current scheme does not
perpetuate the names of Kartasov (1971). Because
autochthonous placers are distinguished by erosive
scour at their base rather than distance from the source,
the term denudation is preferred in this paper while
accumulation is preferred for allochthonous placers.
These two fundamental placer-forming processes have
also been described by Garnett and Bassett (2005) using
the essentially synonymous terms lag and accumulation
but in this paper the presence of a scour surface is
emphasised because this is feature that can be readily
observed in the field.
There are examples of denudational placers where the
relevant minerals have been transported to their present
location in quite different fluid flow regimes from those
that were responsible for reworking and concentrating
the mineral. Aeolian placers are one such example;
another example would be placers derived from glacial
debris followed by river or beach reworking.
Mathematical analysis of mineral grain interactions in
water have led to equations and concepts applicable to
denudational placers (Slingerland and Smith, 1986).
They describe several mechanisms of failure to entrain
that include trapping where denser grains in a traction
load test the bottom more often and thus more often fail
to entrain. Other failures to entrain include: overpassing
or winnowing where the smallest, largest and densest
particles lag; armouring, where smaller grains are
winnowed; and hiding, where smaller size and greater
density act to resist further motion effectively ‘hiding’
grains in bed roughness (Table 1).
In the description of the ten placer deposit models and
five sedimentary environments that follow, it is impor-
tant to recognise that multiple models and environments
can occur in one sedimentary sequence or basin.
Aeolian placers
Deflation placers
These form when a strong unidirectional wind removes
sand, silt and other fine non-cohesive material, leaving
larger grains such as pebbles and dense grains like
gemstones, on an indurated, pebbled, rough or wet
surface. Minerals that have failed to entrain, because of
size, density or shape, accumulate as lag on the eroding
surface. This placer mechanism may contribute in a
minor way to the formation of heavy mineral beach
placers should a strong onshore wind entrain dry quartz
Table 1 A placer deposit classification
Dispersal
Source proximity
dominant
Sorting
dominant
Environment Aeolian Washout/weathering River Beach Continental shelf
Denudation dominates Deflation Colluvial/eluvial Denudation/trap placer Transgressive Transgressive lag
Accumulation dominates Dune Sheetwash/debris flow Accumulation/bed placer Regressive Subsiding shelf
Stanaway Ten placer deposit models from five sedimentary environments
44 Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 1
grains. Pebbles subjected to prolonged sand blast on
exposed surfaces may become faceted. The diamond
grains reportedly seen and collected early in the
twentieth century on the surface along parts of the
Skeleton Coast of Namibia may have been concentrated
by such a mechanism from original pebbly sand beach
or river concentrations (Cornell, 1920; Garnett and
Bassett, 2005).
Deflation placers are denudation features that arise in
two steps. The first is sediment deposition by an older
flow regime such as a river or beach, followed by failure
to entrain by a new fluid flow regime which in this case is
the wind. Deflation placer grains may ultimately come
from hundreds of kilometres away, but source for the
placer creation is proximate.
Dune placers
Coastal dunes are storage and preservation sites for
concentrations created on beaches. These kinds of
storage have built up where prevailing onshore winds
blowing sand off beaches have piled dunes against water
or land barriers, such as lagoons, swamps, steep hillsides
or fault scarps; thus preventing dispersal of the grains.
Vegetation in warm humid climates can also bind dunes
and prevent their reworking. Coastal dune placers
always form within 5 km of the site of beach concentra-
tion. Without barriers to their migration, the wind
would have dispersed sand inland to form a sand plain
with numerous small dunes as the author has observed
on the Foxton Flats north of Wellington, New Zealand,
in west Senegal, and inland of the Orangeburg scarp in
South Carolina, USA.
Examples of economic dune placers are the Waikato
North Head and Taharoa titano-magnetite sands of the
North Island of New Zealand. These are the largest and
highest grade terrestrial placers on the planet today in
terms of tonnage of valuable mineral content. At
Waikato, resources and reserves currently stand at 900
million tonnes of sand grading 22% magnetic minerals
(Barakat and Drain, 2006). At Taharoa, resources and
reserves reach almost 400 million tonnes at 45 to 50%
magnetic minerals (Barakat and Ruddock, 2006). The
deposits have Pleistocene and Holocene sands draping
over them (Mauk et al., 2006) and have formed from
sand blown two to three kilometre inland from beach
concentrations by the dominant south-westerly winds.
Longshore drift has swept beach magnetite concentra-
tions along shores with coastal cliffs preventing inland
dispersal until they reached coastal indentation traps
caused by river erosion or block faulting.
Other examples of dunal storage placers are the one to
four kilometre wide dunes along the east coast of Africa
from east of the Cape of Good Hope to Somalia
(Stanaway, 2005). Dunes along the east African coast
extend only a few kilometres inland from the shore,
except east of the Limpopo where five sets of one to four
kilometre wide dune systems are parallel to the coast
and represent former sea stands on a coastal plain.
Washout placers
Colluvial and eluvial placers
These classic placers form near sites of chemical and
mechanical weathering and washout of weathering-
resistant constituents from rock. Enclosing host miner-
als are hydrolysed and wash downslope as solutes,
hydrolysate clays, oxides and talus. Once liberated, the
higher density resistate minerals concentrate in the talus
on sloping surfaces, and as smaller grains hiding in lag
deposits. Colluvial placers form on scoured and
erosional hill-slopes, and belong to the denudation class
with strong provenance control. Downslope they grade
into sheetwash and alluvial placers.
The layered harzburgite massifs of New Caledonia are
associated with an example of this placer type. Colluvial
placers form on steep hillsides from dunite layers with
50% or more chromite (podiform chromitites). Meteoric
water driven by gravity on hill-slopes, removes lateritic
cap-rock, weathered rock, clays, and colloidal hydro-
lysate iron-manganese oxides. Chromite is more resis-
tant to hydrolysis than are the source rock minerals of
olivine, pyroxene, amphibole and plagioclase, and so the
chromite accumulates by lag on the steep drainage
slopes.
Sheetwash and debris flow placers
Sheetwash placers are sediment accumulations sourcing
their valuable minerals from a nearby weathered rock.
In vegetation-poor sites, high precipitation can cause
flash flood, debris and mud flows that deposit poorly
sorted sediment in drainage depressions. These processes
can operate either on low gradient peneplain surfaces
that have undergone deep chemical weathering, or on
higher gradient alluvial fans. Factors that result in
sheetwash placers include:
Nsufficient concentration of the mineral in a source
rock
Nenough rock weathering to liberate the mineral
Npreferential entrainment of hydrolysate clays and silt
fines in the suspended load of the stream flood or
debris flow
Nlag of larger clasts at the point of provenance.
Desert margin climates with alternating humidity when
weathering takes place under forest cover, followed by
arid un-vegetated conditions when sheetwash and
intermittent stream flows occur are a favourable
environment for this class of placer. Several placer
accumulations of gold, silver and cassiterite in the Andes
are sheetwash-related (Garnett and Bassett, 2005).
Initially, the flow leaves coarse clasts behind, then
selectively entrains sand with mud, and finally preferen-
tially de-trains sands leaving the mud to flow away (this
all occurs as one operation).
Excellent examples of sheetwash placers on a weath-
ered peneplain are the rutile placers near Gbangbama in
Sierra Leone (Stanaway, 1992). Here, 250 million tonnes
of sediment contains 3?5 million tonnes of rutile at
grades of 1 to 2%; this forms in six shallow drainage
depressions averaging 8 m deep. No part of any deposit
appears more than 6 km from the source rock of
weathered charnockite which has 0?5 to 1% rutile. The
placers consist almost entirely of poorly sorted silty and
clayey sand with only a few quartz pebbles, rare thin
lateritic sieve gravel layers and scattered plastic clay
beds.
River placers
River denudation placers
Denudation placers lie on scour surfaces. In upstream
gullies or gulches, bedrock forms the scour surface and
gives rise to gulch placers and bedrock placers, referred
Stanaway Ten placer deposit models from five sedimentary environments
Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 145
to also as bottom placers if covered by alluvium.
Downstream in aggrading alluvium, lag gravel, granules
coarse sand, and high density minerals such as ilmenite
and magnetite delineate the scours, and these may be
only a single layer thick. These mafic minerals are visible
at the margins of sets of current-beds, especially near the
basal margin. Such scour can form extensive surfaces
within thick sequences of alluvial valley fill. Modern
academic studies tend to use flumes and rivers and focus
on flow regimes but miss the vital exploration target
relation between denudational placer deposits and scour
surfaces.
Mining of Miocene-aged gravels of the lower Orange
River in southern Africa has revealed excellent examples
of diamond concentration in irregular bedrock scour
surface potholes and ridges by fluctuating energetic
turbulent river flow. Significantly, the high diamond
gravels display an unusual type of gravel packing that
can only have arisen slowly in the turbulent flow where
the earlier grains act as a sieve that traps or hides
successively finer grains (Jacob et al., 1999). The lengthy
gravel flux required for this packing has also allowed a
larger flux of diamonds to become similarly trapped in
the sieve structure and potholes.
The major concentration of gold in most river gravels
and sands occurs on or within three metres of the major
scour surface, i.e. near bedrock. For example, Peterson
et al. (1968) describing the placer gold in the Tertiary
river channels of northern Nevada County, California
wrote: ‘The drilling records indicate that the highest gold
concentration is near the bottom of the deepest part of
the channel’. Similarly, Yeend and Shawe (1989) writing
of the Fairbanks District of Alaska stated: ‘The bulk of
the placer gold came from gravel from just above to
nearly 3 m above bedrock’. Gold miners refer to river
denudation placers as bottom placers if they lie on river
channel bedrock and, as above–bottom placers if they lie
within alluvial valley sediment. The greater richness of
bedrock placers relates to the frequency of erosion
extending down to bedrock after periods of alluvial
accumulation. Typically, above-bedrock placers record
only one such down-cutting (Kartasov, 1971).
River denudation placers form as the placer minerals
from earlier fluid flow conditions fail to entrain during
later scouring. River denudation placers are condensed
sediments that capture mineral grains from a large
mineral throughput or flux; the valuable minerals fail to
entrain in later fluid flows. These deposits tend to be
closer to source than river accumulation placers.
River accumulation placers
Accumulation placers are created when high specific
gravity and hydraulic-equivalent sand-sized grains are
transported only within that part of water flow with the
energy and other factors best suited to carrying them. If
a river channel is deep enough and if discrete and
differing flow regimes both persist and vary with depth
they will deposit vertically stacked facies defined by
grain-size as gravel, sand, or clay beds. Flow regimes
can produce concentrations based on specific gravity
(Burton and Fralick, 2003). These natural river flow
sorting mechanisms have counterparts in the technolo-
gies used to recover ilmenite and zircon sands; e.g. in the
spiral separator when different velocity flows across the
spiral channel separate and isolate more- and less-dense
grains in two distinct flows, flowing beside each other,
with the high density minerals favouring the inside and
the lower density on the outside of the spiral channel.
The pinched sluice and cone separator operate similarly
to spit two flow regimes and their differing cargoes, but
in a vertical dimension.
The existence of fine grained, flakey gold is well
recorded and known as river bar placers (Dingman,
1932). The first person to record that finer grained gold
grains had concentrated in accumulative point bars,
rather than in the stream channel bedload lag and
bedrock scour, was the Russian placer geologist Bilibin
in 1938 (Kartasov, 1971).
The author has channel sampled a point bar, revealing
7% grades of ilmenite, rutile, zircon in fine-grained well-
sorted sands in the wall of a flood chute on a Congaree
River point bar, near Columbia, South Carolina. The
regional background grade was 0?6%. In the Hinuera
Formation river sands near Hamilton, New Zealand, he
has observed fining upward sequences that show well-
defined black higher density minerals concentrating in
fine well-sorted sands toward the top of the fining
sequences rather than in coarse sands or pebbles at the
basal channel scour.
In brief, river accumulation placers deposit from fluid
flow conditions where certain mineral grains are
selectively entrained, transported and deposited. Flume
experiments using Rio Grande river sands containing
0?38% magnetite (median diameter 144 mm) show that
magnetite concentrates from transition and upper flow
regimes in sands characterised by washed out current
bedding and horizontal lamination (Brady and Jobson,
1973).
Drilling of hundreds of holes in the Cohansey
Formation alluvials of the Neogene of New Jersey in
USA has revealed several heavy mineral deposits
aggregating more than 700 million tonnes of sand grading
3 to 5% heavy minerals (predominantly ilmenite and
zircon). Holes near Upton show seven fining-upward
sequences with high heavy mineral concentrations in the
tops of four of them, and they can be correlated from hole
to hole (Fig. 1). Erosion accompanying the deposition of
the overlying sequence has removed the top of parts of the
river accumulation placer in some drill holes. In these
holes, high heavy mineral values have obviously persisted
into the above-scour basal gravels of the overlying layer
as a river denudation placer. Importantly, these placers
could not have been beach deposits as proposed by Carter
(1978) and Puffer and Cousmiler (1982). It remains
axiomatic in the literature that high density mineral-
enriched, horizontally laminated sands are ipso-facto
beaches. Evidence for a beach origin reported by Puffer
and Cousmiler in the Glidden Durkee deposit consisted
of two thin storm-incursion clay layers with dinoflagel-
lates within a 1?3 m thick peat horizon where 63% of the
taxa found were fresh water types (Carter, 1978). The
seven known ilmenite placer deposits all elongate
perpendicularly or at high angle to any likely coastline
suggesting rivers (Fig. 1). They slope as rivers do along
the axis of deposit elongation at gradients of 1–2 m in
300 m (0?3
o
) and span elevations from 45 m above to
10 m below sea level. In addition, holes drilled by the
author (60 plus holes in the Tabernacle–Fort Dix area,
USA) sit in 1?5 to 4 km wide valleys cut into emergent
marine Miocene grey-black Kirkwood Formation weath-
ered to purple, mauve and red colours. Even Carter wrote
Stanaway Ten placer deposit models from five sedimentary environments
46 Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 1
that he was mystified by the current bedding in his
Cohansey back barrier sands in quarries all over New
Jersey consistently showing SE dips. If these were from a
tidal environment, they should have displayed two
opposing tidal directions.
The relation between denudational and
accumulation placers in river sediments
Channels as deep as 10 m in high sinuosity meandering
rivers (Fig. 1) provide a clear vertical separation for
denudational (at the base) and accumulation placers
(near the top). These streams carry low thin bed loads,
accrete laterally and form classic point bars. However,
increasing the coarser sand and gravel bed-load compo-
nent of the stream demands a faster flow and increased
gradient resulting in a shallower channel and less
sinuosity. Vertical separation between scour and accre-
tion diminishes. Further increasing the bed-load results
in braid or longitudinal bars developing in the river
channel, and the choked channel ultimately becomes a
network of braided shallow channels where both scour
and accumulation become entwined at nearly the same
level. These high-bed-load braided streams feature in
upstream drainages, alluvial fans and choke point
discharge. Going downstream as gravel is lost, sand
deposition increasingly occurs in fining upward facies
leading to vertical segregation of accumulation placers
in sands of hydraulically equivalent grain-size to the
placer grain-size and shape available.
River placer summary
All river denudation placers form in a two-step process.
In the case of upstream gulch placers the valuable
minerals weather from both nearby rock at source and
fragments of source rock broken off and travel down-
stream. After initial deposition the valuable minerals
become trapped and concentrated in scour surface
roughness, whereas the rest of the material with which
they were initially deposited is borne away.
1 River accumulation placers (Top left) location New Jersey, USA (Top right) Orientation of placers showing orientation
in ancestral river valleys prior to south-westward river capture by the present Delaware River (from Markewicz, 1969).
(Bottom) E–W drill cross-section SE of Upton. Vertical arrows show holes and direction of fining with scour surface
gravel grit or coarse sand at base. Fining units are numbered in circles. Accumulation placers are shown with thicker
lines in fine sands toward the top. Numbers show heavy mineral grades, with background at 0?65% heavy minerals.
Topmost silts and clays are generally missing due to erosion accompanying overlay of next unit. Note the river denu-
dation placer above the scour base of units 3, 4 and 5 with mineral eroded from accumulation river placer beneath
Stanaway Ten placer deposit models from five sedimentary environments
Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 147
In contrast, river accumulation placers result from
grain deposition without subsequent entrainment or
removal. They are the present resting places for material
that may have travelled tens to hundreds of kilometres.
Accumulation placers tend to have lower grades because
they lack the large mineral flux from iterative reworking
that concentrates and condenses river denudation
placers. The grade of the accumulation placers depends
on the availability of gangue and placer mineral for
transportation and deposition under the prevailing fluid
flow conditions.
Beach placers
Transgressional beach placers (denudational)
Waves concentrate the higher density sand grains by
selective deposition from advancing waves plus selective
entrainment of lower density gangue grains into retreat-
ing waves. In situations where the sea has transgressed
the easily-eroded sediments of the coastal plain, the
waves erode at the shore-face and remove drowned
terrain to wave-base. This is typically 10 m deep at a
distance of 1 km offshore, and results in a planar sea-
floor surface offshore and long curvilinear wave-
dominant coastlines. Transgressional beach placers are
all landward of and within metres of the trangressional
scour wave cut surface (Fig. 2). They are denudational,
and they derive their mineral from either proximal
sources such as the shoreface and cliffs, or from distal
sources via rivers and longshore drift.
The highest grade strand placers (all highly susceptible
to erosion) lie on shorelines of ultimate transgression on
passive margin continental coastal plains. Transgressional
strand placer deposit thickness depends primarily on tidal
range. An example of denudation beach placers on an
ultimate shoreline are the iron-sand strands on the passive
margin west coast of the North Island, New Zealand.
Like the previously described aeolian, washout and
river placers these transgressive beach placers have
derived their mineral value from a two-step process: an
initial emplacement of the mineral on a beach followed
by the rework of that beach as the sea continued eroding
landward, a process repeated over and over again to give
a high mineral flux and ultimately a very consistent
heavy mineral assemblage along the strand.
Regressional beach placers (accumulational)
Aerial photos of coastal bay infill and wave-dominant
cuspate deltas can show fan shaped sub-parallel sets of
ridge and swale structures called beach ridges roughly
aligned in accord with the coast. These beach ridge
plains consist of accreting beach berms often many
kilometres long, 50–150 m apart and 1–3 m high. They
show how the shoreline has regressed and the sea
retreated with the addition of sand. This sand can
originate from rivers, when such a feature is called a
cuspate delta, but the sand can also derive from the
seafloor as sand swept forward by waves, from long-
shore drift and from biogenic debris (shell hash). Plint
(2010) describes how the asymmetry of breaking
shoaling waves moving up the shoreface pushes the
coarser (and by implication the denser) mineral sand
grains forward up the shoreface to give coarsening
upward sand.
Because beach ridge plains are accumulating systems
not undergoing marine transgressive erosion, the beaches
are not repeatedly undergoing scour and entrainment of
earlier beach concentrations. Achieving economic grades
often requires additional processes. One such process is a
temporary marine transgression. Such an event can give
rise to the Type A mineral sand deposits described from
the east coast of Australia by Roy (1999). These appear
on aerial photos as unconformities or discontinuities
cutting the linear ridge sets. Type B economic placer
deposits appear at the rear portion of a strand plain and
2 Marine transgression onto a tectonically stable, low gradient, sand and mud dominant, coastal plain showing how ero-
sion of the shoreface yields a wave-dominant shoreline. There is a transgressive barrier beach and dune together with
a thin transgressive lag deposit carpeting the new offshore wave-cut surface. Arrows show direction of sediment
movement from the shoreface. On higher gradient coastal plains, cliffs will form behind the beach. Breaker bars, tidal
inlets, ebb and flow tidal deltas as well as incompletely eroded former beach complexes occasionally present as
shoreface ridges (Plint, 2010); these have been omitted for purposes of illustration
Stanaway Ten placer deposit models from five sedimentary environments
48 Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 1
are obviously the sites of ultimate transgression prior to the
commencement of regression. Roy’s beach placer Type C
form following temporary wave and current access to
placer concentrations in the sands of tidal deltas, offshore
ridges and along-shore transgessive beach placers.
The presently mined SE Madagascar ilmenite-zircon
sand placers at Madena, Petricky and Ste. Luce are
regressional beach environments (Dumouchel et al.,
2005) and belong to the Type C regressional placer class
because the adjacent land possesses weathered valuable
mineral rich metamorphic source rock capable of
supplying the ilmenite and zircon with minimal dilution
by quartz and other gangue.
Accumulative regressional beach placers include the
Green Cove Springs series of deposits in Florida USA.
The deposit system there is about 30 km long by 7 km
wide and from 1?5 to 14 m thick. About 210 million
tonnes of sand have yielded 7?25 m tonnes of ilmenite,
rutile and zircon at a grade around 3% (Rose, 2005).
To achieve economic value via a single depositional
event, an accumulation regressive beach placer requires
a richer source material than does a denudational
transgressive beach placer. The Type A and B placers
described above represent transgressive rework zones
within a sequence of overall accumulation. Type C
deposits are true accumulation placers.
Continental shelf placers
Transgressive lag placers
Transgressive lag placers are denudational, and form in
easily eroded intra-cratonic basins and on continental
passive margins with negligible new sediment supply.
During marine transgression, material eroded off the
shore-face undergoes removal in three basic directions;
onto the land, along the coast, and offshore. Scattered
pebbles, shell hash, and higher density minerals together
with sand that fails to entrain in the removing currents leave
a less than metre thick, sediment layer on the seafloor. Such
condensed strata can cover hundreds of square kilometres.
They are commercially viable if they host high value
gemstones and if exploration reveals potholes or traps with
exponential grade increases. Negligible new sediment supply
may allow thin mud which tends to cover the lags. These
placers sit upon a scour surface or platform cut into the
proximate source sediments below (Fig. 2).
A transgressive lag placer partly explored in the 1990s
and formed during several Neogene marine transgres-
sions, has been found off the west coast of South Africa.
The low gradient seafloor 90 m deep 5 km offshore to
140 m deep 40 km offshore is underlain by Cretaceous
and Tertiary delta sediments deposited by an ances-
tral Orange River that transported diamonds from
Cretaceous kimberlite pipes on the Kaapvaal craton.
Several erosive transgressions have flushed away the top
90 m or more of the delta sands and muds, concentrat-
ing rounded quartz pebbles, diamonds, shell hash and
sand as a transgressive lag layer generally less than 1
metre thick. These sediments partly bury some gently
westward dipping alternating harder and softer layer
erosional remnant cuestas, as seen from side-scan sonar.
The diamonds are very high quality because only the
best have survived the long distance transport (.90%
are gem quality). On land, Miocene Orange River
gravels yield their best grades from potholes at 0?1to
0?5 carats per ton (Jacob et al.,1999).Offshore,
diamonds also show highest grades in potholes.
Subsiding shelf placers
Subsiding shelf placers occur in cratonic basins or passive
continental margins and result from an abundant fine
3 Marine regression depositing sand and mud-predominant sediment over a downward tilting continental shelf showing
a zone of normal and storm wave base rework of offshore migrating fine sediment. The thickness of the lag accumula-
tion depends on sediment supply, rate of down-sinking and storm rework frequency. Subsiding shelf placers are not
confined to a scour or unconformity surface as is a transgressive lag
Stanaway Ten placer deposit models from five sedimentary environments
Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 149
sediment supply via coastal and offshore marine erosion.
Covering areas from square metres to tens of square
kilometres with thicknesses from centimetres to tens of
metres, they form in belts 5 to 20 km offshore with
individual placers reaching dimensions tens of kilometres
long and up to 10 km wide (Fig. 3). These offer potential
for mineral sand tonnages even larger than those in
dunes. They comprise very well sorted fine and very fine
sand. Placer minerals include ilmenite, zircon, rutile
kyanite, sillimanite and gold (Patyk-Kara et al., 1999).
All the minerals have smaller grain-sizes than the quartz
gangue at 50 to 120 mm. Subsiding shelf placers appear to
form at depths below fair-weather wave base and above
storm wave base in the near-shore shelf zone where
stronger storm-induced waves stir bottom sediments. A
feature of one of these deposits (WIM150 in Victoria,
Australia) is hummocky cross stratification (Williams,
1990) also recorded by the author in another of these
deposits in the Cretaceous McNairy Formation sands
of Tennessee, USA. Descriptions of structures in fine
sand facies of similar deposits on the Russian Platform
strongly suggest hummocky cross stratification (Savko
et al., 2000). These placers of offshore origin were first
discovered in the 1960s in the Jurassic to Palaeogene fine
sands of the Tethys Sea on the Russian Platform, but their
mode of origin appears to be only recently recognised.
Subsiding shelf placers are initiated by storm-driven
unidirectional currents coupled with wave derived oscil-
latory currents capable of reaching greater depths
because of greater wave amplitudes and periods during
storms. Deposit genesis starts with the formation of a
storm surge when low air pressures and strong winds are
blowing water onshore and piling up water. The returning
bottom-hugging geostrophic flows can reach velocities of
0?7ms
21
enabling them to move and sort sand (Plint,
2010). Each event unit that makes up a deposit, according
to Plint, starts when waves from a rising storm scour into
the muddy seafloor to suspend the fine sand and mud. As
the storm wanes planar-laminated fine sand deposits first,
this gives way to hummocky cross stratified fine sand
which initially may be anisotropic, but as the unidirec-
tional geostrophic flow component weakens this will
become oscillatory eventually declining to two-dimen-
sional ripples. Current velocities over 0?5ms
21
give the
flat bed condition favourable for higher density mineral
concentration (Brady and Jobson, 1973). According to
Plint (2010), each storm unit would be centimetres to
decimetres thick with most of the deposition taking place
with the initial flat bed and anisotropic hummocky bed
structure. Such an accumulation would involve thou-
sands of units just described and doubtless could reach
tens of metres thick with 15 m the maximum recorded in
the literature to date. The required sediment volume
would be derived from gentle continuing down-warp, and
sediment supply in excess of that needed to maintain the
seafloor depth (Fig. 3). Enrichment of placer mineral can
reach 10 to 100 times background values in surrounding
fine sand sediments.
An example of such a deposit is WIM150 in the
Murray Basin, Australia. Depending on the cut off
chosen, this deposit has between 4900 m t of very fine
sand grading 2?8% total heavy minerals (Williams, 1990)
and 542 million tonnes grading 5?9% HM (Australian
Zircon, Report, 2008). According to Williams (1990) it
consists of hundreds of thousands of millimetres thick
lamina and layers enriched in ilmenite, rutile and zircon,
each persisting laterally for hundreds of metres. They
aggregate 6 to 15 m thick and dip about 3 degrees NW.
The WIM150 deposit is part of a series of placers in an
arcuate band 150 km long and up to 25 km wide close to
the southern edge of the Murray basin. These lie at the
base of a coarsening upward Pliocene sand sequence.
The complex history of these subsiding shelf placers is
demonstrated by the Tsentralnoye ilmenite, rutile zircon
and gold placer in central Russia described by Patyk-
Kara et al. (1999). This 15 km long by 5 km wide fine
sand resource is from 1 to 15 m thick. It began to form
in the late Cenomanian as an offshore delta front
mineral sand accumulation as witnessed by glauconite,
the macro and micro fauna and phosphate grain
coatings (Savko et al., 2000), but became emergent as
sand bars. By the Santonian, it had become fully
emergent and a new top surface was established by the
wind (Patyk-Kara et al., 1999) before final burial under
muddy sediments. Mineral sand grades reach as high as
7% heavy minerals.
Exploration for subsiding shelf placers should focus
on sedimentary basins and passive continental margins
exhibiting transgression-regression sequences where
shoreline and river sediments cover marine deposits. A
related target situation is a slowly subsiding marine
realm adjacent to a rising terrestrial sediment supply as
is the case for the WIM150 deposit in relation to the
Western Highlands axis uplift in Victoria, Australia
(Wallace et al., 2005).
Conclusion
Economic placer concentrations have now been recog-
nized in all the major sediment environments except
submarine canyon, submarine fan and abyssal plain.
The genetic classification proposed not only covers all
known placer types, but allows future placer exploration
to proceed using a broad and useful understanding of
geological process. In the past, descriptive classifications
have been the norm for placer exploration and these
have led to prospecting by looking for deposit analo-
gues, principally by sampling in similar geomorphologi-
cal features rather than by understanding of formational
process. This emphasis on analogues has probably not
overly troubled explorers since prospecting has been
successful, despite taking place empirically by sampling
streams, rivers, beaches and dunes. Understanding new
deposit types stumbled upon by chance, such as the
transgressive lag and subsiding shelf placers has taken
time and is as yet far from complete; the question
remains are we blinded to future potential? Subsiding
shelf placers may yet prove to be the largest of all placers
in terms of tonnage of contained valuable mineral, but
have only recently been discovered. The accumulation
placer model is only an application of the principles of
sediment sorting by grain-size to sediment sorting by
grain density. One interesting feature of the subsiding
shelf placer deposits of Russia is the occurrence of fine
gold, probably in hydraulic equilibrium with the other
placer and gangue particles. Could more gold be found
in such environments? Do all subsiding shelf placers
have to consist of fine sand? Could they also result in lag
gravel accumulations with different valuable minerals?
Placer deposits are linked not only by the two-fold
process proposed above, but also in nature. Many sand
Stanaway Ten placer deposit models from five sedimentary environments
50 Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 1
placer deposits occur along ancient shorelines as is
evidenced by the Trail-Ridge-Orangeburg Scarp of the
south eastern Coastal plain of the USA. In at least two
sediment basins, subsiding shelf placers are known to
occur stratigraphically beneath regressional beach pla-
cers, a possibility depicted in Fig. 3. This pairing occurs
both in the Murray basin and in the Oligocene-Miocene
of the Dneipr-Donetz basin of Russia (Patyk-Kara
et al., 1999). It may even occur in the northern Florida
Trail Ridge (fine-medium sand)-Green Cove Springs
(very fine sand) mineral sand pairing, where the uplift
required for a subsiding shelf placer would have been
provided by the contemporary rising Ocala Arch. In this
last case however the offshore placers were reworked
into a regressional beach system to be mined as the
Green Cove Springs sand placer.
Acknowledgements
The author thanks several anonymous reviewers for the care
and effort they applied to correct and clarify his thinking
and writing. He also thanks Rio Tinto Iron, Rio Tinto Iron
and Titanium, BHP Minerals Exploration, Cluff Resources,
Sierra Rutile Ltd, E. I. du Pont de Nemours and Company,
JBL Exploration and Carpentaria Exploration for the
opportunity to investigate many different placer deposits.
References
Barakat, M. and Ruddock, R. 2006. Taharoa iron-sand mining
operation, Geology and Exploration of New Zealand Iron-sand
Deposits, Monograph 25, 225–230, Melbourne, Victoria,
Australia, The Australasian Institute of Mining and Metallurgy.
Barakat, M. and Drain, L. 2003. The development of Waikato North
Head iron-sand deposit, South Auckland, Geology and
Exploration of New Zealand Iron-sand Deposits, Monograph
25, 219–224, Melbourne, Victoria, Australia, The Australasian
Institute of Mining and Metallurgy.
Brady, L. L. and Jobson, H. E. 1973. An experimental study of heavy
mineral segregation under alluvial flow conditions, USGS
Professional Paper.
Burton, J. P. and Fralick, P. 2003. Depositional placer accumulations
in coarse grained alluvial braided river systems, Econ. Geol.,98,
985–1002.
Carter, C. H. 1978. A regressive barrier and barrier protected deposit:
Depositional environments and geographic setting of the late
Tertiary Cohansey sand, J. Sediment. Petrol.,48, 933–950.
Cornell, F. C. 1920. The glamour of prospecting. Reprinted 1992 by
David Phillip Pub.(Pty) South Africa.
Dingman, O. A. 1932. Placer mining possibilities in Montana. Montana
Bureau of Mines and Geology Memoir 5.
Dumouchel, J., Giroux, J., Mead, M. and Yule, W. 2005. The QIT
Madagascar minerals deposits geology, mining and mineral
processing. Heavy Mineral Conference Proceedings, (ed. M.
Akser and J. Elder), 151–156, Littleton CO, Society for Mining,
Metallurgy, and Exploration.
Garnett, R. H. T. and Bassett, N. C. 2005. Placer deposits, Econ. Geol.
100
th
Anniv.Vol., 813–843.
Jacob, R. J., Bluck, B. J. and Ward, J. D. 1999. Tertiary aged
diamoniferous fluvial deposits of the lower Orange River valley,
Southwestern Africa, Econ. Geol.,94, 749–758.
Kartasov, I. P. 1971. Geological features of alluvial placers, Econ.
Geol.,66, 879–885.
Markewicz, F. J. 1969. Ilmenite deposits of the New Jersey coastal
plain, in Geology of selected area in New Jersey and Eastern
Pennsylvania, (ed. S. Subritzky), New Brunswick, NJ, Rutgers
Univ. Press.
Mauk, J. L., Macorison, K. and Dingley, J. 2006. Geology of the
Waikato North Head and Taharoa iron sand deposits, Geology
and Exploration of New Zealand Mineral Deposits, Monograph
25, 231–234, Melbourne, Victoria, Australia, The Australasian
Institute of Mining and Metallurgy.
Patyk-Kara, N. G, Bardeeva, E. G. and Shevelev, A. G. 1999. Titano-
zirconium placers in the sedimentary cover of platforms, Episodes,
22, 89–98.
Patyk-Kara, N. G. 2002. Placers in the system of sedimentogenesis,
Lithol. Miner. Resour.,37, 494–508.
Peterson, D. W., Yeend, W. E., Oliver, H. W. and Mattick, R. E. 1968.
Tertiary gold bearing channel gravel in northern Nevada county
California, U.S. Geological Survey Circular 566.
Plint, A. G. 2010. Wave and storm-dominated shoreline and
shallow-marine systems, in Facies Models 4, (ed. N. P. James
and R. W. Dalrymple), St Johns Newfoundland and Labrador,
Canada, GEOtext6 Geological Association of Canada.
Puffer, J. H. and Cousmiler, H. L. 1982. Factors controlling the
accumulation and of titanium-iron-rich sands in the Cohansey,
Lakehurst area, New Jersey, Econ. Geol.,77, 379–391.
Rose, R. R. 2005. Green Cove Springs deposit geology, in Heavy
Mineral Conference Proceedings, (ed. M. Akser and J. Elder), 1–
6, Littleton CO, Society for Mining, Metallurgy and Exploration.
Roy, P. S. 1999. Heavy mineral beach placers in southeastern Australia:
Their nature and genesis, Econ. Geol.,94, 267–588.
Savko, A. D. Belijaev, D. A. and Ivanov, D. A. 2000. Facial types of
titan-zirconium placers from the central region and prognosis
their study (abstract in English), in Natural and tectogenic placer
and weathered rock deposits at the turn of the Millenium, XII
International Symposium on Placer and Weathered Rock
Deposits, Moscow, Russia, September 2000, 162–165, Russian
Academy of Sciences, Ministry of Natural Resources, Russian
Federation.
Slingerland, R. and Smith, N. D. 1986. The occurrence and formation
of water-laid placers, Ann. Rev. Earth Planet. Sci.,14, 113–
147.
Stanaway, K. J. 1992. Heavy mineral placers, Min. Eng.,44, 352–358.
Stanaway, K. J. 2005. Four world titanium mining provinces, Heavy
Mineral Conference Proceedings, (ed. M. Akser and J. Elder), 47–
59, Littleton CO, Society for Mining, Metallurgy, and
Exploration.
Wallace, M. W., Dickinson, J. A., Moore, D. H. and Sandiford, M.
2005. Late Neogene strandlines of southern Victoria: a unique
record of eustasy and tectonics in southeast Australia, Aust. J.
Earth Sci.,52, 279–297.
Williams, V. A. 1990. WIM 150 Detrital heavy mineral deposit, in
Geology of the mineral deposits of Australia and Papua-New
Guinea, (ed. F. E. Hughes), 1609–1614, Melbourne, Victoria,
Australia, Australasian Institute of Mining and Metallurgy.
Yeend, W. and Shawe, D. R. 1989. Gold in placer deposits, U.S.
Geological Survey Bulletin 1857.
Youngson, J. H. and Craw, D. 1999. Variation in placer style, gold
morphology, and gold particle behavior down gravel bed-load
rivers: An example from the Shotover/Arrow-Kawarau-
Clutha River system, Otago. New Zealand, Econ. Geol.,94,
615–634.
Stanaway Ten placer deposit models from five sedimentary environments
Applied Earth Science (Trans. Inst. Min. Metall. B) 2012 VOL 121 NO 151
