Vertical grade profiles of offshore and coastal placers

Abstract

Accumulation and lag concentration, together with dispersion, are important processes in offshore and coastal placer development. Mineral grains and sediment particles are sorted by flowing water according to their hydraulic equivalence. Distinct placer types are developed. Marine regression and transgression result in reworking leading to modification or destruction. Some placers are stranded on the coast, others are drowned. A vertical grade profile (VGP) graphically depicts the grade variations in the pay section of a placer. A VGP may be plotted for all heavy minerals and every placer type, and two basic patterns are evident. An upward or downward decline in grade indicates that accumulation and lag processes, respectively, were dominant. A reversal in decline direction marks a scour horizon or stillstand. Combinations of the basic patterns exist and some are complex with three grade maxima, revealing multiple mineralisation units. The history of a placer is revealed by its VGP(s) unless part is erased by reworking. Plotting the grade to a common logarithmic scale may assist interpretation. Different placer types have been developed in favoured locations visible on model cross-sections through the coast to the offshore. With minor variations, the VGP pattern displayed by a specific placer type is common to all minerals. Sampling protocol and resource estimation procedures should be guided by the VGP.

Keywords

Placer Vertical grade profile Offshore Coastal Marine Sorting Accumulation Lag

Introduction

During the early 1960s, several mining companies operated approximately 65 bucket-ladder dredges and many open-pits on Malaysian, cassiterite-bearing tin placers. In two such companies attempts were being made to improve ore reserve estimation methods. The objective was to provide a better forecast of the percentage recovery of the estimated grade achievable during production (Broadhurst and Batzer, 1965). The pattern of vertical grade variation appeared to be one of the factors influencing the recovery (Garnett, unpubl., 1964). The in situ grade was never constant. It declined either upward or downward from the bedrock or ground surface, respectively, or from intermediate horizons. Combinations and repetitive patterns existed, especially in the more extensive placers. The individual details were indicative of the manner of formation of the placer. The term vertical grade profile (VGP throughout this paper) was used to describe the plot of grade variability in the vertical dimension of placers being dredged by Tronoh Mines Ltd in the Kinta Valley.

A nearby placer on limestone bedrock was then being drilled and evaluated. The VGPs of the deposit were examined in conjunction with a rudimentary analysis of the tin grade frequency distributions at successive ∼1·5 m elevations. The distributions suggested the existence of three distinct tin-bearing units. Each unit was characterised by a different grade population and by features affecting the recovery of cassiterite. Subsequent statistical work confirmed that the units were present (Clark and Garnett, 1974). VGPs were then used on a variety of tin placer types being explored and mined offshore from southern Thailand.

Placers of both non-marine and marine origin are targeted in offshore and coastal environments for their content of precious metals, tin, diamonds, and other heavy minerals. Their more diverse origins and any subsequent modifications result in a greater variety of VGPs than is displayed by onshore placers.

There are few published examples of VGPs, one such being of some onshore gold placers (Goldfarb, 2007). Others have been interpreted in geomorphological terms (Lintern, 2005). Coastal forms have been identified from the VGPs of palaeoplacers containing heavy minerals (Patyk-Kara et al., 1999). Published VGPs also are available for heavy minerals in Australia (Roy, 1999), offshore Mozambique (Kudrass, 1987) and for a tin placer sited in Ringarooma Bay of Tasmania in Australia (Davies, 2009).

Definitions

To avoid any uncertainty, various terms employed throughout the text are defined in Table 1. It is recognised that there are alternative definitions of some of the terms in the literature, e.g. for coastal, littoral, nearshore, offshore and depth of closure.

Table 1

Definitions of terms used in the text

Term	Definition
Placer	Concentration of valuable detrital minerals formed by sedimentary processes whereby the relatively high density of each mineral serves as the principal cause of concentration (Garnett and Basset, 2005). Reserved for non-lithified placer deposits that have been extensively sampled, demonstrated to be economically viable, or exploited.
Placer type	Placer classified according to the geomorphological feature in which a mineral is concentrated.
Placer field	An area within which there occur numerous large placers yielding the same commercial product. The term ‘mega-placer’ is used in places.
Marine placer	Placer formed entirely by the action of marine agencies.
Deposit	Concentration of valuable detrital minerals which is either part of a placer or is insufficient in terms of grade and/or volume to qualify as a placer.
Mineral	Precious metals (gold and platinum group metals), cassiterite, diamonds, and other ‘heavy minerals’, all of which exceed a density of 2·9 g cm⁻³ and constitute the product of a placer.
Heavy mineral (HM)	Refers to the ‘heavy HMs’ (ilmenite, magnetite, rutile, monazite, garnet, zircon, xenotime, chromite and some other minerals less commonly found in placers) as opposed to ‘light HMs’ which possess a lower density and are of lesser or no commercial value.
Mineral sands	Placer deposit containing an economic concentration of one or more HMs.
Black sands	HMs which accompany, as accessories, precious metals, cassiterite and diamonds in a placer. In South-East Asia black sands are referred to as amang. The term ‘ironsands’ implies magnetite-rich sands.
Grain	Discrete fragment or crystal of a ‘mineral’.
Sediment	Accumulation, either non-cohesive or cohesive, of individual rock ‘particles’ of all sizes from silt to boulders. May, or may not, contain any mineral(s).
Particle	Single, discrete constituent, whatever its size, of sediment.
Strand	Used as a synonym, especially in Australia, for beach.
Coastal	Zone which constitutes the interface between land and sea. Includes the coast and coastline, beach and shoreline, littoral zone and nearshore zone.
Littoral zone	Inter-tidal zone and beach.
Nearshore	Zone seaward from the low water mark to the start of the offshore, defined by the depth of closure (Nicholls et al., 2001). Includes the zone influenced by long shore currents.
Depth of closure	Water depth beyond which the seabed does not change appreciably during storms. May be expressed in terms of wave height and period, and seabed sediment.
Offshore	Seaward of the coastal zone, but in places may be used in a non-specific manner referring to any area seaward of the shoreline.
Seabed	Surface of the sediment (substrate) underlying the sea.
Bedrock	Solid rock which underlies any sediment. May in places include saprolite.
Placer's thickness	Vertical distance between either ground surface or seabed and the base of the placer which may be on bedrock or in sediment (defined by mineral grade and/or sediment lithology).
Depth	Unless described as water depth, is either the vertical distance below the ground surface or seabed at which a placer has been sampled, or mid-depth of a single sample's vertical extent.
Height	Vertical distance above bedrock, a false bedrock, or a datum level and at which a placer has been sampled, or is the mid-height of a sample's vertical extent.
Grade	Degree of concentration of a mineral in sediment, variably expressed as the mass of mineral per unit of in situ volume, wt-%, or ppm. May represent the total mineral present as measured by the equipment used, or the lesser grade expected to be recoverable by a stated metallurgical flowsheet.
Pay section	Any part, or all, of a placer's thickness which is, or expected to be, profitably mined and treated to recover contained mineral(s).
Overburden	Unpayable sediments overlying a pay section.
Unit	A clearly identifiable single stage of mineral accumulation or concentration in a placer.

Placer types

Five groups of placer types exist in offshore and coastal areas:

(i)

placers formed in the littoral zone and nearshore

(ii)

marine placers formed in the nearshore and offshore

(iii)

drowned placers formed on the shore, now seaward of the shoreline

(iv)

drowned non-marine placers, now seaward of the shoreline

(v)

stranded placers formed as a beach or in the nearshore, now above sea level.

Some or all may have formed in a major, placer-hosting offshore and coastal area. Each group contains a variety of placer types most easily defined by their geomorphological form. Some placer types are confined to only one or two groups. Others are more widespread. All owe their origin to some combination of accumulation, lag and dispersion processes (Garnett and Bassett, 2005).

Minerals are introduced into the offshore and coastal zones by fluvial transport and by erosional liberation from coastlines (Fig. 1). Where subjected to flowing water they may be transported and sorted according to their hydraulic equivalence, a combined measure of size and density (Slingerland, 1977). The component particles of non-cohesive sediments behave in a similar manner. The morphology of a very fine-grained mineral, especially of gold, is an additional factor in defining its hydraulic equivalence. Sorting leads to concentration, with or without significant prior dispersal.

Cross-section through the offshore and coastal zones to illustrate some of the terms defined in Table 1

The critical hydraulic equivalence above or below which a particle or grain becomes entrained or settles varies with time and place, governed by the prevailing energy conditions. Grains and particles of less than a critical hydraulic equivalence are entrained and transported seaward or alongshore. An eventual reduction in energy allows selective settling of the grains. A proximal or distal accumulation placer is formed by these processes. The grade declines upward from the base of accumulation if diminishing energy allows increasingly lighter particles to settle. Coarser grains tend to accumulate in, or very close to, beaches (Komar, 2007). Some are concentrated in the swash zone (Komar and Wang, 1984). Exposed, irregular bedrock and coarse gravel layers may provide trap sites for migrating minerals.

Minerals are also released by seabed erosion from exposed bedrock sources and by winnowing from any mineral-bearing assemblage of sediment. A lag concentration of remnant mineral grains is left on the bedrock or among the uppermost particles of the remaining sediment. Within the latter the grade declines downward from the surface. Any coarser, immovable particles may form a protective barrier, as a remnant lag gravel, to inhibit further winnowing. In practice a dominant accumulation process incorporates some lag action. A dominant lag process, likewise, involves some accumulation. A discrete unit results from the uninterrupted operation of each dominant process.

Placers formed in the above manner were both modified and destroyed by marine regression and transgression. Marine transgression, as opposed to regression, tended to increase the number of units in a vertical section. Coastline faulting and warping, cyclic glaciation and erosion were important in areas of uplift (Kudrass, 2000). Repetition of supply, multiphase accumulation, scouring and reworking by the lag process in places rendered a placer complex or changed it beyond recognition. The grade of an accumulation placer may have been increased and its thickness decreased by scouring and reworking by the lag process. That of a lag placer could have been decreased and its thickness increased by superimposed accumulation of lower grade sediments. Many placers now owe their economic grade and mineral content, even their existence, to reworking.

Vertical grade profile (VGP)

A VGP depicts the variation in incremental grade for any number of equal or unequal, contiguous, vertical increments relative to depth or height within the placer. The abscissa represents the grade. The vertical interval used for a VGP usually ranges from 0·25 to 1·00 m. Consistency of interval within a single VGP is not essential. The lowest interval is defined by the minimum vertical sampling interval employed. It may have been designed to assist in determining the required mining selectivity. However, a problem arises if any of the units comprise stacked, mineral-rich strata possessing thicknesses less than 0·25 m. They and their intervening less mineralised layers are consolidated within a vertical interval which exceeds the thickness of each. In the absence of any explanation a package of strata therefore may be indistinguishable on the VGP from disseminated mineralisation over the same vertical interval. In addition, some drilling systems may contaminate underlying unmineralised strata with mineral grains entrained from above. A tail of recorded grades, referred to as drag ore, results. The data falsely indicate that mineralisation extends in depth. Care must be exercised in interpretation.

The source of data for the derivation of a VGP may be a single drillhole. However a more representative VGP for the domain of a mineral may require the averaging of data from a group of adjacent drillholes. Too wide an areal spread should be avoided until lateral uniformity is established. VGPs of contiguous areas in a placer may show very different results which reflect localised energy variations. Experimentation may be necessary. Several representative, but significantly different, VGPs may be revealed in a single placer. Thicker placers frequently possess the most complex VGPs. Specific placer types possess characteristic VGP patterns showing the direction in which grade declines: upward or downward. A pattern reveals the presence of one or more units.

Vertical grade profile patterns

Two basic patterns, and others, evident in all VGPs of offshore and coastal placers are represented by models (Fig. 2). The grade declines either upward (Fig. 2A) or downward (Fig. 2B). The basic ones contribute to an additional four patterns (Figs. 2C, D, E and F). Each of the six is associated with one or more of the placer types, but exceptions occur. A sudden change in direction of grade decline may indicate a scour horizon or a stillstand. Changes in the rate of decline may be caused by the short term intrusion of either lag or accumulation in the prevailing, otherwise dominant process. Some VGPs exhibit a complex, vertical combination of repeated patterns which result from several changes, usually increases, in relative sea level.

Six drawings illustrating model VGP patterns. a basic, single upward decline; b basic, single downward decline; c repeated upward decline; d repeated downward decline; e combination of A followed by B; f combination of B followed by A

Logarithmic grade decline

Grade variation may be depicted graphically as a line chart or a bar chart (Fig. 3). However, a decline in heavy mineral (HM) grade of a dispersion placer has been noted to be logarithmic (Berquist and Hobbs, 1988) and a ‘reversed exponential discipline’ of offshore tin grade was recorded by Kudrass and Schlüter (1994). Many grade populations also approach log-normality. Using a common logarithmic (log₁₀) scale for the abscissa of the VGP (VGP_log) has proved informative for all minerals and most placer types.

Three examples of the results of plotting the common logarithm (log₁₀) of the grade (VGP_log). a, b precious metal-bearing beach in Ecuador. Based on one representative drillhole; c, d two glacio-fluvial, gold-bearing units in the nearshore at Nome, AK, USA. Based on numerous drillholes; e, f nearshore HM placer, Indonesia. Confidentiality prevents disclosure of the HM grades. Based on one representative drillhole

A single unit (Fig. 3A) is frequently represented by a straight line on a VGP_log (Fig. 3B). Subtle changes may indicate the presence, otherwise unsuspected (Fig 3C), of more than one unit (Fig. 3D). Two of three lines on a VGP_log, each representing an individual unit and its rate of grade decline (Fig. 3F), exhibit parallelism which is not obvious on a bar chart (Fig. 3E). Parallelism is not indicative of equal rates of decline.

If a lag event immediately follows accumulation, or vice-versa, the VGP displays a rapid change in decline direction, in grade, and in rate of grade change (Fig. 4). An upward decline reveals that the accumulation process was dominant. A relatively steep decline may indicate that the less dominant lag process also operated to maintain a high grade. A downward decline is created by a dominant lag process during an influx of accumulating sediment and mineral. Rate of decline may reflect the prevailing energy levels and their rate of decay. Sharp reversals in the direction of decline result from a scour horizon or stillstand. Histograms of regional seabed bathymetry and bedrock elevation may support the latter interpretation.

Model VGP_log with interpretations of grade minima and maxima, and grade decline directions for four numbered units

Each form of chart, bar, line and VGP_log, may be more appropriate for different data sets and their interpretation. All three forms are referred to in the remainder of the text as a VGP.

Lithologic control

Individual minerals tend to accompany particular sediment lithologies. Sudden grade changes coincide with lithologic boundaries. The mineral grain size may also display a direct or inverse relationship with sediment particle size. A VGP therefore ideally is viewed alongside any changes in the granulometry of the host sediments (Fig. 5). Gold (density = 15·4–19·4 g cm⁻³) was usually concentrated to a commercial extent in high energy gravels, although fine dispersed gold may exist in sands and, less commonly, in finer sediments. Cassiterite (density = 6·8–7·1 g cm⁻³), being brittle, quickly reduced in size during transport, usually is hosted by sediment ranging from fine sands to pebbles. Diamonds (density = 3·5 g cm⁻³), with a size of approximately 4 mm (around 0·25 carats/stone, or 0·05 g/stone) and higher, are normally found associated with gravels or at their interface with sand. However, diamonds are found in sand alone and, very rarely, in finer sediments. Heavy minerals (density = 4·2–7·0 g cm⁻³) generally are concentrated in sands and silt.

VGPs illustrating the lithologic influence on grade. a nearshore tin placer, southern Thailand, with upward declining cassiterite grade. Based on several drillholes; b offshore gold placer, West coast, South Island, New Zealand. Modified after Lew and Corner (1990); c HM placer in the littoral zone, Peru. Interpretation based on published sampling data (Latin Resources Limited, 2012)

Effect of mineral grain size

The relationship between mineral grade and grain size may be both direct and inverse within different units of the same placer. Therefore the pattern of a VGP may depend on whether the total mineral content, or only a particular size fraction, is reported (Fig. 6). A suite of heavy minerals of different density and range of grain size invariably exhibit very different directions and rates of decline.

VGPs illustrating the effect of mineral grain size on the rate and direction of grade decline. a cassiterite-bearing channel (gully) offshore Billiton Island, Indonesia, showing coarser cassiterite declining upward at 2·5 times the overall rate of finer grains. Based on data from Van Overeem (1960); b dispersion deposit showing opposing directions of heavy mineral grade decline exhibited by different size fractions of grains. Based on data from Hunter (1971); c dispersion deposit showing contrasting grades and rates of cassiterite grade decline exhibited by different size fractions of grains. Modified after Yim (1974)

Examples from major placer fields

For each mineral the various placer groups and types inhabit typical locations in the offshore and coastal zones of major placer fields. Four model cross-sections are drawn to represent a combination of two or more such domains for each mineral: gold (Fig. 7), cassiterite (Fig. 8), diamonds (Fig. 9) and heavy minerals (Fig. 10). Of several tabulated placer types, each exhibits for all minerals a significant similarity of VGP patterns. The patterns are listed, with reference to Fig. 2.

Cross-section through an offshore and coastal gold placer field with VGPs of some typical placer types. The cross-section represents Nome (Garnett, 2000a) and Chilean Tierra del Fuego (Garnett, unpubl., 2013). a remnant lag gravels in the nearshore, Nome. Based on numerous drillholes; b repetitive gold concentration in mobile lag gravels at stillstand elevations within partially eroded diamicton at Nome. Based on 91 grouped drillholes; c gold-bearing diamicton, separated by glacio-fluvial sand and gravels, and overlying the oldest unit of high grade fluvial gravels on bedrock, Lennox Island, Chilean Tierra del Fuego. Based on channel sampling of a cliff (Gaffri, unpubl., 1997; Garnett, unpubl., 2013); d dispersion deposit in the offshore, Nome. Based on an undisturbed sample; e buried channel of pre-glaciation Anvil Creek, Nome. Based on 12 closely situated drillholes; f back beach on Lennox Island, Chilean Tierra del Fuego. Based on a hand-dug pit (Gaffri, unpubl., 2013; Garnett, unpubl., 2013)

Cross-section through an offshore and coastal tin placer field with VGPs of some typical placer types: the cross-section represents Indonesia (Batchelor, 1979; Sundrijo, 1984), Malaysia (Ringis, 1983) and southern Thailand (Garnett, unpubl., 1973). a nearshore, combined channel and mobile lag deposit, southern Thailand. Based on multiple drillholes; b nearshore alluvial fan formed over a cassiterite-rich granite, Dindings district of West Malaysia. Interpreted from a cross-section showing five drillholes (Ringis, 1983); c beach, showing surface concentration of cassiterite, Gwithian, Cornwall. Interpreted from published data (Lee, 1968); d dispersion deposit offshore St. Agnes, Cornwall. Modified after Yim (1974); e two cassiterite-bearing units overlying an offshore channel placer, Laut Tempilang, Indonesia. Modified after Sundrijo (1984); f beach, showing basal concentration of cassiterite, Pantai Belo, Indonesia. Modified after Sundrijo (1984)

Cross-section through an offshore and coastal diamond placer field with VGPs excluding d, of some typical placer types. The cross-section represents the continental shelf offshore Namibia and South Africa. a offshore accumulation; b offshore accumulation (modified palaeobeach). Constructed, with a reasonable probability of being representative, and based on the results from numerous closely spaced sample sites; c offshore, infilled channel. Constructed in the same manner as B, showing two units of diamond-bearing gravels. Unquantified, higher grade sediment exists immediately above bedrock. Based on the results from numerous closely spaced sample sites; d deep water, offshore transgressive lag∼1 m thick. Confidentiality prevents disclosure of the diamond grades

Cross-section through an offshore and coastal HM placer field with VGPs of some typical placer types. a relict strandline in southeastern Australia. Modified after Roy (1999); b strandline deposit in WA, Australia. Based on data from Shepherd (1990); c mineralised dunes, Richards Bay, South Africa, modified after Abzalov et al. (2011); d offshore concentrations on the Mozambiquan inner shelf. In any place either the upper or lower concentration is dominant. An interpretation of >60 core samples (Kudrass, 1987); e dispersion deposit, offshore VA, USA. Based on published data (Berquist and Hobbs, 1988). And dispersed chromite in the nearshore, Port Au Port, NL, Canada. Based on multiple samples. Confidentiality prevents disclosure of the chromite grades; f modern beach placer, Geelwal Karoo farm, Namaqualand, South Africa. Modified after MacDonald and Rozendaal (1995)

Some of the VGPs illustrated with each cross-section have been selected from the author's unpublished work between 1964 and 2013. They are supplemented by VGPs resulting from the author's subjective interpretation of limited, published, mostly numerical material. For a variety of reasons, including confidentiality and sampling difficulties, some placer types for certain minerals are not represented. Unless inhibited by confidentiality requirements all grades are stated in SI units.

Gold placer vertical grade profile

The offshore and coastal gold placers of Nome, Alaska in the USA, have been extensively explored and exploited since 1900 (Garnett, 2000a). Similar, less explored areas include the West coast of South Island, New Zealand (explored offshore and exploited on the coast), Chilean Tierra del Fuego (old, coastal artisan workings, and recently explored onshore) and Siberia. Together, they constitute the model cross-section for gold (Fig. 7). The four locations share an important feature: their extreme latitude. Glacial action eroded proximal and distal primary sources of gold. The mineral was transported in diamicton or coarse gravels to the coast and beyond the shoreline. Placers exist both above and below mean sea level (msl).

At Nome two units of diamicton and two terminal moraines extend well into the nearshore. They incorporate discontinuous remnants of pre-existing onshore fluvial placers, ice-rafted boulder fields, outwash gravels, and the products of marine reworking. Changes in relative sea level combined with high energy storms lead to the formation of several placer types, each represented by a characteristic VGP. During stillstands collapsing palaeocliff faces of diamicton released gold to the existing beaches and into dispersing currents. High grade remnant lags formed on seabed exposures of diamicton.

All economic mineralisation seaward of the modern beach is in less than 30 m of water. Western Gold Exploration and Mining Company Limited Partnership (WestGold) dredged offshore at Nome during the late 1980s (Garnett, 2000a). The depth limit of marine mining is dictated by the difficulty of excavating cohesive diamicton containing large clasts up to boulder size. Onshore deposits, especially parts of raised beaches which are now depleted, possessed the highest, consistent grades.

All five placer groups are represented to varying degrees of commercial importance (Table 2). On Lennox Island, Chilean Tierra del Fuego, two phases of glaciation and an intervening unit of fluvially reworked diamicton, all overlie an earlier, high grade fluvial unit. Elsewhere, worldwide, thin surface concentrations of gold have been recovered from beaches on an artisan scale.

Table 2

Common VGP patterns displayed by gold placers

Group	Placer type	Common VGP patterns	Details
1	Modern beach	A, B, C, E, G Refer to Fig. 2	The highest grade of coarse gold usually is immediately above bedrock (Vorob'yev, 1979; e.g. Fig. 7F), but at Nome gold concentrations lie above a false bedrock of clay (Collier et al., 1908). Finer gold may be non-uniformly disseminated on the surface or over part of the full beach thickness. The finest gold may accompany black sands, garnet and pyrite in laminae and/or thin strata. The existence of one form of concentration does not guarantee the presence of others.
	Remnant lag	B	At Nome high gold grades, with coarse grains, exist as relict lag gravels that form a surface veneer to the diamicton (e.g. Fig. 7A) (Nelson and Hopkins, 1972). They constitute an economic target despite their highly variable grade and difficulty of sampling. Palaeolags are preserved at stillstands within seabed-exposed diamicton (e.g. Fig. 7B).
2	Mobile lag	A, B	Formed on a small scale at stillstands in Nome with a lower grade variability than remnant lags. Elsewhere, formed ‘seams’ in sand and gravel (Vorob'yev and Krapiviver, 1975).
	Dispersion	B	At Nome flattened gold grains <100 μm in size, and sediment particles <2 mm were dispersed into the offshore (Fig. 7D) to become incorporated in otherwise gold-free sand and silt. Grade may decrease by as much as 50% for each 0·2 m increase in depth (Neronsky, 2000, pers. comm.). Finest gold was dispersed tens of kilometres.
3	Submerged beach		At Nome beaches were formed at stillstands but marine modification has obliterated most evidence. Submarine beach remains in part and has been dredged both on the coast and offshore.
4	Channel	A	Anvil Creek at Nome extends at least 2 km offshore. As a V-shaped channel, 350 m wide at its shoulder, it is cut ∼20 m deep in diamicton (e.g. Fig. 7E). Extensive overburden prohibits dredging.
5	Stranded beach	Mostly A	Five raised beaches and connecting, seaward trending topographic depressions and coastal channels at Nome have been extensively mined since 1900. The highest is almost 40 m above msl. Grades decline laterally along the palaeoshorelines.

Tin placer vertical grade profile

The offshore and coastal tin placers of South-East Asia constitute the model for cassiterite (Aleva, 1973; Batchelor, 1979). They are part of the South-East Asian tin belt and comprise the deposits of the Tin Islands of Indonesia (Bangka, Billiton and Singkep), southern Thailand, and the Dindings district of West Malaysia. All lie on the micro-tidal, shallow water margins of the Sunda Shelf over which offshore water depths vary from approximately 30 to 120 m. The three placer fields contribute to the model cross-section (Fig. 8) and share an important feature in common. The tropical climate of their equatorial location has accelerated the alteration and erosion of low lying, coastal granites. These have released cassiterite for transport into the nearshore.

On the shelf an older alluvium comprising cassiterite-rich gravels overlies bedrock in a piedmont fan environment. The alluvium is overlain unconformably by younger flood plain deposits containing lower grade concentrations. Some channels were incised to 120 m below msl during falls in relative sea level (Batchelor, 1979). In places a youngest and widespread unit comprises overburden of mud and sands up to 30 m thick. Selectively eroded, karstic limestone provides very large, seabed trap sites for cassiterite-rich sediment. Elsewhere, exposed granitic bedrock carries thin, high grade mineral concentrations. Nearshore placers have been modified by monsoon storms in Indonesia and tsunamis in Thailand. All five placer groups and many placer types are represented to varying degrees of commercial importance (Table 3).

Table 3

Common VGP patterns displayed by tin placers

Group	Placer type	Common VGP patterns	Details
1	Debris flow	A refer to Fig. 2	Some colluvial tin placers offshore southern Thailand originated from debris flows (Aleva, 1979). Grade declines downward into that of the unwashed sediment, but the VGP can be complicated by reworking and sediment removal.
	Alluvial fan	G	Drowned fans may exhibit an overall upward decline of grade (e.g. Fig. 8B). Grade may be economic.
	Modern beach	A, B, C, E	Cassiterite may be concentrated over true and false bedrock. Vertical grade constancy occurs in younger Indonesian beaches (Sundrijo, 1984). Concentration is common as a thin, exploitable surface veneer (e.g. Fig. 8C).
	Delta	C, G	Cassiterite occurs in economically viable palaeo deltaic sequences of sand and gravel in Malaysia and Indonesia (Sundrijo, 1984)
	Remnant lag	B	Created in the nearshore, especially by tidal scour (Kudrass and Schlüter, 1994)
	Residual		Very thin, high grade concentrations (e.g. Indonesian kulit or skim), formed over seabed outcrops of granite, are exploited. Grade may decline downward into decomposed bedrock.
	Bar, spit	B	Ephemeral deposits exist in the littoral zone
2	Mobile lag		Residual deposits migrated from interfluves into valley bottoms (Aleva, 1985).
	Dispersion	B	Finer cassiterite settled in bays and was dispersed into the offshore where, depending on grain size, it usually penetrated no more than 1 m into the substrate (Yim, 2000; e.g. Fig. 8D).
	Widespread accumulation	C, G	Continued erosion, supply of cassiterite, fluvial transport and downstream sedimentation created cassiterite-rich units, interbedded with low grade sediments (e.g. Indonesian miencan or covered; Aleva, 1985; e.g. Fig. 8E).
4	Channel	A	Cassiterite-bearing channels were formed in the nearshore and offshore in both South-East Asia and Cornwall (Camm, 1999)
	Submerged beach		Submerged beaches and terraces were formed, e.g. at 4 to 34 m below msl, Malacca, West Malaysia (Batchelor, 1983)
5	Stranded beach		Raised beaches have been mined

The offshore and coastal tin deposits of southern Thailand have been extensively explored and mined. From an initial venture in 1908, offshore dredging achieved its highest production in the 1960s and 1970s. Indonesian offshore dredging commenced in 1938 and continues as a major industry by PT Tambang Timah in water depths of up to 30 m. Future production increasingly will come from the nearshore and offshore. The mining limit will be determined by water depth, overburden thickness and pay section grade. Some exploration, targeting submerged channels and dispersion placers, has been conducted around the coast of Cornwall, England.

Diamond placer vertical grade profile

The Namibian and South African west coast is a unique placer diamond field (Bluck et al., 2005). It provides the model cross-section (Fig. 9). Fluvially transported diamonds from numerous, eroded kimberlitic sources in the hinterland entered the South Atlantic Ocean via several rivers (De Wit, 1999; Spaggiari et al., 2006). Some important conditions led to diamonds then being concentrated along the coastline of the two countries. Long period, high amplitude swells on the low gradient shelf combined with very high wind velocities on the exposed, micro-tidal coastline. These conditions created long shore currents which dispersed sediment particles and diamonds northward from the Orange River delta. Placers extend for over 200 km along the Namibian coastline (Garnett, 2000b).

Diamonds were deposited in the littoral zone on proximal barrier beaches, spits, wash-over deltas and more distal linear beaches (Oosterveld, 2003). They also settled in agitated, coarse gravels and were trapped by irregular bedrock surfaces. Finer diamonds were concentrated in more distant embayments and pocket beaches. Thin layers, now drowned, of mobile, diamond-bearing gravels formed over wide areas of exposed bedrock. Diamonds and sand were transported by wind off beaches and into onshore aeolian transport channels and salt pans which were subjected to desert deflation. Many transgressive and regressive events along the entire coastline caused repeated reworking and redistribution of all the placer types. They became, and remain, exposed, stranded and submerged over a vertical range exceeding 200 m. In places on the inner shelf the placers are buried beneath up to 14 m of mud. All five placer groups are represented to varying degrees of commercial importance (Table 4).

Table 4

Common VGP patterns displayed by diamond placers

Group	Placer type	Common VGP patterns	Details
1	Beach	A refer to Fig. 2	Modern day beach and platform are mined into the surf zone where thin, diamond-bearing, gravel units lie under sand.
	Delta		Remnants of a delta remain at the Orange River outflow.
	Bar and spit	A, B	Present in the littoral zone.
	Trap site	A	Traps exist throughout the modern and palaeonearshore zones.
2	Mobile lag	A	Resulted from reworking of fluvial and deltaic sediments at dominant stillstand elevations (e.g. Fig. 9A). Most important is at 120 m water depth, from immediately south, to ∼300 km north, of the Orange River.
	Dispersion		Diamonds were dispersed northward, exponentially declining in size (expressed as carats/rough stone) with increased distance.
3	Submerged beach	A	Several, recorded down to ∼70 m below msl, have been mined (e.g. Fig. 9B).
	Submerged terrace	A	Several recorded down to ∼120 m below msl.
4	Channel	A, C	Fluvial channels were reworked at all water depths to 120 m (e.g. Fig. 9C).
	Aeolian		Drowned salt pans and deflation surfaces, evidenced by ventifacts found more than 100 m below msl.
5	Stranded beach	A, G	Several, raised, storm generated beaches have been mined over long distances on the Namibian coastline up to 10 m above msl. Diamonds are concentrated on, and within crevices in, highly irregular bedrock.
	Terrace	A	Exist up to 90 m above msl.

Representative VGPs are not easily obtained. Many of the units are very thin (y1 m) and the mineral (diamond) concentrations are the lowest of any placer. Some sampling systems may not recover all the diamonds off competent, irregular bedrock (Fig. 9C). Reliable data result from recording the number of diamonds recovered in large volume bulk samples or during production at different water depths (Fig. 9A).

Diamonds were first discovered on the Namibian coast in 1908. Raised beaches have been exploited ever since to yield over 100 million carats of gem quality stones. Mining of the nearshore placers was underway in the 1960s. In 1989 De Beers Marine commenced mining in the offshore at ∼120 m water depths. Production by Namdeb Diamond Corporation (Pty) Ltd has continued at that depth in the Namibian Atlantic 1 lease opposite the Orange River mouth and elsewhere. Other companies recover diamonds from shallower waters, including the surf zone, along the coastline of both countries. Annual, Namibian offshore production surpassed that from onshore in 2001, and by 2012 had reached 1·2 million carats per year.

Heavy mineral placer vertical grade profile

Placer fields exploited for heavy minerals (HM) are found in Australia (Roy et al., 2000; Hou et al., 2003), India and the eastern USA. Together, they form the model cross-section (Fig. 10). Numerous, further mineral sands are productive worldwide. Formation of a mineral sands deposit has required prolonged weathering of HM source rocks and supply, with a low clastic accompaniment, to micro- and meso-tidal environments.

Heavy minerals accumulated in the nearshore in water depths of less than 30 m. Shoreline configuration, seabed bathymetry, and wave climate were important factors in determining the prevailing energy levels and where different placer types were formed. All deposits were modified during major storms. Monsoons continue to be a very effective HM resource generator, by moving grains and sediments towards a beach for winnowing. Very fine HM grains, together with light HMs, were dispersed by currents over tens to hundreds of kilometres and circulated in offshore dispersion zones. Repeated swash action and storms concentrated HMs on beaches established on bedrock or sand. Onshore winds drove HMs into large transgressive dunes behind the beaches. Transgressive migration caused shelf sand, already containing concentrated HMs, to be transported shoreward. Very extensive raised beaches remain as strandlines, parallel to the shoreline. They now exist in places a few hundreds of kilometres inland as multiple lenses with a combined thickness of several metres.

Only three placer groups are well represented to varying degrees of commercial importance (Table 5). Placers overtaken by a transgressive sea generally are reworked to destruction and do not remain as two groups of submerged placers. Strandlines and dunes are presently the source of most HM production. Nearshore deposits are increasingly important.

Table 5

Common VGP patterns displayed by heavy minerals

Group	Placer type	Common VGP patterns	Details
1	Beach	A, B, E, G refer to Fig. 2	High grade laminae and strata are common at the surface (Fig. 10F). Grade is sensitive to changes in sediment lithology, decreasing in coarser sand and slimes, and increasing with finer grain size.
	Dune	B, C, E, G	HM grade gradually decreases with depth below ground surface, but may be enriched at the base and at intermediate elevations (e.g. Fig. 10C).
	Delta	B	HMs accumulated in areas of sandy beach accretion (in the uppermost 1 m) between promontories and on the gently sloping, submerged prodelta.
2	Barrier islands	B	Winnowing along the flanks of submarine topographic ridges concentrated HMs on the fine sand surface. Maximum concentrations may occur as shore-parallel bodies around the fair weather depth of closure.
	Dispersion	E, C	Distal concentrations may infiltrate the substrate for a few metres, grade increasing with depth (Fig. 10E). Sheeted, widespread accumulations, and up to 15 m thick, comprise very fine HM grains in marine sand and silt. Some may be partly mobile and display varying thickness (Fig. 10D).
5	Raised beach	A	Chromite-rich beaches exhibit basal grade enrichment (Drew and Lessard, 2009).
	Strandline	A	HM grades are reported (Baxter, 1976) to increase with depth. The highest grades, exceeding 60 wt-% HMs, are confined to the basal, clay-rich section of each strand (Fig. 10B), whereas the upper sandy fraction usually carries only a few per cent HMs (Welch et al., 1975). Exceptions occur (Fig. 10A). Where multiple, stacked strata exist the uppermost may be the richest.

Some common placer types exhibit the same basic VGP patterns, regardless of the mineral involved (Table 6).

Table 6

Examples of the uniformity of VGP patterns displayed by some placer types

Placer type	VGP pattern (Refer to Fig. 2)	Examples
Channel	Upward declining (A)	Gold (Fig. 7E), cassiterite (Fig. 8E), diamonds (Fig. 9C)
Beach (upper conc.)	Downward declining (B)	Cassiterite (Fig. 8D), HMs (Fig. 10A and F)
Beach (basal conc.)	Upward declining (A)	Gold (Fig. 7F), cassiterite (Fig. 8F)
Dispersion	Downward declining (B)	Gold (Fig. 7D), cassiterite (Fig. 8C), HMs (Fig. 10E)
Mobile lag	Complex	Diamonds (Fig. 9D), HMs (Fig. 10D)

Complex vertical grade profile

A common feature of many complex VGPs is the presence of three distinct mineral grade maxima (Fig. 11). Each maximum follows a unit resulting from the lag process (downward declining grade) and precedes an accumulation unit (upward declining). The author has observed similar, triple grade maxima in major onshore placer deposits worldwide. They include the gold placers of Yuba, California, which for many years were not drilled fully to bedrock. In deeper areas only the uppermost maxima was sampled. Many placers, likewise, historically have not been drilled to the depth necessary to ensure that all the mineralised units in a potential pay section have been sampled. Too often drilling depth has been limited to the excavational reach of the available mining equipment.

Complex VGPs displaying three stages of grade decline or grade maxima. a exploited, deltaic tin placer, Labohan Degong, Selangor State, West Malaysia, showing three stages of upward grade decline (units 1, 3 and 5) with each followed by more rapid decline (units 2, 4 and 6), during an overall rise in sea level. Based on results of widespread drilling; b tin placer in the coastal area of Ringarooma Bay, Tas. An interpretation based on cross-sections (Anon., 1995); c HM mineralisation in the Rivière Natashquan delta, northern bank of the St Lawrence River, Canada. Based on drilling results (Potvin, pers. comm., 1993). Confidentiality prevents disclosure of the HM grades

Application of vertical grade profiles

Few placers comprise a single unit throughout their lateral extent. Outliers can be an exception. The preparation of VGPs reveals whether or not more than one unit comprises the pay section. Cross-sections showing the VGP at each sample site should be prepared, following the example of Abzalov et al. (2011). If more than one unit exists an iso-thickness plan and semi-variogram should be constructed for each. Kriging should not be applied to the entire pay section if it comprises two or more very different units. The volume and average grade should be estimated separately for individual units within the pay section.

Sampling problems can be encountered in some environments. Grades and thicknesses of lag gravels, for example, may vary considerably over 1 or 2 m laterally, resulting in a nugget effect (Saravanakumar et al., 2014). In contrast, an underlying accumulation unit may possess much greater, lateral grade uniformity. A compromise, always influenced by cost, is then required between two different, ideal sample spacings. Diamond exploration possesses its own unique challenges. Different average grain sizes may also suggest that sample support might be tailored for the unit containing the coarser grains and/or lowest grade. Other characteristics such as clay content and sediment particle size frequency distribution determine sampling practice and affect metallurgical recovery. These parameters vary from one unit to another.

Confidence in an interpreted VGP can be improved by plotting the corresponding chart of a more abundant accessory mineral. Coincidence of grade maxima elevations and grade decline directions may validate the VGP of the target mineral (Fig. 12). The accessory mineral can then serve as a proxy in reconnaissance exploration. For example, magnetite and ilmenite may act as proxies for each other or for chromite (Fig. 12A). The VGP of imitating HMs or fine gold may provide supporting evidence of uncertain diamond concentrations. Black sands and magnetite are useful proxies for some gold placers. However, outside certain grain size ranges, the grades of proxy and target minerals may vary inversely. Varying grain size and thus hydraulic equivalence, can give misleading results (Fig. 12B). Changing mineral supplies or energy levels may yield conflicting data (Fig. 12C).

VGPs illustrating similar and dissimilar direction and rate of decline of co-existing minerals. a magnetite, ilmenite and non-magnetics imitating the downward grade decline, but failing to imitate the more rapid rate of decline, of chromite in an Indian beach placer. Based on published data (Gujar et al., 2010); b total absence of any sympathetic relationship between tin and titanium, Ringarooma Bay Australia. Modified from Davies (2009); c zircon imitating two of three titanium grade maxima, Ringarooma Bay, Australia. Modified from Davies (2009)

Placers should be sampled with an appropriate vertical interval which is equal to, or less than, the vertical selectivity of the intended mining tool. Otherwise, mine planning cannot prevent unwanted and unquantifiable vertical dilution or resource loss during mining. Operational flexibility is absent if changing economics dictate that a new cut-off grade is required. However, there is little evidence that the VGP is awarded the importance it deserves in evaluation, resource estimation and mine planning of some placers. Very few published technical reports authored by Qualified Persons provide more than a single grade for the full vertical extent of the pay section at a sample site. Meanwhile, interest is increasing in the exploitation of placers in the nearshore and offshore zones (Garnett, 2011). Plotting and interpretation of VGPs deserve to become a standard procedure in all stages from exploration, through resource estimation, to mining planning.

Conclusions

The pattern displayed by the VGP of a placer reveals the number of discrete units comprising the pay section. It shows the extent to which alternating processes of predominantly accumulation (upward grade decline) and lag concentration (downward grade decline) contributed to the development of a deposit. Reversals in decline direction represent scour horizons or stillstands. The history of a placer is fully displayed unless the pattern is modified or partly erased by reworking. A log₁₀ line chart aids in interpretation. Parallelism of some lines on such a chart is common but does not indicate equal rates of grade change.

The pattern of a VGP may depend on whether the total mineral content, or only a particular size fraction, is reported. The mineral grain size may display a direct or inverse relationship with sediment particle size. Similarly, the relationship between mineral grade and grain size may be both direct and inverse within different units of the same placer. Sudden grade changes may coincide in elevation with lithologic boundaries. A suite of heavy minerals of different density and range of grain size invariably exhibit very different directions and rates of decline. Certain VGP patterns characterise specific placer types regardless of the mineral involved.

Preparation of the VGPs of a placer, preferably at an early stage in exploration, reveals the number of units comprising the pay section and any overburden. Few placers comprise a single unit throughout their lateral extent. For resource estimation they should not be treated mathematically on the basis of such an assumption. The preparation of a semi-variogram for each unit will guide the approach to using geostatistical techniques. It will also indicate the extent to which sample spacing may require a compromise between the demands of separate stacked units. Ideally, the vertical sampling interval should not exceed the intended mining selectivity, especially if the grades are marginally economic.

Economic interest is increasing in the exploitation of placers in the nearshore and offshore zones. Plotting and interpretation of the VGPs presently appears to be neglected. It deserves to become a standard procedure in all stages from exploration.

Footnotes

Acknowledgements

The author has drawn from his operational experience on placer exploration and mining programmes, mostly for tin, gold and diamonds. His intermittent association with placers resulted from his employment by Associated Mines (Malaya) Ltd, ConZinc RioTinto Sdn Bhd, Charter Consolidated Ltd, Anglo American Corporation of South Africa Ltd, and Hudson Bay Mining and Smelting Co, Ltd. His later roles as a director of Diamond Fields Resources Ltd, Diamond Fields International Ltd, and Minera Tierras Australes, plus retention as a consultant by several corporate clients, allowed additional involvement with placers in the marine environment. He gratefully acknowledges the extent to which all the companies have allowed him to expand his practical knowledge of placers. The text has benefitted enormously from the advice of referees.

References

Abzalov

, Domouchel

, Bourque

, Hees

and Ware

. 2011. Drilling techniques for resource estimation of mineral sand deposits, in 8th Int. Heavy Miner. Conf., 27–40, Perth, The Australasian Institute of Mining and Metallurgy.

Aleva

GJJ

. 1973. Aspects of the historical and physical geology of the Sunda Shelf essential to the exploration of submarine tin placers, Geol. Mijnb., 52, (2), 79–91.

Aleva

GJJ

. 1979. Exploration for placer tin deposits offshore Thailand, Proc. 11th Commonwealth Min. Metall. Cong., (ed. Jones

M J

), 59–65; Hong Kong, Institution of Mining and Metallurgy.

Aleva

GJJ

. 1985. Indonesian fluvial cassiterite placers and their genetic environment, J. Geol. Soc. Lond., 142, 815–836.

Anon. 1995. Zephyr Minerals N.L., Perth, WA, Australia, Personal communication.

Batchelor

. 1979. Geological characteristics of certain coastal and offshore placers as essential guides for tin exploration in Sundaland, Southeast Asia, Geol. Soc. Malays., Bulletin 11, 283–313.

Batchelor

. 1983. Sundaland tin placer genesis and late cainozoic coastal and offshore stratigraphy in Western Malaysia and Indonesia, PhD thesis, University of Malaya, Malaysia.

Baxter

. 1976. Heavy mineral sand deposits of Western Australia, Geol. Survey of Western Australia, Mineral Resources Bulletin 10.

Berquist

and Hobbs

. 1988. Reconnaissance of economic heavy minerals of the Virginia inner continental shelf, Virginia Division of Mineral Resources Open-File Report 88-1.

10.

Bluck

, Ward

and De Wit

MCJ

. 2005. Diamond mega-placers: southern Africa and the Kaapvaal craton in a global context, Geological Society, London, Special Publications, 248, (1), 213–245.

11.

Broadhurst JR and Batzer

. 1965. Valuation of alluvial tin deposits in Malaya, with special reference to exploitation by dredging in Open-cast mining, quarrying and alluvial mining, 97–113, London, Institution of Mining and Metallurgy, Elsevier Publishing Company.

12.

Camm

. 1999. Review of Cornish stanniferous placers – with special reference to offshore placers of Quaternary age on the south coast, Trans. R. Geol. Soc. Cornwall, XXII, (2), 57–88.

13.

Clark

and Garnett

RHT

. 1974. Identification of multiple mineralization phases by statistical methods, Min. Ind. (Trans. Inst. Min. Metall. A), 83, A43–A52.

14.

Collier

, Hess

, Smith

and Brooks

. 1908. The gold placers of parts of Seward Peninsula, Alaska, including the Nome, Council, Kougarok, Port Clarence, and Goodhope precincts, US Geological Survey Bulletin 328, US Geological Survey.

15.

Davies

. 2009. T2-MRL – Ringarooma Bay Annual Report, Period ending 31 July, 2009, Van Dieman Mines Pty Ltd, Tas., Australia.

16.

De Wit

MCJ

. 1999. Post-Gondwana drainage and the development of diamond placers in western South Africa, Econ. Geol., 94, (5), 721–740.

17.

Drew

and Lessard

. 2009. Developing a new process design based on the unique geological characteristics of an industrial mineral bearing placer system in south-western Oregon, USA, 7th Int. Heavy Miner. Conf. ‘What next’, 115–122, Champagne Sports Resort, central Drakensberg, The Southern African Institute of Mining and Metallurgy.

18.

Garnett

RHT

. 2000a. Marine placer gold, with particular reference to Nome, Alaska, in Handbook of marine mineral deposits, (ed. Cronan

D S

), 67–101, Boca Raton, FL, CRC Press LLC.

19.

Garnett

RHT

. 2000b. Marine placer diamonds, with particular reference to Southern Africa, in Handbook of marine mineral deposits, (ed. Cronan

D S

), 103–141, Boca Raton, FJ, CRC Press LLC.

20.

Garnett

RHT

. 2011. The new wave, Min. J., 4 March, 17–18.

21.

Garnett

RHT

and Bassett

. 2005. Placer deposits, in Economic Geology 100th Anniversary Volume, 813–843, Littleton, MA, Society of Economic Geologists.

22.

Goldfarb

. 2007. Dynamic classification of alluvial gold placers in the northeast of Russia, Geol. Ore Depos., 49, (4), 241–270.

23.

Gujar

, Ambre

, Mislankar

and Iyer

. 2010. Ilmenite, magnetite and chromite beach placers from South Maharashtra, central west coast of India, Resour. Geol., 60 (1), 71–86.

24.

Hou

, Frakes

, Alley

and Heithersay

. 2003. Evolution of beach placer shorelines and heavy mineral deposition in the astern Eucla Basin, South Australia, Aust. J. Earth Sci., 50, 955–965.

25.

Hunter

. 1971. Heavy mineral analysis of selected Monterey Bay cores, Master's dissertation, United States Naval Postgraduate School, Monterey, CA, USA.

26.

Komar

. 2007. The entrainment, transport and sorting of heavy minerals by waves and currents, in Heavy minerals in use, (ed. Mange

M A

and Wright

D T

), Vol. 58, Developments in sedimentology, 3–48, Amsterdam, Elsevier B.V.

27.

Komar

and Wang

. 1984. Processes of selective grain transport and the formation of placers on beaches, J. Geol., 92, 637–655.

28.

Kudrass

. 1987. Sedimentary models to estimate the heavy-mineral potential of shelf sediments, in Marine minerals: advances in research and resource assessment, (ed. Teleki

P G

, Dobson

M R

, Moore

J R

and von Stackelberg

), 39–56, NATO ASI Series C: Mathematical and Physical Sciences, Vol. 194, Dordrecht, Reidel Publishing Company.

29.

Kudrass

. 2000. Marine placer deposits and sea-level changes, in Handbook of marine mineral deposits, (ed. Cronan

D S

), 3–12, Boca Raton, FL, CRC Press LLC.

30.

Kudrass

and Schlüter

. 1994. Development of cassiterite-bearing sediments and their relation to late Pleistocene sea-level changes in the Straits of Malacca, Mar. Geol., 120, (3–4), 175–202.

31.

Latin Resources Limited, Press release, 30 May 2012.

32.

Lee

. 1968. Prospecting for tin in the sands of St. Ives Bay, Cornwall, Min. Ind., (Trans. Inst. Min. Metall. A), 77, A49–A64.

33.

Lew

and Corner

. 1990. New Zealand offshore gold exploration – the Harvester Project, in Mineral Industry in New Zealand, Proc. 1990 Annu. Conf. Australas. Inst. Min. Metall., 275–280, Rotarua, Australasian Institute of Mining and Metallurgy.

34.

Lintern

. 2005. Zuleika Sands gold deposit, Ora Banda, WA, in Regolith expression of Australian ore systems (ed. Butt

C R M

, Robertson

I D M

, Cornelius

and Scott

K M

), 354–356, Perth, CRC LEME.

35.

MacDonald

and Rozendaal

. 1995. The Geelwal Karoo heavy mineral deposit: a modern day beach placer, J. Afr. Earth Sci., 21, (1), 187–200.

36.

Nelson

and Hopkins

. 1972. Sedimentary processes and distribution of particulate gold in the northern Bering Sea, US Geological Survey Professional Paper 689.

37.

Nicholls

, Larson

, Capobianco

and Birkemeier

. 2001. Depth of closure: improving understanding and prediction, Coast. Eng. Proc., [S.I], (26), January.

38.

Oosterveld

. 2003. Evaluation of alluvial diamond deposits, in Workshop: alluvial diamonds in South Africa, Directorate of Professional Programmes of the Geological Society of South Africa, 1–7, Johannesburg, Geological Society of South Africa.

39.

Patyk-Kara

, Bardeeva

and Shevelev

. 1999. Titano-zirconium placers in the sedimentary cover of platforms, Committee for the co-ordination of joint prospecting for mineral resources in Asian offshore areas (CCOP), 22, (2), 89–98.

40.

Ringis

. 1983. Stratigraphy of the unconsolidated sediments off Lumut, Malaysia, in relation to the occurrence of detrital cassiterite, in Exploration for offshore placers and construction materials, 100–114, Committee for the Co-ordination of Joint Prospecting for Mineral Resources in Asian Offshore Areas (CCOP) Project Office. United Nations Development Programme (UNDP) Technical Support for Regional Offshore Prospecting in East Asia. Report RAS/81/120, CCOP/TP 12.

41.

Roy

. 1999. Heavy mineral beach placers in southeastern Australia: their nature and genesis, Econ. Geol., 94, (4), 567–588.

42.

Roy

, Whitehouse

, Cowell

and Oakes

. 2000. Mineral sands occurrences in the Murray Basin, southeastern Australia, Econ. Geol., 95, (5), 1107–1128.

43.

Saravanakumar

, Brown

and van Eck

. 2014. Sample support size and spacing determination for resource development of a marine placer gold deposit, J. S. Afr. Inst. Min. Metall., 114, (1), 7–17.

44.

Shepherd

. 1990. Eneabba heavy mineral sand placers, in Geology of the mineral deposits of Australia and Papua New Guinea, (ed. Hughes

F E

), 1591–1594, Melbourne, The Australasian Institute of Mining and Metallurgy.

45.

Slingerland

. 1977. The effects of entrainment on the hydraulic equivalence relationships of light and heavy minerals in sands, J. Sediment. Petrol., 47, (2), 753–770.

46.

Spaggiari

, Bluck

and Ward

. 2006. Characteristics of diamondiferous Plio-Pleistocene littoral deposits within the palaeo-Orange River mouth, Namibia, Ore Geol. Rev., 28, 475–492.

47.

Sundrijo

. 1984. Some essential aspects of the geological characters of various secondary tin deposits – a guide for mining operations, Proc. 4th SEATRAD Semin., 455–486, Technical Publication No. 4, Ipoh, Southeast Asia Tin Research and Development (SEATRAD) Centre.

48.

Van Overeem

AJA

. 1960. Geological control of dredging operation on placer deposits, Billiton, Indonesia, Special One Hundredth Year Volume, Geol. Mijnb., 39, 458–463.

49.

Vorob'yev

. 1979. Mineral distribution in a gold beach placer, Int. Geol. Rev., 21, 72–78.

50.

Vorob'yev

and Krapivner

. 1975. Preliminary classification of coastal-marine placers, Int. Geol. Rev., 17, (7), 823–829.

51.

Welch

, Sofoulis

and Fitzgerald

ACF

. 1975. Mineral sand deposits of the Capel area Western Australia, in Economic geology of Papua and New Guinea, Monograph series No. 5, (ed. Knight

C L

), Vol. 1, 1071–1088, Australasian Institute of Mining and Metallurgy, Melbourne.

52.

Yim

WW-S

. 1974. Some aspects of the geochemistry of tin and other elements in sediments off north Cornwall, PhD thesis, University of London, UK.

53.

Yim

WW-S

. 2000. Tin placer deposits on continental shelves, in Handbook of marine mineral deposits, (ed. Cronan

D S

), 27–66, Boca Raton, FL, CRC Press.