Volume Two
May 2014
Appendix 3
Compilation of literature on the Chatham Rise
phosphorite deposit (Boskalis 2013a)
Boskalis
Offshore bv
Chatham Rock Phosphate
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Compilation of literature on the Chatham Rise
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FINAL
phosphorite deposit
Compilation of literature on the
Chatham Rise phosphorite deposit
Final Report
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Table of Contents
1
Introduction .................................................................................................................................................... 3
2
Occurrence and distribution of phosphorite deposits ...................................................................................... 4
3
Genesis of Chatham Rise phosphorite deposit .............................................................................................. 7
4
Age of Chatham Rise phosphorite deposits ................................................................................................... 9
5
Characteristics of phosphate nodules .......................................................................................................... 10
6
Characteristics of matrix material ................................................................................................................. 13
7
Characteristics of substrate .......................................................................................................................... 14
8
Geotechnical parameters ............................................................................................................................. 15
8.1
Grain size distribution ........................................................................................................................ 15
8.2
Density and moisture content............................................................................................................. 16
8.3
Shear strength ................................................................................................................................... 16
8.4
Atterberg limits ................................................................................................................................... 16
9
Implications for mining.................................................................................................................................. 17
10
Total estimated reserves .............................................................................................................................. 18
11
10.1
Valdivia cruise results......................................................................................................................... 18
10.2
Sonne cruise results .......................................................................................................................... 18
References ................................................................................................................................................... 19
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1
Introduction
This document comprises a compilation of publically available literature on the phosphorite deposit of the central
Chatham Rise. This document is subdivided into chapters on the occurrence and distribution of phosphorite
deposits, the genesis of the Chatham Rise phosphorite deposit, mechanisms of phosphatisation, age of the
deposits, characteristics of phosphate nodules, the matrix material and the substrate material, geotechnical
characteristics of the various materials, implications for mining, and estimated reserves.
Phosphorites on the sea floor generally occur on continental margins in low- to mid-latitude areas, at depths of
less than 500 m. Besides continental shelf and slope deposits, which account for the bulk of ocean-floor
phosphate, deposits also occur on topographic highs such as seamounts, rises, and ridges.
Some of the
offshore occurrences immediately adjacent to known land deposits for example, Florida and Baja California, may
be submarine extensions of the onshore deposits. Other deposits, of which Chatham Rise is an example,
probably formed independently of any onshore deposits (Burnett, 1980).
Figure 1 Image of the Chatham Rise (from Wright (2009)).
The Chatham Rise is a broad submarine platform about 130 km wide and 960 km long, which extends eastward
from Banks Peninsula on the east coast of the South Island of New Zealand to slightly beyond the Chatham
Islands (see Figure 1) (Burnett, 1980).
Thin, unconsolidated, surficial deposits of nodular phosphorite - a
potential substitute for imported rock phosphate - occur in water depths of approximately 400 m along some 400
km of the crest of the Chatham Rise. The locus of the area is between 179°08’ E and 179°42’ E.
The
phosphorite deposits are patchy, and are predominantly black, glauconite-coated nodules about 20-40 mm in
diameter. The phosphorite nodules consist of indurated Lower and Middle Miocene (and possibly Oligocene)
chalky limestones, phosphatised late in Miocene times (Wesley Karns (1974); Cullen (1987); Hughes-Allen
(2011)).
The Chatham Rise phosphorite deposit has been estimated at 30 million tonnes of phosphorite, averaging 9.4%
2
2
P (21.5% P2O5) and concentrated at 66 kg/m over an area of 378 km , in between the longitudes mentioned
above (Hughes-Allen, 2011). The total resource in this region is provisionally estimated at 100 million tonnes.
One of the primary reasons for the increase in commercial interest in the Chatham Rise deposits in the 1970’s
and 1980’s was that the Chatham Rise phosphate does not need to be converted to superphosphate to be an
effective fertiliser (Falconer, 1989).
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2
Occurrence and distribution of phosphorite deposits
The average depth of the Chatham Rise platform is approximately 400 m below sea level. The Rise is terminated
abruptly on the north side where the seafloor descends steeply into the Hikurangi Trench; the southern and
eastern slopes are more gentle. The bottom topography is characterized by smooth, gently sloping profiles, with
occasional jagged profiles of up to 10 m of relief. Several shallow banks that shoal up to depths of 51 m below
sea level are present on the Chatham Rise. Channels and ridges, which would suggest a period of subaerial
erosion, have been reported as well (Wesley Karns, 1974).
The existence of nodular phosphorite on the Chatham Rise seafloor has been known since 1952, when New
Zealand Geological Survey scientists identified and described material obtained incidentally in a single dredge
haul by RRS Discovery II from a locality some 80 miles west of Chatham Island. The nodules recovered from this
survey were described as highly concentrated Globigerina ooze, and contained glauconite and some quartz and
schist fragments. The degree of phosphatisation varied in the different nodules examined, in some it was
complete, but most nodules had large unaltered cores. The main phosphate mineral was collophane. At the
time, no commercial interest was shown in the deposits, despite the importance of phosphorus to New Zealand's
agricultural industry. Not only was offshore-mining technology at a relatively rudimentary stage, but New
Zealand's supply of rock phosphate from traditional island sources seemed secure for several decades. Nodular
phosphorite has also been recorded as patches along some 400 km of the E-W crest of Chatham Rise, between
its intersections with longitudes 177°E and 177°W. In 1967 and 1968, Global Marine Incorporated carried out
extensive sampling of the whole Chatham Rise and found that the densest deposits were between 179°E and
180° (see Figure 3) (Wesley Karns (1974); Cullen (1987); Falconer (1989)).
In this region, the phosphorite deposits lie in water depths of 375-425 m. Smaller patches of phosphorite occur to
the east and north-east of Matheson Bank, but require further qualitative and quantitative assessment. The
bottom morphology in these areas is gently undulating, with slopes rarely exceeding 3° (Cullen, 1987).
Figure 2 General distribution of phosphorite on Chatham Rise (from Cullen (1987)).
Across the Rise, phosphorite occurs as a surficial or subsurface uncemented gravel of hard subangular to
subrounded nodules, enclosed in a matrix of completely unconsolidated, glauconitic sandy mud that overlies
Oligocene ooze/chalk. A fine sediment layer, up to 1 m thick and containing sparse granules of phosphorite,
overlies the nodule-rich layer. Frequent exposure of the nodule-bearing layer at the seafloor indicates very low
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sedimentation rates and gentle current winnowing. There is little evidence of sorting by ocean currents. Where
present on the Chatham Rise, the nodule layer ranges in thickness from a few centimetres to a known maximum
of 0.7 m (70 cm). The typical thickness over much of central Chatham Rise is in the order of 0.2-0.3 m (Falconer
(1989); Cullen (1987); Hughes-Allen (2011)).
The nodules vary in size from granules to cobbles (i.e. ~2 mm to greater than 150 mm), and, although there is
little evidence of sorting by currents, nodule size frequency tends to peak in the 10-40 mm range (see Figure 3).
The nodules are composed of indurated and phosphatised, light coloured pelagic oozes, dated by enclosed
foraminiferans as Lower and Middle Miocene. Upper Eocene-Oligocene (mainly Lower Oligocene) foraminiferans
and nannofossils that occur in nodules from some parts of central Chatham Rise are now considered to have
been derived, by erosion, from Paleogene chalky limestones exposed locally on the seafloor during Miocene
times. However, the converse could equally be true, i.e. that younger microfossils were introduced into burrows
in exposed Paleogene chalk during pre-phosphatisation Miocene bioturbation (Cullen, 1987).
Figure 3 Phosphate nodules on the seafloor (left) and separate nodules in detail (right) (from Wright (2009)).
Based on 960 grab samples from the Valdivia and Sonne cruises of 1978 and 1981, the Chatham Rise
phosphorite deposit has been estimated at 30 million tonnes of phosphorite averaging 9.4% P (21.5% P 2O5) and
2
2
with nodule frequencies averaging 63-66 kg/m over an area of 378 km , in between the meridians mentioned
above (see Figure 4). In terms of local supply and demand, this reserve represents could be capable of
sustaining a mining venture, at an economic recovery rate, for 30-60 years. Extrapolation to cover the entire
phosphorite distribution on the crest of central Chatham Rise provides a provisional total in the order of 100
million tonnes of phosphorite between longitudes 179°E and 180° (Hughes-Allen (2011); Cullen (1987)).
Correlation between detailed seismic surveys and numerous bottom samples has allowed ten seismic facies to
be mapped on the rise.
High concentrations of phosphorite occur on only one of these facies, which is
characterized by locally rough seabed, whereas the adjacent facies have a much smoother seabed. The
2
2
concentration averages 66 kg/m in the rich areas and <12 kg/m in other areas. However, even in the rich areas,
2
there are variations between 0-350 kg/m over distances of 100 m or less (Falconer, 1989). The patchy
distribution of nodules on the Chatham Rise is believed to reflect the irregular development of an original
phosphatic ‘duricrust’ wherever the calcareous precursor was exposed at the seafloor with some modification
from original locus of deposition by iceberg scouring during low stands of sea-level (Hughes-Allen, 2011).
For mineable reserves, the vertical distribution is important as it may present problems regarding extraction. The
limiting of yield with increasing depth is due to the fact that all the phosphorite is in the sand overlying the chalk,
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and the sand is not very thick. In four areas of the Sonne cruise, at least 50% of the phosphorite is present
within the upper 25 cm, and 74-94% is present within the upper 40 cm. However, the phosphorite is often
concentrated towards the base of the sand. For example, in Area 2, 40-50% of the phosphorite is within 10 cm
of the chalk/ooze (Falconer, 1989).
Figure 4 Distribution of phosphorite. Light stipple denotes 50-100 kg/m2 phosphorite, heavy stipple denotes 100-150 kg/m2
phosphorite, and solid black denotes 150-350 kg/m2 phosphorite (from Cullen (1987)).
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3
Genesis of Chatham Rise phosphorite deposit
Phosphorite deposits are known to occur in ocean basins around the globe. Several of these deposits, including
the Chatham Rise deposit, have been estimated as being between 5 and 23 million years old (Hughes-Allen,
2011). Although a study in 1950 concluded that the nodules had formed essentially in place by inorganic
precipitation from the Miocene to the present, a later study using uranium series age dating discounted the
possibility of modern formation. Presently, these deposits, like most ocean-floor phosphorites, are believed to be
relict. Paleontological dating generally indicates a Miocene age for these deposits. Two possible scenarios of
phosphorite genesis are:
1) Miocene chalk, formed in the Late Miocene, could have been replaced by direct uptake from phosphorus
dissolved in overflowing water masses, or
2) replacement may also have been caused by phosphorus released in the organic-rich muds of an upwelling
area.
During the Pliocene and Pleistocene, submarine erosion has removed most of the non-phosphoritic parts of
Miocene chalks and created a karst-like topography. During this period, ice-rafted terrigenous debris and
airborne volcanic ashes were added to the surficial lag sediments. During the Pleistocene gouging, icebergs, as
indicated by straight, several-meter-deep furrows, considerably disturbed the uniform distribution of phosphorite
nodules. Consequently, the assessment of phosphorite reserves and the mining of nodules is difficult (Kudrass &
Von Rad, 1984). In Von Rad & Rösch (1984), the complex evolution of Chatham Rise during the last 40 million
years is summarized as follows (see also Figure 5):
1)
After deposition of the Late Eocene to Middle Oligocene nanno oozes and a Late Oligocene hiatus several
hundred meters of Early-Middle Miocene sediments accumulated on the crest of Chatham Rise.
2)
During the Middle to early Late Miocene times, a hardground developed, which was bored and fragmented,
forming a lag deposit of chalk pebbles. Since the Late Miocene, the morphologically high-standing blocks of
the Chatham Rise were eroded down, with phosphatised chalk pebbles being preserved as a lag gravel.
3)
The ferruginisation of the rims of chalk pebbles was produced as goethite and other Fe oxides replaced
calcite in foraminiferal tests and in the matrix; foraminiferal chambers were also filled by goethite.
4)
Later (mid-, to lower Miocene), a gradual phosphatisation of chalk pebbles and cobbles took place by
inward migration of cryptocrystalline apatite (collophane) replacing calcite, pushing the goethite front inward
and filling open pore space (in foraminifera etc.).
5)
In general, glauconitisation (about 7-10 Ma age) postdates phosphatisation. Glauconite replaced and filled
fecal pellets and foraminiferal chambers, and resulted in marginal (and inward decreasing) replacement of
the 0.5-1 mm thick nodule rim of phosphatized chalk pebbles; local glauconite-filled foraminifera are older
than the phosphatisation event. Inorganic opal-A was also precipitated in the nodule rime zone, mainly in
foraminiferal chambers (early diagenetic silification).
6)
The late Neogene was characterized by a later-stage phosphatisation, glauconitisation, and pyritisation,
especially of foraminifera, burrow fills and cracks in phosphorite nodules.
7)
The sandy fill of pockets and burrows in phosphorite nodules was cemented by young collophane and by
authigenic phillipsite.
8)
Slow deposition, reworking, and bioturbation during Pliocene to Quaternary times formed a condensed
glauconite-foraminiferal lag sediment, influenced by glacial erratics from drifting icebergs and sporadic
ashfall, which supplying rhyolitic glass from New Zealand’s’ North Island.
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Figure 5 Left: schematic illustration of probable morphological control of phosphatisation with repeated cycles of partial burial,
phosphatisation, erosion, and hardground formation; Right: Evolution of the Chatham Rise phosphorite deposit and associated
sediment (C1 and C2 are alternative models for the phosphatisation process (from Kudrass & Von Rad (1984)).
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4
Age of Chatham Rise phosphorite deposits
Radiometric dating of the phosphatisation event is not possible, as the age of the deposit is beyond the limit of
uranium-series method (700,000 year). Therefore, the age of phosphatisation has been estimated by studying
the foraminiferal age of the phosphatised chalk, by looking at the phosphatised whale bones and shark teeth
associated with the nodules, and by determining the K-Ar age of the glauconitic rim of phosphorite nodules.
According to this approach, the time of phosphatisation can be limited to a Late Miocene event (Kudrass & Von
Rad, 1984). Along most of the Chatham Rise, the phosphorite rests on a substrate of Upper Eocene-Lower
Oligocene chalky limestone. At its interface with the nodule bearing ‘superficial lag’ deposits, the topmost 15 cm
of the limestone is softened to the consistency of cream cheese. Close examination of this layer has failed to
reveal the presence of any post-Oligocene microfossils and may correlate to a Miocene erosion feature, the
‘Marshall Paraconformity’ and the glacio-eustatic lowstand of sea level. The youngest reliably dated limestone to
have been phosphatised is 12-15 Ma (Middle Miocene) (Hughes-Allen, 2011). Micropalaeontological study of the
Chatham Rise phosphorite nodules reveals the presence of both Oligocene and Miocene foraminiferans. Many, if
not all, of the Oligocene foraminiferans appear to be reworked, and were presumably derived from the Lower
Oligocene chalk that now forms the substrate of the nodule layer on central Chatham Rise. Apart from the
occurrence in a single sample of the foraminifer Sphaeroidinella seminulina, there is no evidence for the
presence on Chatham Rise of Upper Miocene sediment, or for its inclusion among the phosphorite nodules. The
final stage in the complex process of nodule formation - post-dating the boring, fracturing and phosphatisation was the deposition of the surficial glauconite coating. Potassium-argon dating of the glauconite associated with
the phosphorite has provided ages ranging between 5.7 and 11 Ma, with a pronounced prevalence of dates in
the order of 7 Ma (in 1990, this upper limit was pushed to 4.90 (±0.35) Ma) using strontium isotope
87
86
measurements ( Sr/ Sr captured from ambient pore water at the time of formation (Hughes-Allen, 2011). The
older dates may reflect some contamination by material older than the glauconite itself, while loss of radiogenic
argon could explain the younger dates.
Overall, it is apparent that the complex sequence of processes that transformed Tertiary limestone on the
Chatham Rise into spreads of phosphorite nodules occurred in the Late Miocene, and spanned an interval
between approximately 12 and 7 Ma BP. This date range coincides with dates for marine phosphorite formation
off southern Africa, north-west Africa, eastern and western North America, and at several other localities in the
Pacific and Indian Oceans (Cullen, 1987).
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Characteristics of phosphate nodules
The Chatham Rise phosphorites are buff, variably phosphorized foraminiferal chalk pebbles with a complex
compositional, textural, and colour zonation (see Figure 6 below). The core of the nodules consists of light
yellowish-brown, weakly phosphatised nanno chalk (I), surrounded by an increasingly more phosphorized chalk
zone (II). Commonly, the nodule rim consists of three zones: a dark-brown goethite zone (III) grading into a
yellowish, cellophane zone (IV), and an outermost greenish-black glauconite (-collophane) rim (V) (Von Rad &
Rösch, 1984).
Figure 6 Schematic cross-section of a typical phosphorite nodule generalized from thin-section observations and XRD
analysis. Roman numerals explained in text above (from Von Rad & Rösch (1984)).
Phosphorous is delivered to the oceans via continental weathering and fluvial transport. Phosphorus is
processed by plankton and liberated in sediments by the decay of marine organisms. The initial step in the
formation of phosphorites is the super saturation of pore water in organic matter-rich sediments at the
sediment/sea-water interface. In the case of the Chatham Rise, this super-saturation occurred where sediments
of the calcareous Oligocene to Pleistocene Penrod Group were exposed at the sea floor. Replacement of the
calcium carbonate precursor by calcium phosphorite occurs in areas of ocean upwelling and high organic
productivity (Hughes-Allen, 2011). The phosphate nodules display two distinct phases of boring and burrowing.
Vestiges of early filled burrows riddle the phosphatised limestone in most nodules, while a later (postconsolidation but pre-phosphatisation) phase of boring is reflected in the multitude of open tubular cavities that
penetrate - and determine the irregular final shapes of - the majority of nodules. These later borings, in particular,
played an important role by providing access routes for phosphatising and glauconitising solutions. One of the
more prominent features, reflecting the diagenetic origin of the phosphorite, is the crudely concentric structure
observed in many individual nodules. In all but the smallest nodules, a core of relatively soft, pale, lightly
phosphatised chalky ooze is surrounded by the main mass of darker, brown, more completely phosphatised
ooze. The brown colour progressively intensifies outward, with increasing phosphatisation and some
ferruginisation in the form of hydrated iron oxide close to the nodule rim,. The outer surfaces of nodules are
almost invariably coated by a thin (mostly <1.0 mm), continuous, glossy layer of greenish-black cryptocrystalline
glauconite which has replaced the outermost rind of phosphatised limestone. Synchronous inward migration of
the displaced phosphate has created a contiguous inner layer of secondary collophane, while a somewhat paler
green glauconite has penetrated into the nodules along ‘veins’, microfractures, and differing textural zones.
There placement of the original calcareous ooze progressed from the outside inward, and, apart from some
recent minor abrasion and chemical dissolution, emplacement of the glauconite coating represents the final
process in the formation of the nodules as seen today (Cullen, 1987).
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The nodules consist of phosphatized Lower Oligocene pelagic limestone, which became indurated, burrowed,
and fragmented before being phosphatized. Apatite and calcite are the main mineral phases of the nodules, with
glauconite and goethite occurring as minor components in the dark coating. P2O5 concentration of 63 bulk nodule
samples, containing phosphorite particles >1 mm, varied between 20.1% and 23.7%, with most nodules
containing between 20.1% and 22.5%. P2O5 concentration of the nodules ranged between 21% and 22.5%. The
highest P2O5 concentrations occurred in the 4-10 mm grain size fraction. The higher P2O5 concentrations in the
small-sized nodules suggest that the chalk was fragmented before the phosphatisation process occured. The
P2O5 concentration was reduced in smaller nodules by the glauconitic coating, and by calcite in the core of larger
nodules,. The decrease of P2O5 in the larger nodules indicates that apatite did not completely replace the calcite
in cores of the larger nodules. . As apatite is too hard to be bored by marine organisms, the burrows must have
been created before the phosphatisation. Overall, these results suggest that phosphatisation occurred after the
nodules had already attained their present size and shape. A sequence of processes thus can be established as
follows: deposition of foraminiferal ooze in the Oligocene was succeeded by consolidation, partial erosion,
exposure at the sediment-water interface, and modification by burrowing organisms. The final induration
occurred during phosphatisation and was followed by the deposition of the glauconitic coating (Kudrass & Cullen,
1982).
The chemical composition of nodules is a typical example of how the interplay of many factors will affect a mining
prospect. The smaller nodules are richer in Al, Si, and Fe, due to the large proportion of glauconite rim, as well
as external collophane and goethite zones. There is more calcite replacement in the smaller nodules, which also
have slightly higher P2O5 content than the larger nodules. There are differences in size distribution on the rise,
with the western areas having a higher percentage of small nodules than the eastern areas. The Valdivia work in
1978 was focussed on the western area and its geochemical results, compared with the Sonne 1981 data, reflect
nodule size differences with the different areas. Size is not the major factor controlling P2O5 content, but it has a
very significant effect on the other constituents. These constituents may have a major impact on any chemical
processing, and there are indications that calcite content is a factor in performance as fertilizer on pastures
(Falconer, 1989). Table 1 is a summary of elements in the Chatham Rise phosphorite deposits.
Table 1 Distribution of major elements in Chatham Rise phosphorites, based on 94 analyses (summarized in Cullen (1987)).
Component
Max. weight %
Min. weight %
Mean weight %
Standard deviation (s)
SiO2
19.19
1.15
6.72
4.96
Fe2O3
6.37
0.08
3.06
1.76
Al2O3
3.29
0.14
0.96
0.80
TiO2
0.07
<0.01
0.03
0.02
MgO
1.40
0.04
0.63
0.34
CaO
53.31
34.28
44.92
4.87
Na2O
1.57
0.69
0.98
0.18
P2O5
26.70
4.88
20.53
3.50
K2O
2.02
0.13
0.89
0.65
SO3
2.65
0.33
1.53
0.43
F
3.00
1.30
2.46
0.56
38.36
9.27
18.28
5.90
LOI*
*LOI = loss on ignition
The Chatham Rise phosphorites are mineralogically relatively simple. The phosphatic mineral is a
cryptocrystalline or collophanous carbonate-fluorapatite (francolite). Under a scanning electron microscope, the
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apatite appears as hexagonal prisms up to 7-8 μm across, or as globular collophanous aggregates of small (<2
μm) crystals that are sometimes in the process of
recrystalling into larger prisms. The apatite occurs in
association with finely crystalline and biogenic calcite, which is most abundant in the cores of larger nodules
where it has avoided the replacement process. Foraminiferal tests and nannofossils abound, often retaining their
original configuration even after replacement or partial replacement by phosphate. Glauconite is the only other
significant mineral present in the phosphoritenodules. Glauconite occurs as occasional pelletal grains, derived
from the original limestone, as local post-replacement fissure and vein fillings, and, as diffuse cryptocrystalline
masses. Silica is present in small amounts in fossilised diatom and radiolarian skeleta and occasional sponge
spicules, with some secondary quartz and opal. This mineralogy is reflected in the geochemistry of the
phosphorites, except for some compositional differences that may be seemingly related to nodule size. For
instance, the ratio of phosphorus to calcium generally increases with decreasing nodule size as phosphatisation
approaches its maximum in nodules 10-40 mm across. Similarly, SiO2, Al2O3, Fe2O3, and K2O (the components
of glauconite) become more important in particles smaller than 8 mm, reflecting in turn the more complete
replacement by glauconite of the smaller phosphorite nodules during late-stage diagenesis (Cullen, 1987).
Trace-element distributions display an analogous variation between larger and smaller nodules. Some elements,
(e.g., Cu, Mo, Ba, Co, and V) are variably distributed in nodules of all sizes, and are considered to be derived
from the parent limestone. Sr, Th, and U, on the other hand, tend to occur in higher proportions in nodules 8-64
mm across, and may have been introduced during phosphatisation. As, Ni, Pb, Rb, Y, Zn, and Zr all predominate
in nodules <8 mm across and their introduction is associated with late-stage development of glauconite.
Uranium concentrations are usually high, and may even attain ore grade. However, even if the phosphorite were
to be used as an unprocessed, direct-application fertiliser, studies suggest there is little danger of uranium
contaminating livestock or building up in the soil (Cullen, 1987).
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6
Characteristics of matrix material
The phosphate nodules at Chatham Rise occur within a matrix of totally unconsolidated Quaternary greenishgrey muddy sands and sandy muds. Grain size analyses indicate that fine sand predominates (62-250 μm),
typically accounting for 50-60% of the total by weight. Silt (4-62 μm) comprises about 25-35%, and clay (<4 μm)
amounts to 10-20%. Similar fine sediment (up to about 1.0 m thick) on the crest of Chatham Rise and containing
sparse granules (2-4 mm) of phosphorite, overlies much of the nodule-bearing layer. Frequent exposure of this
layer at the actual sediment-water interface is demonstrated by underwater photography, reflecting processes
that combine a low sedimentation rate with bioturbation and very gentle current winnowing. The Quaternary
muddy sands and sandy muds incorporate a variety of autochthonous and allochthonous sedimentary particles
other than the phosphorite nodules. The most ubiquitous and conspicuous type of particle in these sands and
muds is granular glauconite, comprising roughly ellipsoidal greenish-black grains which, with foraminiferal tests,
form the bulk of the medium- and fine sand grades (125-500 μm). These grains yield K-Ar dates in the order of 67 Ma, and are believed to have been derived, by mechanical winnowing and/or chemical dissolution, from very
late Miocene sediments.
There has been much comment on the patchy distribution of the phosphorite deposits on central Chatham Rise,
Given the widespread occurrence of glacial erratics in the area, it has been suggested that gouging of the
seafloor by icebergs may have considerably disturbed the phosphorite distribution. This gouging process means
the estimation of total phosphorite reserves is difficult. The patchy distribution of nodules on Chatham Rise is
believed to reflect the irregular development of an original phosphatic ‘duricrust’ wherever suitable oozes were
exposed, although some local minor modification by iceberg grounding may have also occurred.
A locally important constituent of the nodule deposits is autochthonous concretionary flint, derived from the
Upper Eocene-Lower Oligocene chalk substrate. The majority of flint concretions have somewhat elongate,
rounded shapes and dimensions less than 0.15 m, but occasional slabs up to 0.5 m across have been taken in
dredges. The flint tends to be dark grey or black, with a pure-white siliceous rind when freshly derived, and
petrological resemblance to flints of the European Upper Cretaceous Chalk Formation is marked. Commonly,
however, the outer rinds of the flints on Chatham Rise are impregnated or replaced by dark green glauconite,
and it is evident that these masses must have been exposed on the seafloor during the Late Miocene-Early
Pliocene episode of glauconitisation that succeeded the phosphatisation. Phosphatised and glauconitised
cetacean teeth and bone and shark teeth are a minor, but almost ubiquitous component of the nodule deposits
on Chatham Rise. Most of the bones and teeth are fragmental and bored, and few exceed 0.1 m. In summary,
the sediment that incorporates the phosphorite nodules is a ‘lag’ deposit, representing a substantial sedimentary
hiatus and embodying the winnowed relics of a series of diverse sedimentological processes that span the past
10 Ma or so (Cullen, 1987).
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Characteristics of substrate
Along most of the crest of central Chatham Rise, the phosphorite deposits rest on a substrate of Upper EoceneLower Oligocene chalky limestone that contains some conspicuous layers of concretionary flints. The chalk is
normally well compacted and shows evidence of modest deformation in seismic records. At its interface with the
nodule-bearing superficial lag deposits, the topmost 10-15 cm of the chalky limestone is softened to the
consistency of cream cheese, and riddled with borings and burrows. It was initially thought that this material
comprised reworked / re-deposited chalk, but close examination has failed to reveal the presence of any postOligocene microfossils. The softening of the chalk is therefore provisionally regarded as a Quaternary weathering
phenomenon, perhaps related to the intense bioturbation of the chalk surface. Seismic records indicate the
existence of younger strata (presumably of Miocene age) overlying the chalk on the northern and southern flanks
of central Chatham Rise, and these strata are thought to represent downslope extensions of the formations from
which the phosphorite was derived. Sedimentary formations with similar seismic signature blanket the EoceneOligocene chalk west of 177°E, and have also been sampled in the vicinity of 178°W, close to the Chatham
Islands. The only true outcrop of Miocene rock known on Chatham Rise (43°32.9'S, 178°13.6'W) is an unbored,
superficially phosphatised pavement outcrop of Lower Miocene limestone. In the vicinity of Matheson Bank, and
at a few other localities along the crest of the Rise, chlorite-zone schist and greywacke occur surrounded or
partly surrounded by narrow strips of Paleocene and/or Lower Eocene deposits. Any of these strata could form
the substrate for nodule deposits over limited areas, but such a relationship has not, as yet, been observed
(Cullen, 1987).
Figure 7 Schematic section to demonstrate the supposed interrelationships between the various facies on the Chatham Rise
(from Cullen (1978)).
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Geotechnical parameters
The surface layer of the Chatham Rise phosphorite deposits can be classified as a very poorly sorted fine
sand/silt with a mean d50 of 60 μm, the layer up to 50 cm below seabed as a poorly sorted silty fine sand with a
mean d50 of 90 μm. Most of the phosphorite nodules are larger than 8 mm, reaching maximum diameters of 100
to 120 mm on average. Based on tests performed on grab samples from the Sonne-17 cruise, the following
sediment types can be distinguished (Meyer & Toan, 1984):
-
Clayey fine sandy silt, very soft, often semi-liquid, olive greenish grey, glauconite-rich, with or without
phosphorite nodules. The thickness of this layer seldom exceeds 10 cm below surface.
-
Clayey, silty fine sand and fine sandy clayey silt, loose, sometimes medium dense, olive grey-green,
glauconite-rich, with or without nodules. Depending on the clay content, these sediments may have a tough
consistency. In a few cases purplish-grey fine sand was found to be irregularly interbedded with the green
sands and silts. This layer has a proved thickness of 155 cm at max. Seismic survey however indicates that
this layer may reach >10 m in certain places.
-
Foraminiferal nanno ‘ooze’, white to cream, plastic, stiff or very stiff, bioturbated, with irregular transition to
and intermixed with the superficial sand.
-
Chalk, white to cream coloured, indurated, embedded in ooze, often bioturbated.
The surface sediments exhibit very low strengths and have moisture contents in excess of the liquid limit. This
means that this material has no bearing capacity and can be characterized as slurry. Due to compaction, the
strength of the underlying glauconitic sand/silt (approx. 10 cm below the surface) becomes stronger with
increasing depth. The calcareous ooze has a shear strength nearly twice that of the overlying sand/silt and a
corresponding higher bearing capacity. However, due to a liquid limit slightly above the natural moisture content,
this sediment is also sensitive (Meyer & Toan, 1984).
8.1
Grain size distribution (from Meyer & Toan, 1984)
Glauconitic sands
Grain size distribution curves representing the uppermost 3 to 20 cm of the glauconitic sand/coarse silt show a
very poorly sorted fine/silt with a d50 of 42 to 90 μm. Grain size distribution curves comprising the sequence from
0 to 50 cm show a somewhat broader band, characterizing the sediment as a very poorly sorted, coarse silty fine
sand with a d50 of 47 to 160 μm.
Foraminiferal ooze
Grain size analyses show a d50 between 3.2 and 25 μm (average: 9 μm), which corresponds to medium silt.
Phosphorite nodules
Phosphorite nodules collected during the Sonne cruise were split into two size fractions (1-8 mm and >8 mm)
and were assessed quantitatively by weight. Maximum diameter of nodules found in the 4 research areas varied
between 40 and 180 mm, with 100-120 mm as the most frequent maximum diameter.
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Density and moisture content
Average wet densities of the surficial silt/sand layer (upper 10 cm), the sand layer from 10 to 70 cm, and the
3
underlying ooze were measured as 1.64, 1.72, and 1.81 t/m , the corresponding dry densities as 0.98, 1.15, and
3
1.27 t/m . Moisture contents were 68, 51, and 43% respectively.
3
3
Wet densities of phosphorite nodules ranged from 2.55 to 2.96 t/m and are on average 2.76 t/m . The density
tended to decrease with increasing nodule diameter (larger nodules have lower P 2O5 and higher CaCO3
contents). Moisture content of the phosphorite nodules varied from 2 to 7%, being higher in the larger nodules.
8.3
Shear strength
Undrained shear strengths were determined on board using a hand vane on most of the grab samples in
different depths within the grab, and using a plate bearing test on the surface of the samples. This yielded the
following results:
-
Surface sediments (0-10 cm) showed very low shear strength values of 2-5 kPa (average: 4 kPa),
increasing up to 38 kPa at a depth of 50 cm. Plate bearing measurements yielded figures of 20-50 kPa
(average 23 kPa) for the surface sediments.
-
Ooze showed shear strength values measured with a hand vane of 14-120 kPa (average: 45 kPa), and
plate bearing measurements of 64-250 kPa.
8.4
Atterberg limits
Five sand/silt samples and five ooze samples were tested. The former had a liquid limit in the range of 38-58%
(average: 49%) and a plastic limit of 0-14%, while the latter had a liquid limit in the range of 39-58% (average:
46%) and a plastic limit of 0-6%.
From the Casagrande plasticity chart, the sand/silt and ooze/chalk can be classified as plastic silts. The liquid
limit is close to, or sometimes higher than the natural water content and indicates the sediments may be
sensitive to liquefaction processes.
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Implications for mining
The Chatham Rise deposits are situated on the continental shelf, but they are 200 miles offshore in 400 m water
and in relatively severe seas. Some key features of the deposit with respect to mining are (Meyer & Toan (1984);
Falconer (1989)):
-
The nodules vary in size from 0.5 mm to 400 mm, with a mean size of 20-40 mm. There is very little
material over 200 mm.
-
Large variation in nodule sizes over short distances.
-
The deposit is not restricted to the surface. The nodules are mixed in a silty sand matrix that overlies an
ooze or chalk base.
-
The nodules make up, on average, only 13 percent by weight of the sandy layer.
-
The average concentration of phosphorus is 66 kg/m , but even over distances of less than 10 m, this
2
concentration varies. Areas of high average density can, however, be defined at a scale of several
kilometres.
-
The thickness of the phosphorite bearing sand, i.e. the depth to the chalk/ooze layer, is highly variable over
distances of tens of meters or less.
-
The ooze/chalk is adhesive and difficult to separate from the other material.
-
The seabed in the rich phosphate areas is irregular but relatively smooth, with slopes generally less than 8°.
However, slopes of 15° have been observed in the general area. There are no rock outcrops.
-
The nodules are hard. Once broken, the nodules splinter indicating they are brittle.
Based on data from the Valdivia and Sonne cruises of 1978 and 1981, the Chatham Rise phosphorite deposit
has been estimated at 30 million tonnes (Hughes-Allen, 2011). The total resource in this region alone is
provisionally estimated at 100 million tonnes. This reserve could thus satisfy much of New Zealand's phosphate
requirements for several decades, and, given the impending depletion of the New Zealand’s remaining traditional
supplies of rock phosphate in the West Pacific/Australasian region (Nauru and Christmas Island), the Chatham
Rise phosphorite constitutes a resource with considerable economic potential. Present indications are that the
Chatham Rise phosphorite reserve could provide an alternative source of agricultural phosphate, the main
residual marketing challenges are its effectiveness on high-pH soils, its marketability to the public, the economics
of mining, and some social and environmental impact factors (Cullen, 1987).
One of the primary reasons for the increase in commercial interest in the Chatham Rise deposits in the 1970’s
and 1980’s was the finding that the Chatham Rise phosphate does not need to be converted to superphosphate,
The nodules were shown to be an effective fertiliser when ground, pelletised and applied directly (Falconer,
1989). The average phosphorus content of the nodules (9.4% P = 21.5% P205) is also somewhat higher than that
of the widely used superphosphate fertiliser. Pot and field trials have established that, when finely crushed, the
phosphorite is suitable for use as a direct-application fertilizer on many New Zealand soil types. In addition, the
nodules can be converted, at increased cost, to superphosphate and triphosphate fertilisers (Cullen, 1987).
Although not a large resource, even by ocean floor standards, the Chatham Rise deposits have many features
favourable for marine mining. The deposits occur at relatively shallow depths, have the potential for direct
application on New Zealand soils, and have associated glauconitic deposits, which could be a source for potash
and ferrous iron. Another favourable aspect is that the deposits are within New Zealand's exclusive economic
zone (Burnett, 1980).
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Total estimated reserves
10.1
Valdivia cruise results
A first attempt at estimating the phosphorite reserves in the survey area was made onboard the Valdivia. For
each sample, the phosphorite coverage was calculated by multiplying the phosphorite concentration with the
thickness of the phosphorite sand. Individual samples were thought as being representative of the area around
2
each grab station. In the eastern part of the survey area an average of 1 sample was taken per 0.33 km , in the
2
western part of the survey area it was 1 sample per 0.77 km . The sum of all coverage values, multiplied by 0.33
3
2
and 0.77 respectively, gave an estimated value of 5.3 million m of phosphorite in the 277 km surveyed, or 14
3
2
million tonnes (density of phosphorite 2.65 g/cm ) and a mean coverage of 61 kg/m . However, this method
resulted in high standard deviations of the phosphorite nodules and their log-normal distribution with three subpopulations, which made the calculation (using a mean representative area and a mean coverage) unreliable.
Therefore, a second method, using a geostatistical approach, was implemented to improve the estimation of the
3
phosphorite reserves. Based on the model, the amount of phosphorite was estimated to be 5.4 million m or 14
2
million tonnes (over an area of 207 km ), which is somewhat more than the preliminary estimation. The average
2
coverage was estimated at 69 kg/m phosphorite. The total reserves were increased up to 18 million tonnes
2
2
2
phosphorite for 284 km when a cut-off grade of 15,000 m per km block was assumed (Kudrass & Cullen,
1982).
10.2
Sonne cruise results
Following the Sonne cruise, phosphorite reserves were estimated by two methods. When eliminating blocks with
2
less than 44 kg/m phosphorite, the reserves in the Sonne area were estimated to be 9.5 million tons phosphorite
2
2
in an area of 167 km with an average of 57 kg/m (10 to 17% of the total sediment weight). The overall
patchiness of the phosphorite-rich areas, however, complicated the assessment of reserves (Von Rad, 1984).
The reserves were then calculated separately for each of the four areas surveyed in great detail. These results
2
determined that 7.5 million tons of phosphorite may be present, with an average of 54 kg/m within the 140 km
2
2
large subareas. The phosphorite-rich areas (284 km ) in the Valdivia area was estimated to contain 18.8 million
2
tons phosphorite with an average coverage of 66 kg/m . The overlap of both the Sonne and Valdivia areas
2
reduced the Valdivia area to 238 km with reserves of 17.5 million tons. When Valdivia and Sonne areas were
2
combined, the total area was 378 km with reserves of 25 million tons phosphorite and an overall grade of 66
2
kg/m . Additional reserves of approximately the same magnitude may be found in the neighbouring areas, but
concentrations and coverage were anticipated to be much lower (Kudrass, 1984).
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References
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Cullen, D.J. (1978) The Distribution of Submarine Phosphorite Deposits on Central Chatham Rise, East of New
Zealand - 2. Sub-Surface Distribution from Cores. NZOI Oceanographic Field Report no. 12, May 1978.
Cullen, D.J. (1987) The Submarine Phosphate Resource on Central Chatham Rise. DMFS Reports 2, Mineral
Report Series MR4530.
Falconer, R.K.H. (1989) Chatham Rise Phosphates: A Deposit Whose Time Has Come (and Gone). Marine
Mining, vol. 8, pp. 55-67.
Hughes-Allen, S.L.M. (2011) Genesis of the Chatham Rise Phosphorite; an interpretation from current literature.
Kenex Ltd.
Kudrass, H.-R. & Cullen, D.J. (1982) Submarine Phosphorite Nodules from the Central Chatham Rise off New
Zealand - Composition, Distribution, and Reserves - (VALDIVIA-Cruise 1978). In: Geologisches Jahrbuch, Reihe
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Geologischen Landesämtern in der Bundesrepublik Deutschland, Hannover, pp. 129-178.
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