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Lowe, S. 2013. Potential for magmatic Nickel-Copper-Platinum Group Element deposits (Ni-Cu-PGE) in northern
Guyana. Caribbean Journal of Earth Science, 46, 43-50. © Geological Society of Jamaica. Available online 5th
November 2013.
Potential for magmatic Nickel-Copper-Platinum Group
Element deposits (Ni-Cu-PGE) in northern Guyana
SHERWOOD LOWE
Department of Geological Engineering, University of Guyana, Gyana.
Email: [email protected] or [email protected]
ABSTRACT. The assessment of undiscovered mineral deposits poses special problems, especially in a country
like Guyana where geological and mineral exploration datasets are inadequate. Outside of gold, uranium and a
few other economic minerals, little assessment has been done of mineral resources on a systematic and regional
basis in Guyana. The demand for national assessments of mineral potential has recently increased in urgency
since the country’s adoption of a low-carbon/low deforestation development strategy under the UN’s Reduced
Emission from Deforestation and Forest Degradation (REDD+) initiative. As mining is seen as a significant
contributor to deforestation and degradation, support for its continuation as a major land use will depend
largely on its actual and projected economic value to the nation. Much of Guyana’s potential mineral wealth
remains undiscovered due poor accessibility within the rainforests and a thick weathering profile. Therefore, an
assessment of its value based on current mine production (particularly, of gold) is likely to severely undersell
mining as a main contributor to the future economy. This paper seeks to assess this undiscovered mineral
potential, in particular for magmatic Cu-Ni-PGE deposits in northern Guyana. Given the insufficiency of
detailed geological data, a qualitative approach is employed. The methodology involves three steps: (i) selection
of suitable descriptive deposit models for magmatic Ni-Cu-PGE deposits, (ii) identification and/or delineation of
regions geologically permissive for such mineralization, (iii) within each region, a qualitative assessment of the
extent to which components of the deposit models match ground features identified from available geological
and exploration data. The large number of favorable geological conditions strongly suggests a high probability
of occurrence of this important deposit class in northern Guyana.
Key words: mineral resource assessment, Guyana, Ni-Cu-PGE deposits, low carbon, mining.
1. INTRODUCTION
Magmatic Ni-Cu sulphide and PGE deposits are a
broad group of deposits containing nickel, copper,
and platinum group elements (PGE) associated
with a variety of mafic and ultramafic magmatic
rocks. These deposits provide most of the world’s
Ni and contain substantial reserves. In terms of
PGEs, these deposits are the source of essentially
all of the world output (Eckstrand and Hulbert,
2007). Worldwide exploration for PGEs remains
fairly strong, averaging just over US$215m in 2010
(Metals Economic Group, 2011).
Several occurrences and anomalies of these
deposit types exist in Guyana, the most prominent
occurrence being associated with the ultrabasic
flows or intrusions at Kauremembu in northern
Guyana. In addition, the country possesses large
tracts that are geologically favourable for such
mineralization, though no discoveries with
economic potential have yet been made.
2. METHODOLOGY
In assessing the potential for undiscovered deposits
of these classes in northern Guyana, the
methodology involved three steps: (i) selection of
descriptive deposit models for magmatic Ni-CuPGE deposits suitable for Guyana, (ii) identification
and/or delineation of regions geologically
permissive for such mineralization, (iii) within each
region, a qualitative assessment or rating of the
extent to which components of the deposit models
match local geological factors identified from
available geological and exploration data.
The paucity of, but slowly increasing,
substantive information on Guyana’s mineral wealth
outside of gold, diamonds, manganese and a few
other minerals presents challenges in conducting
such mineral resource assessments. In better
explored regions worldwide, quantitative (instead of
qualitative) assessments have been attempted to
provide estimates of the number of undiscovered
deposits, their sizes and grades. (e.g., Singer, 2007;
Rasilainen et al., 2010). In poorly explored regions,
such as northern Guyana, a quantitative approach
would be highly speculative. Nevertheless, mineral
assessments have become urgently necessary to
support decisions on land use policy and to
calculate opportunity costs within the low
43
S. Lowe – Magmatic Nickel-Copper-Platinum Group Element deposits in northern Guyana
carbon/low deforestation development strategy that
Guyana is presently pursuing. Pending more
abundant data, qualitative assessments can provide
a firmer and more evidence-based understanding of
the country’s mineral endowment.
3. DESCRIPTIVE DEPOSIT MODELS
On the basis of their principal metal production,
magmatic sulfide deposits in mafic rocks can be
divided into two major types: (i) those that are
sulfide-rich, typically with 10 to 90 percent sulfide
minerals, and have economic value primarily
because of their Ni and Cu contents. These ores are
associated with differentiated mafic and/or
ultramafic sills and stocks, and ultramafic
(komatiitic) volcanic flows and sills; and (ii) those
that are sulfide-poor, typically with 0.5 to 5 percent
sulfide minerals, and are exploited principally for
PGEs. These are associated with sparsely dispersed
sulphides in very large to medium-sized, typically
mafic/ultramafic layered intrusions. Layered
intrusions are also major hosts of Ni-Cu-PGE, Cr
and Fe-Ti-V deposits.
The mafic and ultramafic magmatic bodies that
host the Ni-Cu (± PGE) sulphide ores are diverse in
form and composition, and can be subdivided into
the following four subtypes (Eckstrand and
Hulbert, 2007):
• A meteorite-impact mafic melt sheet that
contains basal sulphide ores (Sudbury, Ontario is
the only known example).
• Rift and continental flood basalt-associated
mafic sills and dyke-like bodies (such as Noril’skTalnakh in Russia).
• Komatiitic (magnesium-rich) volcanic flows
and related sill-like (such as Kambalda and Agnew
in Australia).
• Other mafic/ultramafic intrusions (such as
Voisey’s Bay in Labrador, Canada).
The uniqueness of the first subtype (associated
with meteorite impacts) rules out its possible
existence in Guyana. Likewise, the third subtype
(based in komatiites) is unlikely to be of major
importance as such rock types are rare in northern
Guyana. The existence of the second and fourth
sub-types of Ni-Cu sulphide deposits is more likely
and they form one focus of the current study.
For this deposit type, the study used a modified
model derived mostly from the USGS Descriptive
Model 2b for the Merensky Reef PGE (Cox and
Singer, 1986).
PGE-dominant magmatic ores are associated
with mafic/ultramafic intrusions. There are two
principal subtypes of deposits (Eckstrand and
Hulbert, 2007):
44
• Reef-type or stratiform PGE deposits, which
occur in well layered mafic/ultramafic intrusions.
• Magmatic breccia type, which may occur in
stock-like or layered mafic/ultramafic intrusions.
For this second deposit type, the study used the
deposit model for PGE stratabound mineralization
in layered mafic/ultramafic intrusions in the
Australian Precambrian, compiled by Hoatson
(1998). In both the Ni-Cu sulphide and PGE deposit
classes, the chosen models provided descriptive and
genetic information on tectonic setting, ore controls,
host rocks, mineralogy, geochemical and
geophysical signatures among other aspects.
4. METALLOGENESIS AND PERMISSIVE DOMAINS IN
NORTHERN GUYANA
The permissive host environments for this broad
Ni-Cu-PGE deposit type include areas with mafic
and ultramafic flows and intrusives in the northern
Guyana. Domains in northern Guyana can be
demarcated based on this first-order criterion and
then possibly fine-tuned using evidence based on
mineral occurrences and geochemical and
geophysical anomalies.
Four episodes of mafic magmatism can be
identified in northern Guyana (e.g., Gibbs and
Barron, 1993; Choudhuri et al., 1990). These four
episodes can be equated to four distinct
metallogenic epochs and provinces for magmatic
Ni-Cu-PGE sulphide mineralization and are, from
oldest to youngest:
(i) the mafic/ultramafic flows and intrusions of
Guyana’s greenstone belts;
(ii) the flood basalts and intrusions of the
Avanavero Suite;
(iii) the minor intrusions of PAPA (PostAvanavero-Pre Apatoe) dykes; and
(iv) the widespread dykes of the Apatoe Suite.
Table 1 shows the stratigraphic positions and
ages of the four episodes of basic magmatic
intrusions in Northern Guyana. Figure 1 shows their
geographic location, and in terms of age, they can
be placed in two large groups: (i) those emplaced
and metamorphosed during the Trans-Amazonian
Episode (2.2 – 1.9 Ga), an event encompassing the
entire Guiana Shield, marked by regional
metamorphism, deformation, and widespread
granitic emplacement; and
(ii) those emplaced in the post-TransAmazonian period. This latter group of intrusions is
unmetamorphosed.
The first emplacements of magmatic bodies are
the mafic and ultramafic intrusions formed during
the Trans-Amazonian orogeny. In northern Guyana,
these mafic-ultramafic metavolcanics form the
S. Lowe – Magmatic Nickel-Copper-Platinum Group Element deposits in northern Guyana
Table 1: Basic stratigraphic column for northern Guyana
ERA
AGE (Ga)
LITHOSTRATIGRAPHIC
UNITS
TECTONISM
LITHOLOGY
Cenozoic
Apatoe Suite
1.9 – 1.8
Iwokrama
Formation
Paleoproterozoic
2.2 – 1.9
Multi-phase or
continuous plutonism
Muruwa
Formation
lower members of three greenstone belts, known in
Guyana as the Barama-Mazaruni Supergroup.
Gibbs (1980) estimated that 50% of the
metavolcanics in the northern Guiana shield is of
basaltic composition and 2% of ultramafic
composition. Ultramafic rocks are known in over
twenty localities in the greenstone belts of northern
Guyana (Gibbs and Barron, 1993). Considering that
the greenstone belts cover approximately 30,ooo
km2 and have a stratigraphic thickness of at least 9
km, mafic and ultramafic rocks are volumetrically
very significant in this region (Gibbs, 1980).
A second major pulse of mafic magmatism is
related to sills and dykes of the post-Transamazonian
Avanavero Suite of Paleoproterozoic (Statherian) age.
The Avanavero Suite constitutes the most important
Paleoproterozoic mafic magmatism event in the
Guiana Shield, northern Amazonian Craton, and is
described as a Large Igneous Province (LIP) (e.g.,
Reis et al., 2012). LIPs are key hosts for Ni-Cu-PGE,
Cr and Fe-Ti-V deposits (Ernst and Peck, 2010).
Paleomagnetic studies show that there were two
pulses of mafic magma in the Avanavero Suite
corresponding to 1.80-1.84 and 1.61-1.67 Ga., but
there appears to be no geochemical difference
between them (Choundhuri, et al., 1990). Using U-Pb
dates, Reis et al. (2012) support the two-pulse
Dolerite dykes
Basic magmatic Large Igneous Province
(LIP). Gabbro-norite sills and dykes
K’MUDKU EPISODE (1.3 – 1.2 Ga)
1.89
Roraima
Supergroup
TRANS-AMAZONIAN EPIDODE
Avanavero
Suite
Granitoid-greenstone
basement
1.79
Burro-Burro
Group
PAPA dykes
TECTONIC STRESSES RELATED TO THE OPENING OF THE ATLANTIC OCEAN
0.019
Volcano-sedimentary
sequences
Mesozoic
Conglomerate, arkose, orthoquartzite and
smaller amounts of shale and tuff with
jasper, deposited in fluvio-deltaic and
lacustrine environments.
Acid volcanics, volcaniclastics and
associated subvolcanic intrusives.
Quartzose and arkosic sandstones,
siltstones and conglomerates of
fluviodeltaic origin and minor fine-grained
cherty sediments of deeper-water origin.
Granites to diorites
Low-grade metamorphic metasediments.
Felsic- intermediate metavolcanics.
Basic and minor ultrabasic metavolcanics.
hypothesis but provide different ages: a first pulse at
1793–1795 Ma and a second pulse at ca. 1780 Ma.
Both sets of authors, nonetheless, suggest a short life
span for Avanavero magmatism of approximately 15 –
20 Ma.
This short lifespan is significant, as a correlation
exists between the mineralization potential of mafic
bodies and the number of pulses and duration of the
magmatic event.
The sills and the larger dykes of the Avanavero
Suite form thick differentiated bodies varying from
norite to ferrodiorite and granophyres. Sills can
reach thicknesses of around 500 m, and the NEstriking dykes can be followed for several
kilometers. Collectively, these bodies are
volumetrically significant in northern Guyana.
The Avanavero magmatic system is permissive
for both the rift and continental flood basaltassociated mafic sills and dyke-like bodies subtype
of the Ni-Cu type, and for the stratiform PGE
deposits in layered mafic/ultramafic intrusions
subtype of the PGE type.
A third pulse of mafic magmatism produced
relatively few intrusions, which are referred to as
“PAPA” dikes (post-Avanavero-pre-Apatoe) (Gibbs
and Barron, 1993). Their ages spread from
300-1300 Ma.
45
S. Lowe – Magmatic Nickel-Copper-Platinum Group Element deposits in northern Guyana
Figure 1. Geological Map of Guiana (reproduced at 2/3rds original size – see separate file for full sized version).
The
fourth
and
last
phase
of
post-Transamazonian basic magmatism relates to
the dykes of the Apatoe Suite of Jurassic age.
46
These form extensive dyke swarms throughout the
shield, with individual bodies reaching as much as
50 m in width and a length of several hundred
S. Lowe – Magmatic Nickel-Copper-Platinum Group Element deposits in northern Guyana
kilometres (Gibbs and Barron, 1993). The Apatoe
dolerites mark the precursor stages in the opening
of the South Atlantic Ocean during the Mesozoic
(around 200 Ma), and belong to the JACT (Jurassic
Atlantic Continental Tholeiites) association.
These last two magmatic phases could
potentially host the mafic/ultramafic intrusions
subtype of the Ni-Cu type.
5. MINERAL RESOURCE ASSESSMENT
In assessing the mineral potential of the four
major basaltic magmatic systems in northern
Guyana for magmatic Ni-Cu sulphide and PGE
deposits, this study used the following factors from
the descriptive models as the main criteria of
assessment.
• Tectonic setting. Rift systems or reactivated
deep-seated faults are likely to produce large
volumes of magma in repeated pulses. The
emplacement of such large volumes of mafic and
ultramafic magma is attributed to the rise and
impingement of mantle plumes on continental and
oceanic lithospheric plates.
• Age. Older magmatic systems are more
deeply eroded, which increases the possibility that
their more prospective feeder systems are likely to
be exposed as giant dike swarms, sill complexes,
and layered intrusions.
• Volume of magma. The small percentage of
PGE, Ni and Cu in basaltic magmas requires a high
concentration factor (and hence a massive volume
of magma) to produce economic concentrations of
the metals. Lightfoot (2007, p. 643), however,
points out that small (“volumetrically trivial”)
bodies of mafic-ultramafic rock can contain
exceptionally large economic Ni sulphide deposits.
He cites as examples the Ovoid Deposit at Voisey’s
Bay in Canada and China’s Jinchuan Intrusion
which has a projected surface outcrop area of less
than 1.4 km2, yet contains a historic and present
reserve and resource of over 500 Mt of mineralized
ultramafic rock.
• Number of pulses or cycles of magmatism
within one and the same system. The more often
the magmatic system is replenished, the higher the
chances of large economic mineralization are likely
to be.
• Magmatic
differentiation.
Economic
mineralization depends on efficient separation of
metal-rich phases from silicate melts. Layering and
cumulate textures are key signs of magmatic
segregation.
• Presence of deep seated faults. These
deposits are formed from mantle-derived magma.
Faults provide access and a conduit to bring such
magma to the surface.
In addition, the assessment will take into
consideration other factors such as exploration
history, the presence of known mineral
occurrences/deposits and geochemical and
geophysical anomalies in northern Guyana and
in the shield as a whole.
Table 2 provides a matrix to assess the
permissive domains in northern Guyana against
these selected criteria, which are rated as either
optimistic (O) in red cells, if a criterion enhances
the prospectivity of the domain, or pessimistic (P),
if it lowers it. Where no or little information exists
to make a judgment, ND (no data) applies. The
PAPA dykes are not considered as they are poorly
studied and relatively few in numbers.
6. CONCLUSION
In comparing and assessing the descriptive models
against the geological and exploration information
from northern Guyana, optimistic factors outweigh
pessimistic factors by over a two to one ratio. The
likelihood for Ni-Cu-PGE is therefore considered
high. The results reflect only a snapshot of our
current knowledge. We therefore caution that the
limited available data leave open the estimation of
the scale of this potential. With increasing
exploration work and academic studies, assessment
of undiscovered resources can become more
quantitative. To have attempted such an approach
now would have rendered the assessment
excessively subjective.
Among the most favorable factors is the
presence of several associations of mafic magmatic
rocks, in particular the volumetrically-significant
mafic metavolcanics of the greenstone belts and the
mafic magmatism of the Avanavero LIP. The small
number of known commercial deposits across the
entire Guiana Shield counts among the more
pessimistic factors.
In assessing the opportunity costs of
foregoing mineral exploitation as a trade-off for
forest-based carbon credits in Guyana’s low
carbon development strategy, we submit that the
high likelihood of magmatic Ni-Cu-PGE deposits
strengthens the case for mining as the best use of
the land. Any compensation therefore to the
country to forego mineral development would
have to be greater than that calculated for known
deposits of gold (e.g., Office of the President,
2012). Similar studies should be undertaken on
the wide range of deposit types likely or found in
Guyana,
such
as
albitite-hosted
and
unconformity-related uranium deposits and
volcanogenic massive sulphide deposits.
47
S. Lowe – Magmatic Nickel-Copper-Platinum Group Element deposits in northern Guyana
TABLE 2: Assessment of prospectivity of identified domains for magmatic Ni-Cu-PGE deposits
Favourable
Greenstone metavolcanics
Avanavero Suite
Apatoe Suite
conditions
Tectonic
setting
Related to island-arc volcanism, during
the Trans-Amazonian Episode.
Age
Palaeoproterozoic. No mention of
feeder systems.
Continental rifting. Possibly
signifying a failed arm
O
(Choudhuri et al., 1990).
Continental. Intrusions
precursor to opening of
O
O
Central Atlantic ocean
during the Mesozoic.
Paleoproterozoic. Exposure
Jurassic. Mainly small
ND of feeder dykes in some areas O individual dykes.
P
(Gibbs and Barron, 1993).
Volume of
magma
Significant. Metavolcanic rocks in
northern Guyana comprise 50% mafic
and 2% ultramafic. The entire volcanosedimentary sequence is over 9 km
thick covering 30, 000 km2.
Significant. Sills up to 500 m
thick and strike for hundreds
of km. Total volume of sills 30,000 km3 (Gibbs and
O
Barron, 1993).
Dykes up to 50 m in
width, some up to
several hundred of
kilometers in length.
O
Dykes are in swarms of
up to ten. Very
abundant across the
Shield.
Number of
No age differences reported among
pulses/cycles of basic/ultrabasic members of the
magmatism. greenstone belts.
Possibly two pulses
P (Choudhuri et al., 1990; Reis
et al., 2012).
Magmatic
Reported for ultrabasic rocks at
differentiation. Kauremembu in the Barama belt.
Major element plots by
Choudhuri et al. (1990) show
vague differentiated trend.
Differentiation possibly prior
to emplacement, leading to
sills of different compositions,
and within the sills
themselves. Later exploration
O
P
work in 2002 reports only
little evidence of modal
layering in large sills.
Cumulate textures and
rhythmic layering in the
Tumatumari-Kopinang dike
and sill complex (Hawkes,
1966).
Layering/cyclic Commonly form layered complexes in
units/compositi the Guiana Shield.
onal interface,
i.e, ultramaficgabbroic
contacts.
Presence of
deep-seated
faults.
Presence of deep-seated faults.
Exposure of
None reported.
plumbing or
feeder systems
or
conduits/depth
of erosion.
48
Presence of deep-seated
O faults.
P Likely one pulse.
O
P
None reported, but
unlikely given the
smallness of the bodies
and rate of
emplacement.
Dykes are faultO controlled.
Sills are found at the base of
Mainly small dykes.
the sedimentary succession
or exposed when the
overlying rocks of the
ND Roraima Supergroup have
O
been eroded. Massive feeder
dykes exposed (up to a
kilometer in width).
P
O
P
S. Lowe – Magmatic Nickel-Copper-Platinum Group Element deposits in northern Guyana
Exploration
and mining
Geochemical
signature
Relatively little exploration effort
directed toward the search for Ni and
Cu sulphides in ultramafic rocks. Ni
laterites preferred targets. No
commercial deposits identified to date.
Preliminary exploration
No exploration
conducted in 2002, involving
attention received.
geological mapping,
P lithogeochemistry at two
P
sites. No clear signs of PGE
mineralization reported.
Further work recommended.
Most of the examined ultramafic rocks
Presence of palladiumNone reported.
contain only background Ni values (0.2 bearing mineral (potarite) in
0.3%). Only few occurrences of distinctly
stream sediments over the
higher Ni values found (Gibbs and
Avanavero. Significance in
Barron, 1993).
dispute.
However, lowermost mafic
metavolcanic rocks of the greenstone
Ore minerals detected:
belts spatially associated with
ilmenite. magnetite,
anomalous Ni values at Five Stars in
pyrrhotite and chalcopyrite
Baramita and at Kauramembu, where
(Choudhuri, et al., 1990).
ND
O
drilling has indicated a lateritic horizon
containing 10-20 million tons of Ni
Lithogeochemistry in 2002
averaging 1% Ni over serpentinite
returns inconclusive results at
(Walrond, 1980).
two sites.
Stream sediment geochemistry draining
the Tenapu formation returned
anomalous values of Cr, Cu and Zn.
The northernmost sector of the Omai
area show anomalous Cu-Ni-Cr values,
and probably overlie ultrabasic rocks.
Geophysical
signature
Aero-magnetic and aeroelectromagnetic signature detected
over Itaki gabbro stock.
Deep seated faults below the
Avanavero basalts detected
O by Airborne Total Intensity
O
Magnetic survey. (Nadeau,
2009).
Presence of
deposits in
other parts of
the Guiana
Shield.
i) The USGS (1993) estimates there is a 1
None reported.
percent chance of one or more deposits
of nickel-copper-platinum group
elements associated with pyroxenite
and gabbroic intrusive rocks in
permissive domains of the Venezuelan
Guiana Shield.
ii) The large PGM-bearing gabbroicultramafic De Goeje body in E Suriname.
O
Its continuation in French Guiana
(Tampok) dated at approximately 2.15
Ga (Delor et al., 2003).
iii) Chromite deposits in Bacuri maficultramafic complex, in the eastern part
of the Guiana Shield, Amapa State,
Brazil. (Spier and Filho, 2001).
iv) Chromitite deposits also in the
Suriname shield.
P
ND
ND
None reported.
P
P
49
S. Lowe – Magmatic Nickel-Copper-Platinum Group Element deposits in northern Guyana
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