Geostatistical modeling and clarifying control factors
of Ni grade distribution in a laterite nickel deposit
Dissertation
Submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy
By:
Asran Ilyas
Student ID: 102d9101
Department of Life and Environmental Sciences
New Frontier Sciences
Graduate School of Science and Technology
Kumamoto University, Japan
March 2013
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ABSTRACT
Laterite nickel deposit is a deposit formed by rock weathering processes. This deposit
carries valuable Ni metal that can be mined because of the weathering processes acted
in a long geological time. Because of their abundant resources on earth, the
development of exploration techniques to locate the metal is needed. This research tries
to study how to find the metal through the analysis of its grade distribution based on the
relationships among geological factors controlling the formation of this deposit which
are topography in the form of slope, thickness of rock weathering resulted in the form of
laterization zones (limonite, saprolite, and bedrock zones), and groundwater that carried
Ni grade. The slope would influence the surface water into the ground which then
turned into groundwater. The groundwater then reacted with the surrounding rocks and
dissolving Ni element and deposited it in deeper zones. To achieve these requirements,
this study applies geostatistical techniques to spatial modeling of the Ni grade. The
Sorowako area in Sulawesi Island, Indonesia, was chosen as a case study area because it
has a representative laterite Ni deposit with large reserves. Chemical content data from
drillhole cores at 294 points were used for the analysis. From the results of this study, it
was found that the slope gradient has a remarkable correlation with the thickness of the
limonite zone, but there was no correlation between the thicknesses of the zones. One
original thing was found from this study that is the maximum Ni grade in the saprolite
zone has a general relationship with the thickness of the zone. The thicker of the
saprolite zone, the higher of the maximum Ni grade contain on it. Co-kriging was
adopted to incorporate this correlation into estimating the maximum Ni grade in the
saprolite zone. As a result, the maximum Ni grade in the saprolite zone tends to be high
mainly in areas of slight slope. The Ni accumulation at this topographic feature
probably originates from deep weathering by groundwater infiltrating through
well-developed rock fractures. Therefore, how to trace the paleo-groundwater flow
directions that carried Ni grade was also performed in this study. Geostatistical method
was used to find it. From the results of this study, it was found that the
paleo-groundwater flow directions can be traced through the modeling of the limonite
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and saprolite zones thicknesses, the modeling distribution of the maximum Ni grade in
the saprolite zone, and the modeling distribution of the presence of goethite found in the
saprolite zone because goethite formed by the reaction of iron oxide minerals with the
hydroxyl ions of groundwater. All of this study was explained in Chapter I until Chapter
VI. Chapter I explains about nickel and laterite nickel deposit, geostatistical method,
and the research study area. Chapter II describes the process and analysis of Ni grade
distribution model and the modeling results. Chapter III describes the relationship
between Ni and SiO2 grades for precise grade mapping and genesis identification in
order to find out the suitable grade of Ni with low concentration of SiO2 grade that will
be fed into the metallurgical processes related to the maintenance of refractory walls in
the pyro-metallurgical processes of Ni. Chapter IV describes the prediction of
paleo-groundwater flow directions that carried Ni grade related to the distribution of Ni
grade, especially the maximum Ni grade in the saprolite zone. Chapter V extended this
study into 3D model which is explain the results of 3D model of Ni grade distribution
and their relationship with topographic condition and paleo-groundwater flow direction
that carried Ni grade, and Chapter VI is the conclusion of study.
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Table of Contents
Page
Abstract …………………………………………………………………….
Table of Contents …………………………………………………………..
List of Figures ……………………………………………………………...
List of Tables ……………………………………………………………….
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Chapter I Introduction ……………………………………………………...
1.1 Background of study …………………………………………………...
1.2 Objectives of study …………………………………………………….
1.3 Characteristics of Ni and laterite nickel deposit ……………………….
1.4 Geostatistical applications ……………………………………………..
1.5 Study area and data source ……………………………………………..
1.5.1 Geological setting ……………………………………………………
1.5.2 Drillhole data ………………………………………………………..
References ………………………………………………………………….
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Chapter II Geostatistical modeling of Ni grade distribution ……………….
2.1 Introduction …………………………………………………………….
2.2 Data analysis …………………………………………………………...
2.3 Correlation analysis with topographic properties ……………………..
2.4 Co-kriging for Ni grade modeling ……………………………………..
2.5 Discussion ...............................................................................................
2.6 Conclusion ..............................................................................................
References .....................................................................................................
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Chapter III Correlation of SiO2 content with Ni grade in their spatial
distributions for precise grade mapping and genesis
identification …………………………………………………...
3.1 Introduction ……………………………………………………………
3.2 Method and materials ………………………………………………….
3.3 Results ………………………………………………………………….
3.4 Discussion ……………………………………………………………...
3.4.1 Destructions of SiO2 …………………………………………………
3.4.2 Correlation analysis of SiO2 and Ni grade distribution model ………
3.5 Conclusion ……………………………………………………………..
References ………………………………………………………………….
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Chapter IV Estimating paleo-groundwater flow as a control factor on
Ni grade distribution …………………………………………...
4.1 Introduction ……………………………………………………………
4.2 Deposition of Ni by groundwater flow path …………………………...
4.3 Modeling method ………………………………………………………
4.4 Results and discussion …………………………………………………
4.4.1 Estimating paleo-groundwater flow direction from laterization
zone thickness ………………………………………………………..
4.4.2 Paleo-groundwater flow direction estimated from mineral
composition …………………………………………………………
4.5 Conclusion ……………………………………………………………
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References …………………………………………………………………
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Chapter V Extension to 3D Ni grade model ……………………………….
5.1 Introduction …………………………………………………………….
5.2 Geostatistical analysis ………………………………………………….
5.3 Analysis of 3D Ni grade distribution model with topographic surface ..
5.4 2D model of paleo-groundwater flow direction and its relationship
with the 3D model of Ni grade distribution ……………………………
5.5 Conclusion …………………………………………………………….
References ………………………………………………………………….
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Chapter VI Conclusions ……………………………………………………
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Acknowledgments …………………………………………………………
List of publications ………………………………………………………..
Personal identity ……………………………………………………………
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List of Figures
Figure
Page
1.1 World’s land based nickel resources and primary nickel production
(resources distribution by contained nickel) (top) and increasing of
the world nickel production (bottom) (Dalvi et al. 2004) ……………...
1.2 Diagram of correlations between the chapters from Chapter II until
Chapter V for the purpose of the research in this study ……………….
1.3 World distribution of laterite nickel deposit resources
(Brand et al. 1998) ……………………………………………………..
1.4 Laterization zones in a laterite nickel deposit ………………………….
1.5 Location of study area in South Sulawesi Province, Sulawesi Island,
Indonesia with geological map and four principal tectonic belts in
Sulawesi Island ………………………………………………………...
1.6 Detail of study area in which is a mining hill of PT. INCO Indonesia
with the distribution of 294 drillholes in the 1.6 km (E-W) and 1.0 km
(N-S) superimposed upon the contour lines of topography. It is
expressed using a plane rectangular local coordinate system ………….
1.7 Saprolite zone in one location of the study area (vertical red arrow)
which shows extensive distribution of rock fractures ………………….
2.1 Changes of Ni grade with depth in a drillhole site at study area. It
shows (a) limonite zone, (b) saprolite zone, (c) bedrock zone, and the
maximum Ni grade in the saprolite zone (enclosed by green circle) …..
2.2 Histograms of the average of Ni grade data from 294 drillhole sites
in the (a) limonite, (b) saprolite, and (c) bedrock zones, respectively …
2.3 Scattergrams of correlation between Ni and Fe grades, Ni and SiO2
grades, and Ni and MgO grades in the limonite zone (top), saprolite
zone (middle), and bedrock zone (bottom), respectively. These
correlations tend to show a weak correlation between the variables …..
2.4 Schematic map of topographic features classified into five categories
by slope gradient (steep slope, ridge, slope with intermediate gradient,
slight slope, and flat slope associated with a valley areaor cavity) ……
2.5 Drillhole distribution at 294 sites that classified into five categories
of topographic features (see Fig. 2.4) ………………………………….
2.6 Correlation between the category of topographic features (first to fifth
category depending on slope gradient as defined in Fig. 2.5) and mean
thickness of the limonite zones in each category. Slope gradient
decreases from first to fifth category …………………………………..
2.7 Two conditions for selecting drillholes in the same category to identify
relationships between the slope gradient and the thickness of
laterization zone, in particular limonite zone in detail, which are for
drillholes (a) on the same elevation and (b) along a straight line in
the direction of the maximum gradient ………………………………...
2.8 Relationships between the categories of topographic features and the
average thickness of the limonite zone using drillhole data located (a)
on the same elevation and (b) along the maximum gradient of slope
selected by the two conditions in Figure 2.7 …………………………...
2.9 Scattergrams between the thickness of limonite and saprolite zones
using (a) all drillhole data and (b) drillhole data at the same elevations
selected by the method in Figure 2.7a …………………………............
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2.10 Changes in the Ni grades with depth at four drillhole sites at (a) slight
slope, (b) flat slope, (c) ridge, and (d) steep slope areas. Open circle
denotes the maximum Ni grade at each drillhole ……………………..
2.11 Scattergrams between the saprolite zone thickness and the maximum
Ni grade of the zone for every category of topographic feature and for
all drillhole data ………………………………………………………
2.12 Omnidirectional semivariograms of: (a) the maximum Ni grade in the
saprolite zone; (b) the thickness of the zone; and
(c) a cross-semivariogram combining these two variables. Spherical
models were used to fit all the semivariograms. Each dot represents
the semivariances at each separation distance ………………………..
2.13 Distribution of the maximum Ni grade in the saprolite zone by
co-kriging superimposed upon (a) the category of topographic feature
and (b) a perspective view of topography. Enlargement of the highest
grade zone is included in (a) ………………………………………….
2.14 Scattergram for cross-validation of co-kriging accuracy between
measured and predicted co-kriging values of the maximum Ni grade
in the saprolite zone …………………………………………………..
2.15 Distribution of the maximum Ni grade in the saprolite zone by
OK analysis (a) and a scattergram showing the cross-validation of the
OK accuracy (b) ………………………………………………………
2.16 Relations of the mean (closed circle) and the upper and lower
quartiles (top and bottom of bar) of (a) the thickness of saprolite zone
and (b) the maximum Ni grade in the zone to category of topographic
feature ………………………………………………………………...
2.17 Examples of rock fractures in the saprolite zone at ridge area (right),
slight slope area (middle), and steep slope area (left). Almost all
fractures are filled with garnierite …………………………….............
3.1 Scattergram between Ni and SiO2 grades in the limonite zone (left),
saprolite zone (middle), and bedrock zone (right) at the steep slope
(ss), ridge (rg), slope with intermediate gradient (swig), slight slope
(slg), and flat slope (fs) areas, respectively …………………………….
3.2 Scattergram between Ni and SiO2 grades using all mean grades
for each drillhole in the limonite zone (left), saprolite zone (middle),
and bedrock zone (right), respectively …………………………………
3.3 Distribution model of Ni and SiO2 grades in the limonite, saprolite,
and bedrock zones in the study area using OK ………….......................
3.4 Schematic diagram for purpose analysis of the study ………………….
3.5 Elemental mapping results of Ni (middle) and Si (right) which shows
almost the same distribution in a soil sample with resin from
the saprolite zone at ridge area (left photo) ……………………………
4.1 Schematic cross-section of laterite nickel deposit enriched
with migratory solutions (de Vletter, 1978) ……………………………
4.2 Cross section in N-S of groundwater system in Sorowako area
(Notosiswoyo et al. 2012) ……………………………………………..
4.3 Seven observation stations where soils and rocks have sampled at
different topographic slope related to analysis of Ni grade
distribution ……………………………………………………………..
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4.4 Calculation of the limonite zone thickness from drillhole A104677
(located at the center) in Station 1 represented by the range of the
semivariogram models from the changes of the grades vertically of
(a) Ni, (b) Fe, (c) SiO2, and (d) MgO, respectively ……………………
4.5 Prediction of paleo-groundwater flow direction (red arrows) in Station
1 based on the combination of prediction map (right side) and
semivariogram map (left side) of the distribution model of limonite
and saprolite zones thicknesses, and resultant of both using nine
drillhole sites data coded by B1 until B9 ………………………………
4.6 Prediction of paleo-groundwater flow direction (red arrows) in seven
observation stations based on the limonite and saprolite zone
thicknesses relatively from a thin to a thicker thickness
in the study area ………………………………………………………..
4.7 The bedrock of harzburgite in Station 3 represented from its thin
section that shows olivine (ol) and pyroxene (px) minerals with cracks
inside the minerals due to past tectonic process ……………………….
4.8 XRD analyses result from soil sample of limonite zone in Station 3
which shows the present of the goethite mineral’s peak (G) from its 2θ
angle pattern ……………………………………………………………
4.9 XRD analyses result from soil sample of saprolite zone in Station 3
which shows the present of goethite (G) and quartz (Q) from its 2θ
angle pattern …………………………………………………………....
4.10 The bedrock of dunite in Station 2 which shows the present of the
abundance of olivine (ol) and olivine with cracks and alignment
of scratches (ol (1)) which indicate the strong pressure
on the steep slope area due to past tectonic process ………………….
4.11 The present of oxygen from SEM analysis from the soil samples in
the (a) limonite zone and (b) saprolite zone of Station 3, 4, and 7 to
prove the reaction of Fe with oxygen to form iron oxide minerals …..
4.12 Distribution model of the saprolite zone thickness where goethite
found in the Station 3 using 21 drillhole sites data at the station. The
prediction of paleo-groundwater flow direction is shown by big arrow
at the model …………………………………………………………...
4.13 Distribution model of the maximum Ni grade in the saprolite zone at
Station 3 using 21 drillhole sites data at the station. The prediction of
paleo-groundwater flow direction is shown by big arrow at the
model ………………………………………………………………….
5.1 Spatial correlations between (a) Ni grade and Fe grade, (b) Ni grade
and SiO2 grade, and (c) Ni grade and MgO grade by using all the
vertical drillhole data in the three laterization zones in the 294 drillhole
sites …………………………………………………………………….
5.2 Spatial correlations between (a) Ni grade and Fe grade, (b) Ni grade
and SiO2 grade, and (c) Ni grade and MgO grade by using all the
vertical drillhole data in the limonite zone in the 294 drillhole sites …..
5.3 Spatial correlations between (a) Ni grade and Fe grade, (b) Ni grade
and SiO2 grade, and (c) Ni grade and MgO grade by using all the
vertical drillhole data in the saprolite zone in the 294 drillhole sites ….
5.4 Spatial correlations between (a) Ni grade and Fe grade, (b) Ni grade
and SiO2 grade, and (c) Ni grade and MgO grade by using all the
vertical drillhole data in the bedrock zone in the 294 drillhole sites …..
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5.5 Spatial correlations between (a) slope angle of topographic surface
where the drillhole insist and the vertical drillhole data of Ni grade in
the each drillhole in the limonite zone and (b) saprolite zone thickness
and the vertical drillhole data of Ni grade in the each drillhole in the
saprolite zone using all the 294 drillhole sites data ……………………
5.6 Semivariogram model (exponential model) of 3D Ni grade in
horizontal direction (omnidirectional) to find out the modeling
distribution of Ni grade in 3D model ......................................................
5.7 Semivariogram model (exponential model) of 3D Ni grade in
vertical direction (omnidirectional) to find out the modeling
distribution of Ni grade in 3D model …………………………………..
5.8 3D model of Ni grade distribution using OK method of the study area .
5.9 Topographic features in the study area in (a) 2D model and
(b) 3D model …………………………………………………………...
5.10 Vertical exaggeration of 3D model of Ni grade distribution in the
study area from the west to the east side in order to analyze its
relationship with condition of topographic surface ………………….
5.11 Prediction of paleo-groundwater direction in 3D model at Station 7
(red arrow) based on the 2D modeling result shown in Figure 4.6 in
Chapter IV and the 3D model of Ni grade distribution ……………….
5.12 Analysis of 3D paleo-groundwater flow direction in Station 3 (red
arrows) looking above based on the modeling result of 2D
paleo-groundwater flow direction shown in Figure 4.12 in Chapter IV
with the 3D model of Ni grade distribution relatively from low to
higher grade, appeared from (a) 11 m to (b) 19 m depths ……………
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List of Tables
Table
Page
1.1 Physical and chemical properties of nickel (Ni) (British Geol. Survey,
2008) …………………………………………………………………...
1.2 Example of drillhole sample data composed of four chemical
concentrations, weathering layer, rock type, and main constituent
mineral …………………………………………………………………
2.1 Mean thicknesses of limonite and saprolite zones and the maximum Ni
grade in the saprolite zone at each category of topographic feature.
Sample data were classified into two groups by the two methods in
Figure 2.7 ………………………………………………………………
3.1 Mean grades of Ni and SiO2 in the limonite, saprolite, and bedrock
zones for each category of slope gradient ……………………………..
4.1 Calculation results of the zones thickness at nine drillhole sites in
Station 1 ………………………………………………………………..
4.2 XRF, SEM, and thin sections analysis from soil and rock samples ……
4.3 XRD mineral analysis results from soil samples at seven observation
stations …………………………………………………………………
4.4 Calculation results of the limonite and saprolite zones thicknesses at
21 drillhole sites at Station 3 including the maximum Ni grade for
modeling process ………………………………………………………
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