MINISTRY OF EDUCATION VIETNAM ACADEMY OF
AND TRAINING SCIENCE AND TECHNOLOGY
GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY
-----------------------------
PHAM THANH DANG
FORMATION OF NICKEL LATERITE ORE IN SOME ULTRAMAFIC BLOCK, NORTHER VIETNAM
Major: Mineralogy and geochemistry
Code: 9.44.02.05
SUMMARY OF GEOLOGY DOCTORAL THESIS
Hanoi, 2021
The thesis is completed at:
GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY,
VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY
Supervisors:
1. Assoc. Prof. Nguyen Van Pho
2. Assoc. Prof. Pham Tich Xuan
Examination board:
Commenter 1: Assoc. Prof. Do Dinh Toat
Commenter 1: Assoc. Prof. Nguyen Trung Minh
Commenter 1: Assoc. Prof. Nguyen Thi Minh Thuyet
This doctoral thesis will be defended at the GUST-level
Board of Examiner at Graduate University of Science and
Technology, Vietnam Academy of Science and Technology
at ......... on ........./......... /2021
This doctoral thesis can be found at:
- National Library of Vietnam - Library of Graduate University of Science and Technology
INTRODUCTION
Nickel laterite ores are a major source of nickel in the world besides nickel
sulfide ores, they account for 70% of global Ni resources and currently contribute
about 60% of total Ni output, the rest is from sulfide ores (Michael Green, 2019).
Recently, with resource depletion in nickel sulfide mines and the advancement in ore
mining and processing technology, nickel laterite has become a leading global search
and exploration target. They are loose materials formed by the intense and prolonged
weathering of olivine-rich ultramafic rocks, mainly in humid to subtropical tropical
climates (Golightly, 1981; Gleeson and nnk, 1999; Butt and nnk, 2013). During the
weathering process, the primary (primitive) primary constituents of ultramafic rocks
such as MgO and SiO2 are washed away and carried away, while the other components
such as Fe, Al, Ni, Mn and Co are the opposite. , is accumulated and enriched. The
relatively complete weathered section on common ultramafic rocks includes the main
zones: the bottom is the saprolite zone located just above the bedrock, in the middle is
the transition zone and the top is the limonite zone.
Vietnam in general and the North of Vietnam in particular, located in the humid
tropical climate, have a lot of ultramafic intrusive bodies, so the presence of nickel
laterite is entirely possible. In fact, recently, nickel laterite mineralization has been
discovered in the weathered crust of Nui Nua (Thanh Hoa), Suoi Cun and Ha Tri (Cao
Bang), and Ban Phuc (Son La) blocks. However, up to now, there have not been
enough detailed and systematic studies on nickel laterite formations in our country,
leading to difficulties in assessing their prospects and finding them. Therefore, the
researcher selected the thesis topic “Formation of nickel laterite ore in some
ultramafic blocks, Northern Vietnam” to clarify the mechanism of nickel enrichment in
the weathering process of ultramafic rocks as well as The fundamental factor
influencing the formation of this mineral, forms the basis for the assessment of new
nickel resources, aiming to meet the increasing demand for Vietnamese industries.
1. Urgency
1
2. Research scope and object:
- Research scope: Ha Tri ultramafic masses belong to Cao Bang complex
(Cao Bang) and ultramafic masses belong to complex of the same name Nua
Nua (Thanh Hoa).
- Research subjects: The weathered crust developed on the bedrock is
ultramafic in the Ha Tri (Cao Bang) and Nui Nua (Thanh Hoa).
3. Objectives of the study:
- To clarify the geochemical behavior of nickel in the weathering of
ultramafic rocks and the formation of nickel laterite ores;
- To elucidate the formation process of nickel laterite ore types;
Initial assessment on the prospect of nickel laterite ore in the North of
Vietnam;
4. Main research content:
- Additional research on the distribution characteristics, mineralogical
composition, geochemistry of ultramafic rock in Ha Tri (Cao Bang) and Nui
Nua (Thanh Hoa) blocks;
- Detailed study of weathered sections on ultramafic rocks in Ha Tri (Cao
Bang), Nui Nua (Thanh Hoa) rocks and associated nickel laterite mineralization;
- Analyzing and evaluating the premise of nickel laterite ore formation:
bedrock composition, climatic conditions, geomorphology and geomorphology,
tectonic activities;
- Outlook on nickel laterite ore prospects in the North of Vietnam based
on the premise of ore formation;
Chapter 1. An overview of Ha Tri ultramafic masses (Cao Bang)
and Nui Nua (Thanh Hoa)
1.1. Ha Tri intrusion block (Cao Bang)
Ha Tri ultramafic mass has the form of a lens extending northwest -
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southeast with a length of about 0.8 km, width of 0.3-0.5 km. The intrusive
rocks of Ha Tri block consist mainly of lherzolite, lherzolite containing
plagioclase, porphyritic lherzolite (type picrite) and a small part melanogabro
olivine. Lherzolite contains plagioclase with olivine content ranging from 55 to
70%, clinopyroxene (15 - 20%), orthopyroxene (~ 10%), plagiocla (several
percent), auxiliary minerals including Cr-spinel, in addition there are other
sulfide minerals intrude in rock. Secondary minerals include hornblende,
chlorite and serpentine. The rock is serpentineized in the weak to moderate
level.
1.2. Nui Nua super mafic block (Thanh Hoa)
The Nui Nua ultramafic mass is the largest of the ultramafic bodies
distributed along the Ma river fault and is also the largest ultramafic intrusion in
Vietnam. Nui Nua block consists mainly of rocks apohazburgite, apodunite, less
than apolherzolite and diaba dykes. Apoharburgite and apodunite rocks strongly
serpentineized and talcized. In many places, serpentinite are now being mined in
the south of the block in Te Loi commune (Nong Cong, Thanh Hoa). Apodunite
consists mainly of olivine (85-95%) serpentineized, leaving only the central part
of the grain, a small amount of orthopyroxene is also talcized, bastitized, mainly
chromite ore minerals. Apoharburgite consists of olivine (60 - 75%) with the
majority serpentine and it is replaced by minerals antigorite, lizardite, sometimes
chrysotile short strands; orthopyroxene (10-20%) also changed strongly, minor
minerals include cromspinel, the common ore minerals are ilmenite and
chromite. In Nui Nua ultramafic rocks, secondary minerals such as serpentine
and chlorite are common.
Chapter 2. Theoretical basis and research method
2.1. Theoretical basis
2.1.1. Elemental Nickel geochemistry
Nickel has the symbol Ni, element metal which is a member of group 10
3
of the periodic table, along with Pd and Pt. Element has atomic number of 28,
atomic number 59, two oxidation states (II and III). According to the
geochemical classification of Goldshmidt, nickel belongs to the group of
siderophil elements together with Co, Fe, Mo, Pd, Pt, ... Nickel atom has two
electron configurations, [Ar] 4s23d8 and [Ar] 4s13d9, they are energetically
close, where the symbol [Ar] refers to the propagation structure of argon.
However, the atomic computation research has given the nickel background
state configuration of 4s13d9.
Nickel’s geochemical behavior:
During magma: The ionic radius of valence Ni is close to Fe (II) and Mg
(II) ions, allowing these elements to interchangeably in the lattice of several
silicates and oxides. Ni readily substitutes for Mg for Mg in the mantle Mg-rich
minerals and is largely left in the residue during partial melting. Ultramafic
rocks (dunite, peridotite, komatite) usually have the highest Ni content, up to ~
4000 ppm; Poorer is the pyroxenite group with the largest content up to> 1500
ppm. In rocks of basic groups and equivalents such as gabro, basalt or eclogite,
the Ni content does not exceed 500 ppm. In the mantle, the Ni content is also
different, this is due to the different Ni content in the composition of their rock-
forming minerals. Of the mantle minerals, olivine is richer in Ni, followed by
spinel. In the pyroxene, orthopyroxene has a higher Ni content than
clinopyroxene. This behavior of Ni is related to its charge and ionic half. Under
the above mantle and mantle conditions, the most common oxidation state of Ni
is Ni (II). The effective ionic radius of Ni (II) depends on the number of coordinates ([4]Ni, 0,55 Å; [4]Ni square, 0,49 Å; [5]Ni, 0,63 Å và [6]Ni, 0,69Å).
The charge similarity and semi-ionic similarity allow Ni to significantly replace
Mg in olivine, so that olivine is the main Ni-carrying mineral in mantle
minerals.
During weathering: nickel is released mainly from olivine and partly
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from pyroxene and serpentine minerals. Ni is characterized by low mobility
under neutral, alkaline conditions. It accumulates at the bottom of the cross-
section in the local weathering products (pyroxene-derived smectite, the fissure-
filled garnierite) and in the serpentine grains that have not been weathered. In
the supernatant, Ni is closely related with Mn and Fe hydroxyt, mainly in the forms adsorbed by geothite. Ni2+ is relatively stable in water and can travel far
(especially in acidic to near neutral), its precipitation occurs in more alkaline medium (pH> 9). In addition, Ni2+ is stored in clay minerals (smectite) in
favorable environments. With an increase in the degree of weathering, nickel
continues to be washed away from previously formed nickel-containing
minerals (goethite, smectite, garnierite and serpentines) and redistributed in the
newly formed phyllosilicate at the section bottom.
2.1.2. The concept of nickel laterite ore
“Nickel laterite” ore (“supergene nickel ore) are terms referring to a type
of mineral containing an industrial value of nickel formed during chemical and
mechanical weathering of ultramafic rocks (peridotite, dunite, serpentineite, ...)
under prolonged humid tropical conditions (J.P. Goligtly, 1981; Charles R. M.
Butt, 2013; Peter C. 2017). Nickel laterite ores are classified into 3 types based
on the main mineral composition of nickel, including:
Type A (Hydrous Mg silicate deposits): with ore minerals mainly Mg-Ni
silicate hydrate (garneirite), often appearing in the lower part of the saprolite
zone. hydrat silicat Mg-Ni (garneirite). These are the most important epiphytic
Ni ores because they have the highest Ni content of the epiphytic Ni ores with
an average content of 1.53% Ni (Gabriel Aragao Rodrigues Soares et al, 2018)
of which garnierite has Can contain up to 30% Ni.
Type B (Clay silicate deposits): The main ore-bearing mineral is nickel-
containing smectite with an average Ni content of about 1% (Gabriel Aragao
5
Rodrigues Soares and nnk, 2018)
Type C (Oxide laterite deposits): with ore minerals mainly nickel-
containing Fe oxyhydroxides with an average Ni content of about 1,1%
(Gabriel Aragao Rodrigues Soares và nnk, 2018)
In three types of above epiphytic nickel ores, type A is the most important
and significant because the nickel content is much higher than the other types
(up to 30% nickel in garneirite). In fact, most nickel laterite accumulations have
both of the three types of ore mentioned above, consisting of an oxide type and
hydrous Mg silicate or a clay silicate component (Brand et al. 1998; Berger et al.
2011).
2.2. Research methods
2.2.1. Methods of survey and sampling in the field
The research student conducted a field survey to take samples in
weathered sections of ultramafic rocks in Ha Tri block and Phan Thanh block
(Cao Bang), Nui Nua block (Thanh Hoa). Samples were taken at different
depths in each weathered section, corresponding to the different weathering
zones.
2.2.2. Group of analytical methods
+ Method of lithological analysis by polarizing microscope
+ X-ray fluorescence method (XRF)
+ Method of mass spectrometry plasma (ICP-MS)
+ X-ray diffraction method (XRD)
+ Method of scanning electron microscopy - SEM / EDX
+ EPMA micro-detection electronic method
2.2.2. Data processing methods
+ Mass balance calculation method
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+ Method of evaluating the degree of lateritization
Chapter 3. Weathering characteristics of megalithic rocks of Ha Tri (Cao
Bang), Nui Nua (Thanh Hoa) and related nickel laterite mineralization
3.1. Characteristics of weathering of Ha Tri cubic ultramafic rock (Cao
Bang) and related nickel laterite mineralization
3.1.1. The weathered cross section on ultramafic rock in Ha Tri block
3.1.1.1. Section HT-01
Mineral composition
In the saprolite zone, mainly serpentine, chlorite, talc minerals with lower
ratio of olivine, pyroxene, especially at 8m depth, there are quite abundant
minerals like nickel silicate of garnierite group such as willemsite
[(Ni,Mg)3Si4O10(OH)2], nepouit [(Ni,Mg)3Si2O5(OH)4]. At the boundary of the
saprolite and limonite zone, the clay minerals (nontronite, montmorillonite) are
more abundant, along with the expression of iron-rich minerals (goethit,
hematit). In the limonite zone, mainly iron-rich minerals such as goethite,
hematite; serpentine, chlorite, and kaolinite only appear in the lower part of the
limonite zone (near the boundary with the saprolite zone). The closer to the
surface the iron-rich mineral group is dominant.
Chemical composition
The results of mass balance calculation show that elements of silicate and
alkaline groups (Si, Mg, Ca, Na, K) are washed in driftwood, these elements
tend to be removed the most in the region. upper of the weathered section
(limonite zone) corresponding to the coefficient K = −60.1; -95.90 and -100%
for alkaline elements. In addition to the driftwood washed elements, the section
HT-01 has the enrichment of (Ni, Fe, Co, Mn) during the weathering process. Fe
was enriched up to more than 220% (K = 224.52) at the top part of the section
HT01. Co also showed clear enrichment behavior in the limonite zone, where
the highest enrichment coefficient was over 170% (K = 173.2). Nickel is
7
enriched in all two research sections, in the limonite zone they are enriched up
to 136% (K = 136.5); to the Ni saprolite zone was enriched and reached the
maximum value in the lower part of the saprolite zone where the mineral veins
of the garnierite group appeared with the highest enrichment coefficient of
nearly 800% (K = 784.30).
3.1.1.2. Section HT-LK90
Mineral composition
The section HT-LK90 in the northern area of the Ha Tri block also has a
marked change in the mineral composition of the weathered section with the
same depth, similar to that of HT-01, the only difference is the ratio. the
garnierite minerals in the saprolite zone are much less than the samples in the
HT-01 section.
Chemical composition
The mass balance calculation results showed that at the HT-LK90 section,
the elements of the silicate and alkaline groups (Si, Mg, Ca, Na, K) were also
washed the most strongly, along with the enrichment of ( Ni, Fe, Co, Mn) during
weathering. Nickel is also enriched in the entire section, in the limonite zone the
enrichment coefficient is equivalent to that of section HT-01 with the maximum
enrichment level of about 135% (K = 134.4); However, in the saprolite zone, the
enrichment coefficient of Ni was slightly lower than that at the section HT-01
with the maximum enrichment coefficient of only 443% (K = 443.8), half as
much as 800% (K = 784.30) at section HT-01, this is consistent with the mineral
composition results in section HT-LK90 where the percentage of the mineral
"garnierite" present here is much less than in the HT section. -01.
3.1.2. The degree of lateritization in the Ha Tri block
The results of the calculation of S / SAF and UMIA index showed that the
weathering process in the Ha Tri (Cao Bang) blocks belonged to the laterritation
type and was only weak to medium.
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3.1.3. nickel laterite mineralization in the Ha Tri mass
The research results show that in the weathered section in the Ha Tri
block, there exist all three epigenetic Ni mineralization types: type A (hydrous
Mg silicate), type B (Ni-silicate clay) and type C (oxide)
3.1.3.1 Type A oxidation (hydrous Mg silicate)
In the weathered zones of Ha Tri block, the mineral group characteristic
for type A mineralization is "garnierite" with a characteristic green color, which
appears as a form of filling or sticking to the surface of joints developed in
ultramafic rocks. unfinished weathered in the middle part of the saprolite zone
(Photo 1). Garnierite minerals in Cao Bang area include three main types:
serpentine type, talc type and chlorite type (figure 1). The chemical composition
of Garnierite in Ha Tri area is characterized by high Ni content (25.50 -
40.02%), low Fe content (0.09 - 0.9%) and almost no Al (< 0.02%) similar to
Garnierite in famous nickel mines in the world such as New Caledonia,
Dominican Republic and many other places. In particular, the Ni and Mg
content have a clear inverse correlation, showing the substitution in the forming
phases (Villanova-de-Benavent et al, 2014; Wei Fua et al, 2018).
Through the two sections HT-01 and HT-LK90, it can be seen that the
nickel content reaches very high values in the lower and middle part of the
saprolite zone, corresponding to the area where many garnierite minerals are
present, the nickel content in the range from 1 25-3.01%, average 2.1% in cross
section HT-01 and from 0.87-1.74%, average 1.25% in section HT-LK90. With
these results, it can be said that nickel laterite mineralization type A (hydrous
Mg silicate) in the Ha Tri block area is of quite good quality, although it is lower
than the Ni content at grade A mines in New Caledonia. (2.6% Ni) but at this
level equivalent to that of the average nickel content in grade A mines in
9
Indonesia about 1.8% Ni.
Photo 2. Garnierite under scanning electron microscope (SEM). (Gar- garnierit)
Figure 1 The graph of correlation
of Si-Mg- (Ni + Fe) ratio in
“garnierite” in Ha Tri area with
the schools classified by Brand et
al. 1998
Photo 1. The mineral image of the turquoise "garnierite" group in the weathered section of ultramafic rock in Ha Tri mass
3.1.3.2. Type B mineralization (clay silicate ore)
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The clay minerals formed during the weathering process of ultramafic rocks in Ha Tri block consist mainly of nontrolite and montmorillonite. The clay
Photo 3. Smectite clay minerals in Ha Tri area under scanning electron microscope (SEM)
minerals formed by the weathering process of Ha Tri monolithic ultramafic rocks are capable of storing nickel quite well (1.06% Ni in nontrolite and 1.2% Ni in montmorillonite) and this is one of the figures. formula for enriching nickel during the weathering process. However, nickel-containing clay minerals (nontrolite, montmorillonite) are only present at the boundary of the saprolite zone and the limonite zone, with the thickness of only 1 to 2 m. This shows that although the capacity to store nickel is quite good, but in Ha Tri mass weathering zone, the mineralization of Nickel laterite of type B accounts for only a small part compared with type A and type C.
3.1.3.3. C-type mineralization (oxide type)
11
The C (oxide type) nickel laterite mineralization is characterized by ore minerals mainly nickel-containing Fe oxyhydroxides, namely nickel-containing goethite. At Ha Tri block weathering sections, goethite is a fairly common mineral in the limonite zone, they can be seen with the naked eye at the upper part of the section with reddish-brown accumulations. The analysis results of goethite composition in the Ha Tri ultramafic rock weather zone showed that the average nickel content in goethite was 1.03%, although this content was not too high, it showed the ability of goethite to store. nickel is quite good, justifying the nickel enrichment in the entire limonite zone at the sections of the Ha Tri area as described in the previous sections. With the limonite zone with relatively large thickness (from 4 to 6 m) in the Ha Tri block and the average nickel content in
the entire limonite zone at the cross sections about 0.58%, it shows the potential of nickel mineralization. Type C (oxide type ore) is quite large.
3.2. Characteristics of weathering of Nui Nua (Thanh Hoa) ultramafic rocks and related nickel laterite mineralization
3.2.1. Weathered cross section on ultramafic rock in Nui Nua block
3.2.1.1. Cross section NN-01 (developed on bedrock is hazbugite)
Mineral composition
the mineral garnierite the presence of
There is a clear change in mineral composition according to section depth NN-01. The saprolite zone, characterized by serpentine, chlorite, and talc - nepouite minerals and [(Ni,Mg)3Si2O5(OH)4] at depth of 7-8m. Clay minerals (nontronite, saponite) are present at the boundary of the limonite and saprolite zones . The limonite zone is mainly iron-rich minerals such as goethite, hematite;
Chemical elements
The results showed that at the NN-01 section, the elements of the silicate and alkaline groups (Si, Mg, Ca, Na, K) were also washed the most strongly, along with the enrichment of (Ni, Fe, Co, Mn) during weathering. Ni shows the behavior has quite similar characteristics with the sections in the Ha Tri (Cao Bang) area, however, the nickel enrichment in this section is concentrated mainly from the middle saprolite zone to the upper part of the zone. Limonite instead concentrated mainly in the lower saprolite zone as in the areas of Ha Tri (Cao Bang). Here, the maximum nickel enrichment value (1.02%) is also much lower than that in the section of Ha Tri area (3.02%). This difference is due to the fact that the richest nickel-rich mineral "garnierite" is not common here, but instead is the prevalence of nickel-containing clay minerals (nontronite, saponite) in the upper saprolite zone to the middle part. limonite zone.
3.2.1.2. Section NN-03 (developed on bedrock is dunite)
Mineral composition
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The saprolite zone, still characterized by the minerals chrysotil, chlorite, talc, but not the mineral garnierite. The clay minerals (nontronite, saponite) are present at the boundary part of the limonite and saprolite zone with a lower rate
than the section NN-01 . The limonite zone is characterized by iron-rich minerals such as goethite, hematite.
Chemical composition
The elements of the silicate and alkaline group (Si, Mg, Ca, Na, K) are also the most strongly washed driftwood elements, besides there is the enrichment of (Ni, Fe, Co, Mn) during the weathering process. However, Ni exhibits a relatively different behavior compared to that at the NN-01 section and the sections in the Ha Tri (Cao Bang) area when the concentration increases mainly from the boundary area of the two limonite zones and saprolite in the presence of relatively common clay minerals (nontronite, saponite) above (where goethite is abundant) instead of in the saprolite zone where "garnierite" minerals are present as in Ha Tri area (Cao Equal)
3.2.2. Level of lateritization in Nui Nua massifers
The results of S/SAF and UMIA index calculations show that the weathering process in Nui Nua (Thanh Hoa) blocks is of the laterritation type and is only weak to moderate, similar to that in Ha Tri (Cao Bang).
3.2.3. Nickel laterite mineralization in Nui Nua massif
3.2.3.1. Type A mineralization (hydrous Mg silicate)
In Nui Nua block, minerals of "garnierite" group are only discovered at cross-section NN-01, developed on hazbugite bedrock, but they are only present in small content at the depth not too great from 8.5 - 9 m. in weathered sections and difficult to distinguish with the naked eye, they are detected only by X-ray diffraction method. Corresponding to the presence of garnierite minerals, the nickel content ranges from 0.52 to 1.02%, which is quite low compared to the A-type mineralized section in Ha Tri, Cao Bang (1,25 -3.01% Ni), confirming the garnierite poverty of the zone.
3.2.3.2. Type B mineralization (clay silicate ore)
In the Nui Nua monolithic ultramafic weathering zone, nickel-containing clay minerals include nontrolite and saponite, instead of nontrolite and montmorillonite as in weathered sections in Cao Bang area. Nontrolit has an average NiO content of 1.44%, much higher than that of nontrolit in Ha Tri area (1.06%), which is quite high level of nickel in nickel containing clay . Saponite 13
Photo 4. Clay minerals in Nui Nua region under scanning electron microscope (SEM)
has a lower NiO content than nontrolit with an average content of 1.38%. From there, it shows that clay minerals formed during the weathering process of Nui Nua ultramafic rocks have very good ability to store nickel and this is the main form of nickel enrichment in the Nui Nua ultramafic rock weathering zone.
In two sections NN-01 and NN-03, nickel-containing clay minerals have a greater proportion at section NN-01 developed on hazbugite bedrock, where they are present in the range from 7m to 3m in magnitude. corresponding to a nickel content of 0.66 - 0.73% in the section. Whereas in the section NN-03 developed on dunite bedrock, clay minerals are present only at a depth of 3-4 m, corresponding to a nickel content of only 0.53-0.56% in the section. From there, it shows that the mineralization of nickel laterite of type B (clay silicate) has more manifestation in the weathered zone developed on the bedrock - hazbugite with the nickel content of 0.66 - 0.73% in the depth range from 7-3m in weathering crust. It can be said that nickel laterite mineralization of type B is the most potential here.
3.2.3.3. Type C mineralization (oxide type)
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The results of chemical composition analysis showed that goethite in Nui Nua region had an average nickel content of 2.07%, much higher than that of goethite in Ha Tri, Cao Bang (1.03%). With this nickel content, goethite at Nui Nua mass shows very good nickel storage, which is suitable for nickel enrichment in the whole limonite zone at cross sections as described in previous sections.
Although in both the NN-01 and NN-03 sections of Nui Nua area, there is nickel enrichment in the limonite zone, where geothite is very common, but the average nickel content in each section is. different. In the NN-01 section, the average nickel content in the entire limonite zone was 0.53% while in the NN-03 section growing on the bedrock, the average Ni content in the limonite zone was 0.64%. This shows that the nickel drift from the limonite zone in the NN-01 section is better than at the NN-03 section, from which the nickel in the NN-01 section moves downwards better and is enriched in the mineral phases. clay and garnierite at deeper sections in the section. Because the nickel drifting process from the limonite zone at the NN-03 cross section is less, this zone is richer in nickel and has a better prospect for nickel laterite mineralization type C (oxide type).
Chapter 4. Factors influencing nickel laterite ore formation and their prospects in the North of Vietnam
4.1. nickel laterite formation conditions in the study areas
4.1.1. Nickel supply for epigenetic enrichment
a. Olivine
- (4.2)
Olivine is the main silicate mineral with the ratio from 55 to 70% in the un-weathered lherzolite bedrock in Ha Tri (Cao Bang) and 50-59% in hazbugite, 85-95% in dunite in Nui Nua massif area ( Thanh Hoa). The NiO composition in olivine in Ha Tri block was quite high from 0.25-0.42% (average 0.31%), in Nui Nua from 0.22-0.33% (average 0.27%).
Nickel-rich olivines
During the weathering process, olivine is considered to be the least stable mineral and is altered by reaction 4.1 (Freyssinet et al. 2005) and 4.2 (Julie L. Baumeister, 2012): (FeMg1-x)2SiO4 + 4x H2O = H4SiO4 +2xFeOOH + (2-2x)Mg2+ +2xe-+(6x-4)H+ (4.1) Olivine Goethite (Mg,Ni)2SiO4 + 4H2CO3 = 2Mg2+ + 2 Ni2+ + H4SiO4 + 4HCO3
These are the reactions that transform olivine and release the elements Mg and Ni into the weathering solution so that the nickel enrichment process during weathering continues to develop. The relatively high Ni content in olivine in the 15
study area (average 0.31% NiO in Ha Tri and 0.27% NiO in Nui Nua) shows that this is the main source of nickel enrichment in the weathering process. in these areas.
b. Serpentine
In two study areas, serpentine is also a common mineral in Ha Tri and Nua Nua block bedrock, they are the main products of serpentineization, distributed around or gradually replacing olivine particles to form structures. characteristic mesh form. Serpentine is a fairly stable mineral during the weathering process, so at the first stage of weathering, nickel is not mobilized from serpentine as it does from olivine, however with available nickel content (average 0.17% in Ha Tri and 0.23% in Nui Nua) serpentine will be richer in nickel when weathered nickel substitutes for Mg in the structure of serpentine in the following stages, the formation of a serpentine-type "garnierite" mineral.
c. Nickel sulfide minerals
Nickel-containing sulfid minerals are only present in the ultramafic mass of Ha Tri (Cao Bang), including mainly violarite - (Ni,Fe)3S4 và một lượng nhỏ pentlandit - (Ni,Fe)9S8 . However, the nickel supply for epigenetic enrichment from these nickel sulphide diffusions is negligible and is evidenced by most of the major nickel laterite deposits in the world such as in New Caledonia, Philippines, Indonesia, Australia, ... are all formed from ultramafic silicate weathering without involving nickel sulfide accumulation.
4.1.2. Formations of nickel laterite in Ha Tri ultramafic mass
The formation process of epigenetic Ni ore in Ha Tri area can be
summarized as follows (Figure 2):
1. Initially less stable minerals, first olivine, then orthopyroxene and clinopyroxene weathered with the solubility and removal of Si, Mg, alkali elements, releasing Ni into the weathering solution simultaneously. precipitated Fe (goethite) minerals. Ni is adsorbed or isomer replacement for Fe in goethite to form C-type ore (oxide type).
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2. As the weathering continues, the goethite formed in the upper part is dissolved and re-precipitated with the release of Ni into the weathering solution. Ni is released in the process of dissolving goethite with the free Ni in the
weathering solution moving downwards thanks to high porosity and fissures. At the boundary between the upper limonite zone and the lower saprolite it is adsorbed by the common nontronite and montmorilonite clay minerals here to form B-type ore (clay type).
3. Next, the Ni-containing solution by "preferential flow" circulates through the joints or micro-faults in the saprolite zone to form garnierite in the form of adhesion or fill the joints to create type A ore (type silicate).
Figure 2 Model of nickel laterite formation in Ha Tri area (adapted from Wei Fua et al, 2018)
4.1.3. Formations of nickel laterite in Nui Nua
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In the section NN-01 developed on the bedrock, the serpentineized hazbugite is very strong, the mechanism of nickel laterite formation is quite similar to the one in the Ha Tri area developed on the bedrock, the serpentineized lherzolite at the moderation. However, unlike in Ha Tri area, the presence of garnierite minerals here is very limited and the role of A type ore is not great.
At the weathered section NN-03 developed on the bedrock, the very strongly serpentineized dunite, only type B and C ores can be observed, type A is completely absent. Here, in the limonite zone, Ni is enriched by adsorption mechanism or substitute for Fe in goethite, creating C-type ore. Ni content in goethite reaches 2.07%, while Ni content for the whole limonite zone ranges from 0.43 to 0.66%. In the boundary between the limonite and saprolite zones, type B ore develops with the Ni enrichment by adsorption mechanism by clay minerals mainly nontronite and saponite. Ni content in clay minerals reaches 1.4%, while Ni content for this part of the section is 0.55%.
4.2. Factors affecting the formations of nickel laterite ores
Lateritization of ultramafic rocks and nickel laterite ore formation is controlled by many different processes and conditions, both in properties and in scale. Therefore, the section weathered in different locations is very different in details such as: thickness, mineral composition, chemical composition and development of individual zones in the section. There are places where the typical zones of the weathered section are fully developed, and in some places, one or several zones are absent. The main factors that influence chemical weathering and nickel laterite formation include:
- Climate: The climate determines the amount of rainfall and thereby influences the amount of water passing through the soil, affecting the intensity of driftwood washing and removing dissolved components. Besides rainfall, the efficiency of rainwater (the amount of water that seeps down through the section instead of overflow) is very important. In addition, the average rock temperature is close to that of the surface air, increasing the dynamics of the weathering processes (Butt and Zeegers, 1992). The most favorable climate for nickel laterite mineralization is in the humid tropics.
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- Original stone type: The mineral composition, the chemical composition of the parent rock determines their susceptibility to weathering and the elements available for recombination into new minerals. The most favorable and common condition for the formation of nickel laterite mineralization are olivine-rich ultramafic rocks that are serpentineized or to a moderate extent. In which the formation of the highest quality mineral deposits - type A (hydrous Mg silicate) is preferred in the following order:
peridotite rocks (lherzolit, hazbugit) >> dunit
- Geomorphology: The most favorable geomorphology for nickel laterite mineralization is an area with moderately high geomorphology with low topographic cleavage with rounded peaks and slopes, which at the same time facilitate groundwater. Being able to move freely to facilitate the ore-forming process, while ensuring the ability to conserve mineralization before erosion
- Structure - tectonic: Accessibility of water to the bedrock (water circulation - weathering solution) is especially important for epigenetic Ni weathering and ore. Fractures and joints are formed by tectonic activities that act as channels of the weathering solution, the higher the degree of fracture, the greater the capacity to circulate the solution, and accordingly the stronger and stronger the weathering process. has conditions to grow deep into the bedrock. On the other hand, the joints in the saprolite zone are natural "traps" for the accumulation of hydrous Mg silicate minerals, which are an important component of A type ore - the type of ore of industrial significance.
4.3. Prospects of nickel laterite ore in the North of Vietnam
Although these are only preliminary studies on epigenetic Ni in the two regions Ha Tri and Nui Nua and evidently the data to evaluate the prospects of epigenetic Ni ore in the North of Vietnam in particular and in the country, in general, it is very limited. However, the prospect of epigenetic Ni ore can be initially assessed with orientation through the analysis and determination of the premises and factors that control the oreization. According to the research results, the top important premise for finding epigenetic Ni ore in our country can be determined, including the characteristics and composition of parent rock, climatic conditions, topographic features and tectonic structure.
4.3.1. Climate premise
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Among the above factors, the climate factor can be considered as the same for all northern regions of Vietnam, since millions of years ago, our climate was in a tropical climate and subtropical (Madelaine Böhme et al, 2013; Nguyen Xuan Huyen et al, 2004; Pham Nhu Sang et al, 2018), this is the first favorable premise for the search for epigenetic mineral deposits in Vietnam.
4.3.2. Original stone premise
The second important and easily accessible factor is the element of the bedrock characteristics and composition. As discussed in the previous section, ultramafic rocks with the most potential for nickel laterite formation are olivine- rich type variables, in which the highest quality mineral deposits are likely to form - type A ( hydrous Mg silicate) is preferred in the order: peridotite rocks (lherzolit, hazbugit) >> dunite. In the North of Vietnam, ultramafic rocks occupy a small part compared to other magmatic bodies and only show up in large structural zones such as Song Hien, Song Da, Song Ma ... with blocks of several hundred square meters in size up to several tens of square kilometers . In which the blocks have bedrock characteristics favorable for nickel laterite formation, including: ultramafic blocks of Cao Bang complex (Ha Tri, Suoi Cu, Phan Thanh), blocks of Ban Xang (Ban Phuc, Ban Khoa), blocks of Nua Nua and possibly other blocks of this complex in the River zone. Code, ultramafic blocks of the Ba Vi complex.
4.3.3. Topographic and geomorphological premise
The ultramafic masses in the Cao Bang area (Suoi Cun, Ha Tri, Phan Thanh) and the Ban Phuc ultramafic block (Son La) have relatively similar topographic features, with topographic elevation from 200-500m, moderately split with rounded vertices and slopes. These could be favorable areas for nickel laterite mineralization during weathering. Meanwhile, the geomorphology in Nui Nua area has lower average altitude but strongly cleaves it with steep slopes, which are less favorable conditions for nickel laterite ore formation.
4.3.4. Premise of tectonic structure
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The results show that ultramafic masses in Cao Bang area are located in areas with very high lineament density, followed by blocks in Son La area, while in Nui Nua the lineament density is quite low. The lineament density reflects the degree of rock breaking, so the ultramafic blocks in Cao Bang and Son La areas have a higher degree of crushing than that of Nui Nua. Thus, in terms of tectonic structure, the Nui Nua block has less favorable conditions than the blocks in Cao Bang and Son La. Among the ultramafic mass groups
mentioned here, perhaps the blocks in the Cao Bang area have the most favorable tectonic structure conditions.
4.3.5. Prospects of nickel laterite ore in the North of Vietnam
Table 4.2. Synthesize the factors that control the mineralization nickel laterite in a number of ultramafic bodies in Northern Vietnam
Epigenetic Ni ores are influenced by the synthesis of many factors that control the weathering process of ultramafic rocks such as bedrock characteristics (composition, serpentineization level), climate, geomorphology and tectonic structure. Each element has different effects on the weathering and accumulation of beneficial elements. Clearly, then, the more factors favorable for epigenetic Ni accumulation and weathering, the higher the prospects for this mineral. Results of synthetic assessment of the controlling factors of epigenetic Ni ore formation in some ultramafic blocks in the North of Vietnam are shown in Table 4.2.
Climate
Geomorphology
Oreization expression
Original rock characteristics
Structure, construction
Super mafic block
Advantage Advantage
Advantage
Advantage
Advantage Advantage
Advantage
Advantage
Advantage Advantage
Average
Advantage
Ha Tri (Cao Bang) Phan Thanh (Cao Bang) Suoi Cun (Cao Bang)
Advantage Advantage
Advantage
Advantage
Ban Phuc (Son La)
Advantage Advantage
Advantage
Advantage
Type A (hydrous Mg silicate) Finding Garnierite minerals as similar to that in Ha Tri area Finding the relative enrichment of Ni in the weathering crust Finding the type B mineralization (clay silicate) (Nguyen Khac Giang, 1999) ?
Advantage
Normal
Disadvantage
Normal
Type B (clay silicate) and type C (oxide)
Advantage
Normal
Advantage
?
?
Ban Khoa (Son La) Nui Nua (Thanh Hoa) Ba Vi complex blocks
From the results of synthetic assessment of control factors of epigenetic Ni ore, taking into account the premise and search signs, we can comment on the epigenetic Ni prospect as follows:
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- Ultramafic blocks of Cao Bang complex (Ha Tri, Suoi Cun, Phan Thanh) and
Ban Xang complexes (Ban Phuc, Ban Khoa) are the convergence of many favorable factors for laterritization and formation. epigenetic Ni mineralized more than anything.
Therefore, these areas are considered to be the most promising for epigenetic Ni ore.
In fact, remarkable epiphytic Ni ores have been found in these areas.
- Nui Nua and possibly other blocks of this complex in the Ma River zone have less favorable conditions, especially the conditions for the formation of the
mineralization type with the highest content. , type A (hydrous Mg silicate), therefore
may be less promising. However, with the presence of Ni epigenetic types B and C over a large area (~50Km2), Nui Nua block is still the subject of interest in evaluating the epigenetic Ni. In addition, in Nua Nua there is quite a development of ore types B
and C, because they are often located near the surface, they can be eroded and re-
accumulate in the lower adjacent areas to form Ni-rich sedimentary deposits. This is
also the object that needs research.
- The ultramafic blocks of the Ba Vi complex are mostly small in size and have
many unfavorable factors, so there is less prospect of nickel laterite mineralization
than the stated ultramafic blocks.
These are very rough initial statements, only for orientation. To fully evaluate
the prospects of epigenetic Ni in our country, more detailed and comprehensive studies are needed.
CONCLUSIONS AND RECOMMENDATIONS
1. The research results show that the weathering process in the Ha Tri (Cao
Bang) and Nui Nua (Thanh Hoa) blocks is of the laterritation type and is only weak to
medium. The S/SAF and UMIA index calculations confirmed this conclusion.
2. Weathering characteristics on the blocks Ha Tri (Cao Bang) and Nui Nua
(Thanh Hoa) have a clear difference. The weathered section developed on the bedrock
is the moderately serpentineized lhezolite in Ha Tri (Cao Bang), characterized by a thick saprolite zone containing many nickel laterite minerals of the garnierite group at open structures (fissure , fault, ..) with the average Ni content of 1.7%, the limonite zone has much lower Ni content (average 0.6%) with Ni-containing minerals mainly goethite and smectite (nontronite, montmorillonit). Meanwhile, the sections developed on the bedrock are apohazbugite and apodunite, which are strongly serpentineized in
Nui Nua (Thanh Hoa), where saprolite is often poor in Ni (average 0.5% Ni), mainly
enriched nickel. from the boundary of the limonite and saprolite zones to the entire 22
upper limonite zone with nickel-containing minerals mainly smectite (saponite,
nontronite) and goethite (average 0.7% Ni).
3. nickel laterite mineralization in the Ha Tri (Cao Bang) block includes three
types: type A (hydrous Mg silicate ore, with ore minerals mainly Mg-Ni silicate
hydrate (garneirit)), type B (clay silicate ore, mainly nickel-containing smectite) and
type C (Oxide-type ores, with ore minerals mainly nickel-containing Fe oxyhydroxides), the average Ni content is 1.7%, 0.8% and 0.6%, respectively.
However, type A ore is dominant in both size and content.
4. The nickel laterite mineralization in Nui Nua (Thanh Hoa) massif is diverse
depending on the original rock characteristics that the weathering zone develops. In the developed weathering zone on the base rock, apodunite only develops C-type ore
(type oxyt), with ore minerals mainly nickel-containing Fe oxyhydroxides and a part
of B-type ore (clay silicate type), dominated by smectite contains nickel, the average
Ni content is 0.63% and 0.55%, respectively. Meanwhile, in the developed weathering
zone on bedrock, apohazbugite has all three ore types A, B and C, but type A (hydrous
Mg silicate) has negligible scale, The amount of nickel in this ore is much lower than
that in the Ha Tri (Cao Bang) area (0.65%), while the B and C ore types also have
relatively low Ni content, respectively 0, 67% and 0,5%.
5. The difference in nickel laterite mineralization characteristics in the two ultramafic masses Ha Tri (Cao Bang) and Nui Nua (Thanh Hoa) is mainly due to the
differences in the original rock composition, topographical characteristics and
anticatonic conditions of two regions. The Ha Tri block has the base rock composition
of the moderately serpentineized lhezolite, which is more favorable for the
lateritization and epigenetic enrichment of nickel, while the base rock in Nui Nua
block is apohazbugite and apodunite, which is very strongly serpentineized. more
favorable for the lateritization and epigenetic enrichment of nickel. The topographical
conditions - geomorphology and tectonic structures in the Ha Tri block are more
favorable for the lateritization development and formation as well as the conservation of epigenetic Ni mineralization compared to the Nui Nua massif.
6. Epigenetic Ni ores are influenced by the synthesis of many factors that control the weathering process of ultramafic rocks such as bedrock characteristics (composition, serpentineization level), climate, geomorphology and tectonic structure. Each element has different effects on the weathering and accumulation of beneficial
elements. According to the favorable factors for the weathering process and formation
of epigenetic Ni mineralization, the blocks belong to the Cao Bang Complex in
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Northeast Vietnam (Ha Tri, Suoi Cu, Phan Thanh, etc.) and the blocks belong to Ban
Xang complex in Northwest Vietnam (Ban Phuc, Ban Khoang, etc.) is the most promising for epigenetic Ni, especially type A ore. Nui Nua and other blocks
belonging to this complex in the Ma River zone are less promising. Particularly, Nui
Nua block is of low prospect, but due to its large area, the epigenetic Ni ore here is still
the object that needs attention. Ba Vi complex ultramafic blocks are of the least promising type.
Recommendations
The results stated in the thesis are only initial studies on the formation of nickel
laterite ores placed in the combined effects of factors affecting the formations (climatic topographical conditions, especially original
rock points and composition,
characteristics, tectonic conditions, ...) in the two areas of Nua Nua (Thanh Hoa) and
Ha Tri (Cao Bang). There is a necessity for more detailed studies, a broader range of studies on this important mineral.
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