Ni(Co) Laterite Deposits of Southeast Asia: A Review and Perspective

Abstract

Southeast Asia has great potential for mineral exploration, and this region is well-known to host huge economic ore deposits located in complex tectonic terranes. Amongst these ore deposits, the Ni(Co) laterite deposits are mainly distributed in Indonesia, the Philippines, and Myanmar. There are two main types of Ni(Co) laterite deposits consisting of hydrous Mg silicate (or garnierite) and oxide ores, with limited development of clay silicate type. These deposits are influenced and controlled by the lithology of ultramafic bedrock, topography, climate, weathering, structures, and tectonic environment. The degree of bedrock serpentinization has an important influence on the grade of Ni laterite ore. Given the growing demand of modern society for Ni(Co) ore resources, deep research should be focused on a better understanding of the genesis of this laterite deposit and geological features of Ni(Co) ore, as well as its exploration applications in southeastern Asia. Improving current research and exploration methods by means of cutting-edge technologies can enhance the understanding of the Ni(Co) enrichment mechanism in weathered laterite and lead to the discovery of new deposits in Southeast Asia. Ni(Co) laterite deposits from this region, especially Indonesia and the Philippines, have the potential to be a source of scandium, rare earth elements, and platinum group elements.

1. Introduction

Nickel (Ni) and cobalt (Co) are critical metals that are widely used for aerospace, architectural, industrial, marine, military, and transport applications. The Ni(Co) laterite deposits produce significant amounts of nickel compared to cobalt [1]. Nickel metal is generally derived either from nickel sulfide ore or from laterite nickel ore, with the nickel sulfide deposits being the main source of nickel ore due to their high grade, low impurity, and easy metallurgy [2,3]. Recently, the decline in nickel sulfide resources has led to the development of new sources of Ni in anticipation of the future demand for this metal [1,2,3]. Thus, nowadays, the Ni(Co) laterites are important exploration targets, which are stimulated by the strong world economic growth. The Ni(Co) laterites are mid-Tertiary to recent supergene deposits that are widely distributed in southern and northern America (Brazil, Colombia, and USA), the Caribbean (Dominican Republic and Cuba), southeastern Europe (Albania, Bosnia Herzegovina, Cyprus, Greece, Kosovo, North Macedonia, Serbia, and Turkey), western Africa (Cameroon and Ivory Coast), western Asia (Iran and Oman), southeastern Asia (Indonesia, the Philippines, Myanmar, and China), and southern Pacific (Australia, New Caledonia, and Papua New Guinea) [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20].

Southeast (SE) Asia is located at the junction between the Tethys and Circum-Pacific metallogenic domains, and it is well-known for being an important metallogenic belt with huge mineral resources, such as aluminum, copper, chromium, iron, gold, lead, nickel, potassium, tin, and zinc [21]. Laterite nickel deposits are widely developed in SE Asia, with Indonesia containing the world’s largest published Ni ore reserves (21,000,000 tons Ni) and is considered the world’s largest Ni producer with 26% of the Ni global mine production (760,000 tons Ni) [22]. The Philippines is the world’s second-largest Ni producer (~14% of global Ni mined), with an annual production of 320,000 tons and published Ni reserves of 4,800,000 tons [22]. Ni laterite deposits are well-developed in Myanmar, while Ni-Mg laterite deposits are found in China. The later Ni type is beyond the scope of this paperand, thus, is not included.

In recent years, with the increase in nickel demand driven by the strong growth recorded by the Chinese economy, numerous studies have been conducted to investigate Ni(Co) laterite deposits in SE Asia. Previous studies have well documented these Ni(Co) laterite deposits in terms of geological, mineralogical, and geochemical characteristics, as well as the ore-forming process and exploration techniques [23,24,25,26,27,28,29,30,31,32,33,34]. Although these studies have provided a good foundation for further research and exploration work, few have proposed a brief summary of Ni(Co) laterite deposits in SE Asia, whereas most published literature is available in Chinese.

In this work, we provide a review of the tectonic settings, metallogenic belts, and magmatic rock and ore types for the Ni(Co) laterite deposits in SE Asia. A comprehensive analysis of the influence of protolith, climate, geomorphology, and structure on the development of favorable weathering profiles is further discussed in this region, as well as the role of serpentinization on the grade of Ni laterite ore. Then, we highlight the potential of Ni(Co) laterite deposits of SE Asia to be a source of critical metals, such as Sc, rare earth elements (REE), and platinum group elements (PGE). Perspectives are also proposed for future research and exploration work on these Ni(Co) laterite deposits.

2. Geological Background and Metallogeny

2.1. Tectonic Settings

SE Asia has a complex tectonic pattern built from the interaction between the Eurasian, Indian–Australian, Pacific, and Philippine Plates (Figure 1). Detailed tectonic settings of SE Asia are discussed in previous publications [21,35,36,37,38,39,40,41] and are briefly summarized below.

SE Asia consists of complex structural units (Indochina, Nambung, Sibumasu, and West Myanmar terranes) resulting from the subduction and collision between the Pacific and Indian–Australian plates toward the Eurasian Plate, while the opening and closure of the Paleotethys and Neotethys led to multiple suture zones between these terranes since the late Paleozoic [21]. The structural units of SE Asia also comprise separated terranes (Sundaland and Philippine) and associated island arcs and arc-continental collision belts [38]. To the northern edge of SE Asia, the Indochina and Sibumasu terranes collided to form the Paleotethys suture zone, while the Sibumasu and West Myanmar terranes converged to form the Sagaing tectonic zone at the end of Mesozoic [41]. The central volcanic arc belt of Myanmar and the Andaman–Sumatra–Java–Banda volcanic arc belt were formed as a result of subduction of the Indian–Australian plate under the Eurasian plate [35]. To the southern margin of SE Asia, the Philippine Islands arc belt and the Philippine Trench resulted from the subduction of the Pacific and Philippine plates into the Eurasian plate [40]. This compressive tectonic had further caused the development of the Philippine Islands–North Sulawesi volcanic belt and suture zones, such as the Borneo and Sulawesi sutures, which recorded the development, subduction, collision, and extinction of oceanic basins [40,41,42,43]. From the west of the Philippines to Kalimantan, the Philippine Islands subducted and squeezed in both sides of east and west directions, leading to the NW–SE directed subduction of the North Sulawesi Island and the development of the Manila trench and the Northwest Borneo, North Sulawesi, and Sulu troughs in this region [44].

2.2. Metallogenic Belts and Spatio–Temporal Distribution

The Ni(Co) laterite deposits in SE Asia are developed in tectonically active plate collision zones or accretionary terranes, where the host ultramafic rocks and weathering processes range in age from Cretaceous to late Tertiary [2]. A summary of the metallogenic belts and geological features of Ni(Co) laterite deposits in SE Asia is given in Table S1 and discussed below.

The distribution of the Indonesian Ni(Co) laterite deposits is controlled by the spatial occurrence of ophiolites (Figure 2) [45,46,47]. Based on the lithostructural settings of these deposits, they can be regrouped into three metallogenic belts from west to east (Figure 2): (i) the Eastern Kalimantan (e.g., Pulau Sebuku Ni deposit), (ii) Sulawesi (e.g., Sorowako, Kolonodale, and Moraweli Ni(Co) deposits), and (iii) Northern Maluku (e.g., Weda Bay and Irian Barat Ni deposits) metallogenic belts [21,26,27,34,48,49,50,51,52]. The Eastern Kalimantan metallogenic belt is an arc-continental collision belt characterized by Jurassic-Miocene Subduction ophiolite suites, mélange accumulation, unconformably overlain by Neogene, Oligocene granites, and deep-water sediments [21]. The Sulawesi metallogenic belt consists of subduction-magma and an arc-continental belt composed of an eastern magmatic complex, northern Cenozoic arc volcanic rocks, porphyry-related quartz-rich diorites, and western arc-continental collision I-type granites [21]. The Northern Maluku metallogenic belt comprises the Waigeo and Gag Islands, the northern half of the Irian Jaya Island and the western and eastern half of the Halmahera Island (Figure 2), which form magmatic arc and subduction complexes made up of Paleozoic metamorphic rocks (granitic gneisses, mica schists, and phyllites) unconformably overlain by ultrabasic rocks of andesitic and adakitic composition, Tertiary ophiolites and felsic intrusions, and Triassic–Lower Cretaceous to Upper Cretaceous–Upper Tertiary sedimentary rocks, including limestones and clastic rocks [21,40,45,46].

The Ni(Co) laterite deposits in the Philippines are distributed along ophiolite complexes. Based on their lithostructural settings, these deposits are regrouped into two distinct metallogenic belts from north to south (Figure 3): the Philippine Mobile Belt (e.g., Sta. Cruz Ni and Intex Ni-Co deposits) and the Palawan–Mindoro Continental Block (e.g., Berong Ni-Co, Danao Ni, Dinagat Ni, Rio Tuba Ni, and Surigao Ni deposits) [23,28,30,31,32,33,53,54]. The Philippine Mobile Belt was formed from the amalgamation of island arcs and north-south trending, east-dipping ophiolite suites, including a succession of ultramafic and mafic cumulates, residual harzburgites, lherzolites, dike-sill complexes, and volcanic rocks [55,56,57,58]. The Palawan–Mindoro Continental Block consists of Late Paleozoic metamorphic rocks comprising quartzites, schists, phyllites, and slates, which are unconformably overlain by Lower Triassic to Lower Miocene clastic sedimentary rocks such as sandstones, limestones, slates, tuffs, and cherts [21,54,59]. Amphibolites, mica schists, and ophiolites are exposed to the south of this block, and they are associated with Late Cretaceous to Early Miocene shales, mudstones, limestones, and sandstones [60,61,62,63]. The Palawan–Mindoro Continental Block is separated from the Philippine Mobile Belt by the Mindoro suture resulting from their arc-continent collision that occurred during the Miocene [64,65].

To the west of SE Asia, Myanmar is tectonically located between the Indian Plate and the South China Block [35]. Ni laterite deposits are widespread in Myanmar where they are distributed in three sub-parallel N–S-trending ophiolite belts from west to east (Figure 4): the Western Ophiolite Belt (e.g., Mwetaung and Mindinkyin Ni deposits), Central Ophiolite Belt (Budaung and Indawgyi Ni deposits), and Eastern Ophiolite Belt (Tagaung Taung and Tauggadon Ni deposits) [20,66]. These Ni laterite deposits are well distributed along ophiolite complexes consisting of Late Cretaceous–Early Eocene mafic and ultramafic igneous rocks associated with Cretaceous–Quaternary sediments [40,66,67]. These ultramafic rocks are underlain by high-grade metamorphic rocks (gneisses, migmatites, and some quartzites) and granitoids, and they have favorable metallogenic conditions for the formation of high-grade Ni laterite deposits [20,66,67].

2.3. Ultramafic Bedrock Types and Characteristics

Cretaceous ophiolite-related ultramafic rocks are widespread in all metallogenic belts of Indonesia, including the Eastern Kalimantan, Sulawesi, and Northern Maluku belts (Figure 2) [21,68,69,70]. These ultramafic rocks form the bedrock from which the Ni laterite deposits are developed. These rocks mainly comprise 70% peridotites made of harzburgite, lherzolite, and minor dunite and 30% pyroxenites and gabbros, which have undergone regional high-grade metamorphism and strong weathering resulting in the formation of serpentinites [26,27,34,52,69,71]. In the Sulawesi belt, the Kolonodale and Morowali Ni laterite deposits are hosted by harzburgite and lherzolite [26,27,34], while the Sorowako Ni laterite deposit is developed on harzburgite and dunite [71]. The Weda Bay Ni laterite of Northern Maluku belt is also developed on dunite and harzburgite [52]. Serpentine, olivine, orthopyroxene, and, in some cases, clinopyroxene are dominant in peridotites of the Kolonodale and Weda Bay Ni laterites [26,52]. The pyroxenites of the Morowali Ni laterite deposit principally contain orthopyroxene as the major mineral, while serpentine occurs as an alteration product of orthopyroxene [34]. Talc-like phases are also observed in peridotite bedrock of the Kolonodale and Morowali Ni laterite deposits [27,52]. The geochemical characteristics of ultramafic rocks of Indonesia are scarce in the literature.

In the Philippines, the Ni(Co) laterite deposits are developed on the underlying mafic to ultramafic rocks comprising peridotites (harzburgite, dunite, and lherzolite), pyroxenites, and gabbros, which are found in ophiolite complexes of the Philippine Mobile Belt and the Palawan–Mindoro Continental Block (Figure 3) [30,31,32,33,47,72]. The Sta. Cruz Ni laterite deposit of the Philippine Mobile Belt is developed on harzburgite with minor chromitite and sporadic dunite lenses [31]. Harzburgite mainly consists of olivine and orthopyroxene, which are altered to serpentine and talc-chlorite, respectively [31]. At the Mindoro suture zone, the bedrock of Intex Ni-Co laterite deposits comprise lherzolite, harzburgite, and dunite, with serpentine veins cutting primary minerals in all types of bedrock [32]. Lherzolite mostly contains talc, olivine, pyroxene, chromite, and Fe-oxide [32]. Harzburgite is characterized by olivine, orthopyroxene, and clinopyroxene, while dunite mainly consists of olivine and minor orthopyroxene [32]. The Berong Ni-Co laterite deposit of the Palawan–Mindoro Continental Block is developed on the harzburgite bedrock containing serpentine surrounded by olivine, pyroxene, and chromite [33]. The geochemistry of the Philippine ultramafic rocks is poorly studied. Some ophiolite suites, such as the Eocene Zambales Ophiolite Complex, display a bimodal geochemical signature with island arc tholeiite and transitional from a mid-ocean ridge basalt to island arc affinity [56,64].

Petrography and geochemistry of ultramafic rocks from which Ni laterite deposits developed are documented in Myanmar [20,67,73,74,75]. In the Central Ophiolite Belt, the weathering profile of the Budaung Ni laterite is developed on serpentinite, likely metamorphosed harzburgite [20]. Serpentine with a small amount of chromite and magnetite is common in the Budaung bedrock, which contains 36.1 wt.% SiO₂, 40.2 wt.% MgO, and 0.38 wt.% NiO [20]. The Tagaung Taung Ni laterite deposit of the Eastern Ophiolite Belt was formed on massive serpentinites and peridotites composed of harzburgite and dunite [20,67,73,74,75]. The main minerals in these rocks are olivine, serpentine, orthopyroxene, smectite, montmorillonite, and minor amounts of enstatite, magnetite, and chromite [20,67,74]. The Tagaung Taung serpentinites generally contain 38.9 wt.% SiO₂, 40.2 wt.% MgO, and 0.27 wt.% NiO [20]. These rocks generally occur in the mountain ridges and slopes, while the peridotites are exposed at lower topography [67,74].

2.4. Ore Deposit Type and Ore Distribution

Ni(Co) laterite deposits in SE Asia typically display three lithostratigraphic layers of variable thicknesses developed over the ultramafic bedrock, which are from the bottom to the top (Figure 5): the saprolite layer (0.3–24 m), the limonite layer (2–18 m), and the ferruginous cap (0.2–1 m). It should be noted that these lithostratigraphic layers may not be completely developed in all weathering laterite profiles (Figure 5). Ni ore is generally found in the saprolite and limonite horizons and at the transition zone between the saprolite and limonite layers [20,26,27,28,29,30,31,32,33,34,52,53,67,73,74,75].

The Ni laterite deposits of Indonesia mostly consist of high-grade hydrous Mg-silicate type with 2–3 wt.% Ni and exceptionally 5–10 wt.% Ni [26,27,34,52,71]. Low-grade oxide limonite types are further found with 0.97 wt.% Ni and 40.09 wt.% Fe [76]. The Ni-bearing phases in these deposits are mainly composed of serpentine (lizardite, polygonal serpentine, karpinskite, and népouite), talc (kerolite and pimelite), olivine, pyroxene, goethite, hematite, and chromite, with less amount of chlorite and amphibole [26,27,34,50,52,71]. A mixture of serpentine-like and talc-like phases known as garnierite is common in the saprolite horizon of the Sorowako, Kolonodale, and Morowali Ni deposits in Indonesia [26,27,34,50,52,71]. Although high Ni contents are noticeable in garnierite of the saprolite layer, Ni is preferentially enriched in the talc-bearing phases rather than the serpentine-bearing phases [27,34]. The Ni content is generally higher in the saprolite layer than in the limonite layer, where Ni is hosted by goethite and hematite [26,27,34,52]. The limonite layer is dominated by goethite with minor hematite, while the saprolite layer is dominated by serpentine and garnierite with minor goethite [26,27,34,52]. The Ni content of residual serpentine of the saprolite layer can be up to six to seven times higher than that of primary serpentine of the bedrock [26,27]. Higher Ni contents can be found in olivine (0.1–0.3 wt.% Ni) and serpentine (0.3–3.0 wt.% Ni) [27,71].

Besides the traditional laterite Ni deposits of Indonesia, gravel lateritic Ni ore deposits have been reported in the Sulawesi metallogenic belt in northern and western of the Sulawesi Island [77,78]. From the bottom to the top, the structure of the deposit includes fresh to semi-weathered conglomerate, strongly weathered conglomerate, and laterite layer [77,78]. The Ni ore body is mainly distributed in the semi-weathered conglomerate layer [77,78].

In the Philippines, Ni(Co) laterite deposits mostly belong to high-grade hydrous Mg-silicate type, with local development of oxide and clay silicate deposits [28,30,31,32,33]. Ni ore is principally associated with Fe-oxide/oxyhydroxide (goethite and hematite) and Mn-oxyhydroxide (asbolane and lithiophorite–asbolane intermediate) phases in the limonite layer, with Ni contents up to 0.85 wt.%, 1.71 wt.%, and 23.89 wt.% in hematite, goethite, and lithiophorite–asbolane intermediate, respectively [30,31,32,33]. In the limonite horizon, Co ore is also hosted by goethite, hematite, and lithiophorite–asbolane intermediate, with Co content up to 0.90 wt.%, 1.13 wt.%, and 12.59 wt.%, respectively [31,32,33]. In the saprolite layer, Ni ore is hosted by serpentine and garnierite [31,32,33]. Garnierite generally belongs to the sepiolite-like (sepiolite–falcondoite) and talc-like (kerolite–pimelite) series and occurs as vein or fracture fillings in the saprolite layer of most Philippine Ni(Co) deposits [30,31,32,33]. Garnierite consisting of a mixture of serpentine-, kerolite-, and sepiolite-like clay is characterized by higher Ni contents up to 6.11 wt.% [32,33].

The Ni laterite deposits in Myanmar, including the Tagaung Taung, Budaung, Dagongshan, and Mwetaung deposits, are of hydrous Mg-silicate type and locally oxide type [24,67,73,74,75,79,80,81], although the Tagaung Taung deposit is considered as a hybrid of the hydrous Mg and clay silicate type [20]. The later Ni laterite deposit may host clay silicate ore type based on the crystal chemistry of smectite and its occurrence in the weathering profile [20]. Ni orebody is generally distributed in the saprolite and limonite layers developed on serpentinized peridotites [67,74,79,80,81]. At the Tagaung Taung deposit, Ni ore mostly occurs in the saprolite layer, which contains more than 1.4 wt.% Ni hosted by serpentine, smectite, and talc, while goethite is abundant in the limonite horizon, with a mean Ni content of 1.24 wt.% [74,80].

3. Factors Controlling the Development of Ni(Co) Laterite Deposits

3.1. Lithology, Geomorphology, Climate, and Structure

Previous studies have demonstrated that the enrichment of Ni in the weathering profile is controlled by the interplaying of several factors, including the parent rock, climatic regime, geomorphology, drainage, chemistry/rate of chemical weathering, and structure [2,3,82,83,84,85,86,87]. A comprehensive analysis is conducted below to understand how the above factors interplay to produce Ni(Co) laterite deposits in SE Asia.

The distribution of the Ni(Co) laterite deposits is generally controlled by the bedrock type and topography in SE Asia [26,27,30,31,32,33,34,71,74,79,80,81]. The deposits are characterized by underlying unserpentinized ultramafic rocks (lherzolite, harzburgite, and dunite) containing 0.25 to 0.35% Ni hosted by variably serpentinized olivine and pyroxene [26,27,30,31,32,33,34,71,74,79,80,81]. The high content of pyroxene in bedrock has promoted the formation of high-grade Ni ores in Myanmar [20], whereas the abundance of olivine in parent rock is considered to be one of the most important factors in the formation of high-grade Ni ores in the Philippines and Indonesia [26,27,30,31,32,33,34,71]. Most Ni(Co) laterite deposits are developed on serpentinized ultramafic rocks located in plateaus and terraces or areas with moderate to low-lying relief, while hills and ridges or areas with high-lying relief are not conducive to the development of laterite profile [26,27,30,31,32,33,34,71,74,79,80,81]. In the Philippines, terrain with a slope of less than 30° has a well-developed laterite profile above the ultramafic rocks [32]. The tropical climate of SE Asia is characterized by heavy monsoon rain (1300 to >1800 mm) and dry weather (25–30 °C) with some short showers. This climatic regime involving humid and hot tropical conditions of high rainfall and warm temperatures promotes a prolonged and pervasive chemical weathering process of ultramafic rocks to form a thick lateritic profile comprising from the bottom to the top: the saprolite, limonite, and ferruginous cap layers developed over the serpentinized bedrock [26,27,30,31,32,33,34,71,74,79,80,81]. These lithostratigraphic layers are not often developed in all weathering laterite profiles (Figure 5), and some layers locally may not be observed due to the discrepancy in erosion or weathering processes [26,27,30,31,32,33,34].

The weathering is influenced by the geomorphology, climate, Eh and pH of percolating water, and mineralogical characteristics of the parent rock [88]. In SE Asia, the weathering of bedrock is stronger and more aggressive in lower topography and low land depressions, where water percolates downward along preferential biological and geological pathways, which represent the main channel of Ni activation and vertical migration [26,27,30,31,32,33,34,74,80]. Differences in the mineralogical composition of bedrock can result in contrasting types of chemical weathering [2]. Taking the case of the Myanmar Ni laterite deposits, the Tagaung Taung serpentinites have undergone more intensive chemical weathering relative to the Budaung serpentinites, likely metamorphosed harzburgites, due to the slower dissolution rate of serpentine relative to olivine and pyroxene [20]. The saprolite and limonite layers of Ni laterites of SE Asia generally display an increase in Ni contents relative to those of the bedrock, which can be explained by ion exchange processes between the weathering solution and primary minerals [27,30,31,32,33,34,67]. During the chemical weathering process of the parent rocks, circulating water triggers the alteration of primary minerals and the most soluble elements (Mg, Ca, and Si) are leached, whereas the least soluble elements (e.g., Ni, Co, Cr, Fe, Al, and Ti) are concentrated, with Ni leaches from olivine, pyroxene, and serpentine, then reprecipitates to form garnierite [26,27,30,31,32,33,34,74,80,89]. As the pH of percolating water decreases, the mineral solubilities increase, and Ni is prone to be released from limonite into percolating water under acid conditions, leading to Ni enrichment in the saprolite layer [26,27,30,31,32,33,34,74,80,89]. In the Philippine Ni laterite deposits, mildly acidic environmental conditions (pH ≤ 6.57) in the limonite layer have enhanced the mobilization and re-distribution of Ni from the limonite horizon towards the saprolite layer of the weathering profile [23,32].

Significant fracturing was developed in the bedrock under the influence of long-term tectonic processes that have affected SE Asia (see Section 2.1). The existence of faults, fractures, joints, and fissures in the bedrock provides favorable conditions for percolating water that promotes the decomposition and denudation of these rocksand enhances the weathering of primary minerals (e.g., olivine and pyroxene), which are unstable in humid and hot tropical environments [27,31,32,33,34,71,74,80,81,89]. In SE Asia, the thickness of the Ni weathering profile developed over the bedrock is generally controlled by the topography and degree of erosion, whereas the Ni grade is influenced by a combination of the type of bedrock, topography, and structure [26,27,30,31,32,33,34,71,74,80]. For instance, the mean thickness of the limonite layer tends to increase from steep to flat topography, while that of the saprolite layer does not correlate with the topographic slope in the Sorowako Ni laterite deposit of Indonesia [71]. At Sorowako, high Ni grades were found to be commonly concentrated in the saprolite layer developed over dunite (2.36% Ni), having more olivine than harzburgite (0.95%–2.02% Ni), under slight slope topography with a slope angle of 5–19°, typical of the most strongly fractured and deeply weathered areas at Sorowako [71]. In SE Asia, the Ni(Co) ore generally occurs in garnierite present in the joints and fractures at the lower and upper parts of the saprolite horizon and in the limonite layer [23,26,27,30,31,32,33,34,74,80,89]. Garnierite may locally extend into the underlying bedrock layers along the fractured zone where a high density of joints or fractures occurs. Fieldwork and mineralogy indicate that garnierite occurs at the lower part of the saprolite layer of a serpentinite-derived regolith in SE Asia [27,31,32,33,34,89]. A study on garnierite of the Kolonodale Ni laterite deposit of Indonesia has shown that the origin of garnierite is linked to a preferential flow of oversaturated solutions through accessible conduits in the regolith [27]. At Kolonodale, the colloidal nature, high organic matter, and low pH of aqueous fluids along preferential flow pathways in garnierite-bearing rock are suggested to be the key markers in the transport and deposition of Ni ore [27].

In summary, most Ni(Co) laterite deposits of SE Asia are located in hill and low mountain topographic regions, and they formed during deep erosion and strong chemical weathering of serpentinized and unserpentinized ultramafic rocks, such as harzburgite, lherzolite, and dunite. The combined effects of lithology, geomorphology, climatic conditions, and structure commonly produce a vertical sequence with horizons, including the saprolite, limonite, and ferruginous cap layers. The humid and hot tropical climate of SE Asia is the weathering force condition for the formation of a thick Ni-rich laterite profile, while the flat topography combined with significant fracturing in the bedrock provides channels for Ni activation, leaching, precipitation, and redistribution along the laterite profile. The mobility of elements is governed by the preferential flow pathways and the whole structure of the laterite profile, which provide ideal conditions for preferential Ni (Co) enrichment in the weathered layers.

3.2. Degree of Serpentinization of Bedrock and Related Ni-Co Contents

The distribution of Ni(Co) laterite deposits in SE Asia is linked to the spatial distribution of ophiolitic complexes (Figure 2 and Figure 3), where similar protoliths, topography, climatic regime, and structural conditions prevail, especially in Indonesia, the Philippines, and Myanmar [26,27,30,31,32,33,34,52,67]. However, Ni enrichment is strongly related to the development of saprolite layer, which is mainly controlled by the degree of serpentinization and fragmentation of the bedrock [23,26,27,28,30,31,32,33,34,52,67].

The degree of bedrock serpentinization has an important influence on the grade of Ni laterite ore [32,67,89], which is further controlled by the combined effect of bedrock type, structure, and topography (see Section 3.1). Moderate to higher degree of serpentinization provides the condition for the development of high-grade Ni ore, while no or low degree of serpentinization is conducive to the development of low-grade Ni ore [67]. The Ni content is related to the alteration and weathering degree of the rock, and the Ni content can reach above the industrial grade in the parts where serpentinization is strong and Ni silicate veins are well developed [32,33,34,67,74,89]. When comparing Ni laterite developed on different bedrock (Figure 6), the greatest Ni enrichment is generally observed in the saprolite horizon, or rarely limonite layer, developed on the serpentinized harzburgite (Ni up to 4 wt.%), serpentinite (Ni up to 2.9 wt.%), lherzolite (Ni up to 2 wt.%), harzburgite (Ni up to 2 wt.%), harzburgite–dunite (Ni up to 1.9 wt.%), and clinopyroxenite (Ni up to 1.5 wt.%) bedrock [32,74,89]. The Ni contents increase from the lower part to the upper part of the saprolite layer and decrease at the saprolite–limonite transition zone (Figure 6c–e) [27,74,89]. The high Ni gain in the saprolite layer is due to the downward migration of dissolved Ni from the limonite into the saprolite [27,74]. In a few cases, the maximum enrichment occurs at the bottom of the lower saprolite [67].

Co is mainly enriched (up to 8 wt.%) in the lower section of the limonite layer developed on the serpentinized harzburgite bedrock in Indonesia (Figure 6b) [89], while minor Co enrichment (up to 0.16 wt.%) was documented in the upper part of the limonite layer developed over the lherzolite, harzburgite–dunite, and clinopyroxenite bedrock in the Philippines (Figure 6d–f) [32]. Co contents up to 0.9 wt.% were also reported in the lower section of the limonite layer in the Berong Ni-Co laterite profile of the Philippines [33]. In contrast to other deposits of SE Asia, the Ni laterite deposits of Myanmar are not enriched in Co [24,67,74,79,80,81].

4. The Potential for Critical Metals of Ni(Co) Laterite

Apart from Ni and Co ore, Ni(Co) laterite deposits of SE Asia are variably enriched in critical metals (Sc, REE, and PGE) in Indonesia and the Philippines [89,90,91,92,93]. Although their contents are low compared with conventional critical metal ore deposits (Table S2), they may be of economic significance as these critical metals are considered to be cost inexpensive by-products during the Ni(Co) production [94].

Indonesian Ni laterite deposits host scandium, which is generally associated with the occurrence of iron oxide from pyroxene [91,92]. Pyroxene-bearing bedrock (harzburgite in composition) has Sc content of 15 ppm, while it reaches up to 87 ppm in the limonite layer developed on this ultramafic rock [91]. In Southeast Sulawesi, the Lapaopao Ni laterite hosts Sc content of 15 ppm and up to 81 ppm in the ultramafic bedrock and overlying limonite layer, respectively [92]. Based on detailed investigations of four Ni laterite profiles (Konde, Petea, Watulabu, and Willson) in the Soroako and Pomalaa mining areas, Ito et al. [89] have revealed that Sc contents vary from 24 to 81 ppm (mean of 38–64 ppm) in the limonite layer, which is consistent with that in the underlying bedrock. Moreover, significant contents of Sc hosted by goethite were reported in the limonite layer of Ni laterite deposits from the Philippines, such as the Surigao Ni laterite (31.1 to 59.4 ppm, mean = 47.1 ppm), Zambales Ni laterite (67.2 to 79.5 ppm, mean = 73.0 ppm), and Pulang Daga Ni laterite (68.7 to 89.3 ppm, mean = 75.5 ppm) deposits [93,95]. The average Sc content for Ni laterite deposits of SE Asia is comparable to that found in New Caledonia (36.72 ppm for the Koniambo Ni deposit; [96]), Greece (42 and 64 ppm for the Larymna and Evia Ni(Co) deposits, respectively; [18]), Cuba (61.75 ppm for the Punta Gorda Ni deposit; [94]), Dominican Republic (72.65 and 82.08 ppm for the Loma Caribe and Loma Peguera Ni deposits, respectively; [94]), but higher than that (21.71 ppm; [13]) for the Loma de Hierro Ni(Co) deposit in Venezuela (Table S2, Figure 7a).

The Ni laterite deposits in the Philippines contain low total REE (ΣREE) content with an average of 13.8–45.3 ppm [93], which are similar to that reported for the Konde, Petea, Watulabu, and Willson Ni laterite deposits of Indonesia having an average ΣREE content of 3.50–29.83 ppm [89] (Table S2). The average ΣREE content of these Ni laterites is comparable to that found in New Caledonia (Koniambo Ni deposit: 5.61 ppm; [96]) and Venezuela (Loma de Hierro Ni(Co) deposit: 23.17 ppm; [13]), but very low compared to that of the Larymna Ni(Co) (773.88 ppm), Punta Gorda Ni (861.93 ppm), and Loma Caribe Ni (1047.41 ppm) and Loma Peguera Ni (1180.09 ppm) deposits in Greece, Cuba, and Dominican Republic, respectively [18,94] (Table S2, Figure 7a). Therefore, the REE beneficiation of Ni laterite deposits of SE Asia is economically unviable when compared to the REE contents of 500–3000 ppm for ion-adsorption REE (iREE) deposits having substantial economic value [97]. This can be explained by the absence of typical REE-bearing minerals (e.g., allanite, bastnäsite, gadolinite, and titanite) in the Ni laterite deposits of SE Asia, but well distributed in iREE deposits [97]. In the Ni laterite deposits of Indonesia, it is generally suggested that REE were released from goethite near the ferruginous cap of the laterite profile and accumulated by adsorption onto Mn-rich oxyhydroxides in the transition zone between the saprolite and laterite layers, whose processes were governed by the dissolution and recrystallization of goethite enhanced by weathering [89]. This process may have contributed to different degrees of REE enrichment, consistent with the lower REE content of the more weathered profiles (Konde and Petea) relative to the less weathered profiles (Watulabu and Willson) [89].

The Ni laterites of Indonesia display relative enrichment of Pt and Pd (mean content of 32–51.9 ppb; [89]), which is comparable to that for the Ni laterites of Northern Oman Mountains in Oman (46.28 ppb; [14]) but lower than that of Ni laterites in Cuba (109.93 ppb, Punta Gorda) and Dominican Republic (130.90 and 142.21 ppb for Loma Caribe and Loma Peguera, respectively; [94]) (Table S2, Figure 7b). It is commonly suggested that the PGE enrichment in Ni laterites of Indonesia was probably due to the deep oxidative weathering and residual concentration as impurities in weathering-resistant minerals and subsequent remobilization then reduction and precipitation controlled by the presence of Fe- and Mn-rich oxyhydroxides and/or bioactivity [89].

Elevated contents of Cr (up to 2 wt.%) were found in the laterite layer of the Tagaung Taung Ni deposit in Myanmar [20,74]. Recent studies have further reported significant amounts of Cr (up to 5 wt.%) and Mn (up to 0.7 wt.%) hosted by goethite and hematite in the bedrock and saprolite layer of the Intex Ni (Mindoro Island) and Berong Ni-Co (Palawan Island) laterite deposits in the Philippines [32,33].

Given that the weathering process can trigger the remobilization and enrichment of a range of potentially valuable metals [22], the above results suggest that the Ni(Co) laterite deposits in Indonesia and the Philippines and, to some extent, in Myanmarare worthy targets for critical and transition metals although they may not reach economically useful levels for Sc, REE, and PGE.

5. Future Perspectives

5.1. Research Perspectives

Previous studies have raised some questions, and there are still a lot of unknowns regarding the physical and chemical changes in the parent lithology and the Ni-bearing phases during the weathering process. It is important to understand the mineralogy, structure, and crystal chemistry of ore and gangue minerals, along with the mobility of elements through the weathering profile of the Ni(Co) laterite deposits. Such studies can be achieved by applying cutting-edge technologies (e.g., micro X-ray diffraction (µXRD) and TESCAN Integrated Mineral Analyzer (TIMA)) in combination with common techniques (e.g., scanning electron microscopy (SEM) and reflected light microscopy).

The current understanding of the formation and origin of Ni(Co) laterite deposits remains enigmatic to specific areas in SE Asia. At present, there is a lack of theory explaining why some areas host both Ni and Co mineralization (mostly in the Philippines) while other areas only host Ni ore (Indonesia and Myanmar). The geological causes of the abnormal distribution of Ni(Co) grade in this region remain unknown. Hence, in-depth investigations and comparative studies should be conducted on the mineralogical, geochemical, and geobiological mechanisms that govern the Ni and Co enrichment throughout the weathering profile of Ni(Co) laterite deposits in Indonesia, the Philippines, and Myanmar. In this way, it is of crucial importance to combine mineralogical and chemical analytical techniques, such as transmission electron microscopy (TEM), micro X-ray fluorescence spectroscopy (µXRF), laser ablation inductively coupled mass spectroscopy (LA-ICP-MS), X-Ray Powder Diffraction (XRPD), and micro-Raman spectroscopy [98,99,100,101,102]. These methods are useful to acquire the most comprehensive information and provide further insights into the structure, mode of occurrence, and speciation of the Ni(Co)-bearing ore and gangue minerals. The complex textural relationships between these minerals and their formation mechanism can be successfully unraveled by means of TEM and LA-ICP-MS [99,100]. Combined with electron probe microanalysis (EPMA), XRPD-based quantitative phase analysisand automated SEM-based analysis systems (e.g., TIMA and QEMSCAN^®) offer the possibility to assess the mineral abundance and metal deportment in the Ni(Co) laterite ores [101,102,103]. These analytical techniques have helped to discriminate between Ni-rich and Ni-poor goethite in the laterite ores [102]. Recently, the ultramafic index of alteration (UMIA) has been proposed to assess the geochemical variations triggered by chemical weathering in a specific profile [94,104]. To better understand the local enrichment of Ni in the saprolite layer, a combination of 3D modeling with field observations and a borehole dataset can be helpful [105]. A similar method can be implemented to characterize the relationship between the distribution of Ni(Co) and their contents in the lithostratigraphic layers of these laterite deposits of SE Asia.

The Ni-bearing Mg phyllosilicates (garnierites) host significant ore minerals in the Ni(Co) laterite deposits in SE Asia [26,27,31,32,33]. Due to their fine-grained nature, low crystallinity, and common occurrence as mixtures, the characterization and classification of different types of garnierite is complex [27,31,32,33]. However, recent studies on garnierite from the Caribbean Ni(Co) laterite deposits have suggested that the characterization of this mineral phase can be achieved by using a combination of diverse innovative methods, including high-resolution transmission electron microscopy (HRTEM), analytical electron microscopy (AEM), differential thermal analysis and thermogravimetry (DTA-TG), µRaman spectroscopy, and synchrotron microfocus X-ray absorption spectroscopy (µXAS) [106,107,108,109]. A combination of XRD and HR-TEM has been previously applied with the aim of describing the composition and dissolution kinetics of garnierites from the Loma de Hierro Ni(Co) laterite deposit in Venezuela [110]. A comprehensive understanding of the alteration dynamics controlling the source and enrichment of Ni mineralization during saprolitization was achieved through insightful (nano)textural and mineralogical investigations (XRPD, EMPA, and TEM-HRTEM) of the Ni-rich phyllosilicates occurring in the saprolite layer developed on distinct magmatic protoliths from the Wingellina Ni-Co deposit in Western Australia [111,112]. The mobility and accumulation of Ni in serpentine and garnierite across the weathering profile were elucidated by means of µXRD, EPMA, and µXAS [106,107]. The textural relationship between the different garnierite phases and the distribution of Ni at a nanometer scale can be defined and clarified by a detailed TEM-HRTEM-AEM imaging study [100]. This integrated technique combined with DTA-TG, µRaman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR) can contribute to a better textural and mineralogical characterization of garnierite from Ni(Co) laterite deposits in SE Asia and shed new insights into the formation processes of garnierite in lateritic environment [100,109]. The process of Fe and Ni enrichment in Fe–Ni–serpentine of Ni laterite deposits can be better explained by combining the knowledge of the compositional variations of serpentine and detailed mineralogical analyses with thermodynamic and geochemical calculations related to the serpentine stability [108]. Combined with detailed fieldwork, innovative mineralogical techniques can be successfully implemented for the garnierite phases and other Ni-rich minerals (e.g., goethite and montmorillonite) in order to gain a comprehensive understanding of the weathering and enrichment processes throughout the Ni(Co) laterite deposits in SE Asia.

When compared to sulfide Ni deposits, a precise age of mineralization for the Ni(Co) laterite deposits is difficult to constrain since most of these deposits were formed and evolved over long periods under various weathering regimes [3]. This may explain that direct dating of regolith has not been conducted, even on the Ni(Co) laterites themselves in SE Asia. Moreover, the period of intense weathering is not yet constrained. Although the formation of Ni laterite deposits have been linked to climate change [113], there is no clear relationship between the present climate and the ore type, grade, or size of Ni(Co) laterite deposits from this region. It should be of interest to constrain whether or not some localities have undergone cold month mean temperatures of 15–27°C or warm month mean temperatures of 22–31°C [114]. Hence, the history of formation of the Ni(Co) laterite deposits of SE Asia would be clearly defined, while the exploration potential can be better assessed [114].

In addition to Ni ore and economic contents of Co, the Ni(Co) laterite deposits host critical metals or high-tech elements that are increasingly demanded by many industries. These deposits are becoming worth targets for Sc, REE, and PGE [94,113,114,115,116,117]. Recent studies have shown that Ni(Co) laterite deposits from the Caribbean, Balkan, and Pacific regions are enriched in critical metals (e.g., REE, PGE, and Sc) that could be economically extractable [94,116,117,118]. However, few studies have been conducted on the occurrence and distribution of Sc, REE, and PGE in Ni(Co) laterite deposits of SE Asia (see Section 4). Previous investigations were focused on the remobilization/recrystallization and enrichment of critical metals in the laterite profile from Indonesia [89,90,91,92], while recent work has proposed the REE potential of the Ni(Co) laterite deposits from the Philippines [93]. Therefore, further in-depth investigations should be dedicated to assessing the element contents of critical metals in Ni(Co) laterite deposits of SE Asia. Detailed investigation of the minor and trace element behavior of these metals would be of great interest for understanding the mobility of such elements throughout the weathering profile.

5.2. Exploration Perspectives

The southeastern Asia region has considerable potential for further discovery and development of new reserves of Ni(Co) laterite ore in the Philippines, Indonesia, and Myanmar, which have similar features to those in the Southern Pacific, likely in Australia and New Caledonia. Exploration surveys for these deposits in this region have faced many problems, such as the complex regional or local geology, remote areas and lack of road access, complex distribution of ore minerals and ore-forming elements, and limited sampling density and strategy for soil and rock chemical analyses [23,25,67].

The Ni(Co) laterite deposits in SE Asia have been explored for decades by using classic field methods, including mapping and sampling of outcropping bedrock and saprolite or laterite surfaces, manual pit digging, trenching, and later incorporated stereo-photo interpretation, drilling, and geochemical and geomorphological analyses [119,120,121,122]. These classical methods have the potential to hamper the discovery of hidden deposits due to a thick lateritic cover and regolith [67,123,124]. Hence, new investigation techniques integrating regolith mapping, remote sensing, geophysics, and mineralogy coupled with geochemistry have been proposed to improve the finding of Ni(Co) laterite deposits. Integration of these innovative exploration techniques could have important perspectives in SE Asia.

Previous work has revealed that most Ni laterite deposits give rise to distinct vegetation anomalies, which can be readily identified by remote sensing methods [124]. Regolith mapping is a recent depth non-invasive technique proposed to enhance the prospecting and drilling survey of hidden Ni(Co) laterite deposits [105,125,126]. Airborne electromagnetic (AEM) has been applied in combination with mineralogical and geochemical analyses in order to clarify the relationship between regolith materials, petrophysical properties, hydrogeology, and mineralization at the Cawse Ni laterite deposit in Western Australia [125]. Accurate analysis of remote sensing data using ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) images and different airborne systems (e.g., Landsat TM imagery) provide useful and high-resolution mineral information, which is applied in regolith-landform mapping [126]. Further enhancement to the data using temporal merging can create images that are both easier to interpretand more reliable indicators of lithology and alteration [126]. Thus, regolith-landform mapping can be a useful guide for geochemical sampling strategy during the exploration of Ni(Co) laterite deposits since geochemical anomalies in the weathering profile can result from the formation of secondary ore such as Ni laterite [105]. Highly improved regolith mapping helps to identify major controls on metal dispersion, select suitable geochemical sampling areas, and assist anomaly interpretation [105]. Dense vegetation would particularly affect regolith mapping and not electromagnetic or other airborne surveys.

The application of aerial photos combined with aeromagnetic surveys at different scales is useful in delineating more accurately the distribution, lithology change, and favorable location of structures and ultramafic bedrock [105]. For instance, 2D regolith (down to >80 m) imaging has been successfully applied by means of electrical resistivity tomography [127,128]. This useful tool has been proven efficient insignificantly reducing the number of drilling holes for Ni ore exploration in Zimbabwe and New Caledonia [123,129]. Heliborne electromagnetic surveys of different areas have accurately outlined structure images of the weathering mantle and underlying bedrock down to 300 m [129]. Three-dimensional regolith mapping is an innovative tool that uses airborne electromagnetic measurements, remote sensing technology, and various software, such as Mining Visualisation System (MVS), ArcView 3D Analyst version 9.3, and Vulcan version 8.2, to visualize and quantify geochemical, geological, and regolith data [105].

Since geochemical analyses have insufficient contribution in the first steps of the Ni(Co) prospection, the combine with mineralogical identification by means of portable X-ray fluorescence and Raman spectroscopy devices has been suggested, which helps exploration geologists to grasp knowledge on the chemical and mineralogical features of samples in the field [108]. Geological mapping can also be coupled with geochemical halo secondary scanning using the above devices to determine the Ni(Co) mineralization zones [124].

When exploration activities reach a high level that produces a large amount of data, digitalization with statistics could help to improve the prediction of exploration targets through a better understanding of the data gathered. A suitable statistical analysis of large sample data has been proven to be an effective method for analyzing and discussing the geochemical features of Ni laterite deposits in Indonesia [130]. Although this statistical method is promising, further work is required to explain the geological causes of the abnormal distribution of Ni grade [130]. Moreover, geostatistical techniques (e.g., ordinary kriging for a single variable) have been successfully applied for modeling the Ni grade in Indonesian laterite Ni deposits, with regard to the topographic effects (slope gradient and thickness of the limonite and saprolite layers) and paleo-groundwater system [51,71]. This helps to obtain a more precise estimation of available Ni grade and an identification of factors controlling the grade distribution in these laterite deposits [51], while the relationship between the degree of weathering, groundwater–rock interaction, strata thickness, and the Ni content and distribution can be clarified [71]. The geostatistical modeling methods have good perspectives for the exploration of Ni(Co) deposits in SE Asia.

6. Conclusions

In recent years, laterite Ni(Co) ore resources have become widely attractive due to the decrease of Ni sulfide ore resources. The laterite Ni(Co) deposits occur within 20 degrees of the equator, with a few exceptions, and develop from the progressive weathering of ultramafic bedrock found in accretionary terranes and cratonic areas.

Southeast Asia has extensive ophiolite distribution and a hot humid climate, which makes it an ideal environment for the development of Ni(Co) laterite deposits, especially in Indonesia, the Philippines, and Myanmar. Generally, a typical Ni(Co) laterite profile includes four lithostratigraphic layers comprising from the bottom to the top: the serpentinized bedrock, saprolite, limonite, and ferruginous cap layers. In this region, Ni and Co were formed from the pervasive chemical weathering of ultramafic rocks. Topography, tectonic setting, lithology, alteration, climate, and structures are the main ore-controlling factors of the Ni(Co) laterite deposits. Hydrous Mg silicate (or garnierite) and oxide are the major ore types in Southeast Asia, while clay silicate ore is locally developed. Ni enrichment is strongly related to the development of saprolite layers, which is mainly controlled by the degree of serpentinization and fragmentation of the bedrock. Besides Ni and Co, the laterite ore body can contain a reasonable concentration of critical metals, such as scandium, rare earth elements, and platinum group elements.

In Southeast Asia, the role of conventional methods and cutting-edge technologies in the exploration of Ni(Co) laterite deposits has become more and more important for the discovery of concealed deposits. Good prospecting sites for Ni(Co) ore can be discovered in this region by using a combination of advanced remote sensing technologies, geophysical surveys, and chemical analyses.