2.1. Loma De Hierro Ni-laterite deposit

The Loma de Hierro unit, where the mining area exists, is aligned W-E and is part of an elongated, deformed belt in the southern Caribbean Plate margin (Figure 1; ^{Lewis et al. 2006}). The southern margin of the Caribbean Plate is represented by the late Cretaceous Dutch and Venezuelan Islands and the northern Cordilleras of Venezuela (^{Giunta et al., 2002a}). Like the northern margin, the southern margin consists mainly of transpressive or strike-slip shear zones, and includes Jurassic to Cretaceous ophiolitic complexes cropping out along suture zones, such as that of Venezuela (^{Giunta et al., 2002b}). This southern margin is formed by a thrust belt with a set of late Cretaceous south-vergent oceanic and continental tectonic units that have been progressively affected by brittle and ductile deformation related to W-E strike-slip faults (e.g., La Victoria Fault, Figure 2; ^{Giunta et al., 2002b}). The Franja-Costera unit (not shown in Figure 2), is an early Cretaceous ophiolitic mélange that overthrusts southwards the northern border of Serranía de la Costa (^{Giunta et al., 2002a}). The Serranía de la Costa comprises the Paleozoic metamorphic association (orthogneisses and schists) of the Ávila unit (^{Urbani, 2005}) and the late Jurassic carbonate-terrigenous sequences of the Caracas unit (Figure 2).

Figure 2 Simplified geological map of the study area showing the location of the Loma de Hierro laterite deposit. Simplified from ^{Hackley et al. (2006)}. 

The Caucagua-El Tinaco unit, in the Serranía del Interior, overlies the Serranía de la Costa. It is a pre-Mesozoic basement covered by Cretaceous volcanosediments, basalts, dolerites, and gabbros (Figure 2; ^{Giunta et al., 2002a}). The Loma de Hierro unit appears to the south of the Caucagua-El Tinaco unit. It is an oceanic sequence of serpentinized mantle peridotites and Cretaceous metavolcanic sediments (^{Giunta et al., 2002a}). The Loma de Hierro unit is overthrusted by the Villa del Cura unit, composed of early Cretaceous wehrlite-clinopyroxenite cumulates and metamorphosed serpentinized mantle peridotites, massive metabasalts and metavolcanic sediments (^{Giunta et al., 2002b}).

2.2. The Loma de Hierro Ni-laterite deposit

The Ni-laterite deposit of Loma de Hierro is located 50 km SW of Caracas, in the northern-central part of the country, and extends roughly W-E in a series of hills in the mountain ranges of Serranía del Interior. The area mined for nickel by the state company Loma de Níquel C.A. has a length of about 15 km with WSW-ENE direction, and a width from 1 to 7 km, between the states of Aragua (west) and Miranda (east). Its coordinates are 10°09′-10°10′ N and 67°08′-67°06′ W, and has an altitude of ~1300 m.a.s.l.

In 1941, laterite soils from Loma de Hierro were discovered as a potential source of nickel. They were exploited until 1960, when the work was stopped as the hills were declared an area of special natural interest. After several studies on the Ni-extraction economic interest and viability, exploitation was resumed in 2001 by Anglo American PLC. In 2015, the exploitation was assumed by the Venezuelan State, creating the company Loma de Níquel C.A., with an estimated reserve of 40 Mtons of nickel. In 2017, due to a technical shutdown of the exploitation and of the factory of ferronickel pellets attached to it, the mine could be visited, allowing the sampling of a complete laterite profile. A panoramic view and several details of the mining front are shown in Figure 3.

Figure 3 (a) Panoramic view of the laterite profile of Loma de Hierro, showing the location of the sampled area (b). (c) Detail of the lowest part of the profile, showing the sampled saprolite horizon (d). (e, f, g) Detail of the serpentine-dominated saprolite horizon. (h) Detail of the goethite-dominated limonite horizon. 

3.1. Sampling

A total of 33 samples were collected from an exploitation front. The selected profile is ~25 m thick and is characterized by three zones (from bottom to top): the serpentinised peridotite protolith, a well-developed saprolite zone, and a thin limonite zone (Figure 3). For the present work, 17 of these samples were selected for detailed chemical and mineralogical analyses.

3.2. Analytical and computation techniques

Samples were powdered, sieved below 75 µm particle size, dried, and homogenized. Major, minor and trace element compositions were determined at the Actlabs Laboratories (Ontario, Canada) by X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS; after acid dissolution).

The same samples were analyzed using X-Ray powder diffraction (XRPD). Diffractograms were obtained at Centres Científics i Tecnològics of the Universitat de Barcelona (CCiT-UB) in a PANalytical X’Pert PRO MPD Alpha1 powder diffractometer in Bragg-Brentano θ/2θ geometry of 240 mm of radius, nickel filtered Cu Kα1 radiation (λ = 1.5406 Å), 45 kV and 40 mA. The samples were scanned from 4 to 100º (2θ) with a step size of 0.017º and measuring time of 50 s per step, using an X’Celerator detector (active length = 2.122º). Mineral-identification was facilitated by X’Pert High Score search-match software using the powder diffraction database of the International Centre for Diffraction Data (ICDD). Mineral semi-quantitative analyses were performed by the full profile fitting method (Rietveld method) using the TOPAS V4 software and the American Mineralogist Crystal Structure Database.

Minerals and textures were examined on polished thin sections under optical microscope (OM) with both transmitted and reflected light, and under a scanning electron microscope (SEM) Quanta 200 FEI, XTE 325/D8395, with an INCA Energy Dispersive Spectrometer (EDS) microanalysis system at the CCiT-UB.

Electron microprobe analyses (EMPA) of representative samples were performed with a JEOL JXA-8230 electron microprobe, equipped with five wavelength-dispersive spectrometers and an energy-dispersive spectrometer. The operating conditions were 20 kV accelerating voltage, 15 nA beam current, 2 μm beam diameter and counting time of 20 s per element. The calibration standards used were wollastonite (Si, Ca), corundum (Al), orthoclase (K), hematite (Fe), periclase (Mg), rhodonite (Mn), NiO (Ni), metallic Co (Co), rutile (Ti), albite (Na), Cr₂O₃ (Cr), V (V), and Sc (Sc). Structural formulae were calculated on the basis of 1.5 oxygens (goethite), 4 oxygens (olivine, magnetite, Cr-spinel), 6 oxygens (pyroxene), 7 oxygens (serpentine), and 11 oxygens (garnierite). Structural formulae of primary serpentine (Srp-I) were calculated taking into account all Fe as Fe(II), while those of secondary serpentine (Srp-II) and garnierite were obtained taking into account all Fe as Fe(III), based on XANES data on Srp-I, Srp-II and garnierites from the Dominican Republic (^{Roqué-Rosell et al., 2017}). In addition, the molar percentage of the talc-like phase (kerolite-pimelite) in the garnierite mixture was calculated using the equation proposed by ^{Soler et al. (2008)}.

7.1. Elemental mobility in the weathering profile

In the protolith, the distribution of the relevant major element oxides among the main minerals indicate that Mg and Si are contained in forsterite, enstatite, and Srp-I; Fe is present in the above phases plus magnetite and Cr-spinel; and Al, very scarce (1 wt.% Al₂O₃), is contained in minor chlorite and Cr-spinel (both less than 1% modal). The Fo/En molar ratio is about 3, indicating that complete serpentinization would yield an excess of forsterite, which will be eliminated by dissolution during the supergene stage.

Forsterite dissolution by acidic weathering, according to reaction 1, results in an increase of porosity, even if silica were retained as an amorphous silica gel. This is reflected by the increase of the Si/Mg molar ratio, from 0.7 in the protolith to 0.95 at the top of the saprolite horizon, near the Mg discontinuity.

The preservation of rock structure in the saprolite suggests a replacement of forsterite and enstatite by serpentine (Srp-II) and/or kerolite. Assuming total retention of silica and maximum conservation of mass in the minerals (which is not the case), the net reactions from forsterite (dominant in the protolith) can be written as

The equivalent reactions from enstatite would be given by

The formation of serpentine (Srp-II) from forsterite and enstatite weathering according to r.2 and r.4, and that of kerolite, more favorable under higher Si/Mg aqueous concentration ratios (^{Galí et al., 2012}), (r.3, r.5), may explain the formation of the secondary phyllosilicate mixture identified in the field.

The newly formed Srp-II is quite different from Srp-I. Srp-I, which is only observed in the protolith, indicating that its composition is essentially unstable in weathering conditions, is Al-poor, and has half the Fe and more than one order of magnitude less Ni compared to either the coexisting forsterite or enstatite (Table 2). In the saprolite horizon, Srp-II is enriched in Fe by a factor of 1.5 (from 3 to 5 wt.% FeO) and in Ni by more than two orders of magnitude (from 0.02 to 6 wt.% NiO) compared to Srp-I. It has been shown (for a review, see for instance ^{Villanova-de-Benavent et al., 2017}) that this change in composition stabilizes serpentine in the lateritic environment. In particular, Srp-II may be described as a three-component solid solution of the end members lizardite, Mg₃Si₂O₅(OH)₄, népouite Ni₃Si₂O₅(OH)₄, and Fe(III)-serpentine Fe₂Si₂O₅(OH)₄, that lowers the equilibrium constant for dissolution by 2 to 5 log K units. The detailed mechanism of formation of such a solid solution is controversial.

Fe released in the dissolution of olivine and pyroxene precipitates as Fe(III)-oxy-hydroxides. Magnetite is also frequently oxidized to goethite afterwards (r.6).

The transition to the limonite horizon is marked by a clear Mg-discontinuity (Figure 4). This limonite horizon concentrates the highly insoluble elements Fe, Al, and Cr included in inherited minerals that resisted weathering, such as magnetite and Cr-spinel, or in newly formed phases such as goethite, hematite, and gibbsite (Figure 4, Table 1).

SEM images show an astonishing variety of microstructures and reactions in all these minerals, which include several processes of cementation of quartz by oxy-hydroxides, alteration of Cr-spinel (post-magmatic), transformation of goethite to hematite and vice versa, phyllosilicates pseudomorphed by goethite and gibbsite, and a variety of mixtures of amorphous Fe-oxy-hydroxides (Figures 8g and 8h). Mn is retained in black, poorly crystalline nodules identified as minerals of the lithiophorite-asbolane series and asbolane (Figure 9h).

The remarkable Al₂O₃ content in the limonite horizon, which is related to the high amount of gibbsite detected by XRPD, has also been observed in other Ni-laterites worldwide [(e.g., the hydrous Mg silicate-type deposits of Cerro Matoso, Colombia (^{Gleeson et al., 2004}) and Loma Caribe and Loma Peguera, Dominican Republic (^{Aiglsperger et al., 2016}), and the oxide-type laterites of Nkamouna, Cameroon (^{Lambiv Dzemua et al., 2013}) and Moa Bay, Cuba (^{Aiglsperger et al., 2016})]. In all cases, the protolith is not substantially rich in Al (<2 wt.% Al₂O₃), but the Al content increases upwards in the profile. In particular, Al₂O₃ reaches 6-10 wt.% in the ferricrete and 8-10 wt.% in the limonite of Cerro Matoso, 32 wt.% in the ferricrete and 10-23 wt.% in the limonite of Loma Caribe, 26 wt.% in the ferricrete and 4 wt.% in the limonite of Loma Peguera, 20-25 wt.% in the limonite and ferricrete breccia of Nkamouna, and 15 wt.% in the duricrust and 8-10 wt.% of the limonite of Moa Bay. This is correlated with important amounts of gibbsite and kaolinite (e.g., up to 22 wt.% gibbsite in the ferricrete and 7 wt.% kaolinite in “tachylite” of Cerro Matoso). In Cerro Matoso, the protolith is a lherzolite with harzburgite and dunite, containing forsteritic olivine, orthopyroxene, and clinopyroxene. In the Dominican laterites, the protolith is mainly harzburgite and dunite, and in Moa Bay, it is harzburgite. Hence, they may all contain significant amounts of Al-bearing pyroxene, explaining the higher Al contents, which, together with Fe, Ni, Co, and Mn, would ultimately concentrate in the upper horizon.

The overall REE contents in the Ni-laterites are very low when compared to conventional REE ore deposits (e.g., ^{Chakhmouradian and Wall, 2012}), but they are similar to REE contents measured in other laterite deposits of the Caribbean area. In Moa Bay (Cuba), ∑REE concentrations are low in the unweathered harzburgite (~0.1 ppm), vary from 1 to 3 ppm in the saprolite, and are higher in the limonite (15 to 68 ppm) (^{Aiglsperger et al., 2016}). In Loma Caribe (Falcondo, Dominican Republic), the parent rock contains 0.6 ppm ∑REE, the saprolite contains 0.9 to 1.6 ppm ∑REE, and the limonite concentrates 37 to 335 ppm ∑REE. In Loma Peguera, the contents of REE in the parent rock, saprolite, and limonite horizons are 0.2, 0.5 to 6, and 122 to 171 ppm ∑REE, respectively (Aiglsperger et al., 2016). In all cases, REE contents in saprolite and limonite horizons are enriched with respect to the protolith, as a result of the decomposition of REE-bearing minerals due to the weathering process. REE aqueous complexes move down the profile but are removed from the aqueous phase by mineral precipitation or sorption processes once pH increases.

However, note that in Loma de Hierro, the highest ∑REE contents (up to 81 ppm) are found in the upper saprolite horizon, just below the limonite horizon, while in the aforementioned Caribbean laterites and in other deposits such as Wingellina Ni-Co oxide-type laterite deposit (Australia) REE are mainly concentrated in the limonite horizon (e.g., ^{Aiglsperger et al., 2016}; ^{Putzolu et al., 2019}). ^{Aiglsperger et al. (2016)} and ^{Putzolu et al. (2019)} observed a positive correlation between REE and MnO or Fe₂O₃ content in the limonite horizon. In Loma de Hierro, despite the highest REE concentration in the saprolite horizon, this correlation is also observed (Figures 5c and 5d) suggesting that REE can accumulate in Fe/Mn-bearing oxy-hydroxides. These oxides have been identified, and despite being in low amount, the preferential Fe/Mn-oxy-hydroxides scavenging of LREE instead of HREE (due to the highest stability of HREE-humic acids aqueous complexes) may explain the negative continuous slope for LREE observed in Figure 6 (^{Sanematsu and Watanabe, 2016}). Moreover, in this limonite horizon, there is a slight positive Ce anomaly, which indicates the oxidation of Ce(III) to Ce(IV) and probably precipitation of Ce(IV) solid phases under the oxidizing conditions prevailing in this horizon (e.g., ^{Aigslperger et al., 2016} and references therein). Ce content positively correlates with Fe₂O₃ (Table 1), which is coherent with the Ce fractionation and concentration in the upper horizons of the deposit (^{Mongelli et al., 2014}). The REE accumulation at the top of a developed saprolite horizon with significant negative Ce anomalies below a thin REE-leached zone with positive Ce anomalies, as observed in Loma de Hierro, is similar to the ion-adsorption-type REE deposits of China described in ^{Sanematsu and Watanabe (2016)}. These deposits developed over granites and are characterized by REE being preferentially sorbed onto kaolinite and similar minerals. According to ^{Sanematsu and Watanabe (2016)} and references therein, REE fractionation due to sorption is negligible at the ionic strength of soil water, and this would explain the planar slope observed in Figure 6. However, more information is needed to locate the mineralogy of REE, and to constrain the weathering conditions that occurred in the Venezuelan laterites. In addition, more data on worldwide distribution of REE in oxide, clay, and hydrous Mg silicate-type Ni-laterites, together with a detailed mineralogical characterization, is paramount.

Co contents are linked to Mn contents (Figure 12d) as in other laterite deposits (^{Llorca and Monchoux, 1991}; ^{Roqué-Rosell et al., 2010}; ^{Lambiv Dzemua et al., 2013}; ^{Aiglsperger et al., 2016}; ^{Putzolu et al., 2019}). However, while in Moa Bay and Falcondo the Mn-Co association is mainly found in the upper saprolite, in Wingellina, as in Loma de Hierro, the most significant contents of Mn and Co are found in the limonite horizon. According to ^{Putzolu et al. (2019)}, precipitation of Mn-oxy-hydroxides is controlled by the variation of Eh and pH in the profile. Accumulation of Mn-oxy-hydroxides in the transition between saprolite and limonite horizons has been frequently described (^{Brand et al. 1998}). ^{Dublet et al. (2017)} suggested that weathering of the protolith promotes the oxidation of Mn(II) to Mn (III) and finally to Mn(IV), which precipitates forming Mn-oxy-hydroxides rather than being incorporated into goethite. This precipitation seems to be favored by a high humidity and an alkaline pH, conditions that are likely to be found at some depth from the surface, close to the Mg discontinuity.

Sc contents in the limonite horizon of Loma de Hierro (40-70 ppm) are not as high as values reported by ^{Aiglsperger et al. (2016}) in Moa Bay (Cuba) and Falcondo (Dominican Republic) laterites (70-80 ppm and higher) or by ^{Teitler et al. (2019)} in New Caledonia (40-100 ppm). However, they are similar to those measured in laterites from New Caledonia (up to 70 ppm; ^{Audet (2008)} in Aiglsperger et al., 2016), Oman (35-85 ppm; ^{Al-Khirbash, 2016}), and Australia (13-80 ppm; ^{Putzolu et al., 2019}). In the Australian laterites, however, higher Sc concentrations were measured in the saprolite horizon, contrary to that observed in this study and in the other cited references.

Sc is considered to be initially present in pyroxene in the peridotite (^{Aiglsperger et al., 2016}; ^{Teitler et al., 2019}), substituting for ferrous iron and later concentrating in Fe(III)-bearing minerals during laterization. This association with Fe(III) has also been observed in a lateritic deposit in eastern Australia (^{Chassé et al., 2017}) and in a bauxite deposit in Greece (^{Vind et al., 2018}). Sc may substitute for Fe(III) in the goethite structure (^{Levard et al., 2018}; ^{Ulrich et al. 2019}) or be sorbed onto Fe-oxides minerals (^{Chassé et al., 2017}), but after recrystallization of goethite to hematite, Sc concentrates downwards leading to further Sc enrichment (^{Teitler et al., 2019}). ^{Putzolu et al. (2019)} suggested that Sc may also substitute for Fe(III) in Fe(III)-bearing layered silicates (e.g., montmorillonite or chlorite), although more studies must be done to clarify this point. In this study, only a few EMPA measurements have shown Sc in goethite. Considering its low concentration and the complexity of Fe-oxyhydroxide texture and crystallinity, more efforts should be focused (e.g., dedicated LA-ICP-MS analyses) on the mineralogy of Sc, and to better determine Sc contents in given minerals in order to improve the Sc extraction methods.

7.2. Mineralogy of Ni

In the harzburgitic protolith, Ni is mainly found in forsterite with values lower than 0.41 wt.% NiO; also Ni content in enstatite is below 0.21 wt.% NiO, while Srp-I is very poor in Ni (<0.04 wt.% NiO). As seen in Figure 4, the most Ni-enriched horizon in Loma de Hierro is the saprolite horizon. This horizon is rich in Srp-II, with NiO contents close to 6 wt.%. Moreover, kerolite-pimelite dominated garnierites are also present, with Ni contents as high as 22 wt.% NiO. Goethite, the main constituent of the limonite horizon, is poorer in Ni, showing a mean NiO content of 0.5 wt.%, although in some cases it can reach values of 2 wt.%. Meanwhile, Mn-oxy-hydroxides have, in general, higher Ni contents (~15 wt.% NiO). Assuming this average NiO content and that the estimated amount of these Mn-oxy-hydroxides in the samples is 3 wt.%, Mn-oxy-hydroxides would account for approximately 80% of the NiO contained in the limonite horizon.

According to the classification of Ni-laterites by ^{Brand et al. (1998}) and ^{Butt and Cluzel (2013)}, which is based on the principal Ni-bearing minerals, the Loma de Hierro deposit can be classified as a hydrous Mg silicate-type, which is consistent with the thickness of the saprolite horizon compared to that of the limonitic horizon (Figure 4). Fe and Ni contents in Srp-II are comparable with those of other Caribbean hydrous Mg silicate deposits. In the Yamanigüey deposit (Cuba), developed on serpentinized harzburgite and dunite, Ni and Fe contents in Srp-II are in the ranges 1.99-4.72 wt.% NiO and 2.2-15.6 wt.% Fe₂O₃; while in Loma Caribe, from the Falcondo mining area (Dominican Republic) and developed on a serpentinized harzburgite, Ni and Fe contents in Srp-II range from 1.8 to 7 wt.% NiO and from 3.8 to 13.5 wt.% Fe₂O₃ (^{Villanova-de-Benavent et al., 2017}). The contents of NiO in Srp-II and kerolite-pimelite dominated garnierites in Loma de Hierro are also similar to those measured in the Loma Ortega deposit, also from the Falcondo area and classified as a hybrid-hydrous Mg silicate-clay deposit (<2.5 wt.% NiO in Srp-II and 23-36 wt.% NiO; ^{Tauler et al. 2017}).

7.3. Controls on the formation of the weathering profile

As stated by many authors (^{Golightly, 1981}, ²⁰¹⁰; ^{Freyssinet et al., 2005}; ^{Butt and Cluzel, 2013}), the main factors governing the formation and evolution of Ni-laterites are the parent rock (mineral composition, degree of initial serpentinization), climate (temperature, rainfall), water table, topography (hills, slopes, plateau), tectonics (uplift, fractures), and age (exposure time). The present climatic conditions in Loma de Hierro are characteristic of a semi-humid tropical high forest, with annual precipitation of 950 mm and mean temperature of 19 °C, with a dry season from December to March. Tectonic uplift has been important, probably early after the emplacement of the ophiolitic unit, which represents an exposure period lasting for several My. Its altitude (~1300 m.a.s.l.) combined with rather high erosion rates has resulted in a mountainous relief, with slopes up to 25° (46%).

In the studied profile, the Mg discontinuity is also a marked physical discontinuity as there is an irregular, sharp surface (Figure 3a) between the saprolite and limonite horizons, easily seen by the contrasting color and the change in the mechanical consistency, texture and mineralogical composition of the materials. This irregular surface suggests that erosion left the terrane exposed and the residual inconsistent limonite material was locally removed and redeposited, replenishing the depressed areas of the eroded surface, where fragments of duricrust (ferricrete) can be found. As seen in Figure 7, the amount of Mg-bearing secondary minerals (e.g. kerolite) is still high at the upper saprolite, consistent with a significant mechanical erosion. After uplift rejuvenated the relief during important elapsed time periods, erosion transported weathered material downwards, being erosion the dominant process. However, when uplift ceased, weathering became the dominant process and the eroded surface was covered again by limonite material. The blocks of fractured harzburgite, separated by several centimeters, are often coated by garnierite that may have precipitated due to oversaturation of the percolating solutions, perhaps more accentuated during the dry periods.

The ultramafic index of alteration (UMIA), defined by equation 1 (^{Aiglsperger et al., 2016}; inspired by the mafic index of alteration (MIA) by ^{Babechuk et al., 2014}), has been calculated for each sample and is shown in Table 1. UMIA for the protolith in Loma de Hierro has a value of 3 while that of saprolite samples is around 6, consistent with values for unweathered peridotites and saprolites shown by ^{Aiglsperger et al. (2016)} in laterites of Moa Bay (Cuba) and Falcondo (Dominican Republic) (Figure 14a). Limonite samples of Loma de Hierro, however, show values from 53 to 73, slightly lower than those calculated by ^{Aiglsperger et al. (2016)} in the Caribbean area (Figures 14a and 14b).

Figure 14 Molar ternary plots in the Al-Fe-Mg-Si space showing the weathering trends of ultramafic rocks and the ultramafic index of alteration (UMIA), with data from this study and from Brazil (^{Colin et al., 1990}), Greece (^{Eliopoulos and Economous-Eliopoulos, 2000}), Colombia (^{Gleeson et al., 2004}), Cameroon (^{Ndjigui et al., 2008}; ^{Ndjigui and Bilong, 2010}), and Cuba and Dominican Republic (^{Aiglsperger et al., 2016}). (a) AF-S-M ternary plot illustrating the general weathering trend of peridotites with initial loss of MgO followed by loss of SiO₂, and concomitant enrichment of Al₂O₃ and Fe₂O₃. (b) A-SM-F ternary plot illustrating the weathering trend with respect to Al₂O₃ enrichment (bauxitization) or Fe₂O₃ enrichment. 

Although there is a clear loss of Mg and Si in the upper parts of the profile, the enrichment in Fe in Loma de Hierro laterite is not as high as in other laterites.