Garnierites and garnierites: Textures, mineralogy and geochemistry of garnierites in the Falcondo Ni-laterite deposit, Dominican Republic

Highlights

• Garnierites from Falcondo precipitated in a tectonically active regime.

• Five types were identified: serpentine-, talc-, sepiolite-like and mixtures.

• Garnierite precipitation occurred in successive stages progressively.

• The main garnierite in Falcondo is talc-like, and shows one of the highest Ni contents.

• The variation of garnierite mineralogy has implications on ore processing.

Abstract

Garnierites (Ni–Mg-bearing phyllosilicates) are significant ore minerals in Ni-laterites of the hydrous silicate-type. In the Falcondo Ni-laterite deposit (Dominican Republic), garnierites are found within the saprolite horizon mainly as fracture-fillings and thin coatings on joints. Field observations indicate an important role of active brittle tectonics during garnierite precipitation. Different greenish colours and textures can be distinguished, which correspond to different mineral phases, defined according to X-ray diffraction (XRD) and electron microprobe (EMP) analyses: a) talc-like (10 Å-type), b) serpentine-like (7 Å-type), c) a mixture of talc- and serpentine-like, and d) sepiolite-like types. Compositional data indicate continuous Mg–Ni solid solution along the joins lizardite–népouite (serpentine-like), kerolite–pimelite (talc-like) and sepiolite–falcondoite (sepiolite-like). In general, talc-like garnierite is dominant in Falcondo Ni-laterite and displays higher Ni contents than serpentine-like garnierites. EMP analyses showing deviations from the stoichiometric Mg–Ni solid solutions of serpentine and talc are best explained by talc- and serpentine-like mixing at the nanoscale. A detailed textural study by means of quantified X-ray element imaging provides a wealth of new information about the relationships between textural position, sequence of crystallization and mineral composition of the studied garnierite samples. These results indicate several stages of growth with variable Ni content, pointing to recurrent changes in the physical–chemical conditions during garnierite precipitation. In addition, our detailed mineralogical study of the Falcondo garnierites revealed that the different types identified have characteristic H₂O content and SiO₂/MgO ratios, which play important roles during the pyrometallurgy process.

Introduction

Although Ni (± Co) laterite deposits account only for about 40% of the current world's annual Ni production, they host over 60% of the world land-based Ni resources (Gleeson et al., 2003, Kuck, 2013), and the amount of Ni being extracted from laterite ores is increasing steadily (Mudd, 2010). About 10% of the world's Ni resources are found in the Caribbean region, mostly in the northern part, and include the Moa Bay and Punta Gorda deposits in eastern Cuba and the Falcondo deposit in central Dominican Republic (Dalvi et al., 2004, Lewis et al., 2006, Nelson et al., 2011). Other Ni-laterite deposits in the region include the Gloria deposit in Guatemala (Golightly, 2010) and the Meseta de San Felipe deposit in Camagüey in Central Cuba (Gallardo et al., 2010a). On the other hand, the major Ni laterite resources in the southern Caribbean include the Cerro Matoso in Colombia (Gleeson et al., 2004) and Loma de Hierro in Venezuela (Soler et al., 2008), both presently exploited.

Ni (± Co) laterite deposits are regoliths formed by the chemical weathering of ultramafic rocks, mainly in tropical and subtropical latitudes (e.g. Elias, 2002, Freyssinet et al., 2005, Golightly, 1981). Under the high temperature and intense rainfall, typical of these environments, the most soluble elements (especially Mg and Si) are leached from primary rock-forming ferromagnesian minerals, and the least mobile elements (especially Fe, Al) accumulate in successive horizons of the lateritic profile (e.g. Freyssinet et al., 2005, Golightly, 2010).

Although there is no widely accepted terminology and classification, Ni (± Co) laterites are commonly classified into three categories, according to the main Ni ore assemblage (Brand et al., 1998). These include: i) Oxide laterite deposits in which the ore assemblage is principally Fe oxyhydroxides; ii) Clay silicate deposits dominated by Ni-rich smectites; and iii) Hydrous Mg silicate deposits in which the ore is mainly Mg–Ni phyllosilicates (including garnierites). The hydrous Mg silicate deposits generally have the highest Ni grade (1.8–2.5 wt.% Ni) and are characterised by a thick serpentine-dominated saprolite horizon covered by a relatively thin Fe-oxyhydroxide-dominated limonite horizon (laterite sensu stricto horizon). These deposits are formed under conditions of a low water table and continuous tectonic uplift (Butt and Cluzel, 2013, Freyssinet et al., 2005).

In terms of production and reserves, the Falcondo deposit is the largest hydrous Mg silicate-type Ni-laterite deposit of the Greater Antilles, with estimated Ni reserves of about 79.2 million dry tonnes at a grade of 1.3 wt.% Ni (Falcondo Annual Report, 2010; http://www.falcondo.com.do/ES/Publicaciones/brochures/Memoria_FALCONDO_2010.pdf). As other hydrous Mg silicate-type deposits worldwide, Ni-bearing serpentines and particularly garnierites are concentrated in the lowermost part of the saprolite horizon, toward the base of the profile (Freyssinet et al., 2005).

The term garnierite is generally used to refer to the group of green, fine-grained poorly crystallized Ni-bearing magnesium phyllosilicates, which include serpentine, talc, sepiolite, smectite and chlorite; often occurring as mixtures (e.g. Brindley, 1978, Brindley and Hang, 1973, Faust, 1966, Springer, 1974). Although garnierite is not recognised as a mineral species by the International Mineralogical Association (IMA), it is a convenient field term used by mine geologists for all green Ni-silicates when more specific characterization is not possible (Brindley, 1978).

The classification and nomenclature of garnierite-forming minerals remain controversial because of their fine-grained nature, poor crystallinity and frequent occurrence as intimate mixtures of different mineral species. After various studies (Brindley, 1978, Brindley, 1980, Brindley and Hang, 1973, Brindley and Maksimović, 1974), Brindley and co-authors distinguished two groups of garnierites: i) 1:1 and ii) 2:1 layer minerals. The first group includes the Mg–Ni series of the serpentine group minerals: lizardite–népouite, chrysotile–pecoraite and berthierine–brindleyite. The second group comprises the following Mg–Ni series: talc–willemseite, kerolite–pimelite, clinochlore–nimite, and sepiolite–falcondoite.

The most common garnierite minerals found in nature are lizardite–népouite and kerolite–pimelite (Brindley, 1978). For these minerals, the terminology “serpentine-like” (or 7 Å-type) and “talc-like” (or 10 Å-type) garnierites, respectively, has been widely used (Brindley and Hang, 1973, Brindley and Maksimović, 1974, Galí et al., 2012, Wells et al., 2009). It is important to note here that the term “talc-like” does not refer to the normal composition and/or structure of talc (Brindley and Hang, 1973). Actually, the characterisation of talc-like garnierites is controversial and both the Mg and Ni talc-like end members kerolite and pimelite, respectively, are discredited mineral species by the IMA. Despite being historically described as hydrated talc-like minerals, kerolite and pimelite were classified into the smectite group by Faust (1966). In contrast, other authors proved a structure with talc affinity, since kerolite and pimelite do not exhibit intracrystalline swelling (e.g. Brindley and Hang, 1973, Kato, 1961, Maksimović, 1973, Slansky, 1955). In addition, an intermediate phase between talc-like and serpentine-like end members, karpinskite (Mg,Ni)₂Si₂O₅(OH)₂, was described by Rukavishnikova (1956), however it is not accepted as a mineral species by the IMA.

Several mineralogical studies using different techniques have been published on garnierites since their discovery in 1864, most of them during the late 1960s to early 1980s. The majority of the publications since the 1960's focussed on the composition of garnierites from New Caledonia (Caillère, 1965, Pelletier, 1983, Pelletier, 1996, Troly et al., 1979), Indonesia (Golightly, 1979) and Australia (Elias et al., 1981). It is only in the past few years that detailed additional information, including mode of occurrence in the field, petrography and relations between garnierites and their host rocks, is provided (Cluzel and Vigier, 2008, Wells et al., 2009).

In the case of Falcondo Ni-laterite deposits, as a result of the Al-poor nature of the ultramafic protolith, previous results have shown that garnierites consist mainly of the combination of three solid solutions: serpentine-like [(Mg,Ni)₃Si₂O₅(OH)₄], talc-like [(Mg,Ni)₃Si₄O₁₀(OH)₂·(H₂O)] and sepiolite–falcondoite [(Mg,Ni)₄Si₆O₁₅(OH)₂·6(H₂O)] (Galí et al., 2012, Proenza et al., 2008, Tauler et al., 2009). However, except for the sepiolite–falcondoite series (Springer, 1976, Tauler et al., 2009), little work has been done on the mode of occurrence, textures and composition of these Ni-phyllosilicates.

In this paper we summarize the information on the garnierites from the Falcondo Ni-laterite deposit up to the present, and provide detailed descriptions of their occurrence in the field and textural relationships, as well as new results on mineralogy and mineral chemistry. Our study focuses on serpentine- and talc-like garnierite, presents data from the saprolite host and also includes new information on the sepiolite–falcondoite garnierite. The aim of this work is to gain further insight into the origin of garnierites in the Falcondo deposit.