Comments on the paper “Ochreous laterite: a nickel ore from Punta Gorda, Cuba” by Oliveira et al.

Abstract

The paper, “Ochreous laterite: a nickel ore from PuntaGorda, Cuba,” by Oliveira et al. (2001) presents acharacterization of oxidized nickel ore from the PuntaGorda deposit in the Moa district (eastern Cuba). Theauthors assume that the nickel ore derives from an ophioliticassociation (serpentinized ultramafic rocks, olivine gabbros,and plagiogranites) strongly affected by lateritization. Theynote that ‘no extensive study has so far been published aboutthe Ni mineralization in regard to the mineralogical andchemical nature of the ore’ and reach the conclusion thatnickel is associated with goethite and cobalt with manga-nese minerals. The authors find two ore types, Ni-rich(1.56–2.06%) and Ni-poor (0.24–0.96%), and infer that thetwo types are related to differences in the parent rocks.Oliveira et al. contend that the Ni-rich ore derives fromperidotites containing Ni-bearing orthopyroxenes and thatthe Ni-poor ore forms from peridotites containing Al-clinopyroxene depleted in Ni. Despite their interestingattempt, we believe these points require some criticalcomments that point to possible alternative interpretations.First, the Moa-Baracoa ophiolitic massif consists of alower unit of mantle tectonites made up of serpentinizedharzburgite with minor dunite, ‘impregnated peridotite’(plagioclase- and clinopyroxene-bearing peridotite), wehr-lite, and pyroxenite and a crustal sequence composed oflayered gabbro (olivine gabbro and gabbronorite) anddiscordant pillowed basalts and sediments of the Quivija´nFormation (Guild, 1947; Wadge et al., 1984; Ri´os andCobiella, 1984; Fonseca et al., 1985; Iturralde-Vinent, 1989,1996, 1998; Keer et al., 1999; Proenza et al., 1999a,b).Towards the top of the mantle tectonites, the harzburgitecontains increasing amounts of dunite, gabbro sills, andchromitite, which form elongated pseudotabular bodiesparallel to the foliation of the host harzburgite, as well asdiscordant dikes of gabbro and pegmatitic gabbro (Fig. 1).This upper zone has been considered as forming part of theMoho Transition Zone (MTZ) (Proenza et al., 1999a,b).Second, Oliveira et al. (2001) ignore many publishedstudies on the geological, mineralogical, and chemicalcharacteristics of the Cuban lateritic nickel deposits(Kudelasek et al., 1967; Vera, 1979; Formell and Oro,1980; Vershinin et al., 1984; Quintana-Puchol, 1985;Ostroumov et al., 1985, 1987; Cordeiro et al., 1987;Rojas-Puro´n et al., 1993; Rojas-Puro´n and Carballo, 1993;Capote et al., 1993; Almaguer and Zamarzry, 1993;Rojas-Puro ´n and Beyris, 1994; Rojas-Puron and Orozco,1995; Almaguer, 1995, 1996; Lavaut, 1998). These studiescontain extensive results about the Ni mineralization in theMoa district and conclude that Ni in ochreous laterite isassociated with goethite and Co with manganese minerals(asbolanes).Third, the differences in the mineralogical and chemicalcomposition of the Ni-rich and Ni-poor ores cannot beexplained by the presence of Ni-bearing orthopyroxene orNi-depleted, Al-rich clinopyroxene in the parent rock, asOliveira et al. (2001) conclude, because the Moa peridotitesare made up of only harzburgites and dunites (Proenza et al.,1999a,b), neither of which contains significant clinopyrox-ene. The average forsterite and NiO content of olivine in theharzburgites is 91.0 and 0.39 wt%, respectively, whereas indunites, these values are 91.5 and 0.38 wt% (Table 1). TheNi content of orthopyroxene is very low (NiO , 0.1 wt%),usually below the detection limit of the electron microprobe(Table 1). However, clinopyroxene-bearing harzburgites arevery scarce in the whole Moa-Baracoa massif. Therefore,the Ni-rich ore can only derive from harzburgite and dunite