Ni-laterites represent one of the main Ni sources worldwide, with about 40% of the annual production (Gleeson et al.,2003). The Punta Gorda Ni laterite deposit is part of a larger province of nickel laterites in northeast Cuba (Moa Bay district) (Lavaut, 1998) developed from serpentinized peridotites

Domènech Ortí, Cristina

Galí Medina, Salvador, 1949-

Proenza Fernández, Joaquín Antonio

Villanova de Benavent, Cristina

English

Diposit Digital de la Universitat de Barcelona

 macla nº 19.  julio ‘14 revista de la sociedad española de mineralogía Reactive Transport Modelling: the Formation of Ni-laterite Profiles (Punta Gorda, Moa Bay, Cuba)  /CRISTINA DOMÈNECH (1*), SALVADOR GALÍ (1), JOAQUIN PROENZA (1), CRISTINA VILLANOVA-DE-BENAVENT (1) (1) Dept. Cristal·lografia, Mineralogia i Dipòsits Minerals, Facultat de Geologia, Universitat de Barcelona, c/Martí i Franquès s/n, E-08028 Barcelona ,Catalonia (Spain) INTRODUCTION  Ni-laterites represent one of the main Ni sources worldwide, with about 40% of the annual production (Gleeson et al., 2003). The Punta Gorda Ni laterite deposit is part of a larger province of nickel laterites in northeast Cuba (Moa Bay district) (Lavaut, 1998) developed from serpentinized peridotites.   This laterite profile is composed of four principal horizons, from bottom to top: (1) serpentinized peridotite, (2) saprolite, (3) limonite and (4) ferricrete (Proenza et al. 2007). Based on the mineralogy of the principal ore-bearing phases, the Punta Gorda deposit is classified as oxide-type. In this case, the upper part (limonite zone) is the main ore horizon and is mainly composed of goethite (>50 wt%), and minor hematite, maghemite, gibbsite and Mn-Ni-Co oxyhydroxides (lithiophorite and “asbolane-lithiophorite intermediates”). The nickel content in goethite varies from 0.3 to 4.5 wt.% (average value = 1.4 wt.%) (Proenza et al., 2007).   Nevertheless, the exact location of nickel in goethite is still unclear. Several authors (Arai, 2008 and references therein) suggest that nickel is sorbed onto goethite surface, while others defend that Ni is incorporated into goethite structure (Singh et al. 2002, Landers et al. 2009). Both processes are thought to occur, simultaneously.  In this study a 1D reactive transport model is applied to simulate the formation of this deposit aiming at improving the understanding of the different factors governing the process of lateritization of ultrabasic serpentinized rocks. Specifically, in this model, the sorption of Ni onto goethite surface has been tested as the main process governing Ni accumulation in the goethite of this profile.    CONCEPTUAL AND NUMERICAL MODEL  The formation of the Punta Gorda Ni laterite deposit is simulated by assuming the weathering of a column of partially serpentinized peridotite due to the meteoric water flow. This peridotite parent rock is assumed to be composed of Ni-bearing forsterite (∼0.4%wt Ni), enstatite, oceanic (primary) serpentine (∼0.4%wt Ni) and magnetite (Table 1). The dissolution of the forsterite and enstatite is assumed to be kinetically controlled (Palandri & Karakha 2004) whereas that of primary serpentine and magnetite is considered to occur according to local equilibrium. As a consequence of the dissolution of these minerals, goethite (chosen as representative of Fe-bearing oxides) and a secondary Ni-rich serpentine (>1 wt% Ni) are allowed to precipitate if oversaturated, following local equilibrium (Table 1).   Once released from primary minerals Ni is considered to sorb onto the goethite surface and/or to precipitate with the secondary serpentine.    Information concerning the sorption of Ni onto goethite is scarce (Marcussen et al. 2009, Rajapaksha et al. 2012). In the model presented here, sorption of Ni onto goethite surface is modelled according to the one-site DLM proposed in Marcussen et al. (2009), where Ni sorbs onto goethite surface following reaction r.3. The authors used for r.1 and r.2 the values of the equilibrium constant (K) proposed by Richter et al. (2005) but an update of these values (Richter & Brendler, 2008) is used in this model.  Goet_OH + H+ = Goet_OH2+  r.1 Goet_OH = Goet_O- + H+  r.2 Goet_OH + Ni2+ = Goet_ONi+ + H+ r.3  Assuming elongated prisms in the c direction, a surface area of 84.8 m2/g is calculated from XRD analyses. The BET specific surface is 65.2 m2/g. The Ni sorption capacity of goethite has been calculated from the maximum % of Ni content in goethite and the surface area of goethite, resulting in a value of 5 sites/nm2 if surface area is 84.8 m2/g and of 6.5 sites/nm2 if surface area is 65.2 m2/g. This last pair of values has been chosen for the numerical calculations.   As logK values were all provided for a site density of 2.31 sites/nm2, constants are recalculated to be consistent with the site densities of Moa goethite (Table 1).    LogK  Site/nm2 2.31a 5b 6.5b r.1 7.00 6.7 6.6 r.2 -9.39 -9.7 -9.8 r.3 -0.35 -0.69 -0.80 Table 1. LogK (equilibrium constant) values for reactions r.1, r.2 and r.3. aValues from literature (see text). bValues recalculated in this study to account for the site density found for Moa goethite.   Calculations are done at 25ºC with the code PHREEQC v. 3.0 (Parkhurst & Appelo, 2013). Lo K values for minerals listed in Table 2 have been recalculated from end-member minerals to account for their chemical composition.   The system consists of a column of 35 m discretized in 100 cells of 0.35 m each. The total computing time is 10 My. Boundary water is a tropical rainwater with a constant infiltration rate of about 2000 mm/y. The system is considered fully water-saturated. The porosity is equal to 0.2. Due to code limitations, this porosity is constant throughout the calculations. However, as porosity is a key factor in the system evolution and is palabras clave: Laterita, Níquel, Transporte Reactivo, Cuba key words: Laterite, Nickel, Reactive Transport, Cuba  resumen SEM 2014 * corresponding author: cristina.domenech@ub.edu   macla nº 19.  julio ‘14 revista de la sociedad española de mineralogía  known to increase as laterite deposit forms, several sensitivity cases have been done to study the influence of this parameter.   RESULTS AND CONCLUSIONS  Modelling results reproduce the mineral succession observed in the field. After 10 My, serpentinized peridotite has been partially weathered. Due to the infiltration of an oxidizing rainwater, enstatite, magnetite and forsterite dissolve. Porewater pH and Eh evolve from values of 9.5 and -463 mV in the peridotite domain to values of 4.5 and 400 mV close to the surface. A goethite rich layer develops at the top of the column, where conditions are more acidic and oxidizing, and a layer rich in secondary serpentine is formed at the contact with the initial peridotite. Small amounts of goethite are also present in contact with the secondary serpentine layer (fig.1). The thickness of the goethite rich layer is close to 7 m, although it increases up to 12 m if porosity is 0.4.   Once released from primary serpentine, Ni is sorbed onto goethite surface with values up to 2% wt Ni, especially in the goethite precipitated with the secondary serpentine. Close to this layer, pH is higher, enhancing sorption of Ni (fig.1). At lower pH, as the one prevailing in the upper goethite layer, Ni sorption is not favored and Ni is released to the porewater. In the field, however, it is thought that the flocculation of goethite particles with sorbed Ni onto their surface would prevent Ni desorption from goethite due to pH changes. However, this process as described here has not been implemented in the model.   After 10 My and for a porosity of 0.2, near 16% of Si and Mg are released from the column, while near all Ni and Fe are still there. For higher porosities, the amount of Si and Mg released from the system may reach values up to 35 % but Fe and Ni still remain in the system.  The obtained results show that both the mineral succession and the Ni content in goethite observed in the field can be reproduced by considering the conceptual model described in this work. They also show that sorption of Ni onto goethite surface can account for the concentration of Ni measured in this horizon.  Nevertheless, there are some points that should be improved in the future. Discrepancies between the observed and the calculated thicknesses of the different layers might be due to a combination of different factors such as porosity changes, uplift, changes in soil water saturation that have not been considered in the present model. This kind of studies is thus to be considered as a useful tool to improve the understanding of the different factors governing the process of lateritization of ultrabasic serpentinized rocks.   ACKNOWLEDGEMENTS  This research has been financially supported by the Spanish project CGL2012-36263.   REFERENCES   Arai, Y. (2008): Spectroscopic evidence for Ni(II) Surface speciation at the iron oxyhydroxides-water interface. Environmental Science and Technology 42, 1151-1156. Gleeson, S.A., Butt, C.R., Elias, M. (2003): Nickel laterites: A review: SEG Newsletter, 54, 11-18. Landers, M., Gilkes, R.J., Wells, M.A. (2009): Rapid dehydroxylation of nickeliferous goethite in lateritic nickel ore: X-ray diffraction and TEM investigation. Clays and Clay Minerals 57, 751-770. Marcussen, H., Holm, P.E., Strobel, B.W.,  Hansen, H.C.B. (2009): Nickel Sorption to Goethite and Montmorillonite in Presence of Citrate. Environ. Sci. & Tech. 43 (4), 1122-1127. Palandri, J.L. & Kharaka, Y.K. (2004): A compilation of rate parameters of water-mineral interaction kinetics for application to geochemical modelling, U.S. Geological Survey, Open file report 2004-1068.  Parkhurst, D.L. & Appelo, C.A.J. (2013):  Description of input and examples for PHREEQC version 3—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Techniques and Methods, book 6,chap. A43, 497 p., available only at http://pubs.usgs.gov/tm/06A43/. Proenza, J.A., Tauler, E., Melgarejo, J.C., Gali, S., Labrador, M., Marrero, N., Perez-Nelo, N., Rojas-Puron, A.L., Blanco-Moreno, J.A., (2007): Mineralogy of oxide and hydrous silicate Ni-laterite profiles in Moa Bay area, northeast Cuba. In: Andrew, C.J., et al. (Ed.), Mineral Exploaration and Research: Digging Deeper, 2. Irish Association of Economic Geology, pp. 1389–1392. Rajapaksha, A. U., Vithanage, M., Weerasooriya, R., Dissanayake, C.B. (2012): Surface complexation of nickel on iron and aluminum oxides: a comparative study with single and dual site clays. Colloids and Surfaces A: Physicochem. Eng. Aspects 405, 79-87.  Richter, A., Brendler, V., Bernhard, G. (2005). Blind prediction of Cu(II) sorption onto goethite: Current capabilities of diffuse double layer model. Geochim.Cosmochim. Acta, 69, 2725–2734. Richter, A & Brendler, V. (2008) Blind Prediction and Parameter Uncertainty – A Sorption Test Case. In M. O. Barnett & D. B. Kent (Eds). Developments in Earth & Environmental Sciences, 7, Chapter 10. Singh, B., Sherman, D.M., Gilkes, R.J. Wells, M.A., Mosselmans, J. F. W (2002):  Incorporation of Cr, Mn and Ni into goethite (α-FeOOH): mechanism from extended X-ray absorption fine structure spectroscopy. Clay Minerals 37, 639-649.  Reaction Log K End members  Forsterite Mg9Fe0.95Ni0.05Si5O20 + 20 H+ = 9 Mg2+ + 0.95 Fe2+ + 0.05 Ni2+ + 5 H4SiO4 134.9 Forsterite, Fayalite, Ni2Si2O4 Enstatite Mg9FeSi10O30 + 20 H+ + 10 H2O = 9 Mg2+ +  Fe2+ + 10 H4SiO4 109.4 Enstatite, Ferrosilite 1a Serpentine  Mg2.85Fe0.14Ni0.01Si2O5(OH)4 + 6 H+  =  2.85 Mg2+ + 0.14 Fe2+ + 0.01 Ni2+ + 2 H4SiO4 + H2O 33.1 2a Serpentine  Mg2.78Fe0.16Ni0.06Si2O5(OH)4 + 6 H+  =  2.78 Mg2+ + 0.16 Fe2+ + 0.06 Ni2+ + 2 H4SiO4 + H2O 32.7 Serpentine, Nepouite, Greenalite Table 2. Mineral reactions and corresponding log K used in the calculations.  fig 1. Laterite profile obtained after 10 My with a porosity of 0.2. *%wt Ni sorbed onto goethite.  

Reactive transport modelling: the formation of Ni-laterite profiles (Punta Gorda, Moa Bay, Cuba).

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Reactive transport modelling: the formation of Ni-laterite profiles (Punta Gorda, Moa Bay, Cuba).

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