macla nº 9. septiembre ‘08 revista de la sociedad española de mineralogía 197

Garnierite Mineralization from Falcondo Ni-Laterite Deposit (Dominican Republic)

/ JOAQUÍN. A. PROENZA (1, *), JOHN. F. LEWIS (2), SALVADOR GALÍ (1), ESPERANZA TAULER (1), MANUEL LABRADOR (1), JOAN CARLES MELGAREJO (1), FRANCISCO LONGO (3), GIOVANNI BLOISE (3)

(1) Departament de Cristal.lografia, Mineralogia i Dipòsits Minerals. Facultat de Geologia. Universitat de Barcelona, C/ Martí i Franquès s/n, E– 08028 Barcelona (Spain) (2) Department of Earth and Environmental Sciences, The George Washington University, Washington, D.C. 20052 (U.S.A) (3) Falcondo XStrata Nickel, Box 1343, Santo Domingo (Dominican Republic)

INTRODUCTION. accounts for 40% of the total ore colloform texture, characteristic of the reserves. fill of open spaces from a solution. Mine geologists use the term In the Dominican Republic upward However, in most samples, the colloform “garnierite” for the green Ni-rich silicate movement of the serpentinized garnierite is fractured and brecciated, minerals that occur in many Ni-laterite peridotites occurred in the late and the fragments are cemented by a deposits. However, garnierite is not a Oligocene and it is generally agreed that second generation of garnierite. Quartz mineral species recognized by the the peridotites were exposed to and chalcedony fill the remaining pore Commission on New Mineral and weathering and erosion in the early spaces in the vein. Mineral Names (CNMMN). Actually, Miocene. Lateritization began at this garnierite is a general name for the Ni- time, and continues today but it is not Mg hydrosilicates that usually occur as suggested that optimum conditions for an intimate mixture that commonly laterization have necessarily been includes two or more of the following continuous since the Early Miocene, but minerals: serpentine, talc, sepiolite, that they remained favorable for a smectite, and chlorite (e.g. Brindley and mature lateritic soil profile to develop. Hang, 1973; Springer, 1974; Brindley et This Miocene land surface was al., 1979; Gleeson et al., 2004). subsequently broken into blocks by vertical movements associated with Here, we report new data on the transpressional movement along major mineralogical composition and mineral faults (Lewis et al., 2006). chemistry of garnierite veins in Ni laterites presently being mined from the GARNIERITE MINERALIZATION: MODE OF fig 1. Garnierite veins in saprolite horizont. S: Falcondo Mine in the central Dominican OCCURRENCE. saprolitized peridotite. Republic. Samples were investigated using X-ray diffraction (XRD), optical Most of the garnierite ore is found in the microscopy, scanning electron saprolite horizon (Fig. 1) as tension microscopy (SEM), transmission electron fracture-fillings (mm to cm-thick veins). microscopy (TEM), and electron probe It also occurs as a boxwork fabric, thin microanalysis (EMPA). coating on joints, and in small tension gashes in the limonite zone. Garnierite DOMINICAN Ni-LATERITE PROFILE ores also occur in unweathered peridotite, as vein fillings and thin Ni-laterite is well-developed over the coatings. Loma Caribe serpentinized peridotites, exposed in the Cordillera Central of the Three types of breccia are distinguished, fig 2. Garnierite fragments (G1: mainly Ni-rich Dominican Republic. The Loma Caribe all with complex textures and sepiolite and quartz) cemented by later generation peridotite is one of the occurrences of mineralogy: of garnierites (G2: mainly Ni-rich talc). Wide of ophiolite-related ultramafic rocks that (i) fault breccias mainly composed of photography 4.5 cm. crops out along the northern plate sepiolite-falcondoite and quartz. MINERALOGICAL CHARACTERIZATION. margin of the Caribbean Plate (Lewis et (ii) breccias consisting of hard al., 2006; Proenza et al., 2007). saprolitized peridotite clasts cemented The mineralogy of the garnierite by garnieritic material. mineralization is highly variable. XRD In Dominican Ni-laterites most of the (ii) breccias of garnierite fragments patterns of garnierites show peaks at ~7 nickel is found in the saprolite zone cemented by a later generation of Å and ~10 Å, characteristic of the below a relatively thin cover of limonite. garnierite of different composition (fig. structures of serpentine group minerals These deposits are classified as the 2). and talc-like minerals. The dominant hydrous silicate-type. The saprolitic zone Commonly garnierites display typical serpentine mineral is lizardite 1T. In

palabras clave: Níquel, laterita, garnierita, nepouita, falcondoita, key words: Nickel, laterites, garnierite, nepouite, falcondoite, Republica Dominicana, Caribe. Dominican Republic, Caribbean.

resumen SEM/SEA 2008 * corresponding author: [email protected]

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addition, XRD results show that Si garnierite veins include sepiolite- Talc-willemseite falcondoite and quartz, commonly associated with “chrysoprase” (a green Lizardite-nepouite cryptocrystalline variety of quartz with ~ 2 wt% Ni). Sepiolite-falcondoite Ni-Sepiolite

Powder diffraction profile refinement and TEM analysis confirmed the Falcondoite Ta l c structure type for both sepiolite and falcondoite, with refined cell parameters Willemseite of 13.385(4), 26.955(9), 5.271(3) Å and

13.33(1), 27.03(2), 5.250(4) Å and Sepiolite series space group Pncn. Estimated Talc series crystallinity for sepiolite (average size 14 nm) is much better than that observed

in falcondoite (average size 9 nm). TEM Ni-lizardite Serpentine series images of sepiolite display aggregates Nepouite with a characteristic fibrous morphology

(fibers > 3 μm).

The “garnierite”-like minerals have Mg Fe+Ni composition ranging from: (i) Ni--bearing fig 3. Si-Mg-(Fe+Ni) ratios for Ni-bearing hydrosilicates in garnierite ores from Falcondo Mine talc to willemseite (up to 25 wt% Ni), ii) (Dominican Republic). Compositional fields are taken from Brand et al. (1998). Ni-lizardite to nepouite (up to 34 wt% Ni), and iii) Ni-sepiolite to falcondoite (up REFERENCES Our results show that Dominican to 24 wt% Ni) (fig. 3). Systematically, Ni- rich phase are iron-poor (< 0.5 wt% Fe), garnierites consist of fine-grained Brand, N.W., Butt, C.R.M., Elias, M. (1998): indicating a secondary (neoformed) mixture of three solid solutions: lizardite- Nickel laterites: classification and features. Journal of Australian Geology & origin. The amounts of Al, Cr, Ti, Ca, Na nepouite, talc-willemseite, and sepiolite- Geophysics, 17(4), 81-88. and K are very low. falcondoite. From XRD and EMPA, the Brindley, G.W., Hang, P.T. (1973): The nature components of the mixtures can be of garnierite-I Structure, chemical Some EMPA show significant deviations identified or the mole fractions of the compositions and color characteristics. from the composition of talc, serpentine diverse phase can be calculated. Clays and Clay Mineral., 21, 27-40. and sepiolite series, Brindley, G.W., Bish, D.L., Wan, H.M. (1979): (octahedral/tetrahedral cation ratios Characterized talc-like minerals belong Compositions, structures, and properties of nickel-containing minerals in the and the water contents calculated by to the willemseite [(Ni,Mg)3SiO4O10(OH)2] kerolite-pimelite series. Am. Mineral., 64, - talc [(Mg)3SiO4O10(OH)2] series. Very difference). These deviations are 615-625. consistent with the presence of quartz often Ni rich talc-like minerals (d001 Cluzel, D., Vigier, B. (2008): Syntectonic intergrown with garnierites and the fine- ~10Å) in Ni-laterites have been referred mobility of supergene nickel ores from grained mixture of Ni-containing to as the kerolite-pimelite series New Caledonia (Southwest Pacific). serpentine and talc (Brindley and Hang, [(Mg,Ni)3Si4O10(OH)2.H2O] (e.g. Brindley Evidences from faulted regolith and 1973; Springer, 1974 Brindley et al., et al., 1979; Gleeson et al., 2004). garnierite veins. Resource Geology, 58, 1979). From fig. 3 it is evident that However, kerolite and pimelite are not 161-170. garnierite samples with an intermediate mineral species recognized by the Genna, A., Maurizot, P., Lafoy, Y., Augé, T. CNMMN. (2005): Contrôle karstique de composition, between talc (high minéralisation nickéliferes de Nouvelle- Si/(Mg+Ni ratio) and serpentine (low Calédonie. C.R. Geoscience, 337, 367- Si/(Mg+Ni ratio) correspond to mixtures Finally, chemical compositions of 374. of talc-like minerals and serpentine, Dominican sepiolite-rich garnierite cover Gleeson, S.A., Herrington, R.J., Durango, J., consistent with the XRD results. a large interval of falcondoite-sepiolite Velázquez, C.A. (2004): The mineralogy solid solution (Fal3 and Fal70). These and geochemistry of the Cerro Matoso DISCUSSIONS AND CONCLUSIONS. compositions suggest a complete S.A. Ni laterite deposit, Montelíbano, miscibility along the sepiolite- Colombia. Economic Geology, 99, 1197- falcondoite join. 1213. Fault breccias containing garnierite Lewis, J.F., Draper, G., Proenza, J.A., Espaillat, clasts in turn cemented by garnierite is J., Jimenez. J. (2005): Ophiolite-Related evidence of syn-tectonic precipitation of ACKNOWLEDGMENTS. Ultramafic Rocks (Serpentinites) in the supergene Ni-Mg hydrosilicates. This Caribbean Region: A Review of their relationship between brittle tectonic This research has been financially Occurrence, Composition, Origin, structures and generation of laterite supported by the Spanish projects Emplacement and Ni-Laterite Soils profile has been well documented in CGL2006-07384 and the by SGR00589 Formation. Geol. Acta, 4, 237-263. New Caledonian lateritic weathering of the Catalonian Government. JAP and Proenza, J.A., Zaccarini, F., Lewis, J., Longo, JFL gratefully acknowledge the help and F., Garuti, G. (2007): Chromite composition profile (Cluzel and Vigier, 2008). and platinum-group mineral assemblage According to Genna et al. (2004), the hospitality extended by the staff of of PGE-rich Loma Peguera chromitites, dominant deformation mechanism is of Falcondo mine (XSTRATA). Loma Caribe peridotite, Dominican the crack-seal type. Republic. Can. Mineral., vol. 45, 211-228. Springer, G. (1974): Compositional and structural variations in garnierites. Can. Mineral., 12, 381-388.