Serpentine As a Natural Nickel Scavenger in Weathering Profiles of the Aguablanca Ni-Cu-(PGE) Deposit (Spain)

Comunicación 142 Ni-Serpentine in the Aguablanca Deposit, SW Spain

Serpentine As a Natural Nickel Scavenger in Weathering Profiles of the Aguablanca Ni-Cu-(PGE) Deposit (Spain). A SEM and HRTEM Study / SAIOA SUÁREZ BILBAO (1, 2)* / FERNANDO NIETO GARCÍA (2) / FRANCISCO VELASCO ROLDÁN (1) (1) Departamento de Mineralogía y Petrología. Universidad del País Vasco, 48940 Lejona (Vizcaya). (2) Departamento de Mineralogía y Petrología e Instituto Andaluz de Ciencias de la Tierra. Universidad de Granada-CSIC. Campus de Fuerteventura, s/n. 18002, Granada. Desintegration of Ni-Cu-(Platinum group elements) sulphides during supergene alteration in the Aguablanca deposit (Badajoz, SW Spain) has resulted in the partial release of base-metals at low temperature conditions. Specially Ni and Cu are being variably retained by several inherited and neoformed mineral phases throughout the developed weathering profiles. A detailed study of these phases using electron microscopy has allow to describe and analyze accurately a neoformed serpentine that represents the main Ni-bearing silicate within the profiles. This serpentine resembles Ni-lizardite and reaches contents of up to 42%wt. NiO. The different mechanisms for metal retention in serpentine are also discussed since nickel occurs both structurally bounded and sorpted as pure native particles. This study examines the processes that contribute to the natural retention of metals in sheet silicates during the exogenic cycle. INTRODUCTION tes that may form the distinctive garnie- Country). The carbon coated samples (< rite in the supergene domain. However, 2 mm) were examined by a LEO 1430- Fine-grained minerals developed during the common fine-grained and inhomoge- VPSEM instrument with a 3.5 nm spa- supergene alteration of silicates in base- neous character of those minerals tial resolution and operating at an acce- metal sulphide deposits might play a sig- made their study very difficult and few lerating voltage of 20 kV, using BSE nificant role in controlling the heavy metal detailed compositional analyses in imaging and EDX analysis, (CIC, fate originated from the oxidation of sul- natural samples have been published. Granada University). phides during the exogenic cycle. Iron and manganese oxides or hydroxides, prefe- This study examines the formation of a Due to the very fine grained nature of rably developed in the upper parts of the Ni-rich serpentine in the weathering profi- the serpentine crystals, two samples weathering profiles, are known scaven- le of the Aguablanca Ni-Cu-(PGE) magma- were selected for a detailed HRTEM gers of heavy metals in ore deposits (e.g. tic sulphide deposit (Ossa Morena Zone, study. The thin sections were prepared Thornbern, 1979). Nevertheless, sheet SW Spain) (e.g. Tornos et al., 2006 and with Canada balsam, ion-thinned using silicates concentrated in the lower parts references therein). The use of high-reso- a Gatan 600 ion mill and carbon coa- of the profiles could also play a potentially lution techniques has allowed a detailed ted. HRTEM lattice-fringe images and important influence on metal retention in analysis of almost monomineralic areas electron diffraction patterns from selec- mining areas due to their well-known sorp- of Ni-serpentine and the evaluation of the ted areas (SAED) were obtained in a tion capacity. different sorption mechanisms of Ni rela- Philips CM20 (STEM) with a LaB6 fila- ted to the present weathering conditions ment, equipped with an EDAX solid Minerals of the serpentine group are at low temperature. state EDX detector operating at 200 kV, common examples of highly Ni-enriched (CIC, Granada University). phases during weathering processes MATERIALS AND METHODS and have been reported in worldwide WEATHERING PROFILES. mineral deposits (e.g. lateritic nickel Four representative samples from the OCCURRENCE OF SERPENTINE deposits from Brazil, New Caledonia, Aguablanca weathering profiles were Australia, etc.). The Ni analogues of selected for this study. They were Supergene alteration in the Aguablanca lizardite and chrysotile (nepouite and collected in the southern orebody of the deposit has formed immature, 6-8 pecoraite, respectively) can retain much deposit, above the unaltered host gab- meter-thick weathering profiles above more than a 30% of NiO, becoming fre- bronorite intrusive of the Aguablanca the unaltered host gabbro and below quently one of the main Ni-bearing pha- Stock. X-ray diffraction studies of both the gossan (10m thick), where base- ses in alteration zones of mineral depo- the < 2 mm and the < 2 µm fractions metal sulphide oxidation took place. sits. Especially interesting are those were carried out using a Philips PW These profiles typically show several nepouites in intimate mixture with kero- 1710 Powder Diffractometer with Cu-Kα horizons related to the main physical lites (10 Å talc) or other hydrous silica- radiation, (University of the Basque and chemical properties of the underl-

palabras clave: niquel, serpentina, HRTEM, yacimiento de key words: nickel, serpentine, HRTEM, Aguablanca deposit Aguablanca

Resumen WORKSHOP 2008: Suárez et al., Macla 10 (2008) 142-144 *corresponding author: [email protected] macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 143

ying host rocks and variable degrees of (Fig. 1A). It is mainly located at the packets and well developed 7 Å packets, supergene metal enrichment. Thus, a edges of chlorite crystals, filling spaces suggesting some intergrowth between ~40 cm thick clay-rich horizon (with con- between layers, along cracks or as a both phases at this scale. Serpentine tents up to 56% of phyllosilicates) was coating in cavity walls or another textu- seems to crystallize mainly in monomine- developed overlying the unweathered ral defects along the layers. In detail, ralic stacks although occasionally, SAED gabbro. Compared to other horizons serpentine displays a microcrystalline patterns in more disrupted areas have within the profile, this clay-horizon character forming aggregates often shown some smectite contamination. shows a clear Ni-Cu enrichment (con- crumbled and mixed with small frag- This is a very common feature of Ni-enri- tents up to 6000 ppm and 3900 ppm, ments of chlorite. Low magnification ched serpentines, but well defined fringes respectively) even though the lack of TEM images normally show vuggy edges with spacings of 10 Å were not detected sulphides (< 0.3% S). Chemical proper- of chlorite filled with disordered arran- in the lattice-fringe images. ties are characterized by a neutral pH gements of serpentine layers folded at [6.7 - 7.0], with a moderate humidity the edges. AEM analyses of the serpentine (n=12) percentage (35%) and a cation inter- were carried out in apparently unconta- change capacity up to 56.28 cmolq/K. minated areas showing regular stac- kings of the layers. The following avera- The mineralogical and geochemical study ge structural formula was obtained cal- of these horizons showed that commonly culated on the basis of a total cation neoformed secondary phases such as valence of 28 positive charges and smectites, vermiculites or Fe-oxides and assuming all the Fe as Fe+2: oxyhydroxides are here acting as Ni-Cu- (Ca 0.02-0.19, K 0-0.05) 0.02-0.24 carrying phases. Values of up to 3.8% (Mg AlVI Fe+2 NiO and 4.0% CuO were measured for 1.22-2.36 0.18-1.16 0.20-1.37 +2 +2 +4 those phases. However, neither the com- Ni 1.58-3.49 Cu 0-0.19 Ti 0-0.17 +2 +4 mon inherited primary silicates nor their Mn 0-0.02) 5.51-6.02 (Si 2.96-4.06 IV hydrothermal alteration products contain Al 0-1.04) 4-4.06 O 10 (OH) 8 significant Ni or Cu in their mineral com- position. Commonly, this mineral mixture This serpentine is characterized by high is dominated by amphiboles, pyroxenes, Ni contents of up to 3.49 afu, with an chlorite, talc, phlogopite and plagiocla- average value of 2.65 afu and high stan- ses. Most of these mineral phases show dard deviation of 0.66. In many cases, NiO and CuO contents of up to 0.31% and nickel occurs as the dominant divalent 0.17%, respectively. octahedral cation, with lower Mg, Fe and Al contents. Cu, Mn and Ti contents are The only exception is the well preserved low whereas Co and Cr were not detec- hydrothermal chlorite, which displays the ted. The total octahedral cations range maximum contents of Ni and Cu detected from 5.51 to 6.02 with an average value in both neoformed or inherited silicates of 5.72 and a low standard deviation of within the profile (values of 12.5% NiO 0.18. Nickel and magnesium show a and 9.06% CuO). A supergene retention clear inverse tendency (r = -0.811, of Ni is deduced since chlorite from the Pearson correlation coefficient) pro- lower fresh host rocks does not display bably being isomorphously replacing significant Ni contents (maximum values each other in different proportions. of 0.76% NiO). This trioctahedral Ni-cli- Besides, Ni shows a good negative nochlore occurs widespread throughout correlation with AlVI (r = -0.713) but is the weathering profile as detrital frag- poorly correlated with other chemical ments dispersed in the horizons (300 µm parameters. Tetrahedral substitution is in size) and to a lesser extent as centime- variable; some analyses showed a tric crystals filling thin veins that cross substantial excess of silica and were cut the profiles (Suárez et al., 2005). rejected. Total Al and Si are moderately correlated (r = -0.621). A detailed high-magnification study of this chlorite has revealed the formation of ser- This studied serpentine is classified as pentine minerals intergrown within the an intermediate phase between the Mg chlorite, reaching contents of up to 42% in and Ni end-members and should be con- NiO. This serpentine is not related to the sidered as the nickeloan form of the serpentinisation process underwent by the magnesian mineral (Ni-lizardite) when deposit which is restricted to the ultrama- Mg > Ni and Ni is ≤ 3.0 afu Fig. 1. A) BSE image of an ion-thinned chlorite sample. B) fic enclaves within the Aguablanca Stock. Lattice fringe image of a monomineralic Ni-serpentine and R Si O (OH) (≤ 30% wt. NiO). Other its corresponding SAED (inset). C) Pure native Ni particles in 6 4 10 8 The following section describes the main the surface of serpentine layers. crystals fall closer to Ni end-members characteristics of the identified serpentine. of the isomorphous series when Ni > High resolution images of those regions Mg and Ni > 3 afu. In such cases, the CHARACTERIZATION OF THE SERPENTINE (Fig. 1B) reveal large areas in which well- serpentine shows here Ni contents bet- defined 7.1-7.3 Å packets are developed. ween 3.13 and 3.49 afu, and they can Serpentine occurs at the submicrosco- Boundaries between chlorite and serpen- be considered as nepouites (Brindley, pic scale penetrating into chlorite laye- tine are characterized in most of the 1978). The chemical composition of ring and growing at the expense of it cases by regular alternations of thin 14 Å this serpentine falls into the “garnieri-

depósito legal: M-38920-2004 • ISSN: 1885-7264 Comunicación 144 Ni-Serpentine in the Aguablanca Deposit, SW Spain

tes field” delimited by Gleeson et al. Chemical and textural studies of the nic- influence of disequilibrium conditions, (2003), ranging from kerolite to pimelite keliferous serpentine have revealed two and moreover, to the different weathering (Mg, Ni-talc minerals) and from serpen- different mechanisms for the Ni reten- conditions at low temperatures, which tine to Ni-bearing serpentine (nepouite). tion during the exogenic cycle: certainly play an important role in the pro- cesses of Ni supergene enrichment Here, serpentine seems to occur prefe- (i) Ni metal structurally bounded (Manceau and Calas, 1985). rably as a nickel lizardite-nepouite rather than as another polymorphic Serpentine shows different Ni contents The correct determination of the sorp- form of the serpentine mineral group. between the Mg and Ni end-members as tion mechanism of metals on clays and Particularly useful in their identification the result of a variable degree of replace- other mineral surfaces must be an are the morphological characteristics ment of Mg+2 by Ni+2, presumably due to important target for understanding the and the XRD patterns since nepouites their similar ionic size and equal charge. fate of such pollutants and could facili- and pecoraites have the same chemical In general, these intermediate minerals tate successful environmental remedia- formula. In this case, serpentine occurs do not conform exactly to the theoretical tion procedures. This research adds to in aggregates displaying a platy morpho- formula of a normal serpentine structure, the understanding of the processes that logy with a flat layering, which is in and it is quite difficult to supply comple- affect the natural retention of metals in strong contrast to the typical fibrous tely satisfactory formulae. In this case, the subaereal environment. nature and cylindrical layering of the some analyzed areas showed some silica chrysotile-pecoraite species. excess and deficiency of R+2 ions. Some ACKNOWLEDGEMENTS authors (e.g. Brindley, 1978) suggested XRD data are less conclusive since an that this deficiency is possibly caused by We thank to Río Narcea Recursos S.A. effective separation of the serpentine and leaching at the edges of the clay parti- Assistance by M. M. Abad, I. Guerra and I. the host chlorite was not achieved. The cles. The leaching not only would remove Nieto (UGR) was essential for this work. basal spacing of the serpentine is close to soluble R+2 ions and OH- from the edge SGIker technical support (MEC, GV/EJ, half that of chlorite and the reflections coin- positions, but also could leave a residue European Social Fund) is also gratefully cide almost exactly with all even-order chlo- of silica. Nevertheless, impurities from acknowledged. Financial support was sup- rite peaks. This situation produces a ratio- silica-rich phyllosilicates in some areas plied by research projects CGL2007- nal 00l diffraction pattern fixed, in which cannot be totally excluded. Smectite for- 66744-C02-01/BTE and CGL2005- rational positions are unaffected by the mation in some areas could explain the 00798/BTE (MEC) and by the research common treatments. On the basis of the silica enrichment or the minor quantities fellowship AP-20034667 (MEC). weak reflections, the presence of the 1.50 of Ca+2 or K+ occasionally recorded. Å peak might indicate the occurrence of REFERENCES nepouite since pecoraite only displays a (ii) Ni metal as pure native particles strong reflection at ~1.53 Å. Regarding to Bailey, S.W. (1969): Polytypism of trioctahe- the layer stacking sequence, the strong dral 1:1 layer silicates. Clays and Clay Min., These nanoparticles do not share the 17, 355-371. 2.387 Å reflection or the invariable absen- typical properties shown by metallic nic- ce of the 2.49 Å peak are probably indicati- kel particles commonly found in high tem- Brindley, G.W. (1978): The structure and che- ve of the group A (2M1>1M=3T) defined by perature environments (e.g. Nickel, mistry of hydrous nickel-containing silicate Bailey (1969) for the polytypism of triocta- and aluminate minerals. Bull. du B. R. G. M. 1959). Besides, Ni enrichment is here Section II, 3, 233-245. hedral 1:1 layer silicates. The most intense restricted to the oxidized rocks within the reflection at 2.326 Å of the group B and the weathering profiles, so their origin is Gleeson, S.A., Butt, C.R.M., Elias, M. 1.945 Å peak of the group B and D are also more congruent with the supergene (2003): Nickel laterites: a review. Soc. Econ. absent on the XRD patterns. According to domain. Nickel-rich products associated Geol., 54, 9-16. Bailey (1969), group A is characteristic of a with sheet silicates during weathering are Manceau, A. & Calas, G. (1985): relative low structural stability. hardly documented. Their physical and Heterogeneous distribution of nickel in chemical properties make difficult an hydrous silicates from New Caledonia ore Nickel was also found occasionally as anhe- accurate classification with the usual deposits. American Mineral., 70, 549-558. dral particles of pure native Ni on the surfa- techniques, so nickel oxides and hydroxi- Nickel, H. (1959): The ocurrence of native ce of the analyzed specimens (Fig. 1C), par- des are commonly deduced due to the nickel-iron in the serpentine rock of the ticularly in highly altered areas. These parti- oxygen detection during the analytical pro- Eastern townships of Quebec province. cles have a globular-elongated shape vie- cedures. In Aguablanca, part of the nickel Canadian Mineral., 6, 307-319. wed parallel to the serpentine sheets, and seems to occur as a pure native metal Suárez, S., Velasco, F., Yusta, I. (2005): small sizes between 3 and 30 nm. Areas of suggesting that the local weathering con- Caracterización química y mineralógica de 100-150 nm display a high density of supe- ditions might have been critical. Further suelos en el yacimiento magmático Ni-Cu de observations are required to determine Aguablanca, Badajoz (España). Macla, 3, rimposed particles following almost the 201-202. same direction of the sheets. EDX analyses the precise environmental and geochemi- suggest that no other elements apart from cal conditions of this process. Thornber, M.R. (1979): Supergene alteration Ni are present. Therefore, a native structu- of sulphides. V. Laboratory studies on the ral state can be deduced. The characteristics of the Ni-serpentine dispersion of Ni, Cu, Co and Fe. Chem. Geol., 26, 135-149. developed at Aguablanca evidence the DISCUSSION AND CONCLUSIONS variability of the substitution of Mg by Tornos, F., Galindo, C., Casquet, C., transition metals during the weathering, Rodríguez Pevida, L., Martínez, C., Martínez, with heterogeneous distribution of the Ni E., Velasco, F. & Iriondo, A. (2006): The Serpentine occurs as an accessory Aguablanca Ni–(Cu) sulfide deposit, SW mineral in the middle-low parts of wea- along the hydrous silicate layers. These Spain: geologic and geochemical controls thering profiles but represents the main common heterogeneities are thought to and the relationship with a midcrustal laye- Ni-bearing silicate in the supergene be related to the different local growth red mafic complex. Mineral. Deposita, 41, domain of the Aguablanca deposit. conditions of the minerals with possible 737-769.

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