Geochemistry and Mineralogy of PGE in the Falcondo Ni-laterite Deposits, Dominican Republic T. Aiglsperger, J.A. Proenza, Manuel Labrador Departament de Cristal.lografia, Mineralogia i Dipòsits Minerals. Facultat de Geologia Universitat de Barcelona, C/ Martí i Franquès s/n, E-08028 Barcelona, Spain
[email protected];
[email protected];
[email protected] F. Zaccarini, G. Garuti Department of Applied Geological Sciences and Geophysics,The University of Leoben, P. Tunner Str, 5, A-8700 Leoben, Austria
[email protected] F. Longo Falcondo Xstrata Nickel, Box 1343, Santo Domingo, Dominican Republic
Abstract. Two laterite profiles from the Falcondo Nilaterite deposit in the Dominican Republic have been analysed to their PGE contents. General PGE enrichment is observed in both profiles close to the exposed limonitic horizon as well as within the saprolitic horizon. Chondrite normalized patterns of the saprolite and limonite show flat PGE trends similar to average mantle peridotites. PGE enrichment is mainly controlled by the presence of chromitites, however, supergene processes have influenced the re-distribution of PGE, leading to local enrichments of these elements, especially in the upper limonite. One limonite sample (total PGE content 212 ppb) and one saprolite sample (total PGE content 62 ppb) have been investigated for PGM using hydroseparation (HS) technique. In both samples PGM were found included in bigger, progressively weathered awaruite and chromite, whereas irregular shaped free grains of PGM were limited to the saprolite sample. All detected PGM grains are smaller than 20 µm in diameter and mainly consist of Ru-Os-Ir-Fe phases. The possibility of finding precious metals as mineral compounds within the highest horizons of Ni-laterites can play an important role for exploration projects in the future. Keywords: Platinum group elements, platinum group minerals, Ni-laterite, Dominican Republic
1 Introduction Recently, high contents (up to more than 4 ppm) of Platinum group elements (PGE) and Au have been reported from Acoje (Philippines) and Musongati and Kapalagulu (Burundi) laterites (Rusina, 2006; Bandyayera, 1997), thus showing high potential of PGE accumulation related to weathering of ultramafic rocks. However, the question how lateritization can influence PGE enrichment and platinum group minerals (PGM) distribution is still matter of debate. The model of PGM in situ growth within laterite profiles, as proposed by Bowles (1986), faces the possibility of supergene dissolution of pre-existing PGM. High total PGE contents up to 3 ppm, accompanied by the presence of a large number of PGM, were previously reported in the Falcondo Ni-laterite deposit of Dominican Republic, related to small chromitite bodies associated with
serpentinized dunite (Proenza et al., 2007; Zaccarini et al., 2008). However, data available on the PGE concentrations and PGM distribution in the Falcondo laterite hosting these PGE rich chromitites are limited (Proenza et al., 2010). In this contribution we report a detailed study of PGE geochemistry and mineralogy from two different laterite profiles of the Falcondo Nilaterite deposit, with the aim to understand the origin of PGM and the PGE behaviour in the supergene environment.
2 The Falcondo Ni-laterite deposit Located in the central part of the Dominican Republic, the Falcondo Ni-laterite deposit is developed on the Loma Caribe serpentinized peridotite, which mainly consists of lherzolite, clinopyroxene-rich harzburgite and harzburgite containing small masses of dunite (Proenza et al., 2007). The ore minerals, serpentine and “garnierites”, are found within the saprolitic horizon of the Falcondo Ni laterite deposit, which is classified as a silicate-type deposit (highest grade Ni laterite deposits). Falcondo geologists have defined six zones of ore grade, which divides the Falcondo Ni-laterite with increasing depth from the surface as follows: • • • • • •
zone A zone B zone C zone D zone E zone F
→ → → → → →
chocolate-brown limonite ochre-brown limonite soft serpentine hard serpentine serpentinized peridotite unweathered peridotite
However, lateral and vertical distribution of any of these ore types varies randomly within short distances in the laterite profile. For simplification, with view to other Nilaterite deposits in the world, zone A and zone B correspond to the upper limonite, whereas the zones C to E represent the lower saprolite.
3 PGE distribution in weathering profiles Seven laterite samples of different horizons from the chromitite bearing laterite Loma Peguera (LP) as well as
eight laterite samples from the chromitite free laterite Loma Caribe (LC) were analysed for platinum-group elements (PGE) in the Genalysis Laboratory.
zone A to zone B (IPGE < PPGE) and within zone C (IPGE > PPGE) (Fig. 2).
3.1 The Loma Peguera weathering profile Samples were collected in the horizons B, D and E (Fig. 1), whereas one PGE-rich chromitite sample in horizon D (3.6 ppm total PGE) was excluded from the PGE distribution diagram. Total PGE contents vary between 37 ppb and 212 ppb. The maximum concentration is observed near the exposed limonite, whereas the minimum value occurs in the central saprolitic horizon (Fig. 1). Ruthenium has the highest values among all PGE, ranging from 9 (zone D) to 71 ppb (zone B) (Fig. 1). IPGE dominate slightly within the exposed limonite horizon and are clearly enriched relatively to PPGE in one sample within the lower saprolite zone (Fig. 1).
Figure 2. Laterite profile from Loma Caribe with general PGE distribution trends (1) and comparison of IPGE and PPGE contents (2) in ppb; laterite profile includes zones A to F. Black spots mark sample locations; (a) clay rich transition zone; (b) shear zone.
3.3 PGE distribution in the different lithologies
Figure 1. Laterite profile from Loma Peguera with general PGE distribution trends (1) and comparison of IPGE and PPGE contents (2) in ppb; laterite profile includes zone B, D with chromitite body (a) and E. Black spots mark sample locations, white spots mark locations of samples processed by HS.
The chondrite normalized patterns of Fig. 3 show that the saprolite and limonite have a rather flat PGE trend, comparable with those of worldwide distributed mantle peridotite. However, the limonite patterns are shifted up to about one order of magnitude and the saprolites display a Pd positive trend. Chromitite has the highest PGE content and display a high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio.
3.2 The Loma Caribe weathering profile Two samples from the upper limonite (zone A), two samples from the clay rich transition zone from zone A to zone B, and one sample each from zone B, C, D and E were analysed (Fig. 2). Total PGE values of 152 and 165 ppb are observed within the upper part of zone A. The highest PGE concentration is reached within zone B (198 ppb), whereas the lowest occurs in the clay rich transition zone (34 ppb) (Fig. 2). Ruthenium contents are usually the highest among all PGE within the profile and vary from 5 ppb (transition zone A to B) to 47 ppb (zone A) (Fig. 2). Palladium does not correlate with the general trend of the other PGE and reaches the highest value at the generally PGE depleted clay rich transition zone (Fig. 2). IPGE and PPGE follow a similar trend (Fig. 2). Differences can be observed at the highest levels of the limonite (IPGE > PPGE), at the transition zone from
Figure 3. Characteristic chondrite-normalized patterns of samples from Loma Peguera and Loma Caribe laterite profiles. Gray field = mantle peridotites. Normalization values from Naldrett and Duke (1980).
4 The assemblages and compositions of PGM: preliminary results One saprolite sample and one sample from the exposed limonite horizon from Loma Peguera were processed by hydroseparation (HS) in the HS-11 laboratories in St. Petersburg and at the University of Barcelona, respectively. Two PGM, containing Os-Ru-Ni-Fe and Os-Ir occurring included in awaruite (Fig. 4A) and in the contact awaruite-altered chromite, respectively, were found in the limonite sample. In the saprolite sample free PGM grains with high porosity and irregular shape (Figs. 4B and 4C) were found beside PGM grains included in chromite. Chemical composition of these grains is dominated by Ru-Os-Ir-Fe. All the described PGM are smaller than 20 µm.
5 Concluding remarks PGE enrichment in the Falcondo Ni-laterite deposit is mainly controlled by the presence of chromitites. However, PGE concentration in limonite is quite high compared with mantle peridotite not affected by lateritization. Therefore, supergene processes can influence the re-distribution of PGE, producing local enrichment in these elements. PGM were found in the saprolite and in the exposed limonitic horizons of the Falcondo Ni-laterite deposit. They represent a good example of PGM in a Ni-laterite profile derived from ophiolite-related mantle peridotites. According to their texture and mode of occurrence, most of the PGM, although partially altered, have a primary origin. Free grains of PGM with corroded surface textures occur only in the saprolite. Hydrolysis processes could have led to chromite dissolution and therefore to liberation of these PGM grains. Our results suggest that PGM-bearing lateritic profiles are mainly the residue of a primary enrichment in bedrock.
Acknowledgements This research has been financially supported by the Spanish project CGL2009-10924 and a PhD grant to TA sponsored by the Ministerio de Educación (Spain). The authors gratefully acknowledge the help and hospitality extended by the staff of Falcondo mine (XSTRATA). Many thanks to the University Centrum for Applied Geosciences (UCAG) for the access to the E. F. Stumpfl electron microprobe laboratory.
References Bandyayera D (1997) Formation de laterites nickelifères et mode de distribution des éléments du groupe du platine dans les profils lateritiques du complexe de Musongati, Burundi: PhD thesis (unpubl.): Université du Quéebec à Chicoutimi: 440 pp. Bowles JFW (1986) The development of platinum-group minerals in laterites: Economic Geology 81: 1278-1285. Naldrett AJ, Duke JM (1980) Platinum metals in magmatic sulfide ores: Science 208: 1417–1428. Rusina Mining NL (2006) Quarterly report for the period ended 30 september 2006: http://www.rusina.com.au Proenza JA, Galí S, Labrador M, Tauler E, Zaccarini F, Garuti G, Lewis JF, Longo F (2010) Platinum Group Minerals in the Saprolitic Horizon of Falcondo Ni-laterite Deposits, Dominican Republic: 11th International Platinum Symposium: Ontario Geological Survey: Miscellaneous Release-Data 269. Proenza JA, Zaccarini F, Lewis JF, Longo F, Garuti G. (2007) Chromian spinel composition and the Platinum Group Minerals of the PGE-rich Loma Peguera chromitites, Loma Caribe peridotite, Dominican Republic: Canadian Mineralologist 45: 631-648.
Figure 4. Back scattered electron images of PGM from Loma Peguera; (A) in awaruite included PGM from limonite sample; (B and C) free PGM from saprolite sample; aw = awaruite, chr = chromite.
Zaccarini F, Proenza JA, Rudashevsky NS, Cabri LJ, Garuti G, Rudashevsky VN, Melgarejo JC, Lewis JF, Longo F, Bakker RJ, Stanley CJ ( 2008) The Loma Peguera ophiolitic chromitite (Central Dominican Republic): a source of new platinum group mineral (PGM): Neues Jahrbuch für Mineralogie 185/3: 335-349.
