Mineralium Deposita (2000) 35: 260±281
Ó Springer-Verlag 2000
ARTICLE
B. Stribrny á F.-W. Wellmer á K.-P. Burgath T. OberthuÈr á M. Tarkian á T. Pfeier
Unconventional PGE occurrences and PGE mineralization in the Great Dyke: metallogenic and economic aspects
Received: 12 September 1998 / Accepted: 7 December 1999
Abstract Platinum group elements (PGE) are strategic materials because 96±99% of the world production is derived from just ®ve mining districts and because they cannot be replaced as catalysts in many chemical processes. In order to lessen the strategic character of PGE, both conventional deposits and unconventional PGE mineralizations were investigated in an attempt to locate viable deposits which would diversify the supplier base. In the Great Dyke, conventional PGE mineralization occurs in the form of pristine sul®de ores mined underground and oxidic surface ores. New observations such as bimodal distributions of the PGE in the Main Sul®de Zone (MSZ), elevated Pt/Pd ratios in the oxidized MSZ compared to the sul®dic part and distinct dierences between the platinum group mineral (PGM) assemblages of the MSZ and stream sediments of adjacent rivers emphasize the fact that even though the Great Dyke seems to be the second or third largest PGE occurrence in the world, the complicated PGE distributions and supergene redistributions should be kept in mind during planning and mine operation. Investigations of unconventional PGE occurrences in ophiolites, Alaskan-type intrusions, porphyry copper deposits and in the Kupferschiefer show that economically exploitable PGE concentrations can be expected in a broader variety of host rocks than considered favourable in the past. In the Albanian Mirdita ophiolite average contents of 860 ppb Pt and 60 ppb Pd were detected. Flotation concentrates of porphyry copper deposits, for example from Mamut, Malaysia, Santo Tomas, Philippines, Editorial handling: F. M. Meyer B. Stribrny (&) á F.-W. Wellmer á K.-P. Burgath T. OberthuÈr á T. Pfeier Bundesanstalt fuÈr Geowissenschaften und Rohstoe (BGR), Postfach 510153, 30631 Hannover, Germany e-mail:
[email protected] Tel.: +49-511-6432583; Fax: +49-511-6433664 M. Tarkian Mineralogisch±Petrographisches Institut der UniversitaÈt Hamburg, Grindelallee 48, 20146 Hamburg, Germany
Elacite, Bulgaria, and Ok Tedi, Papua New Guinea, contain between 827 and 1860 ppb Pd + Pt. In selected pro®les of the Polish Kupferschiefer average contents of 255 ppb Pt, 94 ppb Pd, 2.4 ppm Au and 13.0 ppm Ag were analysed. The distribution of the PGE resources in the world and the annual production rates, however, underline the fact that the chances for a signi®cant change in the supplier base are relatively low. The Bushveld Complex will remain the largest producer, followed by Noril'sk-Talnakh, Sudbury and Stillwater. If the operations in the Great Dyke reach their planned capacities, the Great Dyke will rank in third place among the PGE-producing deposits in the world.
Introduction Platinum group elements (PGE) were selected as strategic mineral commodities for the International Strategic Minerals Inventory (ISMI) (Sutphin and Page 1986). ISMI uses the term ``strategic mineral'' for mineral ore and derivative products that are largely or entirely from foreign sources that are dicult to replace and that are important to a nation's economy. PGE belong to the strategic mineral commodities because 96±99% of the world production is in just ®ve mining districts linked to ®ve intrusive complexes (Stribrny 1996) and cannot be replaced in their function as catalysts in cars and many industrial processes, which in part are made necessary by increasingly stringent environmental standards. In 1997 the world platinum supply amounted to 154.5 t (Johnson Matthey 1998 and Wilson and Prendergast 1998; Table 1). The mines operating in the South African Bushveld Complex produced 115.2 t Pt. At Noril'sk-Talnakh, in northern Siberia, about 20.2 t Pt were produced, followed by Sudbury, Canada, with 4.85 t Pt, and Stillwater, USA, with 2.61 t Pt (Fig. 1). The initial production from mines in the Great Dyke of Zimbabwe has yielded 2.1 t Pt. The remaining 9.54 t are sales from Russian stockpiles or Pt production from other deposits, for example, placers in Colombia. In the
261 Table 1 World platinum and paladium production in 1997. (Data from Johnson Matthey 1998 and Wilson and Prendergast 1998) Year Pt Pd Pt rel.% rel.% (t/a) Bushveld 1997 Noril'sk-Talnakh 1997 Sudbury 1997 Stillwater 1997 Great Dyke 1997 Russian stockpiles and others 1997 World production 1997
74.6 13.1 3.14 1.69 1.35 6.17 100
Pd (t/a)
24.8 115.2 56.0 27.6 20.2 62.2 3.4 4.85 7.58 3.6 2.61 8.16 0.7 2.1 1.56 39.9 9.54 90 100 154.5 225.5
last few years, the contribution of placer deposits to the overall supply has not exceeded 1%. Signi®cant increases in Pt production of alluvial mines have been reported from eastern Russia. World palladium supplies decreased by 18.3 t to 225.5 t in 1997. In contrast to platinum, most of the world production of palladium came from Russia: an estimated 62.2 t from Noril'sk-Talnakh and 87 t from stockpiles. The mines of the Bushveld Complex contributed 56.0 t Pd. Stillwater, Sudbury and the Great Dyke, produced 8.16, 7.58 and 1.56 t Pd. The remaining 3.0 t Pd came from other deposits. The strategic character of PGE becomes even clearer when the global resources are considered. The identi®ed Fig. 1 Location of the conventional PGE deposits and the unconventional PGE mineralizations under investigation in ophiolites, Alaskantype intrusions, porphyry copper deposits, and the Kupferschiefer
economic resources [R1 E, known economically exploitable deposits, and R2 E, extensions of known economically exploitable deposits, of the United Nations resource categories (Schanz 1980), which can be compared with the proved mineral reserve (code 111) and the probable mineral reserve (codes 121 + 122) of the UN International Framework Classi®cation for Reserves/ Resources (Kelter, in preparation)] of the above-mentioned ®ve major PGE deposits have been estimated by ISMI (Sutphin and Page 1986) to amount to 61,428 t of Pt and Pd (Table 1). The Bushveld complex contains 49,970 t Pt + Pd (81.4%). In Noril'sk-Talnakh, the PGE resources amount to 5980 t Pt + Pd (9.7%). The Great Dyke deposits contain 4320 t Pt + Pd (7.0%). The resource estimations for Stillwater and Sudbury are 940 t and 218 t Pt + Pd (1.5% and 0.4%) respectively. Investigations for new deposits in both conventional and unconventional host rocks were carried out in order to lessen the strategic character of PGE and to diversify the supplier base. The term ``conventional PGE mineralization'' covers PGE enrichments in: ± layered intrusions (e.g. Bushveld, Stillwater, Great Dyke), ± magmatic nickel sul®de deposits (e.g. Noril'sk-Talnakh, Sudbury), ± placer deposits (e.g. Colombia, Russia). ``Unconventional PGE mineralizations'' are all types of PGE occurrences in sedimentary, metamorphic and magmatic rocks that do not belong to the above-men-
262 Table 2 Ore tonnages, total metal contents and grades of conventional and unconventional PGE mineralization. (Data from Buchanan 1979; Sutphin and Page 1986; Vermaak 1995; Naldrett 1999; and present work) Deposit
By-products or main products
Ore tonnage Pt Pd Pt Pd Pd/2.21 PtE (t ´ 106) tonnage (t) tonnage (t) grade (g/t) grade (g/t) =Pt+Pd/2.21 (g/t)
Bushveld, Merensky Reef, Rep. S. Africa Bushveld, UG2, Rep. S. Africa Bushveld, Platreef, Rep. S. Africa Noril'sk-Talnakh, Russia Stillwater, USA Sudbury, Canada Great Dyke, Zimbabwe Kupferschiefer, Poland Bregu i Bibes, Albania Mamut, Malaysia Ok Tedi, Papua New Guinea Elacite, Bulgaria Sum Mean
Cu, Ni, Co
2136
10,300
4380
4.82
2.04
0.923
5.74
Cu, Ni, Co
3726
13,600
11,300
3.65
3.05
1.380
5.03
Cu, Ni, Co
695
4960
5430
7.14
7.82
3.538
10.68
1642 50 1648 1128 1040 15 179 275
1560 210 106 2592 83 14.0 2.7 0.5
4420 730 112 1728 42 0.9 8.2 7.4
0.95 4.24 0.34 2.30 0.08 0.86 0.015 0.002
2.70 14.83 0.63 1.53 0.04 0.06 0.046 0.027
1.222 6.710 0.285 0.69 0.018 0.027 0.021 0.012
2.17 10.95 0.63 2.99 0.10 0.89 0.036 0.014
185 12,719 1060
1.0 33,429 2786
4.6 28,163 2347
0.0055 24.41 2.03
0.025 32.80 2.73
0.011 14.84 1.24
0.017 39.25 3.27
Cu, Ni, Co Cu, Ni, Co Ni, Cu, Co Cu, Ni, Au, Co Cu, Ag, Pb, Zn, Au Cr Cu, Au Cu, Au Cu, Au
tioned types. The following occurrences were investigated (Fig. 1): ± new conventional PGE deposits just entering production, for example those of the Great Dyke, Zimbabwe, ± potential alternatives in the form of unconventional PGE occurrences in ophiolites, Alaskan-type intrusions, porphyry copper deposits and in the Kupferschiefer, ± the mineralogy and worldwide distribution of platinum group mineral (PGM) enrichments in placer deposits (Cabri et al. 1996).
Metallogenetic aspects of PGE mineralizations PGE in layered intrusions: the Great Dyke, Zimbabwe Geological setting The ca. 2.58-Ga-old Great Dyke is a layered intrusion of linear shape that strikes over 550 km NNE at a maximum width of about 11 km, and cuts Archean granites and greenstone belts of the Zimbabwe craton (Worst 1960). In its present level of erosion, the Great Dyke comprises a slightly sinuous, and locally faulted, line of ®ve layered ultrama®c±ma®c complexes called the Musengezi, Darwendale, Sebakwe, Selukwe and Wedza subchambers. Stratigraphically, each complex is divided into a lower Ultrama®c Sequence of dunites, harzburgites, olivine bronzitites and pyroxenites, together with narrow layers of chromitite basal to cyclic units, and an upper Ma®c Sequence mainly consisting of a variety of plagioclase-rich rocks (norites, gabbronorites, olivine gabbros). Economic concentrations of PGE, Ni and Cu are found some metres below the transition from the Ultrama®c to the Ma®c Sequence, in the ``Main Sul®de
Zone'' (MSZ), several metres thick, in pyroxenitic host rocks. The MSZ is characterized by disseminations of mainly intercumulus Fe±Ni±Cu sul®des. Although the Great Dyke of Zimbabwe constitutes the world's second or third largest reserve of PGE after the Bushveld Complex in neighbouring South Africa (Sutphin and Page 1986), mine production of PGE has only commenced recently at the Hartley Platinum mine (Broken Hill Proprietaries Ltd, BHP, and DELTA GOLD, now owned by ZIMPLATS and currently under care and maintenance) some 70 km south-west of Harare, and at Mimosa mine (ZIMASCO) near Zvishavane. Further exploration projects are those at Ngezi (ZIMPLATS) and Unki (Anglo American Corporation, AAC Zimbabwe). Analytical methods Cores and underground samples covering the MSZ were obtained from the various prospects and mines. The ®rst aim was to determine the mineralogy of PGM and the mineralogical siting of the PGE in the ores. Therefore, the samples were subjected to detailed mineralogical work including optical microscopy, and electron- as well as limited proton- (PIXE) and ion-probe (SIMS) microanalysis (OberthuÈr et al. 1997a, b, 1998; Weiser et al. 1998). However, new scienti®c targets have emerged in the course of the work. Both the supergene redistribution of PGE in weathered MSZ ores and the ®nal product of element remobilization in the form of detrital PGM in recent rivers draining the Great Dyke have been investigated (OberthuÈr et al. 1997b, 1999). Results Distribution of PGE and PGM in MSZ ores Investigated samples of the MSZ from Hartley mine (OberthuÈr et al.
263
1997a, b) contain about 3±4 vol% sul®des and traces of oxides, which mainly occur interstitially between the grains of cumulus orthopyroxene. The sul®de assemblage consists of pyrrhotite, pentlandite, chalcopyrite and subordinate pyrite. Oxide minerals include rutile, ilmenite, chromite and loveringite. Unusual compositions for the oxide minerals were reported by Johan et al. (1989) and were substantiated by microprobe analyses. The rutile is Cr- and Zr-bearing (up to several wt% each), chromite has up to 2.82 wt% TiO2, together with low Mg contents (0.67±3.23 wt% MgO), and loveringite is a collector of a wide spectrum of incompatible elements. These mineral compositions are attributed to crystallization from residual magmatic melts, possibly with participation of a late-magmatic ¯uid phase. Previously, it was assumed that the PGE are generally present as discrete platinum group minerals (PGM). However, our studies (OberthuÈr et al. 1997a, b; Weiser et al. 1998) disclosed a bimodal distribution of PGE, which occur as (1) discrete PGM, and (2) also in appreciable though variable amounts in various sul®des. 1. At Hartley mine, about 20 dierent PGM species, and some hitherto unnamed compounds have been detected so far. Most of the PGM are (Pt,Pd)-bismuthotellurides followed by PGE sul®des and sulfarsenides. Grain sizes range from 20 to 150 lm in diameter (Fig. 2A). In addition, anhedral grains of gold were observed in and close to chalcopyrite. 2. PIXE investigations showed a pronounced partitioning of the PGE into dierent sul®des: pentlandite constantly carries major contents of Pd and Rh (up to 2236 and 259 ppmw respectively), whereas chalcopyrite and pyrrhotite only occasionally have PGE contents above their limits of detection. SIMS analysis revealed that pyrite is a constant carrier of Pt (0.4±244, mean 35.5 ppm). Pentlandite has a mean content of 8.5 ppm Pt. An estimate based on the Hartley data set indicates that most of the Pt is present in the form of discrete PGM. In contrast, Pd mainly appears to be hosted in pentlandite. Oxidized MSZ ores The oxidized MSZ ores taken between 3 and 10 m below surface are competent, light to dark brownish in colour and locally show some greenish or bluish staining. The orthopyroxenes of the pyroxenites show only incipient alteration; however, the interstitial network is ®lled by iron hydroxides and brownish smectites. Rare relict sul®des are surrounded by rims of iron hydroxides. Whereas Pt grades and grade distributions are grossly similar in sul®de and oxidized MSZ ores, Pt/Pd ratios in the oxidized ores are elevated relative to their sul®de counterparts. The PGM present are Pt phases (sperrylite and cooperite/braggite), which can be regarded as representing relict phases of the primary MSZ ores. The PGM of the moncheite group appear to have been disintegrated. Evans et al. (1994) proposed that Pt±Fe alloys
formed from (Pt,Pd)-bismuthotellurides in oxidized ores from the Zinca area. However, so far our studies have found no support for this hypothesis, and the fate of the ``missing'' Pt (ca. 75%) and Pd (100%) not present in a particulate form remains open. Both elements appear to be dispersed either in iron hydroxides or in smectites, probably also in the form of ``PGE oxides''. The larger sizes of gold grains found in the oxidized ores (50± 280 lm) compared to those from sul®de MSZ (1000 ppb PtE shows the area that can be economically mined
complexes'' in the western Cordillera. On the basis of a ``placer index'' of 100 Pt/(Pt + Ir + Os) > 90, Cabri and Harris (1975) concluded that the PGM derive from ma®c±ultrama®c intrusions of the Alaskan type. The major placers are concentrated in the area of the Rio Condoto and the Rio San Juan rivers. A combined heavy-mineral and pebble survey along the Rio Condoto upstream from the village of Condoto and along the tributaries of the catchment area led to the discovery of a zoned ultrama®c intrusion at the drainage divide between the Rio Condoto and the Rio Tarena on the west side of the Western Cordillera (Fig. 5). The ``Alto Condoto intrusion'' (Salinas and Tistl 1992; Tistl et al. 1994) has an area of 5 ´ 8 km, gives rise to mountainous terrain and is covered by dense rain forest. It shows a zonal arrangement with a dunite core surrounded by wehrlite, olivine clinopyroxenite and hornblende(-magnetite) clinopyroxenite (Fig. 5). In contrast to ophiolitic basic and ultrabasic rocks, orthopyroxene is extremely rare. Geochemical and isotopic data suggest a ma®c tholeiitic melt as parent magma (which is approximately
represented by widespread hornblende±plagioclase dykes: zone 5 in Fig. 5). The intrusion caused an intense contact metamorphism of the surrounding wall rock. An outer biotite±hornfels zone and an inner hornblende±hornfels zone can be distinguished. Anatectic hornblende±plagioclase veins with comb-like layers of hornblende (``needle diorites'') are present close to the contact. Panning to ®nd primary mineralizations By extensive panning (30 pans equivalent to about 300 kg material from each site), the origin of the PGM was traced back to the dunitic core of the Alto Condoto intrusion, where 155 shallow holes and 37 test pits with an average depth of 3.5 m (maximum 4.5 m) were sampled. The typical section (from top to bottom) included the following layers (Fig. 6A; average thickness in parentheses): ± black soil rich in organic matter (0.4 m), ± greyish clay (0.2 m), ± iron pan (concretionary iron hydroxides; 0.35 m), c Fig. 4 Geological models of the PGE mineralization in Bregu i Bibes used for estimating the Pt resources. Model 1 shows the mineralized chromitite layer as an inclined layer, in model 2 the sequence forms a syncline
267
± yellow clay (decomposed dunite; 0.7 m), ± weathered dunite with original structure (variable thickness), continuously changing to ± partially weathered dunite in situ with original structure and blocks of unaltered dunite with schlieren and small pods of chromitite.
Four vertical channels were sampled in each test pit, and each layer was sampled separately. A circular channel sample was collected at the base of each test pit (Fig. 6B). Using this sampling procedure, about 25 samples were taken from each test pit. The samples were preconcentrated by panning and examined under a ®eld
268
its, grains with growth defects) and sizes between 700 °C and
