Kupferschiefer)

ELSEVIER Journal of Geochemical Exploration 66 (1999) 17±25 www.elsevier.com/locate/jgeoexp

Exploration for Au and PGE in the Polish Zechstein copper deposits (Kupferschiefer)

A. PiestrzynskiÂ a,Z.Sawlowiczb,* a Department of Ore Deposits, University of Mining and Metallurgy, Mickiewicza av. 30, 30-059 Krakow, Poland b Institute of Geological Sciences, Jagiellonian University, Oleandry Str. 2A, 30-063 Krakow, Poland Accepted 25 February 1999

Abstract

Results of preliminary exploration for possible economic accumulations of gold and PGE in the working Cu±Ag mines of southwest Poland are presented. High contents of Au (avg. 0.5 ppm), Pt (avg. 0.2 ppm) and Pd (avg. 0.1 ppm) occur in oxidized shales and in places in the underlying sandstones. Maximum contents of Ru, Rh and Ir are 22, 12 and 2 ppb, respectively. These shales are characterized by low contents of TOC and sulphides, degraded organic matter, and lower Re-contents in comparison with typical Cu-rich shales (Au 20 ppb, Pt <5 ppb, Pd 4 ppb). Samples with high contents of Pt and Pd are typically rich in Au. Gold is present as native gold, electrum, in native silver, possibly as organometallic compounds, and in some Cu and Ag sulphides. Some of the Pt and Pd occur as diarsenides or Pt-alloys, but most occur in unknown carriers. The most promising areas in the Lubin±Sieroszowice mining district seem to be the transition zones between sulphide-rich facies and the oxidized Rote FaÈule facies developed both laterally and vertically in the lower part of the shales and=or the upper part of the sandstones.  1999 Elsevier Science B.V. All rights reserved.

Keywords: Au; PGE; Kupferschiefer; Cu-deposits; exploration

1. Introduction local but high enrichments in these elements in the mining area of the Polish Kupferschiefer stimulated Possible economically signi®cant accumulations a detailed study. Occurrences of Au and PGE in the of Au and PGE in anoxic sediments have been re- Permian Kupferschiefer from Germany were already ported in the past few years, including the Mo± reported in 1948 by Goldschmidt et al. (in SchuÈller, Ni±PGE deposits in Cambrian black shales of 1959). New studies from these areas reveal various China and Devonian black shales of the Yukon, concentrations in different lithological types, ranging Canada (Coveney et al., 1992), and Zechstein Cu from 43 ppb PGE in the Kupferschiefer to about deposits of Poland (Kucha, 1982; Sawlowicz, 1993b; 1250 ppb PGE in underlying conglomerate and over- PiestrzynskiÂ et al., 1996) (see also Pasava, 1993, for lying limestone (Pfeifer et al., 1997). In the mining a review). A growing interest in the exploration for area of the Polish part of the Kupferschiefer, high precious metals in black shales and reports on very concentrations of noble metals (maximum values: Au 3000 ppm, Pt 340 ppm, Pd 1000 ppm and Ru 53 Ł Corresponding author. E-mail: [email protected] ppm) were described from a very local and thin layer

(few mm to cm) at the base of the Kupferschiefer of the KGHM±Polska Miedz Company, 474 kg of by Kucha (1981, 1982). In ores, BanasÂ and Kijew- gold and 95 kg of Pt C Pd sludge were recovered in ski (1987) found maximum values of 0.21 ppm Pt, 1995. 0.63 ppm Au and 0.08 ppm Pd. A signi®cant enrichment of Au and PGE (10 to 100 times) in bulk samples from the oxidized facies (Rote FaÈule D RF), 2. General geology present within the mining area (Sawlowicz, 1993b), stimulated an exploration program for economically The geology of the Lubin±Sieroszowice dis- important accumulations of these metals. Data and trict was described by Oberc and Serkies (1968). recommendations presented in this paper are the The basement is composed of Proterozoic gneisses, results of a preliminary stage of research and are schists, phyllites and granitoids, overlain by de- based on underground ®eld observations, ®re assay± formed Carboniferous conglomerates, sandstones ICP chemical data for bulk samples and on ore and and mudstones and by clastic Permian rocks (mainly SEM±EDS microscopy. sandstones and conglomerates). The Lower Permian Three underground (700±1200 m deep) Cu±Ag red beds have been divided into lower and upper mines (Lubin, Rudna, Polkowice±Sieroszowice) are units (Klapcinski et al., 1984). The lower red beds in operation in the Fore-Sudetic Monocline (south- (up to 1000 m thick) are composed of conglomerate, west Poland, Fig. 1). Gold and PGM's are recovered sandstone and mudstone and contain in some places from copper concentrates as by-products during met- bimodal volcanics (rhyolites, trachybasalts and rhy- allurgical processes. According to the annual report olitic tuffs) (Ryka, 1981; Pokorski, 1981). The upper

Fig. 1. Lateral distribution of the oxidized facies (Rote FaÈule) and precious metal zones in the Lubin±Glogow mining district: 1 D boundary of the oxidized facies; 2 D Au-, Pt- and Pd-rich (above 1 ppm) zones (after PiestrzynskiÂ et al., 1997, modi®ed). A. PiestrzyÂnski, Z. Sawlowicz / Journal of Geochemical Exploration 66 (1999) 17±25 19 unit (up to 500 m thick) contains mostly ®ne-grained to the deposition of massive ore-bearing sandstone sandstone with local conglomerates. In the upper in the Middle±Late Jurassic (Sawlowicz and Kosacz, part of this unit, the Weissliegende sandstone (up 1995). Oxidized chlorine-rich brines formed in the to 45 m thick) has been distinguished, overlain in underlying thick molasse rocks and entered the places by the so-called boundary dolomite (0±20 cm Kupferschiefer sulphide-rich bed probably in the thick), the Kupferschiefer and evaporitic cyclothems Rote FaÈule areas during several pulses (Rentzsch et (limestones, dolomites, anhydrites, rock salts and al., 1976; Jowett, 1986; Oszczepalski, 1989; Wodz- clays) (up to 200 m thick). icki and Piestrzynski,Â 1994). A metal-bearing lower Zechstein black shale formation (Kupferschiefer Ð T1) covers about 54.5% (170,000 km2) of Poland, while the Lubin± 3. Field investigations of the noble metal zone Sieroszowice polymetallic ore deposits on the Fore- Sudetic Monocline embrace only 0.4% of the total Exploration is focused on economically barren Kupferschiefer area. Economic Cu±Ag mineraliza- zones (avg. Cu <1.1%) within Cu-rich areas. The tion occurs at the border between Lower and Upper typical lithology of the sections containing noble Permian sediments in the southwestern part of the metal occurrences is shown in Fig. 2. The red-brown Polish Permian basin (Fig. 1). and grey shales are characterized by low contents of The Kupferschiefer is characterized by highly Cu and organic matter (Sawlowicz, 1991), but native variable thickness, with an average of ¾0.27 m in gold was observed on the surface of crushed and the ore district (Mayer and Piestrzynski,Â 1985). It cut red shale samples. The transition zone between is composed of clay (illite), dolomite and calcite, Cu-rich and Cu-poor zones is observed both laterally organic matter and terrigenous quartz and feldspars. and vertically (Oszczepalski, 1989; Speczik et al., The shale was deposited in deep and shallow shelf 1997). Its most common feature is the presence of environments (Oszczepalski and Rydzewski, 1987). red coloured layers or spots in sandstone (Fig. 3), The ore zone is 0.4±26 m thick and contains three shale or dolomites, or greyish layers in the black types of ore: the black shale (Kupferschiefer), the shales (Fig. 4). In some areas a red layer of dolomitic underlying white sandstones (Weissliegendes), and sandstone and shale is present. The width of the the overlying Werra carbonates. It transgresses the transition zone varies from several cm to ¾1m. underlying rocks at low angles (avg. 6ë). Within It continues horizontally or diagonally, crossing all the Kupferschiefer, oxidized facies (Rote FaÈule) are lithologies (Fig. 2) for up to a few hundred metres. known from several places, one of the largest oc- Macroscopic sulphides are rare, although some hand currences being in the western part of the deposit specimens of red shale contain 1-cm-long diagonal (Rydzewski, 1978) where it forms the western bor- veinlets with chalcopyrite, bornite and galena, and der of the Lubin±Sieroszowice area. vertical veinlets of pyrite occur in red sandstone. To date, 115 ore minerals have been recorded in Au±PGE-rich sections occur locally within the mining area, with the main assemblages consist- mapped Cu-poor areas although it is not clear if ing of chalcocite±covellite group, bornite, chalcopy- a Cu-rich zone occurs above. They have never been rite, pyrite and tennantite (Mayer and Piestrzynski,Â found below massive copper-rich accumulations in 1985), forming disseminations, veinlets, lenses, sandstones. veins, spots and massive ores. The main sampling method for noble metals in- The present controversial genetic models of Cu volves the collection of ¾1 kg samples taken in mineralization range from syn-diagenetic to epige- vertical pro®les (every 20 cm or less in the case of netic, depending mainly on the type of mineralisation changes in lithology), crushing, milling and splitting considered (Wedepohl, 1964, 1994; Jowett, 1986; to 30 g. Au, Pt and Pd were determined by ®re assay Wodzicki and Piestrzynski,Â 1994). The mineralising and ICP. Selected samples of 50 g were are analyzed process was probably complicated and long-term, for Au, Pt, Pd, Rh, Rh, Os and Re by INAA after lasting from the deposition of the Kupferschiefer FA±NiS preconcentration (ACTLABS Canada). (dispersed mineralisation) (Sawlowicz, 1990, 1992) 20 A. PiestrzyÂnski, Z. Sawlowicz / Journal of Geochemical Exploration 66 (1999) 17±25

Fig. 2. Cross-section through the contact between Cu-rich and Rote-FaÈule zones, Polkowice mine: 1 D white sandstone; 2 D carbonate-rich red sandstone; 3 D carbonate-rich brownish sandstone; 4 D red shale; 5 D black shale; 6 D dolomite; 7 D red spots.

Fig. 3. Weissliegendes sandstone with red spots, underlying dark shale (Kupferschiefer) and dolomites (badge D 3cm). A. PiestrzyÂnski, Z. Sawlowicz / Journal of Geochemical Exploration 66 (1999) 17±25 21

Fig. 4. Typical Au±PGE-bearing section containing white sandstone (A), red-brown shale (B), grey shale (C), black shale (D)and dolomite (E).

4. Geochemistry and mineralogy Pfeifer et al. (1997). Maximum contents of Ru, Rh and Ir in a few samples studied are 22, 12 and 2 ppb, High concentrations of Au (max. 100 ppm, avg. respectively. These shales are characterized by low 0.5 ppm), Pt (max. 2.5 ppm, avg. 0.15 ppm) and contents of TOC (2±5%), sulphides (0.2±0.7% S), Pd (max. 4 ppm, avg. 0.08 ppm) (Table 1) occur degraded organic matter and Re (1 ppm) in compari- in the oxidized facies (RF) composed of shales, and son with the Cu-rich shales (Table. 1). Samples with locally in overlying dolomites and in the uppermost high contents of Pt and Pd are typically rich in Au, part of the underlying sandstones (Fig. 2). A signi®- but the reverse relationship is not so obvious. The cant correlation between the noble metal distribution highest enrichment is observed for platinum, with and RF oxidation front has recently been found by Pd=Pt ratios of 1.3 for rich samples (above 500 ppb

Table 1 Contents of Au, Pt, Pd and some other elements in the Lubin±GøogoÂw mining district

Facies Au Pt Pd TOC S Cu Re n (ppb) (ppb) (ppb) (%) (%) (%) (ppm) Cu-rich 48876.52.28.35.325 Secondary oxidized Rote FaÈule 460 145 74 1.8 0.2 0.8 1.5 56 Lithological types: a Zechstein limestones (dolomites) 50 2 1.5 0.5 nd 2.5 nd 27 Kupferschiefer (shale) 2500 186 88 6.5 nd 7.1 nd 23 Weissliegendes (sandstones) 500 54 30 0.2 nd 0.2 nd 19 n D number of analyses; nd D not determined. a From Pfeifer et al. (1997). 22 A. PiestrzyÂnski, Z. Sawlowicz / Journal of Geochemical Exploration 66 (1999) 17±25

Fig. 5. Comparison between gold analyses by FA±ICP (30 g sample) and INAA (¾5 g sample).

Au, Pt and Pd) and 2.2 for those low in Au, etc. of up to 1000% were found for individual sam- (<500 ppb). There is no correlation between noble ples, although with a statistically large population, metals and other elements, except rhenium. The Ag- the difference is less marked. The average Au-con- content in these samples is low. In sections rich in tent for 70 samples was 2.2 ppm with INAA and noble metals, the maximum concentrations of Au are 3.2 ppm with FA±ICP. Thus, for preliminary geo- typically present in the uppermost part of the pro®le chemical mapping, the less-costly INAA analyses and those for Pd are in the lowermost part. However, are suf®cient. Reproducibility of FA±ICP using only it must be stressed that exceptions to this are locally three samples of shale and sandstone splits was poor, common. A typical cross-section through a transition with differences up to 2000% for a sample with low zone is shown in Fig. 2. Au-contents (Table 2). A few samples with red spots were split into red Gold is present as native gold, electrum, as iso- spotted material and grey host rock. Contents of Pt morphic substitutions in native silver, and in some and Pd were higher and contents of Au lower in the Cu and Ag sulphides (Kucha, 1974, 1976; Salamon, red spots, suggesting a close relationship between 1976; Kucha and Piestrzynski,Â 1991; PiestrzynskiÂ et haematite and Pt and Pd. al., 1996; Speczik et al., 1997). Some Pt and Pd oc- Areas with high concentrations of Au, Pt and Pd cur as Pd arsenides or Pt-alloys, but most are within are present mainly in the southwestern part of the mining district, proximal to the extensive Rote FaÈule Table 2 area (Fig. 1). Individual samples with abnormal con- Comparison between determinations of two splits (30 g each) of centrations of Au and PGE were also found in other the same samples by FA±ICP method small secondary oxidized areas within other parts of Sample Au Pt Pd the mining district. (ppb) (ppb) (ppb) A comparison between different methods of gold SPEB1 1722 1496 127 determination was carried out to ®nd a reliable, SPEB1 3747 1583 288 reasonably priced, method of analysis. About 70 SPEE3 99 17 8 samples were determined by ICP with FA precon- SPEE3 5 <5 <3 centrations (30 g sample) and INAA without precon- SPI2 17 31 6 < < centration (5±10 g) (Fig. 5). Signi®cant differences SPI2 15 5 3 A. PiestrzyÂnski, Z. Sawlowicz / Journal of Geochemical Exploration 66 (1999) 17±25 23 unknown carriers (Kucha, 1976, 1982). Mineralog- plex at the contact with a reducing environment of ical studies of noble-metal-rich ores indicate two coal. There is a general agreement that the met- paragenetic groups. The ®rst is composed of native als in the Kupferschiefer were carried in oxidizing, gold, haematite and covellite. This gold is of very chloride-rich solutions which leached underlying red high ®neness (PiestrzynskiÂ et al., 1996). The second beds and volcanics (Rentzsch et al., 1976; Wodzicki group consists of gold, electrum, digenite, half-bor- and Piestrzynski,Â 1994). The Rote FaÈule facies were nite, and covellite, occurring in the uppermost part probably the areas where such solutions entered the of the Weisliegende sandstone and in the transition Kupferschiefer (Rentzsch et al., 1976), and expanded zone of the Kupferschiefer. Electrum is the more during the diagenesis (Oszczepalski, 1989), similar common gold mineral; and the native gold has a to oxidation rolling fronts in the uranium deposits of lower ®neness (¾87%). In addition to Pd-arsenides, Wyoming. Subsequent processes including dissolu- traces of Pt and Pd were found in some gold-bearing tion of Cu and Cu±Fe sulphides, degradation of or- minerals such as haematite and bornite, and in native ganic matter coupled with its sulphidation (Sawlow- gold (PiestrzynskiÂ and Pieczonka, 1997). icz, 1993a), dissolution and reprecipitation of car- Based on ore microscopy, there is a correlation bonates (Sawlowicz and Halas, 1990), formation of between the number of gold grains and the chemical sulphates and thiosulphates (Kucha and Piestrzynski,Â analyses for Au, and this method should be applied 1991), created speci®c physico-chemical conditions as a rough back-up to the analyses as well as for in such areas. The transition zone between the Rote recognizing the character of its occurrences. FaÈule zone (oxidation) and Cu-rich zone (reduction) was the place where very complex reactions leading to Au and PGE concentration could proceed. 5. Genesis of precious metal enrichments Two mechanisms of enrichment can be envis- aged: (1) release of Au and PGE from earlier Au and PGE enrichments are relatively common deposited Cu±Fe sulphides during an enlargement in shaly and organic-rich sediments associated with of Rote FaÈule areas and their deposition at the thick complexes of sandstones and conglomerates oxidation=redox front; (2) late stage introduction (often with volcanics) or carbonates (Kupferschiefer; of Au±PGE-rich solutions and deposition in the tran- Zambian Copper Belt Ð Unrug, 1985; Jedwab, sition zone. 1988; Cu±Ag mineralization in the Proterozoic of Namibia Ð Borg et al., 1988). Organic-rich, often pyritic-rich, sediments serve as a low-permeability 6. Exploration model redox barrier. The transport of Au and PGE in so- lution is now generally accepted especially in view Documented areas of low Cu-contents bordering of increasing experimental data. These noble metals or within Cu-rich areas should be the ®rst choice can be transported as chlorine, cyanide or thiosul- in the search for noble metals. The most promising phate ions (Mountain and Wood, 1987, 1988; Wood target seems to be the transition zone between the et al., 1989; Jaireth, 1992) and a signi®cant role in sulphide-rich facies and the typical oxidized Rote transport can also be played by organic acids (Wood, FaÈule facies which developed both laterally and ver- 1990; Bowles et al., 1994a,b). The precipitation of tically (5±50 cm), in the lower part of the shales Au and PGE from oxidizing solutions could result and=or the upper part of the sandstones. Primary from higher pH and Eh potential at the redox barrier oxidized facies which were never mineralized do not of the Kupferschiefer black shale. One of the main seem to be promising for exploration. reducing factors was probably the organic matter Features signi®cant for precious metal exploration (Kucha, 1982; Sawlowicz, 1993b). The reduction of in the Kupferschiefer are as follows: PGE by sedimentary matter has been described by (1) Low content of Cu and lower contents of Baranger et al. (1991) and Bowles et al. (1994a). TOC, S and Re in shales (usually clay±carbonate Van der Flier-Keller (1991) suggested precipitation shales). of PGE from groundwaters from an ultrama®c com- (2) Dark-red or brownish colour of shales and=or 24 A. PiestrzyÂnski, Z. Sawlowicz / Journal of Geochemical Exploration 66 (1999) 17±25 sandstone, typically below Cu-rich black shales (oc- Accompanying Metals in Copper Ores. CUPRUM, Wroclaw, casionally shales are macroscopically black, but nu- pp. 49±63. merous red internal re¯ections from ®nely dispersed Baranger, P., Disnar, J.R., Gatellier, J.P., Ouzounian, G., 1991. haematite are visible under the ore microscope). Metal reduction by sedimentary organic materials: In¯uence of medium parameters on the reaction rate. In: Pagel, M., (3) Presence of sulphide relics, mainly chalcopy- Leroy, J.L. (Eds.), Source, Transport and Deposition of Metals. rite and covellite. Balkema, Rotterdam, pp. 511±514. (4) Presence of coarse grains of haematite of Borg, G., Tredoux, M., Maiden, K.J., Sellschop, J.P.F., Way- hydrothermal origin in red coloured strata. ward, O.F.D., 1988. PGE- and Au-distribution in rift-related (5) Red spots and poor Cu mineralization in the volcanics, sediments and stratabound Cu=Ag ores of Middle = upper part of sandstones. Proterozoic age in Central SWA Namibia. In: Prichard, H.M., Potts, P.J., Bowles, J.F.W., Cribb, S.J. (Eds.), Geo-Platinum (6) Presence of degraded, aromatic compounds Ð '87. Elsevier, Amsterdam, pp. 303±317. rich organic matter Ð in black shales. Bowles, J.F.W., Gize, A.P., Vaughan, D.J., Norris, S.J., 1994a. (7) Vertical separation of Au and Pt, Pd, with Au Development of platinum-group minerals in laterites Ð initial enrichment overlapping Cu-mineralization. comparison of organic and inorganic controls. Trans. Inst. We suggest that before carrying out a costly stage Min. Metall. (Sect.B) 103, 53±56. involving Au, Pt and Pd analyses, samples should be Bowles, J.F.W., Gize, A.P., Cowden, A., 1994b. The mobility of the platinum-group elements in soils of the Freetown Penin- examined by ore microscopy, Cu analyses and sim- sula, Sierra Leone. Can. Mineral. 32, 957±967. ple organic geochemical analyses (solvent extraction Coveney, R.M., Murowchick, J.B., Grauch, R.I., Nansheng, and IR analysis of aliphatic versus aromatic bands, C., Glascock, M.D., 1992. Field relations, origins, and re- Sawlowicz, 1989). This procedure should help in source implications for platiniferous molybdenum±nickel ores delineating a transition zone. in black shales of south China. Explor. Min. Geol. 1, 21±28. In addition, very local (on mm±cm scale) occur- Jaireth, S., 1992. The calculated solubility of platinum and gold in oxygen-saturated ¯uids and the genesis of platinum± rences with high concentrations of precious metals palladium and gold mineralization in the unconformity-related may be found in samples with high radioactivity, and uranium deposits. Mineral. Deposita 27, 42±54. enrichments in Hg, Ag, Bi, As, P, and organic mat- Jedwab, J., 1988. Mineralogie des Metaux du Groupe du Platine ter, and with low Cu contents. A radiometric survey au Shaba, Zaire. Mem. Acad. R. Sci. Outre Mer, pp. 1±25. could be useful in such cases. Jowett, E.C., 1986. 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