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Economic Geology Vol. 84, 1989, pp. 1003-1027

The Kupferschiefer: An Overview with an Appraisal of the Different Types of Mineralization

D. J. VAUGHAN,

Department of Geology, The University, Manchester, M13 9PL, England

M. SWEENEY,

Manchester Business School, The University, Manchester, M13 9PL, England

g. FRIEDRICH, R. DIEDEL,

Institut fiir Mineralogie und Lagerst/ittenlehre, Technische Hochschule Aachen, Wiillnerstrasse 2, 5100 Aachen, West Germany

AND C. HARANCZYK,

Institute of Geological Sciences, Jagellonian University, 2a Oleandry Street, Krakdw, Poland

Abstract

The Kupferschiefer, a thin (<4 m) bed of marine bituminous marl of Upper Permian (Zech- stein) age, occurs over a large area of north-central Europe and has, in certain areas, been exploited for silver and some base metals, notably copper, since medieval times. It has been regarded as the type example of a shale-hosted, strata-bound sulfide deposit and theories regarding the origins of Kupferschiefer mineralization have exerted considerable influence on theories of ore genesis.

The Kupferschiefer sediments were deposited following a rapid marine transgression over an area that had been subject to a very long period of arid to semi-arid conditions. In many areas the Kupferschiefer overlies red-bed sediments (Rotliegende), but in others it overlies Carboniferous sandstones and marls or the bleached and reworked equivalents of earlier rocks (Grauliegende and Weissliegende). Within the Kupferschiefer, lithologieal variations can be related to detailed paleogeography. Overall a euxinie, sapropelie facies predominates, but changes occur in the region of paleohighs where more carbonate and elastic-rich facies are developed. Strong evidence exists for the development of chemical stratification in the Zeeh- stein sea (from which the Kupfersehiefer was deposited) with oxidizing conditions in the upper part and reducing conditions in the lower part. Everywhere the Kupferschiefer grades upward into overlying dolomitie limestones.

Although large areas of the Kupfersehiefer contain only average concentrations of base and precious metals compared to other shales and marls, in certain areas the concentrations reach ore grade. Historically, the Mansfeld district (SE Harz Mountains) was important for copper and silver mining, but at present-day mining is undertaken only in the Spremberg-Weisswasser area (East Germany) and in Lower Silesia (Poland). The ores in such regions contain sulfides of Cu, Pb, and Zn and may be enriched in a variety of other elements, notably V, Mo, U, Ag, As, Sb, Bi, and Se; Cd, T1, Au, Re, and the platinum-group metals are also reported; lateral and vertical zoning of Cu, Pb, and Zn may be observed; and in some areas, a reddening of the rocks adjacent to ores (Rote Fiiule facies) is a useful exploration guide. The Kupfersehiefer in Poland, in two contrasting regions in Germany (the Lower Rhine basin and the Hessian depression) and in England (where it is termed the Marl Slate) are compared and provide evidence for four types of mineralization.

The first (and oldest) is a weakly mineralized type exemplified by the English Marl Slate. Average base metal content of this type is • 100 ppm. Detailed mineralogieal, geochemical, and isotopic studies indicate that the mineralization is synsedimentary; these studies have enabled a model to be developed in which precipitation of the various mineral phases can be related to stratification of the early Zeehstein sea and oscillations in water and oxie-anoxie boundary levels.

The second is an average mineralization involving base metal content at the 2,000-ppm level. In this ease, the study of German examples indicates the important influence of strata underlying the Kupfersehiefer, as stressed by subdivision into a basin type that overlies thick Rotliegende sediments, and a schwellen type that overlies Paleozoie basement. The two subtypes differ in mineralogy and overall base metal ratios, indicating the importance of underlying

0361-0128/89/951/1003-25 $a.00 100 3

1004 VAUGHAN ET AL.

strata as a source of metals. The relationship between barium concentrations in the Kupfer- schiefer and barite mineralization in underlying rocks is also clear from geochemical (including Sr isotope) studies of German examples. The evidence points to an early diagenetic origin for the average mineralization, with sulfur derived bacteriogenically interacting with low-temperature solutions deriving metals largely from immediately underlying rocks.

The third is an ore mineralization where the average base metal concentration reaches • 3 percent. Zonation is clearly developed, as is the association of the Rote F[iule facies with mineralization, although the detailed zoning patterns are more complex than commonly be- lieved. The ore mineralization type is generally restricted to the regions representing the margins of Rotliegende basins, and models of origin associated with late diagenetic processes and the introduction of metal-rich brines (possibly associated with basin compaction) are in line with the geologic and geochemical evidence. Preliminary fluid inclusion data on strata associated with the mineralization point to temperatures of • 120øC, and organic maturation studies can also be used to support models involving introduction of oxidative metal-rich brines during late diagenesis. Again, sulfur isotope data point to fixation of the metals by sulfur derived from bacterial reduction of sulfate.

The fourth mineralization associated with the Kupferschiefer is a much later (postdiagenetic) structure-controlled mineralization (Riicken) involving Co, Ni, Ba, As, and Ag phases, genetically distinct from the types mentioned above and of probable hydrothermal origin. Other possible episodes of mineralization can be identified on a local scale; in many cases insufficient data are available to assess their genetic significance.

The Kupferschiefer is a deposit that appears to be the product of a variety of mineralizing processes influenced by the environment of deposition of the host rock and the underlying geology, but there are many unifying features; notably, that the bulk of the evidence still points to fixation of metals as sulfides by bacteriogenic processes.

Introduction

THE "Kupferschiefer" (copper slate) is a stratigraphic term for a thin bed (approx 0.3-4.0 m) of marine bituminous marl, Upper Permian (Zechstein) in age, and occurring over a very large area of central Europe.

From Poland in the east, through the northern regions of the German Democratic Republic (GDR) and Ger- man Federal Republic (FRG), the Kupferschiefer stretches across the North Sea to northeast England where it is known as the Marl Slate (see Fig. 1). This is a distance of over 1,500 km east to west and an

Probable boundary of Zechstein deposits

' North Sea

Oslo J

Durham

Stockholm

0

Paris

km

FIC. 1. Geographic location of the Kupferschiefer (and Marl Slate).

KUPFERSCHIEFER OVERVIEW 1005

estimated area exceeding 600,000 km 2 (Wedepohl, 1971).

It was one of the first stratigraphic sequences ever described (Lehmann, 1756) and its economic exploitation, chiefly for copper and silver, dates from medieval times. Since the very beginning of the scientific study of ores, the Kupferschiefer has exerted great influence in regard to theories of ore genesis. Nev- ertheless, although some specific aspects of Kupfer- schiefer geology have been discussed in the recent literature (e.g., Harwood and Smith, 1986a; Jowett et al., 1987b), detailed accounts combining geologic setting, modern studies of the petrology, mineralogy, and geochemistry, and current theories regarding the origin of the Kupferschiefer are generally not available in English language journals. Here, we set out to provide a review incorporating new data and assess theories of origin of Kupferschiefer ores.

Overall Geologic Setting

Tectonics and stratigraphy

The Variscan orogeny at the end of the Westphalian C stage of the Carboniferous was an important event on a regional scale in central Europe. The center of this orogeny is now represented by the metamorphic rocks of the Bohemian Massif, surrounded by the Sax- othuringian and Rhenohercynian zones (Fig. 2). At the end of this orogenic phase, large depressions (e.g., the Schneverdinger graben in northern Germany, the Hessian depression and the Thuringian basin, and the southwestern and southeastern Harz Foreland, see Fig. 2) were filled with •1,000 m of clastic (Rotlie- gende) sediments eroded from the Variscan Moun- tains. Autunian (Lower Permian)-age (bimodal) volcanics are intercalated with the clastic sediments of

the lower Rotliegende. The Rotliegende (Autunian) sediments, as first-cycle sediments formed during the evolution of an intracratonic basin, are immature and contain debris of granites, schists, limestones, and volcanics from the adjacent mountains. Certain authors (e.g., Jowett and Jarvis, 1984) have noted similarities between these Rotliegende troughs and the upper Cenozoic Basin and Range province in the western United States, proposing plate tectonic models for their formation in environments ofintracratonic

rifting in the foreland area of major cordilleran-type orogenies.

At the end of the Rotliegende, the Zechstein sea invaded from the north. Glennie and Buller (1983) have proposed a period of only 10 years from the onset of the Zechstein transgression until its greatest extension was reached. The exposed rocks were reworked, sometimes bleached by the influence of the euxinic environment of the sea floor resulting in the grauliegend sediments. Along with the Grauliegende are developed the Zechstein conglomerate and the

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FIG. 2. Location of German Kupferschiefer occurrences in relation to major geologic features.

Weissliegende, strata that represent, in different regions, reworked underlying rocks, fiuviatile, or eolian sediments. Overlying these clastic rocks is the Kup- ferschiefer, a bituminous calcareous or dolomitic shale that represents the first completely developed marine sediment after a long period of arid to semi-arid conditions. The Kupferschiefer is thus underlain in different regions by Rotliegende, Zechstein conglomerate, Grauliegende, or Weissliegende rocks. In northern England, the Marl Slate rests variously on a thin basal conglomerate, on eolian dune sands (the Yellow Sands), or on red sandstone (Penrith sandstone). Everywhere, the Kupferschiefer grades upward into overlying dolomitic limestones, the Zech- stein first-cycle carbonate--Zechstein limestone or Zechsteinkalk (Smith et al., 1986).

Rifting and lithospheric thinning, associated with the opening of the Tethys ocean to the south, occurred throughout the region during the Triassic and early Jurassic (Ziegler, 1982). Thermal anomalies associated with this event may have played a role in the mineralizing process (Jowett, 1986).

Zechstein depositional environments The surface over which the Zechstein sea trans-

gressed was one of strong relief, although the depths of water probably did not exceed 1,000 m. The con-

1006 VAUGHAN ET AL.

ditions then prevailing at the floor of the Zechstein sea were probably comparable, in part, to those now existing in parts of the Ostsee and the Black Sea, with strongly reducing conditions. It has been proposed by many authors (e.g., Pompeckj, 1914, 19'20; Bron- gersma-Sanders, 1971) that a chemical stratification or "chemocline" developed which divided oxidizing conditions in the upper part of the Zechstein sea from reducing conditions in the lower part, where the Kupferschiefer was deposited.

Lithological variations within the Kupferschiefer can be related to detailed paleogeography, with paleohighs, paleorises, and subbasins controlling the observed sediments. Overall, a euxinic, sapropelic facies predominates. The Kupferschiefer of the basin floor (e.g., Lubin, Richelsdorf, Mansfeld, North Ger- man basin; see Figs. 1 and '2) is a thin (0.3-0.5 m) clay-rich bituminous shale, in effect, a basinal facies. Changes in facies occur in the region of paleohighs, reefs, and sandbars and are related to pre-Zechstein relief controlling the former shoreline and position of the chemocline. This marginal facies (found, for example, in the Lower Rhine basin, Spessart, and Rh6n) is characterized by a higher concentration of carbonates and of fine-grained clastic material, indicating increasingly oxidizing conditions and the influence of the adjacent landmasses. The thickness of the Kupferschiefer deposited in the marginal facies reaches as much as 4 m. The suggested paleogeo- graphical environments of some Kupferschiefer occurrences in central Europe are shown in Figure 3 (after Speczik et al., 1986).

Mineralization and Mining Activity

The metal enrichment associated with the Kupfer- schiefer is not, in fact, restricted to one lithostrati- graphic unit. Depending on the thickness of the Kup- ferschiefer and its lithology, the mineralization can be contained within the Zechstein Conglomerate, Weissliegende, Kupferschiefer, and Zechsteinkalk.

Metals, minerals, and their zonal distribution

The Kupferschiefer, despite its name, contains average lead and zinc concentrations more than ten times greater than that of copper (Wedepohl, 1971). It is also true that, whereas in certain areas concentrations of copper and other metals reach ore grade, large areas of the Kupferschiefer contain only average (or even less than average) Zn, Pb, and Cu concentrations relative to normal shales and marls. The copper occurs as the sulfide minerals bornite, chalcopyrite, chalcocite, covellite, and idaite (in roughly de- creasing order of abundance). Lead and zinc occur as galena and sphalerite, although the most abundant sulfide overall, as in every other type of organic matter-rich sediment, is pyrite. The average grain size of the sulfides is very small (20-200 #m).

In addition to Cu, Pb, and Zn, a number of other elements occur in remarkably high concentrations in certain areas within the Kupferschiefer, notably V, Mo, U, Ag, As, Sb, Bi, and Se, with Cd, T1, Au, Re, and the platinum-group metals also having been reported (Wedepohl, 1971; Kucha, 1981, 1982). These occur both within the more common sulfides and as

Rheinisches Schiefergebirge Niederrheinische Bucht

C2 Upper Carboniferous PI Palaeozoic older than Carboniferous

C 1 Lower Carboniferous

,,'• Base of Weissliegende -'"•' sediments

FIG. 3. Paleogeographic reconstruction of some Kupferschiefer environments in Germany (after Speczik et al., 1986).


discrete phases (several of which are shown in the paragenetic diagram, Fig. 4).

Much has been written about the lateral and ver-

tical (bottom to top of sediment column) zoning of Cu, Pb, and Zn in the Kupferschiefer (Richter, 1941; Kautzsch, 1942; Rentzsch, 1964; Wedepohl, 1964; Freese and Jung, 1965) that includes concentration of copper in areas near to the palcoshore and (along with silver) in the lower layers of the sediment column with lead and zinc in the upper layers. Rentzsch and Knitzschke (1968) developed a paragenetic scheme related to mineral zonation as shown in Figure 4. Here, the ore minerals are divided into 10 assemblages, each reflecting different redox conditions prevailing during mineral formation. Thus, paragenesis (1), hematite type, occurs under oxidizing conditions, (2) and (3) under weakly oxidizing conditions, types (4), (5), and (6) under weakly reducing, and (7) to (10) under strongly reducing conditions. It is important to note that only the assemblages (2), (3), (4), (5), and (6) form copper ore concentrations of economically exploitable grade.

Redox - str. weak weak strong state ox. ox. reducing reducing

Para•lenesis I 2 3 4 5 6 7 8 9 10 Hematite • Magnetite

Chalcocite • • /• Neodigenite

Covellite • l Idaite • Bornite • • ••• Chalcopyrite •--• ••• • • • Galena .... •• / • • • Sphalerite -- • • •1 • •

Pyrrhotite

Arsenopyrite

Tennantite

Enargite

Stromeyerite

nat. Silver

Linneite ,,

Millerite

Bravoite

FIG. 4. Ore minerals and their paragenesis in the Kupfer- schiefer (after Rentzsch and Knitzschke, 1968): i -- hematite type, 2 = covelline-idaite type, 3 = chalcocite type, 4 = bornite-chalcocite type, 5 = bornitc type, 6 = bornite-chalcopyrite type, 7 = chalcop•rite-pyrite type, 8 -- galena-sphalerite-chalcopyrite type, 9 = galena-sphalerite type, and 10 -- pyrite-type; str. ox = strongly oxidizing; I major component, I accessory component, trace component.

The Rote F•iule facies

It has been known for some time from the mining of the Kupferschiefer that high-grade copper mineralization is always associated with the so-called Rote F•iule facies. The term "Rote F•iule" was first used

by miners in the Mansfeld area to describe barren, red-colored rocks found in the vicinity of the ore. It is now applied to rocks of the Weissliegende, Kup- ferschiefer, and Zechsteinkalk layers which exhibit various types of red coloration caused by disseminated hematite and goethite. A lot of subtypes of this red coloration have been described by Erzberger (1965) and Freese and Jung (1965). Depending on the lithology, it occurs as lenses, schlieren, spots, and layers, and sometimes covers areas as great as several hundred square kilometers (e.g., Brandenburg, GDR; Rentzsch, 1974); in other cases, it is restricted just to locally developed sandbars, against which the Kupferschiefer pinches out (Schmidt, 1985). The Rote F•iule facies, clearly an oxidized facies, has been interpreted by some authors as representing the shallow-water equivalent of the black copper-bearing shales, with a distribution reflecting that of redox potential during sedimentation and diagenesis (Rentzsch, 1964; Konstantynowicz, 1965; Jung and Knitzschke, 1976). A diagenetic origin is suggested by, among other things, hematite pseudomorphs after (syngenetic) pyrite, and the case for a late diagenetic origin by the action of convecting brines has recently been argued by Jowett et al. (1987c).

Beside the red coloration caused by the Rote F•iule, other red colorations occur mostly in the hanging wall (Zechsteinkalk) and have been attributed to the action of oxidized surface water (Paul, 1982). Unlike the Rote F•iule, they are not of value as exploration guides; the former can often be distinguished by the fact that its boundary crosscuts the different Lower Zechstein stratigraphic units at a low angle (1ø-2ø). The Rote F•iule distribution and the mineral zoning in relation to the Zechstein lithologies, as shown schematically in Figure 5, are useful as exploration guides for Kup- ferschiefer-type mineralization.

Mining and mineral exploration

Economic interest in the Kupferschiefer dates from Medieval times when, in the Mansfeld district at the southeast margin of the Harz Mountains (Figs. i and œ), mining of Cu-Ag mineralization was undertaken. Initially, exploitation was focused on locally developed near-surface zones rich in native silver. Subse- quently, mining followed the thin (30-60 cm) Kup- ferschiefer seam to greater depths and mining extended to the Richelsdorf area at the southwestern margin of the Harz Mountains and farther afield to the Kupferschiefer occurrences in the North Sudetic syncline, Lower Silesia, Poland (see Fig. 1). At the

1008 VAUGHAN ET AL.

WERRA ANHYDRITE

V v V v

v v

ZECHSTEIN LIMESTONE

KUPFER- SCHIEFER

SAN DSTONE

DOLOMITE

Pb-Zn

Mineralization

LIMESTONE

-' '-' ß .

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ß

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Basin - - ' . .

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ß '. "q.• . • Shor•e ! approx. 2 K m FIG. 5. Schematic cross section to show the position of Rote FSule in relation to lithological types

and mineralization.

present time, mining is undertaken in the GDR in the Spremberg-Weisswasser region, and in Poland where the Lubin deposit was discovered in 1957 in the Fore- Sudetic monocline. The latter carries • 2 percent Cu and 30/g/ton Ag and is responsible for Poland being the major European copper producer (• 400,000 tons per annum). Although mining in the FRG in the Rich- elsdorf area was discontinued in 1950, recent exploration has been undertaken for Lubin-type mineralization in the Hessian depression (Figs. i and 2). In this area, nearly 30 million tons of potential ore grading at 1 percent Cu and 15 ppm Ag (over a 2-m thickness of Kupferschiefer) have been proved, but the quantities involved and sporadic distribution of the mineralization prevent profitable mining in the FRG at present (Schumacher and Schmidt, 1985; see also earlier work of Richter, 1941).

The Kupferschiefer in Germany

The Kupferschiefer occurs over wide areas in Ger- many. From west to east, the following occurrences should be mentioned (see Figs. 1 and 2):

1. The Lower Rhine basin, situated near the border of the Westphalian (Subvariscan foreland) and Rhenohercynian zone of the Variscan externides. Due to the absence of Rotliegende sediments, the Kup- ferschiefer directly overlies folded and faulted shales, sandstones, and marls of the upper Carboniferous (Westphalian A-C). It occurs at depths between 200 and 1,000 m.

2. The North German basin, stretching nearly the whole distance from west to east in the FRG, contains Kupferschiefer overlying clastic sediments of Rotlie- gende age as well as Carboniferous rocks (Osnabriick rise). The depth of Kupferschiefer now reaches up to 5,000 m.

3. The Hessian depression represents the southern continuation of the eastern part of the North German basin. It is bordered in the west by upthrusted De- vonian and lower Carboniferous rocks of the Rhenish

Massif and in the northeast by the elevated Paleozoic strata of the Harz Mountains. The occurrences at

Richelsdorf, Rh6n, and Spessart, which have been the target areas for recent exploration activities in the FRG, are situated at the border of the Saxothuringian (Rh6n, Spessart) and the Rhenohercynian zone (Rich- elsdoff) of the Variscan Mountains (see Fig. 3).

4. To the east, the Hessian depression is connected with the Thuringian basin, which is located in the southernmost part of the GDR. At its northwestern margin in the transition zone to the Harz Mountains, the formerly exploited deposits of Mansfeld and San- gerhausen are located.

5. In the eastern part of the GDR, near the frontier with Poland, Kupferschiefer ore is mined in the Spremberg-Weisswasser region.

The occurrences mentioned above are distinctive

in terms of the precise character of the mineralization (and the total amount of metals involved) and details cannot be presented here for all of these. Instead, the


Lower Rhine basin and the Hessian depression are discussed to illustrate the geology, mineralogy, and geochemistry of the German Kupferschiefer. The former involves Kupferschiefer overlying upper Car- boniferous strata, and the latter, Kupferschiefer overlying red-bedlike Rotliegende sediments.

Lower Rhine basin

Geology: The geologic structure of the Lower Rhine basin is characterized by a folded and faulted Carboniferous basement and a gently northward-dip- ping (from 2o-5 ø ) Permian, Mesozoic, and Cenozoic sedimentary cover. The Carboniferous strata, up to several thousand meters in thickness, consist of limestones, clay, sandstones, and shales, with countless mineable coal seams, mostly in the upper part of the section (Westphalian A and B). After Westphalian C, the Carboniferous strata were folded. The intensity of folding increases to the south. The folding was ac- companied by the development of northwest-southeast-striking vertical faults, along which some elements of the structure were uplifted by up to 400 m. Post-Variscan movement along these faults resulted in horst and graben morphology.

Hydrothermal vein-type base metal and barite mineralization occurs in the Carboniferous strata of

the basement, especially in the rock sequences of Westphalian A and B. It is of Variscan and probably also of post-Variscan age. Some occurrences of high- grade lead-zinc ore were mined until 1960, notably at the southwestern margin of the Lower Rhine basin.

The discordantly overlying Zechstein strata of the Lower Rhine basin represent the southernmost ex- tremity of the former Zechstein sea which, in this area, flooded a relatively fiat plain covered by a thin bed of residual and fiuviatile sediments. The thin con-

glomerate bed underlying the Kupferschiefer is coeval with the Weissliegende sandstone. The main com- ponents of the conglomerate are very poorly rounded and sorted fragments of Carboniferous limestones, sandstones, and schists, and rarely, Upper to Middle Devonian limestones. The cements mainly consist of calcite, dolomite, ankerite, anhydrite, gypsum, and barite. Barite also occurs in the form of veinlets and

nodules up to 3 cm in diameter. The upper parts of the Zechstein sequence of Zl to Z4 cyclothems show facies thickness development and distribution typical of marginal basins (Wolburg, 1957). They are overlain by about 300 m of Triassic Buntsandstein, succeeded by Paleocene, Oligocene, and Miocene strata (about 300 m thickness).

Miheralogy and geochemistry: Due to the marginal facies, the Kupferschiefer (up to 4 m thick) is characterized by a high content of carbonates, forming fine-grained laminae of about 0.5-mm thickness, sometimes growing up to several centimeters where sedimentation was located above basement paleohighs

(Fig. 6). The carbonate laminae contain clayey-bituminous and sandy-silty intercalations, the occurrence of the latter depending on the distance to the former shoreline to the south.

The three lowermost strata of the Zechstein cycle (Zechstein conglomerate, Kupferschiefer, Zechstein- kalk) exhibit more or less intensive barite mineralization in the form of nodules, thin layers, and fillings of drusy cavities (Diedel and Friedrich, 1986).

The sulfide mineralization evident within the Kup- ferschiefer is very monotonous. Pyrite and martasite predominate, followed by sphalerite and galena. Only minor amounts of chalcopyrite, covellite, and mala- chite have been observed, particularly in the underlying Zechstein conglomerate.

The base metal content of the Kupferschiefer in the Lower Rhine basin has been investigated by Die- del (1986). The most remarkable features of metal distribution are the following:

1. The copper content is always very low and ranges from a few ppm to 270 ppm. These values correspond to those of noimal black shales.

2. The lead content normally varies between 20 and 350 ppm. Anomalously high values up to 1,900 ppm are observed in 10 drill samples (of a total of 32).

3. With the exception of four drill samples, anomalously high zinc contents occur in all samples. The

FIG. 6. Typical intercalations of clayey bituminous (black) and calcareous (gray) layers of a marginal facies; Kupferschiefer (after Diedel, 1986).

1010 VA UGHAN ET AL.

highest values are 1.25 percent in one sample and 0.55 percent in another, over vertical sections of 10 and 75 cm, respectively.

The distribution of both lead and zinc is charac-

terized by two populations (Diedel, 1986), representing the background (population 1) and the anomalies (population 2). Population i ranges from 0 to 350 ppm for lead and from 0 to 750 ppm for zinc. These values correspond with those given by Knitzschke (1966) for normal black shales. The second population covers a wide field from 350 to 1,900 ppm (lead) and from 750 to 12,500 ppm (zinc), representing an anomalous enrichment of both elements. Within a vertical section, the anomalously high lead and zinc contents occur without exception in the basal part (1 m) of the Kupferschiefer, although where the Kupferschiefer is less than 50 cm thick, the increased base metal concentrations are vertically displaced and occur in the basal part of the Zechsteinkalk. A vertical zoning (Zn below, Pb above) is evident and contrasts with the more usual Cu-Pb-Zn zoning characteristic of the well-known German deposits (Richelsdorf, Mansfeld) and Polish deposits (Lubin, Polkowice, Lena, Konrad; Speczik et al., 1986).

The distribution of barium within the Kupferschie- fer is strongly related to the underlying Carboniferous stratigraphy. The Westphalian A and B strata, com- prising clay-sandstones and sandstones, contain hydrothermal lead-zinc-barium vein-type deposits with up to 12 percent barium. Within the overlying Kup- ferschiefer, anomalously high barium contents of up to 5 percent Ba over 1.8 m are found (Fig. 7). By

contrast, the Kupferschiefer above the clastic sediments of the Westphalian C (sandstones) only exhibits barium contents in the range recorded for normal black shales. As with the lead and zinc, it is possible to interpret the barium concentrations in terms of a bimodal distribution (population 1:0-450 ppm Ba; population 2:450 ppm-12% Ba). Wedepohl (1964) has suggested that such high barium values are the result of hydrothermal solutions that fed into the Kupferschiefer horizon.

Coprecipitation of barite and galena out of one hydrothermal solution is supported by the occurrence of Pb-bearing barite (up to 1.7% PbO) intergrown with galena, within small vertical fissures. The conditions under which such a process could occur (at low temperature) are illustrated by the stability dia- grams in Fig. 8. The barite mineralization and the major component of the sulfide mineralization are thus interpreted as being of the same genetic origin.

These observations suggest that barium- and metal- bearing hydrothermal solutions could have originated in the basement rocks and moved upward until they reached the geochemical trap of the overlying Kup- ferschiefer. The observed barium distribution cannot

be readily explained by a fluvial input of barium, or by direct, synsedimentary precipitation out 6f seawater. The hypothesis of descending Ba-bearing brines, derived from the overlying Zechstein evaporites, is also improbable (as further shown below by Sr isotope studies).

Maturation of organic material: The Carboniferous rocks of the southern part of the Lower Rhine basin exhibit an anomalously high vitrinite rank of up to 4

FIG. 7. Barium content of the Kupferschiefer in the Lower Rhine basin and its relationship to the underlying lithology (after Diedel, 1986).


1.4.

1.2.

1,0.

0,6.

0,4-

0,2.

-0,2-

-0,4-

-0,6-

-0,6-

-1,0-

-1,2-

- 1,4 0 -1,0 -1,2

- 1.4 I 2 3 4 5 6 ? 8 9 10 11 12 13 14

1,2

1,0

0,8

0,6

0,4

0,2

0,0

-0,2

-0,4

-0,6

-0.8

FIG. 8. Stability relations involving Ba, Pb, and S species in the system Ba-Pb-S-H20 at 25øC and 1 atm total pressure. Total activity: Ba = 10 -4, Pb = 10 -6, S -- 10 -2 (after Tischendorf and Ungeth6m, 1984, slightly modified). Field I: Pb-bearing barite; field II: field of cogenetic barite and galena; field III: Ba-bearing galena. Diagonal lines indicate the field of the Pb-bearing barite and the arrow the changing conditions of mineral precipitation.

percent Roi• (Teichmiiller et al., 1979). This anomaly is caused by a basic pluton, the Krefeld high, that rose up to about 3.5 km below the surface of the upper Carboniferous rocks (Niem611er et al., 1973; Bunte- barth et al., 1982).

The Kupferschiefer, which occurs at depths of between 200 and 1,000 m, has a virtrinite reflectance (Roil) which reaches a maximum of only 0.5 percent. A more sensitive parameter indicating the low maturation stage of these bituminous sediments is the stereochemistry of pristane (Patience et al., 1978). The maturation of sediments is primarily dependent on depth of burial so that the isograms of maturation should parallel the isobaths. Above the Krefeld high, anomalous maturation values dominate and the iso-

grams of maturation cut the isobaths at a high angle. Also, the basic pluton beneath the Krefeld high has caused a coalification anomaly in the upper Carbon- iferous strata (Westphalian A). Here, the isograms of maturation of the Kupferschiefer are oriented parallel to those of the Westphalian and the Kupferschiefer contains the largest metal contents of the Lower Rhine basin (Diedel and P6ttmann, 1988).

Isotopic investigations: The only published isotope (S, O, C) studies on the Kupferschiefer in northwestern Germany and the Lower Rhine basin were carried out by Marowsky (1969). He examined concentrates of combined Cu-Pb-Zn sulfides, pyrite, calcite, dolomite, gypsum, and bituminous matter. Highly 32S- enriched metal sulfides (•4S avg -30 to -35%0) were

found and interpreted as due to fixation of H2S derived from bacterial production. The oxygen isotope values of the carbonates were found to vary from •1sO = +1 to -12 per mil (avg = -4.5%0 PDB). Because the isotopic composition of oxygen from calcite and from dolomite is the same, the dolomite was interpreted as a conversion product of calcite. Isotopically light carbon in carbonates was observed in association with

light sulfur in the horizons enriched in metals. Such light carbon has also been attributed to the activities of sulfate-reducing bacteria, hence these isotopic data have been used to support a synsedimentary origin for the Kupferschiefer base metal sulfides (Marowsky, 1969). Wedepohl (1971) has further argued, on the basis of both isotopic compositions and chemical compositions (e.g., association of bituminous carbon and sulfide), that these Kupferschiefer sulfides have originated by precipitation in a closed basin.

However, the extent to which the data of Marowsky (1969) are representative of Kupferschiefer sections showing such phenomena as Cu-Pb-Zn zoning and the development of Rote Fgule facies is uncertain. The case for a biogenic origin of the sulfides other than framboidal pyrite has also been challenged by Jowett et al. (1987a, b), who point out that copper sulfides which formed at low temperatures in isotopic equi- librium with pyrite should exhibit lighter •34S values than the coexisting pyrite, not heavier (•4S = -23 to -38%0 for copper sulfides, -28 to -41%o for pyrite) as determined by Marowsky (1969). Nevertheless, the fact that the work of Marowsky (1969) was undertaken on combined Cu + Pb + Zn sulfide concentrates

may mitigate against these arguments. Recent strontium isotope data for the Kupferschie-

fer in the Lower Rhine basin have been presented by Diedel and Baumann (1988). The S7Sr/•6Sr ratio of barite in the basal Zechstein strata significantly differs from the S7Sr/S6Sr ratio of the Werra anhydrite above (Fig. 9). Therefore, the precipitation of barite from seawater or its formation from descending brines is very unlikely. The most probable source for the strontium and barium are the rocks of the Carbonif- erous.

The Hessian depression

Geology: The Hessian depression (Fig. 2) is part of a large-scale depression that developed during the Permian as the result of rifting between the Euro- Asiatic and the Greenland-North American conti-

nents. It is subdivided by two parallel, southwest- northeast (Variscan)-striking ridges (Hunsriick-Ober- harz-Schwelle, Spessart-Rh/fn-Schwelle) into three troughs (Fig. 3). During early Rotliegende time (Au- tunian) the first northwest-southeast-striking transverse ridges and troughs were formed, subdividing the great Variscan structures into similar ones of Her- cynian age (Kulick et al., 1984).

In the Richelsdorf area, the basement is formed by

1019, VAUGHAN ET AL.

oSa

L •5

•4

0707 708 709 710 711

875r/86Sr

FIC. 9. Sr isotope ratios for barite and rocks of the lower Zechstein strata, lower Rhine basin (after Diedel and Baumann, 1988). 1 = Dinant limestone; 2 to 4 = barite (2 = hydrothermal, Ba-Pb-Zn vein-type deposits of the upper Carboniferous; 3 = Zechstein conglomerate; 4 -- Zechsteinkalk), 5 to 6 = rocks (5 = Zechsteinkalk; 5a -- limestone within Kupferschiefer horizon; 6 = Werra anhydrite).

the Hunsriick-Oberharz ridge and the Baumbacher Transverse ridge and belongs to the Rhenohercynian zone of the central European Variscides. The rocks are phyllites, strongly folded mica-bearing sand siltstones, and graywackes. The Rotliegende sediments are more than 1,000 m in thickness, composed of al- luvial fans in the north and sequences of red-colored sandstones and conglomeratic intercalations in the south.

The basement rocks of the Spessart-Rh6n area (Spessart-Rhi3n ridge) belong to the Saxothuringian zone of the central European Variscides. They are principally of early Paleozoic age (Cambro-Ordovi- cian) and have been metamorphosed by the Variscan orogeny. Several types of ortho- and paragneisses, micaschists, amphibolites, and marbles occur; rarely, intrusive rocks also occur. During the Rotliegende, the Werra-Fulda basin was gradually filled by fluvia- tile and sabkha-type deposits. The sediments consist of conglomerates and sandstones; they are cyclic red beds, representing fining-upward sequences. The sabkha sediments are dominantly red colored and consist mainly of clays and siltstones with sandy intercalations. Characteristic features are nodules of

carbonates and anhydrite. Two facies types of Weis- sliegende, sandbars and conglomeratic sandstone layers, both of fiuviatile origin, can be recognized. The sandbars can form huge mega-crosslaminated structures more than 50 m in thickness (Schumacher, 1985).

Mineralogy and geochemistry: The Kupferschiefer in the Richelsdorf area consists of a black, thinly laminated, bituminous shale, which contains a high pro- portion of quartz, particularly in its basal part. Its thickness ranges between 0.15 and 2 m. The mineralization is hosted in the upper part of the Weisslie-

gende as well as in the Kupferschiefer and in the lowermost Zechsteinkalk. In the southern part of the Richelsdorf area, red-colored rocks (Rote FSule) occur in the basal Zechstein strata. This Rote F•iule represents the oxidized part of a redox boundary that is paralleled by a distinct metal and mineral zonation. At the more reduced part of this boundary, the copper facies is developed, and farther away, the lead and zinc facies. The metal and mineral associations of these different facies are described in detail by Schmidt (1985) as follows:

1. The rocks influenced by the Rote F•iule facies are highly depleted of their former base metal content (now about 100 ppm Cu + Pb + Zn). The dominant ore mineral is hematite, often pseudomorphic after pyrite. Copper-rich sulfides subordinately occur only in the vicinity of the redox boundary.

2. The copper facies is characterized by high copper contents, resulting from the presence of copper- rich sulfides like chalcocite, covellite, idaite, and bornite. The copper mineralization is mostly concentrated in the region of the transition zone between the clastic Weissliegende and the overlying clayey bituminuous Kupferschiefer. Where the Kupfer- schiefer is thin (about 50 cm), maximum copper contents are vertically displaced and may occur in the overlying Zechsteinkalk.

3. Within the lead-zinc facies; the contents of lead and zinc are greater than 50 percent Cu + Pb + Zn; copper values decrease at least to 50 ppm. Galena and sphalerite dominate and chalcopyrite is the most common copper sulfide.

Veins with Co-Ni-As-Ba mineralization, so-called "Riicken," occur in the northern part of the Richels- dorf as well as in the Mansfeld area (Gunzert, 1953; Messer, 1955) and are probably of Mesozoic age. Crosscutting the Kupferschiefer with its reducing character, they are enriched with Co-Ni arsenides such as skutterudite, safilorite, and rammelsbergite.

The base metal mineralization within the Kupfer- schiefer of the Spessart-Rh/3n is again preferentially concentrated in the basal part of the Kupferschiefer (Schmidt, 1985; Schumacher, 1985). Dominant ore minerals are Cu-As sulfides (tennantite, enargite) and arsenides (loellingite, arsenopyrite) that replace earlier copper sulfides (chalcocite, bornite, chalcopyrite; Fig. 10). Whether this generation of earlier formed and now replaced copper-rich sulfides is related to a Rote FSule process is uncertain at present. This type of Cu-As mineralization appears similar to the Co-Ni- Ba-As (Riicken) mineralization previously described from the Richelsdorf area (Schmidt and Friedrich, 1988), but in contrast to the latter (vein-type) mineralization, it is characterized by a disseminated ore mineral distribution and by the occurrence of Ag-Sb- bearing tennantite, where Cu is also replaced by Fe


FIG. 10. Polished section of a sample oftennantite (light gray) replacing bornitc (medium gray), from Richelsdorf, Hessian depression (FRG); oil immersion; width of field = 0.1 mm (after Friedrich et al., 1984).

and Zn. Additionally, an increase of silver and anti- mony from bottom to top of the Kupferschiefer is observed (Diedel, 1984).

From data on the stabilities of Cu I and As TM in aqueous solution at 25øC (Fig. 11), it is possible to suggest the type of solution conditions (shown shaded in Fig. 11) under which tennantite may have formed, namely, from the introduction of relatively acid and oxidizing metal-bearing solutions. Schmidt (1985) also

Eh (Vol t)

1,6] '1,6 1,4 .1,4

1,2 .1,2

o,q --_ t .....

-o,l

As '0'8 -0,8.

t-',0 -1 2 -12

pH

FIG. 11.' Eh-pH diagram showing the boundaries of stability (at 25øC) for various copper and arsenic species in aqueous solution. Hatched area indicates the field in which tennantite may be stable.

has provided some evidence supporting input of oxidized solutions into the Kupferschiefer by using geochemical data. He established high correlations between elements stable under differing redox potentials (Fig. 12), correlations that cannot be explained by a synsedimentary fixation of the metals. Schmidt (1985) also showed a bimodal distribution of copper and silver within the Kupferschiefer, with a background population and a population of anomalously increased values. These data agree with the petrographic observations of Friedrich et al. (1984), who proposed several stages of Kupferschiefer mineralization. Thus, the disseminated Cu-As sulfide mineralization found

at Spessart-Rh/Sn is thought to be due to precipitation from introduced (relatively low-temperature) fluids and not to be synsedimentary in origin.

Maturation of organic material: Recently, bitumens extracted from the Kupferschiefer of the Sangerhau- sen area have been investigated by neutron activation analysis, chromatography, and IR spectroscopy (Hammer et al., 1988). The observed high concentrations and lateral distribution of certain elements

(e.g., Se and Au), as well as the variations discovered in the composition of hydrocarbon groups and their structures, were used as arguments to support the idea of a supply of oxidative metal-rich brines (pro- ducing the Rote F/iule), and as evidence against syngenetic-early diagenetic metal fixation within the Kupferschiefer. Similar results have been reported for the Kupferschiefer from Lower Silesia, Poland (S. Speczik et al., pers. commun.). Results of vitrinite and liptinite reflectance measurements and gas chromatography of saturated and aromatic hydrocarbons point to an increase of organic matter maturation approaching the redox boundary, also shown by elevated phenanthrene/methylphenanthrene ratios and the loss of saturated hydrocarbons. These phenomena are ac- companied by significant copper and silver enrichment, interpreted as being a result of redox reactions between late diagenetic ascending metal-bearing brines and the reducing Kupferschiefer sediment.

Correlatio• - coefficient r

1X)-

08-

06-

OA-

Copperfacies Lead - Zincfacies

• Weil•liegende • Kupferschiefer • Zechsteinkalk

FIG. 12. Geochemical data for the Kupferschiefer and associated strata of Spessart-Rh6n showing high correlations (as evi- denced by the correlation coefficient, r) between elements stable under differing redox conditions (after Schmidt, 1985).

1014 VAUGHANETAL.

Isotopic investigations: Lead isotope measurements to determine the age of the Kupferschiefer mineralization and the source of metals were first performed by Kautzsch (1942) for the Kupferschiefer in Mans- feld and Spremberg. The calculated ages were 300 Ma for Spremberg and 320 Ma for Mansfeld; different sources were suggested by the authors for the two districts.

Later investigations were carried out by Wedepohl et al. (1978) based on a modified three-stage model, following Stacey and Kramers (1975). They con- structed a secondary isochron intersecting the lead evolution line at 250 and 1,700 Ma and interpreted these data as the age of denudation of Paleozoic sediments and the age of erosion of Precambrian rocks. A similarity between the lead of vein-type deposits in the Harz Mountains and that of the Kupferschiefer was established, so that the weathering of Paleozoic rocks, the dissolution of their metal contents, and metal transport by surface waters into the euxinic Zechstein sea were the processes invoked. However, Jowett et al. (1987b) draw attention to the uncer- tainties in the fits to the eight German data points presented by Wedepohl et al. (1978), noting that the intersection could give an age anywhere between 260 and 330 Ma.

The most recent lead isotope data are given by Hammer et al. (1987) for the Kupferschiefer of San- gerhausen (SE Harz Mountains), indicating an age from the lead separation of 240 to 270 Ma. Comparing the lead isotope ratios with the plumbotectonic model of Doe and Zartmann (1979), Hammer et al. (1987) interpret the Kupferschiefer lead as a mixture of mantle and crustal lead.

Fluid inclusion studies: Results of the first micro-

thermometric investigations have been published by Tonn et al. (1987). Fluid inclusions, incorporated in the carbonate cement of the clastic Weissliegende strata, show a homogenization temperature of about 120øC and a salinity of about 23 equiv wt percent NaC1. These first tentative results require both further

confirmation and extension to other horizons, including the Kupferschiefer itself.

The Kupfersehiefer in Poland The Kupferschiefer and overlying Zechstein

(evaporite) sediments are found over two-thirds of the land area of Poland. However, economically significant mineralization is known from only three much smaller areas, in the first two of which exploration is currently taking place.

The Fore-Sudetic monocline ore district is situated

near the border (formed by the Odra fault) of the Rheno-Hercynian zone of the Variscan with the Saxo- Thuringian zone (see Figs. 13 and 14). Here, a thin layer of dolomite and copper-bearing shale overlies a 600-m-thick sequence of Lower Permian sandstones and pyroclastics, that are in turn underlain by folded Devonian and Carboniferous strata (Oberc and To- maszewski, 1963; Oszczepalski, 1980; Klapcinski et al., 1984). Kupferschiefer ores are exploited at depths of between 600 and 1,200 m.

The North Sudetic syncline or depression ("Old Ore") district is situated in the Saxo-Thuringian zone of the Variscan orogen. Here, lead-bearing marls and below them, copper-bearing marls and red-stained marls, overlie Lower Permian beds that range in thickness from 10 m in the east where the Zechstein

rocks crop out, to 500 m in the west (see Fig. 13). The underlying folded rocks of the basement include early Paleozoic greenschists, greenstones, pillow la- vas, spilites, and keratophyres. The mineralized horizon extends from outcrop to depths of 2,000 m. Further details are provided by Konstantynowiez (1965), Lisiakiewiez (1969), Oszezepalski (1986), and Speezik et al. (1986).

The Northern Copper zone of the Fore-Sudetie monDeline, situated from 50 km north of Lubin to 120 km northeast of this locality, lies in the deepest part of the Fore-Sudetie monDeline, above the central part of the Rheno-Hereynian zone of the Varisean orogen. The Kupfersehiefer and mineralized sand-

0 10 20 i

km

SW HE

• ,14 ...... '• , ................................

• , •t , , •-t .... .....,'.:.'.: .... X/ I I I I II I II I I -• ' •JJJ•

I North Sudetic Depression Foresudetlc Block Foresudetic Idonocline

• ........ Buntsandstein • Crystalline rocks •'• Keuper •,::• Zechstein •--• Tertiary and • Muschelkalk Quaternary

[]•TT• Rotliegende • Cretaceous i Faults i

FIG. 13. Geologic cross section through North Sudetic depression, Fore-Sudetic block, and Fore- Sudetic monocline.


• Copper mlneralisatlon _:.:• Oxidised facies • Zechstein outcrops

• Tectonic lineaments O.F. Odra Fault zone

X--Y Line of section

FIG. 14. Sketch map of distribution of the copper-lead-zinc facies and of oxidation zones in the Fore-Sudetic region, Poland (modified after Rydzewski, 1978).

stones found here occur at depths between 1,500 and 2,000 m (Klapcinski et al., 1984) and are underlain by a thousand meters of Lower Permian red beds and pyroclastics, beneath which are folded Devonian to Carboniferous strata.

Geologic setting

As noted in the general discussion of the geologic setting of the Kupferschiefer, the molasse troughs of the Variscan orogeny were filled with Lower Permian pyroclastic sediments, lava flows, and clastic sediments before the transgression of the Zechstein sea. There are, of course, typical Permian red beds (Rot- liegende), whose upper part has generally been reworked by the Zechstein transgression; the first deposits of these red beds are generally dolomites or limestones (Peryt, 1976). In Poland (Lower Silesia), the Zechstein sea would have extended southeastward

as far as Wroclaw (Fig. 14) where a thin bed of dolomite or limestone was laid down under oxidizing conditions (Peryt, 1976; Rydzewski, 1978; Haran- czyk, 1986). In the area of the North Sudetic syncline, the first sediments formed were red-stained marls

(Konstantynowicz, 1965). Following the major transgression in the area, probably only Zary Island (Fig. 14) was not submerged.

The Zechstein transgression appears to have been followed by a period of evaporation and lowering of sea level by '•70 m that has greatly influenced facies distribution and the formation and preservation of euxinic sediments (Haranczyk, 1970, 1984, 1986). In the Lubin-Polkowice area (Fig. 14), for example, a lagoonal environment developed and it appears that copper-bearing shales which formed initially were removed from one-third of the area by subaerial oxidation and weathering or were redeposited by wave

action. Some of the copper removed in this way seems eventually to have infiltrated underlying sandstones. Specifically, a number of environments can be identified from analysis of the sediments:

1. Sandstone bars with no cover of copper-bearing shales and no reef-capping but with marginal zones of redeposited shale (Blaszczyk, 1981; Haranczyk, 1986).

2. Sandstone bars with no copper-bearing shales but capped with a reef dolomite which formed during regression of the sea (e.g., the West Lubin bar). These bars are surrounded by areas of copper-bearing shale (Blaszczyk and Prymka, 1973; Haranczyk, 1986).

3. Sandstone barriers like that marking the margins of the Rudna lagoon (see Fig. 15). This barrier is capped by a 2-m-thick dolomite which formed during regression; the steep slopes show signs of abrasion, and on shallow slopes, the occurrence of tidal flats. Here, lenses (<1.5 m thick) of redeposited shales (known as Feine Lette) occur. On the lagoon side of the barrier, above the zone of preserved lead-bearing shales, sandstones have been filled in and replaced by chalcocite-anhydrite or galena-anhydrite (Haran- czyk, 1970, 1986).

4. The lagoon, as examplified by the Rudna lagoon, which forms part of a large basin that was episodically separated from the rest of the depositional area. Here, lead- and zinc-bearing shales are overlain by dolomites with clay laminae and rare lead and zinc sulfides and, in the upper parts, only framboidal pyrite. The deepest parts of the lagoon are covered by dolomite with scarce clay laminae, some anhydrite, and just traces of ore minerals, overlain by dolomite interlayered with anhydrite.

More generally, it can be said that only within the lagoonal environment is the carbonate interlayered with anhydrite and the dolomite lead-zinc-bearing shale found. In the open areas of the Zechstein sea, calcareous copper-bearing shale is overlain by copper- bearing limestones and dolomites which, although containing lead and zinc, usually do not have economic amounts of these metals. The sequence then passes up into the Zechstein evaporite sediments, with cycles including shale, dolomite, anhydrite, and rock salt totaling '•600 m. In turn, these are overlain by the Lower Triassic red beds and the marine sediments of the Muschelkalk.

Lithologies

The lithologies of importance in relation to mineralization in the Polish areas include two that were

formed in euxinic environments: a calcareous (illite- rich) shale containing copper (to which the term Kupferschiefer can be applied) grading, with the gradual disappearance of the clay minerals and laminations, to a limestone deposited on the bottom of

1016 VAUGHAN ET AL.

$udetes North-$udetlc Szprotawa alevation Fore-$udetlc Monocllne

$yncllnorlum • __

C 2 Upper Carboniferoua ,';,• Autunlan voltanice

C• Lower Carbonlferoue Pc Precambrian

,.,•., Base of Welsallegende PI Palaeozoic older than Carboniferous • eedlmente

FIG. 15. Palcogeographic reconstruction of some Kupferschiefer environments in Poland (after Speczik et al., 1986).

an open shallow sea, and a dolomite shale containing lead and zinc (to which the term Bleischiefer is sometimes applied) with clay (montmorillonite-illite) laminae grading to a dolomite deposited in basins of high salinity.

As a result of weathering and redistribution of the ores contained in these two lithological environments, two further lithologies of importance as ores have been formed in the Fore-Sudetic monocline where

they are intensively exploited:

1. A laminated sandstone ore that occurs mainly near the top of the sandstone bars and areas of emergence. The greater the event of emergence, the deeper the laminated sandstone ore is now located (Haranczyk, 1970). This sandstone ore consists of numerous laminae with sulfides replacing the matrix (most commonly chalcopyrite, bornitc, or chalcocite); the laminae have sharp bottom contacts and diffuse tops. This type of copper-bearing sandstone is particularly important at the Lubin mine in the Fore-Sudetic monocline ore district.

2. A massive sulfide-anhydrite sandstone ore that occurs on the lagoon side of sandstone barriers, specifically in association with the Rudna lagoon where it forms the richest ores of the Rudna mine. Here, chalcocite in some areas, galena in others, forms the matrix of the sandstones to produce irregular ore- bodies sometimes several meters thick and tens of

meters wide, associated with larger barren zones in the sandstone where the matrix may be replaced by anhydrite or gypsum (Zalewska et al., 1978).

The Rote F•iule facies is known from the Polish

mineralized areas; for example, bordering the North- ern Copper zone of the Fore-Sudetic monocline. Jowett et al. (1987b) have recently related the Rote F[iule to mineral (sulfide) zonation and have used palcomagnetism to date the associated iron oxides. The palcomagnetic dating suggests a Middle Triassic age

for the oxidizing event associated with the Rote FSule (Jowett et al., 1987c).

Mineralogy

Disseminated within the shale of the Kupferschie- fer in these localities are very small (a few microns)- sized sulfides including framboidal pyrite, chalcopyrite, bornitc, and chalcocite. In the lead-zinc-bearing shales, the sulfides present in addition to framboidal pyrite are galena and sphalerite, whereas copper sulfides occur in separate laminae. These sulfides are regarded as primary ore minerals. Secondary mineralization is characterized by larger grains, veinlets, and zones of ore minerals that, unlike the primary sulfides, cut across the laminae (Haranczyk, 1972; Salamon, 1979; Niskiewicz, 1980; Banas et al., 1985). More than 50 ore minerals of Cu, Ag, Pb, Zn, Co, Ni, and As have been reported in the zones of secondary mineralization, and rare phases containing cobalt and germanium (Haranczyk, 1975) have also been reported. More recently, Kucha (1981, 1982) has reported Pt-, Pd-, Ag-, Hg-, and Pb-containing minerals. However, they occur only in weathered copper-bearing shales preserved in fragments on the West Lubin bar and appear to be products of concentration and enrichment. They are not present in quantities of economic importance.

The horizontal mineral zonation found in the Ger-

man deposits is also present in Poland; vertical zonation of Cu-Pb-Zn is less obvious and shows different

vertical relationships throughout the deposits. Zinc mineralization is more widely dispersed, whereas lead mineralization is more localized and usually found in the overlying carbonate rocks (Tomaszewski, 1986), although it does occur in the shales (Haranaczyk, 1970).

Within the areas of Kupferschiefer mineralization, there are also numerous sulfide-bearing veins. The origin of the veins is contentious and has been ascribed


to diagenetic remobilization (Rentzsch and Knitzschke, 1968; Rentzsch, 1974) or a much later mineralizing event (Jowett et al., 1987b).

Isotopic investigations

Stable isotope studies (S, O, C) have been undertaken by Haranczyk (1980, 1984, 1986). According to this work, all of the euxinic sediments of the Zech- stein in the Fore-Sudetic monocline, and the Kupfer- schiefer in particular, contain sulfides with very light sulfur (•34S = -28%0) providing some evidence of biogenic origin. The laminated sandstone ore from the district shows the lightest sulfur so far encoun- tered with 834S -- -40 per mil (Haranczyk, 1980), suggesting the possibility of two stages of biogenic enrichment: the first during the initial formation of sulfide mineralization in the shale, the second when solutions (enriched in light sulfur) derived from weathering of the shale descended into underlying sandstones during an episode of subaerial emergence. The dolomites occurring in the topmost sediments of the Rudna lagoon contain framboidal pyrite that yields •4S values of-35 per mil (Haranczyk, 1980). On the other hand, the massive sulfide and anhydrite ore in sandstones contains chalcocite with 8a4S = -9.31 to

-23.1 per mil, with the chalcocite replacing reef dolomite having 8a4S = -13.6 per mil and the associated sulfates having 8a4S values between 5.3 and 9.4 per mil (Haranczyk, 1980), suggesting that the sulfide sulfur could have been obtained by the reduction of sulfates from the same source. Different sulfur isotope signatures are obtained for sulfides disseminated in the shale and for those in vein ores, a result used to invoke two sulfur sources by Jowett et al. (1987a).

Basal dolomite underlying the Kupferschiefer in the Rudna mine exhibits values of 81SO = 4.0 to 4.6 per mil and 813C = 0.8 and 1.6 per mil. These results of Haranczyk (1980, 1984, 1986) generally support the conclusions of Marowsky (1969) that the carbonates which formed in the euxinic environment of the

Zechstein basins are enriched in the light oxygen isotope. Magaritz and Schulze (1980) suggested that the carbonates show enrichment in the heavy carbon isotope moving up through a cyclic sequence; this is evident on comparing the above-mentioned values for basal dolomite (•C = 1.6%0) and hanging-wall sty- lolitic dolomite (SlaC = 4.0%0). In contrast to the carbonate carbon isotope values, organic carbon from the Kupferschiefer is very enriched in the light isotope (SlaC = -26.0 to -28.0%0; Marowsky, 1969; Saw- lowicz, 1986).

The data provided by lead isotopes are complex (Wedephol et al., 1978) and problems of their interpretation have already been discussed above.

The Kupferschiefer in England: the Marl Slate

The lateral equivalent of the Kupferschiefer of North Central Europe is found in northeastern

England and is known as the Marl Slate (see Fig. 1). The Marl Slate contains relatively high concentrations of metal sulfides but nothing approaching concentrations of economic interest. However, the mineralogy, petrology, and geochemistry (including isotopic geochemistry) of the Marl Slate have been studied in some detail because of their relevance to the history of the Zechstein basin and development of economically important deposits elsewhere in that basin. Earlier work on the Marl Slate has been summarized by Smith and Francis (1967) and in the introduction to the detailed geochemical study by Hirst and Dunham (1963); later work has been summarized by Smith (1980), Vaughan and Turner (1980), and Sweeney et al. (1987). Surface exposure of the Marl Slate is poor, and consequently, most of the analytical work has been undertaken on core material.

Geologic setting, stratigraphy, and sedimentology

The Permian sequence in northeastern England begins with the Lower Permian Yellow Sands, variously interpreted as eolian dunes and shallow deposits, that rest unconformably on a reddened surface of the Carboniferous (Coal Measures). For the most part, the Marl Slate rests disconformably on the Yel- low Sands, being the first deposits of the Zechstein sea, a sea that apparently originated by the cata- strophic flooding of the southern Permian basin (Smith, 1980). The Marl Slate can be observed in surface exposures in Durham, and in North Sea boreholes it is readily recognized by its strong gamma signatures on wire-line logs. In some marginal areas, it reaches a maximum thickness of 5 to 6 m, but in the central part of the southern Permian basin, it is rarely more than 0.5 m thick. Studies of the relationship between the Marl Slate and the underlying topography (Turner et al., 1978) show that the Marl Slate is thickest in the depressions between the ridges in the Yellow Sands and thins considerably over their crests. The depth of water in the early Zechstein sea was probably less than 250 m but was more than 60 m in the North

Durham area (Smith, 1970); based on the thickness of succeeding carbonates, Smith (1980) suggested that the basinal depth exceeded 200 m. As discussed below, it has been recognized that the water column of the Zechstein sea was stratified (see, for example, Pompeckj, 1914, 1920; Brongersma-Sanders, 1971).

The Marl Slate is gray to black, well-laminated, and' highly carbonaceous carbonate-rich shale. The lower part, immediately overlying the Yellow Sands (Rot- liegende), is often sandy, whereas the upper part usually becomes more carbonate rich and grades into the overlying Lower Magnesian Limestone. A vertical sequence of sapropel, laminite, massive dolostone can be repeated a number of times within the Marl Slate and in thin sections the laminations can be seen to

result from alternations of organic-rich layers and carbonate or clay-rich laminae. The proportions of

1018 VAUGHAN ET AL.

calcite and dolomite vary widely; calcite and dolomite laminae are commonly intimately interlayered. The laminae have been interpreted as annual varves and from their average thickness of about 0.1 mm, a depositional period of about 17,000 yr has been estimated for the Kupferschiefer (Oelsner, 1959) and Marl Slate (Hirst and Dunham, 1963). An important feature of the Marl Slate, particularly in basin margin areas, is the occurrence of small micronodules (<0.3 mm) of dolomites and length-slow chalcedony after anhydrite (Turner et al., 1978; Turner and Magaritz, 1986). Other features, such as the fine lamination, absence of any benthos or bioturbation, and the presence of well-preserved fish and plants, have long been recognized as indications that the Marl Slate was deposited under anoxic conditions.

Mineralogy and geochemistry

Considerable quantities of pyrite occur distributed throughout the Marl Slate in the form of extremely small framboidal aggregates oriented along bedding planes. A second mode of occurrence of sulfide minerals in the Marl Slate is as lenses, often intimately associated with the micronodules of dolomite and

length-slow chalcedony (Turner et al., 1978; Turner and Magaritz, 1986). This association led Turner et al. (1978) to suggest that the micronodules are evidence of replaced evaporites and that the sulfides within them formed, at least in part, by the diagenetic reduction of sulfate. Pyrite is abundant in the lenses, which also contain chalcopyrite, galena, sphalerite, and a number of rare sulfides which have formed later

than the framboidal pyrite. The minor and trace element composition of the

Marl Slate was studied initially by Deans (1950) and Hirst and Dunham (1963). The latter drew attention to the vertical variation in the trace and major element concentrations, to the fact that such variations can be correlated between boreholes, and to an inverse correlation between Mo, Ni, Co (and to a lesser extent, Cu) and the sedimentation rate. This led the authors to suggest that Mo, Ni, Co, and possibly Cu were adsorbed on detrital clays and organic matter, chiefly at the site of deposition. The sources of Pb, Zn, and to a lesser extent, Cu were considered to be more prob- lematic by these authors, who concluded that they may have been introduced into the basin via subma- rine springs. Subsequently, Turner et al. (1978) made a detailed study of a Marl Slate core from offshore Northumberland and observed as follows:

1. A vertical Cu-Pb-Zn zonation exists in the Marl

Slate and is the same as that described by Wedepohl (1964) and Jung and Knitzschke (1976) from the Kupferschiefer (see Fig. 16).

2. Sulfides occur in two distinct forms; as pyrite framboids and as lenses of pyrite, chalcopyrite, galena,

and sphalerite. The second form is intimately linked with the dolomite pseudomorphs after anhydrite and appears paragenetically later than the pyrite framboids.

3. The carbonate component of the core is chiefly a calcium-rich dolomite.

4. Throughout the core, the coppe? content cor- relates closely with that of quartz and suggests a detrital source for that element (see Fig. 16).

Very detailed geochemical studies were also undertaken of another Marl Slate core by Sweeney et al (1987). Lithologically, this core can be divided into a lower, predominantly sapropelic Marl Slate (2 m) and an upper transition zone (0.65 m) of alternating sapropel and calcite-rich and dolomite-rich carbonates. They noted that, compared with the carbonate units, the sapropelic horizons are enriched in all elements except Mn, Sr, and Ba (reflecting the substi- tution of those elements in carbonate minerals). In the sapropelic part, copper, lead, and zinc display the vertical zonation in concentration noted above, and throughout the core, quartz is strongly correlated with all elements except manganese and strontium (these two elements behave antipathetically with all other trace elements).

Onshore exposure of the Marl Slate is poor, but an investigation of the first cycle carbonate which immediately overlies it and is relatively well represented has revealed a number of pertinent mineralogical features that, where exposure permits, can be seen to be related to the underlying Marl Slate. Harwood and Smith (1986b) have identified an unevenly distributed penecontemporaneous and epigenetic mineralization, the first-cycle carbonate, throughout a strike length in excess of 160 km. The penecontemporaneous mineralization consists of galena, barite, sphalerite, and marcasite; its time of formation was based on petrographic observation (Harwo6d, 1980) and stable isotope analysis (Harwood and Coleman, 1983). Strati- form galena mineralization, although not of economic importance, is widespread and was thought to have originated via bacterial reduction of seawater sulfate (Harwood and Coleman, 1983). The epigenetic mineralization comprises barite, fluorite, galena, and sphalerite; sulfur isotope data are not compatible with a Permian seawater sulfate source (Harwood and Coleman, 1983). The mineralization shows distinct geographic groupings which can be spatially related to structures in the pre-Permian strata and to basement blocks.

Organic carbon and the formation of pyrite

As suggested in the work of Leventhal (1983), Ber- ner and Raiswell (1983, 1984), and Berner (1984), ratios of organic carbon to sulfide (pyrite) sulfur can be used as indicators of fresh water versus marine


(a) Fe% o--o (b) Mn% ---,

I I I

Fe 0 2 4 Mn 0.1 0.2

Ca 0

Mg 0

o__o (c) Ca% •%t•

I I I I i I

8 16 24 0 4 8 12

.• Sr(pprn)

i I I I

100 200

(d) • Pb(ppm) ---* (e) Zn(ppm)

I I I I I

Pb 0 400 800 Zn 0 200 400

Cu(ppm) (f)

i I i i i

0 200 400 0

Quartz

I I i i i

40 80

FIG. 16. Geochemical data for a Marl Slate core section (NCB/D4) as a function of depth: (a) Fe and Mn content (wt %); (b) Ca and Mg content (wt %); (c) Sr content (ppm); (d) Pb and Zn content (ppm); (e) Cu content (ppm); (f) pecentage quartz. The core is 1.18 m thick (after Vaughan and Turner, 1980).

sedimentary environments and of the processes lead- ing to the formation of sulfides in sedimentary rocks. Organic carbon and pyrite sulfur contents were determined for Marl Slate core material by Sweeney et al. (1987). The Marl Slate core section has a C/S ratio of 2:2 and a positive intercept of 0.9 percent on the S-axis (Fig. 17). If only sapropelic samples are considered, then the intercept on the S-axis increases to 2.3 percent and the percentage of pyrite sulfur is un- correlated with the percentage of organic matter. Such a result indicates that pyrite was forming in an anoxic water column. Even in the nonsapropelic samples from the Marl Slate, a positive intercept of 0.8 percent S was found, so it would seem that throughout deposition of the Marl Slate, pyrite was forming in an overlying anoxic water column and particularly during sapropelic deposition. The transition zone has a C/S ratio of 1.7 and a positive intercept on the organic carbon axis of 0.4 percent, implying that sulfate supply may have been a limiting factor in pyrite formation. The degree of pyritization is a term introduced by Berner (1970) and is defined as (the percent of Fe as pyrite)/(the percent of Fe as pyrite + the percent ofFe HCI) where the percent ofFe HCI is the amount of iron liberated on treatment with hot hydrochloric

acid. Thus, if pyrite formation is limited by the availability of iron, the degree of pyritization will approach 1. Values for the Marl Slate (Sweeney et al., 1987) of 0.83 on average, and 0.93 if the nonsapropelic samples are excluded, are very high and suggest that iron availability may have been a limiting factor in pyrite formation.

Isotopic investigations

Stable isotope investigations of material from the Marl Slate have been conducted by Magaritz and Turner (1981, 1982), Turner and Magaritz (1986), and Sweeney et al. (1987). Stable isotope variations and their relationship to lithology and to the calcite/ dolomite ratio for a series of core samples are shown in Figure 18 (after Sweeney et al., 1987). The &34S values in pyrite range from -36.7 to -29.1 per mil, with an average of-32.7 per mil. There is an upward vertical trend to more 34S-enriched values, although it is not well developed. The only parameter that cor- relates significantly with •4S is the amount of carbonate in the sample, high carbonate being associated with 34S-enriched pyrite. In the transition zone, much greater variation is observed with &34S = - 15 per mil in calcite-rich units to -29.1 per mil in the sapropelic

1020 VAUGHAN ET AL.

3.0-

03 1.5-

0

0

3.0-

03 1.5-

Marl Slate section +

+ n=24 Corr. Coeff.=0.58

.i. + ++• % Spy=0'93+0'187 (% Corg) • + R Square=O.34

-I-

:•% + + C/S=2.22

I i

3 6

%Corg

0 ' I i

0 3 6

%Corg

-I-

+ Sapropelic samples from -I-

+ •••••.•+++ • the Marl Slate section + n=14 Corr. Coeff.=-0.24 -i-

%Spy=2.33-0.08 (%Corg) R Square = 0.06

C/S=1.80

3.0 Transition zone section -I-

=1=.•,•+ + + + 0 -I- -I-,• +•1. I. i

0 1.5 3

% Corg

FIG. 17. Weight percent organic carbon versus weight percent pyrite sulfur for samples from a Marl Slate core (after Sweeney et al., 1987). Abbreviations: Corr. Coeff. = correlation coefficient, Corg = organic carbon, Svy -- pyritic sulfur.

n=29 Corr. Coeff.=O.63

%Spy=O.43+O.87(%Corg) R Square=0.40

C/S=1.72

fide) and that barite sulfur isotopes are not directly related to Permian seawater sulfate.

The carbon isotope results of Magaritz and Turner (1981, 1982) show an enrichment in •3C vertically upward through the Marl Slate cores studied of between 2 and 4 per mil. This enrichment is accompa- nied by a decrease in organic carbon content (Turner and Magaritz, 1986). Sweeney et al. (1987) observed relatively uniform b13C values over much of the Marl Slate of between 2.6 and 3.2 per mil (see Fig. 18) and with spikes showing depletion to 3.5 per mil cor- responding to calcite-rich horizons. However, there was again observed to be an overall upward enrichment in •3C of 1.3 to 2.0 per mil. As with the Marl Slate itself, all of the b•3C values in the transition zone are enriched in 13C with respect to normal marine carbonates and show a vertical trend to b•3C enrichment. Oxygen isotopes (see Fig. 18) also show negative spikes in b•80 values that correspond to calcite- rich units (Sweeney et al., 1987); Turner and Magaritz (1986) noted low bl80 values coincident with an increase in sample quartz and iron contents, suggested by them to reflect periods of fresh water influx. The b•80 values shown in Figure 18 exhibit a negative correlation with organic carbon content and with high Sr values. Also shown is a remarkably close correlation between b•80 values and the percentage of dolomite: high dolomite being associated with •O enrichment. In the transition zone, depletion in 1•O increases from sapropel to dolomite-rich to calcite-rich horizons and there is a general upward trend toward •80-enriched values.

units. One sulfate sample (barite), found as a vug in- filling, yielded a b34S = 11.6 per mil, in keeping with values reported for Permian marine-water sulfate (Claypool et al., 1980). More extensive analysis 'of barite from the overlying first-cycle carbonate (Har- wood and Coleman, 1983) suggests that this value may be fortuitous (possibly the result of oxidized sul-

General similarities between the Kupferschiefer in the three countries

The Kupferschiefer in the three countries displays a number of similarities that must be taken into ac-

count in any genetic theories.

1. Although the deposits occur in, and transgress,

50 100 I I

Organic Laminite • (Dolomite rich).• (• 34S

_'• carbonate

1.7-

0.9

0,5

0.1

Yellow sands 0 I 2 3 4 -6 -5 -4 -3 -2 -14-18 -22-26-30-34

FIG. 18. Detailed stable isotope variations and their relationships to calcite/dolomite ratio from a Marl Slate core (after Sweeney et al., 1987).


up to seven different lithologies, they are found within a narrow stratigraphic interval. In Poland the ratios of length to width to thickness for the deposit are 10,000:4,000:1 (Tomaszewski, 1986).

2. There exists both a horizontal and a vertical zo-

nation of sulfide minerals, with copper concentration in the lower layers of the sediment column and lead followed by zinc occurring in progressively higher horizons. Although broadly true, the vertical zonation, in particular, shows different vertical relationships throughout the deposit. Stratiform lead occurs in the first-cycle carbonate in Poland and England, whereas in the Lower Rhine basin of Germany, it is confined to the basal section of the Kupferschiefer.

3. There exists a relationship between barium concentration and the distribution and structure of

the underlying upper Carboniferous strata. In Ger- many, a high barium content is found where the Kup- ferschiefer overlies Westphalian A and B strata. In England, the barium concentration can be related to pre-Permian fault systems. It has been established that some Carboniferous formation waters are enriched in

barium whereas others are sulfate enriched (Downing and Howitt, 1969; Edmunds, 1975). Harwood and Smith (1986b) speculate that barium mineralization is a result of fault-controlled discharge of these Car- boniferous formation waters, possibly aided by the high heat flow found by Brown et al. (1980) to occur in the Yorkshire region.

4. In all localities, several phases of sulfide mineralization can be identified as discussed further be-

low. The first is a synsedimentary, dominantly pyrite- forming event (although framboidal copper sulfides also occur at this stage). This is followed by the main base metal mineralizing event. Then, there is also a crosscutting mineralizing event that may be enriched in more exotic elements.

Evidence for the synsedimentary event is best seen in marginally mineralized areas. Most stable isotope data are from such samples and yield data consistent with formation by bacterial sulfate reduction. Pyrite formation may take place in the water column (Swee- ney et al., 1987) or along with minor base metal sulfides, within the upper horizons of the unlithified sediment. The relationships between base metal concentration and detrital quartz (Turner et al., 1978) and clay and organic matter (Hirst and Dunham, 1963) provide evidence for at least a minor detrital input; however, as pointed out by Richter (1941) it is unlikely that the erosion of the hinterland over the short sedimentation period could explain the large accumulations of metals. The occurrence of reworked

mineralized fragments in the Rotliegende immediately underlying the Kupferschiefer provides unam- biguous proof of at least some synsedimentary mineralization.

The origin of the main mineralizing event has been the cause of much controversy. As discussed in greater detail below, this can be divided into two distinct phases.

The crosscutting mineralizing event is clearly late and epigenetic in origin and gives rise to veins (Riicken) further discussed below.

Genetic Theories

Since the first investigations of mineralization and metal distribution in the Kupferschiefer there has been continuing discussion regarding the source of metals and the mechanisms of metal transport and sulfide precipitation. Rentzsch et al. (1976) provided a detailed summary of the different theories advanced prior to 1975 and readers are referred to this work for the names of the numerous authors who have con- tributed to these discussions.

The timing of the mineralization is of critical importance in understanding and placing constraints on the formation of these ores. Lead isotope studies have been ambiguous, yielding dates between 240 to 320 Ma compared with a Kupferschiefer age of 250 Ma (Wedepohl et al., 1978), although the Pb model ages for the Sangerhausen deposit of Hammer et al. (1987) give 240 to 270 Ma and both Wedepohl et al. (1978) and Hammer et al. (1987) report a late Permian average for their model ages. Palcomagnetism (Jowett et al., 1987c) has given a mid-Triassic age to Rote F•iule alteration. Thus, it seems likely that the mineralization was emplaced at some time during a period of between 250 to 230 Ma.

When all of the data available for the Kupferschie- fer mineralization are considered, models involving a variety of mechanisms and processes of metal concentration need to be invoked. Four major types of Kupferschiefer mineralization can be identified with a number of subtypes (Table 1). Each type of mineralization can be characterized by its metal content, zonation pattern, ore mineral paragenesis, and the presence of any alteration effects as well as the broader characteristics of geologic and tectonic setting.

It is possible to identify a "weakly mineralized" category to which the English example (the Marl Slate) may be assigned. From their detailed geochemical and isotopic data, Sweeney et al. (1987) were able to propose a model for Marl Slate formation related to stratification of the early Zechstein sea (Fig. 19). Here, in an upper oxic layer, carbonate was precipitated in the form of calcite, whereas in a lower anoxic layer, framboidal pyrite was precipitated by a process of bacterial sulfate reduction and interaction with dissolved iron. The anoxic layer was also a zone of formation of dolomite which experimental studies

1022 VAUGHAN ET AL.

TABLE 1. Geological, Mineralogical, and Geochemical Features

Types of mineralization Timing Metallogeny

Average base metal

content

Zonation

pattern 1 Paragenesis 2

Weakly mineralized Synsedimentary Fe(Cu, Zn) • 100 ppm Py, cp, sph

Average mineralization Early diagenetic Schwellen type

Zn + Pb > Cu

Basin type Zn > Pb + Cu

2,000 ppm Cu[S1] --• Pb Py, mc, sph + Zn[T1] Cp, gn, tn

Ore mineralization Late diagenetic Cu >> Pb + Zn 3% Fe +a --• Cu --• Hm, Fe-hyd, cc, bn, Pb --• Zn dg, cv, cp, gn, sph through S1, T1, Ca1

Structure-controlled Postdiagenetic Cu > Pb + Zn 7,000 ppm Cu[S1] --• Zn --• Tn, en, 11, ap, py, sk, mineralization Pb[T1] mi, ra, ba (Riicken)

Co-Ni-As-Ba subtype Cu-Ag-As subtype

I S1 = Zechstein Sandstone (below Kupferschiefer), T1 = Kupferschiefer, Cal = Zechstein first cycle carbonate (Zechsteinkalk) z Abbreviations: ap = arsenopyrite, ba = barite, bn = bornRe, cc = chalcocite, cp = chalcopyrite, cv = covellite, dg = digenite, en = enargite, Fe-

hyd = iron hydroxide, gn = galena, hm = hematite, II = loellingite, me = marcasite, mi = millerite, py = pyrite, ra = rammelsbergite, sk = skutterudite, sph = sphalerite, tn = tennantite

have shown was favored by an environment with a low sulfate concentration as would be the case here.

Jannasch et al. (1974) have shown that two maxima of sulfate reduction occur, one near the oxic-anoxic boundary and the other near the top of the sediment layer. The latter would be the locus of concentration of other metals as sulfides, some of which may have been introduced detritally or adsorbed on clay minerals and others from solutions circulating within the sediments during early diagenesis. The distinctive layering observed in the Marl Slate (and the transition zone) sediments would simply reflect oscillations in the oxic-anoxic boundary and the water level.

The "average mineralization" as defined by Schmidt and Friedrich (1988) occurs either in Kup- ferschiefer over thick Rotliegende sediments (basin type) or over Paleozoic basement (schwellen type). The mineralization is characterized by a high (Zn + Pb)/Cu ratio with an average base metal content of 0.2 percent. Typical ore minerals are pyrite, marcasite, sphalerite, galena, and chalcopyrite. Sulfur isotope compositions (Marowsky, 1969) point to a reduction of seawater sulfate within the sediment (or within the anoxic water column).

The formation of Kupferschiefer ore mineralization, the third major type of mineralization, is restricted to the margins of the Rotliegende basins. In

the Richelsdorf area, for example, the deposition of the Kupferschiefer was controlled by the palcoto- pography of the Weissliegende. The areas of ore deposit formation correspond to the flanks of sandbars and show a zonation with Rote Fiiule facies-copper facies-lead/zinc facies (Schmidt et al., 1986). The total amount of base metals is low within the Rote Fiiule

facies; Fe hydroxides, hematite, and gypsum-anhydrite are abundant. The succeeding copper facies displays a high Cu/(Pb + Zn) ratio. The best Cu/Ag grades recorded have been 10.5 percent Cu plus 160 ppm Ag. Chalcocite, digenite, covellite, and bornitc are characteristic ore minerals and they appear to have replaced prior-formed sulfides. Adjacent to the Rote Fiiule facies, the mineralization is principally concentrated within the Zechsteinkalk and the Kup- ferschiefer; with increasing distance away from the Rote Fiiule, the Kupferschiefer and the Weissliegende are mineralized. The influence of oxidized solutions, enriched in Cu and Ag, on the primary composition of the basal Zechstein unit is suggested by the positive correlation of S-2/Se -2, Fe+3/S -2, SO•2/Corganic, and SO•=/Cu + and by pseudomorphs of hematite after pyrite. The weakly alkaline character of these solutions is marked by the contemporaneous precipitation of hematite-digenite and chalcocite-gypsum (Schmidt, 1985).


of the Four Major Types of Kupferschiefer Mineralization

Alteration

pattern Possible formation conditions Geologic setting Palcogeographic feature Geotectonic event

Absent Reducing, framboidal pyrite formed in water column

Absent Reducing, slightly alkaline (T • Moderate relief of 720øC) syngenetic metal influx S1, T1, Cal over underlying S1 due to reworking of Paleozoic basement preconcentrated metals plus S1, T1, Cal over thick some endogenic input (>100 m) (T possibly •20øC) Rotliegende

Oxidation Interplay between oxidizing, Margins of Rotliegende Undulating relief of (Rote FSule) stronger alkaline solutions and basins Weissliegende due to

reducing conditions in host rock sandbar-conglomerate (T = 7120øC), diagenetic metal channel influx by (basinal?) brines from Rotliegende

Kaolinization, Acidic hydrothermal solutions, T Adjacent to palcohighs Slumping in T1 indicating bleaching > 100øC, epigenetic and uplifts syntectonic movement

Transgression of Zechstein palcosea due to breakdown of Ringkebing-Fyn high

Major subsidence of sedimentary basin within Lower Zechstein

Basinwide tectonism Meso-

Cenozoic

The underlying basement rocks--red beds or molasse-type sediments of the upper Carboniferous-- are considered to be the likely source of metals. Dur- ing the transgression of the Zechstein sea, the upper parts of the basement rocks would have been pene- trated by circulating seawater and preconcentrated metals could have been leached and reprecipitated penecontemporaneously with the black Kupferschie- fer in a euxinic environment. Above basement palcohighs, the metals trapped in the basal Zechstein sequence could have been derived directly from the reworked basement rocks. The input of metals during diagenesis into the lowermost Zechstein strata may have resulted from the continuous subsidence of the

sedimentary intracratonic basin. Movement of initially deep-seated formation waters would have been started by compaction of the sediments in the basin center, forcing the brines outward and upward from the basin. Thus, current metallogenic theories favor the main base metal ore mineralization being emplaced during early to late diagenesis. These mineralizing events can be seen as a continuum, with the latest episode being the type described by Jowett (1986) from parts of the Polish Kupferschiefer.

Jowett (1986) explained the genesis of Kupfer- schiefer Cu-Ag ore deposits in Poland by convective late diagenetic flow of Rotliegende brines during

Triassic rifting. He suggested that brines migrating through the Rotliegende sediments leached metals from the volcanic detritus and moved up the flanks of basement highs into the Kupferschiefer. Above there, the thick evaporites of the lower Zechstein precluded a vertical flow-through so that the brines moved back laterally along the base of the Zechstein to the basin center and sank back down into the Ro-

tliegende, completing a convectional cell. Kucha and Pawlikowski (1986) suggested a model involving the mixing of two brines--a lower, low-salinity, high- temperature, metal-bearing brine and an upper, high- salinity, low-temperature, Na-, CI-, Ca-, SOj2-bearing brine. (A similar model is proposed by Harwood and Smith, 1986b, to explain the anomalously rich metal concentrations in the English first-cycle carbonate.)

An epigenetic structure-controlled mineralization (Riicken) is a fourth type that can be identified. It is very important to emphasize that the Riicken-type mineralization, although intimately associated with the other types of Kupferschiefer mineralization, is genetically distinct. It can be divided into a Co-Ni- As-Ba subtype and a Cu-Ag-As subtype. Both subtypes are associated with fault structures of Mesozoic to Tertiary age. A low Cu/(Pb + Zn) ratio is typical. Characteristic minerals are skutterudite, safilorite, and millerite (Co-Ni-As-Ba subtype) and tennantite

1024 VAUGHAN ET AL.

Oxic Calcite formation No Pyrite formed Low availibility of Fe

(•2 t Thermo-halocline fFMI Anoxic • I Dolomite formation Formation of n

I 250'm I Degree of I ) carbonate etching I ) depends on • • residence time

• • nor dolom,te• • Sulphides of Cu Pb Zn Not to scale

!

Anoxic (b)

(c)

-'r?.•.......• '""<•- :• Sapropel • Dolomite rich

'"""?'• E• Calcite rich

FIG. 19. A model for the precipitation of carbonate and sulfldes in the early Zechstein sea, based on data from the Marl Slate (after Sweeney et al., 1987).

(Ag bearing), enargite, loellingite, and arsenopyrite (Cu-Ag-As subtype), respectively. The generation of the Co-Ni-As-Ba subtype by hydrothermal fluids has already been established by Gunzert (1953) and there is evidence for a hydrothermal origin of the Cu-Ag- As subtype (Diedel, 1984). In addition to the Rticken which they consider to be of probable Cretaceous or Cenozoic age, Jowett et al. (1987b) distinguish a second style of veining which they consider to be coeval with the main sulfide mineralizing event and to be of mid-Triassic to late Jurassic age. The Cu-As sulfides and arsenides in the Kupferschiefer of the Spessart- Rh6n area are anomalous and have replaced most of the sulfides which formed earlier during the synsedimentary or diagenetic stage of mineralization.

When considered on a European scale, the Kup- ferschiefer deposit is seen to be the consequence of a variety of mineralizing processes. Apart from the later, genetically distinct, structure-controlled (Riicken-type) mineralization, these processes may

have been active for a period as long as 20 Ma, com- mencing with the deposition of the Kupferschiefer horizon. The environment of deposition and the geology of the underlying rocks appear to have been the major influences controlling the degree of metalli- zation.

Acknowledgments

The financial support of the Natural Environment Research Council (for the work of D.J.V. and M.S.) and the Minster fiir Wissenschaft und Forschung des Landes Nordrhein-Westfalen, Dtisseldorf (AZ: IV B 4-Fa 9828) and of the Deutsche Forschungsgemein- schaft, Bonn (AZ: Fr 240/46-1) (for the work of G.F. and R.D.) is gratefully acknowledged. The manuscript has benefited greatly from the criticisms of Economic Geology referees from the comments of an anonymous referee, and from the editorial advice of D. Rickard.

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