Stable isotopic variations of clay minerals: A key to the understanding of Kupferschiefer-type mineralization, Germany

Gwehimica PICosmochimica Acta Vol. 57,pp. 17% I8I6 Copyright Q 1993Pergamon PressLtd.Printedin U.S.A.

Stable isotopic variations of clay minerals: A key to the understand~g of Kupferschiefer-type mineralization, Germany Miner~o~~h-P~rologi~hes

A. BECHTELand S. HOERNES tnstitut der Universe Bonn, Pop~l~o~er

SchioR W-5300 Bonn 1, Ge~any

(Received March 20, 1992; accepted in revisedform October 15, 1992)

Abstract-Isotopic compositions of silicate minerals and carbonates of selected Kupferschiefer samples from the Hessian Depression and from the Lower Rhine Basin have been studied in detail. In the area of the Richelsdorf ore district, stable isotope analyses of the Cu-mineralized sections in the Kupferschiefer show that the metal enrichment processes are associated with significant changes in the oxygen and hydrogen isotope composition of the clay minerals. The isotopic compositions of the <2-psize fractions reflect a decrease of approximately 4%0 in 6’*O (18.4 to 13.7%~) and an increase of nearly 506 in 6D (-74 to -25%0) with decreasing distance from the “Rote mule” zone. This isotopic zonation in the Kupfe~~hiefer shale is explained by fluid-rock interaction with ascending, oxidizing, hypersaline solutions, which transported high amounts of base metals from the Lower Permian red beds into the Kupferschiefer horizon acting as a geochemical trap. The estimated isotopic composition of the involved water (6180 between +3.0 and +&Sk; SD between -10 and +30/w) supports the characterization of the ore-bearing solutions as basinal brines. internal oxygen isotope fractionation of illite from samples next to “Rote Fiiule” corresponds to a temperature of approximately 13O”C, which is suggested as the temperature of metal precipitation. In the Lower Rhine Basin, the metal content does not exceed values commonly observed in black shales. Enhanced base metal (Pb, Zn) mineralization is observed only in the bottom section of the Kupferschiefer. The results of the oxygen and hydrogen isotope analyses of the selected core samples provide no evidence for correlation between isotopic composition of the silicate minerals and the extent of lead and zinc sulphide precipitation. Futhermore, they do not point towards isotopic exchange reactions between clay minerals within the Kupferschiefer shale and fluids out of isotopic equilibrium with the investigated mineral phases. However, it is shown that the lack of detectable interaction between the rock and external fluids, as reflected by these data, in general coincides with the proposed concept that the base metals Pb and Zn as well as Ba have been coprecipitated from Carboniferous formation waters. Pb, and Zn. The distribution of the metals is discordant to the bedding plane in the Kupferschiefer in these areas (RENTZSCH, 1974; KUCHA and PAWLIKOWSKI, 1986; JowETT, 1986; SCHMIDT and FRIEDRICH, 1988; VAUGHANet al., 1989; OSZCZEPALSKI,1989; KUCHA, 1990). Until now, isotopic investigations of Kupferschiefer samples were performed only with respect to organic carbon, carbonate, sulphides, and sulphate (R~SLER et al., 1968; MAROWSKY, 1969; HARANCZYK, 1972; HAMMER et al., 1989; SAWLOWICZ,1989; JOWETT et al., 199 1,1993; BECHTELand PUTTMANN, 1991, 1992; HEPPENHEIMERet al., 1991). Essential results from recent studies follow. Detailed isotopic investigations of the organic compounds in Kupfe~hiefer samples from Mansfeld-Sangerhausen (HAMMER et al., 1989) and from the Richelsdorf Hills (BECHTEL and PUTTMANN, I99 1) yield significant zonation in 613C around “Rote Fgule,” which correlates with regular changes in chemical composition of the extractable organic matter. Organic geochemical inv~ti~tions on Kupfe~hiefer from southwest Poland and the Hessian Depression (PUTTMANN et al., 1989a, 1990; BECHTEL and PUTTMANN, 1991) revealed a significant alteration of the extractable bitumens, which correlated with the amount of copper mineralization within the Kupfe~hiefer and consequently with the distance from “Rote Fgule.” The alteration of organic matter was characterized by a decreasing amount of n-alkanes in the examined bitumens (PUTTMANN et al., 1988). This type of

THE BASEMETAL ENRICHMENTin the basal Zechstein unit of the European Permian system, referred to as “Kupferschiefer type” mineralization, has for a long time been quoted as the prototype of syngenetic sedimentary ore deposits (WEDEPOHL, 1964, 197 1; BRONGERSMA-SANDERS,1966; MAROWSKY, 1969; HARANCZYK, 1986; SAWLOWICZ,1989). Extensive mineralogical, geochemical, and stable isotope studies during the past few decades resulted in conflicting models of genesis for Kupferschiefer Cu-Ag ores. In summary, the data point out that in most parts of the basin, the metal content does not exeed values commonly observed in black shales (RENTZSCH, 1974; HOLLAND, 1979; VAUGHAN et al., 1989; OSZCZEPALSKI,1989; KUCHA, 1990; PUTTMANN et al., 1990). Significant base metal enrichment is observed in areas near to the Zechstein seashore which are simultaneously related to the tectonic fault systems of the Variscan basement at the border between ~xo~u~n~an and Rhenohercynian zones (RENTZSCH, 1974; JOWETT, 1986; SPECZIK, 1988; WTTMANN et al., 1990; BECHTELand WTTMANN, 199 1). High-grade copper mineralization occurs mainly in the vicinity of areas of post-depositional oxidation, denominate as “Rote Fgule,” which are characterized by replacement of framboidal and euhedral pyrite by haematite (RYDZEWSKI, 1978). Base metal mineralization is arranged in three distinct zones around “Rote Fgule” in the order Cu, 1799

1800

A. Bechtel and S. Hoernes

alteration has been explained as resulting from interaction with ascending oxidizing solutions, which transported high amounts of base metals from Lower Permian red beds into the Kupferschiefer horizon (PUITMANN et al., 1990; BECHTEL and PUTTMANN, 199 1). The observed enrichment of the organic material in “C was interpreted as the result of a preferential release of isotopically light hydrocarbons through progressive degradation and oxidation of organic matter (HAMMER et al., I989; BECHTEL and P~~MANN, 1991). Metal pr~ipi~tion was suggested to be caused by thermochemical sulphate reduction with organic matter acting as a reducing agent and proton donor (PUTTMANN et al., 1990; BECHTEL and PUTTMANN, 199 1). The horizontal and lateral variations in isotopic and chemical composition of the organic matter within the Kupferschiefer of the Lower Rhine Basin, obtained by combined isotopic and organic-geochemical analyses, were attributed to differences in sedimentary facies during Kupferschiefer deposition as well as in thermal history (JOHANN et at., 1989b; HEPPENHEIMERet al., 1991; BECHTEL and PUTTMANN, 1992). The observed compositional and isotopic changes in the organic material do not point towards thermochemical redox processes (PUTTMANN et al., 1990; HEPPENI-IEIMERet al., 1991). Previous results of geochemical studies (DIEDELand FRIEDRICH, 1986) have shown that metal contents are in the range of “normal” black shales (KNITZSCHKE, 1966; HOLLAND, 1979). Anomalously high base metal (Pb, Zn) contents are restricted to the basal Kupferschiefer sections, which are characterized by intensive barite mine~lization. The dist~bution of Ba, Pb, and Zn is strongly related to the underlying Carboniferous stratigraphy (DIEDEL, 1986; VAUGHAN et al., 1989). Within the Kupferschiefer overlying the Westphalian A and B strata, containing hydrothermal Pb-Zn-Ba vein-type deposits, anomalously high Ba contents of up to 5% Ba are found. By contrast, the Kupferschiefer above the elastic sediments of Westphalian C only exhibits Ba contents in the range recorded for normal black shales. In addition, enhanced Pb/Zn accumulation has been observed in the western part of the basin, which was influenced by the basic int~sion that forms the “Krefeld High” (DIEDELand ~~MANN, 1988). The results provide evidence for the accumulation of the base metals Pb and Zn, as well as Ba, from basinal Carboniferous formation waters (DIEDEL and PUT-~-MANN,1988; PUTTMANN et al., 1990), inforced by the thermal influx of the Krefeld High. Copper enrichment was not observed because potential source rocks, such as the volcanics and the elastic sediments (red beds) of the Lower Permian, are missing in this area. Variations in oxygen and hydrogen isotope composition of clays and clay minerafs on a regional scale, between different grain size fractions, as well as fractionations between clay minerais and coexisting mineral phases in sediments (e.g., quartz, carbonate, sulphate) represent a powerful tool in evaluating fluid-rock interactions, low-temperature thermal events, and isotopic exchange in closed and open systems during or after diagenesis (SAVIN,1967; SAVIN and EPSTEIN, 1970; LAWRENCE and TAYLOR, 1972: O’NEIL and KHARAKA, 1976; YEH and SAWN, 1976, 1977; YEH and EPSTEIN, 1978; YEN, 1980; MARUMOet al., 1980; LONGSTAFFE,1984, 1986; Y EH and I&LINGER, 1986; O’NEIL, 1987; BIRD and CHIVAS,

1988; SAVIN and LEE, 1988). In addition, by determing the isotopic fractionation between oxygen of different sites in hydroxyl-bearing silicates, these minerals may provide singlemineral geothermometers (SAVIN, 1967; HAMZA and EPSTEIN,1979; BECHTELand WOERNES, 1990). For the present study, core samples from narrowly sampled Kupferschiefer profiles within the area of the Richelsdorf ore district and from the Lower Rhine Basin have been selected for hydrogen and oxygen isotope investigations. The mineralogical and chemical compositions of the selected drill sites have been investigated intensively (SCHMILIT,1985; SCHUMACHER, 1985; SCHMIDT and FRIEDRICH, 1988; DIED~L, 1986, DIEDEL and PUTTMANN, 1988; PUTTMANN et al., 1990). Furthermore, detailed isotopic and organic-geochemical data are also available for these samples (WTTMANN and ECKARDT, 1989; PUTTMANN et al., 1989b; BECHTEL and PUTTMANN, 199 1, 1992; HEPPENHE~MERet al., 199 1). As mentioned, the carbon isotope and chemical compositions of the organic material, as well as carbon and oxygen isotope compositions of carbonates, provide evidence that the origin of the metals and the processes responsible for metal accumulation and sulphide precipitation were different for the two investigated areas within the Kupferschiefer basin, It could be expected from the presented concepts, for the ore formation processes, that the extent of fluid-rock interactions within the investigated Kupferschiefer sections should be also remarkably different. In order to prove these assumptions, the present study deals with hydrogen and oxygen isotope investi~tions of the clay fractions from the Kupferschiefer shale, as well as with oxygen isotope invcsti~tions of the coexisting quartz and carbonate minerals. In combination with the published isotopic data and the geochemical features of the selected samples, these analyses provide information about fluid-rock interaction and its contribution to Kupferschiefer type mineralization. GEOLOGICAL SEITING AND SAMPLE DESCRIPTION During Zechstein Transgression the Kupferschiefer was the first sedimentary layer covering large parts of the floor of the Permian Basin in central Europe (Fig. 1). Except for marginal positions the Kupferschiefer shale is characterized by a uniform mineralogical composition and a thickness of about 30-50 cm. Near the southern shoreline of the Zechstein basin the Kupferschiefer reaches an average thickness of about I .5 m due to a higher content of carbonates intercalated with argillaceous laminates (PAUL,1982). The famous copper deposits within the Kupferschiefer bed (Fig. 1) are located along the border between Saxothuringian and Rhenohercynian zones (RENTZSCH,1974). In this crustal section an intensively faulted basement was filled with elastics and volcanics of the Lower Permian, where thick volcanic piles are preferentially overlain by red sandstones. A generalized stmti~phic column is shown in Fig. 2. Tectonic activity in the southern regions near the Bohemian Massif(Fig. i), possibty caused by Triassic intraco~t~nen~i rifting, has been suggested to be responsibIe for an enhanced heat flow (JOWETT,1986; SPECZIK,1988; VAUGHANet al., 1989) and upward migration of fluids which transported the metals into the overlying Kupferschiefer strata (JOWETT,1986). The ore district of the Richelsdorf Hills, Germany (Fig. I), is a typical example of this geological situation (SCHMIDT and FRIEDRICH, 1988; VAUGHAN et al., 1989; JOWETT,1993).Thirty-eight drill-core samples from Kupferschiefer and adjacent strata (Fig. 2), from six exploration drill sites in the Richelsdorfdistrict, have been included in this study. The samples were taken from drift sites which are located at different distances to “Rote Fiiule” (Fig. 2). Accordin~y they belong

StabIe isotopes of cfays from Kupfe~hiefer

1801

minemli~tion

FTC. 1. Map of Zechstein Basin in Central Europe including the Variscan zonation of the basement and the location of major Kupferschiefer deposits (modified after RENTZSCH,1974). (-- 1) = Zechstein shore line; LRB = Lower Rhine Basin; Sp = Spessart; Rh = RhSn; Ri = Richelsdoti, M = Mansfeld; S = Spremberg/WeiBwasser; K = Konrad N = Nowy Kosciol; L = Lena; LPR = Lubin-Polkowice-Rudna (adopted from PUT-I-MANN et al., 1990).

to different metal facies types, based on the relation between Cu, Pb, and Zn in the Kupfe~hiefer unit (KMTZSCHKE,1966). Within the investigated profifes, base metals are enriched mainly in the basal Kupferschiefer section (Table I). Taking this section of economic interest into account, “Rote F&rle”-related high-grade mineralization shows numerous zonation patterns with regard to the metal facies (Table I), as reflected by the ore paragenesis and the ratios of Cu, Pb, and Zn in relation to the total base metal content (CufPbfZn). Also the S2-/C,, ratios show a slight variation with the observed base metal zonation pattern; S/C in general decrease with increasing distance from “Rote FUe.” Comparing the Kupferschiefer from the “Rote Fgule” zone with a similar section within the Pb/Zn zone, the most pronounced differences in the rock forming paragenesis are the pr~om~nance of calcite over dolomite (Table I), the higher amount ofquartz, and the occurrence ofanhydrite cement in the groundmass of the Kupferschiefer shale of the “Rote Fkie” zone (SCHMIDT,1985). Additionally, Kupferschiefer samples from the Lower Rhine Basin (Fig. I) have been analyzed because of the completely different geological constraints. In the investigated area (Fig. 3) Lower Permian is only deposited in very few areas as thin layers, and them is a general lack of volcanics (DIEDELand WTTMANN, 1988). From this part of the basin twenty-three core samples of three boreholes (no. 4 1, 132, and 134) have been analyzed (Fig. 3). As indicated by organic-geochemical investigations, the Kupferschiefer from borehole no. 132, tocated in the very western part of the basin, has been influence by heat Sow of the Krefeld High (JOHANN and ECKARDT, 1989). Here, the Kupfe~hiefer directly overlies the Carboniferous basement and an enhanced Pb/Zn and Ba accumulation (Table 2) has been detected at the bottom part of the shale (DIEDEL and FRIEDRICH, 1986; DIEDEL and WTTMANN, 1988). In this drill hole a predominance of calcite over dolomite at the basal Kupferschiefer is detected by the higher CaO/MgO-ratio (Table 2) compared with the results from the two other investigated profiles. Borehole positions no. 4 1and 134 are located in an area where only burial-related maturation of the organic matter is observed (WTTMANNet al., 1989b). Variation in burial depth of the Kupferschiefer according to different distances from the former Zechstein sea shoreline is reflected by these profiles. The contrast in underIying Carboniferous stratigmphy is related to significant differences in the base

metal and Ba contents (Table 2). Samples from drill site no. 4 1 are included because in this area the Kupfe~hiefer is underlain by a few centimeters of Lower Permian red beds. ANALYTICAL

METHODS

Sample Preparation Samples were subjected to the normal cleaning procedure, which prevent affecting of the isotopic composition of the clay minerals during chemical treatment (SAWN, 1967). After gentle mechanical disaggregation, fractions of the core samples were treated with IN hyd~hlo~c acid to remove carbonate. Repeated washings in distilled water mixed with acetone are followed by stirring and boiling the samples in hydrogen-~roxide (HsOz) for several days, in order to destroy the organic material. The residues were washed with distilled water, dried, and homogenized. The samples were then subjected to the NaCl method, as described by MAROWSKY(1969) for removal of sulphate. Disaggregation by stirring the samples in distilled water is followed by grain size separations using the Atterberg method (MOLLER, 1964, pp. 85). For the present study the <2-g, the 2-6.3-p, and the 6.3-20-fl size fractions were collected. These fractions were subjected to dialysis in distilled water and subsequently dried at temperature <50°C. X-my diffraction analyses indicate that the <2-p size fractions consist mainly of ilhte (2M 1 -t 1M), minor constituents are kaolinite (
1802

A. Bechtel and S. Hoernes night for dolomite). The reproducibility of results was in most cases better than 0. I k. Oxygen isotope Investigations

of Silicates

The grain size fractions (~2 Joand 2-6.3 p) and the quartz separates were reacted with fluorine gas to liberate oxygen for isotopic analyses. For decomposition a modified TAYLOR and EPSTEIN-type fluorine line was used (TAYLOR and EPSTEIN, 1962). Fluorine gas was purified as described by ASPREY (1976). Water samples, which were obtained by dehydroxylation of the clay fractions during vacuum extraction (see the following text), were sealed in small silver tubes and subjected to the fluorination method @'NEIL and EPSTEIN, 1966). Determination

Legend

liii

Zechsteinkalk co1 Kupferschiefer Tl WeiOliegend

‘.:.:.,. \ +(Q-l

In order to determine the oxygen isotope compositions of hydroxyl groups in clay minerals from the 12-r size fractions a vacuum extraction technique was used (BECHTEL and HOERNES,1990). Samples were heated up rapidly to 1300°C by a high-frequency induction coil under vacuum. In order to prevent isotopic exchange of the hydroxyl group oxygen with the dehydrated residue, the extracted water was immediately removed from the sample material by freezing in a glass trap cooled with liquid nitrogen. After heating, the experimental device was flooded with dry nitrogen gas until atmospheric pressure was reached. The water was thawed, taken from the glass trap by syringe, and welded in a small silver tube. Water samples and dehydrated residues were used for oxygen isotope investigations following the procedure just described.

Stratigraphic column

Areas vi thou t Rotliegendes

WeiOliegend .; .._-:. sandbar

51

Rotliegendes RO

Boundary of Rote Fiiule Io”e

Paleozoic basement

Hydrogen Isotope Investigations

For mass spectrometric analyses of hydrogen isotope ratios the clay fractions (~2 w) were weighed in platinum crucibles and dried under vacuum overnight at 200°C. Preparation of hydrogen gas was carried out in an experimental device after GODFREY (1962). For the liberation of water from hydroxyl-bearing minerals, the samples were heated by a high-frequency induction coil under vacuum up to 1500°C. The resulting water was reduced to H2 by passing it over hot uranium (BIGELEISEN et al., 1952). The hydrogen gas was trapped in a sample container with activated charcoal at liquid nitrogen temperature.

FIG.

2. Distribution of basement (areas without Rotliegend), Weil3liegend sandbar and Rote F&de in the Richelsdorf district (SCHMIDT and FRIEDRICH,1988). Ro = investigated drill sites. reaction temperature was 25 “C, kept constant by a thermostat. Carbon and oxygen isotope compositions of coexisting calcite and dolomite were obtained from one sample by mass spectrometric analyses of the resulting CO2 after different reaction times (1 h for calcite; over-

Table

Drill hole

1.

of the Oxygen Isotope Ratios of Hydroxyl Groups

General geochemical features within the basal Kupferschiefer (lowermost 2m) section of the Richelsdorf Hills. Data according to SCHMIDT (19851.

metal zone

ore

BM

CU

Pb -

Zn -

S2-

Calcite

(wt-%)

BM

BM

BM

Cora

Dolomite

Ro 41

RF

hm cp

0.47

0.994

0.004

0.002

0.62

2.27

Ro 23 Ro 18 Ro 19

Cu

cc dg cv bn

3.04 4.67 5.71

0.997 0.883 0.577

0.002 0.059 0.210

0.001 0.058 0.213

0.54 0.48 0.45

1.46 1.48 1.36

25 Ro 21

Pb/ Zn

cp gn sph

2.16 3.21

0.448 0.070

0.184 0.519

0.368 0.411

0.33 0.36

0.57 0.79

Ro

BM = Sum of base metals (Cu+Pb+Zn); RF = "Rote Flule"; hm=hematite, cc=chalcocite, bn=bornite, dg=digenite, cv=covellite, cp=chalcopyrite, gn=galena, sph=sphalerite

Stable isotopes of cfays from Kupfe~hiefer

Paleozoic

minemiization

1803

borsmsnf

I Carbonifrrous I n

Zechstain basin

M@ Fault

Cwbnnifraus

bossmnf

FIG. 3. Location map of the Kupfe~hiefer dist~bution in the southern area of the Zechstein as part of the Lower Rhine Basin (~~MANN and ECKARDT, 1989). Major horst and graben stuctures and the horehole locations are

marked.

RESULTS

Mass Spectrometry

The results of the oxygen isotope investigations of the present study are summarized in Table 3 for thesamples from the Richelsdorf district and in Table 4 for the Kupferschiefer of the Lower Rhine Basin. 6’*0 values ofthe calcites, adopted from BECHTEL and JOHANN (199 1, 1992), are

Institute for Chemistry (Mainz, Germany) using a VG PRISM, 60” instrument. The overall reproducibility, referred to the total analytical procedures, was in the range of 0.1 to 0.2% for ai80 and in most cases between I and 2%~for 6D. Quartz and K-feldspar standards, calibrated against NBS-28 quartz, as well as NBS-30 biotite, were used for control of the analytical results.

Drill

hole

41 132 134 BM

2.

also listed in these tables.

General geochemical features within the basal Kupferschiefer (lowermost 2m) section of the Lower Rhine Basin. Data according to DIEDEL (1986).

Underlying Carboniferous strata Westphalian B Westphalian A Westphalian C

= Sum

DISCUSSION

Oxygen Isotope Investigations

Oxygen isotope determinations were carried out using a VG SIRA9 triple-collector, 90”, 9-cm-radius instrument. Mass spectrometric analyses of the hydrogen gas were performed at the Max-Planck-

Table

AND

BM

CU

hwm) 2492 4922 979

Pb

Zn

(pm)

(mm)

(PPrn)

45 70 48

423 675 163

of base metals iCu+Pb+Zn)

2024 4177 768

Ba

CaO

(wt-%I

MgO

0.18 5.03 0.04

1.60 4.38 1.49

1804

A.Bechtel and S. Hoernes Table 3. Oxygen and hydrogen isotope compositions of the silicates from core samples of the Richelsdorf ore district. (Cal)= Zechsteinkalk, (Tl)=Kupferschiefer, tSlI=Wei8liegendes

Satnple no.

clay size fraction'2~ B'~o(sMow) ao(sMow)

2-6.3~size fraction b'*OtSMOW)

quartz (6.3-20~)

calcite

is'eO(SMOWl GBO(SMOW)

26.9 14.2 16.1 17.1 Bo 41/2 (Cal) 26.0 14.3 15.9 16.0 Bo 4113 fCalf 22.7 15.3 Ro 41/6 (Cal) 22.2 13.6 15.4 14.7 -25.0 Ro 41/9 (Cal) 20.6 14.3 14.6 13.7 -24.1 Bo 41/11 (Tl) 14.2 Ro 41/14 (Tl) ________-__--_~_____--~--~~~-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~__~~_~~_______________ 27.9 15.2 16.8 17.1 Bo 23/8 (Calf 24.4 14.5 16.6 Ro 23/21 fCa1) 23.3 15.4 16.8 17.1 -44.9 Ro 23/26 (Tlf 22.4 15.2 16.4 15.7 -39.7 Ro 23/29 fT1) 21.3 15.4 14.3 -29.2 Ro 23/34 (Tl) 20.6 17.5 15.6 14.8 Ro 23/44 (Sl) I___________-_______--~-~~~~~~~~~~~~~~~~-~~~~~~~~~~~~~~~~~~~~~~~~______~___~____ 26.5 14.8 16.2 17.3 Ro 18/2 (Cal) 27.4 14.4 15.9 17.4 -48.4 Ro 18/17 fT1) 26.2 13.9 15.6 16.0 -40.6 Ro 18,'21(Tl) 22.8 13.3 15.6 14.7 -32.5 Ro 18/25 (Tl) 21.9 17.0 15.3 Ro 18/29 (Tl) 20.6 17.9 16.0 Ro 18/29 (Sl) 20.5 17.5 15.6 15.6 Ro 18/33 (Sl) ___f___~____-_______~~~--~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-~~~~~-~_~__~--_~___~~___ 27.2 14.9 15.5 16.8 Ro 1912-l (Call 26.3 14.6 16.1 16.7 -53.5 Ro 19/19 (Tl) 24.4 14.5 16.5 15.8 -44.8 Ro 19/21-atT1) 23.8 14.1 16.0 15.0 -40.6 Ro 19/22-l(Tl) 19.9 16.3 16.7 Ro 19/32 (Sl) -_______-___--______--~-------~~~~~~~~~~~~~~~~~~~~~~~~~-~~~~~-~_-~-__-_~~_______ 13.6 16.6 Ro 25/7 (Calf 29.0 14.4 17.0 17.3 Ro 25/X? (Cal) 26.2 14.8 16.6 17.9 -70.1 Ro 25/26 fT1) 25.6 14.6 17.7 18.3 -67.2 Ro 25/28 (TlI 23.4 13.8 17.1 17.6 -51.9 Ro 25/30 (Tl) 22.0 14.7 15.7 16.6 Ro 25/38 (Sl) ____________-_______~~~~~~~~~~~~~~~~~~~-~~-~~~~~~~~--~______~___________________ 23.8 13.6 15.8 16.5 Ro 21/3 (Cal) 24.9 13.9 15.6 16.7 -74.6 Ro al/l0 (Tlf 25.3 14.1 16.6 17.3 -70.5 Ro 21/16 fT1) 25,7 14.5 17.1 17.6 -68.9 Ro 21/18 (TlI 23.5 14.4 17.2 18.4 -66.5 Ro 21/21 (Tl) 21.6 13.6 17.6 18.3 -60.6 Ro 21/24 (Tl) 21.8 13.4 16.7 Ro 21/26 (Sl) 15.9 Ro 21137 (Sll

Richelsdorf‘Hills

Graphical presentation oftberesults fromthe<2-rand 2-6.3-p size fractions of the samples from the Richeisdorf district are given in Fig. 4b. In the Pb/Zn-bearing zone (Fig. 4a) the clays generally yield 6’*0 values between 16.4 and 18.4%0 (Table 3). From the coarser grain size fractions (26.3 p) of these samples lower 6”O values in the range of 15.6

to 17.7%0 (Fig. 4b)areobtained. Theseresults canbe best explainedbyahigheramountofdetritalcompounds(quar detriilite) inthe2-6.3-p size fractions comparedwiththe

clay size fractions. In this part of the ore district (Pb,fZn facies), a tendency towards higher 6’*0 values with increasing depth can be detected, probably due to an increasing amount of authigenic clay minerals in the basal Kupferschiefer unit. This conclusion is supported by textural evidences from mi-

Stabie isotopes of clays from ~upfe~~i~fer mine~i~tion Table

4.

Oxygen

and hydrogen

isotope

compositions

of

silicates

from core samples of the Lower Rhine Basin. (Cl)= Zechsteinkonglomerat

Sample no.

clay

size

fraction'2~ b'@O(SIwlf ~D(S~~)

2-6.3~

size

fraction ~18O(S~~)

quartz

calcite

(6.3-20~)

aleo(s~~~

al*o(s~~~

41/4 {Cal) 17.0 -53.9 16.6 15.3 41/7 (Cal) 17.0 14.8 29.2 42/13 (Tl) 16.9 -54.5 16.5 16.0 29.4 41/16 (Tl) 17.3 -54.0 16.7 16.1 29.2 41/27 (Tl) 17.0 -58.4 16.3 15.7 28.4 41/32 (Tl) 17.2 -56.1 16.1 15.6 29.1 41/39 (Cl) 17.2 14.9 28.1 __-______________I__~~~~~~~~~-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-132/3 (T11 16.8 -56.5 16.2 15.8 28.8 132113 fT1) 17.1 -58.3 16.3 15.5 28.1 132/19 (Tl) 16.7 -58.4 16.7 15.3 25.9 132/21 IT11 17.8 -61.0 17.2 15.6 25.7 132/27 (Cl) 17.9 16.2 15.5 132/30 (Cl) 15.5 14.8 14.2 ______________________________________I_~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-~--~-134/3 (Cal) 18.7 -56.9 16.8 15.5 30.1 134/10 (Tl) 17.8 -59.4 16.8 15.7 31.6 134/14 (Tl) 17.3 -62.3 16.1 15.5 31.5 134119 (Tlf 17.5 -69.9 17.1 15.1 31.4 134/29 (Tl) 17.9 -70.1 15.8 15.3 31.1 134133 (TUtElI 17-f 15.7 14.7 134/39 (Cl) 16.1 14.9

croscopic studies (SCHMIDT,1985), as well as by variations of the illite crystallinity determined, according to WEBER (1972), from XRD analyses of selected samples. SCHMIDT ( 1985) reported an increase in ss-parallel orientation of clay minerals with decreasing depth. In general, an orientation strongly parallel to the bedding plane could be expected as a result of ~imen~tion of sheet silicates within the water column. Therefore, the decreasing order in Site orientation is an indirect argument for the increasing formation of authigenic clay. Furthermore, this interpretation coincides with the results from determinations of the illite crystallinity. The <2-p size fractions of the samples from the top and the bottom of the Kupferschiefer profile Ro 2 1 (Pb/Zn zone) were selected for measurements of the relative peak breadth at half peak height (Hb,,). According to WEBER(19721, quartz was used as an internal standard. From sample Ro 21/10 (top) an average value of Hb_, = 2 10 is yielded, whereas XRD analyses of the sample from the basal Kupferschiefer section (Ro 2 l/ 24) resulted on average in Hb,i = 280. The results indicate decreasing illite crystallinity within the investigated profile with increasing depth, which is consistent with the conclusions mentioned in the preceding text. In contrast, from Kupferschiefer samples next to “Rote F&de” and from the copper facies, isotopically lighter silicate oxygen compositions are obtained (Fig. 4). In this Kupferschiefer section the clays show a’*0 values between 13.7 and 15.8%, whereas the 6’*0 values of the 2-6.3-p fractions vary

between 14.2 and 16.5% (Table 3). Furthermore, the clay fractions from the Cu-rich zone show isotopically lighter aI80 values than the corresponding coarser grained fractions (Fig. 4b). The &“O values of the clay size fractions within the Kup ferschiefer unit (Table 3) reflect a correlation between isotopic composition and the distance from “Rote F&de” (Fig. 4). Adjacent to the oxidized facies, zones with different 15~~0values can be mapped (Fig. 4b) by interpolating the oxygen isotope data of the investigated profiles. They are arranged around “Rote Fliule” in the following order: zones with isotopically light silicate oxygen are located next to “Rote Fiiule,” and 6’sO values increase with increasing distance from the oxidated facies. Metal facies zonation corresponds with the observed zonation in isotopic composition (Fig. 4). The zone in which the clay size fractions show isotopically lighter S’*O values than the coarser grained fractions is also marked in Fig. 4b. The remarkable coincidence with the part of the Kupferschiefer section characterized by high Cu concentrations (SCHMIDT,1985) is obvious. The relationship between the oxygen isotopic composition of different grain size fractions, as well as the observed regional distribution of the d’*O values (Fig. 4), cannot exclusively be explained by variation in sedimentary facies during Kupferschiefer deposition. In contrast, isotopic exchange ofthe finegrained clay fraction (mainly composed of iilite) with an isotopically distin~ive fluid phase must be taken into account.

1806

A. J3echtel and S. Hoernes

RF --Cu facies RoCl

focier ?I 23

Rqld

w PblZn facialRq’9 Ro25

I

. Metal content

Ro21

a. I

Ro41

Ro23

Role

Rol9

Ro25

Ro21 2- In 2 G2

L 161

1

loo

200 Cal

16.1 154 11.3 16.6 -t-1’16 7.1

Tl

b. ROLl 1c.2x.3-

Role

Ro23

Ro19

Ro21 In

136-

15.2lCb

13.6-

Ro25

lL5-

14-

lM-

16.9 Cal x.4 -

146

13.9-

1Lb-

11.5-

138-

VA-

14.1-

13.9-

14.6

Tl

1L.F

FIG. 4. Cross section through the southern part of the Richelsdorf district (Ro r drill holes). (a) Metal facies and distribution of Cu, Pb, and Zn within the basal Zechstein sequence. Cal = Zechsteinkalk; TI = Kupferschiefer; SI = WeiNiegend (SCHMIDT, 1985). (b) Distribution of the aI80 values of the <2-p size fraction (ill.; kao; chl.) and the 2-6.3 p size fraction (ill.; kao.; qz.) of the investigated samples in relation to Rote FAule (RF). The area in which the clay size fraction shows isotopically lighter 6”O values than the corresponding coarser grained fraction is marked (dotted). “lsolines” were constructed by interpolating the results from the clay size fractions of the Kupfersehiefer profiles. (c) Results of the oxygen isotope investigations of the separated quartz samples (6.3-20 p).

Considering this, the measured variation in 15~~0of the 26.3-p size fraction in comparison with the values obtained from the clay size fraction can be explained by higher resistance of the coarser grained fractions to isotopic reequilibralion.

Isotopic exchange between the illites and an externally derived fluid could have taken place during diagenesis. However, the variation of the 6’*0 values of the clay size fraction within the Kupferschiefer unit, just described, as well as the correlation with metal facies zonation, point towards isotopic ex-

Stable isotopes of clays from Kupfe~hiefer mine~li~~on

change reactions with the ascending oxidizing solutions, which have been suggested to be responsible for high-grade mineralization and associated “Rote Faule” (SCHMIDT and FRIEDRICH, 1988; BECHTELand WTTMANN, 199 1) through fluid-rock interaction. The isotopic shift of approximately 4%~in 6’80 within the <2-r size fractions is accompanied by variation in illite crystallinity. In contrast to the values (Hb,, = 210-280) measured within the profile (drill hole Ko 21) of the Pb/Zn zone, illite crystallinity is lower in the proximity of the “Rote Faule” zone as reflected by Hb,, = 340 obtained from sample Ro 23/34. The decrease in illite crystallinity is suggested to be due to dissoiution/precipitation processes, caused by the hydrothermal activity. These conclusions are further supported by the results of carbon and oxygen isotope investigations of the coexisting carbonate minerals (BECHTEL and WITMANN, 1991). Isotopically light calcites have exclusively been detected in samples next to “Rote Fame” (Table 3). Graphical p~n~tion of the 6”O vaIues of coexisting clay (illite) and calcite from samples within the Kupfe~hiefer unit in a 6-6 diagram (Fig. 5) shows the good correlation of isotopic composition with corresponding metal facies type. The calcites and illites from Cu-rich samples are markedly depleted in “0. According to GREGORY and CRISS (1986), the data array in the 6-6 diagram in relation to the eqdlibrium values in the system calcite-illite, represented for selected temperatures by straight lines in Fig. 5, indicate that the system in general changed its characteristics from a rock-buffered system within the Pb/Zn zone (data array cross-cut equilibrium lines) to a fluid-buffe~d system within the “Rote mule” zone (data array parallel to the isotherms). Therefore, with decreasing distance from “Rote FiiuIe” the system approaches characteristics of a fluid-dominated system and isotopic equilibration (GREGORYand CRES, 1986). Fractionation data between calcite and illite (Fig. 5) do not represent equilibrium values, probably due to insufficient equilibration of the illites, which are known to be more resistant against isotopic exchange with the coexisting fluids (SAWN and LEE, 1988) in comparison with calcite. Accordingly, the observed tendency in the 6-S plot is mainly governed by changes in the oxygen

Legend:

FIG. 5.6-6 diagram of coexisting clays and calcites within the Kug ferschiefer section of the Richelsdorf ore district. ~uilib~urn PO values (calcite-ilIite) for selected temperatures are marked by straight lines.

1807

isotope composition of the calcites. The shift in isotopic composition of the calcites is accompanied by increasing calcite/ dolomite-ratios towards “Rote Faule” (Table 1). Microscopic investigations of the Kupferschiefer samples from the “Rote Faule” zone (FRIEDRICH and KOTNIK, 1985) provide evidence for carbonatization of the silicates and anhydrite prior to the replacement by the ore minerah, as well as for calcitization of dolomite. These observations are in agreement with recent results of geochemical and petrological studies of the Kupferschiefer from southwestern Poland by OSZCZEPALSK~ (1989), indicating the predominance of calcite in the basal Zechstein of the hematite zone, in contrast to the zones more distant from the “Rote Wule,” where dolomite is much more common. OSZCZEPAL,SKI(1989) concluded that this may be the result of calcitization of earlier formed dolomite. The CO2 nfor calcitization and dedolomitization could have been derived from the degra&tion and oxidation of organic matter, as detected in the “Rote F&de*’ zone (BECHTELand PUTTMANN, 1991). Oxidation of pyrite (SCHMIDT, 1985) could have been caused by dissolved Oz within the fluids or by cuprous ions acting as an oxidizing agent (OSZCZEPALSKI, 1989). The pH of the flowing waters is lowered by these oxidation processes. Acidity can also be acquired through the decomposition of organic matter, which releases CO:!. Moreover, protons produced during the oxidation of pyrite can be considered as agents promoting dedolomitization, implying that both hematitization and cakitization processes could have occurred simul~neously. Considering these results, isotopic ~uilib~um between these secondary calcites and the coexisting water should be established during the formation of the “Rote Fiiule” zone. Therefore, the oxygen isotope composition of the ore bearing solutions, suggested to be responsible for the observed isotopic zonation within the clay minerals, can be estimated assuming that equilibrium with the coexisting water will be best ap proximated by the 6’*0-values of the calcites next to “Rote Faule.” From quartz samples, separated from the 6.3-20-p size fractions of the Kupferschiefer shale and the overlying hmestone sequence (Zechsteinkalk), &I80 values between 13.3 and 15.4X0 (Table 3) are obtained (Fig. 4~). These data argue for a mostly detrital origin of the quartz grains originally formed at high temperatures. Higher 6i80 values of quartz in the range of 16.3 to 17.9%0 (Table 3) are obtained from core samples from three boreholes (no. Ro 23, Ro 18, and Ro 19) in the transition zone between WeilJliegendes and Kupferschiefer and at the top of the WeiMiegend sandstone (Fig. 4~). In this part of the Kupfe~~hiefer section higher contents of gypsum and anhydrite have been detected in the cement of the investigated samples (SCHUMACHER,1985), indicating an enhanced salinity of the formation waters. Therefore, the obtained higher S**Ovalues of quartz from these samples can be explained by an enhanced precipitation of authigenic silica (normally characterized by high S’80 values due to its lowtemperature formation), favoured in hypersaline environments (KASTNER, 1979). tower Rhine Basin Stable isotope inv~i~tions of the clay size fraction (<2 y), the 2-6.3-p size fraction, and quartz samples (6.3-20 ~1)

1808

A. Bechtel and S. Hoernes

FIG. 6. Content of Pb, Zn, and Ba in the investigated drill sites of the Lower Rhine Basin (DIEDEL and PUTTMANN. 1988). Data are are given for the lowermost part (I m) of the Kupferschiefer horizon. Iso-maturation lines are derived from the measurement of the proportion of meso pristane (PUTTMANN and ECKARDT, 1989).

were performed on the Kupferschiefer and adjacent strata from three wells in the area of the Lower Rhine Basin (Fig. 3). Mineralogical compositions of the grain size fractions are nearly identical to those aiready described from the samples of the Richelsdorf ore district; silicate fractions are mainly composed of illite, kaolinite, and quartz (-t chlorite) in variable amounts. Coexisting calcites from the selected Kupferschiefer samples were also investigated for oxygen isotope composition. The results of the oxygen isotope investigations are summarized in Table 4. Within the Kupferschiefer unit the clay fractions (~2 II) generally yield aI80 values between 16.7 and 17.9% (Table 4). The 6180 values of the coarser grained fractions (2-6.3 g) from these samples vary from 15.7 to 17.2% (Table 4). Within all samples investigated the 2-6.3-p size fraction yields a lower 6”O value than the corresponding clay size fraction (~2 FL).According to the results of mineralogical investigations (DIEDEL, 1986), this can be best explait& by a greater amount of detritai compounds in the coarser grained fractions. The observed relationship of the oxygen isotope compositions of different grain size fractions, is comparable to the results obtained from the PbjZn-bearing zone in the Richelsdorf area (Fig. 4). An overai? change of the oxygen isotope composition with depth cannot be detected. In contrast to the results in the Kupferschiefer section of the Richelsdorf Hills, neither a consistent variation of the isotopic compositions on a regional scale nor any correlation between base metal content (Fig. 6) and oxygen isotope data exist. The distribution of the 6”O values can be explained by variation in sedimentary facies during Kupferschiefer deposition. A plot of the 6”O values of coexisting clay (Site) and calcite (Table 4) from the samples within the Kupferschiefer unit (Fig. 7) points out that samples which are enriched in base metals show no significantly

distinct

isotopic

composi-

The array of the data shows that the obtained fractionations do not reflect equilibrium values, obviously due to variable amounts of detrital illite in the clay size fractions. Because the detritai iilites probably have preserved their isotopic signatures, originaliy established at higher temperatures, disequilibrium fractionation data are affected by varying amounts of these minerals within the clays. The distribution of the data points in the 6-6 diagram (Fig. 7) is comparable with the array obtained from samples of the Pb/Zn-~a~ng zone in the Richelsdorf ore district {Fig. 5). fi180 values of quartz separated from the 6.3-20-cc size fractions of the Kupferschiefer shale and adjacent strata range from 14.2 to f 6.1 %D(Table 4). These data are in agreement with the 15~~0values of the quartz samples from the Kupfertions.

30 -

cf’*OISMOW)

III

FIG. 7. d-6 diagram ofcoexisting clays and calcites within the Kupferschiefer horizon of the Lower Rhine Basin. Equilibrium d’*O values (calcite-illite) for selected temperatures are marked by straight lines.

Stable isotopes of clays from Kupfe~biefer schiefer section of the Richelsdorf Hills (Fig 4c). The results suggest that the selected samples are mainly composed of detrital quartz with minor amounts of authigenic silica. 6”O values do not correlate with depth, position of the investigated profiles within the basin (Fig. 6), nor with base metal mineralization. Hydrogen Isotope Investigations Isotopic com~sition of the water involved in fluid-rock interaction can be best estimated from the hydrogen isotope composition of coexisting hydrosilicates. Because the hydrogen content of rocks in comparison with water is very low, the hydrogen isotope composition of the fluid phase may change only slightly during isotopic exchange, while the 6D values of the hydrosilicates will be extensively affected (TAYLOR, 1979). In order to prove the proposed explanation for variations in oxygen isotope compositions of the clay minerals, hydrogen isotope investigations have been performed on the <2-p size fractions of selected Kupfe~hiefer samples (Tables 3 and 4). Richelsdorf Hills The results from the Kupferschiefer within the ore district of the Richelsdorf Hills are summarized in Table 3. Within the PbjZn-bearing zone (Fig. 8a) the <2-p size fractions generally yield 6D values between -75 and -45% (Fig. 8b). These values are in the range normally obtained from illitej muscovite samples in most sedimentary, magmatic, and me~mo~hic rocks (TAYLOR, 1974), and can be interpreted as the result of a mixture between detital illi~jmu~ovite, derived from crystalline rocks, and authigenic illite, formed at the site of deposition or by recrystallization during diagenesis, In this part of the Kupferschiefer section a general tendency towards isotopically heavier 6D values with increasing depth exists (Fig. 8b). This might be due to an increasing amount of authigenic clay towards the bottom part of the shale; a result also supported by the vertical variation in &I80 of the investigated clay size fractions from samples of the PbjZn-rich zone, as well as by the measured decrease in illite crystallinity with increasing depth within this profile. As discussed previously, also textural evidences argue for an increasing amount of authigenic clay in the <2-p size fractions towards the bottom part of the Kupferschiefer. Samples from the copper facies and from the “Rote Faule” zone yield higher bD values in the range of -40 to -25%0 (Table 3). By interpolating the hydrogen isotope data of the investigated Kupferschiefer profiles, zones with different 6D values of the clay size fraction can be mapped (Fig. 8b). The observed zonation corresponds with metal facies zonation (Fig 8a) as well as with the measured variation in oxygen isotope composition (Fig. 4b). The bD values from samples next to “Rote Faule” and the zonation in hydrogen isotope composition of illite within the Kupferschiefer section argue for isotopic exchange between the clay minerals and an externally derived fluid, rich in deuterium. This interpretation is also supported by a further decrease in illite crystallinity accompanying the isotopic shiA. The 6180-6D plot (Fig. 9) of the clay size fractions from the Kupfe~chiefer samples clearly indicate the good corre-

minemii~tion

1809

lation between stable isotope data and metal facies of the Kupferschiefer. Accordingly, we assume that isotopic equilibrium will be best approximated in samples next to “Rote Fiiule,” which show the highest 6D values of the clay fractions measured in the Kupferschiefer sequence. Despite the fact that these illites are not in oxygen isotope equilibrium with the fluids, the hydrogen isotope composition of the coexisting water could be estimated from the 8D values of the clays in the “Rote Fiiule” zone using the illite-water fra~ionation data (YEH, 1980; CAPUANO, 1992), because isotopic equilibration between clay minerals and water is much faster for hydrogen than for oxygen @‘NEIL and KHARAKA, 1976).

Lower Rhine Basin The analytical data from hydrogen isotope investigations of the clay size fractions from Kupferschiefer of the Lower Rhine Basin are listed in Table 4. For all samples investigated the t2-JI size fraction yields 6D values which fall in the range of -70 to -54L (Table 4). The results are generally comparabIe with the hydrogen isotope com~tions obtained from the PbjZn-bearing zone of the examined Kupferschiefer section in the Richelsdorf ore district (Fig. 8b). In agreement with these data, a tendency towards isotopically heavier 6D values can be detected with increasing depth (Table 4). This might be due to an increasing amount of authigenic clay towards the bottom part of the shale. In contrast to the results of isotopic analyses of silicate minerals from the Kupferschiefer profiles of the Richelsdorf ore district, the data obtained within the Lower Rhine Basin do not point towards isotopic exchange through fluid-rock interaction. Distribution of base metals is not correlated with the measured isotopic compositions. Temperature Assessments The experimental results of SAvrN (1967) and HAMZA and EPSTEIN ( 1979) mark internal oxygen isotope fmctionations of hydrosilicates as potential single-mineral thermometers. Methodical investigations by BECI-ITELand HOERNES( 1990) indicate that oxygen isotope ratios of the hydroxyl~u~ can be obtained by measurement of the vacuum-extracted water. The already discussed results of the oxygen and hydrogen isotope investigations of the clay fractions from the Kupferschiefer of the Richelsdorf district let us assume that determination of the internal oxygen isotope fmctionations of illite minerals might be successful1 in evaluating the temperature of base metal mineralization. The results of the oxygen isotope analyses from the clays are summarized in Table 5. The #*O values of the whole mineral oxygen and the extracted water (hydroxyl oxygen) are listed. The measured ai80 values of the dehydrated residues of the investigated Kupferschiefer samples are in agreement with the values calculated from aI80 values of the whole mineral and the extracted water by mass balance (Table 5). In concordance with the previously discussed results of isotopic analyses from the Kupferschiefer profiles, the obtained results for internal oxygen isotope fmctionations in the clay size fractions (Table 5) also reflect the correlation between stable isotope data and corresponding distances of the samples from “Rote F&tie” (Fig. 8~). Within the PbjZn-

A. Bechtel and S. Hoemes

1810

Rott

RoZ3

RO2S

Ralf

m2 f

t. FOG.8. Cross section through the southern part of the Richelsdorfdistrict (Ro = drill holes). (a) For explanation see Fig. 4a. (b) Distribution of the &D-values of the clay size fraction from Kupfers~hiefer samples in relation to Rote F&de (RF). “Isolines” were constructed by inte~lati~~ the results of the Kupf~~chiefer profiles (c) Distribution of the internal oxygen isotope fmctination data (illite-OH) of the clay size fractions (~2 II). Copper-rich zone is marked in the figure (dotted).

rich zone the measured values for illi&-UH fractionation generafly faH into the range of9.5 to t 5.74bo(Fig. %),whereas samples from the copper facies yield without exception fractionation data greater than 17%a(Fig. 8~). The highest values for internal oxygen isotope fractionation (-20%0) are obtained from sampIes next to “Rote Fgule.”

of

By ~x~~~oia~ing the results a first empirical calibration of the internal We-OH ~~cti~nati~n (BE~~HTEL and HOERNES, 1990) to lower temperatures a temperature of approximately 130°C is obtained fram the clays of the “Rote F&de” section. The obtained temperature is in good agreement with the results of fluid inclusion mierothe~ome~~

Stable isotopes of clays from Kupferschiefer mineralization

d’eO

(WOW I

FIG. 9. Graphical representation of the 6D, 6’*0 values of the clay size fractions from the Kunferschiefer unit of the Richelsdorf Hills. Metal facies of the samples is marked.

(TONN et al., 1987) and from organic-geochemical analyses (BECHTELand WTTMANN, 199 1). Biomarker investigations of the extracts from samples within the Pb/Zn bearing zone (BECHTELand PUTTMANN, 1991) indicate that, in terms of

Table

5.

Internal clay the

oxygen size

isotope

fraction

Richelsdorf

(<2p)

maturation, the Kupferschiefer has reached the oil window (100-l 50°C) in this part of the basin (TISSOTand WELTE, 1984). These data are also in agreement with the results of microthermometric investigations of fluid inclusions within the carbonate cement from the underlying basal Zechstein unit (WeiBliegendes) in the Richelsdorf area, which yield homogenisation temperatures of 120°C (TONN et al., 1987). The presented results imply that intracrystalline equilibrium values may be reflected by the internal fractionation data measured in the “Rote Faule” zone, although the whole minerals are not in oxygen isotope equilibrium with the coexisting water. This probably indicates that intracrystalline equilibration is not necessarily connected with the establishment of isotopic equilibrium between the clay minerals and water, and might be achieved earlier. low internal fractionation values obtained from the clay size fractions of samples from the Pb/Zn-bearing zone possibly reflect the mostly detrital origin of the illites, originally formed at high temperatures. Insufficient isotopic reequilibration of the silicate oxygen of the illite minerals through fluid-rock interaction is suggested to be responsible for intermediate values and for the observed zonation of the fractionation data. Within the Kupferschiefer of the lower Rhine Basin, the results of the oxygen and hydrogen isotope analyses provide no evidence for isotopic exchange reactions through fluid-

fractionation (illite-OH) of from Kupferschiefer samples

the of

section

Sample

Total-

Hydroxyl

no.

oxygen

oxygen

(illite) 61*o(%o)

1811

Dehydrated measured

6’80(%o)

tP80(%o

residue talc.

)

1000 In (ill.-OH) I%0 1

6’*0(%0)

Ro 41/9

14.7

-5.2

16.5

16.7

19.9

Ro 41/11

13.7

-6.5

15.9

15.6

20.2

Ro 23/26

17.1

-1.0

18.5

18.7

18.1

-3.5 -5.7

17.5

17.4

19.2

16.4

16.1

20.0

18.6

18.8

15.7

Ro 23/.29

15.7

Ro 23/34

14.3

Ro 18/17

17.4

Ro 18/25

14.7

-2.6

16.7

16.3

17.3

Ro 19/19

16.7

18.1

17.9

12.9

Ro 19/22

15.0

3.8 -3.0

16.8

16.6

18.0

Ro 25/26 Ro 25/28

17.9 18.3

5.0 4.6

19.3 19.7

Ro 25/30 Ro 21/16

17.6

19.0

12.9 13.7 17.7

17.3

-0.1 7.8

19.1 19.5 19.2

6.5

18.6 18.9

18.2 18.6

9.5 11.1

4.4

19.9

19.6

13.9

Ro 21/18 Ro 21/24

17.6 18.3

1.7

a

1812

A. Bechtel and S. Hoernes

rock interaction. The presumed influence of hydrothermal activity (especially in proximity of the Krefeld High) on the distribution of base metals in the bottom part of the Kupferschiefer shale, as deduced from stable isotope and organicgeochemical studies (HEPPENHE~MERet al., 1991), is not detectable by isotopic analyses of the silicate minerals. This might be due to an insufficient temperature or time for isotopic reequilibration, in comparison with the conditions adjacent to the “Rote F&tie” zone in the Richelsdorf ore district: an explanation which is in agreement with the low maturity of the organic matter in the area of the Lower Rhine Basin (FW-ITMANNand ECKARDT, 1989; PUTTMANN et al., 1990). From biomarker investi~tions of the extractable organic matter, a m~imum-tern~~tu~ of approximately 80°C is obtained for the Kupferschiefer shale within this part of the basin (BECHTELand W’I”I-MANN, 1992).

CONCLUSrONS Isotopic Compositions of the Ore Bearing SoIutions The results of the Kupfe~hiefer profiles from the Richelsdorf Hills suggest isotopic exchange of the clay minerals with an isotopically distinctive water in zones adjacent to “Rote F&de” at a temperature around 130°C. Correlation of the stable isotope data with base metal zonation indicates that the fluids involved in fluid-rock inte~ction should also be responsible for the formation of high-grade mineralization (“ReicherzZone”) and associated “Rote F&.tle.” As discussed in earlier sections of this article, evidences from microscopic and geochemical studies let us assume that isotopic equilibrium between calcite and the water of the ascending solutions was attained for the samples next to “Rote F&de.” Calcites from samples within the Cu-bearing zone in the bottom part ofthe Kupferschiefer shale next to “Rote Faule” yield 6’sO values of approximately 20% (Table 3). Using the ex~~men~lIy calibrated ~ctionation data for calcite-water @‘NEIL et al., 1969) and the estimated temperatures of 100 and 1SOY, the oxygen isotope composition of the water in isotopic equilibrium with the calcite is calculated to be in the range of 3.0-6.56. The bD values of coexisting fluids can be estimated by ihite-water fractionation data (YEH, 1980) and the temperatures mentioned in the preceding text. Under the assumption that isotopic equilibrium will be best approximated in samples next to “Rote Faule,” which show the highest 6D values and the lowest illite c~stallinity of the clay factions measured in the Kupferschiefer sequence, the hydrogen isotope composition of the coexisting water is calculated to be in the range of -2 to +3%0. By a recent empirical calibration of the illite/smectite-water hydrogen isotope fractionation (CAPUANO, 1992), a broader range and sligthly lower &D-values of the coexisting water between - 10 and +2% are obtained. Comparable calculations, using the data obtained from isotopic analyses within the Kupferschiefer of the Lower Rhine Basin, cannot be performed because there are no indications for isotopic reequihbration, which could be related to the base metal accumulation processes.

Origin and Evolution of the Ore-Bearing Solutions The estimated values for the isotopic composition of the water (6D between -2 and +3%, or according to the calibration of CAPUANO (1992) between - 10 and +2k; 6i80 between 3.0 and 6.5%) which caused isotopic exchange in the Kupfemchiefer of the Richelsdorf Hills through fluid-rock interaction, indicate that the fluids can be characterized as hypersahne basinal brines (SHEPPARD, 1986), probably derived from sedimentary formation waters of meteoric or marine origin. The occurrence of highly saline fluid inclusions (>23.3% NaCl equivalence) in the carbonate cement of WeiBliegend sandstones at the contact with the Kupferschiefer horizon (TONN et al., 1987) supports these conclusions. The reconstruct& isotopic composition also fall within the field of metamorphic waters (SHEPPARD, 1986). However, a metamorphic origin of the solutions can be ruled out because the host rocks of the mine~i~tion and underlying strata were not affected by any metamorphic event. Such brines would have been capable of remobilizing metals (BARNES, 1979; BI~CHOFF et al., 1981) and depositing those now found in the Kupferschiefer (KUCHA, 1990). Furthermore, the results of recent inv~ti~tions of the organic matter (BECHTELand POTTMANN, 1991), as well as the observed metal facies, dependent variations in isotopic composition of the clay fractions, and the carbonates, provide evidence that these brines were also responsible for the formation of high-grade mine~i~tion and associated “Rote F&de.” Taking into acount the presented concept of Kupferschiefer mineralization, the ore-bearing solutions can be stated to be base metal rich (especially enriched in Cu), hypersaline, oxidizing basinai brines. in the vicinity of “Rote Faule,” an upward migration of these brines into the Kupferschiefer is suggested by the results. Underlying red sediments of the Rotliegendes have been suggested as the major potential source of metals (JOWETT, 1986; KUCHA and PAWLIKOWSKI, 1986; HAMMER et al., 1987; SVERJENSKY,1987; BECHTELand ~~MANN, 199 1). The characteristic features presupposed for the migrating brines (metal-rich, oxidizing, hypersaline) can be all fulfilled assuming their source in the Rotliegendes. A model for the evolution of Rotliegendes brines in closed basins has been described by JOWETT (1989) and conforms with the results of several mineralogical and sedimentolo&al investigations (GLENNIE et al., 1978; HANCOCK, 1978; PoKORSKI,I98 I), which indicate a significant amount of diagenetic clay, carbonate, and sulphate cement in the Rotliegendes unit. JOWETT (1989) stated that fresh groundwater dissolves mafic minerals and becomes more concentrated through evaporation. Cements, such as calcite, dolomite, gypsum, and clay are precipitated and groundwater evolves into Na-Ca-Cl brines during early d&genesis. The results of mineralogical studies in the WeiBliegend unit of the Richelsdorf Hills by SCHUMACHER(1985) further support these indications. In the presented model a meteoric origin of the Rotliegendes brines is assumed. The isotopic com~si~on of meteoric water during Lower Permian can be estimated by paleogeographic position of central Europe (MCELHINNY, 1973;

Stable isotopes of clays from Kupfe~~efer

miner~i~tion

---_-_-J

1813

-CAPUANO(19921

Gbtiiegmdes)

__ FIG. IO. Reconstructed isotopic compositions of the metal-bearing, oxidizing brines and Lower Permian meteoric water in the area of the Richelsdorf section. Trajectories for seawater ande~oing evaporation are shown. Curve A represents initial evaporation under relatively humid conditions; curve B is for arid conditions. Curve C is Holser’s estimate (HOLSER, 1979) of evaporating seawater. Trajectory D represents the evolution path for Lower Permian meteoric water into Rotliegend brines. Isotopic trend for meteoric water undergoing exchange with ‘*O-rich minerals (E) are r&c given (modified after SHEPPARD, 1986).

UYEDA, 1978), assuming a variation in SD and 6”O of surface waters similar to the presentday meteoric water systematics (YtJRTsEVERandGAT, 198 1). By theseassumptions, meteoric water of Lower Permian age in central Europe should reflect a”0 vafues between -5 and -3% and 6D values in the range of -30 to - 15%. The isotopic compositions of the meteoric water and of the basinal brines, as reconstructed in this study, are shown in Figure 10. An adequate explanation for the data estimated for the ore-bearing basinal brines (Fig. 10) requires a mechanism for natural waters to become more “0 enriched than SMOW and meteoric waters. As shown in Fig. 10, during the early phases of evaporation of seawater, the lighter isotopes are preferentially removed and the residual fluid becomes enriched in the heavier isotopes. On a bD@O diagram the trajectory taken by the residual brine depends strongly on the humidity and other local climatic variables (CRAIGand GORJXN, 1965; LLOYD, 1966). The trajectories for high and low humidities are shown as curves A and B in Fig. 10. Experiments by GONFIANTINI (1965) and SOEER and GAT (1975), as well as isotopic analyses of an evaporating marine salt pan by HOLSER (1979), indicate that progressive enrichment of the heavier isotopes does not continue indefinitely, but that the trajectury hooks around, as shown by curve C (Fig. IO). Meteoric water which undergoes evaporation will become progressively more enriched in “0 and D similar to evaporating seawater (DANSGAARD,1964). Inasmuch as the starting point of the evaporation trajectory lies on the meteoric water line, curve D in Fig. 10, it follows that the resultant evaporative waters would be depleted in I80 and D relative to evaporating seawater, assuming that the evaporation trajectories for both water types are similar (Fig. 10). However, the concentration of dissolved constituents may be. lower in meteoric waters, and evaporation may have to proceed to greater

concentrations and greater “0 enrichments before the hooked trajectory appears. It has been demonstrated that relatively dilute meteoric waters evaporating under extreme aridity in inland desert environments can become enriched in **Orelative to SMOW by as much as 459bo(FONTES and GONFIANTINI,1970). If the model presented by JOWETT (1989) is correct, it must be possible to interpret the connecting line between the average 6 values of Permian meteoric water and the reconstructed brines as an evolution path, which can be explained by already described mechanisms. As shown in Fig. 10, the deduced curve for the evolution of the Rotliegendes brines is in good agreement with the trajectory for evaporation of meteoric water (D). Therefore, the isotopic composition of the ascending oxidizing brines, which have been suggested to be responsible for isotopic exchange and high-grade mineralization within the investigated Kupferschiefer section, is in agreement with the production of the Rotliegendes brines by evaporation of Lower Permian meteoric water. Altematively, a marine origin of the brines, produced by evaporation or isotopic exchange otthe seawater with “O-rich rocks (Fig. lo), cannot be ruled out. Within the Kupferschiefer of the Lower Rhine Basin, the results of geochemical analyses indicate that barite mineralization in the basal Kupfe~hiefer and the major component of the sulfide mineralization are of the same genetic origin (VAUGHAN et al., 1989). The distribution of Ba within the Kupferschiefer is strongly related to the underlying Carboniferous stratigraphy. Coprecipitation from ascending Baand metal-bearing hydrothermal solutions, originated in the basement rocks is suggested. The s7Sr/8”Sr ratio of barite in the basal Zechstein strata (DIEDEL and BAUMANN, 1988) significantly differs from the 87Sr/86Sr ratio of the Werra anhydrite above. Therefore, the precipitation of barite fmm seawater or its formation from descending brines is very un-

1814

A. Bechteland S. Hoernes

likely. The most probable source for the Sr and Ba are the rocks of the Carboniferous (DIEDEL, 1986; DIEDEL and PUTTMANN, 1988; VAUGHAN et al., 1989; WTTMANN et al., 1990).

Deutsche Forschungsgemeinschaft edged.

(Ho 868/6) is gratefully acknowl-

Editorial handling: D. J. Wesolowski

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Aspects of Ore Deposition

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Within the Kupferschiefer section of the Richelsdorf Hills the observed relationship of the base metal zonation with the isotopic distribution can be explained by the postdepositional formation of the high-grade mineralization in this area. As discussed previously, isotopic and organic geochemical data (BECHTEL and PUTTMANN, 199 1) provide evidence for the contribution of organic matter to copper sulphide precipitation from metal-bearing, oxidizing brines. These ascending brines were responsible for the formation of the oxidated facies within the Kupferschiefer (“Rote Wule”). Upward migration was focused at the transitional zones adjacent to basement highs. In these zones the most pronounced isotopic shift within the clays was caused by the brines. The Kupferschiefer, being itself strongly reducing, is a relatively impermeable sequence compared with the underlying Rotliegend unit, which is both oxidized and permeable. Therefore, the large-scale transfer of gases, liquids, chemical elements and heat from underlying rocks through the Kupferschiefer is restricted. The ascending solutions were retained at the boundary between WeiBliegendes and Kupferschiefer; so the fluids moved laterally beneath and within the basal Zechstein unit (JOWETT, 1993). On the basis of metal ratios, JOWETT (1993) reconstructed the paleoflow direction of the ore-forming fluids in the investigated section of the Richelsdorf Hills. The results indicate a fluid flow to the Northeast away from “Rote Wule.” At the contact and inside the Kupferschiefer unit the brines reacted with the hydrocarbons and successively changed their oxidation potential. During the lateral flow of the metal-bearing solutions, base metal sulphides were precipitated in successive zones, according to their solubility. Also, isotopic composition of the water was affected due to progressive fluid-rock interaction. Therefore,

in zones more distant from the “Rote mule”, the water which came in contact with the Kupferschiefer shale has changed its isotopic signature due to prior equilibration with the mineral assemblage in the pervaded zones. Metal precipitation is suggested to be caused by nonbacterial sulphate reduction with organic matter acting as a reducing agent and proton donor (PUT-I-MANN et al., 1990; BECHTEL and PUTTMANN, 199 I ). By these processes the metal-bearing solutions probably changed their Eh and pH conditions, as well as the isotopic signature with increasing distance from the “Rote FSiule.” This is reflected by the observed correlation between base metal and isotopic zonation within the Kupferschiefer section. Acknowledgments-Support

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