7: Extensional tectonics and the timing and formation of basin-hosted deposits in Europe

Ore Geology Reviews 27 (2005) 241 – 267 www.elsevier.com/locate/oregeorev

7: Extensional tectonics and the timing and formation of basin-hosted deposits in Europe Philippe Muchez a,*, Wouter Heijlen a, David Banks b, Derek Blundell c, Maria Boni d, Fidel Grandia e a

Geodynamics and Geofluids Research Group, Fysico-chemische Geologie, K.U. Leuven, Celestijnenlaan 200C, B-3001 Leuven, Belgium b School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, United Kingdom c Geology Department, Royal Holloway, University of London, Egham Hill, Surrey TW20 0EX Egham, United Kingdom d Dipartimento di Geofisica e Vulcanologia, Universita` di Napoli bFederico IIQ, Via Mezzocannone 8, I-80134 Napoli, Italy e Departament de Geologia, Facultat de Cie`ncies, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Spain Received 30 April 2004; accepted 28 December 2004 Available online 17 October 2005

Abstract In this paper we review a number of the major basin-hosted ore deposits in Europe and argue that most of them are formed in extensional settings. The mineralizing fluids originated as seawater or as evaporated seawater and migrated downwards through the sedimentary basin and into the basement. In regions characterized by pronounced extension and elevated heat production, the fluids were subsequently expelled along extensional faults. During the Palaeozoic, this circulation pattern caused the formation of the sediment-hosted exhalative Zn–Pb deposits of Meggen and Rammelsberg, similar deposits in Sardinia and the mineralization in the Irish basin. The latter could have been formed during a more prolonged period of hydrothermal activity. In the Verviers (Belgium), Upper Silesian (Poland) and other areas, such fluids could have remained in the deeper subsurface for tens of millions of years. During periods of extensional tectonic activity, the fluids were tapped from the basement to form Zn– Pb deposits, hosted by overlying carbonate rocks, and the copper deposits of the Kupferschiefer in SW Poland. D 2005 Elsevier B.V. All rights reserved. Keywords: Extensional tectonics; Fluid chemistry; Kupferschiefer mineralization; Metallogenesis; Mississippi Valley-type mineralization; SEDEX-type mineralization

1. Introduction

* Corresponding author. Tel.: +32 16 327584; fax: +32 16 327981. E-mail address: [email protected] (P. Muchez). 0169-1368/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2005.07.013

During all stages of the evolution of sedimentary basins, Zn–Pb deposits may form in different types of rocks present in the basin. Such mineralization is often classified in two main categories: sediment-hosted exhalative (SEDEX) and epigenetic Mississippi Valley

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type (MVT) deposits (Sangster, 1990). In Europe, the majority of such deposits have long been regarded as having formed in an extensional setting. Recently, however, it has been suggested that the majority of MVT deposits formed in foreland basins mainly during compressional tectonic events at restricted times in the history of the Earth (Leach et al., 2001; Bradley and Leach, 2003). According to Leach et al. (2001), 0°

12°

topographically driven flow models best explain most MVT mineralization in basins all over the world. The aim of this paper is to demonstrate that important Zn–Pb deposits in Europe, including both MVT and other types, resulted from the migration of similar types of fluid during phases of extensional tectonics. The Zn–Pb(–Ba) ore deposits include the SEDEX deposits of Meggen and Rammelsberg (both in Ger12°

24°

56°

London

Berlin

!

Paris

48°

!

!

Rome

Madrid

40°

0 Alpine region

500 km

Major tectonic borders and fracture zones

Variscan region Caledonian region Precambrian region

Irish District Verviers District Meggen

Rammelsberg Upper Silesian District Maestrat District

SW Sardinia District Eastern Alpine District Cévennes District

Fig. 1. Location of the ore deposits and districts discussed in the text. The triangular area centred on London indicated as Precambrian represents a complex area with a cover of Lower Palaeozoic rocks above Precambrian.

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many) and Chaudfontaine (Belgium), the MVT deposits of Upper Silesia (Poland), the Verviers Synclinorium (Belgium) and Maestrat basins (Spain), the eastern Alps (central Europe), the Ce´vennes area (France), the deposits in Ireland and the multi-stage ore district of south–west Sardinia (Fig. 1). The same mechanism also appears to have been responsible for the copper mineralization associated with the Kupferschiefer in SW Poland. We begin by reviewing the general geological and tectonic framework of the different types of ore deposits to provide time and geodynamic constraints on their formation. We then summarize evidence for the similarity in the nature and origin of the mineralizing fluids and for the involvement of the basement in the mineralizing system. Finally, based on these constraints, metallogenic models are discussed and viewed in the light of exploration strategies.

2. Tectonic setting and age of Zn–Pb deposits and the Kupferschiefer Cu deposits 2.1. Palaeozoic sediment-hosted exhalative Zn–Pb deposits in the Rhenohercynian The world-class SEDEX deposits of Meggen and Rammelsberg occur in Middle Devonian siliciclastic rocks (390–375 Ma) in the Rhenohercynian basin (Box 7-1, Schneider, 2005—this volume). At Meggen 60 Mt of ore at a grade of 8% Zn, 1% Pb and 0.04% Cu were present. Rammelsberg was smaller (27–30 Mt) but much richer with 14% Zn, 6% Pb and 2% Cu. The ores are finely laminated and often interlayered with the sedimentary host-rocks. Both ore and sediment were affected by soft sediment deformation,

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indicating a synsedimentary origin for the sulphides. Mineralization is associated with pulses of extension, contemporaneous subordinate bimodal volcanism in the Rhenohercynian basin, and deep synsedimentary faulting (Large, 1988, Large and Walcher, 1999). The tensional pulses were superimposed on a regional subsidence during the post-rift phase of basin development (Large, 2003). The faults provided favourable conduits for the migration of brines into third-order basins during a period of elevated heat flow (Werner, 1989, 1990). The barite (+ Zn, Pb) deposit at Chaudfontaine, located in the western part of the Rhenohercynian basin, is a synsedimentary deposit that formed during the Frasnian (~375 Ma) (Dejonghe et al., 1982; Dejonghe, 1990). Evidence for the synsedimentary formation of this deposit includes rhythmic layering, erosion surfaces, slumping with associated intraformational glide breccias, redeposition of lamellar barite, synsedimentary faults and load casts. Palaeogeographically, Meggen and Rammelsberg were located on the continental shelf that formed the passive north-western margin of the Rhenohercynian Ocean, which evolved with south-eastward subduction (Franke et al., 1995; Oncken et al., 1999; Franke, 2000). The continental shelf rifted during the Early to Middle Devonian (410–380 Ma) but this was followed by convergence from the Middle Devonian onwards. Closure of the Rhenohercynian Ocean occurred during the Late Devonian to Early Carboniferous (Fig. 2, Franke, 2000; Oncken et al., 2000). Franke (personal communication, 2003) recognized three episodes of volcanism in the Rhenohercynian, resulting in the production of basalts with a MORB-like composition during the Late Emsian and of metabasalts during the Middle Devonian to Frasnian and again during the Early Carbo-

Fig. 2. Palinspastic cross-section through the southern margin of the Old Red Sandstone continent in the Middle Devonian, showing the approximate position of Meggen and Rammelsberg (after Franke, 2000).

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niferous. These occurred in the passive margin that later became the foreland. The first greywacke turbidites on the continental foreland were deposited during the Late Tournaisian to Middle Visean, and the main mass of foreland flysch formed between the early Late Visean and the Namurian. All modern tectonic models have stressed the bilateral symmetry of the Variscan belt (for a review see Franke, 2000). Thrusting on either flank was accompanied by dextral displacement, so that the overall tectonic setting was one of dextral transpression. In the western part of the Rhenish Massif (west of the Rhine), which contains the Chaudfontaine mineralization, synsedimentary magmatism was negligible and synsedimentary extension was limited (Franke, 2000). This explains the minor occurrence of ore deposits in this part of the Rhenish Massif compared with the eastern part.

2.2. Mississippi Valley-type deposits Most MVT deposits in Europe have traditionally been related to Mesozoic extensional tectonics, ultimately associated with the break-up of Pangea (Fig. 3) but recent papers have suggested younger ages for mineralization and a closer relation to Alpine tectonism (Fig. 4). For example, the giant MVT mineralization in Upper Silesia (~ 28 Mt Zn and 8 Mt Pb, Szuwarzyn´ski, 1996; Box 7-2, Sass-Gustkiewicz and Kucha, 2005—this volume) has been regarded for a long time as of pre-Middle Jurassic age (Sass-Gustkiewicz et al., 1982; Sass-Gustkiewicz and Dyulyn´ski, 1998). This interpretation is based on the observation that Middle Jurassic sediments rest upon a truncated surface of ore-bearing dolomite (OBD), which is thought to have formed contemporaneously with the sulphide mineralization. The ore-bearing dolomite is

Warsaw London Berlin

Paris

Bucharest

30°N seafloor spreading axis major fault outline of continents

1000 km

Fig. 3. Tectonic framework of Europe during the Middle Jurassic (modified after Ziegler, 1990; Yilmaz et al., 1996).

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Krakow

Paris Bucharest Venice

Tunis

30°N 1000 km

Highlands Lowlands Fig. 4. Tectonic framework of Europe during the Tertiary (modified after Yilmaz et al., 1996).

defined as an epigenetic dolosparite that has been divided into two types: the older type is poor in iron and base-metals and forms large, tabular bodies in the Triassic rocks; the second type occurs as haloes around the ores and is rich in iron and zinc (Szuwarzyn´ski, 1996). Both types of OBD were thought to have formed due to the migration of metal-bearing brines. However, cathodoluminescence studies by Narkiewicz (1993) and Heijlen et al. (2003) demonstrated that the OBD underwent multiple recrystallization. Only the latest dolomite type is associated with the earliest phase of sulphide mineralization. It is not clear if the latter dolomite type is present at the site where OBD was eroded prior to the Middle Jurassic. The main stage of sulphide mineralization took place after dolomite recrystallization and cementation. Leach et al. (1996) argued for a younger age for mineralization, based on palaeomagnetic studies on late-stage dolomite and Symons et al. (1995) suggested a mid-Tertiary age for the sulphide mineralization. According to Symons et al. (1995), this age supports ore genesis from gravity-driven fluid flow associated with the Alpine orogeny (cf. Burchfiel,

1980). Leach et al. (1996) suggested that the deep circulation of fluids from the Carpathian orogenic belt during the waning stages of the Alpine orogeny may have allowed fluids to acquire heat and extract zinc and lead from a large crustal reservoir. In support of a mid-Tertiary age, Symons et al. (1995) quoted evidence presented by Kibitlewski (1991) and Go´recka (1993) that the ores cut Tertiary faults. But Kibitlewski (1991) and Go´recka (1993) mentioned only that the faults have a post-Late Jurassic age, not a Tertiary age. Furthermore, they did not provide clear evidence to support this statement. In contrast, direct Rb–Sr dating by Heijlen et al. (2003) of main-stage sulphides (sphalerite and marcasite) yielded an isochron model age of 135 F 4 Ma for the latest mineralizing event (stages 2 and 3; see Table 1). Stage 1 has not been dated and a pre-Middle Jurassic age cannot be excluded for this generation. The isochron age is further supported by an identical Rb–Sr binary mixing model age (Heijlen, 2002). This method does not depend upon initial isotopic homogeneity between the different minerals analysed (Schneider et al., 2003). The Rb–Sr radiometric age

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Table 1 Compilation of mineralization ages proposed for various carbonate-hosted Zn–Pb districts in Europe (stages according to time scale of Gradstein et al., 2004) District

Field constraints

Proposed age

Method

Reference

Ce´vennes

Cambrian to Recent

different mineralizing events, i.e. pre-Triassic (N251 Ma), Triassic–Jurassic (251 to 200 Ma), post-Jurassic (b200 Ma) Late Palaeocene to Early Eocene (60 to 50 Ma)

ore occurrence and petrography, structural framework

Foglie´rini et al. (1980a,b), Macquar and Lagny (1981), Macquar et al. (1990)

palaeomagnetic dating of host-rock remagnetization ore occurrence and petrography diagenetic studies

Henry et al. (2001), Rouvier et al. (2001)

U–Pb dating or ore-stage calcite ore occurrence and field constraints ore occurrence and structural framework field constraints on the emplacement of the Ore-Bearing Dolomite palaeomagnetic dating of host rock remagnetization and ore minerals Rb–Sr dating of sulphides contemporaneus with Maubach-Mechernich (Schneider et al., 1999) combination of fluid inclusion trapping temperatures and fission track cooling ages of the host rock

Grandia et al. (2000)

Eastern Alps

Middle Triassic to Recent

Maestrat Basin

Aptian to Recent

Sardinia

Cambrian to Early Carboniferous Permian to Mesozoic

Upper Silesia

Middle Triassic to Miocene

Triassic (251 to 200 Ma) Late Triassic toEarly Jurassic (228 to 176 Ma) Early Palaeocene (62.2 F 0.7 Ma) Early Cambrian (542 to 513 Ma) Permian to Triassic (299 to 200 Ma) pre-Jurassic (N200 Ma)

Middle Eocene (46 F 20 Ma)

Early Cretaceous (135 F 4 Ma) Verviers

Late Carboniferous to Late Cretaceous

Middle Jurassic (170 Ma)

post-Middle Jurassic (b170 Ma)

therefore indicates that hydrothermal activity and ore formation occurred well before the Alpine Orogeny. Instead, the main stage of sulphide formation can be related to Early Cretaceous crustal extension preceding the opening of the Arctic-North Atlantic system. Traces of this important mineralizing period are seen in radiometric ages determined for various hydrothermal ore deposits throughout Europe (Schneider, 2000). In the North–West European basin, the Early Cretaceous (135–96 Ma) was a period of major rifting and differential subsidence (Ziegler, 1990). The discrepancy between palaeomagnetic and radiometric age determinations can be explained as due to remagnetization processes that reset the mag-

Klau and Mostler (1983); Schroll (1996) Zeeh et al. (1995, 1997)

Boni et al. (1996) Boni et al. (1992, 2002) Sass-Gustkiewicz and Dyulyn´ski (1998) Symons et al. (1995); Leach et al. (2001)

Heijlen et al. (2003) This paper

Muchez et al. (1997)

netism and would thus invalidate the mid-Tertiary age. There is no firm evidence that remnant magnetism measured in MVT deposits is actually associated with the ore minerals (Kesler et al., 2004). In general, the palaeomagnetic age is the same as or younger than the isotopic age, suggesting that palaeomagnetic measurements frequently reflect later fluid events. Moreover, remagnetization processes occur widely and are not necessarily related to large-scale migration of fluids (Cioppa et al., 2000; Evans et al., 2000). Remanent magnetism can be very sensitive to visually minor changes in recrystallization from mineral instability, heat or pressure. For example, stable isotope data from carbonate rocks indicate that geo-

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chemically closed systems can undergo remagnetization (Cioppa et al., 2000). Therefore, when a major orogenic phase such as the Alpine Orogeny post-dates mineralization, the palaeomagnetic age can be reset, reflecting the age of this orogenesis and not that of ore deposition. In the Verviers district (Belgium), field evidence shows that Zn–Pb mineralization occurred in the Mesozoic. About 2 Mt of Zn metal has been mined in this district. The fault-bounded mineralization crosscuts all Variscan structures but Cretaceous sands and shales rest unconformably on eroded mineralization contained within Dinantian rocks near the deposit of La Calamine (Dewez and Lespineux, 1947; de Magne´e, 1967; Dejonghe, 1998). The ores in the Verviers district are comparable with the sandstonehosted Zn–Pb deposits of Maubach-Mechernich (western Germany), some 40 km to the southeast, and possibly formed at the same time. Direct Rb–Sr dating of sphalerite from these deposits yielded a Middle Jurassic age of 170 F 4 Ma (Schneider et al., 1999). The Middle Jurassic in the North Sea rift was characterized by a period of major rifting and volcanism (Ziegler, 1990). In the Harz Mountains (eastern Germany), banded galena-sphalerite-quartz-calcite veins also formed after the Variscan orogeny (Large, 2003). About 34 Mt ore containing 1.2 Mt Zn and 1.8 Mt Pb and 4700 t Ag were exploited. The age of this mineralization has been estimated by Rb–Sr dating of the minerals in the alteration zones around the veins (Haack and Lauterjung, 1993; Hagedorn and Lippolt, 1993; Schneider, 2000; Schneider et al., 2003) as well as by the crosscutting relationships of the veins with Mesozoic cover sediments and disseminated mineralization hosted in Cretaceous karst-cavity fill sediments (Large, 2003). These analyses and observations indicate that mineralization occurred during discrete periods within the Permian (270 Ma), Triassic (225 and 210 Ma), Jurassic (180, 170 and 150 Ma) and Cretaceous (135 and 90 Ma). The Zn–Pb deposits in the Maestrat basin (eastern Spain) are also associated with rift or post-rift stages (Grandia et al., 2000). Fluid migration leading to Zn– Pb mineralization occurred between the Late Cretaceous and Early Palaeocene (80–60 Ma) as revealed by U–Pb dating of ore-stage calcite, which yielded an age of 62.6 F 0.7 Ma (Grandia et al., 2000). The basin

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itself formed during a rift episode in the Late Jurassic and Early Cretaceous, followed by a post-rift stage at least until the Maastrichtian (Salas and Casas, 1993). This tectonic regime could have remained active until the Late Oligocene, when the basin was inverted in response to Alpine compressive stages. However, the extensive development of lacustrine facies during the Early Tertiary suggests a syn-rift stage. Bradley and Leach (2003) argued that mineralization in the Maestrat basin took place in the foreland of the Pyrenean orogen. Indeed the initial collision started during the latest Cretaceous (Maastrichtian; Puigdefa`bregas and Souquet, 1986) but major deformation, including thrust emplacement, was not before the Eocene. This compressional stage was, however, restricted to the former Pyrenean rift. At that time, the Maestrat basin was located ca. 600 km (450 km plus 150 km shortening) to the south and formed part of the Iberian rift system. Inversion of the Iberian rift system, including the Maestrat basin, started during the Late ´ lvaro, Eocene to Early Oligocene (Guimera` and A 1990). The Basque-Cantabrian basin (northern Spain) also formed during the same rift episode as the Maestrat basin, and most of the Zn-Pb deposits in the basin can be related to extensional tectonics before the onset of the Alpine Orogeny (Velasco et al., 1994). The ore deposits are clearly associated with normal faults that were active before and after ore emplacement (Velasco et al., 2003). Faults that only affect the Lower Cretaceous succession control the position and characteristics of the dolostone host rocks. NE– SW-striking, subvertical faults caused minor displacement of the orebody. Slickensides on these faults indicate predominantly normal movement. In the Catalonian Coastal Ranges (northeast Spain) metal-bearing veins and palaeokarst bodies are as old as Middle Triassic, since the Anisian Muschelkalk is locally mineralized and younger than pre-Alpine because many veins are cut by Alpine structures. Based on a strontium and sulphur isotopic study of low-temperature barite-fluorite veins containing sphalerite, Canals and Cardellach (1993) concluded that mineralization occurred between the Late Triassic and Early Jurassic (225–195 Ma). In conclusion, most MVT mineralization in Europe formed during several periods of extensional tectonism during the Mesozoic, which can be related to the

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pronounced crustal extension that preceded the incipient opening of the Northern and Central Atlantic and Tethys oceans (Ziegler, 1990). This period was associated with high heat flow that could have been the source of heat for mineralizing brines (Littke et al., 2000). In his overview of the tectonic events in the North–West European basin, Ziegler (1990) identified major rifting phases during the Middle Jurassic (~ 170 Ma) and the Early Cretaceous (~ 135 Ma). In the North Sea rift both periods are characterized by anorogenic volcanism. 2.3. Sediment-hosted deposits in Sardinia, the eastern Alps, the Ce´vennes and Ireland 2.3.1. Sardinia Various types of Zn–Pb and Ba ore deposits occur in Southwest Sardinia. They range from preVariscan to post-Variscan in age and from SEDEX and early diagenetic to MVT type (Boni et al., 1996). The pre-Variscan ores were economically the most significant, having produced more than 120 Mt of Zn–Pb and 12 Mt of barite ore over the last hundred years. All deposit types are hosted in Lower Cambrian platform carbonates. Most of the pre-Variscan stratiform to stratabound mineralization was related to several pulses of hydrothermal fluid circulation, coinciding with the main periods of Early Palaeozoic rifting. These deposits are particularly enriched along important palaeotectonic lines that also controlled the occurrence of sedimentary facies during the evolution of the Cambrian platform (Boni et al., 1996). Similar types of deposits are hosted in the Cambrian of southern France (Montagne Noire; Courjault-Rade´ and Gandin, 1988) and northern Spain (Tornos et al., 1996), although they are of much less economic significance. The post-Variscan Sardinian deposits in the southwestern areas, preceded by a strong phase of hydrothermal dolomitization, can be subdivided into veinand palaeokarst-types, both occurring in the same Cambrian carbonates as the pre-Variscan ores (Boni et al., 2002). The metal-bearing veins and palaeokarst bodies are post-Early Carboniferous because these deposits are not folded during the Variscan orogeny and not metamorphosed by Late Carboniferous magmatic bodies (Boni et al., 1992, 2002). Although Jurassic and Cretaceous sediments are rare in Sardi-

nia, the mineralization never crosscuts these sediments where present. Triassic sediments are also scarce, but they contain hydrothermal dolomites and a small barite vein. The bigger ore veins partially follow a N60 to 708E direction, which is also often marked by Permian porphyries (Bakos and Valera, 1972). This direction has been related to Permian strike-slip tectonics (e.g., Arthaud and Matte, 1977). The faults and fractures controlling the vein and palaeokarst ores have been locally reactivated and are filled with Early Tertiary sediments, but not with ores. In addition, extensive Tertiary magmatism also follows this older trend (Valera, 1967). In conclusion, timing of the hydrothermal pulses can be constrained to the interval between Permian and Late Mesozoic (Boni et al., 1992, 2002). 2.3.2. Eastern Alps In the eastern Alps there are more than 200 carbonate-hosted Pb–Zn deposits in Middle to Upper Triassic carbonates that have been referred to as the "Alpine-typeQ or, less frequently, bBleiberg-typeQ (e.g., Maucher and Schneider, 1967; Sangster, 1976). They were of great economic importance in the past, having produced ca. 7 Mt of metal (Klau and Mostler, 1983). The major deposits (Bleiberg in southern Austria, Mezica in north-western Slovenia, Raibl and Salafossa in northern Italy) are located on both sides of the bPeriadriatic sutureQ, a steeply dipping, and deep-seated fault zone between the northern and southern Alps, with both right-lateral and vertical displacement. There has been much discussion during the last three decades concerning the age and type of mineralization. Although the ores clearly show epigenetic features such as vein fillings, breccia cementation and massive replacement bodies, Schneider (1964) interpreted the non-economic equivalents, and the mineralization in its broader sense, as synsedimentary. Schroll (1996) also interpreted the first stage of mineralization as synsedimentary to early diagenetic. This interpretation was based mainly on the presence of very fine-grained sphalerite with anhedral crystal shapes, of spherules, framboids and colloform aggregates and of sedimentary features such as reworked breccias and slumping textures. Although Klau and Mostler (1983) agreed with the sedimentary features, they disagreed with a synsedimentary origin of the

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ores because the mineralized sediments occur in large karst systems oriented more or less parallel to the Triassic bedding. So, they post-date deposition of the rocks as well as karstification and can thus not be classified as synsedimentary with regard to this rock. The cavities are filled with carbonate fragments and finely laminated mineralized sediments. Crossbedding, reworked sphalerites and sphalerite stalactites were observed within the laminated sediments. The cavities were finally filled with Upper Triassic mudstones and siltstones. Therefore, Klau and Mostler (1983) stated that mineralization must have been intra-Triassic. Zeeh and Bechsta¨dt (1994) further questioned the synsedimentary origin of the earliest mineralization stages. Based on a detailed reconstruction of the paragenetic sequence, they concluded that the first main stage of mineralization was initiated after the precipitation of radiaxial fibrous and scalenohedral calcites and dolomite replacement during shallow burial. Typically, marine fluids are responsible for such diagenetic processes. However, the timing of the marine diagenesis is difficult to determine and could have occurred at a depth of only a few metres to tens of metres, still very early in the diagenesis. The second stage of ore deposition (fluorite and sphalerite) post-dates the precipitation of a clear saddle dolomite (Zeeh et al., 1995, 1997). Based mainly on cement stratigraphic arguments, they suggested a Late Triassic to Early Jurassic age for this second stage of ore deposition. The ore deposits in the Raibl mining district are similarly controlled by Late Triassic to Early Jurassic extensional faults (Doglioni, 1988) that were reactivated during Dinaric tectonics in the Palaeogene (Leach et al., 2003). This model of a Late Triassic to Early Jurassic age of emplacement for at least part of the lead–zinc mineralization in the eastern Alps is in agreement with the geodynamic scenario for Europe as a whole, as previously mentioned. 2.3.3. The Ce´vennes area (southern France) The ore deposits in the Ce´vennes Massif in southern France hosted almost 2 Mt of Zn and Pb metal (Michaud, 1980). The main ore deposit occurs at Les Malines. The metallogenesis of the Zn–Pb deposits of the Ce´vennes area has long been regarded as very complex, characterized by a superposition of miner-

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alizing events (Foglie´rini et al., 1980a; Michaud, 1980; Bre´vart et al., 1982; Le Guen et al., 1991). Foglie´rini et al. (1980a) identified four main phases of mineralization at Malines, which are critically discussed below. A widespread Zn–Pb geochemical anomaly coincides with pyrite mineralization in pyroclastic and epiclastic sediments of Middle Cambrian age. Within this geochemical halo, more elevated metal contents occur in low energy, restricted sedimentary environments, including fine-grained euxinic volcanoclastic sediments. At the Noailhac-Saint-Salvy ore deposit, disseminated sphalerite occurs in carbonaceous sandstones and finely laminated sphalerite is present in Middle Cambrian black schists (Foglie´rini et al., 1980b). The black schists and the zinc bearing beds are crosscut by a granodiorite which developed a contact metamorphic aureole. During metamorphism, the originally sediment-hosted sphalerite became enclosed by metamorphic minerals (garnet, diopside) that formed within this contact metamorphic aureole. The granodiorite is dated isotopically at 304 Ma (Pin, 1991) and forms a maximum age for the sphalerite. A second phase of mineralization, more important than the first, is related to karstic cavities and their sedimentary infill in Cambrian dolomite. The breccia in the cavities is polygenetic, the cement is finegrained and contains up to 60% sphalerite. Rhythmic sedimentation patterns, fining upwards, can also be observed. The coarser-grained karst sediments, in particular, are impregnated with clear sphalerite. These mineralized rhythmic sequences are disturbed by slumping and small fractures resulting from differential compaction of the karst infill. Foglie´rini et al. (1980a) therefore concluded that mineralization of the sediments in the karst infill pre-dated these early diagenetic disturbances. However, the age of these karst sediments, their infill and thus of the mineralization is not constrained. A third phase of mineralization was thought to result from the reworking of earlier mineralization phases during the Triassic and formed a mineralized conglomerate. Fragments of pyrite and sphalerite concretions, ovoids of sphalerite and fragmented lamellae of barite resulted from the reworking of an earlier deposit (Foglie´rini et al., 1980a). This indicates a pre-Late Triassic age for at least part of the ore deposit. Although Macquar et al. (1990) do not

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describe such characteristics of the mineralization in the Triassic conglomerates, they do not exclude a Triassic age for the stratiform barites. The fourth phase was thought to have taken place during the Early Jurassic with the formation of fissures encrusted by sulphides and quartz, covered by milky barite (Foglie´rini et al., 1980a). Based on the occurrence of the ores and structural analyses, Macquar et al. (1990) also concluded that mineralization in the western part of the Ce´vennes area took place during the Early Jurassic. Karst sediments with reworked laminated sulphides were tilted by tectonic movements during the Early Jurassic (Macquar and Lagny, 1981). Clauer and Chaudhuri (1995) also suggested an Early Jurassic age for the mineralization at Les Malines using K–Ar dating of illite in the host rocks together with Pb–Pb isotope data. However, according to Leach et al. (2001), this radiometric age does not reflect the timing of mineralization but rather that of widespread diagenesis of the rocks in Early Jurassic time. In contrast to the arguments mentioned above, the four phases of mineralization are not supported by the detailed analyses made by Macquar et al. (1990), who stated that all Zn–Pb mineralization occurred after the Triassic. Indeed the occurrence of ores in karst at the contact between the Cambrian and Upper Triassic does not necessarily imply a pre-Late Triassic age for the mineralization. Also, ore fragments in the Upper Triassic conglomerates do not strictly imply mineralization before this siliciclastic sedimentation. A selective replacement of fragments in the Upper Triassic conglomerate could have taken place after deposition of the conglomerate. The alternation of permeable and impermeable lithologies could have controlled the horizontal migration of the fluids and of ore emplacement. However, if correct, the description of fragments of pyrite and sphalerite concretions, ovoids of sphalerite and fragmented lamellae of barite indicates reworking of an earlier deposit and a preLate Triassic age of at least part of the mineralization (Foglie´rini et al., 1980a). Recent palaeomagnetic studies by Rouvier et al. (2001) and Henry et al. (2001) of the rocks in the Ce´vennes area yielded two styles of characteristic remanent magnetization (ChRM). The first ChRM resulted from the superposition of a dual-polarity primary component and a secondary component. A

well-defined direction of remagnetization forms the second ChRM and corresponds to an Early to Middle Eocene age (60–50 Ma). Rouvier et al. (2001) and Henry et al. (2001) suggested that the magnetic overprint was caused by a chemical phenomenon related to fluid migration that they suggested was also responsible for the MVT deposits in the Ce´vennes area. However, this conclusion completely contradicts the earlier observations and interpretations of Macquar et al. (1990). As proposed by Rouvier et al. (2001) and Henry et al. (2001), the palaeomagnetic age could reflect a widespread fluid flow event during the compressional phase between the Late Cretaceous and Late Eocene, but a relationship with MVT formation is not proven. 2.3.4. Zn–Pb deposits in Ireland The ore deposits in Ireland are closely associated with synsedimentary growth faults and ore grades increase towards these faults (Box 7-3, Ashton, 2005—this volume). It is well known that there was a distinct structural control on the mineralization (for an overview see Hitzman and Beaty, 1996; Johnston, 1999). The exact age of the Irish-type stratabound deposits has been a matter of controversy. A synsedimentary origin of Tournaisian age (355–345 Ma) has been proposed based on the presence of fossil vents and vent biota at Silvermines and Tynagh (Boyce et al., 1983; Banks, 1985). A Tournaisian age corresponds with the onset of extensional tectonics in the central Irish Midlands (Andrew, 1993). At Navan, the ore occurs as complex stratabound, tabular lenses that are offset by faults. An economically minor, but genetically important mineralization is present in the debris flow (i.e., the Boulder conglomerate) overlying an erosion surface. However, mineralized clasts also occur in this conglomerate. Time constraints on the mineralization are given by the erosion surface and the overlying conglomerate and by the style of mineralization, and suggest an Early Visean age (Ashton et al., 1992; Anderson et al., 1998). Recently, the age of the Navan ore deposit was further estimated by lead distribution patterns and a systematic sulphur isotope study in lens 5, across six faults (Blakeman et al., 2002). Metal-bearing fluids are thought to have migrated along nearvertical NNE-, NE- and ENE-trending minor normal faults that were active during Early Visean times.

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These faults predate or are coeval with the major extensional ENE faults, which control the general occurrence of the Irish deposits (Ashton et al., 2003). Reworked clasts of sulphide and dolomitized limestone also occur in breccias in the Waulsortian Limestone Formation at the Coleen zone near Silvermines (Lee and Wilkinson, 2002). The clasts consist of pyrite with a laminar texture, sometimes distorted and sharply truncated at the clast boundaries, and of colloform or layered sphalerite and pyrite. The wedgeshaped geometry of the breccia bodies containing the sulphides is not consistent with a cavity fill-collapse origin. In addition, these breccias are normally not located beneath massive sulphide mineralization, further excluding a gravitational collapse origin (Wilkinson et al., 2003). Mineralization and dolomitization were active at shallow burial depths and hydrothermal fluids followed fault conduits. Fault activity waned before deposition of supra-Waulsortian lithologies. Together with the lack of hydrothermal alteration of the upper Waulsortian, this suggests mineralization within the Coleen zone terminated before deposition of the Waulsortian limestone was complete. Carboni et al. (2003) carried out a detailed structural study of the Lisheen deposit situated along the Rathdowney Trend of the southern Irish Midlands. They concluded that mineralization was strongly controlled by normal faults, which acted both as barriers and conduits for the metal-bearing fluids. The main part of ore deposition is late-, or most likely postnormal faulting. Normal faulting started during formation of the Waulsortian host-rock in the Late Tournaisian until at least the Early Visean. Therefore, economic mineralization formed no earlier than the Early Visean. Chemical remanent magnetization of ore and host rocks at Navan has been assigned a late Early Visean to early Late Visean age of 333 F 4 Ma (Symons et al., 2002). However, Symons et al. (2002) did not show how the remanent magnetism was related to ore minerals. In addition, Wilkinson (2003) has suggested that the error on this age should be larger because a polar wander path for Ireland is not available and the postmineralization tectonic history of the area and the relative translation of the Irish block are poorly known. Based on cement stratigraphy and on the relationship between stylolitization and mineralization in the

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Irish district, a Middle to early Late Visean (335–330 Ma) or even younger age has been proposed (Peace and Wallace, 2000; Reed and Wallace, 2001, 2004). Furthermore, non-planar dolomite cement with a regionally correlative cathodoluminescence pattern occurs throughout the Courceyan (359–345 Ma) to at least the Asbian (337.5 to 333 Ma) in the Midlands (Wright et al., 2000, 2001; Wright, 2001). Where sulphides are present, they post-date this dolomite, suggesting a post-Mid Asbian age for the ores (Wright, 2001). However, a similar cathodoluminescence pattern in carbonate cements does not necessarily imply that they all formed contemporaneously (Miller, 1986) and it is not clear if this dolomite pre-dates the main mineralization phases. Indeed, the dolomite studied by Wright et al. (2000) shows similar textural and cathodoluminescence characteristics to the regionally occurring late pink saddle dolomite that formed postore (Wilkinson, 2003). Minor sulphides are occasionally observed growing on pink saddle dolomite crystals. Wilkinson (2003) also questioned the applicability of stylolites to constrain burial depth and thus the timing of mineralization. The macrostylolites described by Peace and Wallace (2000) and Reed and Wallace (2001) appear to be broadly subhorizontal solution seams with long wavelength, low amplitude undulations. The formation depth of such solution seams is not straightforward and may depend on many factors (e.g., Railsback, 1993). There is no doubt, however, that Zn–Pb mineralization in the Irish Midlands was related to local extensional tectonics. Synsedimenary tectonic activity started during the Tournaisian and two major phases occurred during the Early Visean (Chadian-Arundian, 345–339 Ma) and Late Visean (Asbian; Nolan, 1989) (Table 2). From this review it is also clear that the Zn– Pb deposits mainly formed after sedimentation, during diagenesis of the host rock. A possible explanation for this diagenetic origin can be found in the fact that carbonates are more porous and show a higher reactivity with the mineralizing fluids than the finegrained siliciclastics. Fluids circulating along faults are unable to reach the surface without interacting with the carbonate host-rock. Precipitation in the subsurface could also be due to the mixing of a high temperature moderate salinity external fluid with a locally derived low temperature high salinity fluid (Banks and Russell, 1992).

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Table 2 Compilation of ages proposed for mineralization in the Irish Midlands (ages and stages according to the different authors) Proposed age

Deposit

Method

Reference

Post-Mid Asbian (N335 Ma*) Post-Courceyan (N345 Ma) Late Holkerian to Asbian (333 F 4 Ma) Late Chadian to Early Arundian (~ 345 Ma) Courceyan (359 to 345 Ma*) Courceyan (359 to 345 Ma*) Courceyan (N~352 Ma)

Navan Navan Navan Silvermines Tynagh Silvermines

diagenetic studies diagenetic studies palaeomagnetic studies field indications field indications field indications petrographic and diagenetic studies

Wright, 2001; Wright et al., 2001 Peace and Wallace, 2000 Symons et al., 2002 Ashton et al., 1992; Anderson et al., 1998 Boyce et al., 1983 Banks, 1985 Lee and Wilkinson, 2002

* ages according to Gradstein et al. (2004).

2.4. Kupferschiefer mineralization Within the Lower Silesian basin, near the southwestern margin of the Polish basin, a subbasin developed during Permian times containing a Rotliegend sequence of sandstones, siltstones and rhyolitic volcanics followed by terrestrial redbeds. These, in turn, are unconformably overlain by a transgressive marine Zechstein sequence of carbonates and evaporites (Box 7-4, Oszczepalski and Blundell, 2005—this volume). The thin, black pyritic and organic carbon-rich Kupferschiefer shale, dated at 258 Ma (Menning, 1995), is at the base of the Zechstein sequence. The Permian sequence was followed by a succession of Triassic continental and shallow marine sediments and by a Jurassic succession of shallow marine sediments. Mineralization occurs within the Kupferschiefer shale and the immediately overlying and underlying beds and is associated with a transgressive redox surface, the Rote Fa¨ule hematitic alteration. The highest grade copper ores lie directly against the Rote Fa¨ule surface on its reduced side, with lead and zinc sulphides generally more distal (Box 7-4, Oszczepalski and Blundell, 2005—this volume). The formation of the Rote Fa¨ule and associated ore system is acknowledged to have arisen from a large-scale flow of oxidizing metalliferous brine, concentrating metals in redox-type geochemical traps (e.g., Rentzsch, 1974; Oszczepalski, 1989). Copper mineralization probably started during sedimentation and early diagenesis of the Kupferschiefer (Sawlowicz, 1990). Oszczepalski et al. (2002) explained the association of the copper orebodies with the Rote Fa¨ule as the superposition of early diagenetic low-grade mineralization and dominant late diagenetic stages of high-grade mineralization. The latter formed by ascending brine coupled with the

redistribution of earlier-formed sulphides and suggested that the irregular nature of the Rote Fa¨ule implied variations in the intensity of both upward and lateral fluid flow. Indirect dating by K–Ar of illites and palaeomagnetic measurements indicates that the main stages of mineralization occurred during the Triassic between 250 and 220 Ma (Jowett et al., 1987; Bechtel et al., 1999). This was a time when subsidence in the Polish basin was greatest (diminishing through the Jurassic), indicative of extensional tectonics followed by thermal subsidence (Karnkowski, 1999).

3. Geochemistry of mineralizing fluids Fluid inclusions in iron-rich calcites present in the Kniest (feeder) zone below of the Zn–Pb ore deposit at Rammelsberg have recently been investigated by Stassen (2004). The homogenization temperature of the fluid inclusions ranges between 1308 and 160 8C and salinity is between 4.9 and 10.3 eq. wt.% NaCl. The salinity is two to five times higher than the salinity of seawater. These inclusions could be representative of the SEDEX-type mineralizing fluid at Rammelsberg. A fluid originating from seawater evaporation is indicated by fluid inclusion studies of the pre-Variscan Ba (+ Zn, Pb) mineralization in the Verviers district (western Rheinisches Schiefergebirge). Microthermometric and crush-leach analyses of these fluid inclusions revealed not only the origin, but also the subsequent evolution of the mineralizing fluids (Fig. 5). The fluid inclusions in the barite are singlephase liquid and their salinities vary between 12.6 and 19.5 eq. wt.% NaCl (Dejonghe et al., 1982; Heijlen et al., 2000). These saline fluids are dominated by Na– Ca–Cl resulting from modification of evaporated sea-

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1600 1400

Cl/Br (molar)

1200 1000

lite

n tio olu s dis

ha

800 sw

600 400

T

SE

Maestrat Basin (Grandia et al., 2003) Cevennes area (Viets et al.,1996)

200 0

Verviers district (Heijlen et al., 2001) Chaudfontaine Barite (Heijlen et al.,2000) Upper Silesian district (Heijlen et al., 2003)

Irish district: moderate salinity (Banks et al., 2002) Irishd istrict: high salinity (Banks et al., 2002)

0

200

400

600

800

1000

1200

1400

1600

Na/Br (molar) Fig. 5. Na–Cl–Br composition of mineralizing fluids from various districts discussed in the text. In all districts, evaporation of seawater was the main source of the increase in salinity of the metal-bearing brines. Except for the mineralizing fluids from the Irish district, none of the analyses shows the involvement of a halite dissolution brine. SW = Seawater, SET = Seawater evaporation trend (data from McCaffrey et al., 1987).

water by dolomitization and exchange of Mg for Ca (Heijlen et al., 2000). Similar analyses of fluid inclusions in ore and gangue minerals from the Mesozoic MVT mineralization in the same area also indicate that seawater evaporation was the process by which the fluids were formed (Fig. 5). However, the cation composition of these MVT fluids points to intense dolomitization and interaction with siliciclastic rocks of the Lower Palaeozoic basement (Heijlen et al., 2001). High salinity Na–Ca–Cl brines are observed in the Lower Palaeozoic basement near the Zn–Pb deposits in Belgium (Dewaele et al., 2004). In eastern Belgium, evaporative conditions were present during certain periods of the Givetian to Visean (380–330 Ma), at which time the synsedimentary barite mineralization was formed. Based on these findings, Heijlen et al. (2001) proposed a model in which high-salinity brines were formed by seawater evaporation during the Late Palaeozoic in eastern Belgium. Owing to their high density, these fluids migrated downwards into the subsurface where they caused dolomitization of the shelf limestones (Pre´at and Rouchy, 1986; Nielsen et al., 1994). They migrated upwards along faults and precipitated barite at low temperature (b 50 8C; Dejonghe et al., 1982). Upward migration may

just have been the result of the normal migration pattern along the most permeable zones in a reflux system (e.g., Jones et al., 2003). Similar evaporative fluids migrated through the Devonian and Carboniferous strata and were expelled during subsequent Variscan deformation causing syn- to late-tectonic zebradolomite formation (Heijlen et al., 2000). Brines that migrated in the basement were expelled during the Mesozoic and caused the formation of MVT deposits in the Verviers district. Such brines have been recognized in veins in the basement rocks (Dewaele et al., 2004). A similar model is proposed for the MVT deposits in Upper Silesia (Heijlen et al., 2003). Mineralizing fluids also originated as bittern brines during seawater evaporation (Fig. 5) and interacted with the siliciclastic basement rocks during their downward migration within the subsurface. As discussed above, Rb–Sr dating of the mineralization suggests that the fluids were expelled during the Early Cretaceous. Fluid inclusion temperatures of the post-Variscan (Mesozoic) ores, hosted in the Cambrian carbonates of SW Sardinia, were up to 150 8C, with salinities generally higher than 23.3 eq. wt.% NaCl (Boni et al., 1992). Similar high-salinity fluids (H2O–NaCl–CaCl2) with a slightly lower temperature of ca. 100 8C resulted in

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the deposition, possibly during the Permo-Triassic, of widespread hydrothermal dolomites, pervasively replacing the Cambrian platform carbonates in the same area (Boni et al., 2000). The fluids involved in the formation of Zn–Pb mineralization in the eastern Alps show a compositional variation between H2O–NaCl–CaCl2–MgCl2 and H2O–NaCl–CaCl2, with temperatures up to 170 8C. The calculated oxygen isotopic composition of the mineralizing fluid also reveals values pointing to brines deeply circulating within the basin (Zeeh and Bechtsta¨dt, 1994). At Les Malines (Ce´vennes area), high-salinity fluid inclusions in minerals that formed different types of mineralization show a covariant decrease of salinity and homogenization temperature, indicating that the saline fluids were diluted with a cooler, less saline fluid (Charef and Sheppard, 1988). A few crush-leach analyses again indicate that the fluids most probably originated from seawater evaporation (Fig. 5; Viets et al., 1996). Fluids derived from seawater evaporation deposited Zn–Pb deposits in the Maestrat basin (Grandia, 2001; Grandia et al., 2003). According to Cl/Br ratios (Fig. 5) and stable chlorine isotopes of inclusion fluids the source of ore-forming brines was evaporated seawater that had not reached halite saturation, most likely during the Late Cretaceous and Early Tertiary (Grandia et al., 2003). Except for possibly the Ca and Mg content, the major element composition of these brines was not significantly modified by interaction with the enclosing rocks because the Na–K–Li ratios in fluid inclusions are close to the seawater ratios. This was possible because siliciclastics are very scarce at depth in the Maestrat basin and carbonates do not affect these ratios. Microthermometry of fluid inclusions in ore minerals indicates fluid temperatures up to 150 8C and salinities between 15 and 26 eq. wt.% NaCl. Velasco et al. (1994) distinguished between SEDEX and MVT Zn–Pb deposits in the Lower Cretaceous rocks of the Basque-Cantabrian basin, northern Spain. In this case, however, both types might have formed from the same type of discharge of ore solutions during the Early Cretaceous. The conclusion that a similar fluid could cause the formation of SEDEX and MVT deposits has already been proposed in a comparative study by Sangster (1990). In Ireland, fluid inclusion results from quartz, sphalerite and barite show the involvement of two distinct

fluids in the mineralizing process: a high-temperature, moderate-salinity fluid and a low-temperature, highsalinity fluid (Samson and Russell, 1987; Banks and Russell, 1992). Both fluids represent a predominantly seawater evaporation brine, although some contribution of halite dissolution might be present (Fig. 5; Banks et al., 2002; Gleeson and Yardley, 2002, 2003). However, the high-temperature, moderate-salinity fluid was characterized by a lower degree of evaporation than the low-temperature, high-salinity fluid where evaporation had proceeded past the point of halite saturation. Furthermore, the high-temperature fluid became enriched in K and Li, and depleted in Na due to interaction with Lower Palaeozoic siliciclastic rocks, whereas the major modification in composition of the high salinity brine was an exchange between Mg and Ca during dolomitization. These investigations of the fluid composition indicate genetic similarities between sediment-hosted Zn– Pb deposits. High-salinity fluids form at the Earth’s surface by seawater evaporation and migrate down into the underlying sediments. The passage of such fluids causes water–rock interactions (e.g., dolomitization, sericitization, etc.) and leaching of metals from various rocks. The fluids can subsequently be expelled at or near the surface, or, as is the case in the Verviers district, some of the fluids can remain in the basin or percolate further down into the basement only to be expelled later in the basin evolution (Muchez and Heijlen, 2003). Coupled numerical modelling of fluid flow, heat and salinity by Large et al. (2002) and Yang et al. (2004) has shown that the same process, of sinking evaporitic brines, is important in the formation of the giant SEDEX deposits in the McArthur Basin, Australia.

4. The role of the basement in basin-hosted deposits As explained in the previous section, the fluid chemistry of a number of Zn–Pb basin-hosted deposits indicates the migration of the mineralizing fluids into the basement. For example, the high calcium content of the mineralizing fluid in the MVT deposits in the Verviers district cannot be solely explained by an increase due to dolomitization, but requires an interaction with plagioclase to account for a smaller

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fluid through the vein system (Large, 2003). Large amounts of high-salinity fluids were encountered during the German Continental Deep Drilling Project (KTB) at the western margin of the Bohemian Massif from depths ranging between 2 and 3 km to ca. 9 km (Lodemann et al., 1997; Mo¨ller et al., 1997). The Polish and Irish Zn–Pb deposits contain fluids that are enriched in potassium and lithium and depleted in sodium with regard to evaporated seawater. The processes responsible could be albitization or dissolution of K-feldspar and reaction with phyllosilicates processes that most probably occurred in the basement (Banks et al., 2002; Heijlen et al., 2003). Everett et al. (1999a,b) demonstrated the similarity in composition between the high-temperature end-member fluid that caused Zn–Pb mineralization in Ireland and the fluid that percolated through the basement, forming polymetallic quartz veins. In accordance with the metallogenic model of Russell (1978, 1983), they proposed that the ores formed from fluids that had circulated deeply into fractured basement rocks. The penetration of brines into basement rocks and its importance for Zn–Pb deposits in Ireland and several other mineralized areas in Western Europe has been stressed by Gleeson and Yardley (2002, 2003). They compared the geochemistry of fluid

increase of calcium (Heijlen et al., 2001). The Devonian and Carboniferous rocks are poor in feldspars and only the psammitic Famennian rocks contain up to 45% of largely detrital K-feldspars (Michot, 1963). Plagioclase is, however, abundant in the dacitic-rhyolitic rocks and the detrital arkoses of the Lower Palaeozoic basement (Vander Auwera and Andre´, 1985). Although in general the Pb isotope composition of the lead ore in the Verviers district is very homogeneous compared with that of MVT deposits in the United States (Goldhaber et al., 1995), on a small scale it shows a mixing trend between lead that could have been derived from the Devonian and Carboniferous rocks that make up the sedimentary basin hosting the ores, and lead derived from the Lower Palaeozoic rocks forming the Caledonian basement of the basin (Fig. 6). Similarly, the importance of highly saline, metal-bearing fluids expelled from the basement, causing the post-orogenic, Mesozoic vein-type ore deposits in the Harz Mountains, was stressed by Mo¨ller and Lu¨ders (1993). Sulphide precipitation took place as a result of mixing of the metal-bearing brine with sulphur-bearing low temperature fluids or with wall-rock sulphides. Banding in the veins suggests that a suction pump mechanism may have been responsible for the repeated flux of the hydrothermal 15.90 15.85

207

Pb/204Pb

15.80 15.75 15.70 15.65 15.60 15.55

rve

cu tal r e pe urv Up ec n e og Or

15.50 17.00

s Cru

17.50

18.00

18.50

19.00 206

19.50

20.00

20.50

21.00

21.50

204

Pb/

Pb

Fig. 6. Involvement of basement rocks in the metallogenic system for epigenetic carbonate-hosted Zn–Pb mineralization, as exemplified by Pbisotope characteristics of ore lead from deposits in the Verviers district (modified from Heijlen, 2002). Mineralization in this district is hosted mainly by Carboniferous carbonates. The figure incorporates data from wholerock analyses of Devonian and Carboniferous rocks in the area (Eifelian to Visean; open squares), synsedimentary pyrites in Devonian (Eifelian to Frasnian; filled diamonds) and Carboniferous rocks (Tournaisian to Visean; crosses), and whole rock analyses from Lower Palaeozoic rocks (Cambrian to Silurian; open triangles) of the Caledonian basement. Also shown are sulphide lead data of a metamorphic, polymetallic mineralization in the Caledonian rocks (filled squares) dated at 416 F 2 Ma (Dewaele et al., 2003). The grey field encompasses galena data from 37 epigenetic Zn–Pb occurrences in the district and stretches between the possible lead composition of the Variscan basin (Devonian to Carboniferous) and Caledonian basement rocks. Data are from Pasteels et al. (1980), Cauet (1985) and from analyses performed by J. Schneider (as reported in Heijlen, 2002).

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inclusions in extensional veins in basement rocks with that of Pb–Zn mineralized veins and concluded that the vein minerals precipitated during upward circulation as the fluids cooled, since quartz, fluorite and sulphide minerals in basement veins are unlikely to precipitate during downward fluid movement involving an increase in temperature. The involvement of the basement in the hydrogeological mechanism for the Irish deposits is also apparent from the Pb isotope systematics of the district. The lead isotopic composition within an individual deposit is relatively homogeneous, but is very different from one deposit to another. On the scale of the district, it defines a linear trend. This trend is interpreted to represent a variation in the basement lithology, being less radiogenic in the northwest and more radiogenic in the southeast (Le Huray et al., 1987; Dixon et al., 1990). In contrast to this dominantly vertical fluid flow model involving fluid percolation through basement rocks, Hitzman and Beaty (1996) and Hitzman et al. (1998) proposed a lateral gravity-driven origin for the dolomitizing and mineralizing fluids, based on a limited regional study of variation in the stable isotopic composition of coarse white dolomite. However, the trend observed in the isotopic study was not confirmed by a more extensive regional study of the dolomite (Wright et al., 2000). Furthermore, the lead isotope composition of disseminated Cu-mineralization contained in Devonian red beds is distinctly different (much more radiogenic) from that of the carbonate-hosted Zn–Pb deposits (Kinnaird et al., 2002). This excludes the red bed lithologies as an important source of Pb, while in a gravity-driven flow system, these rocks would have been the preferential aquifers through which fluids were driven from the Variscan Orogeny in the south to the Irish Midlands in the north. Lead isotope data from the ores in the eastern Alps (Ko¨ppel and Schroll, 1988) show a difference in isotopic composition between lead ore (very homogeneous) and trace lead in the Triassic host rock (variable isotopic composition). A strong isotopic similarity between feldspar-lead from the crystalline basement and lead ore indicates that the lead ore was derived from metasediments of the Palaeozoic basement, possibly with a minor amount of lead derived from Permian magmatic and Triassic volcanic rocks (Ko¨ppel and Schroll, 1988). This model assumes that

feldspar was the main source of lead, consistent with the presence of barite and a high thallium content of the ore minerals (Zeeh and Bechsta¨dt, 1994). In a lead isotope study of the Les Malines deposit in the Ce´vennes area, Le Guen et al. (1991) showed that the mineralization underwent a polyphase evolution, with most of the lead being identical to that from Kfeldspar and recalculated whole rock samples from Late Variscan basement granites. The lead isotopic composition of all types of ore present at Les Malines is very homogeneous, with no evidence for inherited lead of Cambrian age. In SW Sardinia, a strong isotopic similarity between feldspar-lead in the Lower Cambrian basal sandstone and ore-lead in the carbonates points to a direct involvement of the pre-Cambrian (not outcropping) crystalline basement as a metal source (Boni et al., 1996). However, the Pb-isotope signatures of the post-Variscan ores show that they are mixtures of the Cambrian (basement-derived) ore-lead and the highly radiogenic, Upper Cambrian to Ordovician sedimentderived lead (Boni et al., 1992). Recent investigations of the Kupferschiefer ore deposits in Poland also demonstrate the importance of the basement in ore formation (Blundell et al., 2001, 2003). Previous genetic models assumed that the copper and other metals were liberated from Permian volcanic rocks within the Lower Silesian basin and from volcanic clasts in the overlying Rotliegend red beds by formation fluids circulating within the basin that evolved into oxidizing Na–Ca–Cl brines. A quantitative fluid flow model was established by Jowett (1986) that includes convective re-circulation of brine within the Rotliegend sequence in a closed system for 5 to 10 million years, driven by elevated heat flow from Triassic rifting, with Zechstein evaporates providing the seal. Cathles et al. (1993) modelled the expulsion of brine from the Rotliegend and underlying Upper Carboniferous sediments by de-watering due to compaction during burial with a slow escape of brine upwards through a low permeability Zechstein evaporate cap. Both models had difficulty in accounting for the total amount of copper present in the Kupferschiefer in the Lower Silesian basin at reasonable concentrations of copper in solution. Blundell et al. (2003) took advantage of basin modelling by Karnkowski (1999), which indicated the presence of a localized high heat flow anomaly

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during the Triassic and Jurassic centred about 50 km NE of the main Kupferschiefer ore deposits, in conjunction with inferred growth faults active during the Triassic that penetrated the basement and the Rotliegend but were sealed in the Zechstein. They proposed a fluid flow model, driven by the heat anomaly, in which pulses of hot brine were expelled rapidly from a network of basement fractures associated with the faults as a consequence of coseismic strain due to earthquake-induced fault rupture. Fluids flowed updip through an aeolian sandstone at the top of the Rotliegend red beds to the location of the Rote Fa¨ule, where metals were precipitated from solution in the reducing conditions of the Kupferschiefer shale. Using data from recent earthquake-induced groundwater flow from faults in the Basin and Range Province, USA, as a modern analogue, (Muir-Wood and King, 1993), Blundell et al. (2003) calculated that multiple earthquake-induced fault ruptures could supply a sufficient volume of brine with a reasonable dissolved copper content (60 ppm) over a period of around 12 million years to account for the amount of copper present in the Lower Silesian basin. Like those of Jowett (1986) and Cathles et al. (1993), the model of Blundell et al. (2003) assumes that the brine initiated as seawater that percolated down through the basin, but differs from them in moving the brine further down into a fractured basement containing Variscan volcanic rocks. There the brine acquired its metal content at elevated temperatures and pressures during its long inter-seismic residence in basement cracks of high aspect ratio, conducive to fluid-wallrock interaction. The arguments discussed above demonstrate that the basement forms an integral part of the metallogenic system responsible for ore formation in the Kupferschiefer copper deposits as well as carbonatehosted Zn–Pb deposits under consideration, whether MVT, sediment-hosted exhalative or any hybrid deposit type. Deep drilling has shown that basement rocks can have significant (fracture) porosities and permeabilities and host highly saline Na–Ca–Cl fluids (Frape and Fritz, 1982; Couture et al., 1983; Kremenetsky and Ovchinnikov, 1986; Nurmi et al., 1988; Pauwels et al., 1993; Bottomley et al., 1994; Mo¨ller et al., 1997). These studies indicate that the basement cannot simply be regarded as a strict hydrological barrier to fluid flow, but that instead it forms a fluid

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reservoir that can be tapped by deep faulting in an extensional tectonic regime.

5. Discussion and implication for exploration 5.1. Tectonic setting According to Leach et al. (2001), most of the world’s MVT mineral deposits formed in foreland basins, mainly during large compressional tectonic events in restricted periods of time. In particular, these authors regarded the Triassic to Early Cretaceous period as devoid of MVT deposits. However, we argue here that MVT mineralization in the Verviers, eastern Alps and Polish districts has a Triassic to Early Cretaceous age. The same can be said for the postVariscan ores in SW Sardinia. The plate tectonic evolution of Europe during this period is such that these deposits formed during large extensional events. The MVT deposits in the Maestrat basin also formed within an extensional regime. During the mineralization in the Maestrat basin, Pyrenean mountain building started ca. 600 km to the north of the Maestrat basin. Bradley and Leach (2003) suggested that mineralization took place within a gravity-driven fluid flow system from the Pyrenees towards the foreland, including the Maestrat basin. However, during the Late Cretaceous, the Maestrat basin formed part of the Iberian rift system and hydrological continuity between the Pyrenees and the Maestrat basin seems unlikely. Moreover, the mineralizing fluids originated in evaporative pools in the same basin, and there is no evidence that they migrated through rocks outside the basin (Grandia et al., 2003). Although the precise tectonic setting of the Irish district during the main stage of ore deposition in the Middle Mississippian is not clear, there is no doubt that Zn–Pb mineralization in the Irish Midlands was related, at least locally, to extensional faulting. The presence of basaltic volcanics of Early Visean age (Blaney et al., 2003) reflects a high geothermal gradient during this period. 5.2. Fluid flow system and precipitation mechanism Within an extensional tectonic setting, several or a combination of processes can explain the expulsion of

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brines from the deeper subsurface. First, a period of high heat flow has been reported or could have been present during the timing of mineralization. This is the case for the sediment-hosted deposits in Germany and Ireland and for the Mesozoic MVT and Kupferschiefer mineralization in Europe. Convective fluid flow may have been induced by this high heat flux, a model proposed by Russell (1978) and Russell et al. (1981) for the genesis of the sediment-hosted Zn–Pb deposits in Germany and Ireland and by Jowett (1986) for the Kupferschiefer mineralization, where the circulation was initiated during extension and driven by a high geothermal gradient. As a result of continued extensional strain and cooling of the rock column the brittle-to-ductile transition zone is depressed and the circulation penetrates to greater depth with time. This model implies a continuous supply of mineralizing fluids. Secondly, large volumes of water are expelled during seismic activity with normal fault movement (Muir-Wood and King, 1993; Muir-Wood, 1994). Based on measurements of fluid discharge from recent earthquakes, Muir-Wood and King (1993) calculated that the total discharge for an earthquake with a fault rupture length of ca. 20 km was around 0.5 km3. To explain these findings, these authors developed a coseismic strain model similar to the seismic pumping mechanism proposed by Sibson (1981). In the coseismic strain model a network of interconnected cracks filled with fluid are forced to close and expel the fluid within the strain field imposed by fault rupture and slippage. This model is generally accepted as a viable and important mechanism for hydrothermal transport (Cox et al., 2001; Sibson, 2001) and has been proposed for the origin of the MVT deposits in Europe (Heijlen et al., 2001, 2003). Blundell et al. (2003) proposed a model in which they combined a period of high heat flow with stress cycling related to normal faulting to account for the expulsion of large amounts of fluids required for the genesis of the Kupferschiefer copper deposits. In discussions about the fluid flow mechanism responsible for the formation of major MVT districts, the volume of the fluids required is a major constraint. In this regard, it is important to note that the argument about bmultiple-basinQ volumes of fluids necessary to generate important mineralization resulted from steady-state temperature calibration of numerical models for gravity-driven flow (e.g., Bethke, 1986)

or by theoretical consideration of metal solubility in the presence of reduced sulphur (e.g., Anderson, 1975, 1977). However, fault-controlled, episodic expulsion of deep fluids can easily explain such temperature anomalies, and direct analyses of fluid inclusions in minerals associated with sediment-hosted base-metal deposits indicates that the metal content can be 10 to 100 times higher than theoretical values and pointing to an extremely low content of reduced sulphur in these fluids (Czamanske et al., 1963; Pinckney and Haffty, 1970; Shepherd and Chenery, 1995). If mass balance considerations can be used as an argument for the viability of the fluid flow mechanism, then the following calculation illustrates that the required volume of mineralizing fluids could be stored in the rocks underlying a district. As an example, we take the Upper Silesian district (~ 3000 km2) where ca. 28 Mt of Zn metal was present originally (Szuwarzyn´ski, 1996). If the concentration of Zn in the mineralizing fluids was 1000 mg/l, the amount of fluid necessary would have been 56 km3 (assuming a 50% efficient precipitation mechanism and neglecting the higher density of the high-salinity fluids). Taking a mean porosity of 1% for the underlying Lower Palaeozoic (Cambrian to Devonian) rocks, in line with in situ measurements in the German KTB deep borehole (Behr et al., 1989), this amount of fluid can be stored in a volume of rock with a thickness of ca. 1.9 km immediately underlying the district. This area is known as the Krakow-Myskow zone and it is made up of several thousands of metres of Lower to Upper Palaeozoic sandstones, shales and carbonates that were strongly faulted and folded, but only slightly metamorphosed (mostly of anchimetamorphic grade and locally up to greenschist facies; Go´recka, 1993). Because the Palaeozoic rocks can be characterized better hydrologically as a fissured aquifer (e.g., Motyka et al., 1998), a porosity of 1% is probably an underestimation. Therefore, these rocks could easily constitute the reservoir from which mineralizing fluids in Upper Silesia were expelled (Sass-Gustkiewicz et al., 1982; Dyulyn´ski and Sass-Gustkiewicz, 1985; Heijlen et al., 2003). This argument does not necessarily apply to other areas in the world, but it can be extended to many of the polydeformed and fractured basements underlying Zn–Pb bearing, Palaeozoic or Mesozoic sedimentary basins in Europe.

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Three main models are proposed for sulphide precipitation in Zn–Pb deposits (Sangster, 1990): (a) mixing of a metal-bearing fluid with a fluid containing significant reduced sulphur, (b) the reduction of sulphate carried together with the metals in the oreforming solution and (c) changes in the physico-chemical conditions of a fluid with reduced sulphur and the metals in solution. The first mechanism has been proposed for most deposits in Europe (e.g., Schroll, 1996; Blakeman et al., 2002; Grandia et al., 2003) and as a main process for MVT deposits in general (Corbella and Ayora, 2003; Corbella et al., 2004). In this model, not only is the amount of metal-bearing fluid important, so too is the availability of the sulphur-rich fluid. MVT deposits that formed early in the diagenesis, when sedimentation was still taking place, are most interesting, since sulphur (originating from the reduction of sulphate) could have been supplied by seawater that circulated through the porous sediments. The results presented here also indicate that in several regions, ore deposition occurred over prolonged periods during crustal extension (e.g., Ireland, eastern Alps and SW Poland) or else in evolving tectonic settings (e.g., Sardinia, Harz Mountains containing the Rammelsberg deposit; Lu¨ders and Mo¨ller, 1992). 5.3. Implications for exploration The overview presented in this paper shows that in Europe, various types of sediment-hosted base-metal deposits formed in areas of the crust undergoing tectonic disruption in an overall extensional or transpressional framework. For example, the major Palaeozoic SEDEX deposits in Western Europe are interpreted to have formed in extensional domains. From a prospective point of view, it is therefore best to focus on those areas in Europe that underwent, at least locally, an important extension with an associated high heat flow. This is not only the case for the SEDEX deposits, but also for Palaeozoic volcanogenic ore deposits (Tornos et al., 2005—this volume). This criterion for prospective areas also results in the selection of the Zn–Pb deposits in Ireland. Within these areas exploration should concentrate on synsedimentary faults and on major fault relay systems (Carboni et al., 2003). These faults and zones of segmentation could have provided the primary vertical conduits for the mineralizing fluids.

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Furthermore, based on the similarity in origin and nature of the fluids (seawater or evaporated seawater formed within the basin itself), on the involvement of the basement, and on the appreciation that migration of the mineralizing fluids was controlled by extensional tectonics, a genetic association can be inferred between Zn–Pb(–Ba) and sediment-hosted Cu deposits (e.g., Kupferschiefer). Which type of mineralization would eventually be formed probably involves numerous variables, including the nature and sequence of the rocks encountered by the metal-bearing fluids, and the presence of suitable precipitation conditions. However, some general boundary conditions can be recognized. These include the opportunity for the formation of evaporated seawater, the possibility for cross-formational migration of the brines into the basin and basement rocks, and for the existence of periods of subsequent important extensional tectonic disruption of the respective areas, which could allow the mineralizing brines to migrate towards places suitable for efficient precipitation. Although sediment-hosted base-metal mineralization in Europe is the subject of this review, the concepts outlined above are applicable to other areas in the world. For example, the formation of evaporated brines and extensional tectonics seem to have played an important role in the epigenetic base-metal mineralization on the Lennard Shelf of Western Australia (Do¨rling et al., 1996; Vearncombe et al., 1996), in the Western Canadian Sedimentary Basin (Nelson et al., 2002) and even in the Copperbelt of Central Africa (Unrug, 1988).

6. Conclusions Although it is not our intention to underestimate the diversity and complexity of processes that lead to the formation of sediment-hosted base-metal deposits, this paper stresses the similarities between a number of important base-metal deposits in Europe. The similarities clearly indicate formation within extensional settings. The mineralizing fluids originated as seawater or as evaporated seawater and migrated downward through sedimentary basins into networks of interconnected fractures within their basement. In regions characterized by pronounced extension and heat production, the ore-bearing fluids were expelled

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back upwards along extensional faults and associated fracture networks during fault rupture. This circulation pattern caused the formation of the SEDEX-type deposits at Meggen and Rammelsberg, the pre-Variscan stratiform deposits in Sardinia, northern Spain and possibly southern France, the Zn–Pb deposits of Ireland, the Kupferschiefer copper deposits of SW Poland and the Zn–Pb mineralization in the Cretaceous basins in the north and east of the Iberian Peninsula. In eastern Belgium, Upper Silesia and other areas, the fluids probably remained in the deeper subsurface for tens of million years. Only during later periods of tectonic activity were the fluids expelled from the basement and caused the formation of MVT Zn–Pb deposits.

Acknowledgements We are grateful to Stephen Kesler and Ross Large for their constructive reviews and thoughtful suggestions and to Peter Cobbold for editing this chapter. We would also like to thank Jens Schneider for permission to use the unpublished Pb isotope data of the Lower Palaeozoic basement in Belgium. This research has been carried out within the GEODE programme of the European Science Foundation. The PhD research of W. Heijlen was financed by the Flemish Institute for the Promotion of ScientificTechnological Research in the Industry (IWT) and a post-doctoral position by the Research Council of the K.U. Leuven.

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