pb-zn deposits and salt-bearing diapirs in southern europe and north africa...

Economic Geology Vol. 80, 1985, pp. 666-687

Pb-Zn Deposits and Salt-Bearing Diapirs in Southern Europe and North Africa

H. ROUVIER,

Universitd P. et M. Curie, Laboratoire de Gdologie Appliqude, 4, place Jussieu, 75230 Paris Cedex 05, France

V. PERTHUISOT,

Laboratoire de Gdologie, Ecole Normale Supdrieure, 45, rue d'Ulm, 75005 Paris, Cedex 05, France

AND A. MANSOURI

Office National des Mines de Tunisie, 26, rue d'Angleterre, Tunis, Tunisia

Abstract

Many diapirs with Triassic evaporite successions in southern Europe and North Africa are accompanied by strata-bound Pb-Zn mineral deposits. These are of two types: (1) epigenetic mineralization filling cavities, exhibiting analogies with Mississippi Valley-type deposits; and (2) syndiagenetic mineralization in sediments rich in organic materials. This second type is very similar to the so-called (volcanic) exhalative type. In most of these ore deposits, diapirism seems to be a determining factor in the development of base metal concentrations. Diapir growth can be divided into four distinct stages: initiation, ascent, piercement, and subsequent deformation. Study of North African and southern European deposits has led to a genetic model of diapir emplacement and associated Pb-Zn mineralization. The source of these metals is unknown. The transport of the metals is provided or facilitated by the brine from adjacent Triassic evaporites. Deposition of sulfides occurs where the deep fluids meet the upper beds. These deposits can be remobilized with each new phase of diapir growth.

Introduction

THE close relationship between certain lead and zinc concentrations and salt-bearing diapir formations of Triassic age in North Africa had already been emphasized by the end of the last century (Levat, 1894). Many of these deposits were being exploited before 1914 and their scientific study incited some interest (de Launay, 1913). Then the economic crisis and the Second World War led to the closure

of many mines and only the most important could be kept active. These deposits enabled Schneider- h/Jhn (1941) to perfect his views on the idea of secondary hydrothermal deposits which he estab- lished from his studies in the Black Forest and

illustrated with Alpine examples (Bolze and Schnei- derh6hn, 1951; Schneiderh6hn, 1954). But interest in North African deposits, which had little economic value, fell rapidly.

The discovery of new metal reserves in the Nefate- Fedj el Adoum • deposit caused renewed interest in peridiapiric concentrations in Tunisia. As a result of this discovery, local metallogenic studies have demonstrated the presence of Pb-Zn concentrations in

•Work by the SociSt$ Tunisienne d'Expansion Mini•re (SOTEMI) and the Bulgargeomine Company, 1976-1979.

Middle Cretaceous strata enveloping the Nefate- Fedj el Adourn diapir (Laatar, 1980). These studies completed the results obtained by the Office National des Mines from the neighboring Kebbouch deposit several years previously. Finally, thanks to the re- sumption and development of exploratory work around the edges of the Jebel Lorbeus diapir (Giot et al., 1981), the Tunisian Office National des Mines (ONM) and teams from the French Bureau de Re- cherche G6ologique et Mini•re (BRGM) have demonstrated the presence of a deposit at Bou Grine which is economically promising.

These discoveries should contribute to a resur-

gence of a number of basic research themes relating to strata-bound deposits. In effect, hypotheses concerning the genesis of base metal deposits in a subsiding basin in an orogen foreland such as that in North Africa are necessarily subject to constraints which differ from the constraints upon deposits formed on the borders of a basin near a terrestrial environment. This has encouraged us to press on with an assessment of both the results obtained on the peridiapiric deposits and their structural and palcogeographic environment and also of the prob- lems which are as yet unresolved. This assessment will be completed by the description of a model of lead-zinc mineralization emplacement around the

0361-0128/85/S9S/666-2252.50 666

DIAPIRS IN S. EUROPE AND N. AFRICA 667

peripheries of diapirs which we hope will form a basis for further discussion and research.

Distribution of Pb-Zn Deposits in Regions with Diapiric Triassic Rocks

North African ranges

Geologic setting: From the beginning of the Me- sozoic, the structural and palcogeographical evolution of Tunisia and eastern Algeria was broadly as follows:

1. During the Triassic, these regions formed a homogeneous palcogeographic unit. It constituted a vast shelf where neritic sedimentation predominated with a marked evaporite component. The complete Triassic sequence only crops out in the south of the country. The Lower and Middle Triassic units are composed of sandstones and dolomites and above these lies a thick sequence of some 1,000 m of

gypsum and rock salt with a few dolomitic interca- latinns (Busson, 1967). Only those Triassic beds uplifted by diapirism are known in the north (Fig. 1). These are the equivalents of the evaporite sequence in the south, which, in fact, they closely resemble.

2. From the Upper Jurassic and during the Cre- taceous, the deepening of the Tethys along the present Mediterranean caused the separation of a deeply submerged zone in northern Tunisia. Two typical palcogeographic units may then be distinguished: the Tunisian trough in the northwest, and the neritic shelf in the south and east which forms the northward extension of the Saharan craton.

In the Tunisian trough, the sedimentary sequence is very thick (about 5,000 m in the Cretaceous alone in the E1 Kef area) and is composed mainly of carbonates or detrital beds (limestones, marls, sandstones).

x----._•MAGHIREB[/////•//l(Iocalil• map ? () I•.,• • ,n• ""w'"" .•'• •

--' -- Z• •29 '-v •

• •" • • 28 • .• • • • rocks ' v• V.• ß ß

•t . • :-' • . • ,•ll •• • • •*•,' • • -- • •/ • (Group I)

• • • ' • H•ofhef/bo//y connected •/?h the Neogene vo/con/sm • ,'• (Group 2 )

50 km

<>o 4,'4,. ,,

i

.,," $4 g !½ASSERIlYE

0 Touchk•9 the Tr/ass/b rocks (Group

[• /n the Middle Cretaceous (neritic facies) touching or not Triassic rocks ( Group 4)

FIG. 1. Pb-Zn and siderite deposits connected with the Triassic evaporites in the northeast Maghreb. Numbers refer to deposits listed in Table 1.

668 ROUVIER, PERTHUISOT, AND MANSOURI

On the southern neritic shelf, the thicknesses are reduced and gaps occur locally in the sequence. The facies of some beds are clearly different from their northern equivalents, especially in the Upper and Middle Cretaceous. Thus the upper Aptian is represented by a reef facies on the shelf and by a deep-water facies in the north (except for a few isolated reef zones associated with the diapirs near the shelf itself).

3. From the middle Eocene onward, the Tunisian trough no longer existed as a paleogeographic unit. The first of the Alpine earth movements caused emergence in the whole area so that areas of deep- water sedimentation persisted only farther north. The stratigraphic sequence is markedly diversified; it became more and more continental as the Alpine progeny took place.

Typical stratigraphic columns for the different paleogeographic units during the Mesozoic are outlined in Figure 2, which thus provides a summary of the essential characteristics of the lithology in

m

PALEOGENE --

1000 --

o TRIASSIC

NEOGENE

Upl•er

Middle

Lower

Turonlen

Upper

Albion

:D 5000 I IiLimestone__ ß U.I O

u I

More than I More than IOO0 m

TRIASSIC • • ß RIASSIC

BASEMENT• BAg E M E NT TUNISIAN TROUGH SHELF ZONE

Upper

Apt•on

FIG. 2. Schematic stratigraphic logs in Tunisia and associated ore deposits. Cap-rock mineralizations are not shown in this figure. Patterns: -- -- argillite, marl; -- = argillaceous limestone; III = limestone; .... sandstone; /k/k = evaporite; blank = conglomerate.

Tunisia: (a) the Triassic evaporites--a thick evaporite layer at the base of the Mesozoic sequence; (b) marine Jurassic and Cretaceous sequences that are particularly thick in the north and contain reef facies in the Middle Cretaceous in the south; and (c) a Cenozoic sequence which varies both in thickness and in facies.

From a structural point of view, the initial deformations which may be detected in the Mesozoic were small and produced simple discordances. They were due to relative movements of blocks which

can be seen in the south and east of the country from the upper Aptian onward. The first diapir ascents can be correlated with these initial move-

ments. Most of the diapirs that crop out at present were emplaced during the close of the Cretaceous, as will be explained in more detail below. Most of them are situated in the northern part of the country, more precisely in the northern part of the southern shelf, in the Tunisian trough, and in even more northerly zones.

The first compressional movements which her- aided the beginning of the Alpine tectonic phases date from the Eocene. They caused minor folding and the return of halocinetic phenomena. During the Neogene, compression was much more important and caused the deformation of the preexisting structures. This deformation was most marked to the

northwest, toward the interior of the range. In these internal zones the deformation phases were accompanied by magmatism.

Thus, four units or structural zones may be distinguished from southeast to northwest: (1) a folded zone with few visible diapirs; (2) a markedly folded zone where the diapirs are especially large and numerous--this is the diapir zone around the Tunis- Teboursouk-E1 Kef line (Perthuisot, 1978, 1981); (3) a zone where tangential deformation is the most marked element, i.e., the nappe zone (Rouvier, 1977); and (4) a zone of numerous diapirs forms the northward extension of the diapir zone (2) and is partly covered by nappes. It should be noted that the boundaries of the structural zones trend obliquely across the boundaries of the paleogeographic zones. The diapir zone occurs in the north in the Tunisian trough and in the south in the southern shelf.

Characteristics of the ore deposits: In Tunisia and eastern Algeria the ore deposits are distributed spatially around the salt-bearing Triassic outcrops which are aligned on northeast-southwest structural trends (Fig. 1). This preferred direction is one of the notable characteristics of maps of the mineral deposits of Tunisia (Sainfeld, 1952; Nicolini, 1968). The characteristics and locations of the ore deposits are summarized in Table 1. An analysis of this table leads us to distinguish four groups of deposits, each


of which is characterized by a particular geologic setting or age. This distribution into four groups closely corresponds to the distribution of mineral formations according to the Bulgarian school (Vas- sileff and Popov, 1979).

Group 1: Pb-Zn mineralization with arsenic and antimony either associated with continental Neogene strata or situated immediately below them (Table 1: deposits 1, 5 to 12, 18).

Group 2: Pb-Zn mineralization with arsenic and mercury in fractures locally injected with Neogene volcanics (Table 1: deposits 2, 3, 4). The genetic relationship between mineralization and volcanism is hypothetical.

Groups 1 and 2 are well developed in the nappe zone. Neogene mineralization is characterized by the importance of karst phenomena in the Neogene beds and even more so below them. The Triassic

rocks behave like any other sedimentary rock in relation to continental alteration. The karstic Triassic

dolomites constitute an excellent receptacle for sulfides, as in the case ofAYn Allega, Bazina, and Djalta. The prototype of the deposits in this zone is that of J. Hallouf-Sidi bou Aouane where the Senonian limestones rather than the Triassic rocks became

karstic (Nicolini, 1968; 1970; Rouvier, 1971; Man- souri, 1980, 1981). In these groups, the Neogene volcanism or continental alteration could constitute

the original source of the metals. Group 3: Pb-Zn mineralization in rocks enveloping

the diapirs or in cap-rock breccias from certain post- Triassic horizons, which may or may not have been reworked (Table 1: deposits 13, 14, 17, 20 to 24, 27 to 30). This group is well developed in the Tunisian trough zone.

Group 4: Mineralization in neritic beds, especially reefs above or against the diapirs, within Pb-Zn deposits (Table 1: deposits 15, 25, 26, 32 to 34) and siderite deposits (Table 1: 16, 19, 26, 31). Group 4 corresponds to the ore deposits of the neritic shelf zone.

It is clear that within each group, a certain relationship exists between the base metal deposits and the salt-bearing bodies. These relationships have been deduced from the following observations: (1) the group 1 Pb-Zn deposits have no direct genetic link with Triassic host rocks; (2) the group 2 Pb-Zn- Hg deposits have only deep fracturing in common with the diapirs--these fractures apparently con- trolled both salt diapirism and Neogene vulcanicity; and (3) group 3 and 4 Pb-Zn deposits in the trough and on the neritic shelf could be genetically related to the Triassic diapirs, if the presence of a diapir is a necessary condition for mineralization. Conversely, group 3 and 4 deposits could have an indirect relationship with the diapirs, if the mineralization

were associated with a particular stratigraphic facies of the host rock, for example, Aptian reefs developed on diapiric shoals.

Pyrenean-Provenqal ranges Geologic setting: The Basque-Cantabrian zone

comprises the southern flanks of the Pyrenees and their prolongation along the northern coast of the Iberian peninsula. It also includes the Cantabrian basin farther south. This vast sedimentary basin trends west-northwest-east-southeast and is the counterpart of the Aquitaine basin in France (Fig. 3). The basin is itself scarcely dislocated, but its northern and northwestern borders were subject to intense Pyrenean folding during the Eocene.

The stratigraphic sequence which may be observed in these regions is very similar to those described in Tunisia. Above a Hercynian basement the Triassic sequence begins with sandstones and then becomes evaporitic. The Jurassic and Creta- ceous beds are marine except for a continental episode at the end of the Jurassic and in the Lower Cretaceous (Wealdian). The Tertiary beds are only well represented in the north of the area along the coast.

As in Tunisia, there is a quite clear contrast between a shallow shelf zone in the north and west and a deeper subsiding zone in the position of the Cantabrian basin. This contrast is marked here in addition by facies variations during the Aptian, with reef facies in the north and west (the Urgonian facies) and deep-water facies in the basin.

Numerous Triassic diapirs occur in the Cantabrian basin where they trend in a west-northwest-east- southeast direction and generally have a regular outline. The initial piercement of these diapirs dates from the Cretaceous (Brinkmann and L6gters, 1967; Kind, 1967). Other diapirs may be seen in the Bilbao and Santander areas where they have irregular out- lines as a result of deformations during the Pyrenean folding.

The Vocontian zone is situated north of the Pyr- enean-Provenqal chain, west of the Alps (Fig. 4). During the Mesozoic this region was occupied by a stable shelf that was prolonged westward by the dry land area of the Massif Central. It corresponded to the foreland of the Alpine orogenic edifice. The stratigraphic sequence closely resembles that of the Basque-Cantabrian zones. Late Cretaceous rocks are absent in places because of the emergence of the Pyrenean-Provenqal orogeny. During the Middle Cretaceous, pelagic sedimentation occurred in the Vocontian basin in the center of the Vocontian zone, while reef sedimentation (the Urgonian facies) occurred on the flanks of the zone.

The Vocontian basin was folded in an east-west direction in the south and in a northwest-southeast

670 ROSIER, PERTHUISOT, AND MANSOURI

z • • • z• z


+• +• +• +• +• +• +• +•


direction in the north. It is crossed by major faults trending northeast-southwest, north-south, and northwest-southeast. The diapirs are infrequent and occur at the intersections of these faults.

Characteristics of the ore deposits: In northern Spain (Fig. 3), in the Basque-Cantabrian zone, the zonal distribution of ore deposits is the same as that of the Tunisian deposits for groups 3 and 4. The shelf zone in the northwest is marked by very important Pb-Zn and siderite deposits in the Aptian reefs which are closely comparable to those in central Tunisia. These occur in the Bilbao (siderite) and Santander-Reocin (Pb-Zn) areas (Monseur, 1961, 1965; Vadala, 1981; Vadala et al., 1981). In the trough zone in the Cantabrian basin, a few Pb-Zn deposits, such as at Altube, occur near the Murguia diapir (Grabert, 1956; von Stackelberg, 1967).

In the Vocontian basin, diapir-related base metal deposits are not common although some Pb-Zn mineralization is found near diapiric Triassic outcrops (Fig. 4). Instead mineral deposits of this type are found in fractures of Upper Jurassic rocks along important dislocations following the C•vennes trends (NE-SW), but there is no systematic relationship of Pb-Zn deposits to Triassic diapirs. For example, major mineralization such as that at Pi•mard (Men- glon mine) is not accompanied by Triassic outcrops (Rouvier, 1962). When such a relationship does occur, the deposit is generally not situated in the immediate vicinity of the diapir. This is the case with the Benivay-Propiac deposits and those with celestite at Condorcet. At Orpierre, the fracture network of the ore mineralization converges toward the Montrond diapir that is several kilometers away.

In the rest of this study, only those deposits will be analyzed which probably owe their existence to the presence of active diapirism.

Pb-Zn Deposits and Their Relationship to the Sedimentary and Tectonic Evolution of a Salt-

Bearing Diapir: Some Tunisian Examples

Mineralization characteristics

The characteristics of base metal stratiform de-

posits surrounding the diapirs or occurring in the cap-rock breccias are illustrated in two Tunisian examples; one, at Bou Grine, which is scarcely deformed or not at all, and the other, at Fedj el Adoun, which has been highly dislocated (Fig. 5A and B).

At Bou Grine the deposit occurs in the northeast- ern periclinal extremity of a fold whose axis is occupied by a Triassic diapir (Orgeval et al., 1981; Sahli et al., 1981). The absence of major concentrations on the flanks of the anticline suggests that the Pb-Zn mineralization took place on the top of the diapiric structure. Only at the periclinal extremity


SAAITAAIDEfi' .•. C A IV T A B R / A IV S œ A

, ,o,m , "'"', • •,l A JV 'B A SIN; I•PLø/vA ß Triassic rocks

• Pb-Zn deposits Domain of

+++ Basement //•Siderite deposits + +

Paleogene

Upper

Cretaceous

Middle

Cretaceous

More than IOOOm

ß

Basement

FId. 3. Pb-Zn-Ba deposits in the Cantabrian basin (northern Spain). See pattern identification in Figure 2.

of the structure has the ore-bearing cover of the diapir been protected from erosion.

At Fedj el Adoum, most of the ore deposit occurs

beneath the main Triassic body. This structural arrangement has been interpreted by Laatar (1980). The mineralized cap rock is only preserved in the

• Pb-Zn and/or Bo-Sr deposits ß Triassic rocks D/EO /

/ EASIN

x

map•,

RANE AN sEA

rn

i upper Cretaceous

Middle

toad Cretaceous ]_

Lower

Cretaceous

200(

upper 5OO(

durossIc

4OO(

M)ddle

Jurossic

5OO(

More hon 3000 rn

r• I ir..• Triassic ß

Basement

FIG. 4. Pb-Zn and Ba-Sr deposits in the Vocontian basin (southeastern France). See pattern identification in Figure 2.


80 m

I • I B o u G r i n e * *\•----'•,•.•-••-•...•__-• J A A A • A

• Upper Senonion: / • • • • I I I iimeslones

/ / ". ••••3 • Lower Senonion:mo,l, •o

/ / • A • •A

/ / i • • .'•// • orgilloceous limestones

I I • • i //•/•/ .• hole • Lower.•,onion: / / I • ; •!//// t I I(Bahloul Formation)

I •0 t I r • /1•/•// • upper Glbian-•n•anian:

I 560 • ' - ß • ' '

Fed j el A d o u m • Triassic: evaporites

Fig. 5. Schematic cross sections through two deposits related to diapirs. A. Bou Grine (after ONM and BRGM documents). B. Fedj el Adoum (after Bulgar Geomine documents and Laatar, I980). See pattern identification in Figure 2.

overturned flank of a syncline beneath an overthrust of upper Miocene age which affects the diapir itself.

Stratiform bodies: Generally the mineralized bod-

FIC. 6. Sphalerite-galena ore of Fedj el Adoum (bedded and mineralized dolarenite).

ies lie conformably on the argillaceous limestones (often dolomitized) or sandstone r_ocks. These host rocks vary in age from Aptian-Albian to the lower Turonian or lower Oligocene. Mineralization also occurs in networks of fissures of all sizes which

permeate the rocks. In the stratiform deposits, sulfide crystallization occurred just after the deposition of the host rock.

In the Albian-Aptian deposits, the minerali?.ation host rock is dolarenite; the clasts of this rock are essentially of Triassic age. They are cemented by calcite, silica, and by Pb, Zn, and Fe sulfides (Fig. 6). This type of ore is included in dolomitic beds of probable Triassic age in the cortical zone of the diapirs and in the cap-rock breccias. The dolarenite is similar to the host rock of solution-disaggregated Pb-Zn deposits in carbonate beds such as those described from the Triassic of Upper Silesia in Poland (Bogacz et al., 1973; Ruckmick et al., 1979), from the Triassic of the eastern Alps (Omenetto and Vailati, 1977; Assereto et al., 1977a and b), and from the Lower Jurassic of the Causses in France (Macquar and Lagny, 1981).

In the Tunisian deposits, this type of sedimenta/y rock is frequenfiy recrystallized and highly fractured, making observation difficult. In some cases attribu- tion to the Cretaceous remains in doubt. It may thus justifiably be asked if any Triassic mineralization occurred before Cretaceous' mineralization. These

first deposits could have been transported upward


by diapirism. The argument against this hypothesis is that the mineralized host rocks are never present in the core of the diapirs but only on their flanks in the contact zone with Cretaceous limestones which themselves are oft6n mineralized. This transition

zone contains the bulk of the Fedj el. Adourn deposits. At Bou Grine, on the other hand, the economic importance of this zone is much reduced.

In the lower Turonian deposits, the mineralization host rock is finely bedded biomiorite. Sulfides, which are sphalerite and pyrite, emphasize the bedding and fill foraminifera tests (Fig. 7); these sulfides crystallized during early aliagenesis (Orgeval et al., 1981). The deposits superficially resemble a black shale in both their color and their high organic matter content (Fig. 8). They form the bulk of the deposits at Bou Orine.

In lower Oligocene deposits (Koudiat-Safra deposits), the host rock is a fine-grained, poorly consolidated sandstone with crossbedding. The dominant sulfide is galena with smaller amounts of sphalerite and pyrite. Galena is the major cement and commonly emphasizes the details of the sedimentary structures (Fig. 9). The early appearance of the sulfides is proved by the reworking of the galena in the burrows (Rouvier, 1967). This host rock has not hitherto produced major economic concentrations, but it would certainly be worth additional pros- pecting.

Vein mineralization: The host rocks of all deposits, whether carbonate or sandstone, are crosscut by mineralized fissures which contain the same sulfides

as the stratiform deposits. In the veins, Pb- and Zn- bearing minerals are generally well crystallized and often automorphic. In the case of the carbonate rocks, sphalerite and pyrite develop in concretionary structures. In the small fractures, the mineralization

ß

4 cl•rl

FIG. 8. Thin bedded biomicrite from the Bou Grine Turonian.

Sphalerite underlines the lamination.

is probably due to a remobilization of sulfides within the rocks, but for the larger fractures, new additions or mineralized solutions cannot be excluded.

In the fractures galena is more abundant than in the stratiform bodies. In the gangue celestite, barite, and kaolinite accompany the calcite. At Fedj el Adoum, sulfur is associated with these minerals in the cap rock. Finally the presence of hydrocarbons

FIG. 7. Planetonic foraminifera (Globigerina) from the Bou Grine Turonian. The interior of the test is occupied by sphalerite (X 200).

FIG. 9. Oligocene sandstone from Koudiat Safra• The crossbedding is emphasized by galena.


may be noted both in the veins and in the stratiform bodies. For example, in the Bou Khil deposit an Upper Cretaceous dolomitized limestone contains sphalerite and is filled with hydrocarbons.

The veins cut indiscriminantly across all post- Triassic sedimentary formations but are particularly well developed at the intersection of the limestone levels of Cenomanian-Turonian and Senonian age as, for example, at the E1 Akhouat and Fedj el Adoum deposits. Mineralized veins are generally banded with a widespread development of schalen- blende. Homogenization temperatures of fluid inclusions of calcite associated with the sulfides give formation temperatures on the order of 100øC, together with very high salinities (Laatar, 1980).

Stages of diapir evolution and the process of Pb-Zn mineralization

Diapiric structures develop in characteristic phases themselves dependent on a variety of the local geologic conditions. Factors which affect diapir development include the thickness of the evaporite- bearing beds, the structural fabric, the basement of the salt-bearing beds, the local and regional pressure- temperature conditions at the time of diapirism, and the composition of the evaporite-bearing horizons. All the diapirs discussed in this section fit into a homogeneous group with the following common characteristics. They all have: a Triassic salt-bearing horizon, a zone of peri-Alpine sedimentation, and a history of strong Alpine tectonic movement.

Moreover, the temporal and structural development of Tunisian diapirs can be analyzed with some degree of precision, notably because of the fossilif- erous and marine nature of most of the post-Triassic formations and because of the relatively weak deformation that the rocks underwent during recent tectonic phases (Perthuisot, 1978). Thus several successive and indeed probably repetitive phases can be distinguished: initiation, ascent, piercement, and subsequent deformation (Fig. 10A to J).

Initiation phase and diapirism localization: The data on this phase are indirect and, to some extent, speculative. The phase probably occurs very soon after the deposition of salt-bearing formations, with accumulations of salt-bearing material directly above faults or such discontinuities (Perthuisot, 1978). In various basins where post-Triassic sediments are very thick, the initiation phase of diapir growth probably occurred throughout the greater part of the Jurassic. Thus, there probably was an important phase of material movement including fluids, during the period both in the salt-bearing body and in its immediate environment. It may be considered that, in all these areas, structural discontinuities in the pre-Triassic basement seem to have influenced the initial migrations of the salt-bearing material. This

explains the fact that the diapirs are localized along faulted zones.

In Tunisia the northeast-southwest to north- northeast-south-southwest alignment of the diapirs suggests that they may parallel transverse basement shear faults which cross the whole Maghreb (Glan- geaud, 1951; Jauzein, 1962). In the Cantabrian basin the diapirs seem to be aligned along dislocations with a Pyrenean west-northwest-south-southwest trend (Fig. 3). As for the Vocontian basin, the few diapirs within it are situated directly above known dislocations in the basement (Baudrimont and Du- bois, 1977), especially those of the C•vennes system (Fig. 4).

The link between the mineralized zone peripheral to the diapirs and certain major dislocations thought to exist in the basement is, to some extent, similar to the relationship between strata-bound Pb-Zn deposits and the border faults which control the stratigraphic facies of the sedimentary host rock. Examples may be cited from the C•vennes dislocations (NE- SW) controlling the Pb-Zn sub-C•vennes zone bor- dering the southeastern basin in France (Bernard, 1958) and the faults in the pre-Cambrian basement of the Viburnum Trend in the United States (Kisvar- sanyi, 1977) or in the Pine Point district of Canada (Campbell, 1967).

Ascent phase: This phase begins when the cover strata fractures under the influence of pressure from less dense salt-bearing beds. Breakage and ascent of the salt may be facilitated by various tectonic movements, notably shearing or extensional movements.

In Tunisia, this phase must have taken place during the Middle Cretaceous in a certain number of diapirs as is demonstrated by the great thickening of the Lower Cretaceous rocks in the interdiapiric basins. It probably also occurred, during the same time interval, in the Spanish Cantabrian country. Evidence is lacking as to whether or not the ascent phase has an identical age throughout the Vocontian zone. However, in the Suzette diapir, the first proven piercement has been related to the Pyrenean-Pro- venial orogenic phase known to occur between the Upper Cretaceous and the lower Eocene (Perthuisot and Guilhaumou, 1983).

The,rise of the diapir leads to development in the covering strata of sedimentary or tectonic wedg- ing that converges toward the intrusive body, and to varying degrees of deformation in the basin floors (Fig. 10B and C). This deformation style has sometimes been invoked to explain the establishment of reef bodies in the Middle Cretaceous in central

Tunisia as at the Jerissa deposits (Mahjoubi and Samama, 1980) and it may be imagined that the appearance of such a reef sheet could be one of the reasons why some intrusive bodies (aborted diapirs) are kept at depth.


SUCCESSIVE STAGES

OF THE DIAPIRIC EVOLUTION

AND ASSOCIATED ORE DEPOSITS

DEPOSITS

(• IN FISSURES

(• IN CAVITIES

(• IN THE SEDIMENTS

G

SA LIFEROUS 'CA P-ROCK

E

FIG. 10. Successive stages of the diapiric evolution and associated ore deposits,

In Tunisia and the Vocontian basin, the ascent phase was accompanied by an intense fluid circulation which allowed the crystallization of potassium and magnesium silicates in the cap rocks (Perthuisot and Saliot, 1979). Study of these minerals and of the inclusions within them has shown that the diapir

zones were the sites of major thermal anomalies caused by the high thermal conductivity of evaporites and by deep-seated thermal anomalies associated with the extensive fracture network in the basement.

These initial stages in diapir development (initiation and ascent phases) characterized by the intensity

678 ROSIER, PERTHUISOT, AND MANSOU-RI

and the permanence of the deformation in certain favored sites are closely comparable to the stages in the evolution of mineralized sites on basin borders

where faulting and flexural movements are frequent during the deposition stage.

Piercement phase: In Tunisia, at the end of their ascent, the Triassic rocks pierced their cover under- water. This piercement was thus accomplished during the Middle Cretaceous under different bathymetric conditions and with varying sedimentological effects. This produced the zone of diapirs which trends obliquely to the facies zones defined by Cretaceous sedimentary rocks (Fig. 10D).

With a deep-water layer (Fig. 10) in the trough zone, the disturbances in the environment brought about by salt intrusion are small compared to those features that can be observed on an ordinary shoal. If the diapir appears on the sea floor, the solution of the saline elements from the diapir causes breccia formation on its outer parts and the liberation into the sedimentary environment of fine insoluble minerals (bipyramidal quartz included in gypsum, for example). This is the case in the lower Eocene in Tunisia.

Emergence of the diapir onto the earth's surface may be preceded or accompanied by the establishment of carbonate reefs that form massifs (Slata, Ouenza, Jerissa) or isolated patches (Lorbeus, Keb- bouch). The temporal relationship of diapir pierceq ment and reef formation has been demonstrated for

the Valle de Mena diapir in northern Spain (Schroe- der, 1979) (Fig. 10E).

Important changes affect the top of the diapir during the piercement phase, especially when the top has emerged. These changes include dolomiti- zation, karstification, and redeposition of insoluble materials in the karst cavities. These phenomena caused the formation of host rocks of the Tunisian

Middle Cretaceous deposits, i.e., Fedj el Adoum (Fig. 1 OF).

The succession of phases outlined above (initiation, ascent, and piercement) can recur, provided that certain conditions are fulfilled which establish a new imbalance. These conditions include: burial, continued supply to the salt-bearing body, or erosional elimination of any blockage. The latter condition seems to have occurred in Tunisia and in northern

Spain for the piercement that occurred during Late Cretaceous to Eocene time. The stratiform miner-

alization of Koudiat Safra in sandstones of upper Eocene to Oligocene age may be correlated with an earlier phase of piercement (Fig. 10G to I).

Subsequent deformation: In most cases, the diapir structures that formed during the Cretaceous in the peri-Alpine zone were subject to Late Cretaceous and Tertiary deformations. The intensity of the deformation varied according to the positions of

these structures in the stress field. In this respect the Tunisian structures are very similar to those in Spain, in that both exhibit a comparable style of deformation between a nappe zone and a more or less tabular shelf zone. On average, the observed result is a more or less asymmetrical and fractured anticline cored by a diapir of salt. The mineralized stratiform bodies are themselves deformed like the

envelope and broken by fractures, which in Tunisia are highlighted by Pb-Zn vein mineralization (Fig. 10J).

The intensity of the regional and local deformation seems to exhibit an influence on the depth to which erosion has reached. This fact can explain the scarcity or even the complete absence in certain areas of outcrops of mineralized deposits. Thus the diapiric structures in the Vocontian zone, which have been eroded down to the Middle Jurassic rocks, are rarely mineralized whereas those in Tunisia, which are but little eroded, are mineralized as at Bou Grine. Con- versely, if tectonic deformations after the initial piercement are weak, then the structures and materials corresponding to the initial diapiric phase often become buried beneath more recent rocks, even if the central part of the diapir has continued its ascent (Fig. 10H and I). This could be the case in most of the diapirs in the Cantabrian basin in contact with the Upper Cretaceous and Tertiary.

Discussion

Certain characteristics of diapir-related deposits are close to those of the Mississippi Valley type which were discussed by Ohle (1980). This comparison has already been made for the concentrations in the cap rocks of the Gulf Coast (Price and Kyle, 1983). In the same area certain structures form traps for hydrocarbons or contain sulfur deposits (Ruckmick et al., 1979; Davis and Kirkland, 1979). In the Tunisian examples, as well as the deposits in the cap rocks accompanied by hydrocarbons and sulfur, concentrations also occur in the peridiapiric beds. This fact justifies further analysis in support of this comparison.

The Tunisian deposits are poor in different mineral types and it is impossible to link their genesis to magmatic phases. The deposits are contained in rocks situated directly above the diapirs. These rocks are thin compared with the thick interdiapiric basin rocks. The host rocks are dolomites or lime-

stones and occasionally sandstones. Zn is most often associated with the carbonate rocks, whereas Pb is associated with the sandstones. Two types of mineralizations may be distinguished: epigenetic mineralization filling cavities and syndiagenetic mineralization in sediments which are rich in organic materials.


In the first type, the filling takes place in cavities of various kinds: fracture voids, solution voids related to unconformities, and porosity in the cap-rock breccia or in dolarenites. These deposits occur essentially under unconformities according to the clas- siftcation proposed by Callahan (1964). Their overall characteristics enable them to be compared with Mississippi Valley-type deposits.

In the second type, the mineralization began in a sediment such as calcareous mud or sand. The bed-

ded minerals are very similar to the so-called (volcanic) exhalative type. They resemble the mineralized H.Y.C. Pyritic Shale of the McArthur River district in Australia (Williams, 1978). Sangster (1970) puts them in the Remac-type ore of the Kootenay arc in southeastern British Columbia.

Deposits associated with diapirs thus form com- posite structures resulting from the coexistence of several phenomena: (1) the formation of the cap rock and its reworking after its emergence in the Middle Cretaceous, (2) mineralization by cavity filling, and (3) in the lower Turonian and in the lower Oligocene, syndiagenetic mineralization which occurred on the top of the diapir. The lower Turonian phase is particularly instructive from the genetic point of view. It was an anoxic phase which has been recognized in the oceans and may be correlated with a high sea level (Jenkyns, 1980). This fact supports field observations which prove that the diapir was submerged and covered with sediments again. It also proves that the solutions carrying the base metals did indeed come up from the depths •tnd have contaminated the sedimentary environment 'on the top of the diapir. This environment was fairly deep and calm as the thinly bedded nature of the host rocks and the ore demonstrates.

Previous Views on the Genesis of the Deposits

From the beginning of the present century, the genetic hypotheses proposed have tried to link the presence of Triassic rocks with the emplacement of mineralization. The absence of chronological data about deformations involving Triassic rocks and the fact that distinctions were not made between different types of association and Triassic rocks and ore deposits led authorities to propose similar explana- tory mechanisms.

One group of hypotheses suggests that emplacement occurred in two stages. In the first stage, the Triassic rocks were mineralized either by a synge- netic process (P. Termier, unpub. rept., 18952; Termier, 1920; Berthier, 1914; Brives, 1972; Mois-

Unpublished report on the Calamine deposit of Jebel el Akhouat.

seef, 1959) or by a hydrothermal epigenetic process related to spilite emplacement (Glangeaud, 1935). In the second stage, the Triassic rocks (including evaporites) were brought up to the surface by tectonic movements (diapirism or nappe formation) and the minerals were redeposited in an oxidized or sulfide form by waters that had percolated into the Triassic rocks.

In the second group of hypotheses, emplacement occurs in a single stage: the fractures bringing the diapir into contact with the surrounding rocks could have allowed deep-seated hydrothermal activity to well up (de Launay, 1913; Berthon, 1922; Solignac, 1927; Sainfeld, 1952). A modification of this hypothesis suggests that surface waters which have been buried and heated at depth could have leached out the base metals in the basement or in the cover.

These solutions could have deposited their contents along the diapir contacts. The resulting deposits were all then considered epigenetic and thus post- date the last orogenic phase in the Quaternary (Bolze and Schneiderh/Shn, 1951). Such deposits have been described by these authors as secondary hydrothermal or regenerated.

Some adjustments have had to be made to the view that the Triassic salt diapirs systematically influence the genesis of lead-zinc deposits. In Tunisia, because not all deposits are associated with diapirs, there must be recourse to explanations which do not involve Triassic rocks (Gottis and Sainfeld, 1952). In France the deposits in Diois and Baronnies have been explained by the washing out of metal traces in the Callovian-Oxfordian by connate waters (Rou- vier, 1960, 1962).

Recently new hypotheses concerning deposits in direct relationship with Triassic rocks have been presented. In Tunisia, the role of Triassic salt diapirs in Cretaceous paleogeography, which was proposed by Bolze (1954a and b), led Nicolini (1968) and Massin (1968) to stress the local influence of shoals and "paleotalus" (Bernard, 1958, 1964; Launey and Leenhardt, 1959; Bernard and Foglierini, 1967). In addition, the idea of a continental cation source has often been suggested. Thus, for Pb-Zn deposits in the Aptian reefs which Handous (1971) studied in central Tunisia, Fuchs (1973) proposed a sudden metalliferous supply to the sedimentary environment. Fuchs suggested that this supply was caused by a reworking of soil horizons enriched with heavy metals as a result of epirogenic movements. In fact, the distance from dry land of any importance which would yield the cations is such that the problem of their actual source has been systematically avoided. Only recently has diapirism as a metallogenic phe- nomenon been suggested (Laatar, 1980; Laatar et al., 1981). In deposits of a very similar type, as at Reocin in Spain, the intervention of saline solutions


from diapir formations as metal vectors has been envisaged (Vadala, 1981; Vadala et al., 1981).

A Model for Peridiapiric Metal Concentration

The model proposed herein takes into account the characteristics of peridiapiric concentrations themselves. First, the deposits were isolated in the midst of a vast basin; the deposits had no direct continental supply. Second, the emerged surfaces of the Thiassic rocks were exposed in only a small area; this makes reworking of pre-Triassic mineralization an unlikely hypothesis. Third, when the salt-bearing formations were mineralized, the mineralization occurred only at the upper tectonic contact with the surrounding sedimentary rocks. In fact, in the case of Turonian mineralization, the absence of reworked Triassic rocks and the great homogeneity of the stratigraphic facies precludes the existence of Triassic outcrops. Emplacement models in which the immediate source of supply is continental and where the trap is located at the geochemical boundary of continental and marine environments (Lagny, 1980) are therefore not applicable. Thus, a different process must be suggested from that of simple supply from atmospheric weathering and leaching of a preexisting metal-rich continental source. The source of the

metal can only be situated at depth. In the proposed model the following four main

factors have to be taken into consideration: (1) an internal and external structural arrangement of the diapir which will channalize fluids from the superface of the salt horizon, the diapir itself, or from deeper

zones within the diapir to the top, (2) an appropriate chemistry and temperature of the mineralization fluids which will mobilize and transport the metallic elements, (3) an abundant source of sulfur, and (4) the existence of a trap for mineralization fluids located at or near the top of the diapir. A very similar model to ours has been proposed for mineral deposits occurring in the cap rock of a Texas diapir (Price and Kyle, 1983). Moreover, our model is a variant of the "pal•insule" model proposed by P•I- issonnier (19,59, 1962, 1967), which brings into play the concept of the "bottleneck." Our model also takes into account the many similarities between peridiapiric Pb-Zn mineralization and Mississippi Valley-type deposits, i.e., the same occurrence of traps which are (systematically) higher than the basin floors, similarities in sedimentological, hydro- logical, and structural characteristics between deposition sites and certain hydrocarbon traps; and compatability of temperatures and salinities of the mineralizing fluids. Finally this model takes into account the data recently obtained about the fun- damental role of compaction in the genesis of strata- bound deposits according to the hypothesis proposed by Noble (1963) and subsequently developed in a recent synthesis by Wolf (1976). Sources and movement of fluids

Postsalt-bearing cover: With compaction fluid present in the pores of the sedimentary rock migrates upward to the top of the diapir structure where the fluid pressure is lower whatever the stage of diapir development may be (Fig. 11). The quantity of fluid

::::::::::::::::::::::::::::::: ¾rnigfation of fluids in ....¾';:.•.."' "•¾:¾

/

iccurnulation

of the fluids in the

enrichment of the brines

ration of fluids in the saliferous body

FIG. 11. Diagram of the fluid circulation around a buried diapir (the arrows with diagonal lines show the movement of Pb-Zn-enriched brines).


that migrated depends in particular on the size of the peripheral zone of the diapir involved in this migration.

In the initial stages, compaction is fairly rapid. Between depths of 0 to 1,000 m, compaction of a clay formation is some 40 percent and only reaches 50 percent at a depth of several thousand meters (Athy, 1930). It may be supposed that most of the fluids related to this rapid change in volume migrate up toward the basin edge. Where a sediment contains 60 percent water at the outset, the loss of 40 percent of one volume results in a new pore water content of 33 percent. Thus, it is probable that a large part of these fluids then migrate toward the edge of the sedimentary basin, as has been envisaged for Pine Point (Jackson and Beales, 1967; Billings et al., 1969), or toward the drain represented by the diapir structure.

In the case of a standard Tunisian diapir during the Aptian, the original thickness of the compactable clay formation between the Neocomian and the Aptian may be about 3,000 m (core OB 101), from Fourni$ and Pacaud (1973). If 100 km 2 is the estimated surface area drained by the diapir, then about 300 km 3 of sediments have undergone pro- gressive compaction; the sediments thus enclosed approximately 200 km 3 of interstitial water. If a third of this volume can migrate laterally, then 70 km a of water will move toward the diapir. With metal contents of similar magnitude to those known in the brines in central Mississippi, i.e., 450 mg/1 Pb-Zn (Carpenter et al., 1974), the sum of the metals potentially transported (some 30 million tons) is well above the tonnage estimated for the two largest deposits of this type known in Tunisia, Fedj el Adoum and Bou-Grine (in all, 1,300,000 tons Pb-Zn).

Evaporite beds: The evaporites contain a great volume of water. It consists of interstitial connate water, brines contained in the inclusions, and waters derived from the dehydration of gypsum bodies.

In principle, evaporite beds are thought to be relatively impermeable. In the case of an evaporite horizon containing a notable proportion of carbonates and detrital minerals, then enough discontinuities can exist in the impermeable beds to enable fluids to pass through. Such is the case in the Triassic evaporites of North Africa and southern Europe which contain sandstone and dolomite beds reaching several tens of meters in thickness. These permeable beds, especially when they are dislocated as is the case in the diapirs, act as drains that direct the fluid movement. Moreover, if the hypothesis of diapir location directly above major faults in the basement is correct, then at the base and the lower parts of the salt diapir there must be a discontinuity favoring the passage of fluids (especially the deep-seated fluids under the salt-bearing series). If such fluids

exist, and if they are related to basement dislocations, then the fluids can move up the structure along fractures, permeable zones, or solution conduits within the diapir body itself.

Channalization of fluids along the contact of the diapir

The diapir contact zone, on either side of the edge of the salt diapir, is marked by intense fracturing and brecciation. This zone is the major drain for fluid migration resulting from the postsalt-bearing cover compaction. These fluids are composed of waters included in the evaporites and waters welling up from basement faults (Fig. 11).

The extent of fluid circulation in this zone is

shown by the abundance of new silicate and magnesium minerals occurring in both the Tunisian and the Vocontian diapirs (Perthuisot and Saliot, 1979). Part of these minerals (micas, talc, and potash feld- spars) date back to the Middle Cretaceous (K/Ar method) (Bellon and Perthuisot, 1977). This age is approximately the date of the first diapir piercement and the first mineralization. Thus it seems probable that the fluid which precipitated the initial base metals is related to the solutions that precipitated these new silicates (Fig. 12).

Mobilization and transport of metallic elements

The problem of the base metal source of peridiapiric mineralization has not yet been solved as is most often the case in many other types of deposits. Therefore only speculation is possible.

It seems probable that metallic cations were orig- inally dispersed throughout the sedimentary formations in the basin, partly in solution in interstitial waters but mainly as cations adsorbed on the clay particles (Noble, 1963; Krauskopf, 1967); the possible existence of base metal cation concentrations

in post-Triassic rocks or even in Triassic strata is not excluded. This is the case in Tunisia where the

Lower Jurassic often contains Pb-Zn deposits (Sain- feld, 1952). A final and very hypothetical possibility would be the presence in the Triassic evaporite basin of mineralized brines identical to those found

in the Red Sea (Bischoff, 1969; Brooks et al., 1969). These brines could have accompanied the opening phase of the Triassic basin. The mobilization and transport conditions of the base metals must also remain speculative.

The return of metallic ions to solution can only occur if conditions change. The important changes include an increase in temperature and pressure, an increase in the salinity of the solutions as they migrate (Neglia, 1979), and the presence of organic compounds from the maturation of kerogen. The increase in the grain size of the clay minerals during late diagenesis necessarily brings about a decrease


FIG. 12. Fluid circulation around a piercing diapir (left) or in the process of piercement (right).

in the surface area which may be used for adsorption. Some of the metallic cations could thus be freed to

reenter the pore-water solution and migrate in fluid channelways in accordance with the methods described above. In this hypothesis, the transportation efficiency of the metals (and hydrocarbons) from the cover strata toward the dome would be assured by a saline solution. The frequent presence of hydrocarbons in the host rocks of certain peridiapiric concentrations in Tunisia (Fedj el Adourn, Kebbouch, Boukhil) suggests that the base metal-bearing saline fluids were oil field brines. This hypothesis is in- creasing in importance at present as a result of the work of Jackson and Beales (1967) and that of Sverjensky (1984) more recently.

In Tunisia, it is, in fact, difficult to decide whether the migrations of the hydrocarbons and the metal- bearing fluid were simultaneous or successive. Nev- ertheless, the presence of hydrocarbons in base metal-mineralized beds proves the hydraulic con- nection between the base metal-mineralized site and

a more or less distant petroleum parent rock. The probable hydrocarbon source rocks are Cretaceous, but Jurassic or even pre-Triassic formations cannot be excluded. As a variant of this hypothesis, the mobilizing and transporting fluids could be trapped in the salt beds after crystallization. These highly concentrated brines can in fact occupy a large volume (up to 50%) in a saline sediment, which preserves a very high porosity as long as salt recrystallization does not intervene.

Such brines could be the origin of saline fluids rich in Ba-Pb-Zn in the petroliferous Mesozoic beds of the Gulf Coast (Carpenter et al., 1974). Highly concentrated brine saturated in NaC1 and rich in K, Mg, and SO4 initially trapped in the evaporite beds (Louan salt) could have migrated slowly through the postsalt cover to a clay formation where potassium would become fixed on the clay minerals. This fixing would be accompanied by a recrystallization of the clays, thus aiding the solution of the adsorbed metals

in the brine. The Pb-Zn-Ba content of these fluids

is sometimes large enough for barite, galena, or Pb metal to crystallize in certain ore conduits. Scattered deposits are already known in Gulf Coast salt domes (Hanna and Wolf, 1934; Smith, 1970a and b). Similar deposits have been demonstrated in the Cheleken oil field on the edge of the Caspian Sea (Lebedev, 1967).

A similar proeess ean be envisaged for the origin of eertain deposits found dose to diapirs. Brine from a Triassie salt-bearing body eould have migrated in part to the lower zones of the postsalt eover and aeeumulated near the diapir strueture. Part of the brine would thus have beeome enriehed in Pb-Zn-

Ba (and depleted in K) as it erossed the day layers. Another part may have remained within the diapir strueture and supplied the potash and magnesium minerals to the earbonates in the eortieal zone of

the diapir. In the Gulf Coast it has been shown that brine

movement has only really been important after burial to at least 1,500 m with temperatures on the order of 50 ø to 75øC (Moore and Druekman, 1981). In all the eases proposed here, sueh eonditions were eertainly reaehed fairly early during the Cretaeeous. This generalized eireulation system deseribed for the whole eover is perfeetly eompatible with the restricted system dose to the diapir, with the latter only being needed for the first phase of mineralization.

In all the eases eonsidered, the fluids tend to aeeumulate in the apieal part of the diapir strueture as long as piereement has not oeeurred. During or a little before piereement, when the diapir eover is suffieiently distended, fluids migrate rapidly toward the outside and can deposit their metal contents where loeal eonditions favor sulfide preeipitation.

Modes of deposition

The possible eauses of the preeipitation of Pb and Zn minerals in Mississippi Valley-type deposits have

DIAPIRS IN S. E•OPE AND N. AFRICA 683

been listed by Heyl et al. (1959). Among these causes may be noted the lowering of salinity levels and of temperatures or pressures. This must occur when the dome approaches the surface. The extension fractures caused by hydraulic fracturing around the intrusions offer a channelway for the fluids and thus accelerate the circulation system at depth (Fig. 12). Mixing of the metal-saturated brine solution with surface water that is colder and undersaturated

in metals then induces the precipitation of the base metals. Metabolic reduction of the evaporites by micro-organisms provides the sulfur for the galena, pyrite, and sphalerite. The lowering of the fluid pressure also contributes to base metal mineral precipitation (P•lissonnier, 1982). The formation of cap rocks and the sulfur often associated with them is generally attributed to mixing of two distinct types of fluids, one, originating at depth, which is highly saline and the other, originating near the surface, which contains little salt (Ruckmick et al., 1979; Davis and Kirkland, 1979). Thus the deposition of base metal sulfides can occur under the same conditions as sulfur itself. 3

The above outline can account for the different types of base metal deposits found around diapirs. The stratiform sulfides in the Turonian and Oligocene beds could have precipitated from the fluids contained in an, as yet, unconsolidated sedimentary bed directly above or adjacent to the domes. About the same time as this syndiagenetic mineralization was produced, sulfide crystallization could occur in the spaces in the cap rock or in the fractures in the consolidated part of the cover.

The different modes of emplacement of this first generation mineralization do not preclude a subsequent redistribution of the metal stock. Remobili- zation could have occurred when a bed bearing an initial deposit was subject to solution or became karstic during emergence. Similarly, remobilization by the ascent of an initial mineralization may also be a possibility during a subsequent diapir phase.

Limits of the model

If the proposed model is correct, it must account for all the possible observations and especially the spatial relationship commonly observed between the salt diapirs and various base metal deposits. Although the previously mentioned relationships have been demonstrated in the area around the Mediterranean, it must be recognized that many diapir zones lack significant Pb-Zn deposits. However, they have the

a In the Tunisian cases studied, zoned cap-rock formations have not been encountered like those described from the

summits of domes in Germany or the Gulf Coast. However, the calcitization often observed on the apical rock of diapirs clearly seems to be the equivalent of that marking the classic cap-rock calcitic zone.

same structures and were probably formed in the same way.

Examples of diapir zones, with few or without sulfide deposits, include the Gulf Coast and the Gulf of Mexico, the Canadian Northern Territories, north Germany and the North Sea, the Persian Gulf and its borders, and the north Caspian depression.

When diapir structures are emplaeed in several stages, as often seems the ease, we have shown that the deposition of significant amounts of base metals could have occurred during the initial phase of ascent and piercement. This initial deposit may later be buried by local sedimentation, with the result that discovery of the mineralized zones can only occur if tectonic movements and erosion enable

them to be exposed. In the areas that are scarcely affected by compressional tectonics, it is possible that Pb-Zn deposits do exist at depth around the diapirs, either in solution or in the form of Pb-Zn- bearing minerals allied to related sedimentary or tectonic structures.

In the case of the Gulf Coast domes, the scarcity of sulfide deposits may be explained by the fact that Zn- and Pb-bearing fluids are ?apped in nearby hydrocarbon deposits. Migrations of these fluids through any deformation phase would cause both the precipitation of sulfides in the upper parts of the structures and the loss of most of the hydrocarbons. Thus it is not in oil fields that important Pb- Zn deposits may be found. The same reasoning could apply to other diapir and oil field areas which are characterized by the relative persistence of sedimentary fluids (Germany, the Middle East). Unfor- tunately, information about the Pb-Zn contents of these fluids is too scarce to conclude that base metal

deposits are absent. Nevertheless, no base metal anomaly seems to be indicated in these zones.

In the ease of northern Germany, it may be observed that evaporite formations differ appreciably from the European and American Tethys formations, especially by the presence of potassium salts. The nature of these brines and the porosity of the evaporite material is therefore very different. The same compositional difference probably applies to the south Ariantie salt diapirs.

Elsewhere, the presence of high local or regional thermal gradients is perhaps necessary for mineralization to occur. Regional thermal gradients are especially necessary for facilitating the movement of fluids and the deposition of their metal load. In this respect, it is dear that the Tethys zone from the eastern Mediterranean to the Gulf of Mexico

was the preferred location of regional thermal anomalies during the Triassic to Lower Jurassic, the Middle Cretaceous and the Eocene to Oligoeene periods. Suggestions as to why some diapir zones are ore bearing and others are not should not hide one fact: the ore-bearing diapir zones are restricted


to geographical domains that are preferentially enriched in certain metal concentrations such as lead

and zinc (Boyer et al., 1975; Routhier, 1983). This fact alone could explain why, outside these domains, diapir-rich zones are devoid of base metal deposits. Nevertheless, the limits of metal provinces must be determined with more precision in many regions around the world.

Conclusions

The principal characteristics of peridiapiric deposits and Mississippi Valley-type deposits show some striking similarities, as previously emphasized by Price and Kyle (1983). In the case of the Tunisian deposits which have been discussed here, some characteristics are similar to the exhalative (volcanic) sedimentary type of deposits.

The processes proposed herein for the genesis of peridiapiric concentrations could be used to explain similar concentrations on basin borders. It would

then be necessary to abandon completely the hypothesis of a nearby continent source of metallic elements and substitute a model involving base metal-saturated fluid ascent toward the basin edges. Within this framework, the present model is but a particular case of the more general model attempting to explain all strata-associated mineralizations. It would be interesting to be able to demonstrate a geochemical kinship between the fluids trapped in the ore minerals and the fluids from the bottom of

the basin. Unfortunately the latter fluids are inac- cessible.

The following have been identified as precondi- tions necessary for the formation of diapir-associated base metal deposits: (1) the existence of a basement complex, an evaporite bed, and a cover; (2) an appropriate arrangement of beds to channel the fluid upward; (3) the existence of saline fluids; (4) the presence of extension fractures that initiate a high thermal gradient; (5) the succession in the same zone of tectonic or epirogenic movements that facilitate remobilization; and (6) the appearance and maintenance of a contact zone (either at a point or along a line) between a deep zone with hot and saline fluids, and an outer zone with cold fresh water.

These conditions could explain the presence of peridiapiric base metal deposits as recurrent concentrations on a diapiric structure in a metal zone that was deeply buried under a thick sedimentary cover. This seems to be true in the case of the

Upper Jurassic deposits in the Vocontian zone and the eastern prolongation of the sub-C•vennes border, which itself is preferentially mineralized in Triassic- and Lower Jurassic-age strata. This is also the case in the Pyrenean-Cantabrian Cretaceous, and even

more in the diapir zone of Tunisia where the concentration reaches the Oligocene beds. Without diapirism it is unlikely that these deposits could have been exposed and thus reveal the presence of a buried lead-zinc zone.

Acknowledgments

We are grateful to Ali Attya, President General Director of the Office National des Mines of Tunisia

who kindly granted permission to publish this paper and to Pierre Routhier for his critical reading. We are also grateful to Alwyn Scarth for the translation of this article.

July 18, 1983; December 13, 1984

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pb-zn deposits and salt-bearing diapirs in southern europe and north africa...

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