Anciennes mines Plomb Zinc Tunisie

THE
LEAD-ZINC-BEARING
PAUL
DEPOSITS
OF
TUNISIA
SAINFELD
CONTENTS
PAGE
Abstract
..............................................................
Introduction
..........................................................
150
150
Geographic distribution of principal deposits ............................
History ..............................................................
Discovery and exploitation ..........................................
Evolution of theories of ore genesis ..................................
Geologic background ..................................................
151
151
151
154
158
Tectonic frame .....................................................
Tectonic units ...................................................
Structural
features
..............................................
Problem of the Triassic ..........................................
158
158
160
161
Stratigraphic distribution ...........................................
Character of deposits ..................................................
Types of deposits...................................................
161
165
165
Oxidation
.........................................................
Tectonic distribution
................................................
Character of ores ...................................................
169
169
170
Major elements .................................................
170
Minor
171
elements
.................................................
Gangue and accessoryminerals ...................................
Origin of the hydrothermalsolutions..................................
Age of mineralization ...............................................
Conclusions:Conditionsof deposition....................................
172
172
174
174
References
176
...........................................................
ABSTRACT
Metalliferous depositsconstitute an important factor in the economy
of Tunis. Iron-lead-zinc mineralization is representedby small scattered
depositsthroughout the northwesternhalf of the country. The deposits
are telethermaland related to the end of Alpine orogeny, Mio-Pliocene
in age. A convenient classificationis afforded by the structural framework; they are along great northeast-southwest
strike faults. The formations most favorable to mineralization are the Miocene (calcareous and
sandyto conglomeratic),the Senonian,the Aptian, the Jurassiclimestones,
and the marginal brecciasof the Triassic.
INTRODUCTION
EXTRACTION
of mineral productshasbeena major elementin the Tunisian
economy
thepast80 years. Mineralproduction,
in thousands
of tons,for the
wholemodernperiodup to January1954is givenin Table 1.
150
THE LEAD-ZINC-BEARING
DEPOSITS OF TUNISIA
TABLE
Products
I
Marketable tonnage
(In thousands of metric tons)
Phosphates of lime ................
151
Metal tonnage
(In thousands of metric tons)
79,365
12,067
Iron .............................
Lead ............................
Zinc .............................
782
315
Lignite ..........................
Pyrite ...........................
Manganese .......................
Copper ..........................
Mercury .........................
972
15
24
5
9
2
0.02
Strontium ........................
Fluorite ..........................
Barite ...........................
2.5
12
3
In market value, nearly 30 percent of the mean annual productionis at
presentrepresentedby lead and zinc.
GEOGRAPHIC
DISTRIBUTION
OF
PRINCIPAL
DEPOSITS
The metalliferousdepositsin Tunisia are locatedalmost exclusivelyin
the stronglyfoldedrocksthat occupythe northernpart of the country,i.e.,
betweenthe 35thand37thparallels,approximately
in a square200 kilometers
on a side. The only exceptionsare somesmall indicationsof lead-zincand
manganese
and mostof the great sedimentarydeposits(iron and phosphates)
of central Tunisia.
The principal depositsof the iron-lead-zincassociationare found, therefore, in the richestand most temperatepart of Tunisia, but also in the
difficultmountainous
regions. In this countrywhere the maximumaltitude
is at 1,544,many depositsare at an elevationabove1,000 m. Nevertheless,
the generaltopographypresentsa seriesof depressions
in a NE-SW direction,
traversedby the great routesof communication;the distanceto the shipping
port (Tunis) from the mostdistantmine doesnot exceed300 km. The railroad systemservesthe principal mines, but the most out of the way are
forty km from a loading station.
The mineralmap of Tunisia showsthe scattereddistributionof the hydrothermaldeposits. In the northernregion,amongthe hundredsof showings,
aboutsixty ore depositshavebeenor are in production. Only a scorecanbe
consideredas small mining districts with a total productionof more than
10,000tonsof metaland,amongthese,threeare really important(more than
100,000tonsof metal). The districtsare not distributedevenlyin the region; most of them are groupedin the roughestzone, that is to say in the
northwesternportionof the square,with a rather uniform distribution,at most
fifty km apart.
HISTORY
Discovery and Exploitation
The first Europeanprospectors
in Tunisia in the last quarterof the 19th
century were guidedby the old workingsthat marked most of the outcrops
152
PAUL S,4INFELD
Map of mine areas
inTUNISIA
,•
•
FIG. 1.
THE LEAD-ZINC-BEARING
DEPOSITS
OF TUNIS'IA
153
Of ore. Certainlead ore deposits,in particular,had dumpsand important
mineworkings(Djebel Ressas,Sidi Amor ben Salem,Bou Jaber,Khanguet).
These remainsare generallyconsideredRoman by our modernminers; but
somehistorians,like St. Gsell (21), think that "the most active epochfor
the mining industry in Barbary was the middle ages and not antiquity."
Howeverdocuments
are scarce;inscriptions
tell little and there is no indication that the North African mineral resourceswere utilized by the
Carthaginians,
or evenat the beginningof the Romandomination. The first
written recordsdatefrom the Christianera: texts datingfrom the 2nd to the
5th centuriesmakementionof sentenceto forcedlabor in mines;on the other
hand,someof the mineworkers'tools (like oil lamps), foundin the mines,
have been dated 2nd and 3rd centuries.
It is probablethat the Arab invasion,in the 7th century,causeda temporary cessation
of mining and it was not until the 9th century,that references
are foundin the Arab literature. With respectto Tunisia, all the texts from
the 9th to the 14thcenturies,mentiononly the minesof Mejjana ("Majjanael-Ma'adin")--referring to a district locatedon the Algerian-Tunisianfrontier, to the north of Tebessa,which may have includedthe minesof Mesloula,
Ouenza,Bou Khadra, Slata andBou Jaber. The latter place,whereextensive
mine dumpshave been found, may have been a center for ore treatment.
Another referenceis in the accountof the journey of Flamand Adome, who
notedthe activityof the lead mineof Djebel Ressasin June 1470.
It appearsthat mining diminishedgreatly in the 16th century,when the
Europeansbeganto importarticlesof metaland leadto North Africa. There
are no further recordsuntil the modern period except in the 18th century,
in theaccounts
by Pessonel(1724) andby Hebenstreit(1732), who described
the mine of Khanguet,and alsoin the accountby the EnglishmanShaw, who
tellsof Djebel Ressas;this is the localitythat yieldedthe Arab relics (lamps
and pottery).
In the 19th century,upon the arrival of the French in North Africa, no
mineswere in operation. Until the treaty of Kassar-Said(1881) only two
concessions
had beengrantedin Tunisia by the Kheredinegovernment:the
Djebel Ressasin 1869 and Djebba, in 1874.
Nearly all the mineraldepositswere discovered
during the modernperiod,
in the courseof the initial phaseof researchand exploration,from 1880 to
1905. The development
phase,from 1894 to 1914, was characterized
by an
almostcontinuous
increasein production(with a maximumof 37,400tonsof
zinc ores in 1912, and 59,500 tons of lead ores in 1913).
Then the war and
fall in pricesbroughton the first productioncrisisfrom 1914 to 1922, and
many mineswere abandonedduring this period. A notableresumptionaccompanied
the rise in prices. Productionincreasedfrom 1922 to 1930, with
a maximum of 40,000 tons of lead ore in 1924, and 30,000 of zinc ore in
1926.
The Tunisianmineswere at that time stronglyaffectedby the depreciation
of zinc prices,and next by the world economiccrisis,whichbroughtaboutan
almostcompletestoppage,from 1930to 1936. A new impulse,startingfrom
154
PAUL SAINFELD
TABLE
PRODUCTION'
II
OF PRINCIPAL
MIN.•-S
(Estimated in tons of metal)
Total production to
January 1st, 1954
Name
Maximum annual
production
Production
for 1953
of mine
Lead
Djebel Ressas
63,600
Sidi Amor ben Salem
Lead
Zinc
63,650
4,902
2,097
108,800
Djebel Hallour
Sidi bou Aouane
Khanguet Kef Tout
Sakiet Sidi Youssef
E1 Grefa
Djebel Trozza
Garn Alfaya-Koudiat
Zinc
el
90,700
72,800
34,700
34,800
56,000
51,000
37,800
5,277
120
38,000
27,700
3,217
3,600
292
1,362
3,950
4,338
6,300
2,819
26,200
505
4,357
2,867
Year
Lead
1902
800
1924
1,900
1934
1953
1900
1912
1951
1916
1916
Zinc
1,080
2,300
3,600
370
1,500
3,900
1,390
700
Hamra
Sidi Ahmed
10,400
Bazina
34,000
Bou Jaber
Fedj el Adourn
Djebel ben Amara
Ain Allega
24,400
16,200
100
13,300
7,000
13,000
24,000
9.400
2,895
348
Ressas-Touireuf
Zaghouan
20,800
1,100
700
16,800
2,000
Djebba
11,700
3,750
Oued Maden
15,200
Djebel Semene
12.200
Diebel el Akhouat
Djebel Lorbeus
Sidi et Tara
3.800
1.300
10.800
1,653
2,150
1,821
840
1912
130
1,743
3,581
1,753
1,964
76
1,457
4.400
7,600
10,000
400
65
620
1898
1,500
1.613
1912
1909
1905
1912
1953
1895
1909
210
400
2,000
120
1941
1,200
1953
4.400
1953
1920
400
1930
230
1.500
1933, was checkedby the secondworld war and by the campaignin Tunisia.
Recoverybeganduring 1944and the productionfor the year 1953 surpassed
the prewar level.
The graphs (Fig. 2) showthe importanceof the TunisianmetaI production in the North African economy. At the first of January,1954,the probable tonnageof metal in reserveswas on the order of 160,000tons of lead and
60,000 of zinc.
Evolution of Theories of Ore Genesis
As early as 1852, Ville and Coquandobserved,in Algeria, the relation
betweenthe lead-zinc-bearingbeds and the Triassic formations,particularly
the ophitesthat they contain. Peron stated,in 1883 (32), "It is to the eruption of these spilites,or rather to the plutonic phenomenawhich accompanied it, that is owed the remarkablepresenceof these depositsof minerals, iron, lead, and copperin the midst of the upper beds of the Miocene
series." On his trip in 1894,Levat was impressedby the depositsof zinc and
the "eruptive gypsums,"associatedwith ophites, in Algeria as well as in
Tunisia (30).
But the Triassic was identifiedas suchby Marcel Bertrand, at the time
THœ Lœ.4D-ZINC-Bœ.4RING DœPO$IT$ OF TUNISL4
155
of the specialsessionof the GeologicalSocietyof France, in Algiers in 1896
and this identificationwas soonextendedto Tunisia by Pervinquiere(33),
who stated,"... The frequentrelationbetweenthe Triassicand the calamine
deposits. . . one conceives,indeedit follows,that the faults which usually
borderthe Trias in this region have given passageto waterschargedwith
mineralizers."
L
TUNISIAN
PLUMBO
-ZINCIFEROUS
PRODUCTION
Thou,sar•d.•
ort0.,
'•
Tunisian
(in market,able ores)
,
,
1900
retailed
'
f9fO
,
,•.... • .•.-.•,--' ,
f9aO
Produc[;on
t930
COMPARATIVE
o½1953
•oYA
three
TOTAL
PRODUCTION
countries
o•
North
on January •st,1954
LEAD
colourless.
f950
•n thousandsoF metal-tons)
o• the
Lead:
f9•O
Zinc:
hatches
with approximative •igures
(in hundredsoF metal-tons)
Fzc.
2.
ZINC
A•rica
156
PAUL oeAINFELD
However, Pierre Termier, then advisor to several mining companies
refusedto admit these faults and in 1895 explainedthis triasificationor
"maladiedu Trias": "... the Tunisian zinc oreshad as their original source
the Triassicrocksthat musthaveincludeda stratumof sedimentaryzincblende
emplacedas an overthrustnappe. This Triassicwas uncoveredand reworked
by erosionat a recentperiod; the blendewould thereforehave been dissolved,
and reprecipitatedin the form of calamine at limestonecontactsin rocks of
all ages." This theorywas recalled,in 1913,by Louis de Launey,who added,
however, that one could "explain the formation of the same ores by the
actionof hydrothermalcirculationalongthe linesof fracturecorresponding
to
the boundariesof Triassicblocks"(29).
Accordingto J. Berthier (2), all the Tunisian metalliferousdepositsare
dueto the Triassic;this stratumwasmineralizedby sulfides,at the periodof
its formation,or shortlythereafterand gaveup a part of its mineralizationto
the adjacent formationsby the action of circulating waters that have penetrated throughthe upper part in many placesby reasonof the inclinationof
the Triassic and the recumbentfolds (2).
In 1918,A. Brivespostulated
that an intensemineralization,
resultingfrom
the destructionof an old Hercynianmountainchain,had accumulated
at the
centerof depressions
andTriassiclagoons;the veinlikedepositsresultedfrom
the tectonicdraggingout of the deepestTriassicdeposits;the impregnation
depositsand replacements
resultedfrom the reaction,on the adjacentbeds,
of the watersthat had circulatedin the Trias; finally the sedimentarydeposits
resultedfrom the erosionof thesesameTriassic deposits(8).
L. Berthon (3), who gavea detaileddescriptionof the Tunisianmineral
depositsin 1932,considered
that "the persistence
of strict relationsof proximity so plainly shownbetweenthe Triassicsediments,
of which the bulk
remainsbarren, and the great majority of developedmetalliferousdeposits,
appearas a geodynamic
ratherthana geneticphenomenon.
It is the great
faults and fracturessubsequentto the formation of the Trias that have
channelled
the hydrothermal
circulations,
chargedwith metallicfluids,and
produced
thealteration
of thecontact
zoneson theborderof theTriassic,the
metalliferousincrustationsof the fractures,and the replacementof certain
stratafavorableto percolationby water" (3).
In a supplement
to histhesis(1927) M. Solignac(41) recalledtheseideas
and admitted that most of the Tunisian depositsare the "abnormalcontact
depositsand the fissureveinsbranchingfrom them....
It is undeniable
that the maximumof mineralizationis observedin the regionswhere the great
lowerTriassicnappeis bestdeveloped
....
This mineralization
thinsout in
proportionasthe nappebecomeautochthonous.In the southwherethe great
tangentialphenomena
are almostabsentthe large depositsof lead and zinc
ßores do not occur" (41).
However,L. Joleaud(25) pointedout in 1929that manyTunisiandeposits
are related to fractures in Miocene
strata situated far from the Trias.
In
1932,he pointedout the role of the Atlas tectonic;"the mineralization,a phenomenonquite late in the Pyrenco-alpinetangentialaction,was here particu-
THE LEAD-ZINC-BEARING DEPOSITS OF TUNLS'IA
157
larly activelaterally(that is to sayin its geographic
form) and alsovertically
(i.e., by reasonof its manifestations
of localandregionalintensity)"(27).
In 1932-1933,E. Bruet (9) attemptedto provethat the accumulation
of
the upperportionof the depositswasdueto a supergene
phasemainlychemical, followinga hypogenephasehydrothermalor otherwiseand that it had
beenfavored,in Tunisia,by climaticinfiuence.s
(9, 10, 11). As a result of
his examinationof the world distributionof lead-zincdeposits,F. Blondel
(5) pointedout that the occurrencein North Africa is entirely in the upper
part of the Mesozoic:"Everything takes place as thoughoriginatingat the
level of the Paleozoic the zinc and lead mineralization becomesdispersed,
scatteringprogressively
as it risesin the youngerzones"(5).
At the sameperiod, L. Glangeaudsuggesteda more or less remote relation betweenthe ore depositsof northernAlgeria and the eruptiverocks,
and proposedthe followingexplanationfor the depositsin relation to the
Triassic: "The mineralizationcould have taken place at either one of two
periods. In the depositswhich I shall call the primitive, the ore was depositedat thetimeof the intrusionof the eruptiverocks,eitherat their contacts
directly in the Triassic, or within the Lias, by fumerolic or hydrothermal
action. In what I shallcall emigrantdeposits,the mineralizationdue to eruptive intrusions,was originallylocalizedin the Triassicgypso-saline
complex,
then it was subsequently
distributed,followingthe uplift of the Trias, in the
diapir folds and by meansof the circulatingwater taking up the chemical
elementsof the Trias" (15).
However,L. Calembert(12) thoughtthat therewas generallytoo much
deferenceto the aberrantrole of the Triassicand showedthe importanceof
the part playedby the lenticularcalcareous
formationsin the localizationof
certainore deposits. Followinga tour of North Africa in 1949, J. Bouladon
(7) presented
a synthesis
on the leadand zinc depositsas a wholein which
he showedthe geographicdistributionof the mineralizedregions,plating
the "lead pole" at Oujda, in northernMorocco,with three satellites(to the
east,Midelt in centralMorocco,to the west, Beja-Mateur and the AlgerianTunisianfrontier); he alsogavea classification
of the typesof deposits
and
attemptedto establisha correlationbetweentheseand the texture of the
ores and also their tenor in minor elements.
Finally, in 1951,followingtheir detailedwork on the depositsand the
B.P.G. (blende-pyrite-galena)
ore of the Teboursouk
region,J. Bolzeand
Schneiderh/3hn
(6) demonstrated
the doublerole, tectonicand chemical,of
the Triassic,and appliedto theseTunisiandepositsthe idea developed
by the
Germanmetallogenist
in 1941on "secondary
hydrothermaldeposits"in zones
of active dislocation. "The fractured zone at the contact of the Triassic,
with its lode walls, formed at the beginningof the Tertiary, gave an easy
passage
to externalwaters....
Thesehot salinewaterseasilydissolvedand
carried away the sulfidesof the mineralizationsof the basement. The secondary depositionin the overlyingseriesis naturally accomplished
after the
finalemergence
of the regionandthe lastorogenicphase,in the samechannels
of circulationat the edgeof the diapirs."
158
PAUL SAINFELD
GEOLOGIC
BACKGROUND
Tunisia is essentiallya sedimentaryterrain; the section, including
Mesozoic,Tertiary and Quaternary deposits,is very thick. Only in the
north are there outcropsof intrusiverocks. These do not seemto be related
to the metalliferousdeposits,whichare all situatedin the sedimentaryformations, with the exceptionof the iron-copperindicationsof the Galite and
Nefza.
There are, in Tunisia,the classictypesof replacementdepositsand veins,
but their distributionis hardly ever sharplydistinguished.Generallythere
is no preponderance
of oneor of the other,but a type havingmixed characteristics;on onehandveinswith filling of the adjoininginterstices,on the other
handimperfectlydevelopedreplacement.Anotherlesscommontype is quite
widespreadin Tunida--the so-calledfissure impregnationdeposit, which,
moreoveris itself associated
with other types of deposition. It is therefore
quite difficultto classifythe Tunisian depositsaccordingto their type, but
examinationof the tectonicframe providesa simpleclassification,
which, in
a certain measuredoes not exclude the convergenceof certain dominant
characteristics
of the deposits.
The TunisianDepositsin Relation to the North ,41•ricanTectonicFramework
Tunisia represents
only the extremeeasternpart of French North Africa
at the borderof the Alpine Zone,.and cannotnaturally,be regardedas part
of the Algerian Moroccanstructures.
TectonicUnits.--Writers have definedthe followingtectoniczones,from
North to South (13, 14):
MOROCCO
ALGERIA
TUNISIA
--
I
Rn*
DORSAL
KABYLYE-EDOUGH
TELLIAN
II
MOYElq
IV
ATLAS
GALITE
KROUMIRIE-NEFzA
and BORDER
ATLAS
HIGH
III
THE
PLATEAUS
•BEJAOU
TUNISIAN Northwestern-•OU•GH•,
ISV•CLINORIma
HIGH ATLAS
S•H•AN ATLAS
.
ATLAS
rAN'TICLIIqORIUM
SoutheasterndCENTRAL1 EASTERN
TUNIS
I.SoUraERN
V
A•rr•-ATLAS
and
SAHARA
SAHARAIq PLATFORM
TUmS
THE LEAD-ZINC-BEARING
DEPOSITS OF TUNISIA
159
In Algeria, the structureis superimposed
on the paleogeography.Between zonesI and II is the Tellian trench, which correspondsto the subRifaine trench in Morocco,and the Tunisian trench to the east. But on this
latter unit, in Tunis, are superimposed
the great obliquetectoniclines,which
have giventhe countryits presentaspect. In effect,the foldsthat trend eastwestin Algeria,swingto the northeastin the vidnity of the Tunisianfrontier.
Their flanks are generally asymmetrical,overturned toward the southeast,
representingthrustingfrom the north againstthe central horst, evidentsince
the Aptian.
This definitivestructureof Tunisia, outlined in a pre-Burdigalianstage,
was emphasizedby folds later than early Quaternary. The mineralization
appearsrelated to this last manifestationof Alpine orogenesis,and as the
depositsare alignedin the northeast-southwest
folds,it is proposedto classify
themhereaccordingto tectonicunits. Theseare, proceedingfrom the northwest to the southeast:
-The Zone of CrystallineMassi.fs,represented
in Tunisia only by the small
archipelagoof the Galite, is formed by the tertiary eruptiverocks (granodioritesand microgranites),with a more or less metamorphosed
Oligocene
cover. Only somesmall iron-coppervein depositsare known there.
The Kroumirie-Nefza Zone, which comprisesthe northeasternfrontier
regionand the coastalregionfrom Tabarka to Bizerte, is madeup essentially
of Oligoceneformations,the lensesor sheetsof "extrusive" Triassic, and the
dacite and basalt spursof the Nefza. This zone containsthe main line of
deposits:Sidi Driss, Ain Allega, D]ebel Diss, Oued Maden, distributedalong
the length of the principal Triassic layer, which takes all the Kroumirie in a
slant,from the Mediterraneancoastto the regionof Souk Ahras. This zone
comprises
the impregnationdepositsand replacements
by blende-pyrite-galena.
The boundaryof this zone,markedby Triassicsheetsand abnormalcontacts,
contains,in its immediatevicinity, the districtsof Bechater,of ztmdouns(Sidi
Ahmed,Khanguet,Ben Amara) and of Fedj •Issene.
All the rest of folded northern Tunisia representsthe Tunisian Atlas,
whichcomprises(1) a northernarea partiallycorresponding
to the Tunisian
trough,characterized
by a thick Cretaceoussequence(6,000 to 12,000m) of
bathyalfacies; this is traversedby sheetsor slicesof "extrusive" Triassic,
in placescroppingout very extensively;(2) a southeastern
part marginal
to the Sahel and to the Saharanplatform.
The NorthwesternAtlas hasbeensubdivided,somewhatarbitrarily,into:
(a) The Sub-Zone of Thrust Sheets (•cailles)/ from Bizerte to the
Ouargha,characterized,especiallyin its northeasternsection,by numerous
thrust sheets(•cailles) of Eocenelimestones.This showstwo alignments
of deposits,having perhapsa commonorigin, toward the Northeastin the
vicinity of Mateur, the districtsof E1 Grefa, and Graria, on the edgeof an
important Triassic sheet. Toward the Southwest,a first branchcontainsthe
z l•caille, literally shell or scale, is here translated as "thrust sheet" for want of a better
term. According to Goguel (Trait• de Tectonique), dcaille is used in several senses: an assemblageof folded bedsboundedby a thrust fault; a fragment of a nappe isolatedbetween two
other units; a secondarynappe from subdivisionof a major nappe by refolding.--Editor.
160
PAUL SAINFELD
deposits
of Sidi Bou Krim, Bazina,Tabouna,then,the Djebel Sabbahand
the Djebel Bou Sebouaon one side,the Chara, Djebel Hallouf-Sidi Bou
AouaneandTouireufon the other;the second
comprises
the Semene,which
1.ikewise
canbe subdivided,
onepart with Ain GrichandE1Haouaria,theother
partwith Djebba,NebeurandZag andTir. This sub-zone,
essentially
high
in lead,terminates
in Tunisiaon the frontier,with the blende-pyrite-galena
depositsof Sakiet-Koucha,
somewhataberrantmineralogically
(the paragenesis
is alliedrathermorecloselywith the deposits
of the followingsubzone), but in tectonicbearingfrom Touirent.
(b) The Sub-Zoneof the Synclinorium,
characterized,
principallyin the
Southwest,
by the presence
of Cretaceous
brachyanticlines
and the great
Eocenesynclines.It alsocomprises
two principalalignments,
markedby the
Triassicsheets,with a commonorigin towardthe northeast. At the latitude
of Medjezel Bab,thealignment
E1Alia-Djedaria-E1
Ahmeradividedinto one
part with the depositsof Fedj el Adoum,Kebbouch,
Dogra, Sidii, Garn
Alfaya; the otherpart with thoseof Kef Lasfar, Knana, Koudiat Safra, El
Akhouat,Bou Khil, Bou Grine,Lorbeus,Slata,Bou Jaber. This sub-zone
comprises
replacement
depositsas well as impregnation
of B.G.P. at the
Triassic contact.
Towardthe southeast,
a strip of nearlybarrencountry,aboutforty km
wideextendingfromTunisto Thala,separates
the unitsdescribed
abovefrom
theSouthwestern
part of the TunisianAtlas. This region,characterized
by
Jurassic
outcrops,
alongthe importantstrikefaults,with impregnations
and
replacementsof lead or of zinc, has likewise been subdividedinto:
(a) The Sub-Zoneof the Za#houan-Kessera
Anticlinoriumis well de-
finedin thenortheastern
part,whereit merges
with the Jurassic
andAptian
massifsof the Tunisiandorsal:Bou Kornine,Ressas,Zaghouan,Djebilet
el Kohol,Serdj; there,it dividesintotwo branches:
Belouta,Sekarna,Azered,
HamraandKef Chambi,separated
by a vastsyncline
havingon its margin
the districtsof Sidi Mabrouk, Ain Khamouda.
(b) The Sub-Zone
of theFoldsof CentralEasternTunisia,whichbegins
at thenorth,ontheflankof thedorsal(HammamJedidi),thenis moreclearly
defined
at theedgeof theKairouantrench,by thealignment
Trozza-LabeiedMhrila-Noubain a southwest
direction,and Loridga-Touila-Mezzouna
with
northerlystrike.
(c) To the south,SouthernTunisiacomprises
foldstrendingE-W, with
the sedimentary
deposits
of iron and phosphates.
Finally, the SaharanPlatform,which occupies
all the southernpart of
Tunisia, is characterizedby nearly flat Mesozoicformationswith considerable
cover. Naturally this undisturbed zone affords few indications of mineralization.
Thus, most of the metalliferousdepositsare locatedin the northernpart
of the TunisianAtlas. Sinceigneousintrusivesare unexposedin this region,
and would be found only at a great depth, this localizationof mineralization
is probablyof purely tectonicorigin.
StructuralFeatures.--It is in this zonethat the two Atlas chainsof Algeria
reunite,after curvingtoward the northeastnear the Algerian-Tunisianborder.
,
THE LEAD-ZINC-BEARING
DEPOSITS OF TUNISIA
161
This curving is especially'markedby two seriesof faults: longitudinalones,
which are the more importantbecausethey are commonlythe loci of Triassic
"injections," and therefore have penetrateddeeply; the others, transverse,
commonlydiminishingto fractures,but perhapshaving reactivatedthe first
set in accentuatingthe uplift of the Triassic. The zonesof rich mineralization
generally occur at intersections,commonly in associationwith two later
systemsof slip fracturesat 45 degreeswith the principaldirections.
It is difficultto date thesefaults; the pre-BurdigalianAlpine orogenywas
precededby preliminary phases,beforethe Cenomanian,and before and after
the Lutetian, and locally there is evidenceof the influenceof thesedifferent
phasesin the tectonicsof the region,in particular,the diapirismwas certainly
started at the Pyrenian phaseat least. On the other hand, the major paroxysmal phase at the close of the early Quaternary was followed by local
subsidence,which still continues. Northern Tunisia has therefore been an
area of continuousinstabilitysincethe beginningof Tertiary time.
Problem of the Triassic.--The Triassicin North Africa posesa problem
that is essentiallystructural. Relationships
betweenthe Triassicmaterialand
the mineral depositshave long beenrecognized,and it has beenthoughtthat
it might have facilitatedthe upward movementof emanations. The Triassic
material is literally injected into longitudinalfaults and the successive
de-
formationsup to the recentparoxysmalphasehaveaccentuated
this structure.
There is reason to believe that it was then that the mineralization
was de-
positedat the contactsandin the vicinityof the Triassic,whichit accompanied
in its upwardmovement. The Triassiclayersvery probablyrepresenttrue
"livingfaults" (36) activefrom Cretaceous
up to the presentperiod,and one
might imagine that they have maintaineda continuousrelationshipwith
the faultsof the basement
rocks,the hydrothermal
introductions
havingbeen
derivedfromold mineralization
in the basement(6, 15) or evendirectlyfrom
deepintrusivemagma. This problemwill be discussed
in detail below, but
it maybe notedherethat the Triassicinjections
andthe hydrothermal
introductionswere undoubtedlytwo resultsof the sametectoniccause. Is there
entire dependence
of mineralizationon extrusivephenomena?One cannot
affirmthatthisis soin everycase. It is obviously
enticing,in a countrylike
Tunisia,whereigneousmagmasare at greatdepth,to substitute
for themthis
perfectsuccession
of the "intrusive"rocksviz., the "injected"sedimentary
Triassic.
Stratigraphic Distribution
The geologicmapof Tunisiashowsthe extentof the Cretaceous,
Tertiary
andRecentoutcrops. The Jurassicis but little knownexceptin the northern
and dorsal massifs,and in somepoints in central Tunisia. The Triassic is
almosteverywhere"extrusive"as diapirsin abnormalposition,emphasizing
the recentorogeny.
If one examinesthe stratigraphicpatternin comparison
with the metal-
liferousdeposits
of NorthAfrica (Fig. 3) onenotesthatin Moroccoonlythe
Paleozoicand Liassicformationsare mineralized.Most of the Algerian
162
PAUL SAINFELD
depositsare foundin the middleJurassicand the lower Cretaceous,
whereas
in Tunisiathedeposits
areconcentrated
in theupperCretaceous
andTertiary
rocks. One seesa shifting of the mineralizationfrom the Paleozoicto the
Mio-Plioceneon passingfrom west to east in North Africa, in accordance
with the statistical
distribution
of outcropsin eachcountry.
STRATiGRAPHiCAL DiSTRiBUTiON
Of
MAiN
LEAD -ZINC
NORTH
MOROCCO
•,
--
AFRICAN
DEPOSITS
ALGIERS
I--.--=•s•,
TUNI$iA
I --
I •=•. , r•l•., I --
I
( 11•estone•
ann
I'•
8ffi
•0•
NummulKic
(•rl,
aria
limestones)
•
O•
I
•;•o•i•
•NG•
(l•mestones
and
I
,
•0•
•o•
•
o=
•KZ• - KO•
G•G0•
C•nomani•
I
(Albo-Ap•i•n
lime,
cones
and
•LO• I•
•O J•B•
•.
(limestones
*
I
(brec•a)
BOU
I'AOUZ
Fro.
3.
THE LEAD-ZINC-BEARING
DEPOSITS OF TUNISIA
163
Althoughthe controlof mineralization
by physicallyfavorablerocks,such
as'disturbedmicro-brecciated
bedsor chemicallyfavorablerockssucha•
limestonesand dolomites,is strictly applicablein Moroccoand the greater '
part of Algeria, certainexceptionsto it can be found in Tunisia.
The Permian cropsout only in the Sahara, in the small anticlinalrange
of Tebaga, where it consistsof fractured dolomitesand limestones,with a
small zinc-bearingvein deposit.
In northernTunisiathe Triassicis almostexclusively"injections"through
the overlyingformationsalongfaults, especiallystrike faults. It commonly
consistsof a bi-coloredargillo-gypsiferous
brecciacontainingblocksof dolomite and sandstone.It is itselfrarely mineralizedbecause
of its impermeable
naturebut,.quite anomalously,
sandyclaysand especiallycertaindolomitic
bars or breccias,particularlyimportantin the al10chthonous
brecciaand always closeto the contact,carry workablestockworksand replacements
(Ain
Allega,Bazina,Bechater,S.idii,Dogra,Zag and-Tir, Loridga); the extension
of suchdeposits,
is alwayslimited. But the marginsof the lensesof Triassic
at their contacts
with highlycontrasting
formations
commonly
carry an autochthonousfriction brecciacontainingenrichedzones.that are commonly
lenticularand are locallycalled"contactdeposits"(g•tesde contact). Most
of the B.G.P. deposits
of the southernpart of the TunisianAtlas belongto
this type,-commonly
associated
with depositsin transverse
veins (SakietKoucha,Fedj el Adourn,Kebbouch,Garn Alfaya, Kef Lasfar, Knana, E1
Akhouat,'BouGrine,Lorbeus).
Jurassicformations
rarelyoutcropexceptin the northernendof the axial
anticlinorium
of the TunisianAtlas; they are composed
of hard limestones,
eitherLiassic(Grand Zaghouan,Djebilet el Kohol) or Portlandlimestone
(Ressas,Sidi et Taia, HammamZriba), very favorable
to lead-zincreplacement deposits.
The Neocomian,
essentially
marty,is rarelymineralized.It is, however,
in limestones
of lowerCretaceous
agethat the petroleum
well drillednearE1
Akhouat,has cut, at 2685 m, a stockworkof sphalerite,calciteand galena.
It is the samewith the bathyalAptJanof the Tunisiantrench,but toward
the south,this formationbecomes
favorableto replacement
depositseither in
the sub-zoneof the synclinorium(hard limestones
of Slataandof Bou Jaber),
or in the anticlinoriumand in centralTunisia (limestonesand dolomitesof
Serdj, of the Azered,of the Hamra, Kef Chambi,Trozza, Ain Nouba and of
Touila).
The Cenomano-Turonian,
very marly in the north, containsthe curious
impregnationdepositof the Djebel Diss. It is morecalcarious
in the northern
part of the Tunisian Atlas, and containsseveralvein depositsand stockworks
associated
with the "contactdeposits"(Fedj Asserie,Djebba, Nebeur, Fedj
el Adoum,Garn Alfaya, E1 Akhouat); towardthe south,it becomes
dolomitic,
with' replacemeht
deposits(Kef Chambi). The smallmanganese
depositsof
southern Tunisia also occur in this formation.
The Lower Senonianis alwaysmarly, and only the impregnationdeposit
of Oued Maden in Kroumirie
is known there.
The.Upper Senonianshowsmany vein deposits(with cavity fillings) in
164
PAUL SAINFELD
THE LEAD-ZINC-BEARING DEPOSITS OF TUNISIA
165
its upperpart whichconsists
of the ratherhard massivelimestones
of the
Maestrichian in all the northern zone of the Tunisian Atlas (Andouns,
Tabouna, Chara, Hallouf, Sakiet, Lorbeus).
The Eocenecontains,in its middlepart, the massivelimestoneswhich,
althoughof favorableappearance,
are rarely mineralized(Touireuf, Sekarna).
However, the baseof this seriescontainsall the thick sedimentarydeposits
of iron (Djebel Anh) and of phosphate.
Depositsin the Oligoceneof the Kroumirie-Nefzazoneare unknown;in
the Tunisian Atlas this formationcontainsthe small replacementdeposits
of KoudiatSafraandof Djebelel Abeid. The sandstones
of the Burdigalian
containseveralsmallreplacement
deposits
in the northernpart of the Tunisian
Atlas (El Alia, Diedaria, E1 Haouaria).
Finally, the Mioceneat the north of the samezone containssmall replace-
ment depositsand relativelyimportantinterstitialimpregnation,
(Djalta,
Semene,Sidi bouAouane).
It appears,then, that there is a distributionof mineralizationthroughout
the whole stratigraphiccolumnof Tunisia; the principal depositsare im-
pregnations
at the "Triassiccontact,"replacement
deposits
in the Jurassic,
the
Aptian andthe Miocene,andvein depositsin the upperCretaceous,
as well as
the cavityfillingsassociated
with all thesetypes. Despitethis distribution,
all
thehydrot•hermal
deposits
areprobably
of thesameage.
CHARACTER
OF'..DEPOSITS
Types of Deposits
Replacement
depositsproper occurmainly in the limestones-and
sandy
conglomerates
in the smallMiocenesynclinesof Bejaoua(Djeb•l- d -Grefa,
Djebel Semene,Sidi bou Aouane) where the ore is very fine-grainedand
almostentirely of lead (Figs. 4.1, 4.2 and 4.3). A more classictype of
replacement
with largecrystalsoccursin association
with otherkindsof depositsin Aptian limestones
and dolomiticlimestones
towardthe southeastern
FxG. 4.
Aa
Various recent cover (crusts, scree,soils).
pq Early Quaternaryand Pliocene(lacustrinebrecciatedlimestones).
m• Mio-Pliocene(clays and hostseriesof sandstone,
conglomerate
and lacustrine
limestones).
exv-Cø Marls at the transition from Cretaceousto Tertiary.
c•
Upper Senonian(host rock series of.massivelimestones,with marly and
marl-lime intercalations).
c7 Lower Senonian(marls with somelimestoneintercalations).
c•q• Aptianhostrock(alternations
of limestones),
qu'•rtzites
/•d marls,with
massive subreef limestone).
C•v-v Neocomian(marls and marly limestones
).
js-, Portlandianhost rock (.massivesubreeflimestone).
t Triassicdiapir and extrusive (gypso-dolomiticbreccias)
Black = mineralized
zones.
(e = Pb; B.P.G. = Blende, Pyrite, Galena)
Vertical scale equalshorizontal, shown at the side.
166
Pat UL S•tlNFELD
- .. '•
.
'%3'
'•
. -• •
•z-. :...•
..
:
:• ..:.
:'•:f•.. •7•
*',•.."*:."';
i C
ii
i
r r
..
r ,it
r
r r
,
,
, rr
:.:.;% ß ...-. .:.:. ß
.'-'
';5' •'•,
'
4...•.... "•.:.':'
.. 7,::./?..'
.•-
....:..":*i' '*•
,, ,,
., ,-
..... '
..'•....
?:::;'::'-
"
.:
.
.
•
•
•.
.
*•'-.* :....
-.;:.;.:.
.3
.
THE LEAD-ZINC-BEARING
DEPOSITS OF TUNISIA
167
end of the TunisianAtlas. Zonesof veinsmayoccuralongwith replacements
as
at Djebel Slata (Fig. 4.5) but a rather commonoccurrenceis galenain caves,
ßpipes, or rugs of karst origin, indicatingthat the mineralizationwas later
'-::than
oneor moreemergences.
Somenodules
of massive
galenaweighing
up to 40 kg (Fig. 6.1) are moldedin karst cavities(Djebel el Hamra). Certain beds (Slata) or walls of cavitiesare lined with galenacrystals,someof
themenormous(Fig. 6.2). One cavity,at Sidi Bou Aouane,yieldedbeautiful stalactites
showingin crosssectionone or two concentric
rings of galena
in calcite(Fig. 6.3).
Simplevein depositsare not very commonbut clean-cutveinsare known
in the Senonianlimestones,
as at Fedj Assene,wherethe mineralization
is
largelyblende,and at Djebel Hallour (Fig. 4.3), whereit is of lead-arsenic
and highly oxidized. But the beautifulclassicB.P.G. lodes,concretionary
and crustified,well knownin the limestones
of the upperCretaceous
of the
Kef region(Fig. 6.5), are associated
with the impregnations
alongfissures
at
the Triassiccontact(Fedj el Adourn,Djebel el Akhouat,Sakiet-Koucha)
(Fig. 4.4).
The type intermediate
betweenveinsand replacement
depositsis well
represented
in North Africa: this is the fissuredeposit(#•te de cassure)of
the Moroccan
geologist
(7). Its aspect
is vein-likewith brecciafillings,but
thereis alsoreplacement
or impregnation
of the lodewalls,as well as filling
of the cavitiesin the vicinityof the fracture;thismixedtypeis common
in
the Senonian
limestones
(Amdouns).
Finally,a fourthtypeof depositpeculiarto Tunisiaandto the Constantine
consists
of fissure
impregnation;
it is nota truereplacement
buta filling,by
way of very fine stockwork,of thin fissuresof millimetersize, without
preferredorientationin the varioustectonicbreccias,either in the broken
zonesat thecontact
of theTrias (commonly
witha vein-likeextension),
or in
broken
thrustplates
ofEocene
(Ressas-Touireuf)
or Jurassic
(DjebelRessas)
limestones
(Figs.4.6 and5.6).
Theseimpregnation
deposits,
aswellascertainveins,maycontain
lenticu-
lar zones
of enrichment
(the"colonnes"
oftheminers),generally
at theinterFxc. 5. Replacementdeposits.
1. ContinentalMioceneof the Djebel Semene.--Redconglomerates,
with idiomorphicand aciculargalena (white), arounda limestonepebble:Reflectedlight.
X 150.
2. Oligoceneof KoudiatSafra.--Sandstone
with bandsof galena,preferentially
in lignitic partings. X 4/5.
3. Cenomanianof the Kef Chambi.--Dolomiticlimestonewith runs of galena.
X 1/3.
4. Aptian of the Djebel bou Jaber.•Limestone with crystalsof galena in the
ground mass.
5. Aptian of Sidi Amor ben Salem.•Dolomitic limestonewith galena and
baguettesof quartz showing rhombohedralforms of the carbonates. Polarized
light. X 140.
6. Depositsof impregnationof disturbedzones. Jurassicof the Djebel Ressas.
mBreccia fragments of white calcite in large granules, and fine cementof black
calcite, galena and sphalerite. X 2/3.
168
?.-• UL S,4IA?FELD
,..
.•.
.:.
*; ;'4i
.... ,•,-•' b-c
, :"
.:¾'•. ai;,•4:..,-,.'-..
':
.., .:...;:.':;.-:"
'{; •,.7' ";;?
"ic,-.f•**'
4
FIG.
6.
....*:'..?::*k•;
•i•/'
:8:'.:'
THE LEAD-ZINC-BEARING
DEPOSITS
OF TUNISIA
169
sectionof faults, veins or fractures. It is possiblethat these openingsare
at least partly of karst origin.
Oxidation
In certain Tunisian deposits,gossansare mostly absent,except for the
outcropsof certainveinswhere they are alwaysferro-calamines,
with galena
boxworks,as at Djebel Hallour and at Sakiet. Nevertheless,oxidizedzones
are generally very important in a limestoneenvironment. They are composedessentiallyof carbonates--smithsonite
(with hydrozinciteand calamine)
andcerusite(with anglesiteand residuesof galena). No zoneof cementation
is recognizedin the Tunisiandeposits.
The limit of the oxidizedzoneis, generally,aboutthe groundwatertable.
However,in a marly environment
the oxidationis generallyvery superficial,
and the sulphidezone in blende-pyrite-galena
or in blende-galena
extends
almost to the outcrops (Oued Maden). There are, however, several lead
depositswith deepoxidation,at the edgesof collapsestructures,in particular
of that of the Medjerda (Djebel Hallour and Touireuf). Here the oxidized
zonereaches
at leastto a hundredmetersbelowthe presentgroundwater
table.
This is the caseonly in a limestoneenvironment;at the flank of Djebel
Hallour, the depositof Sidi Bou Aouane,with marly walls is practically
withoutan oxidizedzone (Fig. 4.3).
Tectonic
Distribution
It is interestingto note that to someextent the mineralogicdistribution
follows the tectonic lines.
The Kroumirie-Nefzazone containsthe B.P.G. and B.G. deposits,with
very fine crystallizationonly slightly oxidized, with mercury, arsenicand
antimony. The margin containsfew deposits,except lead and zinc, and
little iron; theseare in all casesstronglyoxidized.
In the Tunisian Atlas, the sub-zoneof Bdjaoua-Ouargha
is almostexclusivelylead-bearing(at least, at the presentstageof depth exploration).
The synclinoriumcontainsB.P.G. depositsof rather coarsecrystallization.
Finally, the anticlinoriumand centralTunisiagenerallycarry simpledeposits,
FxG. 6.
Deposits of solution cavities.
1. Aptian, Djebel el Hamra.--Kidney of galena, encrustedwith cerusite,showing forms characteristicof karst cavity. X 1/8.
2. Aptian, Djebel Azered.--Group of galena cubes,ericrustedwith calcite, in
the wall of a cave in Aptian limestones. X 1/5.
3. Miocene of Sidi bou Aouane.--Stalactite of calcite (seen in section), with
two concentricrings of galena (solution channel in lacustrine limestone). X 2.
Veins
4. Eocene, Ragoubet Halgen Naga.--Vein of calcite with pisolites of galena,
aragonite and sphalerite. X 1.
5. Senonian,Sakiet Sidi Youssef.--Concretionin cockadeof sphalerite(schalen•
blend) with crystals of galena 'in somebands.
Recurrence of the two minerals around a fragment of limestone. X 1.
170
P/IUL SAINFELD
whether lead ores or zinc ores. SouthernTunisia is characterizedby sedimentary deposits(iron and phosphate).
It is unlikely, in so far as the hydrothermaldepositsare concerned,that
this distribution indicatesdifferent cycles of deposition. In Tunisia the
associationis in all caseslead-zinc-iron although one or the other of the
elementsmay be dominant.
Characterof Ores
MajorElements.--The
Fe-Pb-Zn
association
is represented
on theone
hand by depositsalmostexclusivelyiron-bearing;on the other, by lead-zincbearingdeposits,in most of which lead or zinc dominateover other metals.
Iron is still presentin proportionequalto or evengreaterthan the two other
constituents,in somecappingsand sulphidezonesof depositswith B.P.G.
TABLE
AN•.L¾S•-S
Deposit
..... i
El Grefa
Marketable ore
Pb
61.07
Zn
1.64
Ag (per ton)
'
III
OF OR•-S
BouAouane
Sorted ore
67
5.10
4 gr
SidiAmor
Dj. Hallour
Sakiet
Sorted ore
High grade ore
Coarse ore
62.25
22.06
3.78
0.10
2.84
5.68
5.60
400 gr
Fe20•
2.78
6.84
1.21
5.91
As
$b
0.33
0.20
2.95
1.62
0.01
0.10
0.61
0.05
S
9.89
17.25
7.02
SiO2
0.80
0.72
8.25
0.68
7.48
1 !.80
1.40
0.40
22.23
20.00
Alg)•
.
2.52
6.31
12.87
21.90
CaO
4.50
MgO
0.11
0.10
0.26
0.36
3.48
10.90
3.44
5.76
1.88
22.68
16.15
$O•Ba, $r
C02, 0
,,
paragenesis,
whichrepresenta little lessthana fourthof the lead-zinc-bearing
deposits--some
12 or 13 deposits,locatedentirelyin the synclinoriumof the
Tunisian
Atlas.
This paragenesisin the districtsof Fedj el Adourn and of Djebel el
Akhouat has been the subjectof a detailedmineralogicalstudy by J. Bolze
and I-I. Schneiderhohn
(6) from which the followingpassages
are taken:
"The Blende . . . of the schalenblende
type . . . in all casesshowsbeautiful
concretionarystructures,in cockades... the colloidaltexture shows a
mosaicof veryfinelamellaeof twinnedsphalerite
....
Thereis no indication
of a changefrom the colloidaldepositof Zn to the stateof schalenblende
by
way of wurtzite."
But, in the Algerian'andTunisiandeposits,
the blendeis commonlycompletelyoxidized,and the ore is composed
of smithsonite,
in someplacesac-
Companied
by hydrozincite
and hemimorphite;
theseare the calamines
of
miners,whichappearin very diverseforms. They are rarely crystallized,
generally
massive,
banded,
mammillary
or concretionary
or stalactitic
in shape,
and of a great variety of colors.
THE LEAD ZINC-BEARING
DEPOSITS OF TUNISIA
171
"Galena is generally well crystallizedin simple cubic forms....
The
crystalsare often deformedor even crushed." In the oxidized zones,the
lead appearschieflyin the form of black cerussite,very finely crystallized.
"The sulfideof iron is deposited
principallyas marcasite. . . in beautiful
concretionary
or arborescent
forms....
Pyrite generallyformsthe baseof
simpleradiatingcrystalsof marcasite. Finally, an amorphous
or crystalline
sulphideof iron in the form of melnikovit-pyrite
sometimes
is visiblein small
spherulites
....
The sequence
of the three sulfidesis not constant. Nevertheless,in general the following succession
is observed:galena, blende,
marcasite-pyrite,calcite."
At Sakiet Sidi Youssefan identicalassociation
showsa first deposition
of galenaandquartz,with secondary
calcite,anda second
deposition
of pyrite,
blende,
marcasite
andschalenblende
(in places
replacing
thepyrite),thewhole
accompanied
by latercalciteand quartz. On the otherhand,in anothervein,
the succession
is normal: schalenblende,
then galena. In additionthere are
recurrences
of the threesulphides
providingbeautifulbandedspecimens
(Fig.
6.5). In the B.G. deposits,a reversalis alsocommon;the pisoliticoresof
RagoubetHalgenNaga show,at the centerof the pisolites,
corrodedcrystals
of galena,concentrically
coatedby calcite,jordanitcand blende(Fig. 6.4).
It appearsimpossible,in Tunisia, to use the texture of the ore and of
galenain particular,in determiningthe age or the type of deposit. The
texture seemsto dependsolelyon the environment;the grain size of galena
is very fine in a marly environment(in the.replacement
depositsof the MioPliocene,in someveinsor disseminated
deposits),but the grain of the galena
becomescoarserin a calcareous
or dolomiticenvironment(in the vein deposits,the impregnations
at the Triassiccontactand the replacementdeposits
in the Cretaceous). The fineness
of grain of the galenaandof the cerussite
is
extraordinaryin someTunisian deposits,ranging about a micron and suggestingdepositionas a colloidalgel.
Minor Elements.--It has not been possibleto establishthe relation between the silver contentof the lead ores and the type of depositor its relative age. The Tunisian.ores are sparselysilver-bearingon the whole--the
depositsof impregnationand of replacementin the Portland and AptJan
limestones
seemto havethe highestcontent,quiteregularlyfrom 250 to 500
gramsper ton. The replacementdepositsin the Mioceneare not silverbearing. Betweentheselimits,the silvercontentdoesnot seemto obeyany
law. The otherelementsare lesswidelydistributed,and still lesswell known.
Cadmium,in the form of greenockite,
has beenrecognized
only in some
B.P.G. deposits
of the subzone
of domes,in the regionof Kef (Fedj el Adoum,
Sakiet-Koucha,
Garn Alfaya).
Vanadiumis knownin the fissuredeposits
of Djebba,of OuedKohol and
of Hamman Jedidi (vanadiniteand dechenite),the replacementdepositsof
the Djebel Agab (vanadiniteand descloizite)and the impregnations
of Ain
Allega and of the Djebel Diss (vanadinite).
Molybdenumhas beenfoundsporadically
in the impregnationdepositsof
Touireuf, in the form of wulfenite.
172
PAUL SAINFELD
Mercury is found in small quantitiesat Oued Maden in the form of cinnabar and metacinnabarand alsoat Djebel Arja (cinabre).
Arsenicis presentin someof the deposits
at the boundaryof the Medjerda
trench in the form of realgar (Oued Maden, Fedj Assene), mimetite and
jordanite (Djebei Hallour, Sidi bou Aouane).
Antimony is alsoquite uniformin the depositof Oued Maden in the form
of stibnite and boulangdrite.
Copperis very widespreadin mostof the metalliferousdeposits,generally
in small quantities(berthoniteat Slata, bournoniteand tetrahedriteat Oued
Maden,chalcocite
at the Kebbouch,chalcopyrite,
azuriteand malachitein the
iron gossans).
Manganeseis rather rare in the lead-zinc-bearingdepositsin the form
of cesarolite(Slata, bou Jaber).
Thus all the minor elementsare sporadicand unimportant.
Gan#ueand AccessoryMinerals.--The gangueof the ores is generally
limestone,calcitic,dolomiticand iimonitic,rarely argillaceous
or gypsiferous;
barite appearsquite commonly,especiallyin the upper zonesof the deposits;
fluorite and celestiteare presentin a few deposits. Nevertheless,thesethree
mineralsservein placesto form smallworkabledeposits.
Hydrothermalsilica is rather rare; quartz is present,however,in some
vein depositsor B.P.G. replacementdepositsat the contact of the Triassic.
But the Aptian formationsof the Cretaceousarchesare commonlysilicified;
one seesthere a veritable invasionof the limestonesby rods and needlesof
quartz, forming rocks of lattice structure of an extraordinary toughness
(Fig. 5.5). The samephenomenon
is foundin someJurassiclimestonesand
dolomiticlimestonesof the dorsal. This silicificationin many casesseems
to be precededby dolomitization,perhapshydrothermalalso, and both phenomena antedate the advent of the lead-zinc.
Origin of the Hydrothermal Solutions
Tunisia is a perfect illustration of the problem of telethermaldeposits,
remote from any eruptive magma; the sedimentarycolumn is very thick.
The only outcropsof igneousrocksare found at about60 km off the Tunisian
coast in the archipelagoof la Galite, consistingof granular (granodiorites)
and micro-granular (microgranites) rocks, with only a few iron-copperbearingveinlets;and in the Nefza, where there are someflows of daciticand
basalticrocks. The greaterpart and the most importantof the metalliferous
depositsare locatedin the southand even there the only igneousrocks are
someblocksof ophiteembedded
here and there in the Triassicand they do
not play any metallogenicrole.
We have examined,above,the tectonicproblemposedby the Triassic
and have shown how various authors have interpreted it as intrusional,
intermediate in nature between magma and sediment, or a companion
or source of ascendingthermo-mineralemanations. This associationis
well illustrated in the Tunisian Atlas; it has been establishedthat there is
particularlybetweenthe two mineralizedregionsof the synclinorium
and the
THE LEAD-ZINC-BEARING DEPOSITS OF TUNISIA
173
anticlinorium,
a hiatusforminga sterilezoneabout40 km wide,whichis very
undisturbed,and almost devoid of Triassic outcrops.
The mechanical
roleof the Triassic,unquestionable
as far as the movement
on the "living faults"is concerned,
woulddoubleas a chemicalrole: the
"primitive"deposits,
whetherin the Triassicor Liassic(15), or in the
basement
faults (6), wouldhavebeendissolved
and their elementsretransportedby waterschargedwith salinesolutions
in their movement
to the
contactof the Triassic. We have seen that most of the depositsof the
Triassiccontactin Tunisia showa commonreversalin the B.P.G. paragenesis;
sulfuretted
hydrogelof leadandzincwouldhavedeposited
firstleadsulphide,
thenblende,with a colloidaltexturein rosettesaroundcrystalsof galena(6).
In brief, the Tunisiantelethermaldepositswouldbe the "emigrantdeposits"
(L. Glangeaud)or "secondary
hydrothermals"
(H. Schneiderhbhn).
This hypothesis,
however,calls for the followingobservations:
(1) Morethanhalfof theminingdistricts
arelocated
at a certaindistance
(1 km or more)fromtheTriassicoutcrops,
commonly
withthickimpermeable
formations
between them.
(2) The principalTriassicmassifs(Oued Zarga, Debadib)are not associatedwith importantdepositsin the area.
'(3) We know of no importantmineralization
within the Triassicand
haveno recordin fragments
of the Triassicbreccias
of a formermineralization. On the other hand there may be present,especiallyat the borders
of theophiteblocks,suchmineralsastourmaline,actinolite,hematite,epidote,
andpyrite,representing
a belatedhydrothermal
pneumatolytic
phase. Why
should not the arrival of Fe-Ph-Zn
be another manifestation of the same
kind?
(4) Chemical
considerations
(reversalor absence
of silica),applyto only
a part of the Tunisiandeposits.Furthermoreabsence
of silicais far from
absolute;quartzis presentin someB.P.G., and especially
in mostof the
deposits
locatedin the Aptianformations,
wherean importantsilicification
is
associated with the dolomitization. These latter characteristics recall the
great American replacementdeposits.
(5) Finally,if onestresses,
in a generalway,andwith goodreason,the
telethermalnatureof the Tunisiandeposits,
it is still no lesstrue that there
was eruptiveactivityduringthe courseof the Miocene;and this, though
evidentlyvery local,mustnevertheless
havepreceded
the metalliferous
emplacements.
In summary,
whilethistheorycanaccount
for a certainnumberof impregnations
of B.P.G. of the TunisianAtlas, it appearssomewhat
premature
to extend it to all of the North African deposits.
Moreover,certainhot springsthat rise in the vicinityof somedeposits
havealsobeenexamined,aswell asall the waterspumpedout of the principal
mines. TheTunisianthermalsprings
are,onthewhole,chlorinated,
sulfated,
or chlorosulfated,
and few are knownnear the deposits;the main onesare
thoseof HammanGabrit,nearthe KharguetKef Tout (26ø), HammanLif,
at thefootof DjebelbouKornine(43ø and50ø), HammanJedidi(61ø),
HammanZriba (46ø) andHammanZebbeus,
at the footof Kef Chambi
174
PAUL $AINFELD
(35ø). But the relativelydeepworkingsof somemineshave tappedhot
waters:Djebel Hallouf, at 175 m in depth (29ø); $akiet $idi Youssef,at
150m (23ø); $idi Amor ben Salem,at 220 m (38 to 42ø); Bou Jaber-Est,
at 50 m (38ø). Yet the low leadcontent(lessthena milligramper litre) of
certainhot or cold waterspumpedfrom minesdoesnot seemto bear any
relationto their temperature. Thus, the water of $idi Amor containsonly
0.15 mg of leadper liter,.whereas
the coldwatersof Fedji el Adoumcontain
from 0.4 to 0.8.
There must be, in certain cases,a mild dissolutionof the
sulphates
or carbonates
of leadof the deposits,
andthereis no reasonto think
thatthehotwatersrepresent
theresidues
of themineralizing
solutions.
Age of Minerali, ation
To resolvethis problem,there are availablethe lesstheoreticalfactors.
It has beenseenthat the first knownphasesof Triassic"injection"date, at
least,from the Pyrenneanmovements,
at the beginningof the Tertiary; but
it seemsindeedthat the Triassicwas not then accompanied
by mineralizers.
No indicationshave yet beenfound in the pebblesof the conglomerates
of
the lower Miocene.
The replacement
depositsin the Mioceneof Northern Tunisia give us,
moreover,a preciseindication,for we have shownthat there is no compellingreasonto postulatemanycyclesof deposition,
but ratherseveralstages
in the samecycle(variationsin the B.P.G. paragenesis,
brecciasrecemented
by ore). In particular,examination
of the depositof Douaria(17) shows
that the ferriferous mineralizationthat affects the continentalbeds whosetop
is well datedas Pontian,wasprobablycomplete
at the endof this epoch;one
finds,indeed,in the overlyingnon-mineralized
beds,pebblesof ore derived
no doubtfrom the mineralizedbeds,for it appearsdifficultto imaginethat
thesewould have beenmineralizedafter their deposition. One is therefore
ledto conclude
that theTunisianore deposits
are relatedto the lastparoxysmal
phaseof the Alpinefolding,that is to say,they are of Mioceneage.
Indeed,a final indicationis affordedby a deepoxidizedzonein certain
leaddeposits
at the edgeof the subsidence
trenchof the Medjerda(Djebel
HalloufandTouireuf). Onecanhardlyexplainthisphenomenon
exceptby
the existence of a fossil water table which would be related to the amount
and rate of subsidence.Therefore,the ores must have beendepositedprior
to theisostatic
readjustment
thathascaused
diverselocaltrenches
of subsidencein Tunisiathroughout
the recentQuaternary. Thesefinalmovements
areagainattested
bydistortion,
crushing
andevencuttingof mineralization
by
the later Triassic"injection,"and even by small faults of readjustment,
subsequentto the mineralization.
CONCLUSIONS:
CONDITIONS
OF DEPOSITION
Thegeneral
geologic
andmetallogenic
considerations
permitusto examine
thefactorscontrolling
the localization
of the lead-zinc
oresin Tunisia?
z The percentages
are calculatedaccordingto the importance
of the deposits(tonnage
taken out and reserves at the close of 1953).
THE LEAD-ZINC-BEARING
DEPOSITS OF TUNISI./I
175
(1) The structural factors seem the most important: the metalliferous
depositsare concentrated
in the foldedarea of Tunisia, that is to say, at the
border of the Alpine zone, and particularlyin the northwesthalf of this
region (75 percentof the deposits). There is a quasi-periodic
distributionof
the principaldistricts,followingthe tectonicalignments. The B.P.G. paragenesisis certainly related to the last Alpine phase. A statisticalanalysis
of the relationsbetweenthe visible faults and the mineralizationgives the
followingresults: strike faults (with or without intruded Triassic) appear
in almostall cases(85%), associatedwith the transversefaults (13%) or
with slips (fracturesde glissement)(9%) or, particularly,with one or the
other (50%).
(2) The stratigraphic
factorsdependon the physical-chemical
conditions.
In the absence
of igneousrocks,onenotesa distributionof the mineralizatiofi
in all the Mesozoicand Tertiary sectionor in companywith the layersof the
"extrusive" Triassic. The statisticalanalysisshowsthat there is a direct
relationbetweenthe favoredstratigraphicstagesand the nature of the rocks
that composethem: the upper Cretaceous,
calcareous,containing38 percent
of the known ore deposits,the middle Cretaceous,calcareousand dolomitic,
17 percent,the Jurassic,calcareous,
15 percent(althoughnot very extensive);
the Neogene,conglomeritic
sandstone
13 percent,the Triassicandits brecciated
border,12 percent,and finallythe Paleogene,
calcareous
and sandy,5 percent.
(3) The physical-chemical
conditionsare naturally in accord with the
precedingfactors:the "extrusive"Triassicor diapir probablyplaysa double
mechanicaland chemicalrole, and, if its influenceon the hydrothermalactivity is not demonstrated
in all cases,.it is very probablefor one part, at least,
of the lead-zinfibearingdeposits.The mineralization
in Tunisiais not entirely localizedin favorablerocks,that is to say, in readily faulted horizons,
in shatteredzones with micro-fracturedconstituents,and especiallyrocks
subjectto easyand large scalesolutionand metasomatism
(78% of the instances). If the limestones,in fact, representthe principal host (52%),
next the dolomites(11%), the brecciasof the Triassiccontact(8%) and the
sandstones
and conglomerates
(7%), it is none the lesstrue that there are
substantialimpregnations
in suchrocksas the marls or the clays. This fact
is quite characteristic,
as well as the relativebarrennessof the dolomites,as
comparedto Morocco,for example.
The relativeimportanceof the typesof depositsis not easyto determine,
because
of their complexity. At mostonecansaythat the vein deposits,
with
impregnationof lode walls and filling of cavities,exceedthe depositsof impregnationand of true replacement.
(4) Tunisia forms, with the Constantine,a Mediterraneanmetallogenic
sub-province
characterized
by the uniquelead-zinc-ironparagenesis.Despite
local variationsin the predominanceof one or another of theseelements,one
can considerthat there was only one period of mineralizationwhich was
quite recent,of Mioceneage.
(5) The fineness
of the grain,the concretionary
and ribbonedappearance
of the ores,in placespisoliticor stalactitic,indicatesdeposition
at low tempera-
176
PAUL SAINFELD
ture (on the orderof 100ø). The almostcompleteabsence
of eruptiverocks
makesTunisia a type countryof telethermaldeposits.
DIRECTION DESTRAVAUX PUBLICS,
TUNIS•
October 12, 1955
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