insula is marked by the Badajoz shear zone along which movements were focused during the. Hercynian orogeny. To the north of the shear zone, in the La ...
EconomicGeology Vol. 86, 1991, pp. 1012-1022
TectonicSettingand Fluid Evolutionof AuriferousQuartz Veins from the La CodoseraArea, Western Spain S. ROBERTS,D. J. SANDERSON,S. DEE, GeologyDepartment,The University,Southampton S09 5NH, UnitedKingdom AND P. GUMIEL
InstitutoTecnolJgico GeoMinerode Espar•a,RiosRosas23, 28003 Madrid, Spain Abstract
The boundarybetweenthe CentralIberianandthe Ossa-Morena zonesof the Iberian peninsulais markedby the Badajozshearzonealongwhichmovements werefocusedduringthe Hercynianorogeny.To the northof the shearzone,in the La Codoseraarea,auriferousveins are locatedin a seriesof late Hercyniannorth-south-to northeast-southwest-trending faults whichare antitheticto the mainshearzone.The passage of dilute CO2-richfluidsat 350 ø to 400øC and 2,800 to 3,300 barsalongthesefaultsresultedin mineralizationwheredilatant structures,for example,offsets,intersections, or fault terminationsdevelopedwithin black slatelithologies.By combininggeologicaland geochemicalsystematics, includingvein orientation,lithology,andfluid chemistry,with remotelysensedfracturedata,a testableexplorationmodelfor the regionhasbeendeveloped. Introduction
THE tectonicsettingandfluid evolutionof auriferous quartzveinsin an areaof transpressive tectonicsnear La Codoserain westernSpainare describedin this paper.The areacontainsmanysmallgoldprospects, situatedon former Romanworkings,and lies to the northof the majorleft-lateralBadajoz-Cordoba shear zone, which is thoughtto controlthe lode goldmineralizationof the area.This paperdescribes the developmentof the mineralizedstructuresand the nature of the mineralizingfluid. These data are then discussed to provideinsightsinto the genesisof gold mineralizationandto form the basisof an exploration model for the region. Geologicand Tectonic Setting
The Hercyniancrustof the Iberian peninsulacan be subdivided into a number of tectonic zones or ter-
ranes,the boundariesbetweenwhichare oftenhighly deformed. The La Codosera area lies at the southern
marginof the CentralIberianzone,whichisseparated from the Ossa-Morenazone to the southby the Badajoz-Cordobashearzone (Fig. 1). This is a complex structural lineament along which movementswere focusedduringthe Hercynianorogeny(Arthaudand Matte, 1975; Lefort and Ribeiro, 1980; Burg et al.,
1. A Precambriansequence,locally known asthe ComplejoEsquisto-Grauvaquico (CEG), comprisesa monotonous sequence of slatesandgraywackes. These rocks are presumedto be of late Proterozoic age; elsewherethey are overlainunconformablyby more varied lithologies,including dolostonesand limestones,variouslyattributed to the Vendian or even Cambrian(Bouyx,1970). Most of the outcropto the north of La Codoseracan be equated with a lower serieswithinthe ComplejoEsquisto-Grauvaquico, but highlydeformedmicroconglomerates or porphyroids (Urra Formation)to the southof the southernridge (Fig. 1) may representan upper unit. 2. Paleozoicquartzitesand slatesunconformably overliethe ComplejoEsquisto-Grauvaquico sequence and occupy three west-northwest-east-southeasttrending ridges (Fig. 2): the southern(or border) ridge,whichalsoincludeshighlyshearedPrecambrian to Devonian sediments,the central ridge, and the northern (or SanPedro) ridge. Near the baseof the lower Paleozoicrocksa prominentquartziteunit, the ArmoricanQuartzite, of Arenig age, forms a good mappablehorizon.This is overlainby a succession of dark gray shales(slates)and thin sandstones, which haveyieldeda varietyof fossils(trilobites,graptolites, brachiopods,etc.) of OrdovicianandSilurianage.The succession can be subdivided on the basis of the cor-
relationof severalthicker quartziteswith better studied successions in the Guadarranqueand Tamames Tectono-stratigraphic units areas(Julivert and Truyols, 1974) and at Almaden The geologyof the La Codoseraarea (Fig. 1) can (Ortega, 1988). The Cantera Quartzite (Caradoc) be subdividedinto a numberof tectono-stratigraphic separatesthe underlyingCalymeneShales(Llanvirunits: nian-Llandeilian)from overlying Upper Ordovician 1981).
0361-0128/91/1251/1012-1153.00
1012
AURIFEROUS QUARTZ VEINS, LACODOSERA, WESTERN SPAIN •
Devonian slates &quartzites
•I;h'.¾(•Ordovician/Silurian slates an'd
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CEG
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[•1Precambrian. (quartzites. •..;.•.•_ / •• ...• Alburquerque batholith. •hrust .'-.' •½ N• .•••••••?.:•
Ahseda -
•
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FIG.1. Geologic map oftheLaCodosera area. CEG= Complejo Equisto-Grauvaquico. A-A'= line
ofsection in Figure2. Insetislocation mapoftheLaCodosera area.1 = South Portuguese zone, 2 = Ossa-Morena zone,3 = CentralIberianzone,4 = Badajozshearzone.
shales,which are in turn separatedfrom Silurian shales,sandstones, and volcanicsby the Criadero Quartzite(Llandoverian). TheLa Codosera syncline liesbetweenthecentral andsouthern ridgesandiscoredby redandgrayDe-
zontalfolds,with a steepslatycleavage;(b)
steeplyinclined,gentlyto moderatelyplunging asymmetric foldswitha crenulation cleavage; and(c) extensivelate fracturingand faulting.
Theyoungest rocksaffected bythisdeformation in
vonianslates,with somethin sandstone andlimestone the La Codoseraareaare of Devonianage,but farther
easttowardCaceres,the Hercyniandeformation af3. The Alburquerque Graniteis a lateHercynian fectsCarboniferousstrata.The foldsandcleavage(D• Granite,which batholithintrudedmainlyintothe ComplejoEsquisto- andD2)arecutbytheAlburquerque contains only late Hercynian and younger fractures. Grauvaquico unit,butwhichfartherwestinPortugal Withinthe ComplejoEsquisto-Grauvaquico, the crosscuts the La Codoserasyncline.This predomiunits.
cleavage oftencutsacross (or transacts) nantlyporphyritic, muscovite-bearing granite, witha Hercynian locallydeveloped topazfacies,hasa peraluminousboth horizontaland steeplyplungingfoldsand in earlyquartzveinsareclearlyfoldedaround composition anda whole-rock Rb/Srisochron ageof places suggest thatmanyof the 286 +__ 3.6 Ma (unpub.data,thisstudy).The granite tightfolds.Thesefeatures is typicalof the crustallyderivedIberianHercynian foldsand someof the faultsand veinswithin the Cominitiated priortocleavage granites, whichare commonly associated with tin, plejoEsquisto-Grauvaquico development buthavesubsequently beenmodified tungsten,anduraniummineralization. by the intensenatureof the superposed Hercynian Hercyniandeformation deformation. Severalsetsof latercrenulationcleavages andkink Hercyniandeformation affectsthePaleozoicstrata in theslates, buttheseproduce andthe underlyingComplejoEsquisto-Grauvaquico,bandsaredeveloped
refolding oftheslatycleavage. Steep producing(a) D•--early, steeplyinclined,subhori- littlesignificant SSW
NNE
Southern CentralRidge
Northern Ridge,
a•daa•Oz Ridge .&..•::,-. •. o • o o o oo o o
o o r•-
+ + • +
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FIG.2. Simplified crosssection of theLa Codosera area.
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1014
ROBERTS,SANDERSON, DEE, AND GUMIEL
north-northeast-trending kink bands represent a and2). To the westofLa Codosera,the southernridge widespread,late, regional deformationevent; the strikes northwest-southeast into Portugal. Detailed othercleavages havenotbeencorrelatedthroughout mappingaroundthe gold prospectsof LosAlgarbes the area and may representlocal deformation,some has revealed quartzite units repeated by faulting, in responseto faulting. usuallywith a northerlydirectedreverseseparation. The crosssection(Fig. 2) indicatesthe changing They clearlycrosscut the earlyfoldsandslatycleavage structuralstylein passing fromthe foldingandthrust- and have been reactivated in associationwith D• folding of the CentralIberianzoneintothe strongstrike- ing andcrenulationcleavage.Minor F• foldshavesislipdeformationof the Badajozshearzone(Sanderson nistralor northerlyvergenceconsistent with the fault et al., 1991). Toward the south, the strike faults offsetsand slickensides on fault surfaces,indicating (thrusts)steepenand showincreasingevidenceof phasesof obliquethrust-left-lateralmovement. obliqueor strike-slipmovement,asthe foldstighten To the eastofLa Codoserathe ridgeswingsto strike andthe deformation increases. A strongsubhorizontal almosteast-west,where it becomeshighly sheared, stretchinglineationdevelops,accompanied by left- with a pronouncedsubhorizontal stretchinglineation, lateral shearbands,which form a strongS-C fabric boudinage,and left-lateral shear bands. Deformed with the cleavage.There isa consistent obliquitybe- LowerDevonianbrecciasat the SanAntonioantimony tween the strike of the faults and folds and that of the mine (ArribasandGumiel, 1984) showsubhorizontal main Hercynian cleavage (S•). In addition the S• stretchinglineationsand plane strainswith X/Z •4. cleavageprogressivelyswingsin strike from north- The beds steepentoward the southernpart of the west-southeast in the north to more east-west in the ridge and the left-lateral shearingintensifies.These south.The D2 foldsare markedlyasymmetrical, with changesrepresenta transitioninto the Badajozshear shearedand extended short limbs, and show a con- zone (Sandersonet al., 1991). sistentsinistralvergence.The associated crenulation Stylesof Mineralization cleavage(S2)alsoswingsfromnorthwest-southeast to
All of the goldmineralizationoccursin quartzveins moreeast-west.Crossfaultingbecomescommon,with the earlyductiledeformation(folding large right-lateral offsets on northwest-southeast-whichcrosscut Threemaingroupsof mineralizedveins trendingfaults(Fig. 1 andseebelow).Thesefeatures andcleavage). areattributedto anincreasing left-lateralcomponent have been recognized. accompanying shortening(i.e., transpression) toward High-angleveins the shearzone (Sanderson et al., 1991). The highestreported gold gradeshave been obStructure of the Mineralized Terranes tainedfrom a seriesof milky quartzveinstrendingN The goldmineralization islargelyrestrictedto the 020ø-060 ø, crosscuttingthe strike of the bedding, southernandcentralridgesof the La Codoseraarea. which are well developedwithin the southernridge (Fig. 3a). The veinsvary between 10 and 60 cm in Central ridge thicknessand are most commonlyobservedwithin The centralridge consists of tightly folded,gen- quartzites but only tend to be mineralized when erallysteeplydipping,Paleozoicrocks,lyinguncon- hostedby black slates.Displacementat vein interformably ontheComplejo Esquisto-Grauvaquico (Fig. sections,pinnateveins,andthe developmentofslick1). Severalfaultshave been mapped,which strike ensideson associatedfracture sets(especiallythose parallelto the foldsbut cut across their limbs.Many trending N 020o-040 ø) indicate that the veins deof shear.En echelonarrays of thesefaultshavereversestratigraphic separation velopedwith a component in crosssection(i.e., appearas thrusts)but have im- and rhombicpull-apartstructuresdevelopin shear portant strike-slipcomponents.Large groovesand zonesassociatedwith pressuresolutionseams.The slickenslides pitchinggentlywithinthe fault planes veinscommonlyhavea fibrousinternalstructure,with indicateat leasta late stageof movementwith a left- individualfibersshowingopticalcontinuityacrossthe lateralcomponent. Overallthe faultsappearto ac- vein, an indicationof crack-sealphenomenon(Ramcommodate the folding,ratherthanthe foldshaving say, 1980). In thin sectionthere is evidenceof coninitiatedonthrustramps.Essentially thecentralridge tinuing, postdepositionalshear in the form of sigis a largenorth-verging foldpair,cutby steeplydip- moidaldrusequartzgrainsandundulatoryextinction. ping faults,on the northernlimb of the La Codosera Strikefault-related prospects syncline. Within both ridges, small quartz veins (5-30 cm Southern(or border)ridge thick) are commonlysituatedon minor faults(strikeSteeplydippingandoverturnedOrdovicianto De- parallel,trendingN 120ø anddippinggentlytoward vonianquartzitesandslatesformthehighlydeformed, the SW),whichshowreverseseparation (thrusts)(Fig. southernlimb of the La Codoserasyncline(Figs.i 3b). Structurespreservedon the faultsurfaces include
AURIFEROUS QUARTZ VEINS, LACODOSERA, WESTERN SPAIN
1015
grainsor mayoccurasaggregates (> 1.5 cmin longest dimension) of predominantly arsenopyrite with minor pyrite.Chloriteis locallypresentandsecondary oxidesarecommon.Pyriteis replacedby goethiteand lepidocrociteas cubicpseudomorphs or concentric andnestedboxworks, controlled by { 100} symmetry planes(Andrew,1980).Arsenopyrite is replacedsequentiallyby scoroditeandFe oxideswith pseudomorphsof lozenge-shaped crystalsor diffuseboxworks,controlledby { 110} symmetryplanes(Ramdohr,1969). Visiblegoldis rarelyobserved. Schematic
block
diagrams of mineralization styles in the La Codosera
area
Sulfide Geothermometry
Experiments onthe stabilityof arsenopyrite in the systemFe-As-Shaveled to the useof arsenopyrite compositions as a geothermometer (Clarke, 1960; KretschmarandScott,1976; Sharpet al., 1985). The commonsulfideassemblage of arsenopyrite andpyrite within the auriferous veins of the La Codoseraarea
maythereforeprovideinformationon their temperature of formation. FIG. 3. Schematicblock diagramof the gold prospects(not to scale).A. High-angleveins,typical of the geometryof mineralized localitiesof the southernridge. The veins are at a high angleto the S• fabricwhichisindicatedby dashedlines.B. Strikeparallelveins,commonlyformingon minordetachmentsurfaces present in both ridges. C. East-westveins commonwithin the core of the centralridge anticline.
The experimentalwork of Kretschmarand Scott (1976) outlineda pseudobinary T-X section(Fig. 4) relatingatomicpercentarsenic to temperature. Their data,anda subsequent reevaluation by Sharpet al. (1985), showthat, providingarsenopyritecomposipy+asp no longer co-exist
subhorizontalslickensidesand gutter structures (HancockandBarka,1987)whichindicatestrike-slip movement. Occasionally,the hanging wall to the quartzveinisa highlyimbricatedquartzite.The small size and lack of strike continuityof theseprospects accountsfor the lack of extensiveworkings. East-west-trending auriferousveins A seriesof larger east-west-trending auriferous quartz veins are well developedtoward the core of the anticlinewithin the central ridge, in particular withinthe Ordovicianslates(Fig. 3c).The veinsshow variablethickness(
