Amphibolite vs. banded amphibolite: a case study in the ... · is the case of São Martinho, east...

Cadernos Lab. Xeolóxico de LaxeCoruña. 2003. Vol. 28, pp. 213-229

ISSN: 0213-4497

Amphibolite vs. banded amphibolite: a casestudy in the São Martinho-Arronches area,

Tomar Cordoba Shear Zone, NE OssaMorena Zone, Portugal

Anfibolita versus anfibolita bandeada: el caso deSão Martinho-zona de Arronches, Zona de

Cizalla de Tomar Cordoba, Zona NE de OssaMorena, Portugal

DE OLIVEIRA, D. P. S.1; WIECHOWSKI, A.2; ROBB, L. J.3 & INVERNO, C. M. C.4

Abstract

Amphibolites and banded amphibolites form an integral part of the Série Negrarocks that outcrop within the Tomar Cordoba Shear Zone that forms the boundarybetween the Ossa Morena Zone and the Central Iberian Zone. The terms amphiboli-te and “amphibole schist” (here changed to banded amphibolite) have been regularlyused by exploration geologists when referring to the amphibolitic-type lithologies,with or without a marked schistosity, that outcrop in the Ossa Morena Zone, e.g. inthe vicinity of the São Martinho gold prospect. In this study the term amphibolite isused for homogeneous, massive-texture rocks at the mesoscopic scale whereas ban-ded amphibolite refers to rocks that exhibit a distinct banding at the outcrop scale.Microscopically both rock types show a schistosity, which is better defined in thebanded amphibolites. These rock types have a rather similar mineralogy.Geochemically, in terms of their major element composition, there is little differen-ce between them. However, each exhibits a distinct mineral chemistry: the amphi-boles in the amphibolites are actinolite-magnesiohornblende while those in the ban-ded amphibolites are mostly magnesiohornblende and minor tschermakite.Likewise, the feldspars in the amphibolites plot in the oligoclase-andesine and labra-dorite-bytownite fields while those in the banded amphibolites are largely constrai-ned to the labradorite fields with a few outliers in the andesine and bytownite fields.

Cadernos Lab. Xeolóxico de LaxeCoruña. 2003. Vol. 28, pp. 213-229

Tentative geochemical signatures for protolith nature and tectonic setting indicate a pro-bable basic protolith for both amphibolites and banded amphibolites although the tex-tures preserved in the banded amphibolites may also indicate a sedimentary-tuffaceousorigin.

Key words: Amphibolite, banded amphibolite, Série Negra, Tomar Cordoba Shear Zone,NE Ossa Morena Zone.

(1) Economic Geology Research Institute-Hugh Allsopp Laboratory (EGRI-HAL), University of theWitwatersrand, Private Bag 3, WITS 2050, Rep. South Africa. Present address: Instituto Geológico eMineiro, Apartado 7586, 2721-866 Alfragide (Lisbon), Portugal

(2) Department of Mineralogy and Ore Deposit Research, RWTH Aachen, Wüllnerstr. 2, D-52062 Aachen,Germany

(3) Economic Geology Research Institute-Hugh Allsopp Laboratory (EGRI-HAL). University of theWitwatersrand, Private Bag 3, WITS 2050, Rep. South Africa

(4) Instituto Geológico e Mineiro, Apartado 7586, 2721-866 Alfragide (Lisbon), Portugal

1. Introduction

The terms amphibolite versus “amphiboleschist” (here modified to banded amphibo-lite) have been regularly used by goldexploration companies operating in theIberian Ossa Morena Zone (e.g. Rio TintoZinc, Portuglobal-Explorações Mineiras,Lda., Auspex Minerals Ltd.) when refe-rring to the amphibolitic-type lithologiesthat outcrop in the gold prospect areas, asis the case of São Martinho, east of Alterdo Chão. These amphibolitic-type litholo-gies (s.l.) are locally hosts to (significant)gold mineralisation on their own or at thetransition between them and quartz bioti-te schists of the Série Negra (de OLIVEI-RA, 2001).

In São Martinho, amphibolites andbanded amphibolites at times outcrop-ping adjacent to each other, form part ofthe Série Negra rocks that outcrop withinthe Tomar Cordoba Shear Zone (TCSZ) inthe NE Ossa Morena Zone. The amphibo-lites and banded amphibolites outcropsouth of the Blastomylonitic Belt wherethe Série Negra rocks have reached amphi-bolite facies metamorphism as opposed tothose outcropping north of theBlastomylonitic Belt that have only rea-ched greenschist facies metamorphicgrade. In light of the potential economicimportance of these rocks and the distinc-tion between the terms amphibolite andbanded amphibolite (“amphibole schist”)that have crept into company reports onthe area and other metallogenic studies,this paper, that does not intend to be apaper on fine metamorphic petrology, exa-mines and records differences in texture,bulk geochemistry and mineral chemistry

between amphibolites and banded amphi-bolites and makes tentative observationsregarding the protolith nature of theserocks from samples collected from outcropand trenches in the area (figure 1) to inves-tigate whether the differences observed atthe mesoscopic scale are also evident at themicroscopic scale, in view of the economicgold potential of these rocks in the area.

2. GEOLOGICAL AND STRUCTU-RAL SETTING

The study area is located within a poly-gon that has at its extreme limits thetowns of Alter do Chão and Arronches inthe W and SE respectively (figures 1 and2). The Série Negra (Black Series) is a pac-kage consisting of metasedimentary (meta-arenites and metapelites), basic igneous(amphibolites and banded amphibolites)and felsic volcanic (metarhyolites) rocks(e.g. OLIVEIRA et al., 1991; de OLIVEI-RA, 2001). The Série Negra occurs juxta-posed on both the north and south limbs ofan asymmetric flower structure that con-tains, from north to south, low-grademetamorphic rocks (greenschist facies) tointermediate-grade metamorphic rocks(amphibolite facies) separated by a centralcorridor of high-grade metamorphic rocks(the Blastomylonitic Belt; figure 2) chris-tened initially as the TCSZ. The TCSZis, however, conventionally understoodtoday to be the zone located immediatelysouthwards of the Central Iberian Zoneup to and including the Barreiros tecto-nised granitoids (e.g. OLIVEIRA et al.,1991; figure 2).

The TCSZ is a geologically complexand diverse zone of intense deformation

CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 215

and metamorphism contemporaneouswith a large sinistral displacement, whichmay be due to a large intracontinentalsinistral fault active during the VariscanOrogeny (BERTHÉ et al., 1979) withsinistral displacements of 100 (BURG etal., 1981) to 300 km (ABALOS &EGUÍLUZ, 1992). This displacementcaused mylonitisation and retrogrademetamorphism (under greenschist toamphibolite facies) to all previous structu-res and mineral assemblages (QUESADA& MUNHÁ, 1990). PEREIRA & SILVA(2001) considered the Tomar CordobaShear Zone a major Eohercynian-Hercyniansinistral transcurrent fault overprinting aCadomian arc localised at a convergentmargin of Gondwana.

The Portuguese sector of the TCSZcomprises a series of polymetamorphicstructural-tectonic subdomains defined byPEREIRA in 1995 and 1999. These sub-domains are, from north to south, Urra-Mosteiros-Ouguela Subdomain, Degolados-Campo Maior Subdomain, Arronches-

Morenos-Caia Subdomain, AssumarSubdomain and the Alter do Chão-ElvasSubdomain. Each of these subdomains is athrust fault-bounded package of rocks andthe thrust faults and vergence are arrangedin a flower structure more or less centeredin the Blastomylonitic Belt (figure 1).

The Blastomylonitic Belt (approxima-tely the core of this flower structure) ismade up primarily of Palaeoproterozoicage [2237 and 1700 Ma, by de OLIVEI-RA et al., (2002) and ORDOÑES-CASA-DO (1998), respectively] migmatiticgneisses of the Campo Maior Formation.

Within the TCSZ outcrops theNeoproterozoic Série Negra rocks (maxi-mum age for the final stages of sedimenta-tion, ca. 565 Ma; SCHÄFFER et al.,1993); the name derived as a result of theoverwhelming majority of dark-colouredrocks that make up this series. The SérieNegra outcrops scarcely both north andsouth of the Blastomylonitic Belt.Stratigraphically the Série Negra is made

216 de Oliveira et al. CAD. LAB. XEOL. LAXE 28 (2003)

Figure 1. Polygon of the São Martinho-Arronches area with sample locations.


Figure 2. Excerpt from the 1:500000 geological map of Portugal showing the simplified geology ofthe NE Ossa Morena and SE Central Iberian Zones (adapted after OLIVEIRA et al., 1992). The sha-ded area corresponds to the location of the study area that is located south of the BlastomyloniticBelt in amphibolite facies-grade metamorphic rocks (adapted after de OLIVEIRA, 2001).

up of the (lower) Morenos and (upper)Mosteiros Formations (OLIVEIRA, et al.,1991). The Morenos Formation is madeup of micaceous schists that are locallygarnet-bearing, limestones and calc-silica-te assemblages, meta-arkoses, meta-areni-tes (quartzites) and micaceous and silice-ous schists, amphibolites and pyroclasticrocks (OLIVEIRA et al., 1991). TheMosteiros Formation consists of blackschists/slates, greywackes, black cherts(quartzites?), limestones and amphibolites(OLIVEIRA et al., 1991). North of theBlastomylonitic Belt and unconformablyoverlying the Mosteiros Formation occursthe Urra Formation made up of a lowerporphyry unit and an upper pelite/grey-wacke unit (OLIVEIRA et al., 1991)usually of a pale green colour (de Oliveira,1998). At the TCSZ borders, a (Lower)Cambrian sequence of platform sedimentsis preserved, which unconformably over-lies the Neoproterozoic Série Negra meta-sedimentary rocks (see figure 1) and con-sists of micaceous schists, amphibolites,metamorphosed carbonate rocks and peli-tic schists (OLIVEIRA et al., 1991;PEREIRA, 1995).

The amphibolites of the Série Negrain the vicinity of São Martinho aremostly hidden below a Palaeogenic clast-supported elluvium deposit that coversmost of the area. Hence, in this area stu-dies have mostly to resort to boreholesamples. However, on the edges of thisdeposit further to the SSW of trig. bea-con São Martinho (figure 1; location ofsample DP91), small (< 2m-wide) out-crops of amphibolite occur. The bestexposure of amphibolites and bandedamphibolites is in the railway cutting

ESE of trig. beacon Travesso (figure 1;locations of samples DP42 and 43). Herethe amphibolitic rock outcrops are domi-nantly massive (with local banding) incharacter with a dark colour and locallywith ferruginous staining when slightlyweathered. They outcrop adjacent toquartz-biotite schists, and black quartzi-tes of the Série Negra and are intruded byaplitic veins, rhyolites and several apo-theses of the Barreiros tectonised grani-toid (figure 2.10 in de OLIVEIRA,2001). Further to the E, near Assumar,the distinct banding of the bandedamphibolites is more evident althoughoutcrops are limited to small occurrencesin road cuttings or stream beds.

Two phases of Variscan deformation areidentified in this sector (GONÇALVES etal., 1972a, b). The first phase of deforma-tion (D1) develops folds with WSW ver-ging axial planar schistosity (S1) with atendency of the axial planes becomingmore horizontal as one moves southwes-twards. The second phase of deformation(D2) generated folds with subvertical axialplanes or strongly dipping to the NE andstriking NW-SE. D2 deformation genera-tes refolding of the D1 structures anddevelops crenulation cleavage (S2)[PEREIRA, 1995].

The TCSZ is intruded by several pre-Hercynian [e.g. the Late Cambrian(weighted mean 207Pb/206Pb age of 508 ±8.1 Ma) Barreiros tectonised granitoids,de OLIVEIRA et al., 2002; the (ultra)basicto felsic intrsusions of Alter do Chão-Cabeço de Vide,Campo Maior and Elvas),syn-Hercynian (e.g. the as yet to be datedelongated granitoid bodies E of Alter doChão; figure 1) and late- to post-


Hercynian rocks (e.g. the Nisa and SantaEulália granite batholiths).

3. PREAMBLE

In order to follow our approach, a briefreview of field and microscopic features of theamphibolitic rocks follows. Amphibolitesare rather massive metamorphic rockscomposed chiefly of hornblende and pla-gioclase (with An ≥ 17) and are diagnosticof the amphibolite facies of metamor-phism. They are amongst the most com-mon rocks formed by regional metamor-phism of moderate-to- high-grade(WILLIAMS et al., 1982). Amphibolitefacies metamorphism applies over a tem-perature range of approximately 500-650ºC and pressures of 3-10 kb (figure 2.1 inSHELLEY, 1995).

Amphibolite protoliths are commonlybasalt, dolerite and gabbro. Bandedamphibolites can be formed from a varietyof rocks mostly from (ultra)basic to inter-mediate igneous rocks but also fromimpure calcareous and dolomitic sedimen-tary rocks (HYNDMAN, 1985).

In thin section amphibolites show apreferred orientation of the hornblendeprisms (WILLIAMS et al., 1982) defininga schistosity, a lineation or both (SPRY,1969; SHELLEY, 1995), better defined inthe banded amphibolites, where it is alsowell expressed mesoscopically. Banding inthe banded amphibolites may be producedby segregation of constituents duringrecrystallisation or, alternatively, may beinherited from bedding or layering insedimentary or igneous rocks, respectively(BATES & JACKSON, 1987).

4. DEFINITION WITHIN THESTUDY AREA

Within the study area the distinctionis made between amphibolite and bandedamphibolite. This distinction stems fromthe more pervasive and penetrative schis-tosity (parallel to the schistosity in enclo-sing quartz-biotite schists) of the bandedamphibolites vs. the more massive, homo-geneous character of the amphibolites atthe outcrop scale. The banded amphiboli-tes show better-defined segregation ofquartz and plagioclase crystals into discre-te bands. Figure 3 exemplifies these featu-res. However, these two lithologies oftengrade into each other and no sharp con-tacts between banded amphibolites andamphibolites are seen. Banded amphiboli-tes extend for up to tens of metres withineither amphibolites or quartz biotiteschists. Though both banded amphiboli-tes and amphibolites host gold mineralisa-tion, it is more common in the former.

5. MINERALOGY AND TEXTURES

The mineralogy of the amphibolites isgenerally similar in samples collectedfrom outcrops and from trenches andboreholes. The only real difference betwe-en samples is the amount of alterationthese have suffered as a result of thevarious prograde and retrograde metamor-phic events.

In both the amphibolites and bandedamphibolites the grain size is extremelyheterogeneous and can range from 10 µmto 250 µm in diameter. This value takesinto account not only the amphibole sizebut also that of quartz, plagioclase andepidote (when present).


Mineralogically the banded amphiboli-tes are made up of amphibole, plagioclaseplus minor quartz with titanite and apati-te with very few opaques set in a nemato-blastic texture. No biotite was observed inthe thin sections but discrete bands of bio-tite from intervening schists have beenobserved in borehole core. Microchemicalanalyses of the hornblendes and feldspars(average analyses shown in appendix 1 andappendix 2 respectively) allow these mine-rals to be classified, respectively, as predo-minantly magnesiohornblende (with aweak shift towards the actinolite andtschermakite fields) (figure 4A) and predo-minantly labradorite with a reduced num-

ber of samples also falling in the andesineand bytownite fields (figure 4B).

The amphibolites are more massiveand consist of strongly coloured amphibo-le with mostly untwinned xenoblastic pla-gioclase, epidote plus minor quartz, tita-nite, magnetite, ilmenite and rare rutile.Garnet is common in higher-grade amphi-bolites (SHELLEY, 1995), but these haveonly been found in one sample nearAssumar in a hornblende-garnet schist(sample DP69, figure 1; de OLIVEIRA,2001,) gneiss similar to an amphibolite inmineralogy but interpreted as a retrogres-sed eclogite by PEREIRA (1999).Microprobe analyses of the amphiboles


Figure 3. Photomicrographs highlighting the textural differences between banded amphibolite (A)and amphibolite (B). The former has a strongly developed schistosity with clear separation betwe-en alternating quartz/feldspar-rich bands (light) and amphibole-rich bands (dark) while the latterhas a distinctly massive character. (Adapted after de OLIVEIRA, 2001).

and feldspars in the studied amphibolitesallow these minerals to be classified asactinolite-magnesiohornblende, whilethe feldspars range from oligoclase tobytownite clustering into a oligoclase-andesine group and a different labradori-te-bytownite group (figures 4A and B),both amphibole ands plagioclase dia-grams being distinct from the bandedamphibolites case.

From this purely mineralogical pointof view it would seem that both amphibo-lites and banded amphibolites in this areacould be derived from basic protoliths.However, other alternative protoliths forthe latter would also have to be suggested,since the presence in the banded amphi-bolites of discrete biotite bands may indi-cate a possible tuffaceous protolith(WILLIAMS et al., 1982) and in othercases, thin, variable bands rich in quartz,(carbonates) and biotite suggest a sedi-mentary origin (SHELLEY, 1995).

6. GEOCHEMISTRY

6.1. Methodology

The samples were analysed by X-rayFluoresecence (XRF) at the Dept. ofGeology, University of the Witwatersrandin Johannesburg using a Philips PW 1400with a rhodium X-ray tube at 50kV and50mA. Strict quality control for majorelements was done using the method laidout by NORRISH & HUTTON (1969)while quality control for the trace ele-ments was carried out using the methodlaid out by FEATHER & WILLIS (1976).Absolute percentage errors and the stan-dard deviation are set out in table 1.

6.2. Protolith signatures throughgeochemistry

Since the contacts between the amphi-bolites and banded amphibolites are gra-dational, the geochemical analyses refermostly to amphibolites and only in threecases where clear meso- and microscopictextural relationships were observed, ban-ded amphibolites were analysed. All sam-ples were free of analysis-contaminatingfeatures, e.g. quartz veins.

Geochemically these rocks range fromintermediate to basic in terms of theirSiO2 content (table 1). Several plots,shown in figure 5, highlight some of therelationships obtained for these rocks interms of their possible protoliths and tec-tonic setting.

Using volcanic rock nomenclaturediagrams, all samples plot in the basicfields of the diagrams shown by figures5A and B. The diagram in figure 5Ashows a spread of the amphibolite sam-ples from the alkaline basalt to subalkali-ne basalt fields while the samples of thebanded amphibolites plot on the borderbetween the andesite/basalt and subalka-line basalt fields. The diagram in figure5B mirrors the spread of amphibolitesamples across the basalt and basalt-ande-site fields whereas the banded amphiboli-tes all plot within the basalt field. Thisapproach corroborates, in a way, the pre-vious considerations about a dominantbasic protolith for these rocks.

In order to evaluate the possible magmatype the amphibolites may have been deri-ved from, the set of samples has been plot-ted on the FeO(t)-(Na2O+K2O)-MgO(AFM) diagram (figure 5C). The spread of



Figure 4. A- Mineral classification diagram for the calcic amphiboles in amphibolite vs. bandedamphibolite. Although both sets of amphiboles plot on the same field of the LEAKE et al. (1997)diagram, the amphiboles in the banded amphibolite, mostly classified as magnesiohornblende,while the amphiboles in the amphibolites cluster loosely across the actinolite and magnesiohorn-blende fields. B- Classification diagram for the feldspars in the amphibolites and banded amphi-bolites. The distribution of the types of feldspars that occur in the two lithologies is different. Alldiagrams plotted from data whose averages are shown in appendices 1 and 2.

the amphibolite samples across both thetholeiitic and calc-alkaline fields of the dia-gram is observed. The banded amphibolitesamples group tightly within the tholeiiticfield (figure 5C). The diagram in figure 5Dshows a plot of the samples with respect totheir possible tectonic setting, tentatively

suggesting a within-plate versus a moreMORB tectonic setting for amphibolitesand banded amphibolites, respectively.However, the large scatter and lack of dis-crete grouping in any of the designatedfields makes this diagram hard to interpretand is not conclusive.


Table 1. Major and selected minor element geochemistry used for generating figures 5A, B, C andD. Fe2O3 was converted to FeO(t) and all major element values recalculated to 100% for the pur-poses of plotting the AFM diagram. Major element values expressed in wt% and minor elementvalues in ppm. Absolute errors for analytical methods are shown: average of 5 calibrations formajor elements and 3 calibrations for trace elements.

7. DISCUSSION

Amphibolite and banded amphiboliteconstitute a suite of rocks that are clearlydistinguishable in the field on a mesoscopicscale. However, the above mentionedapproaches and results, as well as the analy-tical data show that these rocks have a highdegree of variability that is dependant onthe mineralogy of each individual sample.

Geochemical data are inconclusivewith respect to establishing a tectonic set-ting perhaps due to an insufficient num-ber of samples collected. The observedgeochemistry and mineralogy indicate aprobable basic protolith for both amphi-bolites and banded amphibolites, but theprotolith could be more variable for thelatter since they may also exist texturesakin those found in rocks of sedimentary-


Figure 5. Geochemical relationships (after de OLIVEIRA, 2001) obtained for banded amphibolitesand amphibolites. Figures A and B deal with the nomenclature of the various rocks (A - after WIN-CHESTER & FLOYD, 1977; B - after COX et al., 1979). The discrimination of tholeiitic from calc-alkaline series is shown in the AFM diagram (C - after KUNO, 1968). The Zr/Zr-Y diagram for dis-criminating between within-plate, MORB and arc basalts is shown in D (after PEARCE ANDNORRY, 1979).

tuffaceous origin as exemplified by thepresence, in the banded amphibolites, ofboth discrete biotite layers and also theabundance of distinct amphibole-richlayers juxtaposed by plagioclase andquartz layers. Therefore, perhaps, thissuite of rocks should be generically treatedas amphibolite (s.l.) from a geochemicalpoint of view but from a textural andmineralogical point of view these amphi-bolite and banded amphibolite should betreated as individual entities as they havebeen and continue to be by explorationgeologists working in the area and in allthe Ossa Morena Zone.

ACKNOWLEDGEMENTS

This work represents a small part of amuch larger Ph.D. undertaken at theUniversity of the Witwatersrand(Republic of South Africa) by the senior

author and supervised by the second andfourth authors. We would like to ackno-wledge the contribution of Prof. F. M.Meyer of the Institut für Mineralogie undLagerstättenlehre (Aachen, Germany) insupplying microprobe facilities and R.Solá of the IGM who helped with the dia-grams. The senior author benefited froma Praxis XXI PhD bursary (BD/15877/98)awarded by the Fundação para a Ciência ea Tecnologia. Messrs. A. Gouveia and A.Verde are thanked for polished thin sec-tion preparation. Mr. J. L. Pinto is than-ked for help with sample collection andfield work.

Recibido: 07-XI-02Aceptado: 20-I-03



Appendix 1. Average microanalytical data for amphiboles. Structural formula calculated on ananhydrous basis to cations per 23O after RICHARD & CLARKE (1990); Fe2+ and Fe3+ calculatedafter ROBINSON et al. (1981). [Probe: JEOL JXA.8900R; 25 nA beam current; 15 kV acceleratingvoltage; 1 µm beam diameter; 10 and 5s counting times (peak and background respectively)].


Appendix 2. Average microanalytical data for feldspar. (Probe data as per appendix 1).

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