Borg, Gregor

Ehser, Anja

Oliveira, Daniel P. S.

Rosa, Diogo R. N.

Salgueiro, Rute

de Oliveira, Daniel Pipa Soares

Rosa, Diogo

Repositório do LNEG

11 th SGA Biennial MeetingLet’s Talk Ore Deposits26-29th September 2011 Antofagasta, Chile814Characterization of placer gold deposits from centralPortugal: preliminary results Anja Ehser, Gregor BorgInstitute of Geosciences, Martin-Luther-University Halle-Wittenberg, Von-Seckendorff-Platz 3, 06120 Halle (Saale),GermanyDaniel P. S. de Oliveira, Diogo R. N. Rosa, Rute Salgueiro Laboratório Nacional de Energia e Geologia, Laboratório de Geologia e Minas, Estrada da Portela, Apartado 7586, 2721-866 Alfragide, PortugalAbstract. The streams in the Lower Tagus Basin andalong the contact of the Central Iberian and Ossa Morena zones in central Portugal carry significant amounts ofgold. In this study, four placer gold deposits have beencharacterized using geochemical, morphological and heavy mineral data and compared with the styles of goldmineralization in the vicinity. The geochemical similarities of the Tagus palaeo-placer and the Tripeiro and Ergesgold placers suggest a primary gold source likely to besimilar to the mineralization found at Sarzedas andPomar. The more heterogenic composition of the Caiagold placer probably reflects the different styles ofmineralization found at La Codosera, Los Algarbes and Alburquerque to the east.Keywords. Central Portugal, placer gold, LA-ICP-MS1 IntroductionThis study uses minor and trace element data,determined by LA-ICP-MS during a provenance studyon the gold of the unique Early Bronze Age Nebra Sky Disk, central Germany. The data is used to characterize four placer gold deposits of potential prehistoricimportance in central Portugal together with grainmorphology and the associated heavy mineral fraction.The results are discussed further with regard to the stylesof gold mineralization found in the vicinity of the placerdeposits. Similar studies on the geochemical signature ofgold have been carried out since the early works by Watling et al. (1994) on numerous lode and placer golddeposits worldwide, also by studying the microchemicalcomposition of the gold particles (e.g., Chapman et al.2009). More detailed investigations on further goldsamples of these and other, also primary gold deposits,are currently carried out to obtain additional informationon the geochemical and microchemical variability of thedeposits.2 Geological settingThe study area comprises mainly the Lower Tagus Basin(LTB) which extends from the littoral western part of theSetúbal Peninsula to the region of Castelo Branco at theborder to Spain. The Tagus estuary in the Seixal area(Fig. 1a) exhibits auriferous Pliocene sand and graveldeposits of detrital origin, which are located exclusivelyat the southern Tagus margin and are partially covered by Quaternary alluvial and detrital deposits. Primarygold mineralization is unknown for the greater Tagusestuary.  Figure 1. Locality maps of the study area, modified after the Carta Geológica de Portugal Scale 1:500.000 (1992). (a) Simplified geological map of the Tagus estuary containing theTagus palaeo-placer (1). (b) Simplified geological map of thesouthern CIZ and BCSZ in the Castelo Branco regioncontaining the Tripeiro (2), Erges, Rosmaninhal (3-6), Erges,Vale Feitoso (7), and Caia (8-9) gold placers.In contrast, mineralization in the Castelo Brancoregion (Fig. 1b) in the southern Central Iberian Zone(CIZ) and along the left-lateral Badajoz-Cordoba ShearZone (BCSZ), separating the CIZ in the north from theOssa Morena Zone (OMZ) in the south, is mainlycharacterized by post-metamorphic quartz veins that areoften associated with late brittle faults of Hercynian toLate Hercynian age (Murphy and Roberts, 1997). Thefluids result from devolatilisation and dehydration duringprograde metamorphism of a several kilometers thicksequence of Precambrian greywackes and shales(Complejo Esquisto Grauvaquico or CEG) and are ofvarying aqua-carbonic and meteoric mixture (Murphyand Roberts, 1997). At Sarzedas, Pomar, and Mari Rosa(Fig. 1b), the gold mineralization is hosted in the CEG11 th SGA Biennial MeetingLet’s Talk Ore Deposits26-29th September 2011 Antofagasta, Chile815and mainly associated with arsenopyrite and minorpyrite (As-Au, Sarzedas and Pomar) or stibnite (Sb-Au, Mari Rosa). Late stage remobilization at Sarzedas andPomar resulted in the deposition of gold and Cu-Pb-Znsulphides. At La Codosera, Devonian slates and sandstones form the main lithological host of theauriferous quartz-pyrite-(arsenopyrite) veins (Murphyand Roberts 1997) while several kilometres to the westat Los Algarbes, Devonian quartzites contain Sb-Aumineralization. Stratabound mineralization occurs atAlburquerque, where gold is disseminated and found parallel to the bedding of Ordovician-Silurian blackshales or in gossanous veinlets within quartzites(Murphy and Roberts 1997). At São Martinho within theBCSZ, post-metamorphic gold mineralization is alsomainly disseminated and runs parallel to the foliation ofLate Proterozoic schists, with arsenopyrite, pyrite, andchalcopyrite as main mineral phases (Oliveira et al.2007). Late stage remobilization during graniticmagmatism resulted in the deposition of gold withpyrrhotite, pyrite, chalcopyrite and loellingite. 3 Samples and analytical techniques The recent placer gold samples derive mainly from a rareearth reconnaissance survey on Portuguese stream sediments, which included sampling, determination andquantification of heavy minerals (Inverno et al. 2007)with grain sizes below 3 mm. From the rivers with aprehistoric potential for recoverable gold placers, tenrepresentative particles per deposit were selected tocharacterize the morphology and chemical composition.The Tagus palaeo-placer (Salgueiro et al. 2000) andTripeiro gold placer were sampled separately. For the study of the grain morphology, a light microscope and scanning electron microscope were used. Concentrationsof major, minor, and trace elements were determined byLA-ICP-MS at the Curt-Engelhorn-ZentrumArchäometrie (CEZA), Mannheim, Germany. Fulldetails on the operating conditions for the ICP-MS andlaser ablation systems are given in Schlosser et al. (2009). Due the low thickness of most of the goldsamples and based on the initial aim of the analyses toobtain the average composition of the natural gold forcomparison with artefact gold, line ablation with a spotdiameter of 50 �m and energy of 45% was performedover a length of 300 to 600 �m on all samples. A pre-ablation time of 20 to 40 s using a spot diameter of 75�m and energy of 30% was applied to eliminate surfacecontaminations and to diminish the influence ofsupergene rims on the results.  4 Grain morphologyThe grain morphology has been investigated using the criteria by Townley et al. (2003) with the obtainedresults summarized in Table 1. The gold from the Taguspalaeo-placer and Tripeiro placer shows a very regular toregular morphology with strong signs of folding orhammering. In contrast, placer gold from the river Ergesshows a regular to very irregular morphology suggestinga shorter distance to its primary sources compared withthe Tagus palaeo-placer and Tripeiro placer. The largestvariation in morphological characteristics, also in grainsize, is observed for placer gold from the river Caia withthe number of populations, present at both samplelocalities, to be investigated. Table 1. Morphological characteristics of the studied placergold deposits using the criteria by Townley et al. (2003). Tri. =triangular, rect. = rectangular, elong. = elongated, reg. =regular, irreg. = irregular.Deposit Shape Outline Surface Assoc.mineralSub-ordinateTagus oval-round;rect.(10%)very reg.;reg.(10%)very reg.(folded); reg.(hammered)(10%)- - Tripei-rorect.-ovalvery reg.-reg.very reg.(hammered)qtz folding Erges,Rosm.tri.-rect.reg.-very irreg.reg.-irreg. qtz folding,welding  Erges,ValeFeitoso  rect. reg.;irreg.(20%)reg.;  irreg. (20%)-;  qtz(20%)ham-meringCaia elong.-rect.very reg.-irreg.very reg.-irreg.qtz -5 Chemical compositionFrom the 25 elements measured by LA-ICP-MS, theconcentrations of Co, Ru, Os, Ir, Pt, Tl, and Bi werebelow the detection limit of 5 ppm. Cr, Mn, Zn, Pd, andPb were only determined occasionally and, therefore, notconsidered further for chemical characterization. The remaining elements and their general significance for thechemical composition of the placer gold deposits aresummarized in Table 2. The variation of the minor andtrace elements is illustrated in Figure 3.Table 2. Trace elements of the studied placer gold,discriminated by their median concentrations. References forheavy minerals: 1 = Inverno et al. (2007), 2 = Salgueiro et al. (2000). and = andalusite, c = cassiterite, cy = cyanite, hem =hematite, il = ilmenite, lx = leucoxene, r = rutile, sil =sillimanite, st = staurolith, tu = tourmaline, zr = zircon.Deposit Trace elements(median >25 ppm)Trace elements(median 25-5ppm)Main heavyminerals(>5%)Tagus Fe, Cu, Ni Se, Ti il, st, tu, and, sil,cy, zr, r 2Tripeiro Cu, Fe Ni, Se not determinedErges,Rosm.Cu, Fe Ni, Te, Se, As, Cdhem, lx, zr, tu 1Erges, ValeFeitosoFe, Cu, Ni Te, Se -Caia Fe, Sb, Cu, Ni Sn, Te, Se c, il, lx, r 1Ni and Se show the smallest variations inconcentration with the concentrations being similar forall placer gold deposits. The Tagus palaeo-placer andTripeiro gold placer are characterized by low Ag and Teconcentrations, with the Tripeiro gold placer being, intotal, most depleted in trace elements (Fig. 3a). The relatively high Ti and Fe concentrations in the Taguspalaeo-placer may result from secondary mineralformation or inclusion during the long weatheringprocess. However, the main geochemical associations ofthe Tagus palaeo-placer and the Tripeiro and Erges gold11 th SGA Biennial MeetingLet’s Talk Ore Deposits26-29th September 2011 Antofagasta, Chile816placers, namely the median Ag, as well as (Fe), Cu, Ni,and Se concentrations, can be considered similar giventhe natural variability in composition within one golddeposit, as well as the influence of supergene formationand modification of the primary gold and its chemicalcomposition during weathering and transport. Incontrast, the Caia gold placer also contains significantamounts of Sb or Sn, with the corresponding goldsamples deriving from different localities located twokm apart from each other (No. 8 and 9 in Fig. 1). However, at both localities, the heavy mineral fractioncontains more than 25% cassiterite (Table 2). The heterogenic composition of the Caia gold placer is alsodisplayed in the large variation in Ag concentrations,ranging from 0 to 15 wt.% at the locality 8 and from 5 to32 wt.% at the locality 9.  Figure 3. Variation of minor and trace elements in gold from(a) the Tagus palaeo-placer and Tripeiro placer, as well as (b) the Erges and Caia placers. Concentrations are in ppm, exceptfor Ag and Au. n = number of data sets acquired.6 DiscussionThe geochemical similarities of the minor and trace elements in the Tagus palaeo-placer and the Tripeiro andErges gold placers suggest a similar style of goldmineralization for the primary gold sources, althoughdifferences in the Ag and Te concentrations between the Erges gold placers and the Tagus palaeo-placer andTripeiro gold placer have been detected. However, sincethe very regular to regular morphology of the Taguspalaeo-placer and Tripeiro gold placer suggests a muchlonger fluvial transport compared with the Erges goldplacers, a stronger depletion of elements such as Ag ispossible. In addition, different mineralization events, investigated for the Sarzedas and Pomar primary golddeposits in the vicinity of the Tripeiro and Erges goldplacers, promote the formation of gold with different initial chemical compositions. As the rivers Tagus,Tripeiro and Erges cross the same lithology found atSarzedas and Pomar (CEG), similar post-metamorphic (As-Au) to late stage mineralization is likely to befound, with the Tagus palaeo-placer being locatedfarthest away from known primary gold deposits. Theheterogenic composition of the Caia gold placer may reflect the heterogenic mineralization of As-Au and Sb-Au found to the east, including the primary gold depositsof La Codosera, Los Algarbes and Alburquerque. Thehigh Sn concentrations in the Caia gold placer mayresult from the influence of the Sn-W mineralization inthe aureole of the adjacent Alburquerque Late Hercyniangranitic batholith, which will have to be investigated byobtaining more data, also on the primary gold deposits.AcknowledgementsWe thank Ernst Pernicka and Nicole Lockhoff from the CEZA for their support during the course of this study,which is embedded in the framework of the researchgroup 550 funded by the DeutscheForschungsgemeinschaft (DFG). The manuscriptbenefited from the valuable comments of Rob Chapman.ReferencesChapman RJ, Leake RC, Bond DPG, Stedra V, Fairgrieve B (2009)Chemical and mineralogical signatures of gold formed inoxidizing chloride hydrothermal systems and their significancewithin populations of placer gold grains collected duringreconnaissance. Econ Geol 104:563-585 Inverno CMC, de Oliveira DPS, Rodrigues L (2007) Inventariaçãoe prospecção de terras raras nas regiões fronteiriças da BeiraBaixa e do Norte Alentejo: INETI, Internal Report, 2982 pMurphy PJ, Roberts S (1997) Evolution of a metamorphic fluidand its role in lode gold mineralisation in the Central Iberian Zone. Miner Deposita 32:459-474Oliveira DPS, Robb LJ, Inverno CMC, Charlesworth EG (2007)Metallogenesis of the São Martinho and Mosteiros GoldDeposits, Tomar Cordoba Shear Zone, Portugal. Int Geol Rev49:907-930 Salgueiro R, Chichorro M, e Martins L (2000) Ocorrência de ouronos depósitos pilocénicos da zona da Cruz de Pau (Seixal). 1ºCongresso sobre o Cenozóico de Portugal, pp 171-178 Schlosser S, Kovacs R, Pernicka E, Günther D, Tellenbach M(2009) Fingerprints in Gold. In: Reindel M, Wagner GA (eds)New Technologies for Archaeology. Multidisciplinary Investigations in Palpa and Nasca, Peru. Springer BerlinHeidelberg, pp 409-436 Townley BK, Hérail G, Maksaev V, Palacios C, Parseval P,Sepulveda F, Orellana R, Rivas P, Ulloa C (2003) Gold grainmorphology and composition as an exploration tool:application to gold exploration in covered areas. Geochem-Explor Env A 3:29-38Watling RJ, Herbert HK, Delev D, Abell ID (1994) Goldfingerprinting by laser ablation inductively coupled plasmamass spectrometry. Spectrochim Acta 49B:205-219

Characterization of placer gold deposits from central Portugal : preliminary results

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Characterization of placer gold deposits from central Portugal : preliminary results

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