Gossans, the brightly coloured oxidation products of sulphide mineralised rocks, have acted as an exploration target for base and precious metals and sulphur for thousands of years. They are easily identified from remote sensing and field-based reconnaissance, and once found may be drilled to determine the character of mineralisation below. The number of targets drilled could potentially be reduced if gossans overlying significant mineralisation can be discriminated from their field relations, mineralogy and geochemistry. Previous such studies have focussed on porphyry-type systems, with less attention on the generally much lower tonnage volcanogenic massive sulphide (VMS) deposits. However, VMS continue to provide an economically important source of metals in Europe and elsewhere. The Troodos Massif in Cyprus was chosen for this study as it hosts a currently active Cu mine along with historically worked VMS, is little deformed and has a relatively well understood geological framework. Of particular interest are secondary Cu deposits (SCUD) which form due to weathering of primary massive sulphides (PMS). These can be worked at relatively lower financial and environmental cost, and at much lower grades (down to around 0.1 % Cu). The only currently mined SCUD in Cyprus is the Phoenix ore body at Skouriotissa, which lies immediately adjacent to, and structurally below the Phoukasa PMS. The questions addressed in this study are: 1) Do Cypriot PMS that were mined for Cu show original Cu enrichments, or is their elevated Cu content a result of supergene enrichment to form an SCUD? This was addressed by comparing the mineralogical, chemical and S isotopic compositions of PMS mined for Cu with those mined for pyrite only from across the Troodos; 2) Do gossans formed from Cu-rich sulphides show distinctive mineralogical and chemical signatures? The characteristics of gossans known to overlie prospective sulphide bodies were compared with those from barren PMS; 3) What circumstances promote the formation of SCUDs? In particular, did sulphide oxidation occur on the sea floor or in a terrestrial environment? It was considered likely that SCUD formation may require sea floor oxidation because this will result in limited Cu dispersion, due to both sharp pH and redox gradients and limited fluid flow when compared with terrestrial weathering, where the depth to the water table can be considerable. The question was addressed by comparing the field relations, chemistry and S and O isotope compositions of gossans thought to have formed on the sea floor (Skouriotissa - Phoenix) with those generated in a terrestrial setting (Kokkinopezoula, Mathiati and Sia). The remnants of primary VMS deposits mined for Cu in Cyprus (Phoukasa, Sia and Troulli) almost exclusively contain primary Cu sulphides such as chalcopyrite. Secondary Cu sulphides, mainly chalcocite and covellite, are only present in significant concentrations at Phoukasa and Troulli, with Cu oxides being found in Phoenix. At Phoukasa, secondary Cu sulphides have a mean δ34S = 3.69±0.08 ‰ similar to primary pyrite and chalcopyrite (mean δ34S = 3.78±0.08 ‰) suggesting formation from Cu-rich fluids that scavenged S from primary sulphides. Sulphide material collected from copper mines has Cu = 840 to > 10,000 ppm at Phoukasa; 167 to 3573 ppm at Sia; 288 to > 10,000 ppm at Troulli, while the Cu-barren deposits have generally lower Cu grades (Cu = 170 to 433 ppm at Kokkinopezoula; 327 to 1303 ppm at Mathiati north). There are no systematic differences in the S isotope compositions of pyrite between deposits mined for Cu and those not (average δ34S = 1.68, 3.74 and 7.1 ‰ for Cu-rich Sia, Lysos and Phoukasa, and 5.03 and 3.70 ‰ for Cu-poor Kokkinopezoula and Mathiati North sulphides, respectively). No consistent chemical differences (including chalcophile elements) could be identified between gossans overlying Cu-rich as opposed to barren PMS. Gossans overlying the Lysos and Sia Cu-rich PMS, however, show an enrichment in Pb and Zn not observed in other gossans, and umbers, which are chemical sediments associated with VMS systems, often overlying gossans, show strong Cu enrichments in the vicinity of Cu-rich PMS. Umber samples from near the Cu-rich Phoukasa sulphide body contain > 10,000 to 35,400 ppm Cu, while those around Cu-poor Mathiati North contain 669 to 819 ppm Cu. There were no differences in the S isotope compositions of gypsum from sulphide bodies which were Cu-rich (δ34S = 5.9 to 6.9 ‰ for Sia, Phoukasa and Troulli) and Cu-poor (δ34S = 5.0 to 7.3 ‰ for Kokkinopezoula, Mathiati North). Regarding the environment of formation of SCUDs, an initial submarine oxidation of the Phoukasa VMS is considered likely as it is immediately overlain by marine pelagic sediments, while all other deposits studied are overlain by volcanics. In addition, volcanics in the vicinity of Phoukasa show large negative Ce anomalies (Ce/Ce* = 0.90 to 0.38, average = 0.71), consistent with sea floor alteration, compared with other localities such Kokkinopezoula (Ce/Ce* = 0.89 to 1.08, average = 0.97) and Sia (Ce/Ce* = 0.92 to 1.03, average = 0.99). Unfortunately, the S isotope composition of gypsum could not be used to determine the nature of the gossan-forming environment. Gypsums from all locations (average δ34S = 6.74±0.08 ‰) have δ34S values similar to, but slightly 34S enriched compared with their associated sulphides (average δ34S = 2.9±0.08 ‰) which indicates that their S isotope signature largely reflects that of S released during sulphide oxidation, as opposed to evaporation of sulphate-rich waters or direct precipitation from a similar solution (i.e., seawater). However, the oxygen isotope composition of gypsum (average δ18O = 6.2 ‰) from Sia (average δ18O = 2.4 ‰) reflects a mixture of atmospheric O (δ18O = 23.6 ‰) and Mediterranean meteoric water O (δ18O ≈-5.0 ‰), indicating a terrestrial environment of formation. Gypsum from Skouriotissa has an average δ18O = 6.6 ‰ which most likely indicates a combination of seawater and seawater-dissolved O (δ18O ≈23.5 ‰), despite some overlap with the composition of meteoric water and atmospheric O. In summary, it is proposed that the currently unique nature of Skouriotissa as hosting the only major SCUD in Cyprus is due largely to initial sea water alteration of the Phoukasa PMS resulting in limited Cu dispersion and localised Cu enrichment within the primary ore body. Subsequent uplift and alteration of the Phoukasa PMS led to the formation of a relatively high grade SCUD in the Phoenix deposit. The main outcomes of the study are a series of models for the development of gossans and associated lithologies in terrestrial and seafloor weathering environments in Cyprus. These incorporate a new term (retali) for acid leached volcanics in the footwall of PMS, and exploration-relevant field, mineralogical and chemical criteria for their discrimination from gossans, which overlie PMS. In agreement with an existing model, the formation of the Phoenix SCUD is interpreted as having been due to the downward migration of Cu-bearing acid fluids from the seafloor oxidation of the upper parts of the Phoukasa deposit. Secondary Cu mineralisation is thought to have taken place within the relatively reducing environment below the water table in lavas stratigraphically below the Phoukasa deposit. That the formation of SCUDs may require seafloor sulphide oxidation, and that this can be recognised in the mineralogy and chemical compositions of associated volcanics and gossans, provides new exploration criteria for SCUDs. However, it should be noted that the Phoenix deposit was the only SCUD examined in this study, and that this model should therefore be tested elsewhere.

Submitted by Edisangela Bastos (edisangela@ufpa.br) on 2017-03-14T12:27:35Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_EvolucaoMineralogicaGeoquimica.pdf: 13592539 bytes, checksum: dfda6e62c8b7c75992710ab0bdcdb032 (MD5) / Approved for entry into archive by Edisangela Bastos (edisangela@ufpa.br) on 2017-03-17T11:52:03Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_EvolucaoMineralogicaGeoquimica.pdf: 13592539 bytes, checksum: dfda6e62c8b7c75992710ab0bdcdb032 (MD5) / Made available in DSpace on 2017-03-17T11:52:03Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_EvolucaoMineralogicaGeoquimica.pdf: 13592539 bytes, checksum: dfda6e62c8b7c75992710ab0bdcdb032 (MD5) Previous issue date: 1995-07-31 / PADCT - Programa de Apoio ao Desenvolvimento Científico e Tecnológico / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / Para estudar a gênese dos solos desenvolvidos sobre crostas lateríticas foram selecionados cinco perfis, localizados na região de Carajás (N5 e igarapé Bahia) e nos municípios de Paragominas (Mina de Camoaí) e Belém (balneários de Outeiro e Mosqueiro), todos no Estado do Pará. A estruturação dos cinco perfis (N5, Igarapé Bahia, Camoaí, Outeiro e Mosqueiro) permitiu individualizar quatro horizontes: crosta sã, crosta parcialmente intemperizada, crosta intemperizada e solo. A crosta sã é compacta vacuolar, constituída por óxi-hidróxidos de Fe e Al. A crosta parcialmente intemperizada compõe-se por relictos da crosta sã envolvidos por uma matriz argilosa amarelada ou avermelhada, dependendo da composição mineralógica da crosta sã. A crosta intemperizada caracteriza-se pela predominância da matriz argilosa em relação a seus relictos enquanto o solo é formado exclusivamente por material argiloso. A estruturação dos perfis mostra que a passagem da crosta sã para o solo sobreposto é gradual, havendo expansão de volume no inicio da fragmentação e posterior colapso do perfil, quando os relictos se transformam totalmente em um material terroso e microagregado. Os perfis de N5 e Igarapé Bahia são derivados a partir de urna crosta ferro-aluminosa matura. Caracterizam-se pelos teores mais elevados de caolinita e hematita na crosta e seus relictos. No solo concentram-se a gibbsita, Al-goethita e anatásio com tendência a formação de um solo inicialmente bauxítico, que passa a caolinítico em direção ao topo do perfil. No Igarapé Bahia tem-se acúmulo de quartzo no solo. Quimicamente observa--se que a modificação mineralógica leva a um decréscimo nos teores de Fe203 e 6i02 e aumento de Al203, TiO2 e PF. Os elementos-traço (V, Cr, Ni, B, Mo, Zr, Ga, Sc, Y, Mn, Cu e Pb, além de Au no perfil do igarapé Bahia) e os ETR mostram concentração de B e Zr, enquanto os demais tendem a serem diluídos da crosta para o solo. O balanço isotitânio mostra que a transformação da crosta em solo se dá por lixiviação de SiO2 e Fe2O3 nos dois perfis, enquanto o Al2O3 se enriquece no perfil de N5 e é lixiviado no perfil do Igarapé Bahia. A formação do solo de Camoaí, a partir de crosta aluminosa matura, difere dos perfis de N5 e Igarapé Bahia pela diminuição nos teores de gibbsita e aumento nos de caolinita e quartzo, enquanto os teores de hematita, goethita e anatásio, muito baixos, não tem variação significativa. Acompanhando a composição mineralógica tem-se diminuição nos teores de Al2O3 e aumento nos de SiO2, enquanto os de Fe2O3 e TiO2, muito baixos, não tem uma distribuição característica. Os elementos-traço e os ETR, com teores bem mais baixos que nos demais perfis, mostram aumento de Zr e Mn em direção ao solo e diluição dos demais elementos. Em Camoaí a transformação da crosta aluminosa em solo, segundo o balanço isotitânio, dá-se pela lixiviação de Al2O3 e enriquecimento em SiO2 e Fe2O3. Nos perfis de Outeiro e Mosqueiro, derivados de crostas lateriticas silico-ferruginosas imaturas, a formação do solo se com diminuição nos teores de hematita+goethita, de modo que no solo predominam quartzo e caolinita. Conseqüentemente tem-se decréscimo nos teores de Fe2O3 e aumento nos de SiO2 e Al2O3. A distribuição dos elementos-traça mostra, assim como em N5 e igarapé Bahia, concentração de Zr e diluição dos demais. Com base no balanço isotitânio a transformação da crosta dos perfis de Outeiro e Mosqueiro se dá pela lixiviação de Fe2O3, enquanto o SiO2 e o Al2O3 se enriquecem no perfil de Outeiro e são lixiviados no de Mosqueiro. Com base na estruturação dos perfis, na mineralogia e geoquímica, foi possível caracterizar um acentuado processo de desferrificação das crostas ferro-aluminosas e sílico-ferruginosas e de desaluminizaçâo das aluminosas, levando a geração de solos. No perfis derivados das crostas ferro-aluminosas e silico-ferruginosas tem-se a substituição da hematita por Al-goethita, uma fase intermediária onde predomina gibbsita nos perfis ferro-aluminosos e, finalmente, caolinita no topo dos dois perfis. Em todos os perfis observou-se acúmulo em quartzo. O enriquecimento em quartzo e o provável aporte externo de SiO2, via decomposição de material vegetal, levou a neoformação da caolinita a partir da gibbsita nos perfis de N5, igarapé Bahia e Camoaí. Nos perfis de Outeiro e Mosqueiro a presença de caolinita no solo deve-se ao seu enriquecimento relativo a partir da crosta. Como consequência da transformação mineralógica houve a lixiviação de ferro nos perfis de N5, igarapé Bahia, Outeiro e Mosqueiro e de alumínio no de Camoaí, além de V, Cr, Ni, Mo, Ga, Sc, Cu, Pb e ETR, forte enriquecimento em Zr e em parte de 13 e Mn em todos os perfis. As transformações que levaram à quebra no equilíbrio das crostas, a sua desagregação e cominuição, formando relictos e uma nova fase mineral argilosa, são consequências da ação da matéria orgânica, que torna o ambiente mais ácido e menos oxidante, em um processo similar ao da formação de perfis intempéricos em ambiente tropical úmido. As diferenças mineralógicas e geoquímicas observadas entre os perfis são conseqüência das variáveis composicionais das crostas que, em parte, refletem também o seu grau de maturidade. As características mineralógicas e geoquímicas dos solos nos perfis estudados permitem que eles sejam correlacionados a Argila de Belterra, podendo-se portanto admitir que a Argila de Belterra tem origem autóctone em relação as crostas lateríticas subjacentes. / With the aim of studying the genesis of soils developed on lateritic crusts, five profiles located in the region of Carajás (N5 and Igarapé Bahia procpects) as well as in Paragominas (Camoaí Mine) and Belém (Mosqueiro and Outeiro Beaches) municipalities all them located in Pará State, Brazil - have been selected. Based on the structures described in the profiles (N5, igarapé Bahia, Camoaí, Outeiro and Mosqueiro), it has been Possible to distinguish four differents horizons: fresh crust, partially-weathered crust, weathered crust and soil. The fresh crust is compact, vacuolar and composed of iron and aluminium oxyhydroxides. The partially-weathered crust is made up of fresh-crust relicts enclosed in yellowish or reddish clayey matrix, depending on the mineralogical composition of the fresh crust. The weathered crust is characterized by dominance of clayey matrix in relation to relicts, while the soil is composed exclusively of clayey material. The structures of the profiles show that the change from fresh crust to the overlying soil is gradual with volume expansion at the beginning of the fragmentat ion processes. This expansion is followed by the collapse of the profile as the relicts are entirely transformed into an earthy micro-aggregated material. The profiles of N5 and Igarapé Bahia prospects have been derived from a mature iron-aluminous crust, being characterized by the highest contents of kaolinite and hematite either in the crust or in the relícts. Gibbsite Al-goethite, anatasse and quartz are concentrated in the soil, which tends to be initially bauxitic, at the base of the profile, and to become kaolinite-rich toward the top. The mineralogical changes led to chemical modifications such as decrease in Fe2O3 and 8iO2 and increase in Al2O3, TiO2 and PF. The trace elements (V, Cr, Ni, R, Mo, Zr, Ga, Sc, Y, Mn, Cu and Pb besides Au in the profile of Igarapé Bahia) and REE's exhibit concentrations of 8 and Zr whereas the remaining ones tend to be progressively diluted from the crust toward the soil. The TiO2-mass balance shows that the transformation of the crust into soil took place by means of leaching of SiO2 and Fe2O3 in two profiles above mentioned, while Al2O2 underwent enrichment in the profile of N5 and leaching in the profile of igarapé Bahia. The soil formation from a mature aluminous crust in Camoaí differs from those described in the profile of Igarapé Bahia and of N5 in that it shows a decrease in gibbsite contents and an increase in kaolinite and quartz, whereas the verey low contents of ,hematite, goethite, and anatase do not present a characteristic distribution. The trace elements and the REE's, with contents much lower than those observed in the others profiles, show an increase in Zr and Mn toward the soil horizon and a dilution of the remaining ones. In the Camoaí, the transformation of the aluminous crust into soil, based on the TiO2-mass balance, took place through leaching of Al2O3 and enrichment in SiO2 and Fe2O3. In the profiles of Outeiro and Mosqueiro, deriveci from immature silicoferruginous lateritic crust the soil formation occurred by means of decreasing in hematite+goethite so that, in the resulting soil, quartz and kaolinite predominate. In consequence of this, a decrese in the Fe2O3 contents and an increase in SiO2 and Al2O3 have been observed in those profiles. The trace elements distribution presents, as in igarapé concentration of Zr and dilution of the remaining ones. Based on TiO2-mass balance, it could be noticed that the transformation of the crust in the profiles of Mosqueiro and Outeiro took place through leaching of Fe2O3, whereas SiO2 and Al2O3 underwent enrichment in the profile of Outeiro and Leaching in the one of Mosqueiro. On the basis of structures, mineralogical composition and geochemical data of the profiles, it has been possible to characterize an accentuated process of Fe loss in the iron-aluminous and in the silicoferruginous crust as well as Ai removal in the aluminous ones what led to soil generation. In the profiles derived from the iron-aluminous and the silicoferruginous crust, it has been observed substitution of hematite by Al-goethite, an intermediated segment - where there is predominance of gibbsite in the iron-aluminous profiles -- and presence of kaolinite at the top of both profiles. In every profiles, it has been reported quartz acumulation. The enrichment in quartz and the S1O2 being the latter likely come from externai sources -via vegetable-material decay - caused the neoformation of kaolinite from gibbsite in the profiles of N5, igarapé Bahia, and Camoaí. In the profiles of Mosqueiro and Outeiro the kaolinite presence in the soil is a consequence of its relative enrichment in the relation to the crust from which it has been derived. As a consequence of the mineralogical transformation, Fe has been leached from the profiles of N5, Igarapé Bahia, Mosqueiro, and Outeiro and Al from those of the Camoaí, as well as V, Cr, Mo, Ga, Sc, Cu, Pb and REErs. On the other hand, a strong enrichment in Zr and, in part, in 8 and Mn has been reported in every stud ied profiles. The transformations that caused the break down of the balance of the crusts, their desmantlement and comminution, forming relicts and a new clayey mineral phase, are consequence of organic matter influence which turns the environments conditions more acid and less oxidizing, in a process similar to that of weathering-profiles formation in humid tropical environments. The mineral and chemical difference observed between the profiles are consequence of compositional variation of the crusts which, in part, also reflect the degree of maturity of theirs. The mineralogical and geochemical characteristics of the soils in the investigated profiles allow us to correlate them to the Belterra Clay and, in consequence of this, to admit that Belterra Clay had an autochthonous origin in relation to the underlying lateritic crusts.

Submitted by Edisangela Bastos (edisangela@ufpa.br) on 2017-03-16T12:24:42Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_MineralogiaGeoquimicaGossans.pdf: 14742312 bytes, checksum: 5be48bb9b746015745db9174a14c2db5 (MD5) / Approved for entry into archive by Edisangela Bastos (edisangela@ufpa.br) on 2017-03-17T12:21:52Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_MineralogiaGeoquimicaGossans.pdf: 14742312 bytes, checksum: 5be48bb9b746015745db9174a14c2db5 (MD5) / Made available in DSpace on 2017-03-17T12:21:52Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_MineralogiaGeoquimicaGossans.pdf: 14742312 bytes, checksum: 5be48bb9b746015745db9174a14c2db5 (MD5) Previous issue date: 1996-03-20 / DAAD - Serviço Alemão de Intercâmbio Acadêmico / Deutscher Akademischer Austauschdiens / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / FINEP - Financiadora de Estudos e Projetos / A mina Igarapé Bahia e o prospecto Águas Claras são exemplos de mineralizações de ouro supergênico relacionados a gossans e lateritos. Ambas as áreas estão situadas na região de Carajás, Estado do Pará, pertencem a Companhia Vale do Rio Doce e foram pesquisadas pela Docegeo. Neste trabalho foram estudados a mineralogia e a geoquímica dos perfis laterito-gossânicos dessas duas áreas, com ênfase para a distribuição do ouro e outros elementos associados. As duas áreas em questão apresentam estilos de mineralização primária semelhantes e dentro do mesmo contexto geológico regional, a saber, ouro associado a zonas de sulfetos maciços ou disseminados, ligados a processos de alteração hidrotermal em zonas de cisalhamento, cujas rochas hospedeiras são seqüências metavulcano-sedimentares do Arqueano-Proterozóico. Os produtos supergênicos são divididos em dois grupos distintos: os do sistema gossânico e os do sistema laterítico, onde foi evidenciada a superposição do último sistema sobre o primeiro. Na descrição dos perfis supergênicos, através de amostras e informações de superfície e sub-superficie, os seguintes horizontes e zonas foram caracterizados, da base para o topo: (1) no sistema gossânico: zona de sulfetos primários, zona de cementação e espessa zona de oxidação; (2) no sistema laterítico: crosta lateritica com fragmentos dos gossans, crosta laterítica desmantelada ou linhas de pedras e latossolos. O perfil laterítico se desenvolveu sobre gossans pré-existentes, com obliteração das suas feições originais e promovendo remobilização química e física do ouro e dos outros elementos. No quadro geomorfólogico atual, a área Igarapé Bahia apresenta essa estruturação completa, enquanto que na área Águas Claras, o perfil laterítico sobre os corpos mineralizados foi truncado e os gossans estão aflorantes. A composição mineralógica da porção superior dos gossans e dos lateritos é essencialmente à hematita, goethita (com teores variáveis de Al), maghemita, gibbsita, caulinita e quartzo, em diferentes proporções. Nos gossans é nítido o domínio da hematita sobre os demais minerais. Nas porções mais profundas dos gossans, em direção a zona de sulfetos primários, foram identificados: malaquita, cuprita e cobre nativo, predominantemente, e associados a hematita, além de azurita, crisocola e quartzo; na zona de sulfetos primários observou-se uma paragênese um pouco distinta, entre as duas áreas. Em Igarapé Bahia dominam: calcopirita, magnetita, clorita, siderita e quartzo, enquanto em Águas Claras foram descritos: calcopirita, pirita, arsenopirita, cobaltita, magnetita, quartzo, wolframita e turmalina. O ouro primário ocorre finamente disseminado, incluso nos sulfetos, apresentando diferentes graus de pureza. Na área Águas Claras, ocorre associado a uma grande variedade de teluretos de Bi, Ag, Pb e Bi nativo. Ainda nesta área, turmalina (dravita) e wolframita (do tipo ferberita) são importantes minerais acessórios, comportando-se como resistatos, durante o desenvolvimento dos perfis, enriquecendo-se nos gossans e nas crostas, na forma de agregados centimétricos, e servindo como importantes guias na prospecção desses corpos. A composição química dos perfis, em termos dos elementos maiores, é caracterizada por teores extremamente elevados de Fe nos gossans, que diminuem, progressivamente, em direção aos latossolos, e inversamente, Si, Al, Ti e H₂O (perda ao fogo), enriquecendo-se para o topo dos perfis. Cálcio, Mg, Na e K estão completamente empobrecidos na maioria das amostras estudadas. Em relação aos elementos-traço, as associações geoquímicas são bastante variáveis, entre os perfis das duas áreas, refletindo, fundamentalmente, as variações químico-mineralógicas das zonas primárias. Nos corpos gossânicos mineralizados, as seguintes assinaturas geoquímicas foram caracterizadas: Au, Cu e Mo, na área Igarapé Bahia; e Au, Cu, As, B, W, Sn e Bi, na área Aguas Claras. Diferentes partículas de Au de diversos pontos dos perfis, associadas a sulfetos, veios de quartzo, gossans, crostas lateríticas e latossolos foram observadas ao Microscopio Eletrônico de Varredura e analisadas com o Sistema de Energia Dispersiva, com grandes variações observadas, em termos da morfologia e da composição química das mesmas. Prata, Pt, Pd, Fe e Cu foram freqüentemente encontrados nas análises, onde os teores de Ag variavam de menos de 1% até a composição do electrum. As partículas estudadas foram divididas em: (1) Partículas de ouro primárias (associadas aos sulfetos primários); e (2) Partículas de Au secundárias ou supergênicas, associadas aos gossans, crostas lateriticas e latossolos, sendo essas últimas classificadas como (2.1) residuais, aquelas, em geral, com mais de 30 gm de diâmetro médio, núcleo primário e bordas lixiviadas em Ag; e (2.2) autigênicas ou neo-formadas, de elevada pureza, e extremamente diminutas (< 5 pm), via de regra na periferia dos grãos maiores, residuais. Em todas as partículas de ouro relacionadas aos perfis laterito-gossânicos estudadas, as formas e os contatos delas com os principais minerais hospedeiros, goethita e hematita, indicam uma cristalização contemporânea do ouro com esses minerais. Os resultados obtidos levaram a interpretação do desenvolvimento dos perfis laterito-gossânicos em quatro fases principais, de abrangência regional, onde cada uma dessas fases desempenhou um importante papel na redistribuição do ouro: A fase I, denominada de Fase de formação dos gossans, está relacionada ao desenvolvimento dos gossans, em condições climáticas tropicais semi-áridas a sazonalmente úmido (savana), e considerados neste trabalho como anteriores ao Terciário Inferior. Durante essa fase, o ouro foi remobilizado das zonas sulfetadas através, principalmente, de soluções ou complexos Au-tiossulfatados, reprecipitando na zona oxidada, junto com os óxidos 'e hidróxidos de ferro. As partículas neoformadas, resultantes, apresentam granulação fina e pureza média (teor algo elevado de Ag); A fase II foi denominada de lateritização Matura e está relacionada ao marcante processo de intemperismo laterítico que aconteceu na região Amazónica, como um todo, durante o Terciário Inferior. Perfis lateríticos maturos se formaram, indistintamente, sobre os gossans, e sobre as suas encaixantes, com o desenvolvimento de crosta laterítica brechóide contendo fragmentos dos gossans. Com essa superposição de processos, o sistema gossânico foi aberto, e uma nova remobilização aconteceu, dessa vez em condições mais oxidantes e, certamente, com uma importante atuação dos complexos orgânicos, cianetos e complexos aquo-hidrolisados na mobilização do ouro. Além da mobilização química desse elemento, importante dispersão fisica aconteceu, com o início da formação da feição morfológica tipo "cogumelo". Na fase três, descrita neste trabalho como pós-lateritização Matura, assiste-se a uma retomada de condições favoráveis a lateritização, semelhantes as da fase anterior, com o intemperismo dos perfis lateríticos maturos, a partir do Mioceno Médio. Os principais produtos deste período são os latossolos da área Igarapé Bahia. Com a nova abertura de sistema, o ouro é novamente remobilizado, através dos mesmos mecanismos fisico-químicos e com a atividade orgânica desempenhando um papel mais intenso em relação a fase anterior, com forte dispersão fisica, no sentido do espalhamento ou abertura dos halos de dispersão do Au e diminuição do sinal deste elemento. A intensidade deste ciclo de lateritização foi menor que o do Terciário Inferior, já que a mudança para condições mais secas no Plioceno e início do Pleistoceno, levou a uma intensa denudação da paisagem, com a erosão e truncamento dos perfis na área Águas Claras e exposição dos gossans. Importantes depósitos coluvionares (na área Águas Claras) e aluvionares auríferos, a nível regional, são relacionados a esse período. A fase IV estão associados todos os processos de destruição/intemperismo do quadro geomorfológico estabelecido no final da fase III, em função das condições, predominantemente, úmidas, que passaram a prevalecer a partir do final do Pleistoceno e início do Holoceno, dando origem a novos níveis de latossolos, linhas de pedras, colúvios e aluviões. / The Igarapé Bahia mine and the Águas Claras prospect are examples of supergene gold mineralization in gossans and latentes. They are located in the Carajás mining district, Pará state, Northern Brazil. These areas belong to Vale do Rio Doce Company and all the exploration programs were conducted by DOCEGEO. In this work, mineralogical and geochemical studies were performed in the weathering profiles of both areas focussing on the behaviour and distribution of gold and associated elements. The two areas exhibit similar primary geological context, with gold-bearing sulphide zones associated with shear zones and intense hydrothermal alteration, related to Archaean to Proterozoic metavolcano-sedimentary sequences. The supergene products are divided in two main groups: The gossan system and the lateritic system with evidences of superimposition of the latter on the former. The profiles were studied after different surface and subsurface sampling. The following horizons and zones were described, from base to top: (1) in the gossan system: primary sulphide zone, secondary sulphide zone and a thick oxidation zone; (2) in the latente system: a brecciated lateritic iron crust, a dismantled iron crust or stone-lines and latossols. The lateritic iron crust developed over the pre-existing gossans, resulting in a complete obliteration of the primary textures and structures and promoting a new remobilization of gold and other elements. This structuration can be observed today in the Igarapé Bahia area while at Águas Claras the latente profile over the mineralized bodies was truncated and exposing the gossans. The mineralogical composition of gossans and latentes is mainly represented by hematite, and variable amounts of goethite, Al-goethite, maghemite, gibbsite, kaolinite and quartz. Hematite predominates in the gossans and goethite becomes progressively enriched toward the latentes. In the deepest parts of the gossans the following minerais were identified: malachite, cuprite and native copper, mainly associated with hematite, besides azurite, chrysocolla and quartz; the Aguas Claras area presents a broader paragenesis in the primary sulphide zone, that includes: chalcopyrite, pyrite, arsenopyrite, cobaltite, quartz, magnetite, wolframite and tourmaline. Primary gold occur as diminute particles finelly disseminated in the sulphides and with different compositions in the Au-Ag alloy. In the Águas Claras area it occurs associated with a wide range of Bi-, Ag- and Pb-tellurides, besides native bismut. Tourmaline (dravite) and wolframite (ferberite) also occur as important accessory minerais, both in the primary and secondary environment. In the gossans they occur as centimetric cumulates, acting as important guides for gossans identification. Major element geochemistry of the profiles is mainly characterized by very high iron contents in the gossans, that progressively diminish toward the latossols. On the oder hand, the contents of Si, Al, Ti and LOI increase toward the top of the profiles. Calcium, Mg, Na e K are completely depleted in the gossans and laterites. Geochemical associations of trace elements are variable for the two areas and reflect mainly the chemical and mineralogical variations from the primary zones. In the mineralized bodies (gossans + iron crust) the following geochemical signatures were characterized: Au, Cu and Mo, for the Igarapé Bahia area; and Au, Cu, As, B, W, Sn and Bi, for the Águas Claras area. From the various horizons and zones of the profiles, different gold particles were separated and analised by Scanning Electron Microscope with Energy Dispersive System. Strong variations were described in terms of morphology and chemical compositions in the Au-Ag alloy. Silver, Pt, Pd, Fe e Cu were frequently detected, where Ag contents range from less than 1% to more than 25%. The studied grains were divided in two groups: (1) Primary particles associated with primary sulphides; and (2) Secondary or supergene particies, associated with gossans, latentes and latosols. These were further divided in two groups: (2.1) residuais particles, generally with more than 30 grn of mean diameter and exhibiting a primary core with Ag-depleted rims; and (2.2) authigenic or neoformed particles, which are extremely fine (< 5 1.un) and of very high fineness, frequently associated to the coarser and residual grains. The results obtained allowed us to interpret the supergene evolution of the area in four main phases, each one associated with or related to a major period of gold remobilization: Phase I - Gossan formation: related to the development of gossanic bodies in tropical climatic conditions which ranged from semi-arid to seasonally humid (savannas). In this work this is considered as prior to Lower Tertiary. During this phase, gold was remobilized from lower primary zones through thiosulphates complexes and reprecipited in the upper oxidized zones associated with iron oxy-hydroxides. The reprecipitated gold is fine-grained and of medium fineness. Phase Mature Lateritization: related to the broad lateritic weathering processes that took place in the whole Amazon region during Early Tertiary times. Mature lateritic profiles were formed above the gossans and their wall-rocks, with the development of a brecciated lateritic iron crust that includes gossans fragmenta. The gossan system was obviously oppened during this phase resulting in physical and chemical dispersion of gold. The role of organic matter related to biological activity was very important in the chemical remobilization of gold. Phase 111 - Post-Mature lateritization: related to all weathering processes that took place in the region after the establishment of the lateritic profiles during the trànsition Upper Oligocene-Middle Miocene. The main supergene products of this phase are the upper latosols of the Igarapé Bahia area. After the weathering of gold-bearing lateritic crusts, this element is once again remobilized following the same chemical mechanisms of phase II, but under increasing biological activity. This resulted in an intensive physical dispersion, broadening of geochemical haloes and weakening of gold signals. This new lateritic cycle was less intensive as compared to the previous one. It took place in the transition to more and conditions during the Plio-Pleistocene, resulting in an intense denudation of the landscape with erosion, truncation and exposure of the Aguas Claras gossans. Widespread gold-bearing coluvium (in the Águas Claras arca) and Placer deposits are inportant supergene products regionally related to this phase. Ali the weathering processes that took place after the establisment of the landscape in the end of phase III are considered in this work as phase IV. These are related to prevailing humid conditions that become dominant after the end of Pleistocene and during the Holocene, giving rise to new latosols, stone-lines, coluvium and aluviums.

L'objectif de cette thèse était d'utiliser les propriétés magnétiques des roches (paléomagnétisme, anisotropie de la susceptibilité magnétique (ASM) et différentes techniques d'analyse de la minéralogie magnétique), complété par d'autres méthodes classiques (microscopie pétrographique et électronique, goniométrie de texture, spectroscopie Raman, statistiques d'orientation préférentielle de forme et cathodoluminescence) pour aborder des thématiques récurrentes relatives à la formation des gisements métalliques. Cette approche a été appliquée à deux problèmes métallogéniques : (i) le traçage de la circulation du fluide ayant produit la métasomatose ferrifère des carbonates de La Florida (Cantabrie, Nord de l'Espagne) et la mise en place des minéralisations à Pb–Zn associées (MVT) ; et (ii) l'étude de la structuration des gossans et le décryptage de l'histoire de l'altération des amas sulfurés de la Ceinture Pyriteuse Sud Ibérique (CPSI, Andalousie, Sud de l'Espagne). Dans le premier cas, nous avons pu démontrer expérimentalement que la circulation des fluides produit une anisotropie de forme dont on peut retrouver l'empreinte dans le signal de l'ASM. Cette propriété nous a permis de définir une direction NE–SW de circulation du fluide métasomatique à La Florida. Dans le deuxième cas, le couplage entre l'ASM et le paléomagnétisme nous a permis de définir deux fabriques magnétiques dans les gossans primaires ; (i) une fabrique, d'âge ancien, de « compaction », caractéristique des parties supérieures des gossans, probablement associé au réchauffement de l'Oligocène supérieur et (ii) une fabrique de « mélange », plus récente, localisée dans les parties inférieures des gossans, formée probablement lors de l'événement messinien. La fabrique de «compaction » refléterait donc une altération plus mâture comparée à la fabrique de « mélange ». Par ailleurs, l'analyse de la minéralogie magnétique des gossans et des terrasses de la CPSI a révélé la présence « inhabituelle » de la pyrrhotite, une présence que nous avons attribué à l'activité de bactéries sulfato-réductrices dans des microdomaines réducteurs, disséminés dans un environnement globalement oxydant et très acide.